Physics & Math / Subatomic Particles

There’s a time machine on the Stanford University campus, and it runs day and night. It won’t hurl anyone into the past or future, but it does something almost as audacious: It reenacts events that occurred just after the Big Bang, when some of the pure energy that filled the cosmos became all the matter that now exists. Inside an 18-foot-high, 1,200-ton particle detector, matter and antimatter moving at nearly the speed of light smash into each other billions of times a second, shattering into subatomic debris that hasn’t existed for about 14 billion years. “We have the gall to believe that we can prepare a situation that is very analogous to what you had at the beginning of time,” says physicist Jonathan Dorfan, director of the Stanford Linear Accelerator Center. “We’re trying to understand what happened during an extraordinarily energetic part of the birth of the universe—and we do a pretty good job.”

For four years, Dorfan and a small army of 600 physicists from three continents have been using two of the world’s biggest and most complex machines—the two-mile-long Stanford Linear Accelerator and the BaBar particle detector—to solve one of the ultimate cosmic mysteries: Why is there any matter in the universe at all?

Strange as it may seem, the laws of physics suggest that immediately after the Big Bang, all the matter suddenly created should have been obliterated by an equal amount of antimatter, the strange and slightly distorted mirror image of normal matter. An electron, for example, which has a negative electric charge, also has a twin, called a positron, with a positive charge. Whenever a normal particle and an antiparticle meet, they annihilate each other, converting all their mass into energy in a pyrotechnic demonstration of Einstein’s famous law, E = mc².

And therein lies the source of one of the greatest dilemmas of science. Physicists believe that by the time the universe was just 10-33 of a second old (that’s a millionth of a billionth of a billionth of a billionth of a second), the temperature had dropped from unimaginably hot to a mere 18 million billion billion degrees. That was cool enough for the first particles of matter and antimatter to condense from pure energy. But to balance the cosmic energy books—and to avoid violating the most fundamental laws of physics—matter and antimatter should have been created in exactly equal amounts. And then they should have promptly wiped each other out. Yet here we are. Somehow a bit of matter managed to survive.

To understand why, Dorfan and his colleagues spend their waking hours creating antimatter particles and smashing them into regular matter. Time and time again, the researchers have documented that antiparticles and normal particles decay at different rates. However, the difference is far too small to explain the amount of matter that exists in the universe.

So some particle physicists are beginning to bet that the key to solving this conundrum may have been right under their noses all the time. If they’re correct, the potential solution is passing through their bodies, billions of times a second, in the form of elusive, ghostly particles that rival antimatter for sheer weirdness: neutrinos. A surprising theory holds that all the matter in the universe may have started out as neutrinos, which is no small irony, considering that until very recently neutrinos were thought to be entirely without mass and as immaterial as the light illuminating this page.

If you’re the sort who believes in the balance of nature, or in a harmonious, rational cosmos, think again. The universe is profoundly out of whack. Physicists realize things are out of kilter because they can literally count the number of photons—particles of energy—in the universe today and compare that with the total number of matter particles. Photons outnumber matter by a billion to one. If matter and antimatter had completely destroyed each other after the Big Bang, the universe today would contain only photons, because every time antimatter and matter self-destruct, their combined mass is transformed into a small unit of energy, a single photon.

“Since we know how many photons there are compared with ordinary matter, that tells us that most of the matter and antimatter did annihilate, and only a little tiny bit of matter was left over,” says David Hitlin, a physicist at Caltech and the founding director of the BaBar team. But why would nature favor matter over antimatter?

Before physicists knew about the Big Bang, no one spent much time worrying about this cosmic imbalance. Antimatter was simply seen as exotic stuff in an eternal cosmos. The renowned British physicist Paul Dirac first posited the existence of antimatter in 1928, and four years later researchers at Caltech detected the first documented antiparticles—positrons produced by the impact of cosmic rays on the atmosphere. Since then antimatter has been observed many times in cosmic rays, in particle accelerators, and during the radioactive decay of elements, but usually as isolated, short-lived particles like positrons or antiprotons rather than as whole atoms. Antimatter has even been used as a medical diagnostic tool in positron-emission tomography, which uses positrons to find tumors. Although there are no naturally occurring antimatter atoms, in 1995 physicists at the European Center for Nuclear Research (CERN) in Geneva cobbled together a few atoms of antihydrogen by linking a positron to an antiproton and have since made tens of thousands more. The rarity and expense of producing antimatter make it the most costly substance on Earth, $1,750 trillion per ounce by one estimate.

Despite the scarcity of antimatter on Earth, Dirac and other early theorists speculated there might be antiplanets, antistars, even an antiuniverse. But astrophysicists have searched in vain for signs of antimatter galaxies, which would reveal their presence in a very dramatic way: Any matter hitting an antimatter star, for instance, would create a titanic burst of energy. “We know if there were an interface somewhere in the universe between antimatter and matter, then there would be annihilation going on at this interface,” says Hitlin. “Astronomers can look for signs of this, but they don’t see it. The limits are about as far away as we can see—about 14 billion light-years. And we just don’t see anything. You can’t say the antimatter is hiding in a corner. In the good fraction of the universe that we can see, there’s no antimatter.”