Over at Starts With a Bang, Ethan has a posted How Good is Your Theory? Open Thread I, in which he categorizes the spectrum of theories, from Scientific Law at one end, through Validated, Speculative and on to Ruled Out at the other end. My reaction is like that of Mammy’s in Gone With the Wind: it ain’t fittin’. It ain’t fittin’, it ain’t fittin’, it just ain’t fittin’.

First of all, theories do not grow up to be laws, which is one way this spectrum would be interpreted.

Scientific Law: This is really an elite category, reserved for the most thoroughly tested, rock-solid theories and ideas that we have. These are theories that have stood the test of time, as well, making many new predictions that have all been confirmed experimentally and observationally, where there’s practically no room for dispute other than making extensions or variations to the law itself.

If there are variations, then how rock-solid is it? No, that’s not the criterion. A law denotes a straightforward mathematical relationship, i.e. an equation. A law is closer to being synonymous with equation than it is with rock-solid theory. We have Ohm’s law, but we also have non-ohmic devices. We have Newton’s law of gravitation, but we know that fails under some conditions, and is just a subset of relativity. Laws have limits to their applicability.

We also have far-reaching and very well-established theories that are not called laws, simply because there is no simple equation associated with it. The theory of evolution is no less well-established than many laws that exist, for example. And this is a common debating tactic, to denigrate the theory of evolution because it isn’t a law, to make it sound like it has less support and more open to doubt.

The second objection I have is that the spectrum is actually two-dimensional. Ethan mentions how some speculative theories are not testable, or at least not currently testable. If it isn’t testable it really shouldn’t be considered a theory at all, but even ignoring that issue, this points toward the idea that there is a spectrum of the quality of a theory as well. Some theories are better than others, because they do a better job of precisely predicting behavior and/or explain a wider range of phenomena. Some very elegant theories are on the “Ruled Out” side of things, because they were the proverbial beautiful theory slain by the ugly fact, but by virtue of being testable, they were still higher quality than some other “theories” that cannot be (easily) checked. Phlogiston was a better-constructed theory than Brontosauruses being thin at one end, thicker in the middle, and thinner again at the other end, even though the former is ruled out and the latter is true except for a naming issue. The Balmer, Lyman, Paschen et. al series of Hydrogen are less complete than the Rydberg formula and Bohr model, and the Bohr model is tossed into the “ruled out” heap in favor of quantum mechanics. So the breadth of a theory’s reach has to be considered as well — a model that explains one thing is not as highly regarded as one that encompasses many phenomena.

The true spectrum is in the amount of evidence which supports the theory, weighed against evidence that contradicts it. Keeping in mind, of course, that contradiction comes in two flavors: those which kill the theory outright, and those which narrow the boundaries of the theory or require it to be modified. The spectrum of quality is similar to the high jump or pole vault — set the bar at some level of prediction/falsification, and then see if you can make it over the top. Nobody will be impressed by a theory that makes only obvious predictions that are trivially fulfilled (OK, excepting the fans of John Edward and his ilk, who are utterly impressed by “You’ve lost someone recently” at a meeting of people wanting to talk to recently-departed loved ones). If you don’t make predictions, you don’t get to play.