High School Time dilation problem question

[Bartolomeo]

@Ibix. I'm thinking. You say that - something is wrong with setup of experiment. I break my head for quite a long time and I cannot figure out how to conduct such an experiment, so us to get redshift for the both observes in the same experiment. I mean observation of frequency. I would be most grateful for your advice.

 

[stevendaryl]

If mirror gains kinetic energy, it will start moving and recede from another mirror. But, if the mirrors are rigidly fixed (on the opposite sides of a room) the same amount on energy it had swallowed on reception of a photon it will transfer to the photon when spitting it out.
Otherwise it is not a mirror, but a well-polished boots, for example.

No, it is not possible for a photon to bounce off a mirror without imparting momentum to the mirror. It doesn't matter whether the mirror is perfect, or not. I just went through the math of this.

The correct conclusion is that, relativistically, it is impossible for something to be "rigidly fixed". Any object will compress slightly if you crash something into it. Any object will stretch slightly if you jerk on one end.

 

[Battlemage!]

If mirror gains kinetic energy, it will start moving and recede from another mirror. But, if the mirrors are rigidly fixed (on the opposite sides of a room) the same amount on energy it had swallowed on reception of a photon it will transfer to the photon when spitting it out.
Otherwise it is not a mirror, but a well-polished boots, for example.

You can't have it perfectly rigidly fixed, however, since there can be no perfectly rigid rods, if that matters.

 

[Ibix]

@Ibix. I'm thinking. You say that - something is wrong with setup of experiment. I break my head for quite a long time and I cannot figure out how to conduct such an experiment, so us to get redshift for the both observes in the same experiment. I mean observation of frequency. I would be most grateful for your advice.

I don't think it's possible with a single light pulse. I was only commenting that your description of the experiment was incomplete - you said "along the y axis" without saying according to whom.

I think the best you can do is require both parties to follow the same experimental procedure. Both fire a light pulse along their own y-axis and both reflect the other's pulse back. Assuming the pulses have the same proper frequency at emission both observers will agree on the frequency of the pulse they receive from the other and on the frequency of their own pulse reflected back. (Also they will agree that the reflection times were simultaneous and that the other ship emitted the pulse first and received the reflected pulse last.)

This is the reciprocity we expect.

 

[Bartolomeo]

I don't think it's possible with a single light pulse. I was only commenting that your description of the experiment was incomplete - you said "along the y axis" without saying according to whom.

I think the best you can do is require both parties to follow the same experimental procedure. Both fire a light pulse along their own y-axis and both reflect the other's pulse back. Assuming the pulses have the same proper frequency at emission both observers will agree on the frequency of the pulse they receive from the other and on the frequency of their own pulse reflected back. (Also they will agree that the reflection times were simultaneous and that the other ship emitted the pulse first and received the reflected pulse last.)

This is the reciprocity we expect.

Hmmm... IMHO there is one minor problem. If Bartolomeo and Ibix want to release photon along his own Y axis each, we both have to keep our tubes a right angle to direction of travel. One straight up and another straight down. This is the case of Joe and Aliens we started from. We simply will not see each other, I am afraid to say.

 

[Ibix]

So use two tubes - one for the emission and return of your own pulse and obe for the reflection of the other guy's pulse. Or rotate one tube after you emit and before you reflect, then rotate back in time to receive your own pulse reflected back.

 

[Ibix]

Actually you can't use a single tube for reflection because you don't send it back the way it came. You need a v-shaped tube with a mirror at the point, and a single tube for your emission and reception of returned pulse.

 

[Bartolomeo]

Actually you can't use a single tube for reflection because you don't send it back the way it came. You need a v-shaped tube with a mirror at the point, and a single tube for your emission and reception of returned pulse.

It is exactly moving mirror case. Mirror receives photon through one tube which looks into front and spits out through another which tilted back. This tube back will not release photon at right angle in mirrors frame
I think that you understand everything very well now. You will put all pieces together and will see the full picture very clearly.

 

[Bartolomeo]

So use two tubes - one for the emission and return of your own pulse and obe for the reflection of the other guy's pulse. Or rotate one tube after you emit and before you reflect, then rotate back in time to receive your own pulse reflected back.

It is not physics already. It is rather psyhics. It is the same as to conduct one experiment at 10 AM and another at 2 PM.

 

[Bartolomeo]

I will post another trick. If you are not bored yet. Would you mind?
The idea behind all this communication is very simple. Reciprocity of observations occurs, when you change reference frame. I introduce my rest frame first, you introduce your rest frame then. If we conduct observations on one chosen frame, there is no recoprocity of observations. Transverse Doppler effect makes troubles to that (change of frames I mean). It shows that reference frame is mutual property but not private.

 

[Ibix]

I already knew all of this. It doesn’t invalidate any point I am making. Both parties have a v tube to reflect the other's light pulse and a straight tube to emit and receive their own. This is the reciprocity that is expected.

I'm slightly at a loss to understand what it is that you expect.

 

[Ibix]

It is not physics already. It is rather psyhics. It is the same as to conduct one experiment at 10 AM and another at 2 PM.

Of course it's the same, if you do the same experiment. As I noted earlier, you keep running antisymmetric experiments, swapping participants, then acting surprised when the results swap. That is the reason for the difference between your 10am and 2pm experiments.

 

[Ibix]

Reciprocity of observations occurs, when you change reference frame.

Not true in general, as we've discussed at length. The fact that the laws of physics are the same in any inertial frame does not mean that all physical situations must be described the same way in all inertial frames. If it did there would be no need to consider frames.

It shows that reference frame is mutual property but not private.

I have no idea what this is meant to mean.

This conversation isn't going anywhere, so I'm out. Nothing you are saying invalidates anything I understand about relativity. If it causes problems for your understanding, then it would seem to be your understanding at fault.

 

[Battlemage!]

Not true in general, as we've discussed at length. The fact that the laws of physics are the same in any inertial frame does not mean that all physical situations must be described the same way in all inertial frames. If it did there would be no need to consider frames.

I have no idea what this is meant to mean.

This conversation isn't going anywhere, so I'm out. Nothing you are saying invalidates anything I understand about relativity. If it causes problems for your understanding, then it would seem to be your understanding at fault.

I wish you wouldn't as this conversation is helping my own understanding, lol.

 

[stevendaryl]

The original poster, @IvicaPhysics, seems to be satisfied that his questions have been answered, so I'm going to use that as a justification for closing this thread.