when NASA etc talk about things moving at X speed in space, what is that compared to? What’s the relative point in space used and how can you know yours inert against it?

Question

This is a weird one, but it boils down to people saying you can’t travel faster than light. But speed is only relative to another object isn’t it? I mean, I’m sat on the sofa now doing 0mph, but compared to the sun, we’re travelling thousands of mph.

So if speed is only relative to another object, does that mean two objects travelling close to SoL are actually going faster than light relative to each other?

Edit: there’s been some incredible responses which – to my endless surprise – have made perfect sense somehow! Thanks for the response!

In: 20

Answer 1

> does that mean two objects travelling close to SoL are actually going faster than light relative to each other?

I see this wasn’t really answered. The answer is actually no. As it turns out from Special Relativity, nothing with mass can move at the speed of light compared to anything else. And things without mass, such as light, always move at the speed of light.

If you send a rocket away from Earth at 0.8c, and another rocket away from Earth in the opposite direction also at 0.8c, you’d expect their speed compared to each other to be 1.6c, i.e. faster than light. But nope, that’s not how the universe works. Compared to each other they are actually moving at 0.9756c. All kinds of weird shit happens when speeds get closer to the speed of light, and this is just one of them.