The physicist Yoichiro Nambu (南部 陽一郎) was born #OTD in 1921. He developed a theory of spontaneous symmetry breaking to explain superconductivity, paving the way for electroweak symmetry breaking via the Higgs mechanism in the Standard Model.
Image: AIP Emilio Segre Archives

Nambu gave a simple example of spontaneous symmetry breaking. Folks in a crowd might be looking in all different directions, with no direction preferred. Occasionally, one person's choice propagates through the crowd. Soon everyone is looking in the same direction. That's SSB.

In physics we try to formulate theories (the rules for a physical system) in a way that makes symmetries manifest (obvious to anyone reading the rules). For instance, compatibility with special relativity means the rules should take the same form in any inertial reference frame.

So we're not surprised when a physical system is in a configuration that is consistent with the symmetries built into its rules! If there's no reason for a crowd to all look in some direction, we're not surprised if everyone is more or less looking in random directions.

Nevertheless, sometimes a system just ends up in a configuration that seems to express a preference not apparent in its rules. Someone in the crowd gasps, and interactions between people quickly lead to them all looking in the same direction, even if there was nothing to look at.

That's why it's called *spontaneous* symmetry breaking. The rules of the system are agnostic about something — the theory has a symmetry — but the dynamics of the system lands you in what appears to be a configuration with a preference.

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