06 August 2005

Predictions by Darwin – I – The Popular Image of Scientific Prediction

A persistent claim in the creationist literature is that evolution is not scientific because it doesn’t make testable predictions. I think that there are responses that can be made to this which have not been made a lot, but to justify them, I need to lay some foundations. In this post and the next, I’ll discuss what I think the problem is, and then I’ll try out some predictions that I think should be cited.

Of course, the argument that the neo-Darwinian theory does make testable predictions has been made at length, e.g. by Douglas Theobald. In addition to providing (considerable!) detail on evolutionary evidence, Theobald sketches out the basic form of the scientific method and the characteristics expected of testable hypotheses. This is all fine, but I fear that it hews to a narrow view of how scientists come to be confident about their theories, in that it emphasizes the concept of the critical prediction: The scientist, having formulated a hypothesis to account for the existing empirical data, including the problematic observations that necessitate a new theory, predicts that a certain critical experiment will have a particular result. Confirmation is thus dramatic – even cinematic. (Think of Paul Muni as Pasteur, returning to the farm to see whether his vaccinated sheep have survived.)

The modern image of the critical experiment is, I think, dominated by Einstein’s prediction of the bending of light by the gravitational field of the sun.

Einstein developed the General Theory of Relativity over the period 1907 to 1915. Although the argument was basically theoretical, justified as putting gravity in the form of a field theory, he did highlight its power for the explication of anomalous data. In particular, he was able to account for the perihelion advance of Mercury, itself a very public problem with the purely Newtonian model of the solar system. Mercury has a more elliptical orbit than most planets, and for some time it had been known that its closest approach to the Sun was shifting. (Imagine standing on the Sun and marking where in the zodiac Mercury appears to be at perihelion. That apparent position marches along the zodiac, at the rate of 0.04% of a constellation per century.) To account for this, astronomers had postulated the perturbing influence of a planet closer to the Sun, leading to well-publicized searches for (and some erroneous discoveries of) the elusive planet Vulcan.

In general relativity, the perturbation of Mercury’s orbit comes from an effective “tug” by the Sun, reflecting differences between Newtonian and Einsteinian gravity that appear when the gravity field is strong. Because Mercury is so close to the Sun and has such an elliptic orbit, it probes the gravitational field of the Sun more deeply than any other planet, and experiences the relativistic deviations from Newtonian gravitation more than the others.

Einstein’s 1915 paper included another observational test. In both Newtonian and Einsteinian theories, light rays passing close to the Sun will be deflected, so that the position of distant stars will appear to shift as the Sun passes by them. Of course, the only time you can see stars very close to the edge of the Sun is during a solar eclipse. Thus, there was a major expedition to plant instruments in the path of totality of the 1919 solar eclipse, on an island off the west coast of Africa, to measure the effect. The famous Arthur Eddington, of Cambridge University, was in the lead. Einstein predicted a deviation twice the Newtonian value (after correcting a mistake in the 1915 paper). Lo and behold, that was the result Eddington found and trumpeted about the world.

Most physicists, though, were already convinced by the 1915 paper. And Einstein himself was not so hung up on the experiment. When asked how he would feel if the confirming deviation was not measured, he said, “I should be sorry for the dear Lord. But the theory is correct.”

In my next post, I will talk about less dramatic, but more common, forms of scientific prediction.


There's a good review of the Eddington expedition in Donald Fernie's Marginalia column in American Scientist.


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