Astronauts from the space shuttle Endeavour recently attached the Alpha Magnetic Spectrometer, or AMS, to the International Space Station. It will attempt to detect the presence of antimatter in outer space. Since the device has the potential to change the way we think about the universe, this is a good time to brush up on what, exactly, antimatter is.
In the 1920s, British physicist Paul Dirac was trying to make Einstein’s special relativity principle jibe with some of the rules of quantum mechanics — a mathematical system that explains the behavior of small particles. No matter how many times Dirac ran his equations, he couldn’t eliminate a pesky negative sign that he thought didn’t belong there. Dirac ultimately decided the negative sign wasn’t a mistake, but a revelation. For the calculations to work, there had to be an undetected particle with the same weight as a negatively-charged electron — one of the basic building blocks of matter — but with a positive charge. Dirac thus became the first physicist to prove, albeit theoretically, that antimatter existed.
Four years later, Carl Anderson, a 27-year-old graduate student at the California Institute of Technology, detected the positively charged electron in a lab experiment by observing its trail and dubbed it the positron. Both Dirac and Anderson would go on to win the Nobel prize in physics.
Since Anderson’s experiments, scientists have discovered several other forms of antimatter. There’s the antiproton, a negatively charged proton, and the antineutron. (Neutrons have no charge, but the negative of zero is zero, so the theory still holds. Just go with it.) And there are even smaller antiparticles.
Anderson’s experiments might not have been the first time antimatter was created in a lab. His detection methods were the innovation. You see, antimatter and matter don’t get along very well: When a particle and its antiparticle meet, both are annihilated, releasing a bunch of energy. So, any antimatter that appears on Earth disappears almost immediately. Using a device called a cloud chamber, Anderson managed to identify the fleeting traces of the particles before they vanished.
Once the existence of antimatter was proven, a world of potential experiments opened up. At this very moment, scientists at Fermilab in Illinois and CERN, the European Organization for Nuclear Research, are poking and prodding at antimatter particles trying to answer a bunch of fundamental questions.
“If we find an unexpected difference between particles and antiparticles,” says Harvard physicist Gerald Gabrielse, “our most fundamental description of reality [quantum mechanics] could be wrong, and, the implications would ricochet through all of our theories. Every physical law is potentially at stake.”
Do the physicists have your attention? Now here’s how they make the stuff, er . . . anti-stuff.
“We take a proton beam and slam it into a target,” says Keith Gollwitzer, who works with antiprotons at the Illinois laboratory. “Off comes a series of particles and antiparticles, some of which are antiprotons that can be captured electrically and magnetically.”
Capturing antimatter long enough to experiment on it turns out to be a pretty neat trick, since it’s difficult to contain something that disappears whenever it touches anything.
To understand how the storage process works, imagine a square with concrete posts in each corner. Now, place yourself in the middle of the square, attached to each post by a tightly-stretched bungee cord. Try to get out of the square. You might be able to take one step out of the center, or two if you’re really strong. But, the further you get from the middle, the harder the cords pull you back.
An antimatter trap works that way, but with electromagnetic rather than physical restraints. Scientists catch the antiparticle, and then build an electromagnetic field around it that increases in strength the further the particle gets from the middle. That way, they can hold it in place without having it touch anything.
It’s a clever concept, but it takes a lot of energy to maintain the field. The particle also has to be kept really, really, cold. “If you allow the particle to get more than a degree above absolute zero, it will gain enough energy to escape,” says Gabrielse, who experiments with antihydrogen, the antimatter equivalent of the first element on the periodic table.
Meanwhile, as researchers try to generate and trap a handful of particles here on Earth, NASA is looking for them out in space. That’s because, astrophysicists postulate that the Big Bang — the event that created our universe — might not have been so different from the high-energy collisions used to create antimatter in the lab. If they’re right, it should have produced not only matter, but lots of antimatter. And yet, there’s very little detectable antimatter in the known universe.
Even more perplexing is the fact that all our high-energy experiments on Earth produce matter and antimatter in equal proportions. Even if the Big Bang did produce tons of antimatter, and it was simply destroyed as it interacted with matter, why was there all this matter — that’s you, me, and everything around us — left over?
The AMS is one step toward finding the answer. It’s going to sit up in space and try to trace the origins of the antimatter that is floating around the cosmos. Is it possible it’s all coming from the same direction, and that there’s an antimatter universe somewhere?
If neither a potential revolution in every physical law we hold dear, nor insight into birth of the universe, interests you, there’s a potential practical use for antimatter: energy production. Every time an antiparticle meets a particle, energy is produced with no harmful leftovers.
“If we could bottle antimatter, you wouldn’t need nuclear reactors, you wouldn’t need gasoline; you wouldn’t need anything,” according to Mike Shara, an astrophysicist at the American Museum of Natural History. “You’d have the perfect source of energy.”
Unfortunately, the engineering is way, way behind our imaginations right now. Fermilab manages to produce about two one-billionths of a gram of antiprotons per year. That’s not enough to solve the energy problems of a small village, let alone the world.