137 - Arthur I. Miller [118]
The downfall of parity
That same month—June 1956—two Chinese American physicists, T. D. Lee and C. N. Yang, sent Pauli an article they had written in which they argued that perhaps mirror symmetry—parity—might not always be conserved. They had studied the scientific literature and were convinced there was very little actual experimental evidence to support it, added to which certain puzzling phenomena in elementary particle physics could be clarified if it was considered to be not generally valid. They suggested specific experiments for testing their proposal. To suggest that the law of parity might not be inviolate was outrageous. Pauli chuckled and put the article aside.
Nevertheless the two presented such a powerful case that other physicists became curious and started carrying out experiments. On January 17, 1957, Pauli wrote to Weisskopf that he was “ready to bet a very high sum” that the experiments would fail. Little did he know that the previous day The New York Times had carried a front-page report on what it called the “Chinese Revolution” in physics. A group of physicists from Columbia University headed by a woman, Chien-Shiung Wu, had carried out a very beautiful experiment proving beyond a doubt the overthrow of parity in the case of weak interactions.
Weak interactions are interactions between elementary particles—such as electrons, protons, and neutrinos—occurring with a force far weaker than the nuclear force, which binds the nucleus together, or the electromagnetic force between charged particles. On a scale of one to ten, the nuclear force is magnitude 1; the strength of the electromagnetic force, given by the fine structure constant, is 0.00729—which can also be expressed as 1/137;* while the weak force is a mere 0.00000000000001, very weak indeed.
In their experiments Wu and her co-workers monitored the number of electrons emerging from the nuclei of a radioactive isotope (an alternative form) of cobalt undergoing beta-decay. The beta-decay process involved the transformation of a neutron in the nucleus of the radio active sample of cobalt into a proton, electron, and neutrino. To define a direction—in this case up and down—they aligned the spins of the nuclei by placing them in a magnetic field. If parity really was a universal law, if nature really did not distinguish one direction from another, then one would expect precisely equal numbers of electrons to be emitted upward and downward. But this was not what happened. In fact, the electrons came out asymmetrically. In other words, Wu’s experiment was not the same as its mirror image. The beauty of this experiment—which scientists found so impressive—was its precision. The apparatus had to be set at the lowest possible temperature, close to absolute zero. This was necessary to eliminate any movement of the nuclei due to heat agitation, which would have ruined the alignment of their spins. Thus Lee and Yang were proved to be right.
So parity had been overthrown in the weak interactions—an event which struck Pauli like a bolt of lightning. He was, he confessed “very upset and behaved irrationally for quite a while.” He wrote to Weisskopf that he was glad that in the end he had not made any bet. “It would have resulted in a heavy loss of money (which I cannot afford); I did make a fool of myself, however (which I think I can afford to do),” he said.
He wrote humorously to Bohr about the event (a slip of the pen led him to use the wrong date for the overthrow of parity):
It is our sad duty to make known that our dear female friend of many years
PARITY
had gently passed away on January 19, 1957, following a brief suffering caused by experimental treatment. On behalf of the bereaved.
e,,.
(e, , —electron, muon, and neutrino—are three of the many particles that participate in the weak interactions.)
In real life, of course, our bodies are not symmetrical; our heart is on the left, for a start. But