How does CMBR differ from ether (SR self study - Question from W. Rindler)

[maverick280857]

How does CMBR differ from ether (SR self study -- Question from W. Rindler)

Hello everyone

First of all, apologies for a previous post which was against forum rules. I was unaware that I had violated the rules by posting a link to a paper that wasn't from a proper peer-reviewed scientific journal (something I discovered much later).

Anyway, I have the following question, which is actually from Wolfgang Rindler's book on Special Relativity. This isn't homework (I'm teaching SR to myself).

If the universe were filled with a very rarified transparent fluid in which light propagated at speed c' < c, how would that affect special relativity?

The universe is filled with a diffuse 'photon gas' constituting the so-called microwave background radiation (a vestige of the 'big-bang' origin of the universe), which defines a preferred test frame at each point in the universe. How does that differ from an ether?

My random thoughts for each part...

Part 1: Unless light is known to interact with such a fluid, slowing it down or speeding it up, such a fluid will not affect special relativity.

Part 2: CMBR itself comprises of photons, whereas ether (nonexistent) is comprised of ___ (?). CMBR is itself radiation that travels at light speed, whereas ether is static. I know this may sound silly, but I wanted to discuss this part out here. I know CMBR isn't ether. But what really is the difference, formally? Also, what is the significance of this question?

Thanks and cheers.

 

[psycherevolt]

Ether doesn't exist, it was ruled out by the http://en.wikipedia.org/wiki/Michelson-Morley_experiment" . CMBR was theorized to be created by recombination shortly after the big bang when it was cold enough for protons and electrons to couple and create hydrogen.

An ether is defined as something such as air except in a vacuum. In air sound travels, in an ether light would travel. In SR lorentz transformations are used because the speed of light is the same in all inertial reference frames. In an ether the speed of light would be non-constant in all inertial reference frames.

 

[psycherevolt]

Also remember the speed of light is always constant, it does not speed up or slow down as classically described. On average when light passes through a medium it "slows" down by being absorbed and re-emitted

 

[Fredrik]

First of all, apologies for a previous post which was against forum rules. I was unaware that I had violated the rules by posting a link to a paper that wasn't from a proper peer-reviewed scientific journal (something I discovered much later).

You're not alone. I got busted too.

Part 1: Unless light is known to interact with such a fluid, slowing it down or speeding it up, such a fluid will not affect special relativity.

Light has to interact with the fluid in order to "slow down" to c' (relative to the fluid), but this wouldn't change the invariant speed c. So the fluid has no effect on SR.

Part 2: CMBR itself comprises of photons, whereas ether (nonexistent) is comprised of ___ (?). CMBR is itself radiation that travels at light speed, whereas ether is static. I know this may sound silly, but I wanted to discuss this part out here.

It still defines a rest frame, because you need to have a spefic velocity (by definition 0 in that frame) in order to observe background radiation with the same wavelengths from all directions.

I know CMBR isn't ether. But what really is the difference, formally? Also, what is the significance of this question?

The idea behind the aether was that it seemed reasonable to assume that a wave (anything that satisfies a wave equation) always consists of small displacements of the component parts of a medium (like when sound travels through air, or when ripples move across the surface of water). The question is significant because the answer means that the existence of this "preferred" rest frame doesn't invalidate the ideas behind SR.

 

[maverick280857]

Thanks psycherevolt and Fredrik.

(PS -- I was searching for this thread on the SR/GR forum.)