system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If motion is relative, then why is light speed the speed limit?

If I'm off in space somewhere with nothing around me, then how would I determine whether or not I was moving?  How would I determine whether or not I was accelerating, or what speed I was moving at?

So some object comes whizzing by me at half the speed of light.  If I decide that it was stationary and I was moving at half light speed, then I could not possibly accelerate to another half light speed, or I'd be going the speed of light which is impossible.

On the other hand, if I decide that it was moving at half the speed of light, and that I was stationary, then I could accelerate to half the speed of light or faster.

So, what gives here?
gpt
The concept of "moving" only exists when comparing two things. So if there is nothing to compare yourself to, "moving" and "stationary" are the same thing. It's not just that you can't tell if you're moving or not, there isn't any distinction between the two at all.

The concept of the speed of light is very confusing. The important thing is that light in a vacuum always travels at the same speed *relative to you*. However fast or slow you are going, light will always appear to be going at the speed of light compared to you, and nothing with mass can reach this speed.

This sounds like it could cause all kinds of contradictions like the one you've mentioned. But they don't happen because travelling at those speeds causes time to pass at a different rate and distances contract.

If you saw a space ship fly past you at near the speed of light, it would look squashed in the direction its travelling in. Also if it was possible to see the people walking around inside the ship, it would look like they were going in super slow motion.

They would see the same thing if they could see inside your ship too, because from their point of view, you are flying past them at near the speed of light. But inside their own ship everything would seem normal.