Vulcan vs Neptune

There’s an argument that plays out every so often in linguistics the goes as follows:

Critic: This data falsifies theory T.
Proponent: Not necessarily, if you consider arguments X,Y, and Z.
Critic: Well, then theory T seems to be unfalsifiable!

This is obviously a specious argument on the part of the critic, since unfalsified does not entail unfalsifiable, but I think it stems from a very understandable frustration—theorists often have an uncanny ability to wriggle free of data that appears to falsify their theories, even though falsificationism is assumed by a large majority of linguists. The problem is that the logic falsificationism, while being quite sound, maybe unimpeachable, turns out to be fiendishly difficult to apply.

At its simplest, the logic of falsificationism says that a theory is scientific insofar as one can construct a basic statement—i.e., a statement of fact—that would contradict the theory. This, of course, is an oversimplification of Karl Popper’s idea of Critical Rationalism in a number of ways. For one, falsifiability is not an absolute notion. Rather, we can compare the relative falsifiability of two theories by looking at what Popper calls their empirical content—the number of basic statements that would contradict them. So if a simple theoretical statement P has a particular empirical content, then the conjunction P & Q will have a greater empirical content, and the disjunction P v Q will have a lesser empirical content. This is a useful heuristic when constructing or criticizing a theory internally, and seems like a straightforward guide to testing theories empirically. Historically, though, this is not the case, largely because it is often difficult to recognize when we’ve arrived at and accurately formulated a falsifying fact. In fact, it is often, maybe always, the case that we don’t recognize a falsifying fact as such until after one theory has been superseded by another.

Take for instance the case of the respective orbits of Mercury and Uranus. By the 19th century, Newtonian mechanics had allowed astronomers to make very precise predictions about the rotations of the planets, and based on those predictions, there was a problem: two of the planets were misbehaving. First, it was discovered that Uranus—then the last known planet from the sun—wasn’t showing up where it should have been. Basically, Newton’s mechanics predicted that on such and so day and time Uranus would be in a particular spot in the sky, but the facts were otherwise. Rather than cry “falsification!”, though, the astronomers of the day hypothesized an object on the other side of Uranus that was affecting its orbit. One such astronomer, Urbain Le Verrier was even able to work backwards and predict where that object could be found. So in September of 1846, armed with Le Verrier’s calculations, Johann Gottfried Galle, was able to observe an eighth planet—Neptune. Thus, an apparent falsification became corroboration.

I’ve previously written about this story as a vindication of the theory first approach to science. What I didn’t write about, and what is almost never discussed in this context is Le Verrier’s work on the misbehaving orbit of Mercury. Again, armed with Newton’s precise mechanics, Le Verrier calculated the Newtonian prediction for Mercury’s orbit, and again^[1]Technically though, Le Verrier’s work on Mercury predated his work on Uranus Mercury didn’t behave as expected. Again, rather than throw out Newtonian mechanics, Le Verrier hypothesized the planet Vulcan between Mercury and the sun, and set about trying to observe it. While many people claimed to observe Vulcan, none of these observations were reliably replicated. Le Verrier was undeterred, though, perhaps because observing a planet that close to the sun was quite tricky. Of course, it would be easy to paint Le Verrier as an eccentric—indeed, his Vulcan hypothesis is somewhat downplayed in his legacy—but he doesn’t seem to have been treated so by his contemporaries. The Vulcan hypothesis wasn’t universally believed, but neither does it seem to be the Flat-Earth theory of its day.

It was only when Einstein used his General Theory of Relativity to accurately calculate Mercury’s orbit, that the scientific community seems to have abandoned the search for Vulcan. Mercury’s orbit is now considered a classical successful test of General Relativity, but why don’t we consider it a refutation of Newtonian Mechanics? Strict falsificationism would seem to dictate that, but then a strict falsificationist would have thrown out Newtonian Mechanics as soon as we noticed Uranus misbehaving. So, falsificationism of this sort leads us to something of a paradox—if a single basic statement contradicts a theory, there’s no way of knowing if there is some second basic statement that, in conjunction with the first, could save the theory.

Still, it’s difficult to toss out falsification entirely, because a theory that doesn’t reflect reality, may be interesting but isn’t scientific.^[2]Though sometimes, theories which seem to be empirically idle end up being scientifically important (cf. non-Euclidean geometry) Also, any reasonable person who has ever tried to give an explanation to any phenomenon, probably rejects most of their own ideas rather quickly on empirical bases. We should instead adopt falsificationism as a relative notion—use it when comparing multiple theories. So, Le Verrier was ultimately wrong, but acted reasonably—he had a pretty good theory of mechanics so he worked to reconcile it with some problematic data. Had someone developed General Relativity in Le Verrier’s time, then it would have been unreasonable to insist that a hypothesized planet was a better explanation than an improved theory.

Returning to the hypothetical debate between the Critic and the Proponent, then, I think a reasonable albeit slightly rude response for the proponent would be “Well, do you have a better theory?”