A report in the Guardian recently quoted a British solicitor (a lawyer) speaking about the successful outcome of his client’s case: “The next stage will be to find out what liability there is and the quantum of damages”. Such is the association of the word with nuclear physics that one’s first instinct is to wonder what the lawyer thought he was saying.
But, as every legal person knows, and as even a cursory glance at the dictionary confirms, quantum has been a perfectly good word in English for nearly four centuries, in exactly the sense in which the solicitor was using it. A quantum is an amount — the word comes from the Latin quantus, how much or how great, from which we also get quantity. In law it means a sum of money, specifically one due by way of damages, and the phrase quantum of damages is long established.
It’s never been what you might call common outside the law, though Robert Burns, for example, used it in one of his poems: “I waive the quantum o’ the sin” and the introduction to Travels Through France And Italy by Tobias Smollett remarks that it includes “a respectable quantum of wisdom fit to become proverbial”.
Scientists started to use it in English about 1910, following the pioneering papers by Max Planck and Albert Einstein in the first years of the century. In German, the word began in compounds like Elementarquantum, an elementary or fundamental quantity of energy (Planck borrowed the word direct from Latin). The key idea was that energy could only exist in discrete amounts or chunks: quanta. To shift from one energy level to another required a sudden change, for which physicists coined quantum jump in the 1920s. Such a change was by normal standards an imperceptibly tiny alteration in energy levels.
It was really only a short step — a quantum jump, perhaps — between the idea of a sudden change and a sudden large change. That’s the meaning of quantum leap, which sounds like total nonsense to anybody who knows about quanta. But before you complain about non-specialists using it wrongly, consider the fact that its first recorded use in 1970 was in — the science journal New Scientist.