3 K background radiation

[Bowman_David/tiger_mpc@tiger.gtc.georgetown.ky.us]

Alex Burr writes:

> Since it is possible to measure the velocity of the earth with respect
> to the 3 K blackbosy radiation left over from the big bang, why
> cannot one use it as the basis for an absolute reference frame?

Who said that one cannot "use it as the basis for an absolute reference
frame"?  If you want to use this frame as an absolute frame, its your
privilege.  Just remember that this frame that you use has nothing special
about it as far as the LAWS of nature are concerned.  It certainly is a nice
pretty looking frame considering that the cosmic background radiation is
isotropic when viewed from it.  The Hubble expansion of the universe is also
isotropic as well when viewed from this frame.  These special symmetries
possessed by the distribution of the matter and radiation of the universe when
viewed from this frame DO give this frame an absolutely different character
from all the other reference frames which don't have this symmetric viewpoint.
It's just that the laws of nature are still the same in this frame as they are
in any other one.

Suppose we had a flat empty spacetime/universe and then added into some region
of it a bunch of interacting low mass particles (low enough mass so that they
don't cause any appreciable curvature of the spacetime over distances typical
of their spacing and we can ignore any general relativistic gravitational
effects) moving with random initial velocities with respect to each other.
Then, in principle, the center of mass of this "universe" could be found in a
straightforward way, and then an inertial frame in which this center of mass
is at rest could then be found as well.  When this "universe" is viewed from
this frame it is found that it has the special absolute property that whenever
the total momentum of all the particles is added up the answer is always zero.
All the other frames moving with respect to this one don't have this special
property.  In addition, suppose a particular coordinate system is chosen in
this special frame whose origin is located at the center of mass of this
universe.  When viewed from this origin it is found that, on average, the
motions of the particles are such that the universe appears isotropic on a
coarse enough space/time scale.  The laws of nature are STILL the same when
written in terms of this special coordinate system as they are when written
in terms of any other inertial coordinate system.

The case for our actual universe is not so different from the toy example
above.  In our case we have to consider (besides special relativity) general
relativity with a Friedman/Robertson/Walker metric for spacetime inhabited by
a distribution of matter which is homogeneous on a coarse enough length scale,
but the point is the same.  The matter/radiation in the universe may possess
more symmetry in one frame than another in an absolute sense, yet the laws of
nature are the same for that frame as they are for any other one.

David Bowman
Georgetown College
dbowman@gtc.georgetown.ky.us