By Ron Cowen
The 2008 Nobel Prize in physics has been awarded to three theoretical physicists for advances involving the concept of symmetry breaking. The theory highlights how three of the four seemingly disparate forces in nature fall under the same umbrella. The work forms a cornerstone of the standard model of particle physics.
Half of the $1.4 million prize goes to Yoichiro Nambu of the University of Chicago’s Enrico Fermi Institute. He began formulating his mathematical description of a type of symmetry violation, known as spontaneous broken symmetry, as early as 1960.
The other half is shared by Japanese researchers Makoto Kobayashi of the High Energy Accelerator Research Organization in Tsukuba and Toshihide Maskawa of KyotoUniversity’s Yukawa Institute for Theoretical Physics. Kobayashi and Maskawa discovered the origin of another type of symmetry violation that had been observed but not explained. Their work successfully predicted that nature must have at least three families of quarks, which are the fundamental building blocks of matter such as neutrons and protons.
The accomplishments of the winners tie in to the “most essential ideas in our understanding of modern physics,” says physicist Brian Greene of ColumbiaUniversity in New York City.
“The basic laws of physics seem to be incredibly symmetric,” Greene adds, “but to get the kinds of things that we’re used to in the word around us — stars, planets and people — that symmetry needs to be reduced in order for that kind of structure to emerge.”
It’s like adding paint to a blank canvas, notes Greene. On a bare canvas, every point is the same as every other — there’s complete symmetry. But to see the beauty of a painting emerge, a painter adds splashes of color, which reduces the symmetry, “and that’s what needed to happen in the universe,” he says. The cosmos began as a hot uniform sea of particles in which all the laws of physics had melded into one, but transformed and cooled into a rich tapestry.
Nambu discovered that symmetries in nature can be hidden — and spontaneously broken. That idea of hidden symmetries has now become a guiding principle in understanding nature at its deepest level, says Turner.
One way to understand spontaneously broken symmetry is to imagine a round dinner table at which the place settings are symmetric. There’s a napkin to the left and right of each dinner plate, so either side looks the same. But once a diner reaches for a napkin to the left, he determines the choice for everyone at the table, and the symmetry is broken.
In the early 1960s, Nambu was studying the phenomenon of superconductivity, in which electric current, below a certain temperature, suddenly flows without any resistance. Below this critical temperature, electrons, which normally repel each other, abruptly bind up in pairs. It took Nambu two years to develop the concept of spontaneous symmetry breaking in order to explain how superconductivity works. He then rapidly applied the idea to particle physics.
“Nambu was the first to apply the idea of a spontaneously broken symmetry in elementary particle physics — that is, a symmetry that is an exact property of the underlying equations of the theory, but is not realized in the solutions of these equations, and hence not easily apparent in the properties of elementary particles,” says Steven Weinberg of the University of Texas at Austin, who shared the 1979 Nobel Prize in physics. Nambu’s idea “has proved crucial in understanding the properties of particles that interact through the strong nuclear force, in particular pi mesons,” he says, adding that it has also helped unify the weak and electromagnetic interactions.
Nambu discovered a mechanism embedded in the laws of physics that allowed the character of symmetries to change as the universe evolved. In technical parlance, Nambu introduced a scalar field, which Greene likens to a ubiquitous mist. “We don’t know it’s there, it has no manifest features, but the laws of physics know about that mist and it plays the role of reducing symmetry,” says Greene.
“His study of this broken symmetry not only paved the way for hidden symmetry in particle physics more broadly,” Turner says, “but also explained why the pi meson is so much lighter than all the other mesons.”
Kobayashi and Maskawa examined a very different sort of symmetry violation. They were trying to explain a set of puzzling experiments, first performed by James Cronin and Val Fitch in the mid 1960s. In those experiments, subatomic particles called K mesons didn’t behave the same if the particles were replaced by their antiparticles and the same experiment took place in a looking-glass universe, where right and left were interchanged. (Cronin and Fitch went on to win the 1980 Nobel Prize for the experiment.)
In 1972, Kobayashi and Maskawa found that this puzzling asymmetry could be explained if the family of elementary particles was expanded to include at least three families of quarks. At the time, only three quarks were known — up, down and strange. The up and down form one family. Missing members of the other families were subsequently discovered in experiments. The charm quark (partner of the strange quark) was discovered in 1974; the bottom quark (1977) and the top quark in (1994) make up the third family.
Their theory also suggested that physicists could observe a symmetry violation in another type of elementary particle, the B-meson, which is ten times heavier than a K meson, or kaon. Because the broken symmetry involving the B meson occurs rarely, physicists built giant “B factories,” one at the Stanford Linear Accelerator Center in California and the other at the KEK Accelerator Laboratory in Tsukuba, Japan. These factories each produced more than a million B mesons a day. In 2001, both experiments confirmed the B meson violation that Kobayashi and Maskawa had predicted nearly three decades earlier.
