Presentation Speech by Professor Professor Lars Brink, Member of the Royal Swedish Academy of Sciences, Member of the Nobel Committee for Physics, 10 December 2008
|Professor Lars Brink delivering the Presentation Speech for the 2008 Nobel Prize in Physics at the Stockholm Concert Hall.
Copyright © The Nobel Foundation 2008
Photo: Hans Mehlin
Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,
“The Earth is round.” This simple sentence contains so much. It shows how we human beings see the objects around us in symmetric shapes. We have done so for as long as we can remember. Long ago the ancient Greeks classified geometric objects and introduced the concepts we use today. The sentence also shows the importance of symmetries in determining the laws of physics. These laws do not allow the earth to be flat or square. They have a built-in symmetry. But the earth is not exactly round. Its radius at the equator is slightly bigger than at its poles. There are mountains and valleys. So physicists would say that symmetry is “weakly broken”. There are parts of the laws that determine the shape of the earth that break symmetry. When we look in a mirror we accept the image we see as a faithful image of ourselves. But is it? Are we symmetrical if we draw a line from our head downward, dividing the body into a left and right part? No, not completely. Indeed, it is often said that this slight lack of symmetry makes a face look more interesting. Pablo Picasso experimented with this in his paintings of Dora Maar, where the two halves of her face represented different moods. But a real human face is more mirror symmetrical than that.
We often find situations where symmetry must be broken. If we find ourselves at the top of a completely symmetric mountain, almost like the Matterhorn, we are in an unstable situation. If we do not hold onto something, we will fall in one direction and slide down to the foot of the mountain. We do not know beforehand in which direction we will slide. Symmetry, which says that all directions down the mountain are equally probable, has been broken spontaneously. The laws are still symmetrical, but our position somewhere at the foot of the mountain, the “ground state” as physicists say, breaks this symmetry. We do not see the symmetry, but it is there.
How is this reflected in the fundamental laws of nature, the ones that govern physics at the smallest distances? When elementary particles and the forces that act between them were studied in experiments from the 1950s onward, it was found that everything that the underlying symmetries allow can happen. Symmetries must therefore impose enormously strong restrictions for the laws to be meaningful. How can this lead to our universe, with its four kinds of fundamental forces and great variety of elementary particles? The decisive idea came from Yoichiro Nambu, who in 1960 showed that the fundamental laws of nature can exhibit spontaneous broken symmetries, as illustrated by the person falling off the Matterhorn. There can be strong restrictions from large symmetries that are not seen directly in physical experiments, at least at the energies that can be produced in today’s large particle accelerators. The symmetries are there as a consequence of the fundamental laws of physics, but for a system in its ground state there are no direct signs of the restrictions that symmetry should impose. This means, for instance, that the electromagnetic force and the weak nuclear force do not appear related, even though the underlying theory says so. This is just one example of Nambu’s ideas that permeate all of modern physics and have been tested in numerous experiments.
Mirror symmetry is important for fundamental laws, too. It caused a major sensation when Lee and Yang pointed out in 1956 that this is not necessarily true in radioactive decays, a fact that was very quickly confirmed. Nevertheless, it was thought that mirror symmetry would indeed apply to all particle reactions if it were combined with the operation of changing a particle to its antiparticle. However, in 1964 Cronin and Fitch discovered, very surprisingly, that this combined symmetry could also be very weakly broken in certain special processes. During the late 1960s and 1970s the Standard Model that is now used to describe elementary particle physics was developed, partly based on Nambu’s ideas. However, it could not explain this weak symmetry breaking, so the question arose of how one could extend the model without destroying the results that had so successfully been compared to experiments. In 1972 Makoto Kobayashi and Toshihide Maskawa examined the possibility of introducing more fundamental particles − quarks − and found that if there are six different quarks, the theory can indeed break the symmetry. It was a daring assumption, since at that time the everyday world seemingly needed only two different kinds of quarks, whereas three had already been discovered. Kobayashi and Maskawa suggested that there should be three more. These were eventually discovered, the last one in 1994. During the past decade, elementary particle physicists have measured Kobayashi and Maskawa’s theory with great precision and found that it really does fit the data. Nature is endowed with six different quarks, at least at the energies that have so far been accessible. This also leads to an imbalance between matter and antimatter, a fact that Andrei Sacharov pointed out early on. Thanks to this imbalance, we are here today. Symmetry breaking allows for a world of matter.
[Professor Kobayashi, Professor Maskawa,
Together with Professor Nambu you have been awarded the 2008 Nobel Prize in Physics for your seminal works on broken symmetries, which have been instrumental for the modern theories of elementary particles. It is an honour for me to convey the warmest congratulations of the Royal Swedish Academy of Sciences. I now ask you to step forward to receive your Nobel Prizes from the hands of His Majesty the King.]