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First excuse me if this question is very simple for you but for me and for my friend is not. Recently we were discussing what path a light beam will follow in a box moving at a constant velocity in empty space. We are still discussing.

The height of the box is $10$ light minutes. The box moves at a constant velocity in empty space (I am not sure what is exactly the empty space but considering there is no any gravitational distractions). The observer in the box is isolated and I don't know if there is a way for it to know the box is moving, this is another question.

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The observer send a light beam from point $A$, from the center of the floor. Point $B$ is at the center of the ceiling. What happens in 10 minutes later; will the light hit $B$ or not?

If it is possible please explain the answer like you explaining to your grandmother, even simpler. I am sure they are smarter than me.

Edit: What would happen if the box start to accelerate from $t=0$? Can we talk about acceleration without a frame of reference?

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  • $\begingroup$ If the light failed to hit point B, you'd know you were moving. But you have just as much right as any other inertial observer to claim you're standing still. Therefore the light must hit point B. (The same would be true if you'd launched a basketball instead of a light beam.) $\endgroup$
    – WillO
    Commented May 21, 2016 at 14:55
  • $\begingroup$ That's why I guess, but I am not sure about basketball because it has mass however light doesn't. $\endgroup$
    – newzad
    Commented May 21, 2016 at 15:00
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    $\begingroup$ Which part of the argument did you think relied on masslessness? $\endgroup$
    – WillO
    Commented May 21, 2016 at 16:01
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    $\begingroup$ The size of the box is irrelevant and there is no such thing as motion without a second reference. Since you don't have such a second reference, there is no motion in this problem, to begin with. If you are asking what an outside observer for whom the box is moving would see, that's a different matter: they will see the box rotated with the light still hitting B. $\endgroup$
    – CuriousOne
    Commented May 21, 2016 at 19:56

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The simple answer is yes. Ten minutes later in box time the light hits the target. The observer sees the light travel in a straight line for ten minutes then it hits the target ten minutes later on his watch.

But you wouldn't even ask the question if you were in the box, so what do you see from outside, not moving along? You see something similar but subtly different.

You see his torch pointing at the target as they move past you.

You see the beam heading diagonally but always in line with his torch, as you would if he fired a bullet from a rifle. The target also stays in line with his torch, and you see the beam hit the target.

But while his watch shows it only took ten minutes, yours says it took longer. You saw the light take a longer, diagonal path, and when you calculate the speed of light for you, you get the same answer.

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  • $\begingroup$ Yes, and if the box is moving at the speed of light you as an outside observer will never see the beam hit the target. It will appear to you to be moving horizontally. $\endgroup$
    – Wookie
    Commented Nov 28, 2019 at 13:36

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