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Re: The beginning of space-based light pollution?

Subject: Re: The beginning of space-based light pollution?
From: "Mike Kreuzer"
Date: Thu, 23 Jun 2005 10:56:53 +1000
Newsgroups: sci.astro.amateur
"Chris L Peterson" <clp@xxxxxxxxxxxxxxxxxx> wrote in message 
news:jrrjb1hca9nesrj85ifhofev1jk27gkvs1@xxxxxxxxxx
> On Thu, 23 Jun 2005 08:57:07 +1000, "Mike Kreuzer"
> <mikekreuzer@xxxxxxxxxxxxx> 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>

Regards,
Mike Kreuzer
www.mikekreuzer.com 



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