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Author Topic: Does Gravity do any work?  (Read 69595 times)

Offline lightarrow

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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

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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 »
 

Offline lightarrow

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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

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« 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 »
 

Offline lightarrow

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« 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

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« 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 »
 

Offline Farsight

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« 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

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« 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

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« 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 = 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
« Last Edit: 28/01/2010 14:34:08 by lightarrow »
 

Offline Geezer

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« 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 »
 

Offline Farsight

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« 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

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« 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

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« 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

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« 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

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« 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

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« 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

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« 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

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« 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 »
 

Online yor_on

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« 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 »
 

Offline Geezer

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« 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 »
 

Offline JP

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« 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

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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

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« 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

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« 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

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« 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.
 

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Does Gravity do any work?
« Reply #174 on: 01/02/2010 15:32:42 »

 

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