Antimatter shares many properties with ordinary matter. Antiparticles
have the same masses as their matter counterparts, and they fall in a
gravitational field. So yes, antimatter is a sort of matter. It is
still puzzling to us why there is so much matter and so little
antimatter in the universe, since they must be made in equal quantities
in high-energy interactions. There are small, observable differences
between particles and antiparticles, which are under intense study at
particle physics laboratories, to try to explain why this is.
As for the name, it is arbitrary. If we were all made of
antimatter, then we probably would still call it "matter", and call the
other stuff we aren't made of "antimatter". The universe would still
work just fine if all the matter were swapped for antimatter. This
raises an interesting situation. Suppose we detect signals from a
faraway, friendly alien civilization. They tell us they would like to
come and visit us. We would like to check first if they are made of
matter or antimatter, as the latter situation would prove disastrous if
they try to shake our hands -- boom! They insist that they are made of
matter, but that isn't convincing enough -- how do we know that what
they call matter isn't what we call antimatter?
It turns out that there are some high-energy particle reactions
which do in fact distinguish these two. These involve kaons and B
particles. You can follow the links in our other answers to find out
more.
In particle physics experiments, a beam of particles which is
accelerated to high energies is smashed into either a stationary target
or another beam going the other way. In the latter kind of experiment,
with colliding beams, one of the beams is usually made of the
antiparticles of the ones in the first beam. This is done for two
reasons. The first is convenience -- you can get the two beams to go
around a circular ring of magnets in opposite directions if they
consist of oppositely charged particles of the same mass. The other
reason is that when a particle annihilates with its antiparticle, both
disappear, leaving all of the energy available for making new and
interesting stuff. Some accelerators collide electrons with positrons.
Others collide protons with antiprotons. (the protons are actually made
of pieces called quarks, and usually not the whole proton interacts,
just one of the quarks, and so not all the energy is available in these
experiments).
Another experiment in Germany collides a beam of electrons and a
beam of protons going in opposite directions. This is harder to do
because electrons and protons weigh different amounts and so two
separate rings of magnets had to be built, which intersect at the
collision point. If an electron and a proton hit each other, you
usually get a complicated spray of new particles. What comes out has to
add up to what came in in various quantites. The total electrical
charge has to add up to zero. The number of electrons has to add up to
one (electrons minus anti-electrons that is, and some other particles,
neutrinos, can be made too which can carry electron-number). The number
of baryons has to add up to one. A proton is an example of a baryon --
so is a neutron. Other more exotic baryons exist (excited states of
protons and neutrons and even more interesting stuff) are made in these
interactions. You can make proton-antiproton pairs in these
interactions, as long as it all adds up at the end.
A simple interaction is an electron hitting a proton, and you get
out an electron, a proton, and a couple of new particles, like an
electron and a positron, or maybe something else, like a positive pion
and a negative pion. Most interactions are even more complicated.
For your last question, no, a positron is not an antiproton. For
one thing, a positron is positively charged and an antiproton is
negatively charged. Another detail is that an antiproton weighs about
1800 times as much as a positron. And interacts differently. A positron
will actually orbit around an antiproton to make an antihydrogen atom
(which has been produced in the laboratory!). The positron is actually
an anti-electron.
Back in the old days when Dirac proposed the existence of
antimatter and before it was experimentally confirmed, there was some
speculation that electrons and protons were antiparticles of each
other. But they differ in ways that antiparticles should not differ,
and the later discovery of the separate particles the positron and the
antiproton confirmed Dirac's original model.
Tom
(published on 10/22/2007)
