What does it all mean?

[This is the pre-edited version of my latest Muse for Nature News.]

Science depends on clear terms and definitions – but the world doesn’t always oblige.

What’s wrong with this statement: ‘The acceleration of an object is proportional to the force acting on it.’ You might think no one could object to this expression of Newton’s second law. But Nobel laureate physicist Frank Wilczek does. This law, he admits, ‘is the soul of classical mechanics.’ But he adds that, ‘like other souls, it is insubstantial’ [1].

Bertrand Russell went further. In 1925 he called for the abolition of the concept of force in physics, and claimed that if people learnt to do without it, this ‘would alter not only their physical imagination, but probably also their morals and politics.’ [2]

That seems an awfully heavy burden for a word that most scientists will use unquestioningly. Wilczek does not go as far as Russell, but he agrees that the concept of ‘force’ acquires meaning only through convention – through the culture of physics – and not because it refers to anything objective. He suspects that only ‘intellectual inertia’ accounts for its continued use.

It’s a disconcerting reminder that scientific terminology, supposed to be so precise and robust, is often much more mutable and ambiguous than we think – which makes it prone to misuse, abuse and confusion [3,4]. But why should that be so?

There are, broadly speaking, several potential problems with words in science. Let’s take each in turn.

Misuse
Some scientific words are simply misapplied, often because their definition is ignored in favour of something less precise. Can’t we just stamp out such transgressions? Not necessarily, for science can’t expect to evade the transformations that any language undergoes through changing conventions of usage. When misuse becomes endemic, we must sometimes accept that a word’s definition has changed de facto. ‘Fertility’ now often connotes birth rate, not just in general culture but among demographers. That is simply not its dictionary meaning, but is it now futile to argue against it? Similarly, it is now routine to speak of protein molecules undergoing phase transitions, which they cannot in the strict sense since phase transitions are only defined in systems that can be extrapolated to infinite size. Here, however, the implication is clear, and inventing a new term is arguably unhelpful.

Perhaps word misuse matters less when it simply alters or broadens meaning – the widespread use of ‘momentarily’ to indicate ‘in a moment’ is wrong and ugly, but it is scarcely disastrous to tolerate it. It’s more problematic when misuse threatens to traduce logic, as for example when the new meaning attached to ‘fertility’ allows the existence of fertile people who have zero fertility.

Everyday words used in science
In 1911 the geologist John W. Gregory, chairman of the British Association for the Advancement of Science, warned of the dangers of appropriating everyday words into science [5]. Worms, elements, rocks – all, he suggested, run risks of securing ‘specious simplicity at the price of subsequent confusion.’ Interestingly, Gregory also worried about the differing uses of ‘metal’ in chemistry and geology; what would he have said, one wonders, about the redefinition later placed on the term by astronomers (any element heavier than helium) which, whatever the historical justification, shows a deplorable lack of self-discipline. Such Humpty Dumpty-style assertions that a familiar word can mean whatever one chooses are more characteristic of the excesses of postmodern philosophy that scientists often lament.

There are hazards in trying to assign new and precise meanings to old and imprecise terms. Experts in nonlinear dynamics can scarcely complain about misuses of ‘chaos’ when it already had several perfectly good meanings before they came along. On the other hand, by either refusing or failing to provide a definition of everyday words that they appropriate – ‘life’ being a prime victim here – scientists risk breeding confusion. In this regard, science can’t win.

Fuzzy boundaries
When scientific words become fashionable, haziness is an exploitable commodity. One begins to suspect there are few areas of science that cannot be portrayed as complexity or nanotechnology. It recently became popular to assert a fractal nature in almost any convoluted shape, until some researchers eventually began to balk at the term being awarded to structures (like ferns) whose self-similarity barely extends beyond a couple of levels of magnification [6].

Heuristic value
The reasons for Wilczek’s scepticism about force are too subtle to describe here, but they don’t leave him calling for its abolition. He points out that it holds meaning because it fits our intuitions – we feel forces and see their effects, even if we don’t strictly need them theoretically. In short, the concept of force is easy to work with: it has heuristic value.

Science is full of concepts that lack sharp definition or even logic but which help us understand the world. Genes are another. The way things are going, it is possible that one day the notion of a gene may create more confusion than enlightenment [7], but at present it doesn’t seem feasible to understand heredity or evolution without their aid – and there’s nothing better yet on offer.

Chemists have recently got themselves into a funk over the concept of oxidation state [8,9]. Some say it is a meaningless measure of an atom’s character; but the fact remains that oxidation states bring into focus a welter of chemical facts, from balancing equations to understanding chemical colour and crystal structure. One could argue that ‘wrong’ ideas that nonetheless systematize observations are harmful only when they refuse to give way to better ones (pace Aristotelian physics and phlogiston), while teaching science is a matter of finding useful (as opposed to ‘true’) hierarchies of knowledge that organize natural phenomena.

The world doesn’t fit into boxes
We’ve known that for a long time: race and species are terms guaranteed to make biologists groan. Now astronomers fare little better, as the furore over the meaning of ‘planet’ illustrated [10] – a classic example of the tension between word use sanctioned by definition or by convention.

The same applies to ‘meteorite’. According to one, perfectly logical, definition of a meteorite, it is not possible for a meteorite ever to strike the Earth (since it becomes one only after having done so). Certainly, the common rule of thumb that meteors are extraterrestrial bodies that enter the atmosphere but don’t hit the surface, while meteorites do, is not one that planetary scientists will endorse. There is no apparent consensus about what they will endorse, which seems to be a result of trying to define processes on the basis of the objects they involve.

All of this suggests some possible rules of thumb for anyone contemplating a scientific neologism. Don’t invent a new word without really good reason (for example, don’t use it to patch over ignorance). Don’t neglect to check if one exists already (we don’t want both amphiphilic and amphipathic). Don’t assume you can put an old word to new use. Make the definition transparent, and think carefully about its boundaries. Oh, and try to make it easy to pronounce - not just in Cambridge but in Tokyo too.

References
1. Wilczek, F. Physics Today 57(10), 11-12 (2004).
2. Russell, B. The ABC of Relativity, 5th edn, p.135 (Routledge, London, 1997).
3. Nature 455, 1023-1028 (2008).
4. Parsons, J. & Wand, Y., Nature 455, 1040-1041 (2008).
5. Gregory, J. W. Nature 87, 538-541 (1911).
6. Avnir, D., Biham, O., Lidar, D. & Malcar, O. Science 279, 39-40 (1998).
7. Pearson, H. Nature 441, 398-401 (2006).
8. Raebinger, H., Lany, S. & Zunger, A. Nature 453, 763 (2008).
9. Jansen, M. & Wedig, U. Angew. Chem. Int. Ed. doi:10.1002/anie.200803605.
10. Giles, J. Nature 437, 456-457 (2005).