suppose two rockets with a passenger are nearing speed of light velocities and are going opposite directions. Relative to eachother one rocket will seem stationary while the other rocket will look like it’s going almost twice the speed of light. What do both passengers see?

Question

I can’t wrap my head around it. But maybe it might be a very silly question with a simple answer.

In: Physics

Answer 1

First: ALL MOTION AND SPEED IS RELATIVE. Speed isn’t something absolute. Your two rockets can only be moving at near the speed of light *relative to something else*.

So let’s stick an observer on each rocket, and one somewhere in the middle to watch them.

The person in the middle says that (a) they themselves are stationary, whereas (b) the two rockets are hurtling away in opposite directions at close to the speed of light.

The people on the two rockets both say that (a) they themselves are stationary, (b) the observer in the middle is hurtling away at close to the speed of light, and (c) the person in the other rocket is hurtling away at even closer to the speed of light*.

**Not “faster than the speed of light” because relativity tells us that, when you combine speeds, you don’t, actually, just add the numbers together – sure, that’s more than “close enough” for the sort of speeds we see on Earth, but it goes horribly wrong at relativistic speeds. The correct formula has a factor in it that has to be taken off the sum, that gets bigger as the speeds get bigger, and that keeps the final result under the speed of light. (It follows directly from the fact that everyone sees the speed of light as being the same, however they’re moving. It’s counterintuitive to us, but it’s eminently testable, and it’s what happens.)*

If you take no other message away, though, it’s this. ALL THREE PERSPECTIVES ARE EQUALLY CORRECT. Each observer sees themselves as standing still – they’re in their own “inertial frame”. And no-one is “more correct” than the others. All three see the same laws of physics; all three can do all the measurements and calculations they want, and their view of “how things are” will be self-consistent – it just won’t match the results that the other two get. Spacetime simply doesn’t have a prefered frame of reference.

(Don’t get misled by the things we see in space – planets, stars, galaxies and so on. They clearly are, to a degree, moving in something approximating to the same frame – the CMB (Cosmic Microwave Background) is often used to provide that frame of reference, apparently. But it’s important to understand that, in terms of the *physics*, that frame isn’t in any way more “special” than any other – if all the things in the universe apart from you and your rocket suddenly disappeared for a few hours, you’d have absolutely no way of telling whether or not you were moving relative to that frame or not until things reappeared.)