Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Bill S on 06/11/2013 22:47:07
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I’ve been doing some thinking about frames of reference in relation to the CMBR.
Perhaps someone would be kind enough to comment on my line of thinking so far.
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We cannot assign a rest frame to a photon.
The CMBR is composed of photons; therefore it cannot be assigned a rest frame.
However, an observer who is moving at sufficient speed relative to the CMBR will see its spectrum blueshifted in the direction of motion, and redshifted in the other direction.
Thus, an observer can claim to be at rest relative to the CMBR if the radiation is measured as being isotropic.
If an observer can be judged to be at rest relative to the CMBR, relativity holds that it must be equally valid to say that the CMBR is at rest relative to the observer.
How does this differ from assigning a rest frame to the CMBR?
Because the CMBR is composed of photons, although the observer/CMBR may according to the above reasoning, each be considered to be at rest relative to the other, the observer will always see the radiation approaching at c from every direction.
Thus, the observer is not actually stationary relative to the CMBR, but is simply maintaining a position of equilibrium in which the observed wavelengths of the radiation are the same in every direction.
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I’ve been doing some thinking about frames of reference in relation to the CMBR.
Perhaps someone would be kind enough to comment on my line of thinking so far.
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We cannot assign a rest frame to a photon.
Quite true.
The CMBR is composed of photons; therefore it cannot be assigned a rest frame.
Not true. The CMBR can correctly be thought of as a gas of photons. The rest frame is then that frame in which the momentum density of the photons is zero.
Does that help?
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Careful measurement of the Cosmic Microwave Background Radiation (CMBR) suggests that our galaxy is moving with a velocity of 360Km/sec in the direction of Leo. This does provide a standard frame of reference however you cannot say the converse and interpret it as an absolute frame of reference with respect to some origin.
The relative motions and rotation axes of galaxies are in general random and show no preferred direction once the effects of universe expansion and local velocities have been removed