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

>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?

Only if you think of speed in newtonian relativity. When we are talking einsteins relativity you can’t just add velocities like this. You have to use an extended formula that takes speed compared to SoL into account.

u=v+u′/(1+(v*u′/c^2))

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The way it all adds up makes it so that it doesn’t matter what object you are moving relative to, it all calculates the same, as you would expect.