What is spontaneous symmetry breaking?
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Spontaneous symmetry breaking is a physical phenomenon by which something balanced becomes permanently unbalanced. One example might be a ball sitting on top of a hill: It is exactly balanced, but if the ball suddenly rolls down the hill to the bottom, then the system is no longer balanced; because the ball will not roll uphill by itself, the system is now permanently unbalanced. Most of us can think of symmetry from, say, the example of paper folding. From a more general point of view, symmetry can be viewed as a measure of the order or complexity of a system: for example, a crystal.
Theoretical cosmologists hypothesize that the fundamental forces of the universe split off from one another in a form of spontaneous symmetry breaking. If a single, unified force existed with a certain “symmetry” just after the Big Bang, and if that symmetry were somehow “broken” so that the unified force were fractured, then the result might be several fundamental forces. Also, there would be a huge release of stored-up energy that might power hyperinflationary expansion or other kinds of activity in the early universe.