# Inertial frame

I am new to relativity. I noticed that there is a general concept that someone sitting in the inertial frame (a typical one) will not be able to decide whether he/ she is at rest or moving with a constant velocity. Is this true. Are there no advanced devices that can help that person to decide whether he is at rest or in motion.

thanks

## Answers and Replies

Ibix
In short, no.

The whole point of relativity (both the Galilean/Newtonian kind and the Einsteinian kind) is that, as long as you aren't accelerating you can consider yourself to be at rest. There is no sense in which one inertial frame represents "really at rest" and all the others represent moving ones.

Gireesh
Thanks. So that means we don't have any means of proving or disproving that the entire universe (not any planet or galaxy ) is moving at a speed.. I am asking this to clarify my concept ...not necessarily suggesting that the world is moving

Ibix
We always measure velocity relative to something. The galaxy is moving with respect to other galaxies, but it's equally correct to say we are stationary and the other galaxies are moving around us.

But what could the entire universe be moving with respect to? There's nothing else, by definition. So the question you've asked is incomplete because you haven't specified what it is the universe might be moving with respect to, and it cannot be completed because there's nothing outside the universe.

Gireesh
HallsofIvy
Homework Helper
Thanks. So that means we don't have any means of proving or disproving that the entire universe (not any planet or galaxy ) is moving at a speed.. I am asking this to clarify my concept ...not necessarily suggesting that the world is moving
There is a bit more to it than that. It is not just that "we don't have any means of proving or disproving that the entire universe (not any planet or galaxy ) is moving at a speed" it is that there is no such thing to measure. "Speed" or "motion" only exists "relative" to some other object.

FactChecker
Gold Member
Are there no advanced devices that can help that person to decide whether he is at rest or in motion.
It's not a question of whether you can detect motion with respect to another inertial reference frame. It's a question of whether you should prefer one reference frame over another and call it "stationary". The point is that there is no preference for either one. There are ways to see that you are moving versus other objects, but you can assume that you are moving or that you are stationary and the physics using special relativity theory will work out consistently either way. General relativity carries this idea even beyond inertial reference frames.

Gireesh
If you consider the CMB we are pretty close to comoving with that.

FactChecker