Does Gravity do any work?

  • 210 Replies
  • 71011 Views

0 Members and 1 Guest are viewing this topic.

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #150 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.

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #151 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.)
« Last Edit: 26/01/2010 06:31:55 by Geezer »
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #152 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.

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #153 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.
« Last Edit: 27/01/2010 00:20:47 by Geezer »
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #154 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.  [:-\]

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #155 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.
« Last Edit: 27/01/2010 08:10:38 by Geezer »
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #156 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".

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #157 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.
« Last Edit: 27/01/2010 12:39:17 by Farsight »

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #158 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 = ˝mv2 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
« Last Edit: 28/01/2010 14:34:08 by lightarrow »

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #159 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."
« Last Edit: 27/01/2010 18:26:13 by Geezer »
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #160 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.
« Last Edit: 28/01/2010 15:12:07 by Farsight »

*

Offline PhysBang

  • Hero Member
  • *****
  • 598
    • View Profile
Does Gravity do any work?
« Reply #161 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.

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #162 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.
« Last Edit: 29/01/2010 00:17:47 by Farsight »

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Does Gravity do any work?
« Reply #163 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...)

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #164 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.
« Last Edit: 29/01/2010 09:14:30 by Farsight »

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Does Gravity do any work?
« Reply #165 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.

*

Offline litespeed

  • Sr. Member
  • ****
  • 419
    • View Profile
Does Gravity do any work?
« Reply #166 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?


*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #167 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.
« Last Edit: 29/01/2010 19:46:29 by lightarrow »

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12342
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Does Gravity do any work?
« Reply #168 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 :)
« Last Edit: 29/01/2010 22:14:07 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #169 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.
« Last Edit: 30/01/2010 01:34:37 by Geezer »
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Does Gravity do any work?
« Reply #170 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.

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.
« Last Edit: 30/01/2010 02:31:27 by JP »

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #171 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.    

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #172 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.

« Last Edit: 01/02/2010 14:40:05 by Farsight »

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #173 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".
« Last Edit: 01/02/2010 15:09:29 by lightarrow »

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #174 on: 01/02/2010 15:32:42 »
I'm talking about scientific evidence, lightarrow. Search on:

Pair production

electron angular momentum

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.

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #175 on: 01/02/2010 18:13:08 »
I'm talking about scientific evidence, lightarrow. Search on:

Pair production

electron angular momentum

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?
« Last Edit: 01/02/2010 18:15:15 by lightarrow »

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #176 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.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #177 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, 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.
« Last Edit: 01/02/2010 23:58:05 by Farsight »

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Does Gravity do any work?
« Reply #178 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.

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #179 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.


*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #180 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.
« Last Edit: 02/02/2010 21:21:21 by Geezer »
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12342
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Does Gravity do any work?
« Reply #181 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

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

:)
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #182 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.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Does Gravity do any work?
« Reply #183 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.
« Last Edit: 03/02/2010 07:48:52 by JP »

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #184 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.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12342
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Does Gravity do any work?
« Reply #185 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."

« Last Edit: 03/02/2010 15:35:12 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline Joe L. Ogan

  • Sr. Member
  • ****
  • 476
    • View Profile
Does Gravity do any work?
« Reply #186 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

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #187 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)?

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #188 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 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.     

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Does Gravity do any work?
« Reply #189 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 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 section of the forum).

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #190 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".

There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #191 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.

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #192 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.

*

Offline yor_on

  • Naked Science Forum GOD!
  • *******
  • 12342
  • (Ah, yes:) *a table is always good to hide under*
    • View Profile
Does Gravity do any work?
« Reply #193 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.
« Last Edit: 04/02/2010 21:25:45 by yor_on »
"BOMB DISPOSAL EXPERT. If you see me running, try to keep up."

*

Offline Geezer

  • Neilep Level Member
  • ******
  • 8328
  • "Vive la résistance!"
    • View Profile
Does Gravity do any work?
« Reply #194 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"?

« Last Edit: 04/02/2010 19:48:41 by Geezer »
There ain'ta no sanity clause, and there ain'ta no centrifugal force ćther.

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #195 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.

*

Offline JP

  • Neilep Level Member
  • ******
  • 3366
    • View Profile
Does Gravity do any work?
« Reply #196 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."?

*

Offline PhysBang

  • Hero Member
  • *****
  • 598
    • View Profile
Does Gravity do any work?
« Reply #197 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.)
« Last Edit: 05/02/2010 13:50:51 by PhysBang »

*

Offline Farsight

  • Sr. Member
  • ****
  • 396
    • View Profile
Does Gravity do any work?
« Reply #198 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. 

*

Offline lightarrow

  • Neilep Level Member
  • ******
  • 4586
    • View Profile
Does Gravity do any work?
« Reply #199 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...