Dear Steve Willner:

On Tuesday, October 25, 2016 at 11:30:27 AM UTC-7, Steve Willner wrote:
In article ,
dlzc writes:
Individually, no. But in groups, with a center of
momentum frame, [photons] do have rest mass.

So the sum of a bunch of zeroes is non-zero? That's
new physics.

Quite old, actually. I first heard of it in "Spacetime Physics" by Taylor and Wheeler. You can read about it he
http://physics.stackexchange.com/que...less-particles
.... or just realize for two oppositely-directed, equal-energy photons:
E^2 = (sigma( p1, p2) = 0)^2 + (mc^2)^2
If the photons have non-zero energy (E=/=0), their momenta cancel, and they *must* have rest mass.

I have no idea what "with a center of momentum frame"
is supposed to mean.

Standard physics says photons have momentum and energy
but zero rest mass.

Individually, yes. As a system of particles (not all even have to be photons), no.

Photons react to gravity and (in principle, but I
don't think it has been measured) create gravity,

.... and this is the discussion here, to what extent that photons that have not yet propagated past, say 10,000 light years from the center of a spiral galaxy, contribute to the "pull" outside that distance?

but neither of those properties requires rest mass.

Gravitational mass, is actually the question, and so far gravitational mass = inertial mass = rest mass.

David A. Smith