Wednesday, February 13, 2008

Imperfect symmetry

Remember little moron jokes? They may be politically incorrect these days, but here's one I remember from my mother's Treasury of American Folklore:
A little moron is nailing shingles on a house and tossing away every other nail. Another little moron comes along and asks, "Why are you throwing away half the nails?" "The heads are on the wrong end," says the first little moron. "You moron," says the second little moron, "those are for the other side of the house."
Well, I thought it was funny at the time. And it still has a nice philosophical absurdity about it. Maybe even a dollop of existential profundity.

In 1927, the English physicist Paul Dirac, pondering the equations that govern subatomic particles, predicted that every particle should have a kind of mirror image, or "antiparticle" -- a nail with the head on the other end, so to speak. The electron, with its negative charge, should be complemented by an antielectron, or positron, alike in every respect except for having a positive charge. The positively-charged proton should be complemented with a negatively-charged antiproton. And so on.

If antimatter exists, then antielectrons and antiprotons can form antimatter atoms. Antimatter atoms can make antipeople, antiplanets, antistars, antiworlds -- another whole universe on the other side of the house.

There is just one problem. If matter and antimatter meet, they annihilate each other in a burst of pure energy, called gamma rays.

Five years after Dirac's startling prediction, positrons were produced in the laboratory. Today, at high-energy particle accelerator labs, the production of antimatter is commonplace. These antimatter particles are born into a world of matter. Immediately, they meet their matter complements and vanish in a puff of gamma radiation. It takes lots of energy to make antiparticles. They disappear in a flash.

As far as astronomers know, the observable universe is made almost entirely of matter, not antimatter, and this presents a puzzle.

Matter and antimatter should have been created in equal amounts in the furious first moments of the Big Bang. But if other parts of the visible universe were made of antimatter, we would see a flood of gamma radiation from mutual annihilation at the boundaries where domains of matter and antimatter meet.

So where is all the antimatter today?

It seems that just before the universe was a millisecond old, primeval matter and antimatter annihilated each other in a sweeping extinction. But a tiny asymmetry was built into the universe from the very beginning, so that matter dominated over antimatter by -- oh, say -- one part out of 100 billion. When the epoch of annihilation ended, only matter remained. And that's the universe we live in

Why the initial asymmetry? Why not a perfect balance of matter and antimatter, in which case the history of the universe would have lasted less than a thousandth of a second. Physicists have proposed ideas to account for the "breaking of symmetry," but it's mostly a matter of making up just-so stories to fit the apparent facts. Little morons would suggest that in every bag of nails, half of the nails should have heads on the other end. And maybe, just maybe, there is another side of the house.