# Spontaneous Symmetry breaking-weinberg's chair

1. Sep 24, 2009

### krishna mohan

In Weinberg's book, Quantum theory of fields-II, he talks about a chair in the chapter on spontaneous symmetry breaking. He says that, for a chair, a state with a definite l value is not stable but a state with a definite orientation is.

I do not understand what he means.

An l state can be disturbed by a very small perturbation.

But, for an isolated chair in vacuum, a small perturbation is enough to change its orientation.

What is the meaning of Weinberg's statement?

2. Sep 24, 2009

### hamster143

If you subject a definite-orientation chair to the action of a weak field, it will change its orientation in a proportionally weak manner.

A definite-angular-momentum chair is a superposition of states with different orientations. All it takes is a tiny external field that couples differently to different orientations, to induce the variation in energies of this states on the order of several times $$\hbar^2/I$$, and the chair will cease to have definite angular-momentum. A tiny change in external fields leads to a drastic change in angular momentum spectrum.

3. Sep 26, 2009

### krishna mohan

Yes..that does make it clearer..thanks!