On Mon, 16 Aug 1999, Ron Kean wrote:
> A photon reportedly has no rest mass, yet it behaves as if it has
> mass in the sense that it has momentum and is attracted by a
> gravitational field.
Right. Its mass is not _rest_ mass. It is all kinetic. Photons, of course, are never at rest.
It is not necessary, however, to have zero rest mass in order to be one's own antiparticle. The neutral pion has positive rest mass, but is its own antiparticle.
> I presume that if there were enough energy in the form of photons
> concentrated at one locality, a black hole could form, trapping the
> photons in their own gravity, just as a black hole can form if there
> is enough cold mass in one locality.
There are even (in theory) static objects called geons in which electromagnetic fields are stabilized under their own selfgravitation. I forget the details.
> Keith Lynch has already stated that there is no such thing as
> negative mass in the sense of mass which has a gravitational
> repulsion instead of attraction. So a neutron would have a
> gravitational attraction to an anti-neutron just the same as would a
> neutron to a neutron.
Right. In particular, antineutrons ought to be attracted to the earth, i.e., fall when unsupported. I'm not sure if anyone has ever kept any around long enough to check, but the corresponding experiment has been done for positrons.
> It was recently reported in Scientific American that there is
> evidence that neutrinos may have mass, so that raises the question
> of whether there are antineutrinos and what would happen if an
> antineutrino had an interaction in one of the neutrino detectors.
Antineutrinos exist in any case. They are distinguished from neutrinos by their helicity (direction of spin relative to their direction of motion). When a neutron decays, for example, it yields a proton, an electron, and an antineutrino. However, it seems to me that if neutrinos have rest mass, this distinction must in principle lapse, since you can then choose a frame of reference in which the helicity is reversed. I have never gotten an explanation of this question from a real physicist.
||: Two wrongs don't make a right, but three lefts do. :||