After World War II, physicists in the United States and elsewhere were able to return to basic research. One group used balloons and other devices to lift photographic emulsions high in the atmosphere,, where they recorded cosmic-ray tracks. Cosmic rays include the particles that bombard Earth from space and also the particles that these primary particles knock out of the molecules that they encounter in the atmosphere. Collisions between a cosmic ray and a molecule produce higher energies than the particle accelerators (“atom smashers”) of the day were able to achieve. As Albert Einstein had shown in 1905, energy can be changed into mass. In fact, one result of these collisions at high energies is the production of new particles of masses greater than any observed in ordinary matter.
The new particles often behaved in very strange ways. In fact, Murray Gell-Mann, in the early 1950s, began to call one particular property strangeness, a name that has stuck. As a result, in physics (unlike ordinary life), it is possible to specify completely whether something is “strange” or not. Among the strangest of the strange were certain particles that have undergone numerous name changes. These particles are the neutral K (kappa) mesons, also called kaons. In the 1950s, physicists called one variety of K meson tau and another theta (today tau refers to an entirely different particle). [All of these are letters of the Greek alphabet.] The problem with tau and theta was that every bit of evidence suggested that they should be exactly the same particle; but when, like all heavy particles, they decayed into lighter particles, tau decayed into three particles and theta decayed into two. Furthermore, the products of the two decays implied that the original particles had to be different in an essential property called parity. The fundamental wisdom of the time was that two otherwise identical particles could not have unequal parities.
Parity itself is a simple concept that is based on an either/or situation. The most common representations are in terms of numbers, usually +1 and –1, or in terms of right and left. Since these two seemingly dissimilar ideas are mathematically identical, physicists often use the numbers +1 and –1, even when the actual physical operation is closer to right and left. Parity for particles is an example of this. The tau has a parity of –1 while the theta has a parity of +1. The experiment that finally explained “the tautheta paradox,” however, rested on the difference between right and left.
By 1956 the tau-theta problem was a focal session for a physics conference in Rochester, New York. Physicists Martin Block and Richard Feynman were roommates at the meeting. Block suggested a radical idea to Feynman, which Feynman passed on to the conferees at the session on the tau-theta problem. Block’s idea was that there is some fundamental difference between right and left. This was radical because the idea that right and left are essentially the same, a concept called the law of conservation of parity, was one of the pillars of theoretical physics at the time.
Chen Ning Yang, one of the speakers at that session, and T.D. Lee met a month later at the White Rose Cafe near Columbia University and decided to work out the implications of Block’s idea. In another month or so Yang and Lee were able to point to specific experiments that would resolve the question. Since they were theoreticians, others performed the experiments. By the end of the year, experiments conducted under the leadership of Chien- Shiung Wu had shown that parity is not conserved in interactions that involve the weak interaction, one of the fundamental forces of nature. Specifically, cobalt-60, whose atoms decay as a result of the weak interaction, preferentially decays toward the left, not the right.
The announcement of these results in January 1957 forced a major revision in the ideas of physicists concerning weak interactions. It was a revolution whose implications are not completely resolved today. Wolfgang Pauli captured the idea best in a phrase calling God “a weak left-hander!” He initially used those words to dismiss the results, and did not accept the fall of parity until he read papers produced by physicists who confirmed Wu’s result in various other experiments.
This lack of symmetry in a world dominated by symmetry may be necessary for the universe to exist at all. In most reactions, matter and its exact opposite, antimatter, are produced in equal amounts. Yet the existing universe, as far as we can tell, contains matter and not antimatter. Matter and antimatter interact to destroy each other whenever they meet. If equal amounts of matter and antimatter were produced in the creation of the universe, this reaction would have “uncreated” it. Physicists think that some asymmetrical reaction at the beginning of time must have prevented this from happening, or we would not be here to think about it.
About the same time as conservation of parity was being overthrown, theoreticians established the CPT theorem, the essence of which is that any interaction between particles must remain the same if you change the charge of all the particles (for example, an electron becomes changed to a positron), if you change all the parities (for example, right becomes left), and if you switch time from running forward to running backward. This theorem derives from fundamental principles and no one doubts that is it true. Thus, when in 1964 physicists demonstrated that changing both the charge and parity changes some interactions, it became clear that some particle interactions must be different when run backward in time from when run forward. In 1998 such interactions were finally found. No one made time flow backward, but changing the sequence of events so that the last event of one sequence was used for the first event of another produced a reverse sequence different from the forward version. Before this all such interchanges with the beginning events reversed produced the same sequence, but in reverse.