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

[John Denker <jsd@av8n.com>]

On 03/14/2013 04:23 PM, Bernard Cleyet wrote:
> The very recent Latina(o) comic strip has the bright sister telling
> her older brother, Baldo, "Photons have mass, but their rest mass is
> zero."
>
>
> http://www.uclick.com/client/smc/ba/2013/03/12/

The thing that bugs me more than the kooky physics is the arrogant
snark directed at the guy who sees it as kooky.

It reminds me of another cartoon:
  http://cf.chucklesnetwork.agj.co/items/1/3/7/0/4/not-sure-if-troll-or-just-clueless.jpg

The URL says it all: "Not Sure If Troll ... Or Just Clueless."
It's one thing to be clueless about photon mass.  When you are
clueless and aggressively nasty about it, you get promoted to 
troll.

The physics in this example is in the same general category as the 
speed=c story that started this discussion ... only much worse.

The electrodynamics of continuous media is not particularly fundamental,
but it's not crazy, either.  There's lots of useful stuff you can do
with the idea of so-called "light" moving at a speed less than c.  
Using it to explain Snell's law is a good example.

The idea that multiple viewpoints might coexist is not a problem.
The problems arise people assume that their viewpoint must be correct
/to the exclusion of all others/.

Turning to the newer example:  The rest mass of a photon?  Really????
This is what passes for "bright" nowadays?

There are a couple of different ways to define "photon", but no matter
which one you choose, if a photon can possibly be at rest, the resting
photons have nonzero mass.

  I am aware that there are some folks (even some who subscribe to
  this list) who attribute a so-called "mass" m = E/c^2 to everything.
  This is not a good idea.  The best that can be said is that it 
  produces seemingly-correct answers to /some/ problems.  However, 
  it also produces lots of wrong answers.  Furthermore, it is 
  inconsistent with any modern (post-1908) understanding of special 
  relativity.  It will have to be unlearned as a prerequisite for
  understanding general relativity (among other things).

  From this misbegotten velocity-dependent mass one might try to 
  calculate a so-called "rest mass" namely E/c^2/gamma ... but that
  would be equation-hunting at its worst.  According to one point of 
  view, it is talking about something that does not exist (photons 
  at rest), and according to the other view (standing waves) it is 
  just categorically the wrong answer, obtained by hunting up two 
  equations that are predicated on incompatible assumptions.

Bottom line:  According to a modern (post-1908) understanding of how
things work, mass is a Lorentz scalar.  It is the invariant norm of
the [energy, momentum] four-vector.  For massive particles, this
corresponds to the pre-1908 notion of "rest mass" but it is a mistake
to call it "rest mass" now;  it's just mass.  It applies just fine
to running-wave photons, which don't have a rest mass, because they
cannot be at rest.  They do however have a well defined mass, which
happens to be zero.
  -- 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).

See http://www.av8n.com/physics/spacetime-welcome.htm
and references therein.

===========

Pedagogical remarks:  When introducing special relativity to students
(or to grouwn-up friends), don't even mention time-dilated clocks,
FitzGerald/Lorentz contracted rulers, or velocity dependent mass. 
Instead focus attention on modern notions of proper time, proper
length, and invariant mass.  Similarly, don't even mention so-called
"paradoxes".  If special relativity seems paradoxical, complicated,
or weird, you're doing it wrong.

Again:  http://www.av8n.com/physics/spacetime-welcome.htm
and references therein.

The first step should be to convince people to define length in such a
way that when you rotate the object in the xy plane, the length does
not change.  From there it is a relatively small step to explain that
when you rotate an object in the xt plane, the length does not change.
Length, properly defined, cannot change when the velocity changes.

The modern approach is vastly simpler than the other approach.  It is
simpler yet more powerful.  It is in every way better.

Note the contrast: 
 ++ The electrodynamics of continuous media is not useless.  It is in some 
  sense unsophisticated and non-fundamental, but it's OK for Snell's law.
 -- The unsophisticated pre-1908 version of special relativity is useless.
  It's a waste of time.  There is nothing it can do that cannot be easier
  and better by other means.

Assigning a zero rest mass to photons is just kooky.  It is the wrong
answer to a silly question.