What is antimatter? How was it discovered?


Dear Straight Dope:

What is antimatter? Does it occur naturally? If not, then how was it discovered? Did some scientist say, "I think that there is a such thing as antimatter" and then prove it?

Karen replies:

Finally! A question that requires the full power of a penguinist. When I get done, you’ll have a freaking Ph.D. in antimatter.

Antimatter is composed of particles which are identical to matter particles except that their additive quantum numbers have opposite sign. For instance, an electron has electric charge quantum number -1 and fermion number +1, etc. The positron (=antielectron) has electric charge quantum number +1 and fermion number -1, etc. A proton has charge +1 and baryon number +1, etc., and an antiproton has charge -1 and baryon number -1, etc.

Two things to note:

1) You cannot define antimatter without the context of matter. Antimatter is equal and opposite to matter; matter is equal and opposite to antimatter. Since the universe is composed of electrons, protons, and neutrons, we think of those as the matter, and positrons, antiprotons, and antineutrons as antimatter. But what about pions? The pion+ is equal and opposite to the pion-, but which is the matter and which is the antimatter? For physicists, it doesn’t really matter (uh, no pun intended), each is the antiparticle of the other.

2) When a particle and its equal and opposite antiparticle get together, all the additive quantum numbers add to 0 and they annihilate, giving up both their masses into energy, via E = mc2.

Antiparticles do indeed exist in nature, mostly freshly produced in processes associated with radiation or radioactivity. For instance, fusion processes in the sun produce scads of positrons (e.g., helium-3 + proton –> helium-4 + positron + neutrino). You can also get positrons from radioactive decay such as carbon-11 –> boron-11 + positron + neutrino (beta+ decay). A gamma ray (high energy photon) when going through material may convert its energy to an electron-positron pair via mc2 = E. And cosmic rays are high energy particles or antiparticles from outer space.

Antiparticles do not usually last very long. When a gamma ray passes through a metal, for example, and produces an electron and a positron, the electron is quite at home amongst its fellow electrons in the metal. The positron, however, is a stranger in a strange land, and is quickly annihilated like a German tourist in Miami.

If you wish to view a bunch of antiparticles, your best bet is to produce them yourself. You’ll need to acquire a large particle accelerator. (No one calls them "atom smashers" anymore.) Essentially, the idea is to accelerate some mundane but plentiful particles, like protons or electrons, to enormously high velocities and smash them into something, hoping that they will give up some of their kinetic energy to produce particle-antiparticle pairs via mc2 = E.

If you want to get really fancy, you then take the particles you’ve produced and smash them into the antiparticles you’ve produced, to get even more energy to make even fancier particles. You then need to build an enormous detector around the smash zone to detect the exotic particles, because they are going to decay or beannihilated pretty quickly. If you can’t acquire your own accelerator and enormous detector, you may be able to join groups working at existing accelerators at Fermi National Accelerator Lab near Chicago, the Cornell Electron Storage Ring in upstate New York, Stanford Linear Accelerator Center (SLAC) near San Francisco, the European Laboratory for Particle Physics (CERN) straddling the Swiss-French border, BES in Beijing, High Energy Accelerator Research Organization (KEK) in Japan, or at other accelerators. These labs make antiparticles all day long. And some groups even made some antihydrogen ATOMS by getting positrons and antiprotons to bind together.

The really cool thing about antiparticles is that their existence was predicted before their existence was discovered. In 1928 P.A.M. Dirac found that when you merge quantum mechanics and relativity (Dirac had a strong stomach) you necessarily find that each type of particle has a twin. He tried to explain away these twins using various arguments, but a few years later, in 1932, C. Anderson detected a new, positively charged particle that behaved like an electron. He called this particle the "positron". After a while people put two and two together and realized that this positron was the twin of the electron predicted by Dirac. In retrospect, these two events revolutionized modern physics, but at the time, no one was quite sure what was going on.

Now, you may have another question in the back of your mind. (If not, I hope to plant such a question.) If antimatter is equal, just opposite to matter, why is the universe composed of matter and not antimatter? Excellent question. The universe is in fact (currently) composed almost entirely of matter. If there were significant amounts of antimatter, we would be able to see plenty of energy coming from matter-antimatter annihilation. (In fact, recently a cloud of positrons has been discovered near the center of our galaxy, using just this method.)

The universe has not always been preponderantly matter. At the instant of the Big Bang (for those of you not from Kansas) the universe was exactly half matter and half antimatter (from the same energy –> matter + antimatter pair production process I described above.) Something subsequently happened to the antimatter. And whatever happened, happened dang fast, otherwise the matter part of the universe would have immediately annihilated the antimatter half, and that’s it, end of universe. No stars, no planets, no air, no people, no Gap commercials!

So the fact that we are even here asking the question "What happened to all the antimatter?" is phenomenally cosmic.  Physicists have been trying to find out why and in what ways antiparticles are different from particles. The current theory predicts that dissimilarities between particles and antiparticles may be most visible in decays of B and antiB mesons, due to quantum interference between regular weak decays (where the b quark emits a W boson which subsequently decays), and one-loop decays (where the b quark emits, then reabsorbs a W boson). These one-loop decays are whimsically called (it’s been a long time coming but we finally hit pay dirt) PENGUIN decays! Yay!

There are two particle accelerators/detectors designed specifically to find differences between B and antiB mesons (SLAC and KEK). These accelerators are called "B-factories," and the buzzword for what they are looking for is "CP violation." (CP stands for Charge conjugation and Parity, which describes the symmetry between particles and antiparticles; violation means the symmetry is not exact.)

Since the Web was invented by particle physicists you can find out a lot about particle physics by surfing. For a good introduction to particle physics, visit The Particle Adventure (http://particleadventure.org/). This was designed for school children, but it is truthful, well-organized, and award-winning. You can learn more about the antimatter cloud near the center of the galaxy (and see a photo of it) at http://www.astro.nwu.edu/ astro/purcell/511kev/mcgraw.html. You can read about the first whole antihydrogen atoms at http://www.cern.ch/P ress/Releases96/PR01.96EAntiHydrogen.html. You can read a review of penguin decays of B mesons (co-authored by yours truly) at http://www.slac.sta nford.edu/pubs/slacpubs/7000/slac-pub-7796.html. The birth of the World Wide Web is chronicled at http://www.cern.ch/Public/Welcome.html.

And you’ll find information on penguin-free, Kansas-style theories of the matter/antimatter asymmetry of the universe in a few days in the "Comments on Mailbag Items" message board.

Send questions to Cecil via cecil@straightdope.com.


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