Re: does light travel forever?

On 16 Jan 2006 23:11:38 -0800, in talk.origins , ck19bla@xxxxxxx in
<1137481898.169412.95440@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>does light ever run out of energy and stops?

The question is actually a bit harder and more subtle than people have
given credit. Let us ignore the obvious issues of something in the
way, you were not asking about that. There is a very real sense at
which the answer is yes and a no: the perspective of the photon. Light
has no rest mass and, in a real way, does not experience time. Without
a rest mass you plug it into the Lorenz contraction equation and you
see that time has totally stopped for the photon. For it, it travels
either forever or instantaneously. I doubt you were asking that
though, so lets continue. So let us imagine a universe with a single
photon. If there is only the single photon then it is hard to discuss
it moving, but lets ignore that. If this is a flat universe, neither
expanding nor contracting, then, since the photon experiences no time,
it can't "run down". If travel and time mean anything in such a
universe the light would travel forever. If the Universe were
expanding then it would, from the POV of some non-existent observer,
seem to get larger and larger (lower frequency, redder). From the POV
of the photon, still, nothing will change. I am not sure what happens
in a collapsing universe over very long times. (One photon is also not
heavy enough to make a sizable universe collapse, but that is a
separate issue.) The real answer is more complex than this, so just go
with no and you will be close enough.



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