Re: recoiling photons evidence?

"ralph sansbury" <[EMAIL PROTECTED]> writes:
> [ Dishman: ]
>  then you assume the photon
> > moves at v=c which is impossible if it has non-zero mass.
> 
>   Since light moves at v=c there does appear to be a
> contradiction if the photon like other particles or  at least
> like charged particles increases in mass from a nearly zero but
> as yet undetermined finite mass.
>    Of course as you say if its mass was exactly zero this
> increase in mass or whatever would not occur. In any case all of
> these possible properties for the photon no matter which ones you
> accept make the photon unlike other particles for which the
> conservation of momentum has been shown to be applicable.

It is a *definition* of relativity that only zero rest-mass entities
travel at the speed of light, and that, equivalently, entities which
travel at the speed of light have zero rest mass.  This is also
equivalent to saying that there is no comoving (or "rest") frame for
entities traveling at the speed of light.

In formulating the relativistic equations of motion, expressions for
both massive and massless particles can be found.  These expressions
must be invariant under Lorentz transformations, and being so,
momentum is manifestly conserved.  Therefore your statement is in
error.

The "relativistic mass" is an analogy or crutch for understanding
relativity.  It begins by asking the question: *if* we can express the
momentum by the classical equation p = mv, then what is the mass m
which makes that equation work under relativity.  For massive
particles, the answer is of course m = m0 / sqrt(1-(v/c)^2).  For
massless particles, the premise of the statement is false (we can't
express momentum as p = mv), therefore any conclusions are irrelevant.

The "analogy" line of reasoning begs the question.  Relativity is not
classical physics, and so p = mv is not a requirement.  Under
relativity, it is understood that mass is always the rest mass
(=invariant mass), and the relativistic momentum is redefined to be p
= sqrt((E/c)^2 - (m0 c)^2), which is m0 v / sqrt(1-(v/c)^2) for
massive particles, but E/c for massless particles.

When one says that the relativistic mass increases with speed, one
really means, the relativistic momentum becomes non-linear with speed.

CM