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photo: CERN
This bubble chamber photograph shows an electron and a positron (anti-electron) spiralling in opposite directions.



Antimatter found its way into the popular imagination soon after its discovery in the early 1930s. "Star Trek" fans know antimatter as the high-energy fuel of the Enterprise, the stuff that sends the starship faster than the speed of light. That kind of space travel isn't likely to materialize. But the theoretical possibilities of antimatter have long seduced science fiction aficionados and scientists with promises of amazing revelations about the nature of distant galaxies and the origins of the universe.

Perhaps the most amazing thing about antimatter is that it was conceived of at all. In 1928, British physicist Paul Dirac set out to solve a problem: how to reconcile the laws of quantum theory with Einstein's special theory of relativity. Through complex mathematical calculations, Dirac managed to integrate these disparate theories. He explained how things both very small and very fast -- in this case, electrons near the speed of light -- behave. This was a remarkable achievement in its own right, but Dirac didn't stop there. He realized that his calculations would work for an electron with negative charge, but also for an electron with positive charge -- an unanticipated result.

Dirac argued that this anomaly was in fact the electron's "antiparticle," the subatomic equivalent of the "evil twin." In fact, he asserted, every particle has an "antiparticle" with nearly identical properties, except for an opposite electric charge. And just as protons, neutrons, and electrons combine to form atoms and matter, antiprotons, antineutrons, and antielectrons (called positrons) combine to form antiatoms and antimatter. His findings led him to speculate that there may even be a mirror universe made entirely of antimatter.


art: CERN
Many new particles are produced by the annihilation of an electron and a positron
Dirac's equations marked the first time something never before seen in nature was "predicted" -- that is, assumed to exist based on theoretical rather than empirical evidence -- solely on the basis of theory guided by the human imagination. His prediction would be confirmed in experiments by Carl Anderson in 1932. Both men won Nobel prizes for their efforts.

Physicists have learned a great deal about antimatter since Anderson's discovery. One of the more dramatic findings (custom-made for many a science fiction adventure) is that antimatter and matter explode on contact. Like lovers caught in a doomed relationship, matter and antimatter initially attract (thanks to their opposite charges) and then destroy each other. Because these annihilations produce radiation, scientists can use instruments to measure the "wreckage" of their fatal collisions. No experiments have yet been able to detect the antigalaxies or vast stretches of antimatter in space that Dirac imagined. Scientists still send observatories into space to look for them, though, just in case.


photo: CERN
CERN's Antiproton Decelerator (AD) slows down high-energy antiprotons so that their properties can be studied.
But the question that really confounds physicists today springs from the same fountain that captured the imagination of the public: that matter and antimatter annihilate when they meet. All the theories of physics say that when the universe burst into existence some fifteen billion years ago with the Big Bang, matter and antimatter existed in equal amounts. Erupting from a celestial cauldron of unfathomable temperatures, matter and antimatter materialized and then annihilated repeatedly, finally disappearing back into energy, known as the cosmic background radiation. The laws of nature require that matter and antimatter be created in pairs. But within a millifraction of a second of the Big Bang, matter somehow outnumbered its particulate opposite by a hair, so that for every billion antiparticles, there were a billion and one particles. Within a second of the creation of the universe, all the antimatter was destroyed, leaving behind only matter. So far, physicists have not been able to identify the exact mechanism that would produce this apparent "asymmetry," or difference, between matter and antimatter to explain why all the matter wasn't also destroyed.

Rolf Landua

Is there an anti-universe? What would it look like? Rolf Landua describes the goal of the ATHENA experiment.
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Today, antimatter appears to exist primarily in cosmic rays -- extraterrestrial high-energy particles that form new particles as they penetrate the earth's atmosphere. And it appears in accelerators like CERN's, where scientists create high-energy collisions to produce particles and their antiparticles. Physicists study the properties and behavior of manufactured antiparticles, and the antimatter they form when they combine, hoping to find clues to this asymmetry mechanism.

Most scientists believe that a subtle difference in the way matter and antimatter interact with the forces of nature may account for a universe that prefers matter, but they haven't been able to definitely confirm that difference in experiments. Theories suggest that even if equal amounts of matter and antimatter were created with the Big Bang, disparities in their physical properties -- such as decay rate or life span -- might favor a matter-filled world. In 1967, Russian theoretical physicist Andrei Sakharov postulated several (rather complex) conditions necessary for the prevalence of matter. One required something called "charge-parity" violation, which is an example of a kind of asymmetry between particles and their antiparticles that describes the way they decay. By comparing the way particles and antiparticles move, interact, and decay, physicists have been trying to find evidence of that asymmetry ever since.

John Eades

Is the proton a mirror image of the antiproton? John Eades describes the goals of the ASACUSA experiment at CERN. You will need RealPlayer in order to view this video.
To find that evidence, physicists conduct two types of extremely difficult experiments, in an effort to observe matter and antimatter directly. One produces antiparticles and antimatter from high-energy collisions in particle accelerators, and then makes precision measurements of them; these measurements are then compared with everything we know about their matter opposites to identify any detectable differences.

Whatever the outcome of such experiments, physicists will continue to push the limits of human imagination trying to fix this little hole (albeit not the only one) in their beautiful theory. While theoretical physics manages to explain with extreme precision a good part of what we know about the laws of nature -- as experiments confirm -- so far, asymmetry doesn't quite fit into the framework. But who knows? In their search for that elusive mechanism that would help explain the mystery of why we're here, physicists might uncover something totally unexpected, opening the door to an amazing new discovery no one has yet imagined.


EXTERNAL LINKS

For more information on antimatter:
Antimatter: Mirror of the Universe

For information on the antimatter experiments at CERN:
Antimatter Factory
ATHENA
ATRAP
ASACUSA

The Heart of the Matter

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