The Matter with Matter
by Pearl Tesler
Physicists love to lob deep thoughts into the unsuspecting minds of others, stuff like “energy and mass are equivalent” and “at the speed of light, time stops.” Here’s another doozy, deep enough to trump all the rest: Technically speaking, our universe shouldn’t be here.
That’s because, technically speaking, the matter that sprang into existence 4.6 billion years ago during the big bang—the matter that makes up everything in our universe—should have come arm in arm with an equal amount of antimatter. But for some reason, in the ultrahot, ultradense, rapidly expanding fray of the birth of the universe, matter bested out antimatter. Our matter-filled universe stands as testimony to this victory.
Hold those horses, you say: What the heck is antimatter? Consider the humble electron. Electrons are the negatively charged particles that zoom around the nucleus of atoms. The motion of electrons is what we call electricity. The electron has a twin, an antimatter counterpart called a positron; the two are the same in all important particulars, except that the positron’s electrical charge is positive. Not just electrons, but all subatomic particles (there’s a veritable zoo of them) have antimatter counterparts. Bring a particle and its antiparticle together and—poof!—they mutually annihilate, leaving only a blast of gamma radiation in their stead.
The threat of annihilation aside, antimatter isn’t as wacky as it sounds. A world made entirely of antimatter would look and function just like our own. But the world is not made of antimatter—antiparticles are rare and exist on earth only in minute lab-created quantities—and scientists want to know why. If matter and antimatter had been created in equal amounts, then the universe should have disappeared in a puff of gamma radiation long ago. But we’re here, and the antimatter isn’t.
The fact that matter seems to have emerged at the expense of antimatter violates an expected symmetry in nature. It’s all very well for matter and antimatter to pop out of an energetic void; Einstein’s famous E=mc2 equation tells us that energy and matter are just two sides of the same coin. So, provided you have the energy to pay for it, there’s no law against an electron and a positron appearing together from out of nowhere. But the prevalence of electrons in our universe, and the mysterious dearth of positrons, is an affront to the expected balance between matter and antimatter. Physicists term this discrepancy a charge-parity violation, or CP violation for short.
It’s not just a rhetorical matter, matter. The fact that we live an apparently impossible universe is the kind of thing that really sticks in a physicist’s craw. What’s to be done about it? Smash some particles, of course.
Tucked under grassy hillocks in Menlo Park, California, is a 1,200-ton elephant of a gadget called BaBar that is even now lumbering toward a solution to the mystery of matter. BaBar is part of the famous Stanford Linear Accelerator Center, also known as SLAC (say “slack”), a facility devoted to creating and colliding bits of matter. Specifically, BaBar detects the ephemeral particle shrapnel that results from a head-on collision between a beam of electrons and a beam of positrons. In crafting this collision, scientists effectively recreate the high-energy conditions that existed during the big bang.
Among the shrapnel created are heavy, short-lived particles called B mesons and their corresponding antiparticles, anti-B mesons. Both survive for just about a trillionth of a second after the collision before decaying. What BaBar has detected is a subtle difference in the rates at which B and anti-B particles decay—B particles stick around just a hair longer —in other words, CP violation in the flesh. This discrepancy between the decay rates of B and anti-B particles could help explain how, 4.6 billion years ago, matter alone emerged from a hot soup of matter and antimatter.
Parenthetically, you may be wondering how a heavy-hitting piece of equipment like BaBar got named after a children’s cartoon character, that crown-wearing elephant from Paris. By convention, a bar is drawn over the letter designating an antiparticle; for example, an anti-B particle is written, and pronounced “B-bar.” Thus explains the name of this immense gadget that can detect both a B and a B-bar: BaBar.
The experimental evidence of CP violation supplied by BaBar is nice, but there’s a serious hitch. The amount of CP violation measured is, quite simply, not enough. In other words, the differing decay rates between matter and antimatter are too slight to explain the prevalence of matter in the universe.
Physicists are still at the blackboards, struggling to develop “new physics” to explain the origin—and the fundamental nature—of our universe. Ultimately, the quest is for a deep unification of two heretofore irreconcilable branches of physics: quantum mechanics, which describes the very small, and relativity, which describes the very fast. The battle for understanding is being waged in the overlapping zone of the small and fast, a realm called quantum gravity, where bizarre and exciting theories with names like string theory and quantum loop gravity may save the day. Stay tuned for more doozies from the physicists.