14 August 2005

Predictions by Darwin – III – “Cross your fingers” and “I should have thought of that”

I have been looking at scientific predictions that are outside the popular image of critical experiments, for which the model was Einstein and the 1919 solar eclipse expedition. I argued that the future-directed form of this model distracted from the very useful concept of retrodiction, which is often the form of validation used when new theories are presented. I would claim that there are other valid means by which scientists become confident about theories.

Part of the problem of the critical experiment model is that it focuses on theories that are posed primarily to address anomalous or problem data. But more frequently, scientific hypotheses are extensions, augmentations, or more modest revisions of the large-scale theoretical structure within which the scientist is working, not addressing problem data. Some hypotheses only address a subset of the empirical problems with the theory. And some are only posed for fundamentally philosophical, non-empirical reasons, to mesh with the philosophical leanings of the scientist, or, most important, to make the structure more productive of new empirical questions

Are we only talking about background science, rather than revolutionary science as defined by Kuhn? I think not. Consider the case of Copernicus and the heliocentric theory, which was what Kuhn used as the archetype of revolutionary science.

While there were empirical problems with Ptolemaic theory (predicted planetary positions that weren’t right), the heliocentric theory didn’t resolve them. To a large extent, this was because Copernicus retained the model of circular orbits. It wasn’t until Kepler’s introduction of elliptical orbits that there was any significant improvement. Indeed, Copernicus wasn’t particularly concerned about collecting data to support his theory. Historical studies indicate that what observations he made were done to determine specific orbital parameters.

What the heliocentric theory did was provide a framework that Copernicus considered more sensible than the geocentric theory. He was much more comfortable putting the big Sun at the center because it was so much bigger than the Earth, and so much more glorious. There are plenty of arguments suggesting that he was a closet follower of Hermetic philosophy, which virtually deified the Sun. It also relieved him of having to think of the vast celestial spheres spinning at incredible speed, in order to account for their diurnal motion. Physical plausibility was as important as “saving the appearances”.

This importance of physical plausibility was later emphasized by Kepler in the lengthy letter that he wrote in rebuttal to Ursus’ De astronomicis hypothesibus. This was a serious treatise on the meaning of astronomical hypotheses and the methods one should adopt for deciding among them. It argues that hypotheses must both predict phenomena accurately and be physically plausible. It was only published in the 1858 volume of his works, but there is a translation and analysis of it in Nicholas Jardine’s The Birth of History and Philosophy of Science: Kepler’s A Defence of Tycho against Ursus with Essays on Its Provenance and Significance (Cambridge University Press, 1984). There is also a very nice discussion of these arguments in Owen Gingerich’s recent book, The Book Nobody Read (Walker & Co., 2004).

Of course, what was plausible for Copernicus was not plausible for others. For them, it was physically plausible that, if the Earth were rotating, they should be able to feel the motion. It was physically plausible that falling objects would, in their natural motion, fall toward the center of the universe, which meant that the Earth needed to be at that center. It also seemed implausible (although here I think we fairly part from the physical) that the sinful Earth would be a part of the celestial planetary scheme.

So if it wasn’t an improvement in predicting planetary data or a clear triumph of physical plausibility that made heliocentrism so attractive to Copernicus, then what was it? The current thinking – an argument actually made decades ago by Gingerich and others – is that Copernicus preferred the heliocentric model because it expanded his explanatory reach. It enabled Copernicus to calculate things that couldn’t be calculated before, such as the order of the planets. In the Ptolemaic model, the order was decidedly arbitrary; there was no particular reason for Jupiter to precede Saturn. Mercury and Venus were figured to be between the Earth and the Sun because they were never seen far from the Sun. But the rest was pretty much made up. In the heliocentric model, though, Copernicus could work out a geometric proof that required one particular order.

Here is an example, then, of revolutionary science being developed more for philosophical preference (increased explanatory power and a tip of the hat to Hermes Trismegistus) than to account for problem data. Predictive power in such cases takes different forms.

Copernicus did not make predictions. He did, however, note empirical problems that arose as a result of the new system, which would have to be resolved somehow. For example, even Aristotle had known that, if the Earth goes around the Sun, then we ought to see an annual oscillation in the apparent positions of stars (parallax). Copernicus figured that the fact that astronomers of his day didn’t see parallax had to have some answer, but he went ahead without having it. (He figured that it was some combination of imprecision in the data with a much greater distance to the stars than previously assumed. We now know that the answer is mostly the latter.) As a prediction, this is more like crossing your fingers. It has the form: “Such-an-such an empirical problem must eventually be explainable by some other observation or experiment, the nature of which I can guess at, but the answer I can’t.” (It is often the case that predictions like these aren’t even raised by the originator of the theory, precisely because they are problems that she hopes to deal with on her own terms later on.)

Another odd form of prediction is rooted in “virtual” empirical issues. Later investigators, not engaged in testing the theory per se, discover new phenomena that are surprisingly well explained by it. These predictions are of the form “If Copernicus is right, then we should have expected to see this, although we didn’t realize it until now.” An example is the fact that, as Galileo discovered, Venus shows phases like the moon, but Jupiter doesn’t. Of course, if Copernicus had thought it through, he probably could have predicted such an effect, and Galileo would then have been performing one of those cinematic critical experiments. But that’s not how it happened, and not how it usually happens.

In my next post in this series, I intend to identify some very significant Darwinian “predictions” that fall in these last two categories.


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