The Fall of Parity I Couldnt Understand It

In 1956, two young American scientists of Chinese origin Tsung Dao Lee at Columbia University in New York and Chen Ning Yang at the Princeton Institute of Advanced Studies carefully examined theoretically the radioactive P-decay of light nuclei and reasoned that the parity between left and right is violated here (Lee and Yang 1956).1 In their original publication, they furthermore suggested

1 According to the CPT-theorem, quantum field theory gives us today three 'symmetries' by which particles, antiparticles and their interactions can be characterized (Ellis 2003): The first is the charge C (plus or minus) and the second is parity P (left or right. But attention: Parity or space inversion is not the same as mirror reflection. The former sends a system with (x, y, z) coordinates in front of the mirror to an idealized image with (-x, -y, -z) coordinates, whereas for mirror reflection an object occupying the x, y, z plane will have an image with (-x, y, z) coordinates behind the mirror (Cintas 2007). However, often the result is equivalent). The third is time reversal T (forward or backward). By "time reversal" we mean nothing more than a reversal in the direction a particle or wave is moving. According to the CPT-theorem, a particle that is subjected to a combination of these three transformations shows unchanged behavior: Matter particles should behave the same as their antiparticles, if viewed in a mirror with their internal clocks reversed. A CPT-reversal of experiments in order to test parity non-conservation in these interactions. This observation moved the fundaments of physics dramatically because until 1956 people thought that all the four fundamental natural forces are strictly symmetric. Lee and Yang were immediately honoured for their discovery with the Nobel Prize of Physics in 1957. To that time Lee was 30, Yang 34. The choice was inevitable.

The shock waves were still rumbling when, in 1957, the experimental verification of Lee and Yang's theoretical questioning of parity conservation in weak interaction was performed by Mme Chieng-Shiung Wu at Columbia University, New York. Together with Ernest Ambler and members of the National Bureau of Standards in Washington she selected the P-radiator 60Co, which periodically emits electrons (P -particles) for her crucial and today highly cited experiment: 60Co can be obtained by irradiation of 59Co in a nuclear reactor with thermal neutrons. It is a suitable isotope to study the radioactive decay since its half-life is 5 years and therewith opportune for experiments. Add to this, it is feasible to orient Cobalt atoms in space in order to determine a direction to measure emitted decay products: Cobalt nuclei can be - similar to small magnets - compass-needle like oriented in a magnetic field. Usually these nuclei are not oriented and distributed in all directions. In a strong magnetic field 60Co nuclei orient themselves in parallel to the field and in parallel to neighbour nuclei. The electron cloud in paramagnetic 60Co ions - and this is the main advantage of 60Co compared to other P-radiators - strengthens the magnetic field towards the nucleus. In order to eliminate the thermal motion of the particles, Wu's experimental set-up that is schematically depicted in Fig. 5.2 was cooled down to 0.01 K. Under such conditions, the majority of the cobalt nuclei is well oriented in the direction of the applied magnetic field.

If now, the radioactive decay were symmetric in space, the electrons (i.e., P~ -particles) would be emitted in all directions including the direction of the 60Co nuclei spins. If - on the other hand - one could observe an asymmetry in the angular distribution between 0° and 180° between the orientation of the parent nuclei and the momentum of the electrons it provides unequivocal proof that parity is not conserved in P-decay. Mme Wu and her team observed this asymmetry effect in the case of oriented 60Co nuclei. Electrons were preferably emitted against the direction of polarization of the 60Co nuclei (Wu et al. 1957). Here, mirror symmetry is not conserved. The weak nuclear interaction violates parity!

Physicists and physico-chemists were unwilling to accept the removal of symmetry from their theories.

a glass of milk of Lewis Carroll's Alice (in "Through the Looking-Glass and What Alice Found There" Alice asked her cat: How would you like to live in a looking-glass house, Kitty? ... Perhaps looking-glass milk isn't good to drink.") would mean that all charges would be reversed (making it antimilk), the structure would be mirror-reflected, and every molecular motion would reverse its direction (Gardner 2005). Until today, we assume indeed that CPT preserves symmetry; there is no violation of the CPT-theorem known, even if it was searched for intensively. However, particle interactions were shown to violate each of P and C individually, but also the CP combination of these transformations. This violation was proposed by Sakharov to be responsible for the dominance of matter over antimatter in the Universe today and is equally connected to the question "Can time go backward". Before the fall of parity, physicists believed that if you altered just one of the signs, the new sentence would still describe something nature could do.

Fig. 5.2 Wu's experimental test of parity non-conservation provoked by P-decay of radioactive isotopes. (Left) Schematic drawing of the lower part of the cryostat: 60Co nuclei were oriented in a magnetic field and their orientation was fixed in a cryostat at 0.01 K. The upwards-installed anthracen crystal was used to count the emitted electrons (P~-radiation); sodium iodide NaI was for counting equally emitted y-radiation. (Right) The anisotropy of emitted electrons is depicted in function of the direction of the polarizing field pointing up and pointing down. If the polarizing field points down, the anthracen crystal captures more electrons. Electrons were preferably emitted against the direction of cobalt nuclei's polarization. Since the warm-up time for cobalt in the experimental set-up was about six minutes, the anisotropy of emitted electrons disappeared over time

Fig. 5.2 Wu's experimental test of parity non-conservation provoked by P-decay of radioactive isotopes. (Left) Schematic drawing of the lower part of the cryostat: 60Co nuclei were oriented in a magnetic field and their orientation was fixed in a cryostat at 0.01 K. The upwards-installed anthracen crystal was used to count the emitted electrons (P~-radiation); sodium iodide NaI was for counting equally emitted y-radiation. (Right) The anisotropy of emitted electrons is depicted in function of the direction of the polarizing field pointing up and pointing down. If the polarizing field points down, the anthracen crystal captures more electrons. Electrons were preferably emitted against the direction of cobalt nuclei's polarization. Since the warm-up time for cobalt in the experimental set-up was about six minutes, the anisotropy of emitted electrons disappeared over time

Wolfgang Pauli (who died in 1958) is said to have bet large amounts of champagne against it when the suggestion of parity violation first appeared. "Now after the first shock is over," Pauli wrote to Weisskopf after the staggering news had reached him, "I begin to collect myself. Yes. It was very dramatic. On Monday, the twenty-first, at 8 p.m. I was supposed to give a lecture on the neutrino theory. At 5 p.m. I received three experimental papers on the first three test of parity. I am shocked not so much by the fact that the Lord prefers the left hand as by the fact that he still appears to be left-handed symmetric when he expresses himself strongly. In short, the actual problem now seems to be the question: Why are strong interactions right-and-left symmetric?" (Quack 2002; Gardner 2005).

The reaction of Richard P. Feynman is lively described by Martin Gardner: In April 1956, Gardner and Feynman participated at a conference on nuclear physics at the University of Rochester in New York State (Feynman 1997; Gardner 2005) where Feynman raised the question: "Is the law of parity sometimes violated?" Actually, Martin Block, an experimental physicist with whom Feynman was sharing a hotel room, had suggested the question to Feynman the night before. The answer to the theta-tau puzzle (to that time a fundamental question on two distinct types of neutral K mesons), said Block, might be very simple. Perhaps the lovely law of parity might not always hold. Feynman responded by pointing out that if this were true, there would be a way to distinguish left from right. It would be surprising, Feynman said, but he could think of no way such a notion conflicted with known experimental results. He promised Block he would raise the question at next day's meeting to see if anyone could find anything wrong with the idea. So the next day, at the nuclear physics meeting, when discussions started on the theta-tau puzzle, Robert Oppenheimer said, "We need to hear some new, wilder ideas about this problem." So Feynman got up, prefacing his remarks with, "I am asking this question for Martin Block." He regarded the notion as such an interesting one that, if it turned out to be true, he wanted Block to get credit for it.

Tsung Dao Lee and his friend Chen Ning Yang were present at the meeting. One of them gave a lengthy reply to Feynman's question.

"What did he say?" Block asked Feynman later.

"I don't know," replied Feynman. "I couldn't understand it".

"People teased me later," writes Feynman, "and said my prefacing remark about Martin Block was made because I was afraid to be associated with such a wild idea." I thought the idea unlikely but possible, and a very exciting possibility. Feyn-man made a 50-dollar bet with a friend that parity would not be violated. However Feynman lost that bet after the mentioned experimental proof of parity violation by Wu.

Freeman J. Dyson, a physicist now at the Institute for Advanced Studies in Princeton, called the reaction on the now-classic Lee and Yang paper the "blindness of most of his colleagues". Dyson is cited by Gardner (2005) with the words "A copy of the Lee and Yang paper was sent to me and I read it. I read it twice. I said, 'This is very interesting,' or words to that effect. But I had not the imagination to say, 'By golly, if this is true it opens up a whole new branch of physics.' And I think other physicists, with very few exceptions, at that time were as unimaginative as I."

Robert Frisch, the physicist who was co-discoverer of nuclear fission, reported that on 16 January 1957 he received the following air letter from a friend:

"Dear Robert: HOT NEWS. Parity is not conserved. Here in Princeton they talk about nothing else; they say it is the most important result since the Michelson experiment in 1887" (see Gardner 2005).

It is worth mentioning that the fall of parity in physics was the result of theoretical and mathematical work by Lee and Yang. They told the experimenters what to do. Lee and Yang themselves had no experience in the lab. Lee has never been working in a laboratory. Yang had worked briefly in a lab at the University of Chicago, where he was once kind of assistant to the Italian physicist Enrico Fermi. He had not been happy in experimental work. His associates had even made up a short rhyme about him (Gardner 2005):

Where there's a bang,

There's Yang.

Later on, it turned out that electrons are emitted during the P-decay with a velocity that is close to the speed of light. In our part of the universe, these electrons are always left-polarized, if we regard their spin towards their trajectory. The electrons' spin is antiparallel to their direction of propagation. The equally emitted antineutrino is right-polarized. The two types of electrons (left- and right polarized, where only left-polarized electrons are emitted via P~ -decay) are related by mirror-image symmetry, which is, in principle, the same model as that which gives rise to optical activity in chemical compounds.

In contrast to antineutrinos and neutrinos, electrons can interact with matter and we will now have to continue asking the questions whether and how polarized 'ho-mochiral' electrons generated by the P~ -decay can transfer their inherent chirality towards any kind of organic environment. Did nuclear physics processes give ultimately birth to the asymmetry of life?

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