Today the 2008 Nobel Prize winners were announced for physics. It was given to three physicists who described something called symmetry breaking. Since most people don’t know what symmetry breaking is, but people remember me writing about group theory and symmetry, I’ve been getting questions about what it means.
I don’t pretend to completely understand it; or even to mostly understand it. But I mostly understand the very basic idea behind it, and I’ll try to pass that understanding on to you.
We’ll start with the idea of symmetry. Intuitively, we think of symmetry as a situation where something is identical on both sides of a line. Another way of saying that is that reflecting it in a mirror won’t change what we see. Symmetry is really something much more general than that. Mathematically, we say that symmetry is an immunity to transformation. What that means is that for something symmetric, there is some kind of transformation you can do to it, and the result is indistinguishable from what you started with.
The intuitive symmetry – mirror (or reflective) symmetry – is one example of this: flipping a reflectively symmetric image around a line in indistinguishable from the original image. Another easy example is translational symmetry: imagine that you’ve got an infinite sheet of graph paper. If you move that paper to the left the width of one square, you can’t tell that it was moved: it’s completely indistinguishable.
So what is symmetry breaking?
Sometimes you have a symmetric configuration which has to go through a transformation that results in it becoming non-symmetric. A canonical example of this is a ball on a hill. Imaging a perfectly round hill, with a spherical ball sitting on top of it. It’s completely symmetric reflexively and rotationally. But in gravity, it’s very unstable. Eventually something is going to perturb it, and the ball is going to roll down the hill. Once it does that, it’s no longer symmetric. The symmetry was broken by
the motion of the ball. This is called spontaneous symmetry breaking: the system has what is in some sense an inevitable state transition, and after that state transition, the system is no longer symmetric.
In deep physics and cosmology, there are a lot of basic symmetries. There are also a lot of things that appear like they should be symmetric, but aren’t. For example, if the universe started with a big bang, then at some moment immediately after the big bang, space was uniform. But that basic symmetry broke; space is now very non-uniform.
From looking at some of the basic rules of how things work, and it
seems like the quantities of matter and antimatter should be equivalent, which reflects a basic symmetry in the structure of the basic particles that make up the universe. But from what we can observe, that’s very much not true: there’s a lot more matter than antimatter. At some
point when particles were condensing out of the energy cloud after the big bang, the symmetry broke, and we wound up with a lot more matter than
Finally, the basic fundamental forces in the universe appear to be related on a very deep level. They’re really the same thing, but operating at different scales and different energy levels. At very high energy levels, electromagnetic forces, and the two atomic forces are all really the same thing. There’s a deep symmetry between them. But as the energy level of
the environment goes down, eventually they split, and become distinguishable. The symmetry breaks, and we get different forces.
The Nobel prize in physics this year was given to three physicists. One, Yoichiro Nambu, worked out the mechanism for spontaneous symmetry breaking in subatomic physics. The other two, Makoto Kobayashi and Toshihide Maskawa, worked out the origin of the broken symmetry. (Don’t ask me the difference between the mechanism and the origin in this case; that’s well beyond my understanding of how symmetry-breaking applies to physics. I’m just paraphrasing the press release.)