"Chris L Peterson" <[email protected]> wrote in message
> On Thu, 23 Jun 2005 08:57:07 +1000, "Mike Kreuzer"
> <[email protected]> wrote:
>>OK, out of all this, this is the bit I still don't get. Maybe I didn't go
>>the day this was explained in high school or something, but isn't momentum
>>mass * velocity? So no mass, no momentum?
> But photons _don't_ have zero mass. They have zero _rest mass_. But they
> are never at rest. Their actual mass is not defined, but is real and
> positive (photons have energy, and that is the best way to work with the
> mass equivalence). Consequently they have momentum.
This is the bit I don't get, slightly more refined. Maybe I'm
misunderstanding one of these bits, but my reasoning is:
If mass increases (is dilated) asymptotically, to get to the photon's small
positive mass when it's travelling at light speed, doesn't its rest mass
also have to be non zero, just really, really small? That would mean that
something with an even smaller mass could go faster than light, of course.
I don't get (in a nutshell) why zero mass at rest would be dilated to photon
weight. Why doesn't it just stay zero?
Rest mass is just mass measured when there's zero relative velocity between
the measurer & measure-e, so, ok, to measure the rest mass of a photon you'd
need to use other photons, or something else travelling at light speed like
energy, but presumably that's experimentally possible.
Probably just proving a little knowledge is a dangerous thing. <g>