Time Dilation and Relative Time

[Drakkith]

Hmm, Harry, you mind checking and making sure what I've explained regarding preferred frames is correct? After reading your post I'm not so sure anymore.

 

[harrylin]

Hmm, Harry, you mind checking and making sure what I've explained regarding preferred frames is correct? After reading your post I'm not so sure anymore.

I had the impression that it's incorrect, but instead of criticizing doubtful posts I decided to simply present a clarification to the OP that is correct beyond doubt. ;)

A main cause for confusions is the fact that in the literature "preferred frame" can be found to mean different, incompatible things. For example several people on this forum use it as synonym of "absolute", in direct contradiction with its meaning in the older literature.

Quick check:

This explanation fails with a simple example. Consider 3 observers, A, B, and C. A and B are approaching each other at some velocity while C is stationary with respect to A and B. The amount of time dilation in A and B's frame, as measured by C, are equal, since both are approaching C at an equal velocity. However, from A's frame, B is experiencing more time dilation than C, since B is moving faster than C. B also measures more time dilation in A's frame than in C's, for the same reason. This always holds true as long as the three frames are inertial.

OK

If there were absolute motion and a preferred frame of reference then there should be some situation where although the three observers are moving identically to the above situation with respect to each other, C would measure either A or B as having less time dilation than itself. In other words, there would be a situation where either A or B is moving slower than C with respect to this preferred frame and C would therefore see either A or B's clock as ticking faster than its own.

We've looked for this effect. It does not happen. There is no such thing as absolute motion. [..]

Here you equalized "absolute motion" with "preferred frame". But if there were a preferred frame for the laws of physics, then the Lorentz transformations would be wrong. Inertial motion is "relative" in the sense that absolute motion cannot be detected, if it exists or not; the same is true in Newtonian mechanics. SR says nothing about such metaphysical interpretations.

 

[Drakkith]

Here you equalized "absolute motion" with "preferred frame". But if there were a preferred frame for the laws of physics, then the Lorentz transformations would be wrong. Inertial motion is "relative" in the sense that absolute motion cannot be detected; the same is true in Newtonian mechanics.
As others have explained on this forum, one can choose any inertial frame and pretend it to be in reality an "absolute rest frame" with respect to which all motion "really" exists. That cannot change the predictions - and SR has nothing to say about it.

I was under the impression that if a preferred frame of reference existed, all lorentz transformations would be relative to that frame, making this preferred frame also the frame that absolute motion is measured relative to. Is that incorrect?

 

[harrylin]

I was under the impression that if a preferred frame of reference existed, all lorentz transformations would be relative to that frame, making this preferred frame also the frame that absolute motion is measured relative to. Is that incorrect?

It's a meaning of "preferred frame" that is indeed used in the literature, at odds with its original meaning. In fact, in what way would such a frame be "preferred", if we can take any other inertial frame and obtain the same results? Moreover, one would not be able to measure relative to it, since one cannot identify it...

 

[Drakkith]

It's a meaning of "preferred frame" that is indeed used in the literature, at odds with its original meaning. In fact, in what way would such a frame be "preferred", if we can take any other inertial frame and obtain the same results?

You couldn't in a world with absolute motion. That was my point.

 

[harrylin]

You couldn't in a world with absolute motion. That was my point.

SR says that we cannot observe absolute motion; it says nothing about the existence or not of absolute motion. See also the FAQ: https://www.physicsforums.com/threads/what-is-the-pfs-policy-on-lorentz-ether-theory-and-block-universe.772224/

 

[name123]

It's a useful calculation exercise to check that for different inertial reference systems the same prediction is given for when two clocks are compared side by side.

It sounds as though you are saying that with two bodies at motion relative to each other, either could be considered the rest frame when making the prediction for when they are compared side by side.

I don't understand how this can be the case and if you don't mind me recapping some of this thread, I'll explain why.
I was reading in Clifford M.Will's book "Was Einstein right? Putting General Relativity to the Test" that there was an experiment done where in October 1971 an experiment was done with radioactive clocks, and plane trips taken going with the spin of the earth, and against it. He reports: "The eastward trip took place between October 4 and 7 and included 41 hours in flight, while the westward trip took place between October 13 and 17, and included 49 hours in flight. For the westward flight the predicted gain in the flying clock was 275 nanoseconds (billionths of a second), of which two-thirds was due to gravitational blue shift; the observed gain was 273 nanoseconds. For the eastward flight, the time dilation was predicted to give a loss larger than the gain due to the gravitational blue shift, the net being a loss of 40 nanoseconds, the observed loss was 59 nanoseconds".

Now in the experiment there was the gravitational blue shift (where clocks 'tick' faster when not under gravity, so when flying the plane clock 'ticks' faster than the one on the Earth which is under a higher gravity), and the time dilation (where clocks 'tick' relatively slower with relative motion). The issue at the heart of this thread is how can you tell which clock would appear slower if the experiment were done in flat spacetime (where there is no gravitational blue shift). And here is a thought experiment:

Imagine a prisoner, prisoner A who is a physicist in prison sphere in a space. Prisoner A has a telescope, a clock, and a laser measuring device and can measure the distance to the back of another prison sphere that is in front of them (imagine prison spheres only have a glass front). Also imagine that these prison spheres are within a space within a dense asteroid formation, and that these prevent the stars from being seen. Imagine that Prisoner A then loses consciousness and wakes up to measure the distance between it and the other prison sphere increasing at a fixed rate. Prisoner A loses consciousness again and wakes up to measure that the distance between it and the other prison sphere is decreasing at a fixed rate. Prisoner A loses consciousness again and wakes up, with the other prison sphere back in front of its.

How could prisoner A tell if the clock in its prison sphere or the one in the sphere in front would be the one which had 'ticked' less? It has been said on this thread that you can't, that you'd need to know which had accelerated. If you disagree could you please let me know, if you agree, then we can imagine just a fragment of the prisoner thought experiment episode, say where the prisoner had woken up to see the distance between it and the other sphere closing. And let's re-examine the idea that that with two bodies at motion relative to each other, either could be considered the rest frame when making the prediction for when they are compared side by side. It seems that it doesn't work, one will have gone slower than the other and if you were prisoner A you couldn't tell which it would be.

The answer can't be that it all depends on which rest frame the clocks are compared in because there could be an example of a long spaceship with a clock on the front passing much shorter space ship. It could take a thousand years to pass, but it might not take the shorter ship long to get up to its speed and dock on the back (and so into its rest frame), and then check it by the ships internal video link.

Another issue, though related, is that you said in another post (which shows on mine as #56) that "SR says that we cannot observe absolute motion; it says nothing about the existence or not of absolute motion." What I'm not sure one is why it couldn't be said that the one that actually ends up with slower clock due to SR time dilation when compared was the one with the greater absolute motion?

 

[PeterDonis]

I don't understand how this can be the case

This is a basic feature of SR. It has been explained repeatedly in this thread.

How could prisoner A tell if the clock in its prison sphere or the one in the sphere in front would be the one which had 'ticked' less?

On the information given, he can't, because he can't see the other prisoner's clock. So he needs more information to draw any conclusion. That answer has been given to you repeatedly in this thread.

we can imagine just a fragment of the prisoner thought experiment episode

But for just this fragment, we lose the key feature of the scenario, which is that the prisoners start out together, separate, and then come back together again. Only if that is true is there any absolute fact of the matter about which one ages less. If we just look at one segment of the scenario, where the two prisoners are moving relative to each other and neither one is accelerating, then there is no absolute fact of the matter about which one's clock is running slower; it depends on the frame you choose. That has been explained repeatedly in this thread.

It has been said on this thread that you can't, that you'd need to know which had accelerated.

Because, as has been explained repeatedly in this thread, in flat spacetime, that is the simplest piece of additional information that would allow him to draw a conclusion.

At this point we're just going around in circles. Thread closed.