Can Absolute Velocity be Measured with Cosmic Background Radiation?

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Discussion Overview

The discussion revolves around the concept of absolute velocity and whether it can be measured using cosmic background radiation (CMB). Participants explore the implications of using the CMB as a reference frame for measuring velocity, considering both theoretical and conceptual aspects of absolute frames of reference in physics.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that absolute velocity cannot be known, only relative velocity to other objects.
  • Others propose that the CMB could be blueshifted or redshifted depending on the direction of motion, suggesting it might serve as a reference frame.
  • One participant argues that the CMB is not an absolute frame of reference, as it does not hold special significance in physical law.
  • Another participant challenges this view, suggesting that the CMB's uniformity and historical significance from the Big Bang could imply it has a special status.
  • Concerns are raised about the implications of treating the CMB as a fundamental rest frame, particularly regarding the constancy of the speed of light.
  • Some participants note that while the CMB is a convenient reference for measuring relative velocities, it cannot be considered a perfect absolute frame due to its non-simultaneous generation across the universe.

Areas of Agreement / Disagreement

Participants express differing views on whether the CMB can be considered an absolute frame of reference. There is no consensus, as some argue for its significance while others emphasize its limitations.

Contextual Notes

Participants acknowledge that the CMB's uniformity does not negate the fact that it was not generated simultaneously everywhere, which complicates its status as an absolute frame. The discussion reflects various interpretations of the implications of using the CMB as a reference frame.

CosmicVoyager
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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
 
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CosmicVoyager said:
I have heard that one can not know one's absolute velocity, only one's velocity relative to other objects.
You misunderstand: there is no such thing as absolute velocity. So:
But wouldn't the cosmic background radiation be blueshifted in the direction of motion and redshifted in the oposite direction?
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.
 


russ_watters said:
"But wouldn't the cosmic background radiation be blueshifted in the direction of motion and redshifted in the oposite direction?"
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.

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?
 
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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.
 
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CosmicVoyager said:
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.
The universe did not begin at a point. This is a common misconception about the big bang.
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?
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.
 
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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.
 


Chronos said:
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.
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.
 

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