Nobel Prize in Physics: Broken Symmetry

Why can the Universe exist as it is now? Physicists have been striving to answer this question. A proposed theory is “broken symmetry.” In 2008, the Nobel Prize committee recognized significant contribution of three Japanese scientists in the field of symmetry.

On October 7, 2008, the Royal Swedish Academy of Sciences announced that the Nobel Prize of Physics will be jointly awarded to Toichiro Nambu, Makoto Kobayashi and Toshihide Maswaka. Nambu, professor emeritus at the Fermi Institute of the University of Chicago, worked on the mathematical model of spontaneous broken symmetry, while Kobayashi and Maskawa investigated the origin of broken symmetry. Their work on spontaneous broken symmetry explains “why the universe is made up of matter and not anti-matter” (1). Their theory also hints at why the Universe has managed to survive for centuries.

The Big Bang created both matter and anti-matter. Had the Universe been symmetrical, the world we live in would not have formed. When matter and anti-matter collide, nothing is left but radiation energy. This “broken symmetry” is a central field in physics, as physicists believe it is this asymmetry that keeps the Universe in its current state. The world remains as it is because an excess of matter particles triumphed over ten billion anti-matter particles (2).

Yoichiro Nambu charted the course to broken symmetry, by introducing spontaneous violation of symmetry in 1960. While studying superconductivity, a state in which currents flow without any resistance, he recognized spontaneous symmetry violations that indicated superconductivity. He then translated his computations into elementary particle physics, which soon became a milestone amongst theories about the Standard Model.

The Standard Model is a theoretical model proposed in particle physics that explains how nature works. The Model explains three of four fundamental forces of nature and what types of particles participate in these interactions. First of all, particles are divided into fermions (quarks and leptons, the matter constituents) and bosons (force carriers or force-mediating particles). Fermions are then categorized into quarks, which carry color charge; and leptons, which do not carry it. Because of color charge, quarks are involved in interactions with strong nuclear force, while leptons are responsible for other three interactions. There were previously three quarks: up, down and strange. The other three quarks – charm, top and bottom – were discovered by Kobayashi and Maskawa, to be confirmed a few years later. On the other hand, leptons include three neutrinos (electron neutrino, muon neutrino, and tau neutrino) and three negatively-charged particles (electron, muon, and tau). Bosons include gluons and mesons for strong nuclear forces; W+, W- and Z0 for electroweak forces; finally, a graviton, which has not been observed yet, for gravitational forces. (2)

At the time of his discovery, Nambu’s assumption that spontaneous symmetry violations in electromagnetism can also be applied to elementary particle physics was considered bold (2).  His insight into spontaneous broken symmetry can be explained in a following analogy. Imagine a perfectly symmetrical spinning top representing an unstable state. When it loses its balance the symmetry is broken (3). Also, in terms of energy, the state of a fallen pencil is more stable than a perfectly symmetrical balanced state. Therefore, given that the vacuum has the lowest state of energy in the cosmos, it can be inferred that the Universe does not have a symmetrical quantum field: there was an imbalance between matters and anti-matters when the Universe was formed.

Nambu’s discovery affected the Standard Model in two ways. First, it allowed the model “[to unify] the smallest building blocks of all matter” (4). Second, the effects of the third fundamental force in nature, strong nuclear force, could be incorporated (5).

On the other hand, the other two scientists, Kobayashi and Maskawa, made a different claim as bold as Nambu’s. Their discovery is based on the double broken symmetry during a radioactive decay of kaon particles, named by James Cronin and Val Fitch. This unusual broken symmetry demanded an explanation, as this phenomenon threatens the Standard Model (2). Kobayashi and Maskawa theorized that there must be more than three quarks – at this point three of the six quarks were yet to be discovered – in order for their analysis based on the Standard Model and the model itself to hold.

Kobayashi and Maskawa also calculated the probability that a quark in kaon will transform itself into an anti-quark and vice versa. Their calculations implied that if a similar type of transformation were to happen to matter and anti-matter, further quark families had to exist, besides the first family, up and down quarks (2).

It took three decades for their theory to be confirmed. One of the quarks in the second family, charm, was eventually discovered in 1974; the two quarks of the third family in the model, top and bottom, were discovered in 1994 and 1977, respectively (2). In 2001, the BaBar particle detector in Stanford and the KEK accelerator in Taskaba, Japan confirmed that B-mesons, the cousins of kaons, also experience broken symmetry, if rarely, in the way Kobayashi and Maskawa predicted thirty years ago (2).

While the breakthroughs by three Japanese scientists improved the Standard Model, some answers still remain unanswered. One of nature’s fundamental forces, gravitational force, has not yet been incorporated, and the existence of the Higgs boson has not been confirmed yet. However, Kobayashi believes “the issue of the standard model is almost over,” and now scientists are awaiting “new physics.” (6). Although physicists are still not content with the existing Standard Model, it is no doubt that their work certainly put physics into a new dimension.

References

1. C. Moskowitz, Will the Large Hadron Collider Destroy Earth? (2008). Available at http://www.livescience.com/mysteries/080909-llm-lhc-faq.html (23 Jan. 2009).
2. The Royal Swedish Academy of Sciences, The Nobel Prize in Physics 2008 (2008). Available at http://nobelprize.org/nobel_prizes/physics/laureates/2008/info.pdf (23 Jan. 2009).
3. A. Cho, 2008 Physics Nobel Prize Honors American and Japanese Particle Theorists (2008). Available at http://sciencenow.sciencemag.org/cgi/content/full/2008/1007/1 (24 Jan. 2009).
4. The Royal Swedish Academy of Sciences, Press Release (2008). Available at http://nobelprize.org/nobel_prizes/physics/laureates/2008/press.html (23 Jan. 2009).
5. D. Overbye, “Three Physicists Share Nobel Prize,” The New York Times, 7 Oct. 2008.
6. A. Smith, Telephone Interview (2008). Available at http://nobelprize.org/nobel_prizes/physics/laureates/2008/kobayashi-telephone.html (23 Jan. 2009).