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I recently watched a couple science videos on YouTube and left me with a doubt.

If a spaceship goes to a really fast speed in the (horizontal) $x$-direction, what would happen if it shoots a light photon perfectly vertical to it, will the horizontal speed of the spaceship affect the direction of the photon being shot or will the photon go straight up?

I'm aware the light's speed doesn't get affected, but what about the direction?

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    $\begingroup$ From whose perspective? $\endgroup$ – Kyle Kanos Aug 1 '15 at 20:45
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Imagine sitting on an airplane and playing with a laser pointer. As you fire the laser beam upward perpendicular to the motion of the plane you see it go straight up to the ceiling. The same would be true on a spaceship from the point of view of its passengers. As for the people watching this happen on Earth, they will see you fire the photon at an angle which depends on the spaceships velocity. The greater the velocity the larger this angle becomes. This angled trajectory of the photon from the Earths reference frame means that the light takes longer to travel between the laser pointer and the ceiling of the craft. Put another way, they think time on the spacecraft is slowed down.

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  • $\begingroup$ Nice example, thank you, what about if the airplane is going faster than the speed of light? Will the photon stay behind if it's being shot upward (because it doesn't get pushed and stays in constant speed)? Or will it be landing at the same spot in the ceiling because it's being "pushed" by the greater speed of light? (Suposing we can go faster than light) $\endgroup$ – Carlos C Aug 1 '15 at 20:52
  • $\begingroup$ Keep in mind that you never actually make progress toward the speed of light in your own frame. So, to go "faster" than the speed of light would simply be from someone else's point of view. Also, at the speed of light the universe is infinitely thin in the direction of motion and no time passes. Someone trying to view you from Earth would say you don't even exist. $\endgroup$ – Alex Aug 1 '15 at 21:07
  • $\begingroup$ So, is the constant speed of light relative to the time dilatation of the observer? I mean, if the speed of light is 300,000 km/s, we are using "second" to define it, and inside this really fast airplane seconds are "slower" for some viewers, so will this "second" in the speed of light behave different in every viewer? Assuming the "second" is not the same everywhere. $\endgroup$ – Carlos C Aug 1 '15 at 21:24
  • $\begingroup$ If you are on the spacecraft you will measure light to go the same speed as people on Earth. You will also believe that time is running normal as well. However, you will think time on Earth is moving more slowly. In fact, you feel at rest and like the Earth is the one moving! This is because there is no absolute reference frame. $\endgroup$ – Alex Aug 1 '15 at 21:27

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