On Thu, 17 Sep 2009 21:11:53 +0100, Tim S wrote:

The photon is considered to have zero mass *at rest*

This is something that has always bewildered me.

If something has zero mass, does it exist?

*If* that is the case, then presumably photons can never be at rest,
otherwise they would cease to be.

I suppose the other point of view is that outer space exists yet it
has effectively no mass.

But does it really have zero mass since even outer space
is not a pure vacuum and there are still one or two atoms
per large volume?

However, outer space must be composed of something in certain theories
since in those theories it is argued that it is bent by gravity, and
one cannot bend something which is not there.

(Corrections or further explanation of my misapprehension gratefully awaited.)