Symmetry breaking is an important process in both biological evolution and the evolution of the universe as it moves in time through different eras. Wherever structure becomes more complex, symmetry, or at least the original symmetry, is lost. It is this breakdown of symmetry that theorists are trying to understand in reverse, because at the beginning of the universe, it was perfectly symmetrical; and as it cooled, symmetry breaking took place, creating the more complex and varied world of particle physics.
Concepts of symmetry are very important in physics. Space-time symmetries are all around us in the everyday world: the right and left sides of an animal body, the circular disk of the Sun and Moon, a wallpaper pattern, or even a repeated musical rhythm. And symmetry has become a fundamental way of expressing the laws of physics. If an object has symmetry, it has fewer features to describe and contains less information. We need to describe the theme and the number of ways in which it is repeated, like one house in a row of identical houses. Scientific laws can be put in these terms: Something is the same at all times and all places. Now on the other side of the coin, if a system undergoes a phase transition to a more differentiated state, it has more features to describe and also contains more information. For example, water looks the same in all directions, but when it changes and becomes a snowflake, it becomes more complex. A snowflake crystal looks the same in only six directions. This is an example of symmetry breaking.
Symmetry breaking is equally important in the world of particle physics. It is really the cornerstone concept out of which the theory of supersymmetry was born. Physicists believe that just after the big bang, all of the forces of nature were identical and all elementary particles were the same. But within an instant, symmetry was broken. First the color force between quarks broke away from the electroweak force, and hadrons developed very different masses from leptons. Next, the electroweak force fragmented into two parts—electromagnetism and the weak force … and on and on. I think you get the idea. I don't want to confuse you with further broken symmetries.
All of the elementary particles have a property called spin. It's a hard concept to define because it can only be described within the framework of quantum mechanics. However, to suit our purposes you can think of it as rotation around an axis, like a spinning ball, or the rotation of the Earth.
An important feature of the quantum world and an important bridge between the microcosm and the macrocosm is the connection between the spin of elementary particles and their statistics. It appears as though nature divides the quantum world into two classes of particles. The fermions, matter particles that include all leptons and quarks, are associated with fractional spins, such as 1/2, and bosons, the particles that carry the four forces and have a whole number spin, such as 1 or 2. Each of these two divisions has an accompanying statistical system that helps to define their energy states, which is important to know so that physicists can relate the spin of the particle to its location and how it's interacting with other particles.
One of the strangest features of quantum spin is shown by the behavior of fermions also know as “spin 1/2 particles.” If an object like the earth turns in space through 360 degrees, it returns to where it started. But if a spin 1/2 particle rotates through 360 degrees, it arrives at a quantum state that is measurably different from its starting state. To get back to where it started, it has to rotate through another 360 degrees, making 720 degrees, a double rotation. One way of picturing this is that the quantum particle “sees” the universe differently from how we see it. What we see if we turn through 360 degrees twice are two identical copies of the universe, but the quantum particle is able to discern a difference between the two copies of the universe. This weird notion of a particle having to rotate through another 360 degrees plays an important role in the theory of supersymmetry, as you'll soon see.
Excerpted from The Complete Idiot's Guide to Theories of the Universe © 2001 by Gary F. Moring. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.