Re: How can photons be massless
From: Andrew B. Park (novakyu_at_yahoo.com)
Date: 07/17/04
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Date: 16 Jul 2004 20:22:19 -0700
Jim Greenfield wrote:
(snip)
> In labs bodies are regularly lifted by bombardment with photon
> particles.
> This is good old kinetic energy at work; non-zero mass in motion.
(snip)
You know, this was the hardest thing for me to understand in my third
semester of engineering physics (thermodynamics and intro (as in,
"scratching the surface of") modern physics): Photons have no mass, but
they do have momentum (and yes, they also have kinetic energy). The
"lab bodies being lifted" (whatever that means... it is a very sketchy
description of whatever happens in an experiment) can be explained by
exchange of momentum from photon to the "lab body." (Or, the old
Conservation of Momentum, if you consider the system of lab body and
the photon to be a closed system--and consider that photon gets
absorbed into or bounces off lab body.)
Perhaps, if you are unable to go through the derivation yourself, you
could take this to be an axiom (as it is clearly true even from your
own assertion): photons have momentum (but this does not imply that
they have mass, as p = mv does not hold for photons--rather, it's
something like p = hf, if you know what it means). Whether they have
mass or not... well, let's just say that most of the stuff we know for
electrodynamics and QED will not hold if photons had mass (and from
experimental results, we know that our current electrodynamics works,
at least in the range we tested it).
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