Re: Photons was COLLISION 2

[John Denker <jsd@MONMOUTH.COM>]

At 12:33 PM 11/30/99 -0700, Jim Green wrote:
>  Now if you understand what a "photon" is please
> tutor me -- and perhaps a few others on the list.

I'll pass on that.  Let me explain why:

1) The topic of "photons" is well covered in standard textbooks, and even
in the semi-popular literature such as Feynman's _QED_.  It would not be a
good use of Phys-L bandwidth to parrot the standard discussion.

2) QED is irrelevant to this thread, which started out as a discussion of
the dynamics and energetics of rail-car collisions.

3) A more general and important point is this:  There are many people who
find it useful to use the notion of "photon" and the notion of "local
conservation of energy" to solve physics problems.  OTOH, certain Fizzlers
have repeatedly objected to these notions.  To them I say:

  a) If you aren't interested in solving physics problems, that's fine,
but don't bother the list about it.

  b) Unlike pure mathematics, practically every physical theory is inexact
if you look closely enough.  But let's not panic; "inexact" is not the same
as "wrong".  Normally these physical theories have a large domain of
applicability.  It is good to know where the domain of applicability ends,
but it is bad manners to criticize good applications just because
misapplications are possible.

For example:  there are good applications of classical thermodynamics, and
there are good applications of classical electrodynamics.  We use them even
though we know that the two theories together produce the black-body
paradox.  To say it another way:  Potholes don't keep us from driving on
the road.  We just steer around them.

The notions of "photon" and "local conservation of energy" are on much
better footing than classical electrodynamics or classical thermo.

If you think you have caught somebody using such a notion in a situation
that is clearly outside its domain of applicability, the burden is on *you*
to show that it has led to an incorrect prediction of some measurable
quantity.  (Philosophical objections with no measurable consequences don't
count.)

  c) Perhaps you wish to argue that local conservation of energy, while
not wrong, is just less convenient than some other techniques you
know.  Well, then you should explain those other techniques in a positive
way so that the community may benefit.