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

There are two different questions here. Speeds can be quoted relative to anything – any reference frame – that’s relativity.

In rocket science it is common to quote speeds relative to what you’re orbiting. Earth’s speed to the sun, a satellite’s speed to earth, etc.

There is a limit to this, though. Regardless of your reference frame, nothing can appear to be moving faster than celerity (the “speed of light”).

Objects in motion experience distance and time differently. Since speed is a combination of distance and time, an object in motion may disagree with you on the speed of another object in motion. Time and space both dilate perfectly to allow this.