system
Given the following post, respond as if you're commenting on Reddit:
human
Observed Light Speed from a Rocket Never Changes?

Hello all, I've tried finding answers to a question I have on google but can't seem to find any that make much sense to me therefore I've chosen to write here for some clarification.

Say a rocket moves away from Earth at 99% the speed of light and I then shine a laser towards that rocket. Once that laser catches up to the rocket, will it be observed to travel at 1% the speed of light since we are already moving at 99% the speed? I understand that the speed of light remains constant for everyone but if we observe it to travel at 100% the speed of light from the rocket, doesn't that mean that it is technically moving faster than the speed of light? I know my last point isn't true but I'm just having a hard time wrapping my head around the concept of light speed remaining constant regardless of your speed. Thanks!
gpt
This is basically why time and space have different rules in special relativity. There's a couple of things going on here, but the big thing is that events that are simultaneous in one frame of reference aren't simultaneous in another frame of reference, and the time gap between events can be different in different frames of reference.

So yeah, you're travelling at 99% of the speed of light away from Earth, and Earth fires a laser at you. From Earth's perspective it takes a long time to reach you, because the distance between the laser pulse and your spaceship is shrinking at only 1% of the speed of light.

From your perspective, the Earth is moving away from you at 99% of the speed of light, and then fires a laser at you. From your perspective, you are stationary. This laser approaches you at the speed of light, and should reach you quite quickly. For this to add up correctly, what happens is that Earth fires its laser at a different time from your perspective! Earth fires its laser *later*, so even though the distance between the laser pulse and your spaceship is shrinking more rapidly than from Earth's perspective, you still agree about e.g. how old you are when the laser hits you.