Re: [Phys-L] Rest mass of light (photons) Was: Re: speed c

[Bernard Cleyet <bernardcleyet@redshift.com>]

On 2013, Mar 14, , at 18:10, John Denker <jsd@av8n.com> wrote:

>  -- Ordinary running-wave photons have zero mass.
>  -- Photons in the sense of quantized standing waves of the EM field 
>   in a box have nonzero mass.  A box containing such photons has more
>   gross weight than one that doesn't (other things being equal).


Before my post I decided to do a little lit. search.  I found this, which makes (made) sense, and, furthermore, one may use  "argumentum ad verecundiam".   [Tho we know not always valid!]

" ….
Although the "invariant mass" is indeed invariant under choice of reference frame, it is not invariant under choice of how to group things into objects. For example, take two similar blips of light traveling opposite directions. Each one has energy E, momentum |p|=E/c, and an invariant mass of zero. Since the momenta are opposite, we are already using the reference frame in which the momentum of the two-blip object is zero. The invariant mass of the two-blip object is then 2E/c2, not zero. Even when things have no interaction, the invariant mass of the sum is not the sum of the invariant masses. A big box of photons has energy, zero average momentum, and thus has some invariant mass. It acts gravitationally just like anything else with the same energy and no momentum.

Mike W."  



http://van.physics.illinois.edu/qa/listing.php?id=16351

bc notes both use the box of photons.


p.s. If Gracie had "said"  photons have momentum, I'da not learned anything.