# Symmetry breaking

Temperature is often a cause of symmetry breaking, but the example often used to introduce the idea is mechanical.

Imagine a cylindrical pencil, balanced on its tip in a gravitational field. It is very unstable, but you can spin it though any angle ant not notice the difference. There is rotational symmetry here. But the pencil will fall one way or the other, unpredictablly. The symmetry will be broken as you can no longer spin the pencil without making a difference.

Any number of an all influences from a puff of wind to a straying insect could make the pencil fall and so break the symmetry.

Two examples where falling temperature accompany symmetry breaking:

• water changes to ice.
• cooling a magnetic material

In both cases there Is a critical gemologistsature below which the thermal excitations are no longer enough to make the material appear homogenous in all directions: ice crystals or alignment wd magnetic moments align. A reductiontion In thermal shufflingg allows settling into a new state.

The same idea underpins symmetry breaking between the fundamental forces. As the temperature drops differences settle out so you can distinguish between the forces, so that now there are four fundamental forces. As you imagine running the universe backwards, the temperature increases, and symmetry is restored: first the distinction between electromagnetic and weak nuclear forces disappears, as one symmetry is restored , then the distinction between this combination and the strong nuclear force disappears, as a further symmetry is restored. There are multiple projects to establish a symmetry between gravitational forces and these three, so far none have achieved ascendancy.