Well, we sure didn't define matter in that way. We do know of two kinds of particles which have zero invariant mass (called "rest mass" by some, but if it's zero, you can never get these particles to be at rest -- they always travel at the speed of light, c). The two particles are the photon (already spoken about), and a similar particle, called a gluon, which is a lot like a photon in many ways but carries the strong force instead of the electromagnetic one.
Photons can have energy (E) and momentum (p). Einstein's special relativity says that E2
. If E=pc, then we're all okay with zero mass m but nonvanishing energy and momentum. Some people insist on writing this m as m0
but I don't see the real need.
Now whether you want to call photons "matter" or not is a quibble people in the particle physics business never worry about. Photons are their own antiparticles, so they'd be "antimatter" if they were "matter", so we generally don't use that word when talking about photons.
We used to allow for the possibility that neutrinos are massless, and in fact, even with the latest observations that show that at least some neutrinos are not massless, it still is possible that one species of neutrino is in fact massless (although I give it a plausibility rating of very low). Massless neutrinos also don't cause troubles for the theory; E=pc for them, too, and they are more easily classifiable as "matter", being neutral cousins of the leptons -- electrons, muons and taus are the leptons.
All the elementary particles we know of, even the massive ones, appear to be pointlike. But that may just be tautological -- if we knew an object had some spatial extent, we'd seek the pieces it's made of. At any given time, there is a limit to the sensitivity of our experiments, and we call all the things we cannot split into smaller pieces "elementary".
I think there's a gray area where we can resolve some structure but not enough to describe smaller pieces yet. Maybe we'd call particles in that category elementary and maybe we wouldn't. Right now I guess there aren't any like that, though the proton, for example, was in that category for a while. Anyway, Tom's central point still applies: it's not the names that are important but rather the mathematical properties that they get at. Mike W.
(published on 09/12/06)