# I Time Dilation and photodetectors

1. Nov 17, 2017

### Staff: Mentor

My theory is that the OP bungled the question. Your interpretation of the bungled version is correct.

2. Nov 17, 2017

### jbriggs444

That's impossible.

3. Nov 17, 2017

### ZapperZ

Staff Emeritus
What is "impossible"? That my understanding of what the OP wrote is "impossible", or that the thought experiment indicating that both are moving together with the same acceleration is "impossible"?

Zz.

4. Nov 17, 2017

### jbriggs444

It is impossible for two objects to be moving together with the same proper acceleration and a fixed non-zero proper distance between them in the direction of that acceleration.

5. Nov 17, 2017

### ZapperZ

Staff Emeritus
1. Who said anything about "fixed proper distance" between them? All I said was that the OP gives them both the same acceleration.
2. This was the OP's scenario based on my interpretation. If you don't like it, please reply to the OP. Tell HIM that this is "impossible".

Zz.

6. Nov 17, 2017

### jbriggs444

You are completely missing the point.

The rest frame that can be used to analyze the problem is the momentary tangent inertial rest frame of the detector. In this frame, the source was moving away at the time of emission. Doppler applies.

7. Nov 17, 2017

### ZapperZ

Staff Emeritus
And I think that you are completely missing MY point.

Since you want to deal with an "inertial rest frame of the detector", let's remove completely the acceleration. Let's put the laser and the detector in inertial frame S'. I am in inertial frame S. S' is at a speed close to c with respect to S.

I see the laser wavelength that is different than the wavelength of the laser at rest. However, I also see a different band structure of the material that make up the photodetector. If I look at the crystal structure of the material and E vs k dispersion curves, they will be different. Whether it is in an inertial frame or not, the band structure will be different from the point of view of S.

So what is in dispute here now?

Zz.

8. Nov 17, 2017

### davenn

for the sake of my learning here on a subject I am not really familiar with. I don't understand the impossibility of the situation
of 2 objects not being able to move together with the same acceleration

Front end or back end of a rocket, car or whatever, or in the case of the OP a setup with a laser and a detector
Going from the OP I assumed these were on a "platform" of some sort and the whole assy. is being accelerate.
And what is proper acceleration and fixed proper distance ? as compared to some other definition of acceleration or distance

from Wiki

am finding that a little harder to fully understand .... specially with what follows that section

From Wiki .....

I don't see that (proper distance)as being part of the original OP .... there was no mention of this being on a cosmological distance scale

Dave

just want to learn

9. Nov 17, 2017