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This question already has an answer here:

Extending on the question about the time dilation on Wikipedia, namely this diagram in the accepted answer:

Stationary vs moving frame of reference Image credit: John D. Norton

The thing that I don't understand is, why does the light appear to be travelling at an angle for the observer outside the moving frame (the one containg mirrors)?

Physically speaking, what forces the light beam emitted by a moving source not to go simply straight up vertically, missing the upper mirror completely?

I am fully aware why this works in Minkowski spacetime; all observers must see the spacetime event of the beam hitting the upper mirror, and I understand relativistic aspects like relativity of simultaneity, time dilation and comparing discrete spacetime events in Minkowski spacetime diagrams.

But in this case I am confused:

  1. presuming there is vacuum between the plates, what properties of the "empty space" between these mirrors cause it to be slanted, once it leaves the emitter?
  2. is the light supposed to be treated like a particle, and it's actually affected by the train speed?
  3. is the light supposed to be treated like a wave radiating from the emitter, and the point where it hits the mirror is actually not the top of this sphere wave?

(Update)

My confusion is deepened by the following statement by @JohnDuffield in this answer:

(OP's question:) The reason a wave such as sound would have the trajectory shown in this example is that the medium inside the rocket, air, is moving at the speed of the rocket and the sound wave would take on that velocity as it left it's source. Light does not use a medium to move.

Answer: It does. Have a look at Nobel Laureate Robert B Laughlin here: "It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed".

So, given the link to the wikipedia article on aether, this answer seems to imply that aether in fact not only exists, but is also being dragged by the train?

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marked as duplicate by user36790, John Rennie special-relativity Mar 22 '16 at 6:58

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ The same question could be asked of a chicken trying to cross the road. In the frame of the fox waiting on the other side to eat it, the chicken walks in a straight line. However, in the frame of a dog running towards the fox to save the chicken, the chicken seems to have a slanted path, just as you have described here. Now, what properties of the "road" causes the chicken to travel slanted? PS - The chicken was really excited to have made a new friend in the fox, and therefore, it crossed the road to greet him. Little did he know... $\endgroup$ – Prahar Mar 21 '16 at 15:41
  • $\begingroup$ Basically, what I meant to get across is that this particular part of the experiment has nothing to do with relativity or Minkowski space, etc. It holds true even in everyday life, i.e. in Newtonian mechanics. $\endgroup$ – Prahar Mar 21 '16 at 15:44
  • $\begingroup$ @Prahar: but what confuses me is that, if you throw a ball up in the air in a train wagon, for an outside observer it will seem to be traveling at an angle due to inertia (presuming vacuum and "Newtonian" speeds). I am having trouble justifying this "inertia" for photons, for the outside observer. $\endgroup$ – Lou Mar 21 '16 at 16:11
  • $\begingroup$ It's not travelling at an angle "due to inertia". It's travelling at an angle because that's what a straight line upwards in the train's frame looks like for an observer outside, purely geometrically. $\endgroup$ – ACuriousMind Mar 21 '16 at 18:21
  • $\begingroup$ The deeper answer is that the forces that are holding the mirrors together are the exact same forces that are guiding "the photons" (no matter how nonsensical that picture really is once you are using quantum mechanics proper). The mirrors can't go anywhere else than the photons or they would lose cohesion and fall apart. The problem here is NOT what the "photons" are doing, the problem is (and Einstein knew this when he wrote about it originally) that the entire world is governed by exactly one field symmetry and not two or more. $\endgroup$ – CuriousOne Mar 22 '16 at 0:32
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Forthcoming answers will, I'm sure, invoke equivalency and other Relativity theory to explain the observed: here I intend to begin with observables and derive the theory.

A photon emitted from a particle is emitted in no preferred direction: it has an equal chance of going any direction in space. In order to generate a directed beam such as a laser you have to have some way to block or direct the emitted photons such that the ones that make it out are going the same way. A laser in particular does this with a tube that has an aperture at one end, so that the photons going one direction get out and the photons going any other direction stay in.

Now, if you have a laser pointed straight up but moving to the right, the photons emitted inside the laser that are going straight up do not escape the aperture: they are intercepted by the side of the tube. However, some of the photons going up and to the right do get out. They were emitted toward the side of the tube, but then the side kept moving until, by the time the photon gets to the boundary of the tube, the aperture is there and the photon escapes. And since it was moving somewhat to the right inside the tube, it continues moving to the side as it goes up, which to the stationary observer looks like a diagonal instead of "straight up."

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