What’s in a name?

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    Inertia, entropy, Hilbert space, angular momentum, the Schwarzschild metric… Physics abounds with jargon, technical terms, and specialised vocabulary—something of a double-edged sword. One the one hand, the incomprehensibility of much scientific discourse to all but the experts in the field can serve to further isolate the ideas of physics from mainstream culture. On the other, a specialised language facilitates rapid and effective communication of ideas, whilst minimising the risk of confusion.

    So where does all this come from? What are the sources of our most well-worn and treasured physical terms? And what can the background to these words tell us about the history of their associated concepts?

    The attachment of particular words to certain concepts is, in the main, a lengthy process of historical accident. At the time of coining a term, researchers may not even have a clear idea of what concept it’s supposed to express—indeed, the working out of such definitional issues is itself a key stage of conceptual clarification. Take 17th-century physics for example, where an almighty mess of words were used in relation to a bundle of closely related ideas: force, mass, momentum, inertia, weight, moment, motion, matter, body, extension, speed, velocity, impulse, acceleration… Such terminological overlap is bound to cloud clear-cut comparisons of different claims—how to know if a conservation law is correct, when it’s unclear what is being conserved? As more and more agreement is reached upon which words are to be used for which ideas, we can see our more modern physics filtering out from the distillation of older ideas.

    However, this is not a one-way street; the use of certain words is intimately bound up with the physics being worked with. Partly, this is because the theories themselves provide the conceptual distinctions needed for clear terminology. It’s only with a theory of gravity, for example, that a systematic distinction between weight and mass can be drawn: the former as the force felt by a given body in a gravitation field, and the latter as the resistance of the body to motion. The working-out of theories can also illuminate where a single word is being used to describe two quite distinct phenomena, perhaps because of a superficial similarity. The physics of Leibniz (1646–1716) spoke of two kinds of ‘force’: the vis viva, or ‘living force’, as against the vis mortua, or ‘dead force’. However, to modern eyes these are quite different ideas (and neither of them is a force in the modern sense either): the vis viva is the mass times the square of the speed (so twice the kinetic energy), whilst the vis mortua is a more general idea of an ability to move—roughly equivalent to modern potential energy, but extended to include things like centrifugal force. Once we have succeeded in a theoretical separation of concepts, a linguistic separation is the natural next step.

    However, more recent physics tends to specifically invent the words it wants to use, probably because many of the concepts in modern physics have no everyday analogue. Nevertheless, it is still eminently possible for the meanings of words to migrate far beyond their creators’ original intentions—something which helps explain the strange etymology behind many modern terms. Why, for example, should the disorder of a system—its ‘entropy’—have a literal meaning of ‘a turning towards’? Well, at the time of invention, the entropy was just another variable in the thermodynamic equations which describe how heat, temperature and other forms of energy interact. Since it was simply a variable, on a par with energy, heat, or temperature, the term’s inventor Rudolf Clausius (1822–1888) just invented a term to sound like energy—only including the Greek word trope, for transformation, to indicate entropy’s special quality of always increasing as the system transforms over time. It was only the development of statistical mechanics (the study of how thermodynamics can be explained by the microscopic motion of atoms and molecules inside substances) that recognised that Clausius’ entropy was a large-scale representation of something microscopic and more fundamental—the underlying disorder in a system.

    The phrase-coining has continued apace into modern times. Ironically though, as physics becomes ever more abstract, the fashion for appropriating words wholesale from everyday English has grown. Take the quarks (a name itself taken from the sound made by ducks): there are the strange quarks, charm quarks, truth quarks, beauty quarks… Even the classification of quarks according to ‘flavour’ does its best to make them seem homely. Of course, such terms aren’t a problem—no-one is tempted to mix and match their everyday meanings (licking a quark would be of minimal experimental value). Indeed, the linguistic dislocation of ordinary language in amongst the arch mathematical formalisms arguably helps guard against straightforward assumptions regarding the nature and behaviour of phenomena in the distinctly odd quantum realm. At any rate, as physics carries on growing, its language use will do so too—and the words used will continue to be an insightful guide to what is happening in its theories.

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