TiredJim

I used to think so, since it has mass and takes up space. But recently, someone told me that physicists changed the definition to not include antimatter. Is that true?

NonHomogenized

As a (general) science major, I've heard nothing of the sort. Indeed, it makes no sense: antimatter is exactly the same as regular matter, save that the electromagnetic charge of a given particle is reversed (if I'm slightly incoherent, please chalk it up to the fact that it's 6:30am, and I've been at work for the last 8 hours). It would be about as arbitrary as saying that protons and neutrons are matter, but electrons aren't. Or so it seems to me, anyhow.

Any actual physicists care to comment?

Craig

Yeah it's matter, it just doesn't really exist in "real" life. We can create it in labs, and there are all kinds of virtual antimatter particles (which may be what you are thinking of). The stuff in labs is real, the virtual particles are, well, virtual and not real in the sense that a table or an automobile are.

ceb

Isn't "antimatter" a misnomer?

I would think (or at least hope, in my science fiction fantasies ) that if matter = gravitational attraction, then antimatter = gravitational repulsion.

Apparently, however, this is not the case.

Of course, I'm sure I'm just really pointing at my ignorance and yelling, "look at me! look at me!"

Craig

The anti refers to charge, not gravitational pull.

But yes, sometimes these things do have strange names. But matter and anti-matter are not exact opposites, they are both "matter" in the sense that they can exist and be real.

A very long time ago there was a whole bunch of anti-matter in the universe, until the universe was the ripe old age of about .000000000000000000000000000000000001 seconds old.

It was theorized before it was actually found, so got the name anti-matter before it really had been observed. Other interesting names include "atoms" (which are really NOT the smallest building block, despite the name atom which means "smallest building block").

I think Quark came from some old English novel, and Neutrino mean "little neutral one" in Italian.

Shadowy Man

P.A.M. Dirac first formulated the relativistic wave equation, which when applied to the electron, describes accurately its magnetic moment, and elucidates fine details in the spectrum of hydrogen that are not accounted for by the nonrelativistic theory.

However, there were solutions to the equation that involved negative energy states. Dirac proposed that these states refer to a particle that has the same mass as the electron but with a positive charge. He called it the "positron". He also predicted that the positron and electron would annihilate each other.

Carl Anderson won the 1936 Nobel Prize for Physics for discovering the positron.

This is quite a story, because Dirac basically pulled the idea out of his ass, and the positron's existence was predicted solely on the basis of his theory.

In the last couple of years, scientists at CERN were able to create anti-hydrogen. It will be very interesting to see if the spectrum of anti-hydrogen is the same as hydrogen.

Tetlepanquetzatzin

To Shadowy Man's brief description one can add Dirac's interpretation of the negative energy states: Spin-1/2 particles, as described by the Dirac-equation, have no upper limit on their energy. Any energy -- no matter how large -- is physically possible according to the Dirac equation. Nothing strange there, but the negative energies pose a problem. The states of positive and negative energy are completely symmetric, which means that there is no lower limit to the energy of a particle.

The natural expectation, then, is that a particle with positive energy should "fall" to lower energy states while emitting the excess energy. Since there's no lower limit, one would expect an unlimited amount of energy to be released as the particles "falls". This does not fit with our experience, so Dirac proposed that the reason this doesn't happen is that all the negative energy states are already occupied. By absorbing energy, a negative energy state can be lifted up to the positive half of the energy spectrum, leaving a vacant place in the negative half. This vacant place is what we call an "anti-particle". As long as we realize that an anti-particle behaves in many ways like a particle with mass, it is a matter of taste whether we refer to it as "matter" or by some other name.

Coragyps

James Joyce - Ulysses, I think.
There's a pretty readable article about antimatter, the Kobayashi-Maskawa model (!), and such in the 7 August 2003 issue of Nature - try a college library, perhaps.

Shadowy Man

James Joyce, Finnegan's Wake:

"Three quarks for Muster Mark."

Coined by Murray Gell-Mann.