No turning back

By Charles Seife in Washington DC THE dances of particles have revealed that time has a strange asymmetry. For years physicists thought that if the flow of time was reversed, the laws of physics would remain exactly the same on the tiniest scale. If you videotaped two photons colliding to yield an electron and positron, then reversed the tape to watch a positron and electron colliding to yield two photons, this would also satisfy the laws of physics. You couldn’t tell which tape was the original. This process was called “T symmetry”. In the same way, physicists thought that swapping all positive electrical charges with negative charges and vice versa (called C symmetry) and reflecting the Universe in a mirror (known as P symmetry) would leave the laws of physics unchanged. However, in 1957 Tsung-Dao Lee and Chen Ning Yang won the Nobel prize for discovering that the decay of particles called kaons violates P symmetry. And in 1980, Val Fitch and James Cronin won the Nobel for showing that you can’t switch the charges to compensate for the error in the mirror reflection. In other words, kaons violate CP symmetry. Physicists expected to see T violation because they believe that though CP is not conserved, CPT is. So if you reverse time, swap charges, and mirror-image the Universe all at the same time, the laws of physics remain the same. If CP is violated in a decay, T must be violated to compensate. However, scientists had not observed this asymmetry directly. Now, an experiment at Fermilab near Chicago has recorded decays that show T symmetry violation. Scientists created a beam of kaons by firing protons at a target of beryllium oxide. The kaons can disintegrate into two pions, a positron and an electron. Scientists analysed the angles between the decay products, and mathematically divined that T asymmetry must be at work. “It just confirms what you expect, but when you claim you understand something, it has lots of consequences that need to be tested,” says Cronin, of the University of Chicago. “The work is absolutely beautiful.” An experiment at CERN, the European Laboratory for Particle Physics in Geneva, has also found evidence for T violation. It showed that kaons turn into antikaons less often than antikaons become kaons. A video running forwards would show antimatter slowly turning into matter, while the reverse shows matter turning into antimatter. This may explain why we see lots of matter in the Universe and little antimatter,
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