Time Dilation and photodetectors

[Samson4]

In the image below, we will be accelerating the entire apparatus. It won't ever reach a frame of inertia. Would someone in a different frame of reference be able to detect this acceleration from the output of the photo-detector?

As time slows and the distance the laser travels contracts; would the photodetector display this energy input to a stationary observer?

If we replace the photodetector with a perfect mirror and create a laser cavity, would we find an interference pattern in the middle of the cavity as we accelerate the apparatus?

 

[jbriggs444]

As time slows and the distance the laser travels contracts;

If one adopts an accelerating frame of reference in which the assembly is at rest then the photodetector will see red-shifted light due to gravitational time dilation. This is a frame-independent fact that can be read from the photodetector's output.

If one stays with an inertial frame then the detector is always moving faster away from the light at the reception event than the source was moving toward the light when it was emitted. Doppler then says that a red shift will result.

 

[Samson4]

If one adopts an accelerating frame of reference in which the assembly is at rest then the photodetector will see red-shifted light due to gravitational time dilation. This is a frame-independent fact that can be read from the photodetector's output.

If one stays with an inertial frame then the detector is always moving faster away from the light at the reception event than the source was moving toward the light when it was emitted. Doppler then says that a red shift will result.

So as the apparatus accelerates the photo detector output signal will decrease in voltage. Will it also decrease in amperage?

 

[jbriggs444]

So as the apparatus accelerates the photo detector output signal will decrease in voltage. Will it also decrease in amperage?

It will remain constant in voltage. Why would you expect it to change?

 

[Samson4]

It will remain constant in voltage. Why would you expect it to change?

I guess I misunderstood redshift. What if the photodetector is replaced with a material that is only transparent to light slightly below the laser's frequency. If the laser light is redshifted enough, would it transmit through the material during acceleration?

 

[jbriggs444]

I guess I misunderstood redshift. What if the photodetector is replaced with a material that is only transparent to light slightly below the laser's frequency. If the laser light is redshifted enough, would it transmit through the material during acceleration?

The light is emitted with no red shift. It is received at the photodetector with a red shift. To a first approximation, the red shift is proportional to distance traveled in the accelerating frame. Yes, a chromatic filter attached at the detector could pass the red-shifted light.

For a fixed ongoing proper acceleration and a Born-rigid structure the redshift at the detector is fixed and unchanging. [Born-rigid means that the proper length of the structure is constant]

 

[ZapperZ]

There is a flaw in the arguments given so far.

How are you sure that the band structure of the photocathode that make up the photodetector does not change due to the relativistic effects? It seems that the nature of the detector itself has been neglected.

Zz.

 

[Samson4]

There is a flaw in the arguments given so far.

How are you sure that the band structure of the photocathode that make up the photodetector does not change due to the relativistic effects? It seems that the nature of the detector itself has been neglected.

Zz.

Would a material's transmission spectrum be expected to change as well?

 

[Nugatory]

How are you sure that the band structure of the photocathode that make up the photodetector does not change due to the relativistic effects? It seems that the nature of the detector itself has been neglected.

Can we avoid this concern by analyzing the behavior of the detector using the momentarily comoving inertial frame in which the detector is momentarily at rest? In that frame the only relativistic effect will be the red shift of the incident laser light.

 

[ZapperZ]

Can we avoid this concern by analyzing the behavior of the detector using the momentarily comoving inertial frame in which the detector is momentarily at rest? In that frame the only relativistic effect will be the red shift of the incident laser light.

But if you do that, then why should the detector see a red-shifted light, since it is in the same reference frame as the light source?

Zz.

 

[jbriggs444]

But if you do that, then why should the detector see a red-shifted light, since it is in the same reference frame as the light source?

Zz.

It's not though. Classic rocket paradox.

 

[ZapperZ]

It's not though. Classic rocket paradox.

"It's not" what? That they are not in the same frame?

Remove the acceleration. If they are in some inertial frame, will the detector see a red-shifted light? If it does, how come I don't see a red-shifted light from the UV source that I'm using to emit photoelectrons?

Zz.

 

[jbriggs444]

"It's not" what? That they are not in the same frame?

Correct. In an accelerating Born-rigid structure, the front and back ends are not at rest in a single inertial frame.

[In particular, the tangent inertial frame in which the source is at rest at the time of emission and the tangent inertial frame in which the detector is at rest at the time of reception do not coincide]

Remove the acceleration. If they are in some inertial frame, will the detector see a red-shifted light?

Now they are both at rest in a single inertial frame and there is no red shift.

 

[ZapperZ]

Correct. In an accelerating Born-rigid structure, the front and back ends are not at rest in a single inertial frame.

[In particular, the tangent inertial frame in which the source is at rest at the time of emission and the tangent inertial frame in which the detector is at rest at the time of reception do not coincide]

Now they are both at rest in a single inertial frame and there is no red shift.

Then Nugatory can't get rid of the problem of the band structure of the material also being affected and thus, can't be ignore, which was my original point.

Zz.

 

[jbriggs444]

Then Nugatory can't get rid of the problem of the band structure of the material also being affected and thus, can't be ignore, which was my original point.

Nonsense. The material is at rest in its tangent inertial rest frame in which the arriving light is red shifted.

 

[ZapperZ]

Nonsense. The material is at rest in its tangent inertial rest frame in which the arriving light is red shifted.

Huh?

The band structure along the direction of motion has to change (unless you want to turn off electromagnetic interaction in that direction as well). Is this in dispute?

Zz.

 

[jbriggs444]

Perhaps I am not understanding you. There is no motion in an object's rest frame.

 

[ZapperZ]

Perhaps I am not understanding you. There is no motion in an object's rest frame.

Both the detector AND the laser are moving together with the same acceleration in the OP's post. The question isn't about the rest frame.

If I am in an inertial frame looking at both the laser and the photodetector, and I see the laser being red-shifted, then the photodetector must also be affected by the relativistic effects, resulting in a different band structure based on MY perspective.

Zz.

 

[anorlunda]

I think what the OP meant to ask was this.

If we could remotely observe the instruments measuring clock rate on a spaceship traveling at speed near c relative to us, would we see dilated time? Alternatively, if we could ask a passenger on the spaceship to communicate how fast time passes for him, what would he say?

The answer in both cases, is that we would see or hear that time passes normally on that ship.

I think its a clever question. The spaceship passenger speaks into his microphone, "normal" We hear the message as nnnnnooooorrrrrrmmmmmaaaaalllll because of the red shift, but it does not morph to ssssslllllooooowwww.

 

[ZapperZ]

I think what the OP meant to ask was this.

If we could remotely observe the instruments measuring clock rate on a spaceship traveling at speed near c relative to us, would we see dilated time? Alternatively, if we could ask a passenger on the spaceship to communicate how fast time passes for him, what would he say?

The answer in both cases, is that we would see or hear that time passes normally on that ship.

I think its a clever question. The spaceship passenger speaks into his microphone, "normal" We hear the message as nnnnnooooorrrrrrmmmmmaaaaalllll because of the red shift, but it does not morph to ssssslllllooooowwww.

This is what the OP wrote, and the bold is mine:

In the image below, we will be accelerating the entire apparatus. It won't ever reach a frame of inertia. Would someone in a different frame of reference be able to detect this acceleration from the output of the photo-detector?

As time slows and the distance the laser travels contracts; would the photodetector display this energy input to a stationary observer?

If we replace the photodetector with a perfect mirror and create a laser cavity, would we find an interference pattern in the middle of the cavity as we accelerate the apparatus?

View attachment 215155

So here is what I gather:

1. The frame the laser+detector is the same
2. This frame is accelerating and never an inertial frame
3. The OP wants to know if we simply look at the output of the detector, can we detect that it is accelerating?

So is the above correct or not?

Zz.

 

[anorlunda]

So is the above correct or not?

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

 

[jbriggs444]

Both the detector AND the laser are moving together with the same acceleration in the OP's post

That's impossible.

 

[ZapperZ]

That's impossible.

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.

 

[jbriggs444]

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"?

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.

 

[ZapperZ]

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

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.

 

[jbriggs444]

If I am in an inertial frame looking at both the laser and the photodetector, and I see the laser being red-shifted, then the photodetector must also be affected by the relativistic effects, resulting in a different band structure based on MY perspective.

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.

 

[ZapperZ]

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.

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.

 

[davenn]

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.

Who said anything about "fixed proper distance" between them? All I said was that the OP gives them both the same acceleration.

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

In relativity theory, proper acceleration[1] is the physical acceleration (i.e., measurable acceleration as by an accelerometer) experienced by an object. It is thus acceleration relative to a free-fall, or inertial, observer who is momentarily at rest relative to the object being measured. Gravitation therefore does not cause proper acceleration, since gravity acts upon the inertial observer that any proper acceleration must depart from (accelerate from). A corollary is that all inertial observers always have a proper acceleration of zero.

Proper acceleration contrasts with coordinate acceleration, which is dependent on choice of coordinate systems and thus upon choice of observers (see three-acceleration in special relativity).

In the standard inertial coordinates of special relativity, for unidirectional motion, proper acceleration is the rate of change of proper velocity with respect to coordinate time.

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

In standard cosmology, comoving distance and proper distance are two closely related distance measures used by cosmologists to define distances between objects. Proper distance roughly corresponds to where a distant object would be at a specific moment of cosmological time, which can change over time due to the expansion of the universe. Comoving distance factors out the expansion of the universe, giving a distance that does not change in time due to the expansion of space (though this may change due to other, local factors, such as the motion of a galaxy within a cluster). Comoving distance and proper distance are defined to be equal at the present time; therefore, the ratio of proper distance to comoving distance now is 1. At other times, the scale factor differs from 1. The Universe's expansion results in the proper distance changing, while the comoving distance is unchanged by this expansion because it is the proper distance divided by that scale factor.

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 scaleDavejust want to learn

 

[PeterDonis]

Thread closed for moderation.