Symmetry breaking is a term that comes up a lot in theoretical and quantum physics. In a way it is another branch of chaos theory. Symmetry breaking is when an event happens sometimes one way and sometimes the other (if it is a binary choice) and to an observer it appears random almost arbitrary what the outcome will be. But what is actually causing the diverging effects are tiny fluctuations outside the observers sight which means that if these could be known the outcome would be predictable. The reason it is known as symmetry breaking is because for such a small fluctuation to trigger the event the system must be poised to act anyway, like a ball balanced on a spike, which must be symmetrical otherwise it would fall. The breaking of the symmetry is the tiny push required to send this ball toppling and in doing so chooses a side to fall off of.
Inside an optical cavity light, normally laser light, forms standing waves. These waves are only formed when the the returning reflection matches the frequency of the original and the phase difference remains constant between them. In this state the light is symmetric but due to the strenuous conditions it is not stable here for long periods. Many of the standing waves formed (when the light has a range of frequencies) collapse into travelling waves and only one set frequency of light and its reflection ever remain to form the solitary standing wave. This symmetry breaking has been predicted by theory since 1980 but has only just been proven experimentally by supplying an optical resonator with perfectly equal power levels of light on both sides. The symmetry breaking demonstrated here has been suggested for use in information technology and could be used to integrate optical diodes into computer chips. There is also a possibility the delayed effect that changes to this system have could be used as store information in an optical flip-flop circuit (a circuit which can store two states at a time). In a more general sense the proof of symmetry breaking in a resonating state like the light standing wave has implications across all wave states whether they be acoustic or quantum mechanical.