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According to the typical though experiment to derive the time dilation:

Time dilation experiment

a stationary observer would see that light travels more distance, and since light speed is constant, it must follow that time must slow down.

What I am having a hard time understanding is not the conclusion, but rather the premise:

Why/how does the light emitted "know" to travel in a diagonal from the point of view of the stationary observer?

Why doesn't the stationary observer see the light go straight "north" from his/her own frame of reference?

Where is light getting its "west" - "east" velocity component from (if velocities don't add up when dealing with light)?

What would happen if the light was emitted from a stationary light source instead? Would the moving observer see the photon miss the mirror in a diagonal in the other direction?

How does a photon "know" it was emitted from a source in a train and thus has to move in a diagonal (from the pov of the stationary observer), and to move straight "north" if emitted from a source in the stationary frame of reference?

Image by: By Sacamol - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=48778704

pakman
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2 Answers2

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It has absolutely nothing to do with the light knowing anything.

Consider somebody running and dribbling a basketball. In their frame, the basketball is going straight up and down; in yours, it's going diagonally. It's not that the basketball 'knows' to move diagonally. There's one motion it's doing, and it just happens to look diagonal in your frame. The situation with the light is exactly analogous.

knzhou
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Here is how I think of it.

For an inertial light-clock, treat the first emission as a set (a spray) of light-signals in all spatial directions. Only one of those light-signals will strike that particular light-clock's distant mirror at the spot across from the source...(or think of very short mirrors), then continue to reflect back and forth between the mirrors of that particular light-clock.

VisualizingProperTime y pair A with photons

Here's my video of it: https://www.youtube.com/watch?v=1-K3VDKjMdM

robphy
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