GR response to Sam5 (Moved out of derailed thread)

[SeanF]

This came from here, but I've started a new thread to avoid derailing.

Originally Posted by Sam5

Regarding that other matter, here are a few Russian physicists who agree with my assessment of the cause of gravitational redshifts:

“The gravitational redshift being, both theoretically and experimentally, one of the cornerstones of General Relativity, it is very important that it always be taught in a simple but nevertheless correct way. That way centers on the universal modification of the rate of a clock exposed to a gravitational potential. An alternative explanation in terms of a (presumed) gravitational mass of a light pulse – and its (presumed) potential energy – is incorrect and misleading. We exhibit its fallacy, and schematically discuss redshift experiments in the framework of the correct approach.”

http://arxiv.org/pdf/physics/9907017

Sam5, what do you think these guys mean by the word "universal" in the sentence, "That ['simple but nevertheless correct'] way centers on the universal modification of the rate of a clock exposed to a gravitational potential"?

[John Dlugosz]

I think it's overly complex. Here is my interpretation: the light doesn't change as it rises out of the gravity well, smoothly evolving its state as it changes its position. You can see the red shift from a point down inside the well, too (if it's falling)! Rather, the light's state is not changing, but observers in different reference frames measure a different frequency.

Moving away from a massive body is just one way to change your reference frame.

[John Kierein]

I've always had a problem with the way a gravitational redshift is calculated. According to the Einstein gedanken experiment, an accelerating frame of reference is equivalent to a gravitational field. I.e., if you are on an elevator which is accelerating upward at 1 g somewhere between galaxies, and you shoot a bullet against the wall, it drops the same as if it were in a 1 g field on earth. This is generally given as the equivalence of gravitational and inertial mass. Similarly, a laser should drop when shown against the wall in the elevator by an amount due to the finite speed of the light. If a laser is bent by gravity, then the principle of equivalence should apply to light as well as mass. (I personally doubt this myself for some rather unorthodox reasons that I won't go in to here.)
So the light beam red shift should be an acceleration red shift; yet almost all the texts treat it as a velocity red shift with the red shift being related to the escape velocity of the gravity field rather than the equivalent g level.

[A Thousand Pardons]

Originally Posted by John Kierein

I've always had a problem with the way a gravitational redshift is calculated. According to the Einstein gedanken experiment, an accelerating frame of reference is equivalent to a gravitational field. I.e., if you are on an elevator which is accelerating upward at 1 g somewhere between galaxies, and you shoot a bullet against the wall, it drops the same as if it were in a 1 g field on earth. This is generally given as the equivalence of gravitational and inertial mass. Similarly, a laser should drop when shown against the wall in the elevator by an amount due to the finite speed of the light. If a laser is bent by gravity, then the principle of equivalence should apply to light as well as mass. (I personally doubt this myself for some rather unorthodox reasons that I won't go in to here.)
So the light beam red shift should be an acceleration red shift; yet almost all the texts treat it as a velocity red shift with the red shift being related to the escape velocity of the gravity field rather than the equivalent g level.

That's an interesting point, John.

If light behaved in a gravity well the same as in an elevator, then superficially, you'd say that the beam would follow the same parabola that any mass particle would--that is, the curvature would be the same as for ordinary gravity. We know from the history books that that was Einstein's first claim, that that was what was going to be tested originally by the Eddingtion expedition.

But the first world war got in the way, the expeditions were postponed, and in the meantime, Einstein revised his theory with the result that he concluded that the deflection was twice that. Just in time for Eddington to test that new hypothesis.

[SeanF]

So I started a new thread to avoid derailing the original, and now my new thread has gotten derailed.

And before Sam even responded . . . #-o

[Grey]

(Sorry, SeanF. I'm sure that Sam5 will respond, and we can then continue with the intended discussion. In the meantime, though... )

Originally Posted by A Thousand Pardons

If light behaved in a gravity well the same as in an elevator, then superficially, you'd say that the beam would follow the same parabola that any mass particle would--that is, the curvature would be the same as for ordinary gravity.

But a light ray does behave the same in an accelerating elevator as it does in a gravitational field (or so general relativity claims, at least). Naturally, the amount a horizontal beam is deflected downward may be different from that of a massive particle, but the paths that various massive particles will take will also be dependent on their horizontal velocities, so that shouldn't be a big surprise. Of course, the calculations are made a little more complex by the fact that the speed of light remains constant in all reference frames considered, which may not necessarily be the case for the speed of massive particles, so a simple Newtonian treatment of this situation may fail.

Originally Posted by A Thousand Pardons

We know from the history books that that was Einstein's first claim, that that was what was going to be tested originally by the Eddingtion expedition.

But the first world war got in the way, the expeditions were postponed, and in the meantime, Einstein revised his theory with the result that he concluded that the deflection was twice that. Just in time for Eddington to test that new hypothesis.

Of course, the experiment Eddington performed is a little more complex anyway, since it involves a non-constant gravitational field for the light to pass through.

[A Thousand Pardons]

Originally Posted by Grey

(Sorry, SeanF. I'm sure that Sam5 will respond, and we can then continue with the intended discussion. In the meantime, though... )

No, he got ticked and started his own thread.