For nearly 40 years physicists have had to live with the disconcerting knowledge that the universe favors its left hand. In subatomic interactions involving the weak force--the force of nature responsible for radioactivity--things can happen in one direction but not in the other. For example, all neutrinos formed during radioactive decay spin in the same direction. It’s as if all the tops in the world could only spin clockwise.
Many physicists, however, refuse to believe that the universe was simply born left-handed. They assume that in the first fraction of a second after the Big Bang, the cosmos was ambidextrous, just as it was symmetrical in other ways--for instance, the weak force is thought to have been just as strong as the three other fundamental forces, the strong, the electromagnetic, and the gravitational. The assumption of ambidexterity has been very much an act of faith. But now, thanks to their ability to measure atomic-scale phenomena with unprecedented precision, some physicists think they’re close to proving it true. What we’re looking for is what I like to call a ‘fossil’ of the original symmetry, says Geoffrey Greene at Los Alamos National Laboratory. It’s a lingering relic of what was originally a fully symmetrical system that still remains even though we’ve gone on to become a dominantly left-handed universe.
Physicists refer to spinning objects as being left-handed or right-handed because the terms clockwise and counterclockwise depend on which side of the object you look at. Handedness (or chirality) is determined from the direction in which the object moves as it spins. Most screws are right-handed--that is, as you twist the screwdriver in the direction that the fingers on your right hand curl, you drive the screw in the direction of your right thumb. If the DNA double helix were a screw, it would be right-handed, too, and many other organic molecules have folds or twists that can be described as either left- or right-handed. Subatomic particles don’t have folds, but they do have spin, as physicists discovered early in this century. So when such particles are on the move, as they invariably are, they too have a handedness.
And particles produced in weak-force interactions, it turns out, are preferentially left-handed. Whereas the strong nuclear force holds protons and neutrons together in an atomic nucleus, the weak force transforms one of these two particles into the other, sometimes breaking the nucleus apart. The most frequently observed weak interaction is beta decay, in which a neutron gives rise to a proton, an electron, and an antineutrino (the antimatter counterpart of the neutrino).
In 1957 researchers studying the beta decay of cobalt nuclei found that electrons tended to fly off the spinning nuclei in the left- handed direction more often than in the right-handed direction. (In this case the curl of the fingers indicates the direction in which the nucleus is spinning, and the thumb points in the direction the electron travels.) The effect wasn’t large--56 lefties for every 44 righties. But after subtracting out all the other factors that influence the electron’s motion, physicists realized that the weak force was the source of the asymmetry: its push was exclusively in the left-handed direction. Later this finding was confirmed by physicists examining neutrinos, which are created only through weak interactions: those researchers found that every neutrino had a left-handed spin, while every antineutrino was right-handed. When the standard model of subatomic particles was pieced together in the 1970s, this absence of parity in weak interactions was left as an unexplained given of nature.
Unexplained properties don’t sit well with physicists. Although some of them accept weak-force asymmetry as, in effect, an act of God--a reflection of the conditions under which the universe was created--others believe there was an original symmetry that was broken. Just because screws, clocks, and DNA tend to have a certain handedness, the argument goes, doesn’t mean they must have that handedness. Screws would still work if they had left-handed threads, and clocks would tell time if their hands went backward--indeed, some early clocks did. But over time, standardization set in and counterclockwise clocks and left-handed screws became rare. This is what physicists call a broken symmetry. For the weak force, broken symmetry would mean that for a short time after the Big Bang, the weak force was just as likely to produce right-handed particles as left-handed ones; but as the universe cooled, its original ambidexterity was lost. By chance, left-handed particles came to dominate.
The current resurgence of interest in this issue began in 1990 when a team of Russian researchers made the controversial claim that they had found a faint trace of primordial right-handedness in the weak force. Since then physicists like Greene have been trying to confirm the Russian results. In principle there are two ways to show that the weak force can produce right-handed particles at extremely high energies. You could create such particles from scratch the way chemists synthesize organic molecules not seen in nature--but only if you had a particle accelerator powerful enough to rival the Big Bang. Obviously no such accelerator exists.
Instead physicists must search for the remnants of right- handedness in our dominantly left-handed universe. It’s the same way that you’d look to see if screw threads were all forced by God to be right- handed, Greene says. You start looking around for left-handed threads. In sorting through every screw and bolt in a hardware store, one would eventually find a tiny percentage of left-handed bolts--like those used to keep left bicycle pedals and left-handed toilet handles from coming off-- which would be the trace of a fundamental screw-thread parity. Likewise, by carefully measuring interactions that involve the weak force, one might see that a purely left-handed universe couldn’t completely account for them. Just a smidgen of right-handedness would need to be included in order to make everything add up.
The easiest place to look for a trace of symmetry is in the beta decay of neutrons, where the asymmetry was first discovered. At a laboratory operated by the National Institute of Standards and Technology in Gaithersburg, Maryland, Greene and his colleagues are measuring the lifetime of a free neutron--which, unlike a neutron tucked inside a nucleus, survives only about 15 minutes. If Greene finds that the neutron decays more quickly than is generally believed, then this will imply that the weak force is slightly stronger than physicists think. In that case it must have a small right-handed component to counterbalance its dominant left hand: otherwise it would be pushing even more electrons in the left- handed direction as they fly off the decaying neutron.
The same logic applies to experiments under way at the Chalk River National Laboratory in Canada. There researchers are measuring not the entire strength of the weak force but its vector coupling constant-- an indicator of the strength of a part of the weak force that interacts with protons and neutrons. A stronger-than-expected coupling constant would again imply the presence of right-handedness in the weak force.
Finally, researchers at Lawrence Berkeley National Laboratory are coming at the question from the other side: they’re counting the left- handed and right-handed electrons produced by beta decay more precisely than ever before. If the lefty excess turns out to be smaller than the 56- 44 split first observed in 1957, that too would be evidence for a right- handed component of the weak force.
None of these experiments alone can prove right-handedness, because the different measurements--the strength of the weak force, its coupling constant, and the electron motions it produces--depend on one another. But together the results may show that a left-handed weak force cannot account for them all. The experiments take a long time, though, and the results are not yet in. Greene’s group and the one at Chalk River have seen tantalizing suggestions of right-handedness, but not convincing enough to publish.
Looking at the neutron lifetime data alone, one finds a hint of something a little beyond what you’d expect from statistical errors, says Greene. Now our question is, is this an experimental artifact? Or are we onto a real indication of right-handedness?
Right now parity violation--left-handedness--is part of the standard model of elementary particles, Greene goes on. It’s built into it from the foundation, and if you pardon the expression, that handedness is put into the theory by hand--there is no fundamental reason it should be there. Quite the opposite: Nobody likes it. It would be a truly significant discovery to establish that, in fact, the universe is symmetrical, and what we perceive as a choice of one handedness or another is an accident, not an edict of God.