J G Miller coughed up some electrons that declared:

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?


It's a bit moot with a photon as it cannot be at rest in vacuo in normal
space[1] - and if it were, it would have zero energy and thus zero
frequency so nothing would be able to "see" it anyway.

[1] Better check with Hawking and others for the latest theories in weird
environments like black holes. Physics has always been about having a
theory, then there's a better theory that deals with edge cases the old one
didn't, ad infinitum probably...

I always chuckle when that Star Trek (I forget which sub series) wiffled on
about creating a photon decelerator that could harness the power of light
by bringing a photon to a dead halt. Many folk here have such a device - a
solar heating panel!

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

That would be a reasonable interpretation in the Einstein/Planck world. Feck
knows what theories abound now.

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

Even there, weirdness abounds - I don't understand that stuff...

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?

That's not "space" so much as the contents of "space".

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.

Space (and time) are dimensions, so are conceptually different to matter.

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

Einstein's own book is surpisingly readable. Feynman wrote some pretty good
stuff too and a really fun book is "Mr Tompkins in Wonderland" which is a
fictional (but scientifically valid) look at how the world would be if the
speed of light were 30mph.

Cheers

Tim