Length contraction of falling things

[PeterDonis]

" ['in stationary frames (relative to the gravitating source) the energy [of radiation] increases as it goes down.] 'light's energy is affected by gravity' [...] "
There had been discussions on this in a recent thread, about the energy of light increasing as it goes down. I always thought that the energy of an object falling or rising in a gravitational field doesn't change.

It depends on how you define "energy" and how you define "change". Different people have different definitions they like, and they often will say (as the Okun paper that harrylin mentioned does--btw, harrylin, do you have a link to the paper itself?) that other definitions besides theirs are "misleadinng" or some such. The key thing IMHO is to remember that in GR, "energy" is not a fundamental concept; it's a way of interpreting the physics, but you don't need it to actually calculate what happens. You can calculate everything using covariant geometric objects like vectors and tensors, which have unambiguous definitions and behave in well-defined, unambiguous ways.

For example, take the photon free-falling radially towards a gravitating body. It has a 4-momentum vector $p^{a}$ which is parallel transported along its worldline. Since parallel transport is one common way in GR of defining what it means for a geometric object to "not change" along a curve, this implies that the photon's 4-momentum does not change. And since the norm of the 4-momentum can be defined as the "energy at infinity" of the photon, some people interpret this as saying that the energy of the photon does not change.

However, the energy that the photon will be *observed* to have depends on the 4-velocity of the observer, via the equation:

$$E_{obs} = g_{ab} p^{a} u^{b}$$

Even if we interpret what I said above as $p^{a}$ not changing as the photon falls, $g_{ab} u^{b}$ *will* change, in general, for different observers. In particular, it will change for static observers at different altitudes, who will therefore observe the photon to have different energies, and therefore different frequencies, i.e., gravitational redshift/blueshift. Some people interpret this as saying that the energy of the photon *does* change.

Which interpretation is "right"? Does it matter? It depends on what you are using the interpretation for. Both interpretations agree on the actual physical observables, so as far as physics is concerned, they're equivalent. One may lead to more understanding and less confusion for some people, in some situations. But IMHO that's not a question of physics.

 

[vin300]

And since the norm of the 4-momentum can be defined as the "energy at infinity" of the photon, some people interpret this as saying that the energy of the photon does not change.

Isn't the norm of photon 4-momentum zero?

 

[PeterDonis]

Isn't the norm of photon 4-momentum zero?

Oops, yes, you're right, I was confusing the photon case with the timelike particle case. I should have just said that some people interpret the photon 4-momentum being parallel transported as its energy not changing.

 

[harrylin]

[..] btw, harrylin, do you have a link to the paper itself?

I gave a link to the paper itself
Perhaps you meant a link to a freely downloadable (=pirated) version, but no I don't have that. On the other hand, a personal message can have good results (hint).

[..] In particular, it will change for static observers at different altitudes, who will therefore observe the photon to have different energies, and therefore different frequencies, [..]

This issue may indeed be a matter of language. If you change your reference system from for example the ECI frame to the rest frame of a rocket to the moon, does that also change the distance Earth-moon? I will say no, your change of perspective cannot affect a distance. But some people might say yes to that question.

Both interpretations agree on the actual physical observables

Only if it's just a matter of formulation, and not if it's a case of truly disagreeing interpretation (it may be the one for some, and the other for some others!). Gravitational time dilation is observable independent of redshift; see posts #26 and #32.

 

[PeterDonis]

This issue may indeed be a matter of language.

I think in large part it often is. For example:

If you change your reference system from for example the ECI frame to the rest frame of a rocket to the moon, does that also change the distance Earth-moon? I will say no, your change of perspective cannot affect a distance. But some people might say yes to that question.

That's because the two of you are using the word "distance Earth-Moon" to refer to two different invariants; they are the invariant lengths of two different spacelike curves between the Earth's worldline and the Moon's worldline. Changing your perspective doesn't change either invariant, but it may change which one you think should be called "the distance Earth-Moon". But that's a question of terminology, not physics. When I said everyone agrees on all actual observables, I meant everyone agrees on the values of all invariants. They may disagree on what to call them.

 

[Austin0]

OK, you explained the same as I did when I wrote: "blueshift would be double the predicted and verified amount if [light wave frequency increasing in transit] were right. In other words, [that] presentation denies gravitational time dilation." BTW, I forgot to mention that Einstein concluded the existence of gravitational time dilation in 1911 because the concept of an increasing number of cycles in transit is incompatible with wave theory.

But then you write (and I reinsert from your original quote + add detail for clarity, as the disagreement is about the way in which light's energy is affected by gravity!):

Do you mean that according to you, as measured with a single inertial reference system, the energy of a photon in transit changes with the gravitational potential even though its frequency is not?

No,,,,my interpretation is the same as yours.You are confusing the quote of DaleSpams I inserted with my thoughts. The bolded above is his interpretation along with his assessment of my logic in reaching my conclusion.

 

[Austin0]

There is a possible caveat, stemming from my own earlier considerations in a thread dealing with angular momentum of a flywheel lowered into a potential well. We had assumed the flywheel's own angular momentum was invariant wrt potential, a reasonable assumption in keeping with conservation of angular momentum. Yet from a Machian pov that seems suspect. Time dilation demands that the coordinate determined spin-rate declines with lowering potential, and this in turn naively implies an increased coordinate flywheel mass to compensate. Problem is that would conflict with the two assumptions that rest mass/energy declines with lower potential and that EP holds true (inertial mass = active gravitational mass = passive gravitational mass). A redefinition of angular momentum in gravity well is thus necessary to at all reconcile these factors.
There is however an additional possibility - that angular momentum becomes progressively redistributed between flywheel proper and the rest of the gravitating mass system. Which is in keeping with the Machian position that inertia is a mutual effect between all mass/energy. Thus build a shell of mass around a flywheel and it's coordinate spin-rate declines, but one must expect an action of flywheel's motion back on the shell.

The spin rate declines according to which observer? The local or the observer at the initial elevation?

Could you elaborate on the assumption that "rest mass/energy declines with lower potential " ?

 

[harrylin]

I think in large part it often is. For example: [..].

It's good to see that you agree with my example. What is still not clear to me, if some people here actually have been misled by such language or not. You seem to think that this was clear for everyone - but I doubt that. Let's hope that it's at least now clear for everyone.

Perhaps it's better to give a crystal clear example:

Put an atomic clock in a high tower and synchronise¹ it with the ECI frame, as is commonly done. Tune a crystal oscillator to exactly 1.000 GHz with that atomic clock as reference, and use it for a radio emitter. There is a similarly tuned atomic clock and a radio receiver with frequency analyser on the ground. Send during exactly 24h a 1.000 GHz radio signal to the ground, 500 m below. According to GR the received signal as read on the frequency analyser will be exactly 1.000 GHz and the emission time exactly 24 h (in an ideal² situation). That should be clear and obvious to everyone.

Harald

ADDENDUM, notes:
1. tune it to run in sync, as is commonly done for time keeping
2. also overlooking that the ECI frame isn't perfectly inertial

 

[Naty1]

PeterDonis:

Since parallel transport is one common way in GR of defining what it means for a geometric object to "not change" along a curve, this implies that the photon's 4-momentum does not change. And since the norm of the 4-momentum [it is zero] can be defined as the "energy at infinity" of the photon, some people interpret this as saying that the energy of the photon does not change.

Peter: It was this issue that caused me to in my earlier post try to get the original poster back to basics first. He did not seem interested in that approach. I am still unsure what he wanted to accomplish by 'putting photons in a box'. Maybe he thought they would stay the same color forever if left in place? who knows.

In any case, I had been thinking that slowly raising photons from varying gravitational potential depths would result in a different characteristic of light at the surface versus them following null geodesics as when freely emitted; that some changes in characteristics would result. Can you comment about what you think happens?

 

[harrylin]

Maybe not as precisely presented as some demand, but taken in context the intention of jartsa's points above are more than reasonable, even logically necessary. The first is simply saying conservation of energy requires 'trapped light' to act gravitationally just as for rest matter. Cyclic process of hoisting+free-fall should be a zero-sum game. And that requires no locally observed frequency change when lifting the box (=perfectly reflective cavity resonator).

Apparently you assume that no light ray reflects off the bottom or the top during motion. That's not likely IMHO. And thus its effect should be analysed. Is then work done or not? And equal but opposite?

[..] the trapped EM field energy [..]

I would say trapped radiation energy - not field energy!

Say for a cubical cavity resonator, resolve cavity field into three mutually orthogonal sets of counterpropagating traveling waves. Two sets propagating transverse to radial direction, one along that axis. Sets need not be equal amplitude. It should be obvious the two transverse sets have no appreciable coupling to the only conceivable factor that hauling-up process could bring to bear as locally measured [..]

As discussed in parallel, analyses with "local" measurements easily complicates analysis. It can be useful to develop a theory but when you already have the theory, needless frame jumping is often counter productive.

[..] from a Machian pov that seems suspect. [..]

I find Machian POV's themselves suspect.
And I think that we have largely covered the topic of this thread.

 

[Dale]

According to GR the received signal as read on the frequency analyser will be exactly 1.000 GHz and the emission time exactly 24 h (in an ideal situation). That should be clear and obvious to everyone.

That is incorrect, and this contradicts both the Pound Rebka experiment and also current experiments done with atomic clocks at different heights.

 

[Dale]

Coincidentally this same question came up in another recent thread where I expressed this same view as yourself, but it is seemingly not generally accepted by those here. ;-)

I cannot tell from this comment nor from your follow-up comments whether or not you have any disagreement or any confusion from my quote.

 

[jartsa]

So here's what happens in the box that contains light and is being lifted:


Let us examine light in a mirror lined box, which is being lifted at a constant coordinate speed. The coordinate speeds of the ceiling mirror and the floor mirror are the same. The coordinate speeds of the light near the ceiling and the light near the floor are different.

Now we must study Doppler shift. When an EM-wave pulse and a mirror collide, the mirror receives wave crests at average frequency:
the number of crests in the wave / time that the collision lasts

Now, in the Doppler shift effect there is no change of number of wave crests in a wave pulse, so the change of the duration of the collision between the wave and the mirror is the thing that determines the Doppler shift.

An observer who is staying at constant altitude, and observing our box being lifted, says that near the ceiling the light has a larger coordinate speed, and therefore the duration of the collision between the ceiling mirror and the light is determined by the motion of the light in a greater degree as compared to the collision between the light and the floor mirror, so near the floor mirror the duration of the light-mirror collision is determined by the coordinate speed of the mirror in a greater degree, as compared to the situation near the ceiling.

So this observer at constant altitude says that light blue shifts, when being reflected from the floor mirror, more than it red shifts when reflected from the ceiling mirror, and therefore the observer observes a frequency increase of the light.

 

[Dale]

So this observer at constant altitude says that light blue shifts, when being reflected from the floor mirror, more than it red shifts when reflected from the ceiling mirror, and therefore the observer observes a frequency increase of the light.

Can you quantify that?

 

[PeterDonis]

Put an atomic clock in a high tower and synchronise it with the ECI frame, as is commonly done. Tune a crystal oscillator to exactly 1.000 GHz with that atomic clock as reference, and use it for a radio emitter. There is a similarly tuned atomic clock and a radio receiver with frequency analyser on the ground. Send during exactly 24h a 1.000 GHz radio signal to the ground, 500 m below. According to GR the received signal as read on the frequency analyser will be exactly 1.000 GHz and the emission time exactly 24 h (in an ideal situation). That should be clear and obvious to everyone.

As DaleSpam's comment on this post shows, it still isn't.

First of all, when you say "synchronise it with the ECI frame", I assume you mean not just adopting the standard of simultaneity of the ECI frame, but also its clock rate, correct? In other words, you are artificially setting the frequency of *both* oscillators, at different heights, to different clock rates than they would normally run at, similar to what is done with the clocks on board the GPS satellites. Only if you do this does your prediction of unchanged "frequency" with height make sense.

But of course this "frequency" that you are saying does not change with height is really a "coordinate frequency", not a "proper frequency"; it's not the "natural" frequency that at least one of the oscillators (and probably both) would measure if you hadn't artificially set them to the ECI clock rate. In other words, you are artifically tuning each oscillator so that the "frequency" that it measures is *not* the "natural" frequency that would be measured by an observer with the same 4-velocity. The "effective" 4-velocity of the oscillator for purposes of measuring frequency, what you would contract with the 4-momentum of the photon to compute its observed frequency, is some *other* 4-vector, a 4-vector that is associated with "coordinate time" in the ECI frame.

All of which illustrates that it's tough to be clear, even when you are really trying to be.

 

[PeterDonis]

In any case, I had been thinking that slowly raising photons from varying gravitational potential depths would result in a different characteristic of light at the surface versus them following null geodesics as when freely emitted; that some changes in characteristics would result. Can you comment about what you think happens?

In a simple model where the light inside the box travels freely except when it hits a wall of the box, the periods of free travel can still be modeled as they would be if the box weren't there. But the collisions with the wall of the box change things.

The simplest assumption is that the walls of the box are perfectly reflecting mirrors; this means that each collision of a photon with a box wall can be modeled as perfectly elastic, with the photon's momentum perpendicular to the box wall reversing direction (and a corresponding change in the momentum of the box). In another thread I used this type of model to describe how one could extract work from a box filled with photons by slowly lowering it instead of letting it freely fall, and how extracting the work would lower the "photon temperature" inside the box, whereas a freely falling box of photons would have the same "photon temperature" inside as it fell (relative to the box, in both cases). But in either case, the "average" motion of the photons inside the box is no longer null, but timelike--it's just the motion of the center of mass of the box+photons.

One could try more complicated models, but I'm not sure what the point would be. The key point is already clear from the above simple model: the behavior of light inside a box, when the interaction with the box is included, does change.

 

[harrylin]

As DaleSpam's comment on this post shows, it still isn't.

First of all, when you say "synchronise it with the ECI frame", I assume you mean not just adopting the standard of simultaneity of the ECI frame, but also its clock rate, correct? In other words, you are artificially setting the frequency of *both* oscillators, at different heights, to different clock rates than they would normally run at, similar to what is done with the clocks on board the GPS satellites. Only if you do this does your prediction of unchanged "frequency" with height make sense. [..]

Yes, exactly. That is what is commonly done for atomic clocks in time keeping with the ECI frame: They are corrected for altitude, otherwise our time keeping on Earth would not work. But indeed "synchronise" is ambiguous, from the context it makes no sense to interpret it otherwise but I should have used the word "tune" from the start.
And my example isn't perfect, as the ECI frame isn't perfectly inertial - thus a very small difference will still be there. I now added a little footnote.

All of which illustrates that it's tough to be clear, even when you are really trying to be.

Regretfully yes. I now clarified that for later readers.

 

[Austin0]

It depends on how you define "energy" and how you define "change". Different people have different definitions they like, and they often will say (as the Okun paper that harrylin mentioned does--btw, harrylin, do you have a link to the paper itself?) that other definitions besides theirs are "misleadinng" or some such. The key thing IMHO is to remember that in GR, "energy" is not a fundamental concept; it's a way of interpreting the physics, but you don't need it to actually calculate what happens. You can calculate everything using covariant geometric objects like vectors and tensors, which have unambiguous definitions and behave in well-defined, unambiguous ways.


However, the energy that the photon will be *observed* to have depends on the 4-velocity of the observer, via the equation:

$$E_{obs} = g_{ab} p^{a} u^{b}$$

Even if we interpret what I said above as $p^{a}$ not changing as the photon falls, $g_{ab} u^{b}$ *will* change, in general, for different observers. In particular, it will change for static observers at different altitudes, who will therefore observe the photon to have different energies, and therefore different frequencies, i.e., gravitational redshift/blueshift. Some people interpret this as saying that the energy of the photon *does* change.

Which interpretation is "right"? Does it matter? It depends on what you are using the interpretation for. Both interpretations agree on the actual physical observables, so as far as physics is concerned, they're equivalent. One may lead to more understanding and less confusion for some people, in some situations. But IMHO that's not a question of physics.

i think you may be missing a significant point.

These are two very different descriptions of a single unambiguous physical reality.
The state of a photon during transit. The two descriptions are not themselves mutually exclusive. It could possibly be that both conditions pertain. I.e. The photon starts out with a higher frequency at a higher potential due to dilation and gains kinetic energy during transit to a lower potential.
It is the very fact that there is agreement on the observable results that renders them mutually exclusive.
AFAIK those results exactly correspond to the dilation factor corresponding to the difference in potential.
So if we accept the fundamental dilation concept as applied to clocks and consider it applies equally to electron emissions and receptions, then this both explains and necessitates those results completely, with no observable measurement left over to validate a posited frequency shift due to gravitational influence during transit.
If there was such a gradual change during transit then the results would inevitably be gamma + some additional factor. Such is the case with relativistic Doppler where the result is a composite of two effects.

So it is not simply a matter of semantics or interpretation , but a question of the actual physics and is unequivocal. Does a photon kinetically gain energy during transit or not?

The proposition that it does could only be true if our current conception of time dilation and it's effects on atomic periodicity is not correct.
Otherwise it would seem that it is not merely misleading but actually false. IMO

 

[harrylin]

That is incorrect, and this contradicts both the Pound Rebka experiment and also current experiments done with atomic clocks at different heights.

Sorry for the ambiguity; I now added a footnote.

 

[PeterDonis]

The state of a photon during transit. The two descriptions are not themselves mutually exclusive. It could possibly be that both conditions pertain. I.e. The photon starts out with a higher frequency at a higher potential due to dilation and gains kinetic energy during transit to a lower potential.

No, it's not possible. Neither of these descriptions are "the state of the photon during transit". The state of the photon during transit is that its 4-momentum is parallel transported along its worldline, and its energy as seen by an observer is the contraction of its 4-momentum with the observer's 4-velocity, as I stated. That's the physics; everything else is interpretation.

You can *interpret* the above as the photon gaining energy (or frequency) as it "falls" in the gravitational field, or you can *interpret* it as the photon's energy "staying the same" but getting measured by an observer with a different "rate of time flow" than where it was emitted. Both of these are interpretations; neither one is the physics.

AFAIK those results exactly correspond to the dilation factor corresponding to the difference in potential.

The "time dilation factor" corresponds to the "difference in potential" because that's how the two are defined. Both of them are defined in terms of the metric coefficient $g_{tt}$ in Schwarzschild coordinates. Or, if you want an invariant description, both of them are defined in terms of the metric as seen by observers who follow orbits of the time translation Killing vector field of the spacetime. Calling the difference in this with altitude "time dilation" is an *interpretation* of this particular feature of the metric; so is calling it a "difference in potential".

So if we accept the fundamental dilation concept

The time dilation concept is not fundamental. The EFE and its particular solutions are fundamental--or, equivalently, the spacetime geometry, the metric, is fundamental, as are the geometric objects in it, such as a photon's 4-momentum. "Time dilation" is a derived concept that applies in certain scenarios. So is "difference in potential".

 

[harrylin]

[..] You can *interpret* the above as the photon gaining energy (or frequency) as it "falls" in the gravitational field, or you can *interpret* it as the photon's energy "staying the same" but getting measured by an observer with a different "rate of time flow" than where it was emitted. Both of these are interpretations; neither one is the physics.[..]

Obviously we disagree here. How can in my last example the explanation work that the frequency of the radio wave increases as it "falls" down towards the frequency analyser?

 

[PeterDonis]

Obviously we disagree here. How can in my last example the explanation work that the frequency of the radio wave increases as it "falls" down towards the frequency analyser?

To be strictly correct, I should have specified that I was talking about the "natural" frequency that would be measured by an observer, i.e., the frequency obtained by contracting the photon's 4-momentum with the observer's 4-velocity.

In your scenario you are artificially changing the "frequency" that your analyzer reports when it measures a photon. For your frequency analyzer, the rule I gave, that the measured frequency is given by contracting the photon's 4-momentum with the observer's 4-velocity, does not hold as I stated it; to compute the frequency your analyzer will report, you have to use a *different* 4-vector, basically the unit timelike 4-vector of the ECI frame. This is *not* the same 4-vector as the analyzer's 4-velocity; if it were, you wouldn't have to artificially synchronize the analyzer with the ECI.

 

[jartsa]

Can you quantify that?

Not for many consecutive reflections, too complicated. But reflections at the bottom and at the top go something like this:
light - mirror collision time at the bottom:

k is the time dilation factor, or maybe inverse of that, k < 1
c is coordinate speed of light far away from gravity source
s is coordinate length of light pulse far away from gravity source
v is coordinate speed of mirror

collision speed = light speed+mirror speed=k*c + v
collision length = length of light pulse = k*s
time= collision lenght/collision speed = (k*c+v)/k*s
And here is light-mirror collision time at the top of the box:collision speed = light speed+mirror speed=k*c - v
collision length = length of light pulse = k*s
time= collision lenght/collision speed=(k*c-v)/k*s


And frequency can be calculated as: number of wave crests in the wave pulse / collision time

 

[Austin0]

Originally Posted by Austin0

The state of a photon during transit. The two descriptions are not themselves mutually exclusive. It could possibly be that both conditions pertain. I.e. The photon starts out with a higher frequency at a higher potential due to dilation and gains kinetic energy during transit to a lower potential.

No, it's not possible. Neither of these descriptions are "the state of the photon during transit". The state of the photon during transit is that its 4-momentum is parallel transported along its worldline, and its energy as seen by an observer is the contraction of its 4-momentum with the observer's 4-velocity, as I stated. That's the physics; everything else is interpretation.

You can *interpret* the above as the photon gaining energy (or frequency) as it "falls" in the gravitational field, or you can *interpret* it as the photon's energy "staying the same" but getting measured by an observer with a different "rate of time flow" than where it was emitted. Both of these are interpretations; neither one is the physics.

"state of the photon during transit is that its 4-momentum is parallel transported along its worldline"

Does this not mean that its 4-momentum vector (energy) remains constant (stays the same)along that worldline?

"the photon's energy "staying the same" but getting measured by an observer with a different "rate of time flow" than where it was emitted"

So this description is a literal verbal translation of the definitions of 4-momentum and vector transport in this context. How then could you say this was not the physics of a photon in transit??Originally Posted by Austin0

AFAIK those results exactly correspond to the dilation factor corresponding to the difference in potential.

The "time dilation factor" corresponds to the "difference in potential" because that's how the two are defined. Both of them are defined in terms of the metric coefficient in Schwarzschild coordinates. Or, if you want an invariant description, both of them are defined in terms of the metric as seen by observers who follow orbits of the time translation Killing vector field of the spacetime. Calling the difference in this with altitude "time dilation" is an *interpretation* of this particular feature of the metric; so is calling it a "difference in potential".

The point was not the self evident fact that dilation and potential correspond by definition.
But that the observed results correspond to the expected dilation factor.Originally Posted by Austin0

So if we accept the fundamental dilation concept

The time dilation concept is not fundamental. The EFE and its particular solutions are fundamental--or, equivalently, the spacetime geometry, the metric, is fundamental, as are the geometric objects in it, such as a photon's 4-momentum. "Time dilation" is a derived concept that applies in certain scenarios. So is "difference in potential"

.
this is just semantic antics. Would you say that time dilation was not a fundamental concept of SR? Time dilation is inherent in the metric just as it is in the Minkowski metric.
It does not just apply in certain scenarios but in all cases involving clocks at different altitudes , yes? And it is not just an interpretation but a word describing the relationships of clock rates . Whats that if not physics?

 

[harrylin]

[..] This is *not* the same 4-vector as the analyzer's 4-velocity; if it were, you wouldn't have to artificially synchronize the analyzer with the ECI.

The ECI frame can only work with what you call "artificial synchronization" and I did not refer to your analysis but to what you call an "interpretation and not the physics", as you put it:

"the photon gaining energy (or frequency) as it "falls" in the gravitational field".

Discussions here are about the physics and my example in post #43had a two-fold purpose: I also had in mind to illustrate in a very clear way that the physics of that interpretation is messed up.
So far, it looks to me a clear case of mixing up in one description two different perspectives according to the equivalence principle; in physics one cannot freely jump and mix reference systems*. But if, contrary to my thinking, that interpretation predicts without inconsistencies the observations of my example, then this will be less easy to do than I expected.

*For example it's nonsense if I say that you are gaining energy because I took off in an airplane. A physical interpretation can only be valid if it relates to a valid reference system for description of physics.

 

[Dale]

Does this not mean that its 4-momentum vector (energy) remains constant (stays the same)along that worldline?

First, the four momentum is not the energy. Energy is the timelike component of the four momentum. So even if you interpret parallel transport as keeping the four momentum constant you cannot conclude that the same applies for energy.

Second, it is hard to interpret parallel transport as keeping a four vector constant. If you parallel transport a vector on a closed loop it does not generally wind up in the same orientation. It is an interpretation that can be useful over small regions of spacetime where you can neglect curvature, but it can cause problems if taken too far.

 

[Q-reeus]

The spin rate declines according to which observer? The local or the observer at the initial elevation?

By using the term 'coordinate determined' it is automatically implied the observer is at an initial elevation out at 'infinity'. See e.g. the definition of coordinate time given in this Wiki article: http://en.wikipedia.org/wiki/Schwarzschild_metric#The_Schwarzschild_metric
For elevations further in, one simply applies the appropriate correction factor - observed redshift is somewhat less then in general.

Could you elaborate on the assumption that "rest mass/energy declines with lower potential " ?

Sure. Annihilate a positron-electron pair out in gravity-free space and all we have left is a pair of gamma rays with a given emitted frequency. That is a fully conservative process of conversion of rest mass to EM energy. Now lower (not free-fall drop) the same positron-electron pair down into the potential well of some gravitating mass. Then annihilate as before. The gamma rays received 'at infinity' have been redshifted (a la Pound-Rebka experiment) owing to the annihilation process occurring in that gravitationally effected environment. Which can be equated with that in the process of lowering the electron-positron pair down the gravitational potential well, energy is being extracted to the lowering agency (say a winch). Everything is thus consistent with overall conservation of energy/momentum - but there has been a redistribution going on.

 

[Q-reeus]

Apparently you assume that no light ray reflects off the bottom or the top during motion. That's not likely IMHO. And thus its effect should be analysed. Is then work done or not? And equal but opposite?

Harry - it really pays to read an entire thread before commenting critically on an initial part thereof - see you own further remarks below!

Q-reeus: "[..] the trapped EM field energy [..]"
I would say trapped radiation energy - not field energy!

You wish to make some fine distinction here? As I assume you are a retired comms engineer or similar, you aught to be fully familiar with that a cavity field (leaving aside TW resonators) is just equipartitioned cyclicly alternating and phase orthogonal electric and magnetic fields. Disagree?

Q-reeus: "Say for a cubical cavity resonator, resolve cavity field into three mutually orthogonal sets of counterpropagating traveling waves. Two sets propagating transverse to radial direction, one along that axis. Sets need not be equal amplitude. It should be obvious the two transverse sets have no appreciable coupling to the only conceivable factor that hauling-up process could bring to bear as locally measured [..]"

As discussed in parallel, analyses with "local" measurements easily complicates analysis. It can be useful to develop a theory but when you already have the theory, needless frame jumping is often counter productive.

I have no idea what point you are trying to make here - please elaborate!

Q-reeus:" [..] from a Machian pov that seems suspect. [..]"
I find Machian POV's themselves suspect.

You are in turn welcome to that pov - but how would you then explain the process of assembling a matter shell around a spinning flywheel, without it implying a transfer of angular momentum to that assembled shell? HINT - there is a gravitationally analogous 'magnetostatic' field generated by that flywheel, and considering the shell assembly as having to radially draw in mass from infinity, one might suspect a 'magnetic Lorentz force' analogue is operative on that inwardly moving shell matter. Just my humble outlook of course - not out for a full-on intellectual stoush with your emminence! In the past we have a history of mostly agreeing on things, but hell here's a chance to really differentiate! Go for it Harry!

And I think that we have largely covered the topic of this thread.

Huh?

 

[Dale]

collision speed = light speed+mirror speed=k*c + v
collision length = length of light pulse = k*s
time= collision lenght/collision speed = (k*c+v)/k*s

Where do these equations come from? I don't recognize any of them as standard SR or GR equations.

 

[Naty1]

PeterDonis:

...In another thread I used this type of model to describe how one could extract work from a box filled with photons by slowly lowering it instead of letting it freely fall, and how extracting the work would lower the "photon temperature" inside the box, whereas a freely falling box of photons would have the same "photon temperature" inside as it fell (relative to the box, in both cases). But in either case, the "average" motion of the photons inside the box is no longer null, but timelike--it's just the motion of the center of mass of the box+photons.

Very interesting, but I don't get it. why does the photon temperature change dependent of the rate of fall?

The simple minded approach I thought would work: forget the details... you can't hide from the change in gravitational potential [PE] regardlesss of how the photons are are lowered...freefall, slowly, etc and if the PE changes than so must the KE of the photons as total energy remains constant...

Can you explain the last sentence...what's the context...are you saying the freely moving photons have no reference frame, but bouncing ones do...have an average...??

thanks. always interesting!

 

[harrylin]

[..] I have no idea what point you are trying to make here - please elaborate!

When you mentioned "local measurement" that suggested to me that you use different reference frames, with the risk of mixing them up or not transforming correctly between them. It's often better to avoid that.

[..] how would you then explain the process of assembling a matter shell around a spinning flywheel, without it implying a transfer of angular momentum to that assembled shell? [..]

I'm sorry, I meant that we've already finished discussing the topic (which is Length contraction of falling things!) and I already spent too much time on topics that I chose to discuss (I'm not retired, and I have a life). So I'm now trying to reduce (but physicsforums is like an addiction, worse than TV ).

[..] Annihilate a positron-electron pair out in gravity-free space and all we have left is a pair of gamma rays with a given emitted frequency. That is a fully conservative process of conversion of rest mass to EM energy. Now lower (not free-fall drop) the same positron-electron pair down into the potential well of some gravitating mass. Then annihilate as before. The gamma rays received 'at infinity' have been redshifted (a la Pound-Rebka experiment) owing to the annihilation process occurring in that gravitationally effected environment. Which can be equated with that in the process of lowering the electron-positron pair down the gravitational potential well, energy is being extracted to the lowering agency (say a winch). Everything is thus consistent with overall conservation of energy/momentum - but there has been a redistribution going on.

That sounds like a good analysis to me.

 

[Q-reeus]

When you mentioned "local measurement" that suggested to me that you use different reference frames, with the risk of mixing them up or not transforming correctly between them. It's often better to avoid that.

A misunderstanding then - I simply meant if one rides the box back up elevator style, there should be no detection of frequency shift enroute. Sans point raised later re flywheel!

I'm sorry, I meant that we've already finished discussing the topic (which is Length contraction of falling things!) and I already spent too much time on topics that I chose to discuss (I'm not retired, and I have a life). So I'm now trying to reduce (but physicsforums is like an addiction, worse than TV ).

Tell me about it - are there local chapters of 'PhysicsForums Anonymous' offering addicts any hope of rescue?!

That sounds like a good analysis to me.

Thanks Harold - nice to get some positive feedback now and then.

 

[PeterDonis]

Very interesting, but I don't get it. why does the photon temperature change dependent of the rate of fall?

It's not the "rate of fall" that's critical: it's the difference between the box being in free fall and the box feeling acceleration. Also there is an underlying assumption that the time of flight of photons across the box is short enough that tidal effects are negligible.

Consider a photon that bounces off the upper wall of the box, travels to the lower wall, and bounces off it. Look at this process in the instantaneous rest frame of the box, which, by our underlying assumption, can cover both "bounce" events in a single local inertial coordinate patch.

If the box is in free fall, then it can be considered to be at rest in the local inertial frame for both bounce events, so the momentum exchange of the two events cancels. So there is no energy exchange between the photons and the box.

If the box is accelerated upward, then it is moving upward for one of the bounce events. That means that there is more momentum exchanged when the photon hits the lower wall of the box than when it hits the upper wall of the box. So on net there is work being done. In the case where the box is being slowly lowered, the work will be done by the photons on the box (and that work is extracted by means of the rope that is holding the box), so the photons are losing energy relative to the box; hence the "photon temperature" relative to the box goes down (where "photon temperature" is just the average energy per photon).

I should note that I also assumed in this that the energy of the box itself was negligible. For a real box, of course, that would most likely not be the case, and one would have to consider the motion of the atoms in the box itself, not just the photons in it.

 

[Naty1]

PeterDonis:

If the box is accelerated upward, then it is moving upward for one of the bounce events. That means that there is more momentum exchanged when the photon hits the lower wall of the box than when it hits the upper wall of the box. So on net there is work being done.

That seems clear.

Originally Posted by Q-reeus

[..] Annihilate a positron-electron pair out in gravity-free space and all we have left is a pair of gamma rays with a given emitted frequency. That is a fully conservative process of conversion of rest mass to EM energy. Now lower (not free-fall drop) the same positron-electron pair down into the potential well of some gravitating mass. Then annihilate as before. The gamma rays received 'at infinity' have been redshifted (a la Pound-Rebka experiment) owing to the annihilation process occurring in that gravitationally effected environment. Which can be equated with that in the process of lowering the electron-positron pair down the gravitational potential well, energy is being extracted to the lowering agency (say a winch). Everything is thus consistent with overall conservation of energy/momentum - but there has been a redistribution going on.

"Now lower (not free-fall drop) the same positron-electron pair down into the potential well of some gravitating mass..."

This means lower the pair and then place them at rest at some point down in the potential well? Is this different than letting the pair free fall, stopping their fall, then letting them annihilate?? I don't see how the gamma ray energy is any different at the top and bottom of the well: local observers each see the same energy locally, top and bottom, right?; but such an observer at the top will see redshift from the bottom gammas, at the bottom, blue shift from the top gammas.

 

[harrylin]

[..] If the box is accelerated upward, then it is moving upward for one of the bounce events. That means that there is more momentum exchanged when the photon hits the lower wall of the box than when it hits the upper wall of the box. So on net there is work being done. In the case where the box is being slowly lowered, the work will be done by the photons on the box [..]

Thanks that looks like an accurate (although still qualitative) description to me - and I would like add that the momentum exchange is accompanied by a corresponding Doppler frequency shift. Those are consistent with each other as well as with the analysis of Q-reeus. Such a box will likely never exist, but it can serve as a model for what happens when an object is lowered.