More and more detailed astronomical observations of light emitted long
ago from distant objects make sense in terms of a model in which most
of the basic laws of physics, including the speed of light, have not
changed. That's not to say that the model isn't surprising. It includes
the intuitively weird spacetime of General Relativity, a background
energy which gravitationally makes the universe's expansion accelerate,
and something which for a very brief period after the Big Bang made the
acceleration extremely rapid. None of these effects change the speed of
light with respect to the local material that it's travelling past.
However, if you extrapolate back far enough, to the point where effects
like electromagnetism and nuclear forces were not distinct from each
other, the meaning of 'light' as an electromagnetic wave changes.
As you get farther from the realm in which the current laws work,
it becomes harder to guess what the laws look like and whether there's
some meaningful speed which you could say is the same as the speed of
light. If the laws of physics have changed, it can be a little
arbitrary to say whether the speed limit has changed. That's because
there's some choice in saying whether distances, times, or speeds have
changed. What we can say with more definiteness is whether numbers that
don't depend on how you define units have changed. For example, there's
a number called the fine structure constant which has something to do
with the speed of light and the strength of electromagnetic forces and
has no units. Its numerical value (about 1/137) is known to great
precision, and has nothing to do with how you define meters or seconds
or any such thing. One question that comes up is whether that number
has changed over time.
There doesn't seem to be any way to take the current laws and
extrapolate them back all the way to the Big Bang. General Relativity
and quantum mechanics just don't combine in a logically consistent way,
and that becomes a serious problem in trying to understand short time
scales. So we're confident that the ultimate laws of physics aren't
quite the ones we have now. It seems likely that our current universe
of 3 space and one time dimension is collapsed down from one with nine
or ten space dimensions and one time dimension.
It's always possible that some little remnant of our strange and
unknown history gave small changes in the fine structure constant and
other physical numbers even in the realm where the laws nearly take
their current form. Some people claim there are some observations to
support this idea, but the evidence now is very weak.
Mike (w advice from Tom, but he can't be stuck with blame for the final version)
(published on 10/22/2007)