Picture the night sky on a breezy May evening. Picture the moon, the stars and the other planets. Most of what you see from our corner of the universe is matter, the “stuff” made of atoms with positively-charged nuclei.
As you gaze at the sky, your mind may wander and you could hypothesize: why wouldn’t there be (for the sake of symmetry) atoms with negatively-charged nuclei? And Paul Dirac would have agreed with you.
In 1928, Dirac proposed his own version of Schrödinger’s wave equation, which he adapted to include Einstein’s theory of special relativity. The two solutions that solved the equation, however, initially troubled him: one solution had positive energy whereas the other had negative energy. Dirac interpreted the negative energy solution to correspond to antiparticles: particles with positive energy but whose nuclei are negatively-charged. He thus suggested the existence of the positron—the antiparticle of the electron—four years before it was ever confirmed in the lab.
Antiparticles—or antimatter, for that matter—are hard to come across; we have yet to observe antimatter in the visible universe. But maybe not for long. The shuttle Endeavour started its last space voyage yesterday (May 16 2011), equipped with the “Alpha Magnetic Spectrometer” (AMS), which the astronauts will attach to the International Space Station. The AMS will collect data on antimatter and dark matter among other things. That instrument took nearly 15 years and 2 billion dollars to complete.
Meanwhile, scientists on Earth will use particle accelerators to smash particles together at very high speeds, in the hope of generating antiparticles, which they can fuse together to form antimatter. Take for example, hydrogen: One atom of hydrogen is made of 1 electron and 1 proton. The antihydrogen, then, is made of 1 antielectron (or positron) and 1 antiproton. But it’s not so easy to do because when matter meets antimatter, annihilation occurs. By that I mean that their masses are converted to energy in the form of gamma rays.
So to make antihydrogen, scientists cool their antiparticles and carefully push antiprotons into a cloud of antielectrons, which are contained within a magnetic trap, a way to contain antiparticles using magnetic fields. Even when scientists succeed in doing so, however, antimatter proves very difficult to contain. Last year, scientists at CERN had managed to contain antihydrogen particles for 172 milliseconds but two weeks ago, they succeeded in containing them for nearly 17 minutes!
But if we go back billions of years ago to the Big Bang, how could matter and antimatter have been present without being wiped out altogether? We don’t know yet, but some scientists suggest that a slight imbalance in the amounts of both may have favored matter. The experiments currently being conducted will hopefully enlighten us on this “matter”.
Finally, it has been suggested by several media that the next step for those scientists is to trap antihydrogen, “throw it in the air and watch which way it falls”! If this sounds slightly absurd, a quick search on CERN’s website reveals why: “The gravitational force depends [on] the energy of an object, and since matter and antimatter have both positive energy, gravitation acts on them in the same way.”