# Angled light path in moving clock

1. Dec 15, 2012

### phyti

1. Ann's light clock uses a single photon that oscillates vertically between two mirrors separated by a distance d. She leaves the space station, accelerates to a constant .2c in a random direction labeled x.
For her clock to continue working, the photon must move vertically as before, and simultaneoulsly horizontally along x. If the photon does not aquire the speed of the clock, it must move at an angle relative to the vertical. What causes the angular orientation?

2. Assume the same scenario except the upper mirror is moved vertically to a separation of d/2. Does the clock still work?

2. Dec 15, 2012

### Bill_K

If Ann remains still, an observer moving past at any velocity such as 0.2c will see her clock continue to work properly. The photon's velocity will appear to have a sideways component just as the mirrors in the clock do also.

However if you are serious that Ann accelerates, she must push her clock along with her. She will push the mirrors and they will accelerate, but there is nothing to push the photon and it will fall off the edge.

3. Dec 18, 2012

### phyti

We know an observer moving past the clock cannot affect the operation of the clock, and only perceives/interprets his motion as belonging to the clock.
If the clock only works for a specific speed, i.e. in the frame where it was constructed, this would seem to disagree with the 1st postulate.

This also relates to the laser pointed toward the ceiling of a moving capsule.
Does it work for an observer moving at various speeds?

4. Dec 18, 2012

### zonde

It works for observers moving at various speeds. It doesn't necessarily work for accelerating observers.

5. Dec 18, 2012

### phyti

Thanks for that.
I'm only considering constant speeds.
This is based on the speed of light being constant and independent of the source. If either device, light clock or laser, moves to the right, the original vertical beam must move to the right to function correctly. The goal is to explain how the light signal aquires the necessary angle to intercept the moving mirror or ceiling that is vertical relative to the observer moving with the device. I've never seen an explanation for this.

6. Dec 18, 2012

### zonde

Stationary laser functions differently than similar laser that is in motion. If stationary laser emits light vertically then similar horizontally moving laser emits light at an angle to vertical. This is related to aberration. Only aberration usually describe observer effect but the same way it applies to source.

7. Dec 18, 2012

### ghwellsjr

Aberration has to do with a source and emitter that are moving relative to each other which is not the case with a light clock.

8. Dec 18, 2012

### ghwellsjr

This question was considered in this thread, but not to the OP's satisfaction.

9. Dec 18, 2012

### Bill_K

Aberration is the change in direction of the wave vector when going from one frame to another, and is the correct explanation for the light clock.

10. Dec 18, 2012

### Histspec

That follows from momentum conversation in the electromagnetic field. Light rays or photons have momentum p=E/c, so their direction (not their speed) is affected by the motion of the source. That is, if the photons are sent upwards in a straight line (in the comoving frame), then the rays or photons automatically are moving to the right in a frame where the light source is moving to the right, in accordance with aberration.

This is equivalent to the transverse light path of the Michelson-Morley experiment, where one also expects an inclined path to the right due to momentum conservation (in the aether frame). See the lower image at Wikipedia/Michelson-Morley. There you can see how photons gain momentum and thus direction from the source, without changing the photon's speed. If you transform back into the MMX frame, the path becomes a straight line in transverse direction again.

11. Dec 19, 2012

### Saw

Not in contradiction with the explanations you have received, but as a development of the details, the mechanics of the process, you may want to look at the reasoning I give in this Blog entry. The matter puzzled me at the beginning as well. There are recurrent questions about it. And in that Blog entry I tried to provide a didactic explanation, which I think is in line with what everybody has in mind, I just try to make it clearer.

12. Dec 20, 2012

### phyti

Thanks for all responses.
Histspec's is consistent with the mass-energy equivalence which affects the light path near a massive object as verified in the 1919 experiment. This is also my basic understanding from reading Max Born's explanation in his book on Relativity.
Having read Feymann's book on light propagation, I also leave room for revisions!