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鈥檛 tell which tape was the original. This
process was called 鈥淭 symmetry鈥�.
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.
Advertisement
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鈥檛 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. 快猫短视频s 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. 快猫短视频s analysed the angles between the decay products, and
mathematically divined that T asymmetry must be at work.
鈥淚t just confirms what you expect, but when you claim you understand
something, it has lots of consequences that need to be tested,鈥� says Cronin, of
the University of Chicago. 鈥淭he work is absolutely beautiful.鈥�
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, even though the big bang probably made equal
amounts of both.
