# 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?

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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!

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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.

Speed is always relative to something. A plane’s airspeed is relative to the air it’s passing through; its groundspeed is relative to the ground it’s flying over. (The difference is the speed of the *air* over the ground.)

For rockets, there’s speed relative to the launch site; relative to the center of the Earth; relative to the Moon, Mars, Jupiter, or whatever other body it’s approaching; and relative to the Sun.

Any of these are valid, so choose whichever is convenient.

> 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.

>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.

Usually they use “the background/fixed stars”, i.e. stars so large and far away they don’t appear to move and are thus utile as a fixed, unmoving reference point. They do of course move, but the amount is negligible in calculations.