From: Mark Holm 
To: atm{at}shore.net
Reply-To: Mark Holm 


John wrote:


> Light doesn't interact with itself
>

Well, I'm not so sure about that.  How do you explain interference patterns
if light doesn't interact with itself?

Photons are sure a weird idea when you think about it.  They carry momentum
and energy but have no mass, and ever since General Relativity, they
respond to gravity as well.

One of the squirrely bits of quantum mechanics has to do with the
differences between "particles" that interact with each other
strongly and those that don't.
  Turns out to depend heavily on a quantity called spin.  If the
"particles"
have half-integral spins e.g. +-1/2, +-1 1/2, etc. they interact strongly.
Particles in this class include our familiar matter: electrons, protons,
neutrons.  They obey a set of rules called Bose-Einstein statistics and
thus are
called Bosons.  Bosons act a lot more like our classical idea of particles, they
can be "bounced" off of each other, for example (with, however,
the decidedly non-particle like feature that the bounce patterns will have
diffraction effects).

If the particles have integral spins, e.g.  0, +-1, +-2, etc. they do not
interact strongly.  Particles in this class include photons and neutrinos.
They
obey Fermi-Dirac statistics and are called Fermions.  Fermions can go right
through each other.  Only if you examine at the intersection point do you
find interaction in the form of interference.

--- BBBS/NT v4.00 MP