The mirror is shattered. Inherent asymmetry determines our fate. Solving the mystery of the broken symmetry

The mirror is shattered

The Standard Model is a synthesis of all the insights into the innermost parts of matter that physics has gathered during the last century. It stands firmly on a theoretical base consisting of the symmetry principles of quantum physics and the theory of relativity and has stood up to countless tests. But before the pattern was quite clear, a number of crises occurred that threatened this well-balanced construction. These crises related to the fact that physicists had assumed that the laws of symmetry applied to the Lilliputian world of elementary particles. But this, it turned out, was not entirely the case.

The first surprise came in 1956 when two Chinese-American theoreticians, Tsung Dao Lee and Chen Ning Yang (awarded the Nobel Prize the following year in 1957) challenged mirror symmetry (P symmetry) in the weak force. That nature respected mirror symmetry, the symmetry concerning left and right, was considered, like other symmetry principles, to be a well-established fact.

We need to re-evaluate old principles in the quantum world, where the elementary particles exist, claimed Lee and Yang. They proposed a series of experiments to test this mirror symmetry. And sure enough, only a few months later the decay of the atom nucleus in the radioactive element cobalt 60 revealed that it did not follow the principles of mirror symmetry. The symmetry was broken when the electrons that left the cobalt nucleus preferred one direction to another. It was as if you were standing in front of the Stockholm Central station and saw most of the people turning left out from the station.

Inherent asymmetry determines our fate

It may well be that charge and mirror symmetries are broken separately, but both of them, the so called CP-symmetry, are certainly not broken at the same time. The physicist community consoled itself with the idea that this symmetry remains unbroken. The laws of nature, they believed, would not change if you stepped into a mirror world where all matter was replaced with antimatter.

This also means that if you met an extraterrestrial being, there should not be any way of deciding whether the alien came from our world or from the antiworld. A welcoming hug could then have disastrous consequences. Only a puff of energy would be left when matter and antimatter annihilated each other on first contact.

So it was perhaps just as well that the weak force came back into the limelight in 1964. A new violation of the symmetry laws emerged in the radioactive decay of a strange particle, called a kaon (Nobel Prize awarded to James Cronin and Val Fitch in 1980). A small fraction of the kaons did not follow the current mirror and charge symmetries; they broke the double CP-symmetry and challenged the whole structure of the theory.

Thinking about meeting extraterrestrial beings, this discovery offers a salvation. It might be enough to ask an extraterrestrial before it hugs you to first look carefully at the kaon decay at home and check whether it is made of the same matter as us or antimatter.

The first person to point out the decisive importance of broken symmetry for the genesis of the cosmos was the Russian physicist and Nobel Peace Prize Laureate Andrei Sakharov. In 1967, he set up three conditions for creating a world like ours, empty of antimatter. Firstly, that the laws of physics distinguish between matter and antimatter, which in fact was discovered with the broken CP-symmetry; secondly, that the cosmos originated in the heat of the Big Bang; and thirdly, that the protons in every atom nucleus disintegrate. The last condition might lead to the end of the world, since it implies that all matter can eventually disappear. But so far that has not happened; and experiments have shown that protons remain stable for 10³³ years, a comfortable10 trillion times longer than the age of the universe, which is slightly more than 10¹⁰ years. And still there is no one who knows how Sakharov’s chain of events took place in the early universe.

Solving the mystery of the broken symmetry

It may well be that Sakharov’s conditions will eventually be incorporated into the Standard Model of physics. Then the surplus of matter created at the birth of the universe will be explained. That, however, requires a much greater symmetry violation than the doubly broken symmetry, that Fitch and Cronin found in their experiment.

However, even a considerably smaller broken symmetry that the kaons were guilty of needed an interpretation; otherwise the whole Standard Model would be threatened. The question of why the symmetries were broken remained a mystery until 1972, when two young researchers from the University of Kyoto, Makoto Kobayashi and Toshihide Maskawa, who were well acquainted with quantum physics calculations, found the solution in a 3 x 3 matrix.

How does this double broken symmetry take place? Each kaon particle consists of a combination of a quark and an antiquark. The weak force makes them switch identities time and time again: the quark becomes an antiquark while the antiquark becomes a quark, thus transforming the kaon into its antikaon. In this way the kaon particle flips between itself and its antiself. But if the right conditions are met, the symmetry between matter and antimatter will be broken. Kobayashi and Maskawa’s calculation matrix contains probabilities for describing how the transformation of the quarks will take place.

It turned out that the quarks and antiquarks swapped identity with each other within their own family. If this exchange of identity with double broken symmetry was to take place between matter and antimatter, a further quark family was needed in addition to the other two. This was a bold concept, and the Standard Model received these speculative new quarks, which appeared as predicted in later experiments. The charm quark was discovered as early as 1974, the bottom quark in 1977 and the last one, the top quark, as late as 1994.