# Can one's absolute velocity be determined by measuring cosmic background radiation?

1. Jul 1, 2010

### CosmicVoyager

Greetings,

I have heard that one can not know one's absolute velocity, only one's velocity relative to other objects.

But wouldn't the cosmic background radiation be blueshifted in the direction of motion and redshifted in the oposite direction?

Thanks

2. Jul 1, 2010

### Staff: Mentor

Re: Can one's absolute velocity be determined by measuring cosmic background radiatio

You misunderstand: there is no such thing as absolute velocity. So:
Yes, but then you'd just be choosing the CMB as your frame of reference and measuring velocity with respect to it. The CMB is not an absolute frame of reference.

3. Jul 1, 2010

### CosmicVoyager

Re: Can one's absolute velocity be determined by measuring cosmic background radiatio

Hmm. I don't see why not. I mean the universe started at a point expanded in every direction, radiated the CMB, and has continued to expand since then. It seems obvious to me that it is. I wonder what other's think? I would expect there to be disagreement on this.

Maybe I should ask why isn't it? I can say why other objects can't be used as a reference: because their speeds and directions have changed countless ways since the big bang and we don't know what those changes were. The CMB's has not. It has just been stretched out to a longer wavelength. So why isn't it?

Last edited: Jul 1, 2010
4. Jul 1, 2010

### Staff: Mentor

Re: Can one's absolute velocity be determined by measuring cosmic background radiatio

An absolute frame of reference is one that would hold special significance in physical law, such as having the speed of light be constant only wrt that frame. The CMB has no such special significance.

Last edited: Jul 1, 2010
5. Jul 1, 2010

### bapowell

Re: Can one's absolute velocity be determined by measuring cosmic background radiatio

The universe did not begin at a point. This is a common misconception about the big bang.
You are providing reasons for why the rest frame of the CMB is a convenient frame against which to measure relative velocities. Indeed, as observers comoving with the expansion, this is the frame we use when we talk about the age of the universe, and other frame-dependent quantities. However, you seem to be suggesting that the CMB be treated as a fundamental rest frame. As Russ says above, as an 'absolute' frame of reference, the constancy of the speed of light wrt to all inertial frames would be called into question. Are you suggesting that this be the case? If not, then really what's the difference between what you are advocating and the fact that the rest frame of the CMB is simply a convenient one in which to make measurements? Keep in mind that the CMB wasn't always in existence, and when it was generated it wasn't generated at exactly the same time everywhere in the universe.

Last edited: Jul 1, 2010
6. Jul 2, 2010

### Chronos

Re: Can one's absolute velocity be determined by measuring cosmic background radiatio

I would take minor issue with that, bapowell. I believe the CMB was incredibly smooth and uniform when it formed. That is what WMAP studies suggest. As such, it does make a convenient reference point for mundane issues like relative motion in the universe.

7. Jul 2, 2010

### bapowell

Re: Can one's absolute velocity be determined by measuring cosmic background radiatio

I conclude the same in my post. What exactly do you take issue with? I mention that the CMB wasn't generated simultaneously everywhere in the universe simply to point out that it can't be a perfect, fundamental, absolute frame. The difference in decoupling times across the universe is obviously not sufficient to affect the smoothness and uniformity of the CMB, which I agree with.