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An italic capital S, the integral of the Lagrangian over time.
Also known as: Action integral, Action, Hamilton's principal function
Action has units of energy times time — the same units as Planck's constant, which is not a coincidence: ℏ is the quantum of action, and that is the deepest statement of what quantisation means. Nature takes the path for which the action is stationary.
Denotes the action: the time integral of the Lagrangian along a path.
Contexts: Laboratory
Shares its units with the Planck constant, which is itself a quantum of action; this is the direct link between classical and quantum mechanics.
Contexts: Laboratory
Collides with entropy, also an italic capital S, and with the Poynting vector.
Contexts: Laboratory
An italic capital letter S, a double curve reversing at the middle.