Matter, antimatter, and the origin of everything
PASADENA, CALIF. — In September, scientists in Europe made an incredible announcement: they had produced not one, but tens of thousands of "anti-atoms," atoms made entirely of antimatter. This result rightly generated a good amount of publicity, but I was amused at the media's reaction to the news. Whenever I saw it mentioned, either on television or in papers, there always seemed to be a picture of the starship Enterprise or some other science fiction-type image.
Of course, that seems fitting; antimatter is dramatic, exciting stuff, and is, as well, the fuel that supposedly propels the starship Enterprise across the galaxy. But there's so much more to the antimatter story. Sure, understanding antimatter and how to create it in our laboratories may very well lead us to a super-efficient, almost inexhaustible energy source to power the spaceships in our far future. But the properties of antimatter may also help us understand how we, and in fact, everything in the universe, came into being.
What exactly is antimatter? Well, antimatter wouldn't look any different to us if we could see a chunk of it flying through space. And, although this is highly unlikely, if a distant galaxy were made entirely of antimatter, we'd have absolutely no way of telling from the light it emitted. Every type of particle in the universe has a kind of evil twin, its antiparticle. The only difference between the two is that antiparticles have an opposite electrical charge than their regular-matter mates.
An antiproton is exactly like a proton, except that it has a negative charge instead of positive. An anti-electron has a positive charge, and thus is usually called a "positron." There's even an anti-neutron, which is also neutral, since the neutron has no charge. Antimatter isn't really dangerous in small amounts, as an antiparticle can only annihilate a similar amount of matter.
As it turns out, creating antimatter in a laboratory isn't that hard if you can pump a lot of energy into a small volume. The people at CERN, the European Center for Nuclear Research (the acronym is in French), have gotten quite good at producing antimatter by accelerating a stream of protons to nearly the speed of light, then ramming them into a sheet of heavy metal.
The resulting explosions create temperatures of nearly 10,000,000,000,000 degrees, which is the hottest anything in our universe has been since the Big Bang (luckily for us, these explosions are on tiny, tiny scales, comparable to the size of an atom). At these high energy levels, both matter and antimatter particles are created spontaneously, flying out in random directions all over the accelerator, moving at incredibly high speeds.
About one antiproton is created for every million protons rammed into the metal, but as there are over a trillion proton collisions every minute, plenty of antimatter gets created. Once the antiprotons pop into existence, the race is on to keep them from annihilating ordinary matter. Not only is the accelerator tube evacuated of all air, magnetic fields are used to bend the paths of the antiprotons and keep them safely in the middle of the accelerator, away from any normal matter.
An isolated stream of antiprotons moving at close to the speed of light may be interesting, but it's also difficult to study. In 1995, the scientists at CERN threw a bunch of positrons into the mix, hoping that some would go into orbits around the antiprotons and create the world's first antiatoms. Out of the billions of antiparticles, about nine atoms of antihydrogen formed, lasting for about 20 billionths of a second before evaporating back into pure energy.
That result was intriguing, but the scientists weren't satisfied. Until then, they had been building accelerators designed to whip particles into higher and higher energy levels. Now the problem seemed to be getting the antiprotons to slow down long enough to experiment with them. Instead of a new particle accelerator, they needed a particle decelerator.
And so they built one. Now, once a nice new batch of antiprotons has been created and is obediently flying through the accelerator, a series of magnetic brakes can be turned on. Little by little, each time the antiprotons fly through the magnetic fields, they're slowed down to a pokey speed of 10 percent the speed of light, only 18,600 miles per second. At this leisurely pace, scientists have a much better chance of experimenting on the antiprotons. Now, for the first time, scientists can study antiatoms in bulk.
Why do physicists want to get a very good look at antimatter? One reason is that no one, so far, has been able to figure out why our universe is made of matter instead of antimatter. According to all our best theories about the beginning of the universe, the amounts of matter and antimatter should have been exactly equal.
This would have been very bad for any future prospects of life, or indeed, any structure in our universe. For each particle created by the Big Bang, a similar antiparticle came into existence. Mix this together in the tiny spaces of the new universe, and all matter and antimatter should have annihilated in a fraction of the first second of time.
In truth, we think this very nearly happened. Matter and antimatter were created in almost exactly the same amounts, and did fill the early universe with an intense bath of radiation created by their annihilation. But for some unknown reason, the universe was slightly off balance. For every billion, trillion (who knows, maybe far more than that) particles of antimatter, a billion or trillion or whatever plus one particles of matter were created. We, our galaxy, and all the matter in the universe are just the tiny bit of ash that was left over after almost all of the universe annihilated itself.
What caused this tiny imbalance that we owe our existence to? We don't know, but the folks at CERN and the world's other particle laboratories are hoping to find some kind of clue in the behavior of antimatter. Now that antiatoms can be created in abundance, they'll be poked and prodded for any clues to the intrinsic differences between matter and antimatter.
Maybe, in hundreds of years, we'll end up with warp drives and starship Enterprises. Maybe terrible weapons. Or maybe just a better understanding of how our entire universe came into being in the first place.
Michelle Thaller is an astronomer at the California Institute of Technology. A massive-star specialist by trade, she dedicates most of her time to education and public outreach for the Space Infrared Telescope Facility.