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Q & A: More on antimatter

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Most recent answer: 10/22/2007
Q:
if antiproton is basically a proton with different charges, does it mean that antimatter is still a matter? is ’antimatter’ just a name given to distinguish between the classes? I don’t understand some explanation in ’antimater’(previously answered question). It was mentioned that when electron and proton colided at high speed, they produces antimatter which forms matter-antimatter pair. But protons and electrons are two different particle, does it mean during the collision, both anti-proton and anti-electron are formed? and one last question, is positron anti proton?
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- Anonymous
Singapore
A:
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
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(published on 10/22/2007)

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