Quote by Chalnoth It's arbitrary in the sense that one could make any number of other possible choices of reference frame and the laws of physics would still be exactly the same. That is to say, the fact remains that you can't tell whether or not you're moving with respect to the CMB without looking at the CMB.
So the same as jtbell's scenario -

 Quote by jtbell One way to re-phrase this is: without "looking" at the CMB itself, it is (as far as I know) impossible to decide whether we are moving with respect to the CMB. This is similar to the (hopefully well-known) fact that if we are in a train moving with constant speed along a perfectly straight, level, smooth track, with silent engines, and with shuttered windows, we cannot tell (without being able to "look outside") whether we are moving or not.
Yes?

cheers,

neopolitan

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 Quote by neopolitan ... In other words, thinking of the Earth as stationary is arbitrary but very sensible. ...
I like that way of putting it!

Arbitrary but very sensible.

Just as a quibble, knowing what we do about the universe, we can tell our speed relative to CMB without looking at the CMB.

We can look at other markers to see how we are moving relative to the average matter of the early universe. We can tell the same speed by looking at galaxies' redshifts.

Because we are moving 370 km/s in the direction of Leo relative to bulk, the galaxies in that part of the sky are receding on average slower (by 370 km/s) than Hubble Law says they should. And the galaxies behind us, opposite to Leo direction, are receding on average faster (by 370 km/s) than Law says to expect.

The Hubble law about galaxy recession/distance does not work, is not true, unless you correct for the motion of the solar system relative to ancient matter, or the CMB light from it, or whatever you like to call it.

Of course it is arbitrary!

But it is very convenient so cosmologists use that criterion of rest and associated simultaneity, for nearly everything they do.

If you move too fast in some direction you will be roasted by the CMB doppler hotspot in that direction. So, like you said, being at rest relative CMB is "very sensible".

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 Quote by marcus Just as a quibble, knowing what we do about the universe, we can tell our speed relative to CMB without looking at the CMB. We can look at other markers to see how we are moving relative to the average matter of the early universe. We can tell the same speed by looking at galaxies' redshifts.
Well, that's only because we know these other things also have the same reference frame, at least on average. One might argue that if we didn't know this already through observation, we may not be able to use the galaxies' motion as a means to determine the rest frame of the CMB.

 Quote by Chalnoth Well, that's only because we know these other things also have the same reference frame, at least on average. One might argue that if we didn't know this already through observation, we may not be able to use the galaxies' motion as a means to determine the rest frame of the CMB.
Isn't this the same as for the speed of light? We only knew that the speed of light is always c because we measured it. Then Einstein used the constancy of the speed of light as a postulate, which does not means it is proven. Then using the theoretical framework built from that postulate Einstein was able to get a figure for the precession of the perihelion of Mercury which more closely matched observations than earlier theoretical frameworks.

Notionally, we could now go the other way. Using the equations necessary to give better predictions for Mercury's perihelion's precession, we could then go backwards to show that the speed of light must be constant.

If we didn't know that the speed of light was constant as the result of our observations, we possibly would not have arrived at equations to resolve our problem with Mercury (although really, we probably would have).

What doesn't change is that the speed of light is always c (in vacuo), irrespective of whether we have the means to measure it or not.

Similarly, we can work out the rest frame of the CMB by observing galaxies' redshift, irrespective of whether we have the know-how to work it out. I put it to you that you and I might be able to work it out, given the data, access to a library, time and inclination, but if we hand that same data to one of the average contestants on (or viewers of) Big Brother, that person would not be able to work out the rest frame of the CMB. That person's inability doesn't make working it out impossible.

Doesn't the possibility of working out the rest frame of the CMB without directly observing it make that rest frame a little more than merely arbitrary? (I am entirely happy with it not being "special", in that the laws of physics apply equally to any other less sensible arbitrary rest frame. It's just special in that it is a more useful choice.)

cheers,

neopolitan

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 Quote by neopolitan Isn't this the same as for the speed of light? We only knew that the speed of light is always c because we measured it.
Yes, precisely. And the fact that the CMB rest frame and the average galactic rest frame are identical is an indicator of a shared history. Without knowledge that there was such a shared history, we wouldn't be able to state categorically that the two rest frames are the same one.

 Quote by neopolitan Doesn't the possibility of working out the rest frame of the CMB without directly observing it make that rest frame a little more than merely arbitrary?
No. Because the measurement you're talking about is functionally identical to measuring the rest frame of the CMB: the rest frame of the galaxies and that of the CMB are identical because of their shared history.

G'day from the land of ozzzzzzz

This paper is quite interesting

http://arxiv.org/abs/0806.3758
Searching for non-Gaussianity in the WMAP data

Authors: A. Bernui, M.J. Reboucas
(Submitted on 23 Jun 2008 (v1), last revised 9 Mar 2009 (this version, v2))

 Abstract: Some analyses of recent cosmic microwave background (CMB) data have provided hints that there are deviations from Gaussianity in the WMAP CMB temperature fluctuations. Given the far reaching consequences of such a non-Gaussianity for our understanding of the physics of the early universe, it is important to employ alternative indicators in order to determine whether the reported non-Gaussianity is of cosmological origin, and/or extract further information that may be helpful for identifying its causes. We propose two new non-Gaussianity indicators, based on skewness and kurtosis of large-angle patches of CMB maps, which provide a measure of departure from Gaussianity on large angular scales. A distinctive feature of these indicators is that they provide sky maps of non-Gaussianity of the CMB temperature data, thus allowing a possible additional window into their origins. Using these indicators, we find no significant deviation from Gaussianity in the three and five-year WMAP ILC map with KQ75 mask, while the ILC unmasked map exhibit deviation from Gaussianity, quantifying therefore the WMAP team recommendation to employ the new mask KQ75 for tests of Gaussianity. We also use our indicators to test for Gaussianity the single frequency foreground unremoved WMAP three and five-year maps, and show that the K and Ka maps exhibit clear indication of deviation from Gaussianity even with the KQ75 mask. We show that our findings are robust with respect to the details of the method.

 Quote by Chalnoth the measurement you're talking about is functionally identical to measuring the rest frame of the CMB: the rest frame of the galaxies and that of the CMB are identical because of their shared history.
Shared history?

So what you are saying is that there is a rest frame shared by galaxies and the CMB, and we should treat it is "arbitrary"? Is there really much more to the universe than galaxies and the CMB?

I am not becoming less confused here. I can see how the rest frame of the universe (ok, galaxies and the CMB) is not useful to us on an everyday basis. However, I would see our day-to-day preferred choice of rest frame as arbitrary but sensible, while a rest frame based on pretty much everything in the universe is perhaps the reverse - i.e. the choice of the CMB rest frame would not be arbitrary, but (with relatively few exceptions) it would also not be sensible.

cheers,

neopolitan

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