Time's Tiny Arrow Revealed

In the everyday world, time marches forward. A shattered glass, for example, won't fly up from the floor and unbreak itself. But in the simple world of subatomic particles, most events look plausible even when a movie of them runs backward. Now, for the first time, two groups of scientists have found some scenes that aren't reversible--direct evidence of the hand of time in subatomic decay.

Physicists once thought that the equations of the subatomic world would look the same if time flowed backward. For instance, the remnants of a decayed atom could always, though the chances are remote, converge to reform a full atom. That idea died in 1964 when physicists observed a related event--an odd decay of a particle called a kaon which revealed that nature treated matter and antimatter slightly differently. This puzzling asymmetry--called "CP" violation--could be patched up only if the rules of physics were different with time run backward compared to forward.

One way to test this proposition is by observing the rate for an event, say a particle switching from state A to state B, both forward and backward and seeing if one happens more frequently. Of course you can't actually run the clock backward, so instead you measure the rate of B changing to A. One of the groups, the CPLEAR collaboration at CERN in Geneva, Switzerland, did just this with particles called kaons and their antimatter counterparts, antikaons. As they travel, antikaons can transform into kaons and vice versa.

The team used a large tracking chamber to tally the kaons and antikaons as they decayed--each to an electron, a pion, and a neutrino. The charge of the electron revealed which type of kaon had decayed. The rate for antikaons transforming into kaons was a fraction of a percent higher than for what would be the time-reversed process--kaons becoming antikaons--the team will report in an upcoming issue of Physics Letters B. "This shows that you can't turn the clock backward" and always get the same results, says CPLEAR spokesperson Panagiotis Pavlopoulos.

The other group, the KTeV collaboration at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, found a subtler example of time asymmetry by comparing the rates for kaons decaying to different configurations of particles that flip-flop under time reversal. These rates differed by about 13%. "It's a huge effect," says Fermilab physicist and KTeV collaborator Vivian O'Dell.

The amount of time asymmetry is just about right to fix the CP asymmetry first observed over 3 decades ago. "I don't think anyone is surprised but everybody is very happy," says University of Chicago theorist Jonathan Rosner. Why the decays should look any different forward and backward is still a fundamental mystery. But particles, like falling wine glasses, seem to know that the passage of time cannot be easily undone.