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Light is the fastest thing in universe but the speed of light is still a finite number. Imagine me and my friend are some separated by some distance and I fire a laser at my friend. If my friend is in motion with respect to me, the laser might miss him because my friend has moved by the time laser reaches him.

Now imagine me and my friend are in a spaceship that is moving transverse to us. So, me and my friend are at rest with respect to each other but in motion with respect to some third observer. I fire a laser at him, will it miss him?

If it doesn't, why? Let's say I remove the walls of spaceship. Now me and my friend are moving with respect to some third observer. When my position coincide with the third observer, we both fire a laser towards my friend. My laser should reach my friend coz we are at rest with respect to each other. But laser fired by third observer should miss my friend. Does my motion affect the speed of light?

And if it does miss him, can't I use this fact to tell if I'm in motion?

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    $\begingroup$ If you both are at rest w.r.t. each other, you will not see each other moving, so you won’t miss if you shoot it straight at them. Third/fourth/fifth/etc observers are irrelevant. $\endgroup$ Commented Jun 4 at 22:41
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    $\begingroup$ This question doesn't seem related to the equivalence principle. Perhaps you meant the relativity principle... $\endgroup$ Commented Jun 4 at 23:17

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You have a few silly confusions that could have appeared when reading a vague description of the Special Theory of Relativity (SR).

In a mathematically precise treatment, even as an introduction, of SR, it is typically taken that events in spacetimes are absolute. That is, when two things coincide in both position and time, then this is true in every frame of reference. If your laser hits your friend, then the photon coincided in position with your friend at some certain time, and then all frames will agree that this coincidence happened, even if they all disagree with the exact time and place that this event would have been recorded as.

Now, regardless of the constant uniform motion that somebody else might think you and your friend are moving at, when you aim a laser at your friend, you aim it so that you can hit him. The angle at which you aimed the laser will thus be one that will hit. You cannot be so silly as to aim in such a direction that will miss him.

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If you fire a laser at the position of a stationary target, the light will hit the target. If you fire the laser at a moving target, you will miss if the target has moved out of the path of the light before the light reaches it. That is true in all frames, and does not violate any equivalence principles.

No, you cannot use that effect to determine whether you are moving in any absolute sense- you can only use it to determine whether you are moving relative to the target, which you can do in any case just by looking at it.

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