# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Joe L. Ogan on 12/12/2009 18:59:53

Title: Does Gravity do any work?
Post by: Joe L. Ogan on 12/12/2009 18:59:53
Does Gravity do any work?  I think there is a school of thought that believes that Gravity does no work.  It appears to me that the concept may be in error.  I welcome comments.  Thanks.  Joe L. Ogan
Title: Does Gravity do any work?
Post by: Mr. Scientist on 12/12/2009 19:36:08
It is in error, because gravity is the same as matter, and since matter does work, the postulation speaks for itself :)
Title: Does Gravity do any work?
Post by: Joe L. Ogan on 12/12/2009 21:15:02
Is that really true?  That is a rather clear definition of Gravity.  I wonder why Einstein didn't think of that.  Thanks.  Joe L. Ogan
Title: Does Gravity do any work?
Post by: Mr. Scientist on 12/12/2009 21:36:34
he did
Title: Does Gravity do any work?
Post by: Farsight on 12/12/2009 23:19:53
This is an interesting one, Joe. When we look at something as simple as a waterwheel, we say that gravity does do work. We get hydroelectric power from this, so it looks cut and dried.

But when we look deeper, we see there's something of a problem. Work is the transfer of energy, and if we examine two masses falling towards one another, we know from conservation of energy that no energy is added to the "system" that is those two masses. That's because energy causes gravity, and if energy was really being added, the gravity of the two combined masses would exceed the gravity of the two separate masses.

If the two masses spiralled around one another in a closing orbit and then fused gently and stayed cold, we wouldn't have extracted any work. If however we extracted work from the two masses falling together, the energy we extracted will be dissipated as heat. Then we know from E=mc² that the two masses will then weigh a little less. So the answer is: no, gravity doesn't do any work, but it does allow us to do work.
Title: Does Gravity do any work?
Post by: Joe L. Ogan on 12/12/2009 23:31:20
Is Gravity the same as Matter?  Did Einstein so state?  Thanks.  Joe L. Ogan
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 00:18:07
Yes joe. He eqiuavalated gravity to mass and not only gravity, but acceleration and natural distortions. :)
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 00:20:51
This is an interesting one, Joe. When we look at something as simple as a waterwheel, we say that gravity does do work. We get hydroelectric power from this, so it looks cut and dried.

But when we look deeper, we see there's something of a problem. Work is the transfer of energy, and if we examine two masses falling towards one another, we know from conservation of energy that no energy is added to the "system" that is those two masses. That's because energy causes gravity, and if energy was really being added, the gravity of the two combined masses would exceed the gravity of the two separate masses.

If the two masses spiralled around one another in a closing orbit and then fused gently and stayed cold, we wouldn't have extracted any work. If however we extracted work from the two masses falling together, the energy we extracted will be dissipated as heat. Then we know from E=mc² that the two masses will then weigh a little less. So the answer is: no, gravity doesn't do any work, but it does allow us to do work.

Farsight.. enough. You have seen my reply - your archiac science has no place when considering these things. Obviously if gravity is a physical force, it must have a work associated to it.
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 00:57:40
Farsight.. enough. You have seen my reply - your archiac science has no place when considering these things. Obviously if gravity is a physical force, it must have a work associated to it.

I don't think so. Mr S, I presume you are sitting on a chair. Gravity is exerting a force on you and the chair is reacting to that force to support your mass. While the chair continues to support that mass, no work is done. If the chair collapses and you fall to the ground, work has been done. Just because there is a force it does not mean work is being done.
Title: Does Gravity do any work?
Post by: Karen W. on 13/12/2009 01:56:22
By work do you mean there was no energy expended in sitting down on the chair Geezer?
Title: Does Gravity do any work?
Post by: Joe L. Ogan on 13/12/2009 02:29:38
Yes joe. He eqiuavalated gravity to mass and not only gravity, but acceleration and natural distortions. :)

Mr. Scientists.  As you know, I have great respect for your Scientific knowledge.  You have increased my vocabulary.  You have taught me a lot about Science.  But your statement that Mass and Gravity are the same thing begs the question with me.  Please let me explain.  If you give me some Mass, I can measure it, I can weigh it.  I can tell you if it stinks or if it smells good.  I can't do any of those things with Gravity.  It will be very interesting to see you tell me how I can do those.  Thanks for your comments.  Joe L. Ogan
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 02:34:07
By work do you mean there was no energy expended in sitting down on the chair Geezer?

Of course work was expended in moving your mass to sit in the chair. But once you sit in the chair, no work is being done. However, the gravitational force persists.

If mass is moved along the direction of the force of gravity, work is done by, or against, gravity. While you are sitting in your chair, gravity is doing no work.

"Ah ha!" you might say, "So why am I still orbiting the centere of the planet? Surely the force of gravity is supplying the energy to make this happen."

Simply, it cannot do that! The force of gravity acts towards the centre (of mass) of the earth. At any moment while you are sitting in your chair, your direction of travel is perpendicular to that force. Unless Einstein repealled the basic laws of mechanics, it is not possible for gravity to propel your movement around the center (of mass) of the earth. Your motion is a function of the friction between you and the earth. The friction is a function of the gravitational force between you and the earth, but unless you move along that gravitational line of force, no work is done by, or against, the force of gravity.
Title: Does Gravity do any work?
Post by: Karen W. on 13/12/2009 02:46:48
Is the balancing you do to stay in an upright position constitute force still happening or does it become different from side to side why I ask this is that when I have passed out in my chair my body has slumped down and out of the chair to the ground where I find myself when I have come too, so is there not a constant work happening in just trying to sit upright in a chair to begin with?

I am not trying to be difficult but to understand as I am horrible with gravity and it is a difficult subject even with basic understandings...
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 02:59:57
Is the balancing you do to stay in an upright position constitute force still happening or does it become different from side to side why I ask this is that when I have passed out in my chair my body has slumped down and out of the chair to the ground where I find myself when I have come too, so is there not a constant work happening in just trying to sit upright in a chair to begin with?

I am not trying to be difficult but to understand as I am horrible with gravity and it is a difficult subject even with basic understandings...

No problem. It's quite simple really. If your "center of mass" moved closer to the earth's center of mass when you slumped forward, (which it probably did) then the force of gravity did work. When you realized that you were about to start inhaling your soup, you had to do a corresponding amount of work to straighten yourself up again, and you did that work against the force of gravity.

If you are sitting on an office type chair with wheels, and you scoot across the floor on it, you did some work to do the scooting, but gravity didn't do any work because the distance between your center of mass and the earth's center of mass didn't change (unless your floor has a nasty slope, in which case you should sell your house soon.)

BTW, we are all acutely aware of the force of gravity, particularly as we get older. We tend to take it for granted. We do have a pretty good idea about what it does, certainly in close proximity to our star (the sun), but although we can predict quite accurately what it does, we don't really have a very good handle on how the heck it does it! There are many schools of thought on that one.
Title: Does Gravity do any work?
Post by: Karen W. on 13/12/2009 03:22:01
Thank you Geezer.. I need to contemplate that a while! maybe look up some references..or diagrams..you have any... diagrams that is? I am a visual girl!
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 03:36:31
Le sigh!

[diagram=549_0]
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 05:20:44
As Cyril Fletcher put it, "The effects of gravity can be grave."

Seriously, gravity is a major embarrassment to modern science. We are all continuously aware of the effects of gravity. Scientists can describe very accurately how gravity influences all objects with mass, but (as far as I am aware) there is no consensus in the scientific community that describes how gravity actually manages to exert its invisible force.

We can talk all we want about the other forces in nature, but if we can't even explain gravity, why would we believe any of them?

(I'm heading for my underground bunker right now.)
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 07:01:21
I give up. I am telling you all, there is work assocoiated to the gravity of something. If it didn't, gravitational objects could not move. It's very simple.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 07:04:21
Is the balancing you do to stay in an upright position constitute force still happening or does it become different from side to side why I ask this is that when I have passed out in my chair my body has slumped down and out of the chair to the ground where I find myself when I have come too, so is there not a constant work happening in just trying to sit upright in a chair to begin with?

I am not trying to be difficult but to understand as I am horrible with gravity and it is a difficult subject even with basic understandings...

Hi Karen

It's technically true to say there was a gravitational work being done - such things are possible for objects and its called a gravitational potential. However, most of the atoms in your body are able to sit down because of electrostatic repulsion, where all the electrons that are in your body which meet the chair all repel each other like little magnets, so in most cases scientists prefer to think of it being far more ovewhelming than lets say a gravitational influence.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 07:07:34
Farsight.. enough. You have seen my reply - your archiac science has no place when considering these things. Obviously if gravity is a physical force, it must have a work associated to it.

I don't think so. Mr S, I presume you are sitting on a chair. Gravity is exerting a force on you and the chair is reacting to that force to support your mass. While the chair continues to support that mass, no work is done. If the chair collapses and you fall to the ground, work has been done. Just because there is a force it does not mean work is being done.

Now you're talking rubbish.
To have any physical system do work, there must be a force applied to it. This was simple Newtonian dynamics. The chair collapsing does not because of the overwhelming electrostatic force, not the gravity which almost surely cancels out.
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 07:29:55
Farsight.. enough. You have seen my reply - your archiac science has no place when considering these things. Obviously if gravity is a physical force, it must have a work associated to it.

I don't think so. Mr S, I presume you are sitting on a chair. Gravity is exerting a force on you and the chair is reacting to that force to support your mass. While the chair continues to support that mass, no work is done. If the chair collapses and you fall to the ground, work has been done. Just because there is a force it does not mean work is being done.

Now you're talking rubbish.
To have any physical system do work, there must be a force applied to it. This was simple Newtonian dynamics. The chair collapsing does not because of the overwhelming electrostatic force, not the gravity which almost surely cancels out.

Mr S,
It is unfortunate that you have to descend to terms like "talking rubbish". Kindly review my explanation and provide me with some data that points out a flaw in my reasoning.
G
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 07:32:02
I apologize. I've only just woken up. I have had many things on my mind and it does not excuse my use of language. I will though, point out your mistakes, properly.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 07:32:17
How are you with math?
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 07:39:03
My math is limited. My understanding of mechanics and dynamics is not so limited. Work is a very simple concept. Force applied without movement (displacement) does NOT constitute work.

Get your facts straight before you start rubbishing other posters.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 07:51:03
Well, this won't be too hard. :)

Take the constant g - this is gravitational acceleration. This is the kind of acceleration attributed to moving objects. In fact, we can pretty much tell the motion of how thing move to earth because of this constant, which has a value of about 9.8N.

x=x_0+v_0(t)+1/2(at)^2

where t is the function of time, x_0 is the initial position and v_0 is the initial velocity. Now, you can plug in pretty much any values you want for the time, position, but you can specifically replace (a) which is the acceleration symbol with [g] - ''that'' gravitational acceleration so as you might surmise to understand, force is actually attributed to the gravitational acceleration and we often give it as F=Mg - meaning that force is inversely-related to the acceleration of something, a point you where completely ignoring when i told you.

Suppose we did change the formula. It would now look like:

y=v_{0v}(t) - 1/2 (gt)^2

This means we can work out the height of an object - an the essential word here is being ''work'' - so how is work associated to everything conjectured so far?

well, this can be easier to understand. The energy of any mass is given in terms of a possible work it may do. This is the gravitational potential, and if there is a potential for any mass to move, then it SURELY CAN be said there is an associated possible work.

GPE=Wh

is the Gravitational potential energy (GPE) equation, and W is the weight and h is the height. And so:

GPE=Mgh

Where M is the mass. So i have proven here that the mass is certaily related to the work it can do. Just to prove it with a final set of equations, h/2 will be our units above the ground, so to have something move, we now yield the formula:

W_k=Fh

where W_k is the work due to kinetic energy and this work of any material body i inversely related to the force and hieght of the system from some h/2 from the earth's surface.

Done.

Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 07:57:35
You may try to redefine the definition of work all you want, but it's already been defined. Sorry.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 08:12:58
But it proved you wrong, so the sorry from you was disingeneous. I can assure you, that matter is the presence of gravity, so gravity has its own work associated to it as well. You cannot have a material body without the presence of gravity - the two are fundamentally the same thing in relativity.
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 08:23:45
OK. So let's assume, for a moment, that you are correct and that work is being done in propelling you around the universe.

Where is the source of the energy that is doing that work, and how is it being communicated to your mass?
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 08:26:30
The graviton - and according to most scientists, it's a true particle even though its still not been found. In fact, let's take this conjecture rather than your own. Makes things a hell of a lot simpler.

Do not particles contain a kinetic energy which can provide them with work?

Do not gravitons by theory physically interact all material bodies together attractively?
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 08:28:17
You may try to redefine the definition of work all you want, but it's already been defined. Sorry.

And by the way.. i never redefined anything. The work is basic textbook physics.
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 08:38:14
Yes. Gravitrons may explain how the gravitational force is communicated. But how would that force be able to impart a force that is orthagonal to it?
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 08:46:15
You may try to redefine the definition of work all you want, but it's already been defined. Sorry.

And by the way.. i never redefined anything. The work is basic textbook physics.

That's good. I only understand basic textbook physics.
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 08:59:06
Quote
I give up. I am telling you all, there is work assocoiated to the gravity of something. If it didn't, gravitational objects could not move. It's very simple.

Dearie me, Mr S,

That doesn't strike me as being too scientific. If you'll pardon my unscientific opinion, it sounds a lot more like

"Proof By Loud Assertion" than anything else.

BTW, gravitational objects move by virtue of their kinetic energy. What's to slow them down?

G
Title: Does Gravity do any work?
Post by: Bored chemist on 13/12/2009 10:13:20
It is in error, because gravity is the same as matter, and since matter does work, the postulation speaks for itself :)
The unit of mass is the kilogram.
The units in which gravity gets measured depend on how you look at it but they are either M^3/S^-2/Kg or just Kg M S^-2

two things with different units are not the same thing.

Gravity is a force and matter is what forces act on.
They are plainly different and it's silly to say they are the same.

For what it's worth, Einstein said that mass was the equivalent of energy rather than of gravity.

Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 12:57:58
It is in error, because gravity is the same as matter, and since matter does work, the postulation speaks for itself :)
The unit of mass is the kilogram.
The units in which gravity gets measured depend on how you look at it but they are either M^3/S^-2/Kg or just Kg M S^-2

two things with different units are not the same thing.

Gravity is a force and matter is what forces act on.
They are plainly different and it's silly to say they are the same.

For what it's worth, Einstein said that mass was the equivalent of energy rather than of gravity.

Not according to Einstein. Gravity and matter are essentially the same thing; your arguement consists of what units one wishes to choose to measure something, but math is abstractual that way and can't itself be used as an arguement.

If Einsteins theory was not correct locally, then we would see matter without the presence of gravitational distortions... or atleast, hypothetically-saying, since we wouldn't be here at all if the two where not just different fascets or different sides to the same quantum coin.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 12:59:37
Quote
I give up. I am telling you all, there is work assocoiated to the gravity of something. If it didn't, gravitational objects could not move. It's very simple.

Dearie me, Mr S,

That doesn't strike me as being too scientific. If you'll pardon my unscientific opinion, it sounds a lot more like

"Proof By Loud Assertion" than anything else.

BTW, gravitational objects move by virtue of their kinetic energy. What's to slow them down?

G

A decrease in kinetic energy of course, but for a graviton this will not necesserily happen because it must move at a constant speed of c if it where to actually be a physical attraction from one body of mass to another.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 13:00:47
Yes. Gravitrons may explain how the gravitational force is communicated. But how would that force be able to impart a force that is orthagonal to it?

Vector calculus. You could work orthagonal vectors to suit what you wanted to measure.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 13:01:20
And of course the laws of motion included, along with every other quantum detail you wish or need to have.
Title: Does Gravity do any work?
Post by: Joe L. Ogan on 13/12/2009 13:29:14
It is in error, because gravity is the same as matter, and since matter does work, the postulation speaks for itself :)
Mr. Scientist.  I believe that I have figured out what you meant to say:  "Gravity is an inherent part of Matter.  It can not be separated.  But it does do work."  Thanks, Joe L. Ogan
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 13:32:25
Yes, pretty much :)
Title: Does Gravity do any work?
Post by: Mr. Scientist on 13/12/2009 13:33:07
But Work - in the scientific meaning of it because of the total equality of mass and the presence of gravity.
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 17:34:44
Yes. Gravitrons may explain how the gravitational force is communicated. But how would that force be able to impart a force that is orthagonal to it?

Vector calculus. You could work orthogonal vectors to suit what you wanted to measure.

Mr S, I do understand vectors, and it is quite impossible to derive any force that is orthogonal to another force by vector analysis, vector calculus or anything else. This is not a math problem.

I'll give you a model and you can try to knock it down:

Attach a string to a pebble. Now swing the pebble around your head. It orbits around your hand. Easy! Right?

Now try to repeat the experiment without moving your hand in a circle. It's impossible because you cannot impart any rotational movement to the pebble.

Think of gravity as the string and your hand as the center of mass of the earth. Unless the center of mass of the earth executes a circular path relative to you (and it doesn't), it can do nothing to propel you.
Title: Does Gravity do any work?
Post by: Bored chemist on 13/12/2009 20:28:26
It is in error, because gravity is the same as matter, and since matter does work, the postulation speaks for itself :)
The unit of mass is the kilogram.
The units in which gravity gets measured depend on how you look at it but they are either M^3/S^-2/Kg or just Kg M S^-2

two things with different units are not the same thing.

Gravity is a force and matter is what forces act on.
They are plainly different and it's silly to say they are the same.

For what it's worth, Einstein said that mass was the equivalent of energy rather than of gravity.

Not according to Einstein. Gravity and matter are essentially the same thing; your arguement consists of what units one wishes to choose to measure something, but math is abstractual that way and can't itself be used as an arguement.

If Einsteins theory was not correct locally, then we would see matter without the presence of gravitational distortions... or atleast, hypothetically-saying, since we wouldn't be here at all if the two where not just different fascets or different sides to the same quantum coin.
Just plain wrong.
If I chose to measure it in feet, ponds and days that would be a metter of choice but, watever base units you choose the units of mass and gravity will not be the same.
One is a force and the other isn't.
As has been pointed out before, you are seeking "proof by shouting" and that's not going to work here.

It's true that matter produces gravity but that doesn't mean it's the same thing.
Engines produce smoke, but you wouldn't try to run a car on smoke.
Title: Does Gravity do any work?
Post by: Geezer on 13/12/2009 20:56:46
It is in error, because gravity is the same as matter, and since matter does work, the postulation speaks for itself :)
Mr. Scientist.  I believe that I have figured out what you meant to say:  "Gravity is an inherent part of Matter.  It can not be separated.  But it does do work."  Thanks, Joe L. Ogan

Joe, I think you'll find that gravity is a consequence of matter, rather than being part of it. One theory is that matter distorts space to produce gravity. Another is that gravity is produced by gravitons, although, thus far, gravitons remaim hypothetical particles. There is a lot of money being spent to try to observe them, but as far as I am aware, they have not been observed yet.
Title: Does Gravity do any work?
Post by: Joe L. Ogan on 13/12/2009 22:23:47
Would I be wrong in saying, "Gravity is an inherent function of Matter."?  I am trying to get a clearcut definition of Gravity so I can discuss it intelligently with, not only Scientific people but, with the general Population.  Thanks for your help.  Joe L. Ogan
Title: Does Gravity do any work?
Post by: Geezer on 14/12/2009 03:13:35
Would I be wrong in saying, "Gravity is an inherent function of Matter."?  I am trying to get a clearcut definition of Gravity so I can discuss it intelligently with, not only Scientific people but, with the general Population.  Thanks for your help.  Joe L. Ogan

Joe, you are not alone! We seem to have a lot of good science that describes quite accurately what gravity does, although it is not inconceivable that we may have to make some adjustments to that science in the future.

So the "what" part is very well understood. However, when it comes to the "how" part (as in, how is gravitational force communicated between matter) it is still something of a "work in progress". If you look up gravity on Wikipedia, you'll see the many theories of gravity.

I'm not sure how best to describe it in one sentence.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 16:11:09
Would I be wrong in saying, "Gravity is an inherent function of Matter."?  I am trying to get a clearcut definition of Gravity so I can discuss it intelligently with, not only Scientific people but, with the general Population.  Thanks for your help.  Joe L. Ogan

Gravity is matter - There are both inherent forms of the same thing. Inherent here, is used in the sense of talking about matter as actual fluctuations in the form of distortions. These distortions in spacetime create the curvature that is observed around massive objects in space - and curvature is directly related to acceleration.

In relativity, this means that curvature, acceleration, matter and distortions and also including gravity are all fascets of the same presence of what we observe. Remove one of these, and you cannot deal with the rest.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 16:14:17
It is in error, because gravity is the same as matter, and since matter does work, the postulation speaks for itself :)
The unit of mass is the kilogram.
The units in which gravity gets measured depend on how you look at it but they are either M^3/S^-2/Kg or just Kg M S^-2

two things with different units are not the same thing.

Gravity is a force and matter is what forces act on.
They are plainly different and it's silly to say they are the same.

For what it's worth, Einstein said that mass was the equivalent of energy rather than of gravity.

Not according to Einstein. Gravity and matter are essentially the same thing; your arguement consists of what units one wishes to choose to measure something, but math is abstractual that way and can't itself be used as an arguement.

If Einsteins theory was not correct locally, then we would see matter without the presence of gravitational distortions... or atleast, hypothetically-saying, since we wouldn't be here at all if the two where not just different fascets or different sides to the same quantum coin.
Just plain wrong.
If I chose to measure it in feet, ponds and days that would be a metter of choice but, watever base units you choose the units of mass and gravity will not be the same.
One is a force and the other isn't.
As has been pointed out before, you are seeking "proof by shouting" and that's not going to work here.

It's true that matter produces gravity but that doesn't mean it's the same thing.
Engines produce smoke, but you wouldn't try to run a car on smoke.

It's not wrong. If i can remember our debates correctly, you are associating units to justify your arguement. Gravity (or the acceleration due to gravity) is still a force exerted on the system, just as much as weight is in fact inversely proportional to the force and height from the given surface of a gravitationally-warped object.

And i'm not shouting. I've explained very civilly that you where wrong, including those who still persist not to link gravity as an inherent part and of the same single thing as matter itself. I cannot shout on the internet, and even if i could, i wouldn't shout.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 16:17:40
You can quite easily, for instance, measure weight in Newtons. You can also measure the weight of the earth, because of it being made of units of kilograms. Corresponding the two, 1 kilogram is about 9.8 Newtons. So your presumptious nature was wrong.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 17:54:07
Yes. Gravitrons may explain how the gravitational force is communicated. But how would that force be able to impart a force that is orthagonal to it?

Vector calculus. You could work orthogonal vectors to suit what you wanted to measure.

Mr S, I do understand vectors, and it is quite impossible to derive any force that is orthogonal to another force by vector analysis, vector calculus or anything else. This is not a math problem.

I'll give you a model and you can try to knock it down:

Attach a string to a pebble. Now swing the pebble around your head. It orbits around your hand. Easy! Right?

Now try to repeat the experiment without moving your hand in a circle. It's impossible because you cannot impart any rotational movement to the pebble.

Think of gravity as the string and your hand as the center of mass of the earth. Unless the center of mass of the earth executes a circular path relative to you (and it doesn't), it can do nothing to propel you.

Are we certain on this? Why not show me some of this understanding then...? Teach me something new.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 17:54:59
I can't understand your analogy - so will require some math - you know.. the vector calculus you are aware of?
Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 18:00:09
I'll save you the trouble of the math - lets stick to terminology.

Something which form a right angle is perpendicular meaning that they are orthagonal - Force IS a vector quantity, so having the information of two vectors form a vertical point of saturation. Are you telling me that we cannot associate two vectors which are perpendicular together and not define each other using calculus?
Title: Does Gravity do any work?
Post by: Bored chemist on 14/12/2009 19:43:08
"You can quite easily, for instance, measure weight in Newtons. You can also measure the weight of the earth, because of it being made of units of kilograms. Corresponding the two, 1 kilogram is about 9.8 Newtons. So your presumptious nature was wrong."

Just plain wrong.
You measure mass in kilograms and weight in Newtons (at least in physics you do). If you like you can switch units and use pounds and slugs, but the units of weight and mass are different because (whether you like it or not) they are different things.
That's why on the moon I would remain about 70Kg in mass, but I would only weigh about 12 Kgf.
Coloquially you weigh things in kilograms but strictly you are using the Kgf as a unit, not the Kg.

It would also help if you stopped posting word-salad like "Force IS a vector quantity, so having the information of two vectors form a vertical point of saturation. Are you telling me that we cannot associate two vectors which are perpendicular together and not define each other using calculus?"
Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 19:45:54
The definition of 1kg (which is about the same mass as an apple) is 1 newton.

YOU ARE WRONG. Ok?

Just accept it. You're making a fool of yourself.
Title: Does Gravity do any work?
Post by: Geezer on 14/12/2009 19:56:19
Yes. Gravitrons may explain how the gravitational force is communicated. But how would that force be able to impart a force that is orthagonal to it?

Vector calculus. You could work orthogonal vectors to suit what you wanted to measure.

Mr S, I do understand vectors, and it is quite impossible to derive any force that is orthogonal to another force by vector analysis, vector calculus or anything else. This is not a math problem.

I'll give you a model and you can try to knock it down:

Attach a string to a pebble. Now swing the pebble around your head. It orbits around your hand. Easy! Right?

Now try to repeat the experiment without moving your hand in a circle. It's impossible because you cannot impart any rotational movement to the pebble.

Think of gravity as the string and your hand as the center of mass of the earth. Unless the center of mass of the earth executes a circular path relative to you (and it doesn't), it can do nothing to propel you.

Are we certain on this? Why not show me some of this understanding then...? Teach me something new.

We are certain of this. The only way to produce a component of force in the direction of motion is create a vector that has a component of force in that direction. By moving your hand in a circular arc, that is precisely what you are doing. So the force in the string (the vector) has two components at right angles. One that propels the stone, and another that acts towards the center of rotation (which in this case is not where your hand is) that maintains the orbit of the stone.

Now, when the vector (the string) goes straight to the center of rotation, the component of the force in the direction of rotation is zero.

Does that help?
Title: Does Gravity do any work?
Post by: Bored chemist on 14/12/2009 19:59:00
The definition of 1kg (which is about the same mass as an apple) is 1 newton.

YOU ARE WRONG. Ok?

Just accept it. You're making a fool of yourself.

F***ing big apples where you come from.

The definition of the kilogram is the mass of a cylinder of a platinum iridium alloy in France.
It was originally defined from the mass of a cubic decimetre of water and the metre was, in turn defined as a fraction of the eath's meridian through Paris.

I'm not the one making a fool of myself here, other than because I'm wasting my time arguing with an idiot.
Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 20:02:01
It would also help if you stopped posting word-salad like "Force IS a vector quantity, so having the information of two vectors form a vertical point of saturation. Are you telling me that we cannot associate two vectors which are perpendicular together and not define each other using calculus?

Give me strength - now your supporting him? Just you two keep together. You make a good couple.

Title: Does Gravity do any work?
Post by: Mr. Scientist on 14/12/2009 20:03:08
The definition of 1kg (which is about the same mass as an apple) is 1 newton.

YOU ARE WRONG. Ok?

Just accept it. You're making a fool of yourself.

F***ing big apples where you come from.

The definition of the kilogram is the mass of a cylinder of a platinum iridium alloy in France.
It was originally defined from the mass of a cubic decimetre of water and the metre was, in turn defined as a fraction of the eath's meridian through Paris.

I'm not the one making a fool of myself here, other than because I'm wasting my time arguing with an idiot.

1 Newton i meant to say corresponds to the general mass of an apple. Ok? It's you that's messing with my head.
Title: Does Gravity do any work?
Post by: Bored chemist on 14/12/2009 20:08:06
It would also help if you stopped posting word-salad like "Force IS a vector quantity, so having the information of two vectors form a vertical point of saturation. Are you telling me that we cannot associate two vectors which are perpendicular together and not define each other using calculus?

Give me strength - now your supporting him? Just you two keep together. You make a good couple.

Since we are both right it should be no problem to keep together.
Anyway, while I accept that a typical apple has a mass of about 0.1Kg and a weight (on earth near sea level) of about 1 Newton, that has nothing to do with the fact that weight and mass are different. Ask an astronaut.
Title: Does Gravity do any work?
Post by: Farsight on 16/12/2009 00:13:35
Joe: imagine a cubic light year of space. I nearly said "empty space", but it isn't quite empty, because that space has vacuum energy. But anyway, put a planet in the middle of this apace. In doing this you've added a mass, and since E=mc² you've added a concentration of energy. As a result, you've also altered the surrounding space to add a gravitational field. Now add a second planet, thus adding further energy to your cubic light year. Now let the two planets fall together and coalesce. There will be fireworks, and radiation, and that radiation will escape your cubic light year. This is the transference of energy, and at first glance gravity caused it, so we might say "gravity does work". But instead of making two planets, you could have made a star, and then the radiation would have escaped your cubic light year, and gravity didn't make it happen. Since a concentration of energy causes gravity, and a ooncentration of energy in a star emits radiation, it's better to say that non-uniform energy usually does work. Energy tends to spread out evenly, that's entropy, and when it does this, you have a transfer of energy, which is work.

PS: a black hole is something of an exception, in that then the concentration of energy doesn't spread out.
Title: Does Gravity do any work?
Post by: Pmb on 16/12/2009 00:53:18
Quote from: Joe L. Ogan
Does Gravity do any work?  I think there is a school of thought that believes that Gravity does no work.  It appears to me that the concept may be in error.  I welcome comments.  Thanks.  Joe L. Ogan
I’m amazed at how much discussion there is on such a simple question. The answer to your question is yes. Gravity does work. And that is a result which both Newtonian Gravity and General Relativity (GR) give. If you’d like a textual statement of this then you can look on page 187 in the famous GR text “Gravitation,” by Misner, Thorne and Wheeler. The authors use work to describe gravitational red shift on a photon moving through a gravitational field. On page 187 the authors write

-------------------
The drop in energy because of work done against gravitation implies drop in frequency…
-------------------

Quote from: Joe L. Ogan
Is that really true?  That is a rather clear definition of Gravity.  I wonder why Einstein didn't think of that.  Thanks.  Joe L. Ogan
I disagree with his definition. Hence why Einstein never said it. Gravity is not the same as matter. Matter is the “source” of gravity. Gravity also has potential energy in it and since energy has matter it contributes to itself. So gravity is also a source of gravity in this sense. But to say they are the same is wrong.
Title: Does Gravity do any work?
Post by: Geezer on 16/12/2009 01:09:35
I believe Joe's question may have resulted from a suggestion that gravity somehow does work to maintain bodies in orbit.

Joe, please correct me if that was not the thrust of your question.
Title: Does Gravity do any work?
Post by: Geezer on 16/12/2009 01:15:32
Quote from: Joe L. Ogan
Does Gravity do any work?  I think there is a school of thought that believes that Gravity does no work.  It appears to me that the concept may be in error.  I welcome comments.  Thanks.  Joe L. Ogan
I’m amazed at how much discussion there is on such a simple question. The answer to your question is yes. Gravity does work. And that is a result which both Newtonian Gravity and General Relativity (GR) give. If you’d like a textual statement of this then you can look on page 187 in the famous GR text “Gravitation,” by Misner, Thorne and Wheeler. The authors use work to describe gravitational red shift on a photon moving through a gravitational field. On page 187 the authors write

-------------------
The drop in energy because of work done against gravitation implies drop in frequency…
-------------------

Quote from: Joe L. Ogan
Is that really true?  That is a rather clear definition of Gravity.  I wonder why Einstein didn't think of that.  Thanks.  Joe L. Ogan
I disagree with his definition. Hence why Einstein never said it. Gravity is not the same as matter. Matter is the “source” of gravity. Gravity also has potential energy in it and since energy has matter it contributes to itself. So gravity is also a source of gravity in this sense. But to say they are the same is wrong.

Pmb, please try to direct your comments to the author of the definition. "his definition" is ambiguous. I don't think it was Joe's definition.

Thanks

Mod.
Title: Does Gravity do any work?
Post by: Joe L. Ogan on 16/12/2009 02:48:01
No, I did not say that Einstein said that Gravity was the same as Matter.  Einstein said that he did not know what Gravity is.  Obviously Gravity can not be the same as Matter.  It is rather interesting that there is so much disagreement in the Scientific circles.   Thanks for your information.  Joe L. Ogan
Title: Does Gravity do any work?
Post by: Pmb on 16/12/2009 05:59:19
Quote from: Geezer
Pmb, please try to direct your comments to the author of the definition. "his definition" is ambiguous. I don't think it was Joe's definition.

Thanks

Mod.
Sorry. When I said that and said and followed it by "Hence why Einstein never said it." I was referring to the definition given by Mr. Scientist. That's why I wrote "Gravity is not the same as matter."

I'll be clearer next time. Thanks.
Quote from: Joe L. Ogan
It is rather interesting that there is so much disagreement in the Scientific circles.  I believe that some of you need to go back and review the basics.
There is no disagreement in scientific circles. E.g. pick up any text on general relativity and they all agree on the physics I spoke of.

And thanks, but I don't need to review the basics.
Title: Does Gravity do any work?
Post by: LeeE on 16/12/2009 07:12:21
Wow! - six consecutive posts in this thread from the same person.  I think that must be a record.

Is anyone any clearer on whether gravity does any work yet?
Title: Does Gravity do any work?
Post by: Madidus_Scientia on 16/12/2009 07:37:34
I know, as if the frequent double/triple-posting wasn't enough
Title: Does Gravity do any work?
Post by: Geezer on 16/12/2009 17:39:23
Wow! - six consecutive posts in this thread from the same person.  I think that must be a record.

Is anyone any clearer on whether gravity does any work yet?

Six eh? Good catch! I missed that. BTW, Mr S has elected not to renew his membership at this time.
Title: Does Gravity do any work?
Post by: Farsight on 16/12/2009 23:04:31
Can we all agree on the following:

1) When a spacecraft orbits a planet in uniform circular motion, gravity is doing no work.

2) When a spacecraft performs a "slingshot" gravity-assist, and gains extra velocity that propels it towards Neptune, gravity is doing work.

3) When a spacecraft orbits a planet in an elliptical orbit, the question of whether gravity is doing work is debateable. One might say that gravity is "doing work" and thence "undoing work". Alternatively one might say that this is a stable configuration from which no energy is being removed, and hence gravity is doing no work.

IMHO the moot point concerning 3) above is that this situation is akin to a harmonic oscillator. The question is then "Does a harmonoc oscillator do work?". See http://en.wikipedia.org/wiki/Harmonic_oscillator. Since the photon is a form of harmonic oscillator, we then have to ask whether a photon does work when it simply travels through space. I think it's interesting how such a simple question can be so deep.

Pmb: I think it's better that the discussion should be carried on its merits rather than by deference to a 30-year-old textbook, particularly since at least one of the authors advocates time travel (http://www.google.co.uk/search?hl=en&rlz=1T4ADBF_en-GBGB240GB240&ei=PmUpS_vqH8-K4Qb1uuWcDQ&sa=X&oi=spell&resnum=0&ct=result&cd=1&ved=0CBgQBSgA&q=%22kip+thorne%22+%22time+travel%22&spell=1), which remains an unproven and unscientific conjecture.
Title: Does Gravity do any work?
Post by: Joe L. Ogan on 16/12/2009 23:20:25
Quote from: Geezer
Pmb, please try to direct your comments to the author of the definition. "his definition" is ambiguous. I don't think it was Joe's definition.

Mod.
Sorry. When I said that and said and followed it by "Hence why Einstein never said it." I was referring to the definition given by Mr. Scientist. That's why I wrote "Gravity is not the same as matter."

I'll be clearer next time. Thanks.
Quote from: Joe L. Ogan
It is rather interesting that there is so much disagreement in the Scientific circles.  I believe that some of you need to go back and review the basics.
There is no disagreement in scientific circles. E.g. pick up any text on general relativity and they all agree on the physics I spoke of.

And thanks, but I don't need to review the basics.
Title: Does Gravity do any work?
Post by: Geezer on 17/12/2009 01:05:12
Can we all agree on the following:

1) When a spacecraft orbits a planet in uniform circular motion, gravity is doing no work.

2) When a spacecraft performs a "slingshot" gravity-assist, and gains extra velocity that propels it towards Neptune, gravity is doing work.

3) When a spacecraft orbits a planet in an elliptical orbit, the question of whether gravity is doing work is debateable. One might say that gravity is "doing work" and thence "undoing work". Alternatively one might say that this is a stable configuration from which no energy is being removed, and hence gravity is doing no work.

I think that's a very fair summary of the situation.

At the risk of restating the blindingly obvious (I have a knack for that), would it be true to say that whenever the orbiting body is displaced in the direction of the gravitational force (as is the case for an elliptical orbit or a "slingshot") work is done. The energy to do the work is obtained from changes in the potential and kinetic energies of the components of the system?
Title: Does Gravity do any work?
Post by: Farsight on 19/12/2009 19:34:38
At the risk of restating the blindingly obvious (I have a knack for that), would it be true to say that whenever the orbiting body is displaced in the direction of the gravitational force (as is the case for an elliptical orbit or a "slingshot") work is done. The energy to do the work is obtained from changes in the potential and kinetic energies of the components of the system?
Personally I'd say yes for the slingshot, but not for the elliptical orbit, because the gravity isn't adding any energy to the system, and you're not getting any work out of it. I think of gravity as a "pseudoforce" for this reaon: the total energy of a system of two planets does not increase when they accelerate towards one another. But I guess the question "does gravity do work?" is somewhat ambiguous, so I wouldn't argue too strongly against somebody who said work is being done when  potential energy is changed to kinetic energy and vice versa. Interesting topic.
Title: Does Gravity do any work?
Post by: yor_on on 26/12/2009 16:54:18
Per
Does Gravity do any work?  I think there is a school of thought that believes that Gravity does no work.  It appears to me that the concept may be in error.  I welcome comments.  Thanks.  Joe L. Ogan

Perhaps the real question is.

What is 'forces'?

we like to see 'events' as created by 'forces' that we can count on. But probing their smallest constituents they are nowhere to be found.
Title: Does Gravity do any work?
Post by: Pmb on 27/12/2009 03:12:05
Quote from: Farsight
Pmb: I think it's better that the discussion should be carried on its merits rather than by deference to a 30-year-old textbook, particularly since at least one of the authors advocates time travel (http://www.google.co.uk/search?hl=en&rlz=1T4ADBF_en-GBGB240GB240&ei=PmUpS_vqH8-K4Qb1uuWcDQ&sa=X&oi=spell&resnum=0&ct=result&cd=1&ved=0CBgQBSgA&q=%22kip+thorne%22+%22time+travel%22&spell=1), which remains an unproven and unscientific conjecture.
I see nothing wrong giving a textual example of what I had said. It was not meant as proof of what I said but merely extra reading material for those who'd care for it.
Title: Does Gravity do any work?
Post by: LeeE on 28/12/2009 11:33:31
Quote
Perhaps the real question is.

What is 'forces'?

I think this is a very good question indeed.  While we can explain what causes forces, the forces themselves don't actually seem to consist of anything i.e. they aren't an entity in themselves but just seem to be an effect.
Title: Does Gravity do any work?
Post by: Geezer on 28/12/2009 19:44:23
While we can explain what causes forces, the forces themselves don't actually seem to consist of anything i.e. they aren't an entity in themselves but just seem to be an effect.

I wish we really could explain what causes the force. I think it's fair to say we really don't know at this point. However, we are very good at describing the force in terms of the effects it produces.
Title: Does Gravity do any work?
Post by: LeeE on 29/12/2009 13:48:28
While we can explain what causes forces, the forces themselves don't actually seem to consist of anything i.e. they aren't an entity in themselves but just seem to be an effect.

I wish we really could explain what causes the force. I think it's fair to say we really don't know at this point. However, we are very good at describing the force in terms of the effects it produces.

I think it sort of depends at what level of causality we're talking about.

We can say, for example, that gravity is caused by the presence of non-zero rest-mass matter, but we can't say why non-zero rest-mass matter appears to result in gravity.

An intrinsic problem with top-down analysis is that it can only result in an abstract, which can only be defined in terms of itself i.e. it is what it is and it does what it does because it is what it is.  As you burrow further down through the analysis, each time you reach a new lower, or more fundamental level, you are able to say why the higher level does what it does, defined in terms of the new lower level, but the lowest level can only be defined in terms of what happens.

For example, if we say that the presence of non-zero rest-mass matter results in gravity, and that gravity is the consequence of the mediation of gravitons, we've explained how non-zero rest-mass matter results in gravity by defining it in terms of gravitons, but we're now stuck with explaining why the presence and behaviour of gravitons manifests itself as gravity.

(Edited to remove a redundant 'at' in the penultimate paragraph)
Title: Does Gravity do any work?
Post by: om on 29/12/2009 16:26:23
Those interested in the role of gravity in the cosmos should go to

http://www.thenakedscientists.com/forum/index.php?topic=9197.0

To recap a small part:

1. Gravity is a nuclear force since essentially all mass is in the nucleus.

2. Gravity is a weak force over long distances.

3. Gravity is a very strong force over very short distances, but

4. Gravity cannot overcome repulsive forces between neutrons to convert neutron stars into black holes.

http://arxiv.org/pdf/0905.1667v1

In the Sun and in the cosmos, dynamic competition between long-range attractive gravitational forces and short-range repulsive forces between neutrons powers the objects that release energy and fill interstellar space with Hydrogen, a neutron-decay product.

If the universe is finite, then neutrons themselves may be the particle-sized black holes that were made in a Big Bang and compressed into massive, highly energetic neutron stars.

If the universe is infinite, then it may oscillate between expansion as interstellar space is filled with Hydrogen from neutron decay, and contraction after the neutron stars have evaporated and gravitational forces become dominant over repulsive forces between neutrons.

With kind regards,
Oliver K. Manuel
http://myprofile.cos.com/manuelo09
Title: Does Gravity do any work?
Post by: yor_on on 29/12/2009 20:11:43
Yep LeeE. It's very frustrating :)

Or if you accept the Higgs field then gravity 'accumulates' around mass. But where would that field come from, and why would it work at all. What's guaranteeing mass to exist even if that field existed? When we see two phenomena directly related to each other it is easy to wonder what came 'first', or did they came 'together'?

That's one of the reasons why I like 'emergences', as it allows them to come 'together' begetting new 'property's' (water to ice)

But if we look at it as having a 'beginning' involving 'forces' then we treat it as a chain of occurrences from a beginning to an end, and then that first 'force' must contain it all, in some manner of speaking, as from it all other will come.

Maybe there are other ways to look at it too?
Title: Does Gravity do any work?
Post by: questioner on 29/12/2009 22:16:42
Is gravity doing work if the energy that creates the action is within the atom to start with.
Title: Does Gravity do any work?
Post by: Geezer on 29/12/2009 23:59:48
Is gravity doing work if the energy that creates the action is within the atom to start with.

Gravity can do work on the mass of an atom, but I don't believe there has been any change in the energy within the atom.
Title: Does Gravity do any work?
Post by: questioner on 30/12/2009 02:12:23
Is gravity doing work if the energy that creates the action is within the atom to start with.

Gravity can do work on the mass of an atom, but I don't believe there has been any change in the energy within the atom.
If gravity is a weak force and does not change the energy within the atom but affects the equilibrium of the atom.
Think of an atom moving into space away from the earth, as the gravity signal weakens the atom regains equilibrium.
Title: Does Gravity do any work?
Post by: Geezer on 30/12/2009 07:12:58
If gravity is a weak force and does not change the energy within the atom but affects the equilibrium of the atom.
Think of an atom moving into space away from the earth, as the gravity signal weakens the atom regains equilibrium.

That's an interesting idea. Would I be correct in saying that you are proposing that atoms are subject to "stress" in a gravitational field?
Title: Does Gravity do any work?
Post by: questioner on 30/12/2009 09:57:05
If gravity is a weak force and does not change the energy within the atom but affects the equilibrium of the atom.
Think of an atom moving into space away from the earth, as the gravity signal weakens the atom regains equilibrium.

That's an interesting idea. Would I be correct in saying that you are proposing that atoms are subject to "stress" in a gravitational field?
Perhaps, how else would you explain it's reaction at varying distances to the earth. Maybe the gravity signal is undetectable by us at this stage.
Title: Does Gravity do any work?
Post by: LeeE on 30/12/2009 13:25:04
Yep LeeE. It's very frustrating :)

Or if you accept the Higgs field then gravity 'accumulates' around mass. But where would that field come from, and why would it work at all. What's guaranteeing mass to exist even if that field existed? When we see two phenomena directly related to each other it is easy to wonder what came 'first', or did they came 'together'?

That's one of the reasons why I like 'emergences', as it allows them to come 'together' begetting new 'property's' (water to ice)

But if we look at it as having a 'beginning' involving 'forces' then we treat it as a chain of occurrences from a beginning to an end, and then that first 'force' must contain it all, in some manner of speaking, as from it all other will come.

Maybe there are other ways to look at it too?

One of the main things I've been playing with is a bottom-up synthetic approach as an alternative to the top-down analytic approach: instead of analysing downwards through the hierarchy structure from the top, towards the bottom-level fundamental abstract, you start with the bottom-level fundamental abstract and try to synthesise the hierarchy structure upwards.  It's an interesting exercise.
Title: Does Gravity do any work?
Post by: questioner on 30/12/2009 22:14:01
Yep LeeE. It's very frustrating :)

Or if you accept the Higgs field then gravity 'accumulates' around mass. But where would that field come from, and why would it work at all. What's guaranteeing mass to exist even if that field existed? When we see two phenomena directly related to each other it is easy to wonder what came 'first', or did they came 'together'?

That's one of the reasons why I like 'emergences', as it allows them to come 'together' begetting new 'property's' (water to ice)

But if we look at it as having a 'beginning' involving 'forces' then we treat it as a chain of occurrences from a beginning to an end, and then that first 'force' must contain it all, in some manner of speaking, as from it all other will come.

Maybe there are other ways to look at it too?

One of the main things I've been playing with is a bottom-up synthetic approach as an alternative to the top-down analytic approach: instead of analysing downwards through the hierarchy structure from the top, towards the bottom-level fundamental abstract, you start with the bottom-level fundamental abstract and try to synthesise the hierarchy structure upwards.  It's an interesting exercise.
If you want to start at the bottom forget about gravity forming around mass and consider the existence of star dust. A material that planets and stars form around.
Title: Does Gravity do any work?
Post by: LeeE on 31/12/2009 18:05:28
If you want to start at the bottom forget about gravity forming around mass and consider the existence of star dust. A material that planets and stars form around.

Actually, stuff like gravity and mass are relatively high-level secondary phenomenon viewed from where I've been starting from while playing with bottom-up approaches.  Star dust is just ordinary matter, which is even further up the hierarchy.
Title: Does Gravity do any work?
Post by: Farsight on 05/01/2010 00:18:08
I've gone top-down, and it all seems to work out pretty easily. What causes forces is geometry. But it's dynamical geometry, and whilst it's really simple, people don't understand it. I expect that one day it will be taught, but until then it's somehow alien and unwelcome. It's rather odd actually.
Title: Does Gravity do any work?
Post by: LeeE on 05/01/2010 14:03:05
Lol - such certainty... [;D]

Is there not something oxymoronic in claiming that "it's really simple" yet "people don't understand it?
Title: Does Gravity do any work?
Post by: Farsight on 06/01/2010 02:37:45
It's true Lee. People have this amazing ability to reject something simple even when they themselves can't  elucidate any flaw, simply because it doesn't fit with what they know.
Title: Does Gravity do any work?
Post by: Geezer on 06/01/2010 06:23:00
Then kindly elucidate.
Title: Does Gravity do any work?
Post by: LeeE on 06/01/2010 17:07:41
Actually, I was being a bit facetious re the oxymoron; it's only natural that people require overwhelming evidence to accept a replacement for accepted hard-won, and hard-learned knowledge.
Title: Does Gravity do any work?
Post by: Farsight on 07/01/2010 15:30:24
No problem Lee. The trouble is that all that hard learning isn't always supported by evidence. Then when somebody presents some evidence to challenge it, some people refuse to admit it as evidence. They have become so convinced of something they've learned, that can no longer be impartial and open-minded. We're familiar with this sort of thing from religious folk, or those who have adopted some ideology, but we tend to think it doesn't apply to scientists. Sadly, I'm afraid it's a human trait, and all those affected by it are convinced that they are not.

Geezer, here's something I prepared earlier. There's more, but let's see how you get on with this for now.

Title: Does Gravity do any work?
Post by: Farsight on 07/01/2010 16:17:08
People tend to think the speed of light is constant, and Einstein said it. It isn't exactly true. Yes, he started with this as a postulate in 1905, but in 1911 he wrote On the Influence of Gravitation on the Propagation of Light (http://www.relativitybook.com/resources/Einstein_gravity.html), where you can see his ideas evolving as he talks about c = c0 (1 + Φ/c²). Then in 1916 in section 22 of Relativity: The Special and General Theory (http://www.gutenberg.org/etext/5001) he talks further:

"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light)".

People tend to see the word velocity in the 1920 translation without seeing the context. Many skip over his reference to "one of the two fundamental assumptions", and don't see that he's talking about a serious issue with the SR postulate of the constant speed of light. Many do not realise that Einstein didn't speak English in 1916, and what he actually said was die Ausbreitungsgeschwindigkeit des Lichtes mit dem Orte variiert. This translates into the speed of light varies with the locality (http://translate.google.com/translate_t?prev=hp&hl=en&js=y&text=die+Ausbreitungsgeschwindigkeit+des+Lichtes+mit+dem+Orte+variiert+&file=&sl=de&tl=en&history_state0=#). He was saying the speed varies with position, hence the reference to that postulate. And what he also said, is that this causes the light to follow a curvilinear path like a car veers when the near-side wheels encounter mud at the side of the road.

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fcsep10.phys.utk.edu%2Fastr161%2Flect%2Fhistory%2Feinstein_clerk.gif&hash=0665ac13890119e010246d39138389f1)

People often react badly this. Einstein talking about the variable speed of light does not fit with the relativity they've been taught. They don't appreciate that relativity today is something different to Einstein's relativity. People think Einstein told us about curved spacetime, but when you read The Foundation of the General Theory of Relativity (http://www.alberteinstein.info/gallery/pdf/CP6Doc30_English_pp146-200.pdf) it's simply not there. Yes, he talks about geometry and curvature and space-time, but he's giving the equations of motion, through space. He doesn't talk about "motion through spacetime". You can't move through spacetime, it's just the mathematical space where we plot our lines. There's other things that people aren't taught. Such as how he was still derided by many theoreticians even in 1923. You can see a reference to this on page 53 of Graham Farmelo's Dirac biography The Strangest Man:

"At that time, Cunningham and Eddington were streets ahead of the majority of their Cambridge colleagues, who dismissed Einstein's work, ignored it, or denied its significance".

Many people don't know that despite the media accolades and public adulation, Einstein drifted out of the mainstream from 1927 when he fell out with Bohr and others over quantum mechanics. They don't know that General Relativity was a "cottage industry" until the sixties, when the Golden Age (http://en.wikipedia.org/wiki/Golden_age_of_general_relativity) changed it significantly:

"The Golden Age of General Relativity is the period roughly from 1960 to 1975 during which the study of general relativity, which had previously been regarded as something of a curiosity, entered the mainstream of theoretical physics. During this period, many of the concepts and terms which continue to inspire the imagination of gravitation researchers (and members of the general public) were introduced, including black holes and 'gravitational singularity'. At the same time, in closely related development, the study of physical cosmology entered the mainstream and the Big Bang became well established... A number of simultaneous paradigm shifts characterize the Golden Age of general relativity. First and foremost, the Big Bang became the canonical cosmological model. Other paradigm shifts included a growing appreciation of the: Role of curvature in general relativity; Theoretical importance of the black holes; Importance of geometrical machinery and levels of mathematical structure, especially local versus global spacetime structure; Overall legitimacy of cosmology by the wider physics community".

Nor do most people know that in 1949 Einstein and Godel worked out that time is cofounded with motion through space, not with space. It's there in A World without Time: The Forgotten Legacy of Godel and Einstein (http://www.amazon.co.uk/World-Without-Time-Forgotten-Einstein/dp/0713993871) by Palle Yourgrau. But perhaps the signal most important thing most people don't know, is that whilst aether is a taboo word which is most definitely out of the mainstream, Einstein's gave his Leyden address in 1920. And the title is Ether and the theory of relativity (http://www.zionism-israel.com/Albert_Einstein/Albert_Einstein_Ether_Relativity.htm). There's Einstein, talking about space and calling it an aether:

"Mach’s idea finds its full development in the ether of the general theory of relativity. According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that ‘empty space’ in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν), has, I think, finally disposed of the view that space is physically empty".

All in all it adds up to something rather surprising. Relativity has always been the Cinderella of modern physics, despite his vast reputation, Einstein was hardly in the mainstream at all, and his understanding of gravity isn't mainstream any more. What's especially surprising is how similar it is to the way Newton described it in Opticks:

Doth not this aethereal medium in passing out of water, glass, crystal, and other compact and dense bodies in empty spaces, grow denser and denser by degrees, and by that means refract the rays of light not in a point, but by bending them gradually in curve lines? ...Is not this medium much rarer within the dense bodies of the Sun, stars, planets and comets, than in the empty celestial space between them? And in passing from them to great distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies towards one another, and of their parts towards the bodies; every body endeavouring to go from the denser parts of the medium towards the rarer?" queries 20 & 21

The language is different, but the underlying concept is the same. The energy tied up as the matter of a planet "conditions" the surrounding space to create a non-constant gμν along with a gradient in c which causes curvilinear motion. To many people this is unacceptable, because it isn't what they've been taught. It doesn't matter that it comes from Einstein and Newton and is supported by experimental evidence, they refuse to believe it.

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F3%2F39%2FGodfreyKneller-IsaacNewton-1689.jpg%2F225px-GodfreyKneller-IsaacNewton-1689.jpg&hash=a40ade58ea2d736396ed12e47876e204)

Show them two astronauts carrying parallel-mirror light clocks at different locations, and they will refuse to admit what the different readings on those light clock is telling them. They'll talk about coordinate speed and time dilation and spacetime curvature, anything to avoid what's in plain view: in a place where the gravitational potential is lower, the light goes slower.
Title: Does Gravity do any work?
Post by: LeeE on 07/01/2010 16:48:10
I don't think it's as simple as saying that the speed of light 'c' is not constant, for it's a measure of movement along two axis which themselves are not constant.  The speed of light will always be locally measured as being 'c', but different observers, in different localities will disagree upon how much space and time was covered during the measurement.

For example, if space-time were linear then a distant angular measurement, from a known distance, would give the same distance between two spatially separate points as a direct local measurement between them, however, due to the differing shape of space-time between the local observer making the direct measurement, and the distant observer deriving the distance via the angular displacement, they'll get different results.
Title: Does Gravity do any work?
Post by: Farsight on 07/01/2010 18:05:40
Lee: yes, different observers can get different results, and all observers will measure their local speed of light to be 299,792,458 m/s. But you're the single observer looking at two astronauts, each holding a one-metre parallel-mirror laser light clock. One's down near a neutron star, holding his light clock flat to avoid radial length contraction. The other astronaut is well out in space. Let's say you have excellent telescopes, high-speed cameras, and recording facilities. You can put the two astronauts up side-by-side on a split screen, and in slow motion you can even see the laser light reflecting back and forth between their parallel mirrors. What will you notice?
Title: Does Gravity do any work?
Post by: PhysBang on 08/01/2010 14:50:34
Lee: yes, different observers can get different results, and all observers will measure their local speed of light to be 299,792,458 m/s. But you're the single observer looking at two astronauts, each holding a one-metre parallel-mirror laser light clock. One's down near a neutron star, holding his light clock flat to avoid radial length contraction. The other astronaut is well out in space. Let's say you have excellent telescopes, high-speed cameras, and recording facilities. You can put the two astronauts up side-by-side on a split screen, and in slow motion you can even see the laser light reflecting back and forth between their parallel mirrors. What will you notice?
Well, when you collect the tapes later, you will see that each clock operates identically. One cannot beam real-time information from the two locations without some distortion that makes this exercise useless.
Title: Does Gravity do any work?
Post by: PhysBang on 08/01/2010 14:53:59
Then in 1916 in section 22 of Relativity: The Special and General Theory (http://www.gutenberg.org/etext/5001) he talks further:

"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light)".
And how does he go about showing that sspecial relativity does not ahve an unlimited domain? He uses curved spacetime, where special relativity holds in local manifolds that are combined together through differential geometry. If one does not understand this, then one cannot hope to understand contemporary gravitational theory.
Title: Does Gravity do any work?
Post by: Farsight on 09/01/2010 17:43:59
The recording facilities are at your single observer's location, PhysBang. It's not sufficient to claim there's some kind of distortion between you and the astronauts that invalidates the evidence you observe. You can give each astronaut their own recording facilities, and see from afar that astronaut 1's tape moves slower, just like his light moves slower. When you retrieve their tapes you see that tape 1 has recorded say 999 feet as opposed to 1000 feet for astronaut 2, and that this tallies with what you saw during the experiment. It adds up to direct observable evidence that in a location where the gravitational potential is lower, the light goes slower.

Einstein uses differential geometry, but he doesn't actually mention curved spacetime in The Foundation of the General Theory of Relativity. You can interpret the curvilinear motion as curved spacetime, but you mustn't let this distract you from the evidence that's in accord with what Einstein actually said. Here's a corrected translation from section 22 of Relativity: The Special and General Theory:

"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the speed of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the speed of light varies with location."

Einstein's non-constant guv is observable as a non-constant speed of light in GPS and the Shapiro delay, as well as in the Gedankenexperiment I've described here.
Title: Does Gravity do any work?
Post by: Geezer on 09/01/2010 18:21:01
Are we drifting from the original question here? Should we split some of this great discussion into a new or different topic (assuming I can figure out how to do that without fouling everything up!)

What does everyone think?
Title: Does Gravity do any work?
Post by: PhysBang on 10/01/2010 00:04:00
The recording facilities are at your single observer's location, PhysBang. It's not sufficient to claim there's some kind of distortion between you and the astronauts that invalidates the evidence you observe. You can give each astronaut their own recording facilities, and see from afar that astronaut 1's tape moves slower, just like his light moves slower. When you retrieve their tapes you see that tape 1 has recorded say 999 feet as opposed to 1000 feet for astronaut 2, and that this tallies with what you saw during the experiment. It adds up to direct observable evidence that in a location where the gravitational potential is lower, the light goes slower.
The problem with what you say here is that you have not given any rules for when we collect the tapes. Do we collect the tapes according to how long they have been operating in their respective reference frames? Do we collect the tapes according to what we determine to be simultaneous in the reference frame of a third-party observer? These rules have to be laid out.

It is not possible to simply "see" what is going on at a distant location. The light from these distant locations will undergo a number of effects that must be accounted for to remove distortion. Relativity theory is not a theory about this distortion, it is about the rules for the descriptions of the time and place of events.
Quote
Einstein uses differential geometry, but he doesn't actually mention curved spacetime in The Foundation of the General Theory of Relativity. You can interpret the curvilinear motion as curved spacetime, but you mustn't let this distract you from the evidence that's in accord with what Einstein actually said. Here's a corrected translation from section 22 of Relativity: The Special and General Theory:

"In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the speed of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the speed of light varies with location."

Einstein's non-constant guv is observable as a non-constant speed of light in GPS and the Shapiro delay, as well as in the Gedankenexperiment I've described here.
guv is not something that can simply be observed. It is a factor of the metric used to determine the distance between points of spacetime. It is used in this manner whether or not we are considering things in motion. It is the essence of curved spacetime and indeed can mean nothing else.
Title: Does Gravity do any work?
Post by: yor_on on 10/01/2010 02:41:55
Okay, lets start with defining what work is.

"Work can be defined as transfer of energy. In physics we say that work is done on an object when you transfer energy to that object. If one object transfers (gives) energy to a second object, then the first object does work on the second object. "

work_energy_power (http://www.edinformatics.com/math_science/work_energy_power.htm)

So, what do we mean by 'energy' in physics?

"There is no absolute measure of energy, because energy is defined as the work that one system does (or can do) on another. Thus, only the transition of a system from one state into another can be defined and thus measured."

Energy (http://en.wikipedia.org/wiki/Energy#Measurement)

And what do we mean by 'system' in physics?

Well, as far as I can see a 'system' seem to be about anything you can give a coherent description. So let us assume that gravity can be defined as a 'system'

So, do gravity transfer energy?

If gravity can be defined as a system, then it transfers energy, as energy is defined as "the work that one system does (or can do) on another" Notice how the two statements go into each other.

But does it go both ways?

Can you transfer energy to gravity by motion, that is by accelerating in a gravitational field? Well the gravitational field locally in the rocket will increase, right?

But do you then say that the gravity as a field or 'system' gets an added energy?

============

As for what a 'force' is in physics is rather simple.

"In physics, the concept of force is used to describe how a massive body is affected by acceleration or mechanical stress. Force can also be described by intuitive concepts such as a push or pull that can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform"

And gravity when working from that definition is a 'force' as it affects 'massive bodies'.

"What we now call gravity was not identified as a universal force until the work of Isaac Newton. Before Newton, the tendency for objects to fall towards the Earth was not understood to be related to the motions of celestial objects. Galileo was instrumental in describing the characteristics of falling objects by determining that the acceleration of every object in free-fall was constant and independent of the mass of the object. Today, this acceleration due to gravity towards the surface of the Earth is usually designated as \vec{g} and has a magnitude of about 9.81 meters per second squared (this measurement is taken from sea level and may vary depending on location), and points toward the center of the Earth. This observation means that the force of gravity on an object at the Earth's surface is directly proportional to the object's mass."

Force and Gravity (http://en.wikipedia.org/wiki/Force#Gravity)

So defined like that gravity must be a 'force'

Now, the problem with those definitions is that we normally associate 'force' with matter of some kind transforming into energy, but when we speak about gravity there in no 'transforming'. Seen as a field gravity just is, although it will vary due to mass (black hole), acceleration (rocket), and uniform motion (Earth, more or less)

But if we accept those definitions then gravity has to be a 'force'.
But the definitions all go into each other, and none of them seem to 'isolate' what's happening.
Title: Does Gravity do any work?
Post by: Geezer on 10/01/2010 06:18:40
Yes!

But does gravity do any work?
Title: Does Gravity do any work?
Post by: LeeE on 10/01/2010 14:24:49
Yes!

But does gravity do any work?

Aha, so we're back to that old chestnut [:D]
Title: Does Gravity do any work?
Post by: Farsight on 10/01/2010 15:33:02
Are we drifting from the original question here? Should we split some of this great discussion into a new or different topic (assuming I can figure out how to do that without fouling everything up!) What does everyone think?
I think yes, we were drifting from the original question, though the thread seemed to have quietened. How about if I start a new thread containing my "elucidation" along with some sort of question that invites discussion, and stick to "Does gravity do any work" here. Feel free to modify as you feel fit.

Title: Does Gravity do any work?
Post by: Geezer on 10/01/2010 17:07:46
Are we drifting from the original question here? Should we split some of this great discussion into a new or different topic (assuming I can figure out how to do that without fouling everything up!) What does everyone think?
I think yes, we were drifting from the original question, though the thread seemed to have quietened. How about if I start a new thread containing my "elucidation" along with some sort of question that invites discussion, and stick to "Does gravity do any work" here. Feel free to modify as you feel fit.

OK - That would WORK too.
Title: Does Gravity do any work?
Post by: yor_on on 10/01/2010 17:17:59
Yes!

But does gravity do any work?

As defined there it does, but if gravity is a direct result of mass, with acceleration and uniform motion being a 'secondary result' of us having three dimensions in an arrow of time, creating SpaceTime?

If mass is what creates 'space' (with our arrow of time) then 'gravity' is no ordinary 'force' to me, it's more of an invisible field where matter and 'energy' will introduce strains in that field. But it doesn't explain why gravity can increase locally inside that rocket, does it?

-----
But with the Higgs field included it might.
Title: Does Gravity do any work?
Post by: Farsight on 11/01/2010 12:31:04
OK guys, I think this nice simple explanation nails it. See what you think:

Does gravity do any work? No. Imagine you carry a 10kg cannonball up a 100m tower. You puff and pant all the way up the stairs until finally you make it to the top. You did work, and you can feel the sweat running down the inside of your shirt. The work you've done on the cannonball has now given this cannonball potential energy.  Don't worry about where it is or how it's stored, we can come back to that later. The point is that when you lean over the balcony and let go of the cannonball, that potential energy is converted into kinetic energy as the cannonball falls to earth. Gravity effects this conversion from one form of energy to another, but it doesn't add any energy. It didn't do the work, you did.
Title: Does Gravity do any work?
Post by: Geezer on 11/01/2010 18:30:27
OK guys, I think this nice simple explanation nails it. See what you think:

Does gravity do any work? No. Imagine you carry a 10kg cannonball up a 100m tower. You puff and pant all the way up the stairs until finally you make it to the top. You did work, and you can feel the sweat running down the inside of your shirt. The work you've done on the cannonball has now given this cannonball potential energy.  Don't worry about where it is or how it's stored, we can come back to that later. The point is that when you lean over the balcony and let go of the cannonball, that potential energy is converted into kinetic energy as the cannonball falls to earth. Gravity effects this conversion from one form of energy to another, but it doesn't add any energy. It didn't do the work, you did.

You're joking, right?

By definition, work is the change in kinetic energy of a body.
Title: Does Gravity do any work?
Post by: Madidus_Scientia on 11/01/2010 18:53:00
lol

So the combustion of fuel in an engine doesn't do any work, it was the organisms that decomposed into oil millions of years ago that actually cause a car to move.
Title: Does Gravity do any work?
Post by: Farsight on 12/01/2010 16:23:56
Madidus: yes of course the combustion of fuel does work. The "system" that is a gallon of petrol loses energy to the car and gets spat out as exhaust fumes. It isn't a gallon of petrol any more.

You're joking, right? By definition, work is the change in kinetic energy of a body.
I'm not joking, geezer. Look at what yor-on said above: Work can be defined as transfer of energy. In physics we say that work is done on an object when you transfer energy to that object. We transfer energy from that gallon of fuel to the car, so work is done. But drop that cannonball, and we transfer energy from the cannonball... to the cannonball. Look at http://en.wikipedia.org/wiki/Work_(physics)#Mechanical_energy and the bit where it says:

The mechanical energy of a body is that part of its total energy which is subject to change by mechanical work. It includes kinetic energy and potential energy... The principle of conservation of mechanical energy states that, if a system is subject only to conservative forces (e.g. only to a gravitational force), or if the sum of the work of all the other forces is zero, its total mechanical energy remains constant.

So yes, gravity gives that object some kinetic energy. But it came from the object's potential energy, so gravity didn't transfer any energy to the object. But I suppose it all depends on what the definition of work is. If you say it's a change in kinetic energy, then gravity does work. If you say it's change in an object's total energy, then gravity does no work. If you say it's force times distance, you're then left wondering whether gravity is a pseudoforce. For a pseudoforce, the force is always proportional to the mass of the object, which is the case for gravity. Think principle of equivalence, where Einstein was saying there's no force acting on a free-falling body, and instead the force is acting upward on a body that doesn't fall down. The object on the ground is accelerating, not the object in free-fall. That's why I take the total-energy view.

Title: Does Gravity do any work?
Post by: Madidus_Scientia on 12/01/2010 16:52:23
Quote
Madidus: yes of course the combustion of fuel does work. The "system" that is a gallon of petrol loses energy to the car and gets spat out as exhaust fumes. It isn't a gallon of petrol any more.

The fuel is the stored energy. In the cannonball analogy the stored energy is the gravitational potential energy, which uses gravity to do work to convert the stored energy into kinetic energy, just as the fuel combusts to do the same. "It isn't a gallon of petrol any more" It isn't gravitational potential energy anymore.
Title: Does Gravity do any work?
Post by: Geezer on 12/01/2010 18:29:35
Farsight, the same Wiki that you reference is quite clear on the subject.

Work is equal to the change in kinetic energy of a rigid body.

Gravity clearly altered the kinetic energy of the body. Therefore, gravity did work. The only way to argue that it didn't is to alter the definition of "work".

Title: Does Gravity do any work?
Post by: Farsight on 13/01/2010 13:03:22
Madidus: I know what you mean. Burn the fuel, and the kinetic energy that's released drives the car. Drop the cannonball from the top of the tower and make it drive a wheel at the bottom, and it's the same situation.

Geezer: the definition of work is the problem here. If you look lower down on http://en.wikipedia.org/wiki/Work_(physics) you can see This definition is based on Sadi Carnot's 1824 definition of work as "weight lifted through a height". I had a look elsewhere and found multiple statements. Picking something at random, http://id.mind.net/~zona/mstm/physics/mechanics/energy/work/work.html says In physics we say that work is done on an object when you transfer energy to that object. For introductory thinking, this is the best definition of work. Another one at http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Work/DefinitionWork.html says When a force acts to move an object, we say that Work was done on the object by the Force. For our cannonball, these two statements are contradictory. If you just say W = F * d * cos θ and then ask whether gravity is a pseudoforce, you don't get a clear answer. See http://www.av8n.com/physics/fictitious-force.htm for an interesting article.
Title: Does Gravity do any work?
Post by: Geezer on 13/01/2010 18:25:35
Geezer: the definition of work is the problem here. If you look lower down on http://en.wikipedia.org/wiki/Work_(physics) you can see This definition is based on Sadi Carnot's 1824 definition of work as "weight lifted through a height". I had a look elsewhere and found multiple statements. Picking something at random, http://id.mind.net/~zona/mstm/physics/mechanics/energy/work/work.html says In physics we say that work is done on an object when you transfer energy to that object. For introductory thinking, this is the best definition of work. Another one at http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Work/DefinitionWork.html says When a force acts to move an object, we say that Work was done on the object by the Force. For our cannonball, these two statements are contradictory. If you just say W = F * d * cos θ and then ask whether gravity is a pseudoforce, you don't get a clear answer. See http://www.av8n.com/physics/fictitious-force.htm for an interesting article.

I suspect the notion of transferring energy to a body is not necessarily correct. In the case where a ball is accelerated in outerspace, the ball has gained energy. However, in the case of lifting the ball to a greater height against the force of gravity, the ball only gained energy while it was being accelerated during the lifting process, and by the end of that process, it had lost that energy again. There was a redistribution of energy within the system that includes the ball and earth, but I don't think it's correct to say that the ball has gained energy (although I've probably said it often [:D]). Kinetic energy and potential energy are very different. Kinetic energy exists in the absence of gravitational force whereas potential energy only exists because of gravitational force.

I think the second definition is more accurate. Work was done when the ball was raised by the net force in the direction of motion and when the ball is released, work is done by the force of gravity to accelerate the mass of the ball and the mass of the Earth.

Whether or not gravity is a pseudoforce I'm not sure. But it seems to fit the definition of a force in terms of work.
Title: Does Gravity do any work?
Post by: yor_on on 14/01/2010 15:48:38
Gravity is weird. I don't know what it is really but I believe it to be a 'field'. And as a field it exists in a 'continuum'. When we do something inside that 'field' it reacts 'instantly' as far as I know. Like 'inertia' shows us in outer space, That means if you change your rockets course, inertia will produce a instant 'gravitational effect' inside that rocket (frame of reference).

I really like this one. (http://metaresearch.org/cosmology/speed_of_gravity.asp)

It proves, at least to me :) that there still are some reasonable arguments versus the idea of Gravity propagating at 'c' as a 'force' (gravitons.)

Title: Does Gravity do any work?
Post by: Geezer on 14/01/2010 17:26:11
Gravity is weird. I don't know what it is really but I believe it to be a 'field'. And as a field it exists in a 'continuum'. When we do something inside that 'field' it reacts 'instantly' as far as I know. Like 'inertia' shows us in outer space, That means if you change your rockets course, inertia will produce a instant 'gravitational effect' inside that rocket (frame of reference).

I really like this one. (http://metaresearch.org/cosmology/speed_of_gravity.asp)

It proves, at least to me :) that there still are some reasonable arguments versus the idea of Gravity propagating at 'c' as a 'force' (gravitons.)

Yoron - I agree. I don't happen to think gravity is a direct "force" between two bodies, and I'm not confident gravitons will ever be observed.

However, whether I'm right or wrong about that is of little consequence. When we talk about "gravitational force" in the context of work, it's just a convenient and sufficiently accurate way of describing the net force the bodies will experience due to gravity, or, if you prefer, due to mass interaction in spacetime.

Title: Does Gravity do any work?
Post by: yor_on on 14/01/2010 18:29:04
Yeah, when you look at the definitions I found, I can't help but think of permanent magnetism and wonder, anew, why that's not considered a 'force' too?
Title: Does Gravity do any work?
Post by: Geezer on 14/01/2010 18:54:28
Yeah, when you look at the definitions I found, I can't help but think of permanent magnetism and wonder, anew, why that's not considered a 'force' too?

"If it looks like a duck, and quacks like a duck, it's probably a duck!"

If we can anticipate a force and measure the force, it's probably OK to call it a force, even though we don't fully comprehend what causes the force.

May the Schwartz be with you.
Title: Does Gravity do any work?
Post by: Farsight on 15/01/2010 11:02:15
Geezer: hang the cannonball on a pendulum, and give it a push. You give it kinetic energy. As it swings upwards this gets converted into potential energy. As an alternative you could get some stepladders and carry the cannonball to the top of it swing. As to where this potential energy is, it's in the cannonball, not in the earth's gravitational field. This is obvious when you think of a cannonball way out in free space. It's beyond the earth's gravitational field, so its potential energy can't be in the earth's gravitational field. When however you move it and place it in the earth's gravitational field, it falls down, and then its potential energy gets converted into kinetic energy.

In terms of energy-conservation, a cold motionless cannonball at the surface of the earth is in a time-dilated environment as compared to similar cold motionless cannonball in free space, so all the atoms and electrons of the cannonball are moving at a reduced rate. Hence it has less total energy. The kinetic energy of the falling cannonball accounts for the difference. PS: I'm confident that gravitons won't be observed, because they essentially contradict relativity.
Title: Does Gravity do any work?
Post by: Geezer on 15/01/2010 16:44:46
Farsight: The potential energy exists in the entire system that includes the ball, the Earth and the net force exerted between them.

Elevate the ball, as you say, to some stationary point above the Earth. Now remove the Earth. Does the ball go anywhere? I don't think so. What happened to the energy in the ball?

When you elevate the ball, you increase the potential energy of the ball (if there is a gravitational field) but do not forget that you have also increased the potential energy of the Earth by an equal amount. The Earth wants to fall towards the ball just as much as the ball wants to fall towards the Earth. It's only the inertia of the Earth that limits its acceleration towards the ball.

Potential energy only exists within a system, not within the individual components of the system. When you take the ball into outerspace, you have reduced the gravitational force so much that the ball, to all intents and purposes, is no longer part of the system.
Title: Does Gravity do any work?
Post by: Geezer on 15/01/2010 21:22:00
Farsight: We could also conduct this experiment.

Imagine two trucks sitting on a horizontal railway track. The trucks have very good axle bearings that do not produce any friction. We connect the two trucks by a long extensible spring.

Now we force the trucks apart with, say, a telescopic pole so that the spring is extended. Then we magically collapse the pole and observe what happens.

If the trucks have equal mass, they will both accelerate towards each other at the same rate.

We can load up one of the trucks so that it has a much greater mass than the other and repeat the experiment. The more massive truck will be accelerated less than the less massive truck. We can continue adding mass to the heavy truck to a point where its acceleration is imperceptible and only the lighter truck appears to accelerate.

In this model we did work by putting energy into the spring. The spring then did work by accelerating the trucks towards each other. While the trucks are stationary, the energy is clearly in the spring, not in either of the trucks. If you removed the extended spring from the trucks and kept it extended, it would still retain the energy.

In the case of gravity, there is a difference because the "spring" only exists by virtue of the masses. The energy is stored in the combined system, not in any single element of it.
Title: Does Gravity do any work?
Post by: questioner on 16/01/2010 09:13:41
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!
Title: Does Gravity do any work?
Post by: Geezer on 16/01/2010 17:20:20
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

Questioner: That does not seem to address the question about work. Perhaps you should start a new topic in "New Theories". Thanks!
Title: Does Gravity do any work?
Post by: questioner on 17/01/2010 01:20:54
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

Questioner: That does not seem to address the question about work. Perhaps you should start a new topic in "New Theories". Thanks!
Geezer : I think Gravity does it's work at the atomic level inside the atom. Is there any research in this area.
Title: Does Gravity do any work?
Post by: Geezer on 17/01/2010 02:22:14
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

Questioner: That does not seem to address the question about work. Perhaps you should start a new topic in "New Theories". Thanks!
Geezer : I think Gravity does it's work at the atomic level inside the atom. Is there any research in this area.

It might! But "work" in this context is an old mechanical concept which is defined as force times distance, or the change in kinetic energy of a rigid body. The discussion is not really about how gravity works. That is a very interesting question and there is much debate about it on this forum on other threads.
Title: Does Gravity do any work?
Post by: Pmb on 17/01/2010 06:14:08
Does gravity do any work?
The energy that gravity uses appears to be inside every atom. Think of the energy in an atom as something like a spinning flywheel rotating at close to the speed of light with an insulating layer that stops it from reacting with other atoms around it.
Gravity could be a form of radiation (as yet undetected) that can penetrate all matter and upset this energy's equilibrium creating the action we know as the force of gravity.
This upsetting action is the actual work that gravity does. A weak force!

Questioner: That does not seem to address the question about work. Perhaps you should start a new topic in "New Theories". Thanks!
Geezer : I think Gravity does it's work at the atomic level inside the atom. Is there any research in this area.

It might! But "work" in this context is an old mechanical concept which is defined as force times distance, or the change in kinetic energy of a rigid body. The discussion is not really about how gravity works. That is a very interesting question and there is much debate about it on this forum on other threads.
I don't quite understand your reception to the idea that gravity can do work. Why do you believe this is
Title: Does Gravity do any work?
Post by: Geezer on 17/01/2010 07:22:45
PMB: If you look up the definition for Mechanical Work (here is one example http://en.wikipedia.org/wiki/Work_(physics)  ) you will see that a force that produces a change in the kinetic energy of a rigid body has done (mechanical) work.

When you drop an object, gravity accelerates the object, therefore gravity has changed the kinetic energy of the object, therefore gravity has done work.
Title: Does Gravity do any work?
Post by: Farsight on 17/01/2010 08:55:19
Geezer, sorry to be slow getting back to you, my Internet has been playing up.

Farsight: The potential energy exists in the entire system that includes the ball, the Earth and the net force exerted between them. Elevate the ball, as you say, to some stationary point above the Earth. Now remove the Earth. Does the ball go anywhere? I don't think so. What happened to the energy in the ball?
No problem re the system, and no problem re the ball staying put. But the energy in the ball is still there. The ball is made of molecules / atoms / electrons etc, and they're all moving and spinning faster than they were when the ball was on the surface of the earth, where they were subject to gravitational time dilation.

When you elevate the ball, you increase the potential energy of the ball (if there is a gravitational field) but do not forget that you have also increased the potential energy of the Earth by an equal amount. The Earth wants to fall towards the ball just as much as the ball wants to fall towards the Earth. It's only the inertia of the Earth that limits its acceleration towards the ball.
Agreed. We could talk about separating two cannonballs or two planets etc, or even a shell of objects. We just tend to simplify things because the gravity caused by the cannonball is very slight and has no detectable effect on the Earth.

Potential energy only exists within a system, not within the individual components of the system. When you take the ball into outerspace, you have reduced the gravitational force so much that the ball, to all intents and purposes, is no longer part of the system.
True, we don’t talk about potential energy unless we’re talking about a system, and yes, once in outer space the ball is no longer part of the system. We wouldn’t talk about the potential energy of a ball on its own in space, but nevertheless, we can’t neglect conservation of energy.

We could also conduct this experiment. Imagine two trucks sitting on a horizontal railway track. The trucks have very good axle bearings that do not produce any friction. We connect the two trucks by a long extensible spring. Now we force the trucks apart with, say, a telescopic pole so that the spring is extended. Then we magically collapse the pole and observe what happens. If the trucks have equal mass, they will both accelerate towards each other at the same rate.
No problem.

We can load up one of the trucks so that it has a much greater mass than the other and repeat the experiment. The more massive truck will be accelerated less than the less massive truck. We can continue adding mass to the heavy truck to a point where its acceleration is imperceptible and only the lighter truck appears to accelerate.
Yep, I'm with you.

In this model we did work by putting energy into the spring. The spring then did work by accelerating the trucks towards each other. While the trucks are stationary, the energy is clearly in the spring, not in either of the trucks. If you removed the extended spring from the trucks and kept it extended, it would still retain the energy.
Totally agree.

In the case of gravity, there is a difference because the "spring" only exists by virtue of the masses. The energy is stored in the combined system, not in any single element of it.
I'm sorry geezer, but for gravity, there is no spring. That demands action at-a-distance, which even Newton rejected. Separate the two trucks by a very large amount, and the spring doesn't pull the lighter truck back any more. I suppose one way to take a step from this analogy to what I've been saying, is to say that once the lighter truck gets to a certain distance, the spring comes unhooked from the heavier truck, and stays extended whilst the lighter truck carries it off. That means the spring is part of the lighter truck. If you imagine giving a cannonball 11.2km/s worth of kinetic energy, you’re giving it to the cannonball, not the earth. As the cannonball slows down there’s no detectable energy transfer out of the cannonball, so saying the potential energy is in the Earth’s gravitational field is relying on magic like gravitons. The cannonball doesn’t slow down to zero, it escapes the Earth. As a result the Earth’s gravitational field is slightly reduced, so it’s lost energy rather than gained energy, hence the cannonball must have taken the KE/PE energy with it. As an aside, people sometimes say a gravitational field is negative energy, but it isn’t. It’s a place where you could say the local ground state is lower, but it has a higher energy density than the surrounding space, and hence causes a little bit more gravity of its own.

By the by, I think the reason why we have a difference here is that you're taking a “classical” viewpoint whilst I’m taking the “relativity” viewpoint. Yor-on posted a useful link in another thread, see http://www.aei.mpg.de/einsteinOnline/en/elementary/generalRT/GeomGravity/index.html . Whilst I think this description is a bit too simplified, it does indicate the difference between the two viewpoints.
Title: Does Gravity do any work?
Post by: questioner on 17/01/2010 10:03:18
PMB: If you look up the definition for Mechanical Work (here is one example http://en.wikipedia.org/wiki/Work_(physics)  ) you will see that a force that produces a change in the kinetic energy of a rigid body has done (mechanical) work.

When you drop an object, gravity accelerates the object, therefore gravity has changed the kinetic energy of the object, therefore gravity has done work.
Well put, thanks Geezer I'll work on something to put in new theories
Title: Does Gravity do any work?
Post by: Geezer on 17/01/2010 18:09:20
In the case of gravity, there is a difference because the "spring" only exists by virtue of the masses. The energy is stored in the combined system, not in any single element of it.

I'm sorry geezer, but for gravity, there is no spring. That demands action at-a-distance, which even Newton rejected. Separate the two trucks by a very large amount, and the spring doesn't pull the lighter truck back any more. I suppose one way to take a step from this analogy to what I've been saying, is to say that once the lighter truck gets to a certain distance, the spring comes unhooked from the heavier truck, and stays extended whilst the lighter truck carries it off. That means the spring is part of the lighter truck. If you imagine giving a cannonball 11.2km/s worth of kinetic energy, you’re giving it to the cannonball, not the earth. As the cannonball slows down there’s no detectable energy transfer out of the cannonball, so saying the potential energy is in the Earth’s gravitational field is relying on magic like gravitons. The cannonball doesn’t slow down to zero, it escapes the Earth. As a result the Earth’s gravitational field is slightly reduced, so it’s lost energy rather than gained energy, hence the cannonball must have taken the KE/PE energy with it. As an aside, people sometimes say a gravitational field is negative energy, but it isn’t. It’s a place where you could say the local ground state is lower, but it has a higher energy density than the surrounding space, and hence causes a little bit more gravity of its own.

Indeed there is no spring. That's why I put it in double quotes. But there is, without any doubt, a resultant force that is produced by gravity, and it is that force that is responsible for doing the work that answers this topic - "Does gravity do work?"

BTW, in my analogy, if the spring became detached, the energy in it would be briefly converted into kinetic energy before it was dissipated as thermal energy.
Title: Does Gravity do any work?
Post by: Farsight on 17/01/2010 19:32:33
Geezer: take a look at that link (http://www.aei.mpg.de/einsteinOnline/en/elementary/generalRT/GeomGravity/index.html). It says "In the world of classical physics, if particles diverge from this behavior, it must be because there is a force acting on them". Then later on it says "However, there is another possibility...  In that situation, there is no force making the particles deviate from the straightest possible lines;" That's the difference I was on about between the classical viewpoint and the relativity viewpoint. You're taking the former, I'm taking the latter.
Title: Does Gravity do any work?
Post by: Geezer on 17/01/2010 20:51:49
Farsight:

However, my bathroom scales are unaware of that, and they continue to measure a force. And, as mechanical work is a classical idea that combines force and mass, gravity continues to do work.

BTW, the model in the link attempts to explain why moving particles might come together, but as far as I can see, it does not seem to explain why they continue to be attracted towards each other when they meet. Not that I'm arguing with Einstein of course, but models have their limitations.

Title: Does Gravity do any work?
Post by: Farsight on 18/01/2010 12:59:12
Geezer, maybe this will help: http://www.newton.dep.anl.gov/askasci/phy99/phy99x85.htm

Title: Does Gravity do any work?
Post by: Geezer on 18/01/2010 19:21:50
Farsight: I think it would be more helpful if you could explain, in your own words, why work is not done by a force that we can clearly observe and measure. How that force comes about is interesting, but unless you are saying that it does not actually exist, I fail to see the relevance of your objection.
Title: Does Gravity do any work?
Post by: litespeed on 18/01/2010 22:54:07
Geezer

Work is done when mass is moved from its resting state. The amount of work has to do with how far the mass is moved over a given time. The bathroom scale simply quantifies the force available to do work. The standard unit of work actually done is the joule which is equal to one newton meter.

Believe me. If I step off my bathroom scale and into an elevator shaft, my mass will not only move, it will move faster and faster over time. Accordingly, I think gravity works much like a rubber band. At the center of mass there is no gravity - like a relaxed rubber band. The surface of the earth is like a stretched rubber band. It holds potential to impart kinetic energy in the same way as stretched rubber band.

One analogy is something like this. I stretch a rubber band, an insect lands on it and then I release it. Assuming the insect hangs on, work will be done by accelerating the insect in much the same way falling down a well does the same for a dog.

You might reasonably object gravity is holding you down which is not the case with a static rubber band. Similarly, no work is done when an iron bar is held close by a big magnet. However, if you pull the bar down and an insect sticks to it, work will be done when you release the bar and it jumps to the magnet.

In all these cases potential energy can be converted to kinetic energy in a simple way. However, that energy depends on the independent variable: Specifically, potential energy is created by placing the object away from the center of attraction in the first place. Gun powder works the same way. Chemical energy is independently created and then released.

Both gravity and magnetism exert forces over long distances from many places. The mass that is subsequently accelerated in close proximity to either is simply a subtraction from the other sources.

This is just my simple minded way of thinking about this stuff. I could, and may very well, be wrong.

Title: Does Gravity do any work?
Post by: Geezer on 19/01/2010 01:59:11
Thanks Litespeed. I have attempted to annotate your post with by comments:

LS. Work is done when mass is moved from its resting state. The amount of work has to do with how far the mass is moved over a given time. The bathroom scale simply quantifies the force available to do work. The standard unit of work actually done is the joule which is equal to one newton meter.

GE. Totally agree.

LS. Believe me. If I step off my bathroom scale and into an elevator shaft, my mass will not only move, it will move faster and faster over time. Accordingly, I think gravity works much like a rubber band. At the center of mass there is no gravity - like a relaxed rubber band. The surface of the earth is like a stretched rubber band. It holds potential to impart kinetic energy in the same way as stretched rubber band.

GE. OK, except I’m not quite sure what the surface of the Earth has to do with it.

LS. One analogy is something like this. I stretch a rubber band, an insect lands on it and then I release it. Assuming the insect hangs on, work will be done by accelerating the insect in much the same way falling down a well does the same for a dog.

GE. If you are saying work is done when a dog falls down a well, I completely agree and, I no longer understand why we are even having this debate!

LS. You might reasonably object gravity is holding you down which is not the case with a static rubber band. Similarly, no work is done when an iron bar is held close by a big magnet. However, if you pull the bar down and an insect sticks to it, work will be done when you release the bar and it jumps to the magnet.

GE. Well, yes. Work is only being done while there is motion. When the system is static, no work is being done. If you are saying that gravity is doing no work to keep me in place on my chair, I completely agree.

LS. In all these cases potential energy can be converted to kinetic energy in a simple way. However, that energy depends on the independent variable: Specifically, potential energy is created by placing the object away from the center of attraction in the first place. Gun powder works the same way. Chemical energy is independently created and then released.

GE. Right. Potential Energy only exists by virtue of the position of a body within a system. Some other form of energy had to be expended to achieve that position to create the potential.

LS. Both gravity and magnetism exert forces over long distances from many places. The mass that is subsequently accelerated in close proximity to either is simply a subtraction from the other sources.

GE. OK with the first sentence. You lost me on the second - not sure about "other sources".
Title: Does Gravity do any work?
Post by: litespeed on 19/01/2010 15:01:15
Geezer - After a couple microseconds of simulated thought I subtract my comment about subtraction. However, I do not see work being done when gravity holds us in our chairs. For instance, is any work is done when a magnet holds a steel bar from falling down?

At an atomic scale, I don't see work done by the strong nuclear force until it is broken by, for instance, fission. Then all hell can break loose. I would simply speculate potential energy in all four cases has been created over time [(1) gravity; 2) magnet; 3) strong nuclear force; 4) gunpowder].

Let me speculate one step further. The only reason mass has energy E=MC2 is that at one time there was no mass. The mass 'precipitated' from the cooling big bang, thus transferring actual energy into a nicely compacted and stable form. Just about like gunpowder
Title: Does Gravity do any work?
Post by: namaan on 19/01/2010 16:22:00
Hey all,

If I could chime in I think there is a misunderstanding between Mr. Scientist and Geezer. All Geezer seems to be saying is there is no work if there is no displacement. This is true. Mr. Scientist seem to be saying that all masses at a certain height h have a gravitational potential energy GPE. This is also true. In fact I'm pretty sure you both agree with both of these statements, your arguments just don't seem to be 'in phase'.

Strictly speaking, gravity is an acceleration, not a force. Gravity generated by mass A acts on a mass B to give it a force pointing towards mass A and vice versa, as given by ForceGravity = G(massA * massB)/radius^2.

If mass B is displaced parallel to or anti-parallel to mass A, then there is an associated work, as given by Work = Force * Displacement.

Work is an energy, so here Mr. Scientist would point out that this Work is also equal to mass * gravitationalAcceleration * height, or mgh. This can be rewritten as Wh, since W or weight is a force. This is the same form as Work = Force(weight) * Displacement(height)...the point being that the GPE will not be 'used' unless the mass is actually displaced.

Sorry for the long-winded response, this is actually a good way to see whether I'm prepared for the physics test coming up [:D]

P.S. some how I missed that there are like 6 pages to this thread. I ended up replying to stuff on the first page; hope it's still relevant!
Title: Does Gravity do any work?
Post by: Geezer on 19/01/2010 17:09:22
Geezer - After a couple microseconds of simulated thought I subtract my comment about subtraction. However, I do not see work being done when gravity holds us in our chairs. For instance, is any work is done when a magnet holds a steel bar from falling down?

At an atomic scale, I don't see work done by the strong nuclear force until it is broken by, for instance, fission. Then all hell can break loose. I would simply speculate potential energy in all four cases has been created over time [(1) gravity; 2) magnet; 3) strong nuclear force; 4) gunpowder].

Let me speculate one step further. The only reason mass has energy E=MC2 is that at one time there was no mass. The mass 'precipitated' from the cooling big bang, thus transferring actual energy into a nicely compacted and stable form. Just about like gunpowder

Ahem! If you re-read my last reply, you will see that I completely agree with you that no work is being done while the system is static. I don't recall that I ever implied that it was. Sitting in chairs etc. included.

The only remaining question is: Is work done when a brick falls down a well? (As a dog owner, I prefer to use the brick model.)

If we can agree that it is, I think the question is answered.
Title: Does Gravity do any work?
Post by: Farsight on 22/01/2010 03:15:14
Farsight: I think it would be more helpful if you could explain, in your own words, why work is not done by a force that we can clearly observe and measure. How that force comes about is interesting, but unless you are saying that it does not actually exist, I fail to see the relevance of your objection.
I say work is not being done because I use the "transfer of energy" definition of work, as hinted at in http://en.wikipedia.org/wiki/Work_(physics) where it says "Likewise when a book sits on a table, the table does no work on the book despite exerting a force equivalent to mg upwards, because no energy is transferred into or out of the book". Gravity might appear to add energy to a falling object, but it's just converting some of the object's potential energy into kinetic energy. You can think of the object's potential energy as internal rotational spin energy, or as jiggle or something else, but it has to be there in the object rather than somewhere else. Otherwise as the object falls down there has to be a "magical" inflow of energy into the object, and there's no scientific evidence to support this. But I don't think I'm saying anything new here, and think it's probably best if we agree that there are ambiguities along with definition issues plus a mismatch between the classical and relativity viewpoints, and agree to differ.
Title: Does Gravity do any work?
Post by: Geezer on 22/01/2010 22:39:40
Farsight: Ah! So you are saying that, because no energy was added to the falling brick, no work was done on the brick?
Title: Does Gravity do any work?
Post by: Farsight on 23/01/2010 11:46:15
Yes.
Title: Does Gravity do any work?
Post by: Geezer on 23/01/2010 20:16:40
OK - but that seems to contradict the definition of work that states that work is done when the kinetic energy of a rigid body changes.

Could you perhaps be describing something other than work?
Title: Does Gravity do any work?
Post by: Farsight on 25/01/2010 00:08:36
I don't think I'm describing something other than work, geezer. But the definition of work does seem to be at the core of the issue here. Take a look at at http://www.grc.nasa.gov/WWW/K-12/airplane/work2.html which talks about work done on a gas. The gas isn't a rigid body, and whilst pressing down on a piston involves force x distance and hence work and the addition of energy, we're dealing with pressure rather than kinetic energy. Then see http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heat.html which says "This example of the interchangeability of heat and work as agents for adding energy to a system". It talks about adding energy but goes on to say: "neither the words work or heat have any usefulness in describing the final state of the system - we can speak only of the internal energy of the system." So what do you say if you have a method for converting microscopic internal spin motion, aka "jiggle" motion, into macroscopic linear motion, or vice versa? Called gravity? You're changing the internal potential energy into external kinetic energy, or vice versa, but you aren't actually adding any energy. All very confusing.
Title: Does Gravity do any work?
Post by: Geezer on 25/01/2010 01:10:17
I don't think I'm describing something other than work, geezer. But the definition of work does seem to be at the core of the issue here. Take a look at at http://www.grc.nasa.gov/WWW/K-12/airplane/work2.html which talks about work done on a gas. The gas isn't a rigid body, and whilst pressing down on a piston involves force x distance and hence work and the addition of energy, we're dealing with pressure rather than kinetic energy. Then see http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heat.html which says "This example of the interchangeability of heat and work as agents for adding energy to a system". It talks about adding energy but goes on to say: "neither the words work or heat have any usefulness in describing the final state of the system - we can speak only of the internal energy of the system." So what do you say if you have a method for converting microscopic internal spin motion, aka "jiggle" motion, into macroscopic linear motion, or vice versa? Called gravity? You're changing the internal potential energy into external kinetic energy, or vice versa, but you aren't actually adding any energy. All very confusing.

Well, that's all very nice, however, the work required to accelerate a brick (which is a fairly rigid body) is defined quite adequately by the change in its kinetic energy (and has been for rather a long time).

Unless you are willing to create a new definition for "work" and get it accepted by a rather large body of engineers and scientists, I think you should accept the current definition and try to accommodate it in your thinking. Of course, you can always define an entirely new concept based on your theory and try to get that accepted too.
Title: Does Gravity do any work?
Post by: Farsight on 25/01/2010 03:26:57
Your definition of work is not adequate, and it isn't my theory - it's Einstein's. See http://www.perimeterinstitute.ca/Outreach/Explore_Our_Universe/Essence_of_General_Relativity/2/. This is an outreach article by the Perimeter Institute for Theoretical Physics:

"Newton said that a falling apple is accelerating. Since acceleration requires a force, Newton had to invent the idea of a gravitational force that tugs on the apple while it is falling, making it fall faster and faster. Einstein said that it is precisely when the apple is falling that it is not accelerating (straight line trajectory in our space station example), and there is no need to introduce a mysterious gravitational force.

Newton said that an apple in your hand is not accelerating. No acceleration means no force. To arrive at no force, Newton imagined two exactly counterbalancing forces at work: gravity pulling the apple down and our hand pushing it up. Einstein said there is only one force at work: our hand pushing it up. This “unbalancing” force causes the apple to veer off its natural trajectory (and move instead on the circular trajectory in our space station example).

In short, what Newton got backwards was when the apple is accelerating and when it is not. This false starting point, although it is the common sense one, led him astray and required him to invent the idea of a gravitational force. Einstein took the diametrically opposite perspective, which showed “gravitational force” to be a red herring."

Title: Does Gravity do any work?
Post by: Geezer on 25/01/2010 05:08:29
It's not my definition either. It's the one that been used for a very long time, and at least it's written down as I'm sure you know.

How does the PI define work, or is work too mundane a concept for the PI to theorize about? The article you cite does not provide a single mathematical relationship for anything. If you wish to refute the theory of work, you will have put forward a testable alternative theory, and provide a definition.

Perhaps you would like to say that gravity didn't do work, but the curvature of space-time did? I can accept something like that.
However, the kinetic energy of the brick increased, so, according to the definition of work, something did work to accelerate the brick. Or are you saying the brick didn't accelerate at all, or that the brick really has no mass.
Title: Does Gravity do any work?
Post by: Farsight on 25/01/2010 15:22:41
The rigid-body-kinetic-energy description is the "mechanics" definition, geezer. I don't know how the Perimeter Institute define it, but I'll ask them. I'm not refuting the theory of work, I'm trying to inform of work and  gravity according to general relativity. No, your falling brick is not accelerating: Einstein said that it is precisely when the apple is falling that it is not accelerating.
Title: Does Gravity do any work?
Post by: Geezer on 25/01/2010 18:45:02
I think I see the problem. In absolute terms, the brick may not be accelerating. However, in relative terms, it is. Einstein may have said the brick is not accelerating because it is travelling in a straight line in spacetime due to its inertia, but he didn't say the brick and the Earth were not getting closer to each other at an increasing rate.

Within the Earth/brick system, the distance between the brick and the Earth did change. If you prefer to think of this as the Earth accelerating toward the brick, that's fine. We know this to be true because we can measure the effect as often as we want, and we will always get the same result. So, while the brick may have experienced zero force, relative to the Earth it really did accelerate (or the other way around if you prefer).

The velocity of the brick relative to the Earth changed. That's all we need to prove that work was done. The effect we refer to as gravity was responsible for doing the work, even without a direct force acting on the brick.
Title: Does Gravity do any work?
Post by: lightarrow on 25/01/2010 20:16:05
The definitions of: work, kinetic energy, force, acceleration,..., that you have in mind, Geezer, are non-relativistic definitions. The correct ones are those written by Farsight.

The fact that a body's mass have to vary while falling towards a massive object, is...on the road to convince me.
Title: Does Gravity do any work?
Post by: Geezer on 25/01/2010 20:54:10
The definitions of: work, kinetic energy, force, acceleration,..., that you have in mind, Geezer, are non-relativistic definitions. The correct ones are those written by Farsight.

The fact that a body's mass have to vary while falling towards a massive object, is...on the road to convince me.

I would like to see any other definition of Work. If you know of one, please tell us what it is. By the way, if Farsight is correct, as I suspect he is, there is no "falling towards a massive object".

According to Newtonian mechanics, gravity clearly does "Work". While we can demonstrate that there are some flaws in Newtonian mechanics, that does not relieve us of the obligation to explain the previously observed, and indisputable, phenomena in terms of the revised paradigm. Waving our arms in the air while chanting "non-relativistic" may not constitute a sufficient explanation.

If the simplest of terms, such as Work, cannot be explained in relativistic terms, perhaps we should question relativity.

(Personally, I do not challenge relativity. However, we cannot simply dismiss 150 years of scientific endeavor and entirely valid data without an explanation for that data.)
Title: Does Gravity do any work?
Post by: lightarrow on 26/01/2010 15:28:47
If you define work as ∫F•ds you have to tell me what is F when a body falls towards the Earth. In GR gravity is not a force.
Title: Does Gravity do any work?
Post by: Geezer on 26/01/2010 19:55:13
As you can see, I used the well known change in kinetic energy definition of work, precisely for that reason. All that has changed is distance in time. No force is required. I don't think GR abolished the need for either distance, or time.

BTW, if no work is done when a body "falls" to Earth, isn't it a bit strange that work has to be done to increase distance between a body and the Earth, or does your interpretation of GR dictate that no work is necessary? If so, we might want to let NASA know that they have been wasting an awful lot of fuel for no good reason.
Title: Does Gravity do any work?
Post by: lightarrow on 27/01/2010 01:21:36
As you can see, I used the well known change in kinetic energy definition of work, precisely for that reason. All that has changed is distance in time. No force is required. I don't think GR abolished the need for either distance, or time.

BTW, if no work is done when a body "falls" to Earth, isn't it a bit strange that work has to be done to increase distance between a body and the Earth, or does your interpretation of GR dictate that no work is necessary? If so, we might want to let NASA know that they have been wasting an awful lot of fuel for no good reason.
The kinetic energy theorem is a consequence of ∫F•ds.

About the second question, Farsight explained it: when you lift a payload with a rocket, the rocket engine *does* make work on the payload, increasing its energy and so its mass, from M to M + ΔM; when the payload falls, the ΔM becomes kinetic energy.
Yes, it's very weird... I'm still quite confused about it.  [:-\]
Title: Does Gravity do any work?
Post by: Geezer on 27/01/2010 02:01:20
The kinetic energy theorem is a consequence of ∫F•ds.

And it's a consequence because???

This is just plain silly. If work is done to elevate a body, but no work is done to lower a body, we have just invented perpetual motion. Woopee! We're all going to be rich!

er, or, you don't suppose it's because it's quite complicated to explain what's going on in terms of GR? It's quite simple to explain in terms of Classical Mechanics (CM). GR says there is no "gravitational force", so we can't have it both ways and say that an alternative definition for KE that is in accord with GR is invalid without a rigorous proof.

CM is not so hard to understand, and in a great many situations it's a very good model. It certainly provides a very good first approximation. GR refines the model, but it does not invalidate the CM model.

If GR provides an alternative definition for Work, we should understand what that definition is. Failing that, I suppose we'll just have to keep going with the old CM definition.
Title: Does Gravity do any work?
Post by: Farsight on 27/01/2010 11:49:16
I think I see the problem. In absolute terms, the brick may not be accelerating. However, in relative terms, it is. Einstein may have said the brick is not accelerating because it is travelling in a straight line in spacetime due to its inertia, but he didn't say the brick and the Earth were not getting closer to each other at an increasing rate.
Agreed.

Within the Earth/brick system, the distance between the brick and the Earth did change. If you prefer to think of this as the Earth accelerating toward the brick, that's fine. We know this to be true because we can measure the effect as often as we want, and we will always get the same result. So, while the brick may have experienced zero force, relative to the Earth it really did accelerate (or the other way around if you prefer).
Whether it really did accelerate or not represents the difference bewteen Newtonian mechanics and relativity.

The velocity of the brick relative to the Earth changed. That's all we need to prove that work was done. The effect we refer to as gravity was responsible for doing the work, even without a direct force acting on the brick.
Again, I think the problem here is one of definition. Perhaps the important lesson of this thread is how much there is to discuss about "what things mean".
Title: Does Gravity do any work?
Post by: Farsight on 27/01/2010 12:34:00
Your posts noted lightarrow. Mind you, mass is a whole new can of worms, and I wouldn't say I've given any correct definitions. Every time I've tried to look up a defintion of "work" I find different statements with subtle differences.

BTW, if no work is done when a body "falls" to Earth, isn't it a bit strange that work has to be done to increase distance between a body and the Earth, or does your interpretation of GR dictate that no work is necessary? If so, we might want to let NASA know that they have been wasting an awful lot of fuel for no good reason.
Take a look at http://www.ddart.net/science/physics/physics_tutorial/Class/energy/U5L2a.html which says:

"When work is done upon an object by an internal force (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant. In such cases, the object's energy changes form. For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object is transformed into kinetic energy. Yet, the sum of the kinetic and potential energies remain constant. This is referred to as energy conservation and will be discussed in detail later in this lesson. When the only forces doing work are internal forces, energy changes forms - from kinetic to potential (or vice versa); yet the total amount of mechanical is conserved."

This is agreeing with you in saying gravity does do work, essentially saying it's the transfer of energy from one form to another. But it also says that the total energy of your brick up in space is the same as the total energy of your brick when it's falling to earth at 11.2 km/s. There is no transfer of energy to the brick, and the total system energy is unchanged. Hence the ambiguity as regards work. If you say it's the transfer of energy from one form to another, gravity does work. If you say it's transfer of energy into a system, it doesn't.

Re NASA, if they've got a brick sitting motionless on the surface of the earth, they need to add 11.2 km/s worth of energy to give that brick the same total energy it would have if it was sitting motionless up in space. Thus irrespective of the above, whether you say work is transferring energy from one form to the other or transferring energy into the brick, they have to do work on it.
Title: Does Gravity do any work?
Post by: lightarrow on 27/01/2010 13:06:46
The kinetic energy theorem is a consequence of ∫F•ds.

And it's a consequence because???
Because in CM KE = ½mv2 and F = ma. Evaluating ∫F•ds you have the result.

Quote
This is just plain silly. If work is done to elevate a body, but no work is done to lower a body, we have just invented perpetual motion. Woopee! We're all going to be rich!
No, because you loose mass when the object fall and acquires KE, so its energy stay the same...

Quote
er, or, you don't suppose it's because it's quite complicated to explain what's going on in terms of GR? It's quite simple to explain in terms of Classical Mechanics (CM). GR says there is no "gravitational force", so we can't have it both ways and say that an alternative definition for KE that is in accord with GR is invalid without a rigorous proof.

CM is not so hard to understand, and in a great many situations it's a very good model. It certainly provides a very good first approximation. GR refines the model, but it does not invalidate the CM model.

If GR provides an alternative definition for Work, we should understand what that definition is. Failing that, I suppose we'll just have to keep going with the old CM definition.
In GR not only the concept of work, but even much more "simple" concepts as mass, velocity (and also space and time, if we want) are not immediately evident. In GR you don't have a unique concept of mass, for example:
http://en.wikipedia.org/wiki/Mass_in_general_relativity
All of GR treatment is complex, so what is silly is to pretend to have a definition of work as simply mathematically formalized as in CM.
http://www.physicsforums.com/showthread.php?t=130654
Title: Does Gravity do any work?
Post by: Geezer on 27/01/2010 18:23:25
Within the Earth/brick system, the distance between the brick and the Earth did change. If you prefer to think of this as the Earth accelerating toward the brick, that's fine. We know this to be true because we can measure the effect as often as we want, and we will always get the same result. So, while the brick may have experienced zero force, relative to the Earth it really did accelerate (or the other way around if you prefer).
Whether it really did accelerate or not represents the difference bewteen Newtonian mechanics and relativity.

How do we explain our observations? The position of the brick (relative to the Earth) clearly changed. Or are you saying our measurements of time and distance are defective because of relativity? If we can't measure anything, we can never prove anything.

I'm fairly sure that is not the case, but if it were true, relativity would be an exercise in futility and be as much use as the proverbial chocolate teapot.

If you say the brick did not accelerate within any frame of reference but you cannot explain what our observations mean, I'll be forced to conclude you know even less about relativity than I do. Which, btw, ain't much!

This discussion is beginning to sound like a university student I knew who liked to chat up girls. He'd try to engage them in conversation by claiming he was studying the same subject as them.

One time he claimed he was studying Botany (about which he knew bugger all.)

The girl asked "What kind of plants are you studying?"

Quick as a flash he replied "None actually. I'm a Theoretical Botanist."
Title: Does Gravity do any work?
Post by: Farsight on 28/01/2010 15:09:58
General relativity is all about measurements of time and distance, geezer. It's quite simple really, but it usually isn't explained very well. People tend to start with "curved spacetime", and they don't tell you why it's curved. IMHO it's better to think about it as curvilinear motion caused by inhomogeneous space caused in turn by the concentration of energy tied up as the matter of a planet. This is what Einstein said about general relativity in 1920:

"According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that “empty space” in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν)..."

Space in a gravitational field isn't homogeneous. It has a lower gμν below you, and a higher gμν above. Between them there's a gradient. It's like an "energy density gradient". So when you move through it like a photon, you veer, downwards. Since we measure space and time using the motion of light, we say space-time is curved, and that you follow a null geodesic. You tend not to find this kind of description when you look it up and the internet, but it is in line with what Einstein said. It's accepted physics. What he didn't actually say and what isn't accepted physics is this: if you're just sitting there in space like an electron, the electron has spin, so internally it's moving in little circles. Hence part of your path is subject to veer, so you work yourself down - the brick falls down because it's composed of electrons and things with spin. I don't know why this isn't accepted physics, but I think it will be in time.
Title: Does Gravity do any work?
Post by: PhysBang on 28/01/2010 20:34:00
You tend not to find this kind of description when you look it up and the internet, but it is in line with what Einstein said. It's accepted physics. What he didn't actually say and what isn't accepted physics is this: if you're just sitting there in space like an electron, the electron has spin, so internally it's moving in little circles. Hence part of your path is subject to veer, so you work yourself down - the brick falls down because it's composed of electrons and things with spin. I don't know why this isn't accepted physics, but I think it will be in time.
It's quite obvious why this isn't accepted physics: it cannot be used to actually produce predictions of the motions of electrons.
Title: Does Gravity do any work?
Post by: Farsight on 29/01/2010 00:16:07
PhysBang: some imagined mathematical deficiency is no substitute for the experimental fact that pair production creates an electron and a positron from light, that electrons and positrons exhibit the properties of angular momentum and magnetic moment and can be diffracted like light, and that the product of electron/positron annihilation is light. See The Nature of the Electron by Qiu-Hong Hu (Physics Essays, Vol. 17, No. 4, 2004) at http://arxiv.org/abs/physics/0512265 along with Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime by Ye Xing-Hao et al (Chinese Phys. Lett. 25 1571-1574 2008) at http://www.iop.org/EJ/abstract/0256-307X/25/5/014. You may also wish to peruse The Refractive Index in Electron Optics and the Principles of Dynamics by Ehrenberg and Siday (Proc. Phys. Soc. B62: 8–21 1949). I'm sorry, but these are bona-fide peer-reviewed papers, dismissal and denial is no longer an option.

Now please, can we stay on topic. If you wish to assist on this thread, give geezer the initial understanding of general relativity which he seeks.
Title: Does Gravity do any work?
Post by: JP on 29/01/2010 03:30:16
PhysBang: some imagined mathematical deficiency is no substitute for the experimental fact that pair production creates an electron and a positron from light, that electrons and positrons exhibit the properties of angular momentum and magnetic moment and can be diffracted like light, and that the product of electron/positron annihilation is light. See The Nature of the Electron by Qiu-Hong Hu (Physics Essays, Vol. 17, No. 4, 2004) at http://arxiv.org/abs/physics/0512265 along with Inhomogeneous Vacuum: An Alternative Interpretation of Curved Spacetime by Ye Xing-Hao et al (Chinese Phys. Lett. 25 1571-1574 2008) at http://www.iop.org/EJ/abstract/0256-307X/25/5/014. You may also wish to peruse The Refractive Index in Electron Optics and the Principles of Dynamics by Ehrenberg and Siday (Proc. Phys. Soc. B62: 8–21 1949). I'm sorry, but these are bona-fide peer-reviewed papers, dismissal and denial is no longer an option.

It's a big jump from being able to treat curvature of space-time as an index of refraction for light (or electron) beams to saying that electrons have a helical structure.  The first article that makes claims about the electron having some odd helical structure is published in Physics Essays, which, while peer-reviewed, accepts a lot of papers on fringe topics that don't have mainstream acceptance.

Quote
Now please, can we stay on topic. If you wish to assist on this thread, give geezer the initial understanding of general relativity which he seeks.
I do have a question relating to the thread topic:

The concept of work in classical mechanics is useful in dealing with conservation of energy, since it tells you how you can add or subtract energy from a system, especially a system in a potential.  So my question is this: does conservation of energy hold in GR?  And if it does or doesn't, could someone explain why or why not?  (Just from the fact that gravity isn't treated as a force and that it deals with non-inertial reference frames, I would think you'd run into problems...)
Title: Does Gravity do any work?
Post by: Farsight on 29/01/2010 09:09:51
It's a big jump from being able to treat curvature of space-time as an index of refraction for light (or electron) beams to saying that electrons have a helical structure. The first article that makes claims about the electron having some odd helical structure is published in Physics Essays, which, while peer-reviewed, accepts a lot of papers on fringe topics that don't have mainstream acceptance.
The issue of mainstream acceptance and the exact structure can be debated, JP, but the "made of light" and the rotational evidence remains. Another paper on this theme is "Is the electron a photon with a toroidal topology?" by Williamson and van der Mark (Annales de la Fondation Louis de Broglie, Volume 22, no.2, 133 1997). These are former CERN scientists, and it took them six years to get this into a journal. You can download a version from http://www.cybsoc.org/cybcon2008prog.htm#jw.

..The concept of work in classical mechanics is useful in dealing with conservation of energy, since it tells you how you can add or subtract energy from a system, especially a system in a potential.  So my question is this: does conservation of energy hold in GR?...
Yes. See the link above to http://www.ddart.net/science/physics/physics_tutorial/Class/energy/U5L2a.html which says:

"When work is done upon an object by an internal force (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant. In such cases, the object's energy changes form. For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object is transformed into kinetic energy. Yet, the sum of the kinetic and potential energies remain constant. This is referred to as energy conservation and will be discussed in detail later in this lesson. When the only forces doing work are internal forces, energy changes forms - from kinetic to potential (or vice versa); yet the total amount of mechanical [energy] is conserved."

When a brick falls down, some of the potential energy associated with internalised motion becomes the kinetic energy of macroscopic motion. The reduced rate of internalised motion is accounted for by gravitational time dilation, and gravity is adding no energy to the brick.

All: I'm tied up for the next few days, and will be offline.
Title: Does Gravity do any work?
Post by: JP on 29/01/2010 14:59:17
Farsight, Annales de la Fondation Louis de Broglie is another fringe journal that publishes a lot of non-mainstream physics.  I wouldn't rely on that for citations that a theory is valid.  At any rate, the helical-electron theory is certainly not mainstream and therefore doesn't really answer the question about work in a gravitational field--at least not in terms of accepted physical theories.
Title: Does Gravity do any work?
Post by: litespeed on 29/01/2010 19:32:07
lightarrow - You wrote: "About the second question, Farsight explained it: when you lift a payload with a rocket, the rocket engine *does* make work on the payload, increasing its energy and so its mass, from M to M + ΔM; when the payload falls, the ΔM becomes kinetic energy."

This is were I become confused. We know that GPS satellites need to be corrected for two separate effects. First, removing them further from the center of gravity speeds up their time by a given amount. In addition, accelerating them slows down their time, but by a lesser amount. Accordingly GPS satellite clocks actually run faster then those on the ground.

So here is my question. Have the satellites gained or lost mass?

Title: Does Gravity do any work?
Post by: lightarrow on 29/01/2010 19:43:58
lightarrow - You wrote: "About the second question, Farsight explained it: when you lift a payload with a rocket, the rocket engine *does* make work on the payload, increasing its energy and so its mass, from M to M + ΔM; when the payload falls, the ΔM becomes kinetic energy."

This is were I become confused. We know that GPS satellites need to be corrected for two separate effects. First, removing them further from the center of gravity speeds up their time by a given amount. In addition, accelerating them slows down their time, but by a lesser amount. Accordingly GPS satellite clocks actually run faster then those on the ground.

So here is my question. Have the satellites gained or lost mass?
With respect to when they are still on the Earth surface, they should have gained mass. Anyway, the subject is quite complicated for me, I'm not sure I'll be able to give many more details.
Title: Does Gravity do any work?
Post by: yor_on on 29/01/2010 21:29:42
This is how I see it, SpaceTime's gravity is a geometry we cant see. Take gravitational waves f.ex . No matter their 'velocity' they will still be distortions of SpaceTime and when they pass through us we will deform slightly. Gravity also creates what we call 'gravity wells' where matter rests at its 'bottom'. So to do work you will have to oppose gravity, going out from that well.

The whole discussion of 'potential energy' circles around the effect we can observe when a object in 'free fall' gets stopped by matter on its journey. what we call its acceleration is its balance relative that well, depending on the gravity-wells mass.

All objects move, if uniformly or 'accelerating' doesn't matter, you can always think up another object existing in relative 'non motion' relative it. All of those things are definitions relative something else. 'Work' as an abstract idea is when you do something that's contrary to the 'easiest path'. The more you go of the path of non resistance, the more work you will have to do. It's a general rule, fitting almost anything I can think of :)

Like if photons really, from our perspective, never would bend towards mass, and instead alway keep a straight line. If that would happen in our SpaceTime then that light would 'do work.' That it doesn't do so does not mean that it bends, even though it seems so from our perspective. Light will always take the straight path through SpaceTime as shaped by gravity, it's just us unable to see those dips and bends.

You can f. ex. have an situation where you see it as you are 'doing work' on a object that as seen from another frame just is in a 'free fall'.

To me work will be that what opposes, generally speaking :)
Title: Does Gravity do any work?
Post by: Geezer on 29/01/2010 22:13:14
Farsight - Here's the reference you quoted in your most recent post.

"When work is done upon an object by an internal force (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant. In such cases, the object's energy changes form. For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object is transformed into kinetic energy. Yet, the sum of the kinetic and potential energies remain constant."

It is critical that we understand the meaning of the last sentence. While the brick is changing position relative to the Earth, the last sentence would seem to apply. However, when the body stops changing position relative to the Earth, the last sentence no longer holds.

When the brick comes to rest on the Earth, it has zero kinetic energy, and it has reduced potential energy.

Furthermore, the brick has the same mass that it had when we let it drop. What happened to the mass between the two states may be interesting, but it's unimportant.

At the end of the experiment, the brick has reduced energy. I'm reasonably confident that no energy was annihilated or converted into matter during this experiment, so it's likely that the total energy in the system is conserved, but it sure ain't conserved in the brick!

Energy was expended (or, more accurately, transferred to some other part of the system) in moving the brick. Therefore, work was done.

Don't get me wrong. I would like to repeal the Third Law of Thermodynamics as much as anyone, it's just that I don't think this experiment demonstrates a loophole in that law.

I think it's always important to remember that, whereas the large print giveth plenty, the small print taketh away more. Or, if you prefer, the only way to prevent entropy from winning is to do nothing.
Title: Does Gravity do any work?
Post by: JP on 30/01/2010 02:27:41
So I think that the idea of work isn't the fundamental concept.  The fundamental concept is conservation of energy, since work tells you that you're basically transferring energy from kinetic to potential (or into other forms of energy) via "work."  Therefore, it's probably easiest to start with conservation of energy.

I did a bit of reading up on the conservation of energy in general relativity.  I can follow much of the math, but I only partly follow the physical implications of it.

Basically everyone seems to agree that GR in general does not satisfy conservation of energy if you include all the non-gravitational energy.  If the geometry of space-time gets "flat" (i.e. gravity dies away) as you move infinitely far away, then the non-gravitational energy is conserved over that large volume.  The problem with GR and conservation of energy is that the equations of GR take as their "energy" term what is called the stress-energy tensor, which measures only non-gravitational energy, and says that the curvature of space is caused by this non-gravitational energy.  In order to conserve energy over a small region of space, you also need to include gravitational energy.  You can do this by tacking a  a new term onto your stress-energy tensor to include gravity. (http://en.wikipedia.org/wiki/Stress-energy_pseudotensor)

I think the analogue of this in Newtonian gravity would be that the energy of the brick itself isn't conserved while it's falling, but the energy of the brick plus its gravitational potential is.  Unlike the Newtonian case, the equations for gravity don't include a gravitational energy (or force) term, but instead specify it according to geometry, so I guess that's why you would need to go back after the fact and say that the term you're adding (to get conservation of energy) is due to gravity.  Finally, a problem with this formulation is that although energy conservation derived from this pseudotensor holds in all reference frames, the stress energy pseudotensor (the one with the gravitational energy added) isn't necessarily invariant in different reference frames, while the usual stress-energy tensor is.  I think this means that the term you're calling gravitational energy changes depending on your reference frame.

What does this mean for work?  I think you would be in trouble trying to define it, since it would involve energy transfer between the gravitational field and "everything else."  You could define the "everything else" part just fine, but defining the gravitational field part would probably be problematic because it would depend on your reference frame.

I'm not sure if this all makes sense, as I'm still trying to digest what I've read down to a simple form that makes sense...

Edit: I think a problem with considering classical examples as evidence of why work has to be defined in general relativity is that classical examples necessarily mean that the gravitational effects are weak.  To notice problems with the formulation of work, you would have to have a very strong gravitational field that ends up warping space and time in a way such that classical intuition wouldn't hold--or at least so that you couldn't observe the brick falling in the same way since the way you measure space and time doesn't match with the way the brick measures them.
Title: Does Gravity do any work?
Post by: Farsight on 01/02/2010 13:41:30
Farsight, Annales de la Fondation Louis de Broglie is another fringe journal that publishes a lot of non-mainstream physics. I wouldn't rely on that for citations that a theory is valid. At any rate, the helical-electron theory is certainly not mainstream and therefore doesn't really answer the question about work in a gravitational field - at least not in terms of accepted physical theories.
That was just another example, JP. Like I said, the detail is debateable. But pair production isn't, nor is electron angular momentum, nor gravitational time dilation. So the electron really is made of light, there's some kind of rotation or spin going on in there, and it occurs at a reduced rate down near the surface of a planet. It's all mainstream stuff.
Title: Does Gravity do any work?
Post by: Farsight on 01/02/2010 14:02:54
Geezer: when the brick hits the ground, it kicks up debris etc. It does work doing all this, losing its kinetic energy. There will be heat too, which isn't actually classed as work, but it's the same kind of thing - it's essentially kinetic energy at the atomic scale, and the brick loses it. Yes, once the brick has come to rest on the earth, and cooled down, it has zero kinetic energy, and it has reduced potential energy. Yes, the total energy of the system is conserved, but some energy was transferred out of the brick when it hit the ground. Work was definitely done. Whether you say work was done when the brick hit the ground, or when the brick started falling, depends on the definition of work. Note though that whatever your choice here, and depite the brick appearing to have the same mass as it did when you dropped it, as per Einstein's 1905 paper, its final mass is slightly reduced because it has lost energy.

JP: take a look at the The Foundation of The General Theory of Relativity from about page 182 of document 30. Here it is http://www.alberteinstein.info/gallery/pdf/CP6Doc30_English_pp146-200.pdf, you'll be aware it's 3.6 Mbyte pdf. Einstein talks about conservation of energy and on says on page 185 "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". IMHO a simple form of this that makes sense, is to describe a region of space which exhibits a gravitational field as one that also exhibits a gradient in energy density. The energy density diminishes further away from the planet, so this isn't the the brick's potential energy, because the latter increases further away from the planet.

Title: Does Gravity do any work?
Post by: lightarrow on 01/02/2010 15:07:27
That was just another example, JP. Like I said, the detail is debateable. But pair production isn't, nor is electron angular momentum, nor gravitational time dilation. So the electron really is made of light, there's some kind of rotation or spin going on in there, and it occurs at a reduced rate down near the surface of a planet. It's all mainstream stuff.
You are talking of not officially recognized theories here, certainly not "mainstream".
Title: Does Gravity do any work?
Post by: Farsight on 01/02/2010 15:32:42
I'm talking about scientific evidence, lightarrow. Search on:

Pair production (http://www.google.co.uk/search?hl=en&rlz=1W1ADBF_en-GB&q=pair+production&meta=&aq=f&oq=)

electron angular momentum (http://www.google.co.uk/search?hl=en&source=hp&q=electron+angular+momentum&meta=&rlz=1W1ADBF_en-GB&aq=f&oq=)

gravitational time dilation (http://www.google.co.uk/search?sourceid=navclient&aq=1&oq=gravitational+time+dilation&hl=en-GB&ie=UTF-8&rlz=1T4ADBF_en-GBGB240GB240&q=gravitational+time+dilation)

We really do make an electron from light, it really does exhibit angular momentum aka spin, and gravitational time dilation is for real too. That's the evidence, regardless of the status of any theory.
Title: Does Gravity do any work?
Post by: lightarrow on 01/02/2010 18:13:08
I'm talking about scientific evidence, lightarrow. Search on:

Pair production (http://www.google.co.uk/search?hl=en&rlz=1W1ADBF_en-GB&q=pair+production&meta=&aq=f&oq=)

electron angular momentum (http://www.google.co.uk/search?hl=en&source=hp&q=electron+angular+momentum&meta=&rlz=1W1ADBF_en-GB&aq=f&oq=)

gravitational time dilation (http://www.google.co.uk/search?sourceid=navclient&aq=1&oq=gravitational+time+dilation&hl=en-GB&ie=UTF-8&rlz=1T4ADBF_en-GBGB240GB240&q=gravitational+time+dilation)

We really do make an electron from light, it really does exhibit angular momentum aka spin, and gravitational time dilation is for real too. That's the evidence, regardless of the status of any theory.
There is something I'm missing. What:
-Pair production
-electron angular momentum
-gravitational time dilation
have to do with the fact the electron would be made of light?
When you say the electron is "made of light" I understand that its *inner structure* is light. Have I understood well?
Title: Does Gravity do any work?
Post by: Geezer on 01/02/2010 18:37:52
Geezer: when the brick hits the ground, it kicks up debris etc. It does work doing all this, losing its kinetic energy. There will be heat too, which isn't actually classed as work, but it's the same kind of thing - it's essentially kinetic energy at the atomic scale, and the brick loses it. Yes, once the brick has come to rest on the earth, and cooled down, it has zero kinetic energy, and it has reduced potential energy. Yes, the total energy of the system is conserved, but some energy was transferred out of the brick when it hit the ground. Work was definitely done. Whether you say work was done when the brick hit the ground, or when the brick started falling, depends on the definition of work. Note though that whatever your choice here, and depite the brick appearing to have the same mass as it did when you dropped it, as per Einstein's 1905 paper, its final mass is slightly reduced because it has lost energy.

OK - So the brick did work, but are you saying no work was done on the brick so that it was able to do work? And, if we raise the brick up again, can it recover the energy it lost unless we do work on it? Of course not.

Farsight - Your logic defies the Third Law of Thermodynamics. Even Einstein was not bold enough to attempt that.
Title: Does Gravity do any work?
Post by: Farsight on 01/02/2010 23:42:16
There is something I'm missing. What [does]:
-Pair production
-electron angular momentum
-gravitational time dilation
have to do with the fact the electron would be made of light? When you say the electron is "made of light" I understand that its *inner structure* is light. Have I understood well?
Maybe, lightarrow, but those three things together are to do with how gravity works on matter. Of those three things, only pair production is to do with the fact that the electron is made of light. We can't be sure of the structure, but what we can be sure of is that when we make an electron via pair production, we start with a nucleus and a gamma photon. We end up with a nucleus, and an electron, and a positron. The light has gone. Then when we annihilate an electron with a positron, what we get is gamma photons. That's the light back. Nothing else goes in, and nothing else comes out. So what's the electron made of? Light. Nothing else. There's not a lot to understand, the scientific evidence is there, it's cut and dried. But for some strange reason people don't seem to know about it.

Quote from: Geezer
OK - So the brick did work, but are you saying no work was done on the brick so that it was able to do work? And, if we raise the brick up again, can it recover the energy it lost unless we do work on it? Of course not. Farsight - your logic defies the Third Law of Thermodynamics. Even Einstein was not bold enough to attempt that.
My logic is rock solid, Geezer. Like I said way back (http://www.thenakedscientists.com/forum/index.php?topic=27444.msg294946#msg294946), when you lift the brick, you do work. You add energy to the brick. Drop the brick, and when it hits the ground, the brick does work. The energy you put into the brick by lifting it, goes into licking up a crater and heat etc. Whether work is being done by gravity as the brick is falling is debateable, and depends on your definition of work. If work is the transfer of energy from one form to another, gravity does work. But if work is the transfer of energy into or out of the brick, it doesn't. LOL, it's a bit like this:

Q: Will a fall kill you?
A: No. But the sudden stop at the end of the fall will.
Title: Does Gravity do any work?
Post by: JP on 02/02/2010 09:16:43
JP: take a look at the The Foundation of The General Theory of Relativity from about page 182 of document 30. ... Einstein talks about conservation of energy and on says on page 185 "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy".

This is exactly what I said in my previous post, actually.  Einstein includes an extra term to account for the gravitational field's energy.  This is where I think anyone trying to figure out work would get into trouble because the interaction between the non-gravitational energy (including masses) and the gravitational energy is going to be highly nontrivial outside of the classical limit.
Title: Does Gravity do any work?
Post by: Farsight on 02/02/2010 11:16:27
Noted, JP. I was backing up what you were saying about the energy of a gravitational field, but also trying to say something about where the brick's potential energy resides. Perhaps this will get it across: imagine you're up in space with two spinning disks in front of you. They're both doing 10000rpm, and both are equipped with revolution counters. Leave one up in space, and take the other one down to the surface of a planet. After a while, compare the counters. Because of the gravitational time dilation, you find that the counter on the disk in space shows a higher reading. So it's spinning at a higher rate, and has more angular momentum / rotational energy than the disk on the surface. The difference is the potential energy. Now think of a brick as a cuboid full of "spinning disks", see http://hyperphysics.phy-astr.gsu.edu/Hbase/nuclear/nspin.html.

Title: Does Gravity do any work?
Post by: Geezer on 02/02/2010 17:15:02
Farsight:

Work is defined as a change in KE.

Did the KE of the falling brick change? - Yes

If the brick had not been in a gravitational field, would its KE have changed? - No

The gravitational field altered the KE of the brick.

Therefore, the gravitational field was responsible for doing work.
Exactly how gravity did it is interesting and debatable, but that does not answer the question on this topic.

The only way to prove otherwise is to alter the definition of Work, KE, or both. Of course, you are welcome to provide a proof based on revised definitions.
Title: Does Gravity do any work?
Post by: yor_on on 03/02/2010 00:48:19

The concept of work in classical mechanics is useful in dealing with conservation of energy, since it tells you how you can add or subtract energy from a system, especially a system in a potential.  So my question is this: does conservation of energy hold in GR?  And if it does or doesn't, could someone explain why or why not?  (Just from the fact that gravity isn't treated as a force and that it deals with non-inertial reference frames, I would think you'd run into problems...)

JP that's a lovely question :)
And one I'm wondering over too as it connects to my question of the plate jiggling.

------Quote-----

In special cases, yes.  In general -- it depends on what you mean by "energy", and what you mean by "conserved".

In flat spacetime (the backdrop for special relativity) you can phrase energy conservation in two ways: as a differential equation, or as an equation involving integrals (gory details below).  The two formulations are mathematically equivalent.  But when you try to generalize this to curved spacetimes (the arena for general relativity) this equivalence breaks down.  The differential form extends with nary a hiccup; not so the integral form.

The differential form says, loosely speaking, that no energy is created in any infinitesimal piece of spacetime.  The integral form says the same for a finite-sized piece.  (This may remind you of the "divergence" and "flux" forms of Gauss's law in electrostatics, or the equation of continuity in fluid dynamics.  Hold on to that thought!)

An infinitesimal piece of spacetime "looks flat", while the effects of curvature become evident in a finite piece.  (The same holds for curved surfaces in space, of course).  GR relates curvature to gravity.  Now, even in newtonian physics, you must include gravitational potential energy to get energy conservation.  And GR introduces the new phenomenon of gravitational waves; perhaps these carry energy as well?  Perhaps we need to include gravitational energy in some fashion, to arrive at a law of energy conservation for finite pieces of spacetime?

Casting about for a mathematical expression of these ideas, physicists came up with something called an energy pseudo-tensor.  (In fact, several of 'em!) Now, GR takes pride in treating all coordinate systems equally.  Mathematicians invented tensors precisely to meet this sort of demand -- if a tensor equation holds in one coordinate system, it holds in all.  Pseudo-tensors are not tensors (surprise!), and this alone raises eyebrows in some circles.  In GR, one must always guard against mistaking artifacts of a particular coordinate system for real physical effects.  (See the FAQ entry on black holes for some examples.)

These pseudo-tensors have some rather strange properties.  If you choose the "wrong" coordinates, they are non-zero even in flat empty spacetime.  By another choice of coordinates, they can be made zero at any chosen point, even in a spacetime full of gravitational radiation.  For these reasons, most physicists who work in general relativity do not believe the pseudo-tensors give a good local definition of energy density, although their integrals are sometimes useful as a measure of total energy.

One other complaint about the pseudo-tensors deserves mention.  Einstein argued that all energy has mass, and all mass acts gravitationally.  Does "gravitational energy" itself act as a source of gravity?  Now, the Einstein field equations are

Gmu,nu = 8pi Tmu,nu

Here Gmu,nu is the Einstein curvature tensor, which encodes information about the curvature of spacetime, and Tmu,nu is the so-called stress-energy tensor, which we will meet again below.  Tmu,nu represents the energy due to matter and electromagnetic fields, but includes NO contribution from "gravitational energy".  So one can argue that "gravitational energy" does NOT act as a source of gravity.  On the other hand, the Einstein field equations are non-linear; this implies that gravitational waves interact with each other (unlike light waves in Maxwell's (linear) theory).  So one can argue that "gravitational energy" IS a source of gravity.

===Is Energy Conserved in General Relativity (http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html)

And this one too.

"The Newtonian concept of "potential energy" appears at first glance to work in GR as well when describing the motion of a test mass in the field of a central body, in that the effective rest energy change due to time dilation in a static field matches the potential energy, and the potential energy plus (relativistic) kinetic energy remain constant for motion in a static field.

However, I was surprised to find that it doesn't appear to work if you also consider the energy change of the source mass, or consider two similar masses orbiting around one another, even in a linearized weak field approximation. In that model, each of the objects apparently experiences an equal decrease in rest energy as it approaches the other (because of time dilation due to the other object) matching the Newtonian potential energy, so the rest energy of the system changes by twice the amount in the Newtonian model (where the potential energy is a property of the configuration of the system, not of the individual objects). However, the kinetic energy of the system only increases by the Newtonian kinetic energy, equal to the Newtonian potential energy, so this doesn't seem to add up.

In gravitational Quantum Field Theory, some people apparently assume that the energy of the field increases by the same amount, preserving the total energy, and that works very nicely from a mathematical point of view, giving a result very much like the Maxwell energy density of the field in electromagnetism. However, GR says that there is no energy in a vacuum regardless of the strength of the field, so that appears to mean either that QFT isn't compatible with GR or that the definition of "energy" is not the same in these two cases.

Given that the Newtonian concept of conservation of energy allows us to calculate complex gravitational interactions of many bodies to very high accuracy in non-relativistic situations, it seems surprising to me that there should be a problem with the concept in such a simple two-body situation in a weak GR approximation." By Jonathan Scott

:)
Title: Does Gravity do any work?
Post by: Geezer on 03/02/2010 01:29:11
If GR does not conserve energy, there could be something horribly wrong with GR  [;D]

Mind you, if energy actually is being annihilated, it might explain some current mysteries.
Title: Does Gravity do any work?
Post by: JP on 03/02/2010 07:39:28
Edit: I was basically restating what Baez says in yor_on's post, but in a murkier way.  Boiled down to its simplest form, energy conservation holds in GR if you include an extra term due to the gravitational energy (which is not part of the usual energy term in Einstein's equations, but must be included from the bending of spacetime).  The problem is that while it obeys a conservation law, it does not necessarily in itself have a nice physical meaning as energy density at a point in spacetime, since the the value of this "energy" depends on the coordinate system chosen.
Title: Does Gravity do any work?
Post by: Farsight on 03/02/2010 11:11:16
Geezer: there's a different definition of work here: http://physics.about.com/od/glossary/g/work.htm. It says "Work is defined (in calculus terms) as the integral of the force over a distance of displacement". Here's another one at http://id.mind.net/~zona/mstm/physics/mechanics/energy/work/work.html which says "Work is the transfer of energy. In physics we say that work is done on an object when you transfer energy to that object. For introductory thinking, this is the best definition of work. If you put energy into an object, then you do work on that object". There's another one at http://www.physicsclassroom.com/Class/energy/U5L1a.cfm which says "When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object. There are three key ingredients to work - force, displacement, and cause. In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement. There are several good examples of work which can be observed in everyday life - a horse pulling a plow through the field, a father pushing a grocery cart down the aisle of a grocery store, a freshman lifting a backpack full of books upon her shoulder, a weightlifter lifting a barbell above his head, an Olympian launching the shot-put, etc. In each case described here there is a force exerted upon an object to cause that object to be displaced." The definition of work is the problem. GR does conserve energy, energy is the one thing you can neither create nor destroy.

JP: energy is rather like that. If you think of the kinetic energy of a motionless brick, it's zero. But if you move past it, and then think "motion is relative" and flip things around to say it's the brick moving instead of you, then "the brick has kinetic energy". You haven't actually created any energy, the brick hasn't changed one jot. You've changed your state of motion and adopted a new coordinate system.

yor_on: note "the effective rest energy change due to time dilation in a static field matches the potential energy" from your post above. That's a restatement of what I was saying about your jiggle.
Title: Does Gravity do any work?
Post by: yor_on on 03/02/2010 15:07:24
Ah, JP you knew it?
A most devious mind indeed :)
==

And Farsight? Are you saying that you actually understood that lose sentence
"the effective rest energy change due to time dilation in a static field matches the potential energy"

That's more than what I did :)
But if I put it in perspective like this :)

"The Newtonian concept of "potential energy" appears at first glance to work in GR as well when describing the motion of a test mass in the field of a central body, in that the effective rest energy change due to time dilation in a static field matches the potential energy, and the potential energy plus (relativistic) kinetic energy remain constant for motion in a static field."

Then it seems to say that the effective 'change' of energy for a mass will match that potential energy you get counting on it before the happening, observing the rigors of time dilation, which I take to mean is whether it's moving relative your frame of observation, or not, and of course the gravitational field it is in. And then also that this 'potential energy' plus your relativistic (motion and frame dependent(?)) kinetic energy is constant for motion in a 'static field', which then would be?

But what it really seems to say is that you have two equations that levels out on both sides giving you one consistent answer, making sense. but only as long as you don't "consider the energy change of the source mass, or consider two similar masses orbiting around one another, even in a linearized weak field approximation."

Title: Does Gravity do any work?
Post by: Joe L. Ogan on 03/02/2010 15:18:55
Has anyone heard any information about a new discovery being made about Gravity? Thanks for comments.  Joe L. Ogan
Title: Does Gravity do any work?
Post by: lightarrow on 03/02/2010 18:43:46
There is something I'm missing. What [does]:
-Pair production
-electron angular momentum
-gravitational time dilation
have to do with the fact the electron would be made of light? When you say the electron is "made of light" I understand that its *inner structure* is light. Have I understood well?
Maybe, lightarrow, but those three things together are to do with how gravity works on matter. Of those three things, only pair production is to do with the fact that the electron is made of light. We can't be sure of the structure, but what we can be sure of is that when we make an electron via pair production, we start with a nucleus and a gamma photon. We end up with a nucleus, and an electron, and a positron. The light has gone. Then when we annihilate an electron with a positron, what we get is gamma photons. That's the light back. Nothing else goes in, and nothing else comes out. So what's the electron made of? Light. Nothing else. There's not a lot to understand, the scientific evidence is there, it's cut and dried. But for some strange reason people don't seem to know about it.
So how do you explain the fact that, given enough energy, a photon can produce others particles? And the fact that, given enough energy, a couple electron/positron can annihilate producing photons AND other particles (muon, pions, what you want)?
Title: Does Gravity do any work?
Post by: Farsight on 03/02/2010 19:17:20
This is a bit leading edge and hasn't attracted much publicity yet, but they're all dynamical stress-energy "vortons". See for example Stationary ring solitons in field theory - knots and vortons by Eugene Radu & Mikhail Volkov (Phys.Rept.468:101-151,2008) at http://arxiv.org/abs/0804.1357. A vorton is akin to a vortex. With more energy you can make more complicated vortons. Think of particles with mass as light tied in up knots. Sounds a bit odd I know, but see tying light in knots (http://www.physorg.com/news182957628.html) and note the quote "The study of knotted vortices was initiated by Lord Kelvin back in 1867 in his quest for an explanation of atoms", adds Dennis, who began to study knotted optical vortices with Professor Sir Michael Berry at Bristol University in 2000. "This work opens a new chapter in that history." In a nutshell, the particles with a short lifetime aren't very good knots.
Title: Does Gravity do any work?
Post by: JP on 04/02/2010 03:02:34
This is a bit leading edge and hasn't attracted much publicity yet, but they're all dynamical stress-energy "vortons". See for example Stationary ring solitons in field theory - knots and vortons by Eugene Radu & Mikhail Volkov (Phys.Rept.468:101-151,2008) at http://arxiv.org/abs/0804.1357. A vorton is akin to a vortex. With more energy you can make more complicated vortons. Think of particles with mass as light tied in up knots. Sounds a bit odd I know, but see tying light in knots (http://www.physorg.com/news182957628.html) and note the quote "The study of knotted vortices was initiated by Lord Kelvin back in 1867 in his quest for an explanation of atoms", adds Dennis, who began to study knotted optical vortices with Professor Sir Michael Berry at Bristol University in 2000. "This work opens a new chapter in that history." In a nutshell, the particles with a short lifetime aren't very good knots.

Optical vortices really have nothing to do with particles.  Vortices are interesting features in an optical field, but don't make matter from light.  I've actually talked with two of the physicists you cite in your second article, and they wouldn't claim anything like what you're saying about an electron being made of light.

Back on topic, the original poster was asking about gravity and work, so let's try to keep it somewhat on topic.  If you want to discuss the structure of electrons, please start a new thread (although if you're going to propose a new model for the electron, please do so in the new theories (http://www.thenakedscientists.com/forum/index.php?board=18.0) section of the forum).
Title: Does Gravity do any work?
Post by: Geezer on 04/02/2010 04:31:47
Farsight: You seem to have missed my point.

Work can be measured as the change in kinetic energy of a rigid body.

Kinetic energy is a function of velocity (which we can measure) and mass (which we can measure). Therefore, we can quantify an amount of work done.

You will notice there is no reference to any force, because none is required. You might infer there is a force, but I am not, because it is not necessary to do so.

So, we can predict from theory that work was done, and not only that, we can easily confirm the theory by experiment. (Pretty cool, eh?)

Or are you saying the experiment is flawed? It would be preferable if you can do that without a blizzard of URLs in case people think you are trying to do a "snow job".

Title: Does Gravity do any work?
Post by: Farsight on 04/02/2010 11:14:38
Forget I mentioned it, JP.

Geezer: my response would be some restatement of what I said previously about the ambiguous definition of work and the distinction between classical mechanics and relativity, so I don't think there's any more I can add.

yor-on: yes, I understood that sentence. It isn't to do with whether the object is moving as a whole relative to you. It's saying the rest energy of a motionless object down near the surface is less than the rest energy of the same motionless object up in space because of the time dilation, and the difference is the potential energy. It's to do with E=mc2, see http://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence.
Title: Does Gravity do any work?
Post by: lightarrow on 04/02/2010 14:41:25
Work can be measured as the change in kinetic energy of a rigid body.
How do you know that this is valid in GR? It's your definition? It's a postulate? No, it's only a theorem of CM. How do you prove this theorem? When you'll discover this you have the answer to your question.
Title: Does Gravity do any work?
Post by: yor_on on 04/02/2010 16:27:47
Farsight I never said "It isn't to do with whether the object is moving as a whole relative to you." How ever did you draw that conclusion? The 'you' I refer to counting perhaps? Well to draw any conclusion someone will have to do a 'counting', but the 'you' I used here was thought to relate to the object in itself.

As for what he meant by 'static field' though? I'm still wondering. But as far as I can see its two scenarios he presents :) One being still inside that 'static field' the other moving inside that same 'static field'.

As for the Mass–energy equivalence? That's the question ain't it? Furthermore Farsight, you're nicer without that condescending tone. That you believe you got the answer to the universe time and all, does not mean that you have the one and only answer. I have my own ideas, we all have, but we won't push them, and we will tell what is ours and what is main-stream if and when we do.

To mix it all together will just make a mess, and it will all end up in a shouting match and that you already know from other, let it be unnamed forum. This forum will only work as along as we are rigorous in defining what is ours and what is accepted theory. A lot of people are looking at what you write, some of them might draw the conclusion, as JP pointed out, that all you write is 'main-stream' and the one and only truth. I like this forum and I will not allow it to become another 'infected area'. So take care with the 'differing' please. TNS invites all and try to treat them fairly, but there is a line.

The Farsight I remember differed between those?
==

Rereading myself.

I did not mean that your ideas are a 'infected area' Farsight.
They have every right to be expressed here.

Just tell me when they are yours and where you're talking 'main stream'.
If someone have a theory that's all new and shiny :), then we have the forum 'new theories' for it.

This site is as much a site dedicated to trying to give the best 'main stream answers' we know, as a debating site. Don't mistake it for some American high school debate. And that mean that we need cool heads here trying to give fair answers, according to me at least :).

What I'm talking about as 'infected' are when people loses their civility and treat others as idiots.
I hated seeing that at ** and I won't stand for it happening on this site.

So just to be clear, theories are cool, new ones or old ones, don't really care, as long as one define what they are. And when answering questions from TNS the best approach is to at least start :) with the 'main stream views' and if walking of the beaten path, clearly state that we are doing so.

Like writing "My view" "I think" "According to mine ideas" helping those unused to those discussions to see what really is a 'main stream idea' and what isn't. Doing so I see no real problem with one wanting to present an 'alternative approach' in a thread, as my view that is.

I said it before and I will say it again. TNS is an oasis, let us try to keep it that way.
And I'm sorry if I gave the wrong impression with 'infected'.
Didn't see that before rereading it Farsight.
Title: Does Gravity do any work?
Post by: Geezer on 04/02/2010 19:44:33
Work can be measured as the change in kinetic energy of a rigid body.
How do you know that this is valid in GR? It's your definition? It's a postulate? No, it's only a theorem of CM. How do you prove this theorem? When you'll discover this you have the answer to your question.

Hi Lightarrow: Actually, I'm not saying it's valid in GR at all  [:D] I'm saying it's valid in classical terms, and in classical terms, gravity does work. I'm simply pointing out that (in classical terms) it is not necessary to measure a force. A change in velocity is sufficient. (I understand that in GR, gravity is not viewed as producing a force.)

You have to admit it's a rather simple experiment to conduct. Not only that, we can easily demonstrate that by doing a certain amount of work against gravity on the brick we can recover an equal amount of work from the brick.

The question was not restricted to a GR perspective, so we can explain it in classical terms. I'm not sure I've seen a testable explanation in terms of GR, although I may have missed it. I think we should be able to describe the theory and test it experimentally, or am I just being too "old fashioned"?

Title: Does Gravity do any work?
Post by: Farsight on 05/02/2010 01:03:55
Farsight I never said "It isn't to do with whether the object is moving as a whole relative to you." How ever did you draw that conclusion?
You said "which I take to mean is whether it's moving relative your frame of observation". Sorry if I misread it somehow. Best if you ask Jonathan Scott about it directly. Please note that very little of what I've said isn't mainstream, and hopefully where it isn't, I've pointed it out and backed it up with peer-reviewed papers, experimental evidence, or Einstein quotes. You should reread the thread to check. Note that pair production (wherein a +1022keV photon is used to create an electron and a positron) is most definitely mainstream, so it's perfectly valid to say the electron is made from light, and electrons really do exhibit angular momentum and magnetic moment. This is important for understanding the gravitational potential energy vis-a-vis time dilation.
Title: Does Gravity do any work?
Post by: JP on 05/02/2010 05:37:25
The problem is, Farsight, that although you can create electrons from energy, it doesn't follow that electrons are "made of" photons other than that photons and electrons are both made of energy.  Electrons do exhibit magnetic moment and spin, but I don't see what that has to do with gravitational potential energy.

Putting aside the question of whatever model of the electron you're using, can you provide substance to back up the claim that "electrons really do exhibit angular momentum and magnetic moment. This is important for understanding the gravitational potential energy vis-a-vis time dilation."?
Title: Does Gravity do any work?
Post by: PhysBang on 05/02/2010 13:48:04
Note that pair production (wherein a +1022keV photon is used to create an electron and a positron) is most definitely mainstream, so it's perfectly valid to say the electron is made from light, and electrons really do exhibit angular momentum and magnetic moment. This is important for understanding the gravitational potential energy vis-a-vis time dilation.
Note that these claims about pair production are completely untrue. Please look at any textbook on general relativity or any textbook that discusses pair production for a confirmation of this.

(Indeed, one can do a google search for farsight, "pair production" and "not true" to see this claim and its repeated corrections.)
Title: Does Gravity do any work?
Post by: Farsight on 06/02/2010 09:35:41
JP: It isn't an matter of which electron model one uses, it's a matter of looking at pair production and annihilation, including proton-antiproton annihilatation and the final decay products. But putting that aside, there's a wealth of references to electron angular momentum, I hope that isn't an issue. A quick search on google throws up good authoritative articles:

http://www.google.co.uk/search?hl=en&source=hp&q=electron+angular+momentum&meta=&rlz=1W1ADBF_en-GB&aq=f&oq=

Ditto for magnetic moment, see:

http://www.google.co.uk/search?hl=en&rlz=1W1ADBF_en-GB&q=electron+magnetic+moment&btnG=Search&meta=&aq=f&oq=

I hope we can all agree that the electron involves some form of rotational motion. As for why this is important for understanding the gravitational potential energy vis-a-vis time dilation, perhaps we need another thread for that.
Title: Does Gravity do any work?
Post by: lightarrow on 06/02/2010 10:35:22
I hope we can all agree that the electron involves some form of rotational motion.
I see it difficult to find a generalized agreement on this...
Title: Does Gravity do any work?
Post by: yor_on on 06/02/2010 14:16:12
Yep I agree, it's a hard subject to really encompass :)
What I think is the problem here is how one want to see the 'origin' of particles. Depending on ones choice there are different paths. Heck, if I was to rule it would all be 'emergences' :)

Then you can see it as 'real particles' and waves as a 'extension'.
Or waves as being the 'real thing' and particles as some sort of 'concentration'.
And I don't really rule out anything, I try to keep an open mind to what's possible. So that said..

It's all 'emergences' ::))
===

Apropos electrons..
My take on the subject, and no, I don't understand them :)

Electrons and Magnetism (http://www.thenakedscientists.com/forum/index.php?topic=25747.msg280414#msg280414)

Title: Does Gravity do any work?
Post by: JP on 08/02/2010 02:48:29
I hope we can all agree that the electron involves some form of rotational motion.
I see it difficult to find a generalized agreement on this...

I disagree.  Electrons have spin, but do not rotate necessarily.  There's an important distinction between the two, in that you can calculate the values angular momentum (classical rotation) can take on, and spin does not agree with those values.
Title: Does Gravity do any work?
Post by: lightarrow on 08/02/2010 15:49:44
So you agree with me in thinking that it's difficult that most of us agree about the presence of a form of rotational motion.
Title: Does Gravity do any work?
Post by: Farsight on 08/02/2010 23:50:50
Oh come on, JP. Go with the flow. Look at http://en.wikipedia.org/wiki/Zitterbewegung:

Zitterbewegung (English: "trembling motion", from German) is a theoretical rapid motion of elementary particles, in particular electrons, that obey the Dirac equation. The existence of such motion was first proposed by Erwin Schrödinger in 1930 as a result of his analysis of the wave packet solutions of the Dirac equation for relativistic electrons in free space...

There's some kind of rotational motion in there, whether you consider it to be classical or not, and despite your lack of an electron model. Now, can we move on to gravitational potential energy? It's really very simple.
Title: Does Gravity do any work?
Post by: Geezer on 09/02/2010 00:48:55
Geezer: my response would be some restatement of what I said previously about the ambiguous definition of work and the distinction between classical mechanics and relativity, so I don't think there's any more I can add.

Does that mean, in classical terms, you agree that gravity does do work?
Title: Does Gravity do any work?
Post by: lightarrow on 09/02/2010 19:38:54
Oh come on, JP. Go with the flow. Look at http://en.wikipedia.org/wiki/Zitterbewegung:

Zitterbewegung (English: "trembling motion", from German) is a theoretical rapid motion of elementary particles, in particular electrons, that obey the Dirac equation. The existence of such motion was first proposed by Erwin Schrödinger in 1930 as a result of his analysis of the wave packet solutions of the Dirac equation for relativistic electrons in free space...

There's some kind of rotational motion in there, whether you consider it to be classical or not, and despite your lack of an electron model. Now, can we move on to gravitational potential energy? It's really very simple.
How many physicists can still believe in Zitterbewegung? Maybe you can found some at the waxwork...
Title: Does Gravity do any work?
Post by: Farsight on 10/02/2010 08:24:06
Geezer: in classical terms, yes. But not in terms of relativity.

lightarrow: all of them. See http://physicsworld.com/cws/article/news/41352

"European physicists have won the race to observe zitterbewegung, the violent trembling motion of an elementary particle that was predicted by Erwin Schrödinger in 1930. To observe this phenomenon, the team simulated the behaviour of a free electron with a single, laser-manipulated calcium ion trapped in an electrodynamic cage..."

I take it that everybody is now satisfied that there's some kind of rational motion in a subatomic particle like an electron. At point A up in space this occurs at rate X, but because of gravitational time dilation, at point B down on the surface of a planet it occurs at rate Y, which is less than X. This means the subatomic particle has less energy at point B.
Title: Does Gravity do any work?
Post by: dantheman on 11/02/2010 14:46:28
How’s this from someone who knows very little science.
The way I see is that a mass of matter causes a rift in the fabric of space, pushing it out to make room for it’s self. Therefore causing things to attempt to fall in. The larger the mass (not the size) the larger the rift. With black holes being so massive that they achieve a tear rather than a rift. Gravity does not work only the mass does.

Title: Does Gravity do any work?
Post by: JP on 12/02/2010 02:51:50
If anyone wants to discuss more about the electron model, could you start a new thread please?  It's really confusing the original question.  If not, I'll have to split further electron posts off this thread.

Thanks,
JP (mod)
Title: Does Gravity do any work?
Post by: lightarrow on 12/02/2010 15:56:35
Geezer: in classical terms, yes. But not in terms of relativity.

lightarrow: all of them. See http://physicsworld.com/cws/article/news/41352

"European physicists have won the race to observe zitterbewegung, the violent trembling motion of an elementary particle that was predicted by Erwin Schrödinger in 1930. To observe this phenomenon, the team simulated the behaviour of a free electron with a single, laser-manipulated calcium ion trapped in an electrodynamic cage..."
"According to Christian Roos at the University of Innsbruck, Austria, one of the keys to success was to make their non-relativistic ion behave as if it was a relativistic particle. This is crucial because zitterbewegung is predicted by the Dirac equation, which describes relativistic quantum mechanics".

Note the phrase: to make their non-relativistic ion behave as if it was a relativistic particle.
Title: Does Gravity do any work?
Post by: JP on 14/02/2010 07:03:11
Splitting this is going to be a huge headache, so if you don't mind I've put up a question here about electron rotation:
http://www.thenakedscientists.com/forum/index.php?topic=28707

Thanks,
JP (mod)