Under the Microscope:The term theory has a different meaning in science to its everyday meaning. A scientific theory is an official consensus scientific explanation of how some part of the natural world works, supported by extensive evidence, eg the theory of evolution by natural selection, writes Prof William Reville.
In the everyday world the term theory would be more properly called hypothesis, ie a best guess at an explanation, eg "I have a theory about who killed JFK".
As science progresses, scientific theories are often modified to account for new evidence, and sometimes theories are overturned and replaced by new theories. But, modifying, and especially overturning, a scientific theory is a large matter that is only done after much deliberation. This is elegantly explained by Sahotra Sarkar in Doubting Darwin? (Blackwell, 2007).
First, let me briefly mention two scientific theories that were replaced by new and better theories. It used to be thought that the fundamental nature of heat was a fluid called "caloric" - the caloric theory of heat. This was replaced in the 19th century by the kinetic theory of heat which represents heat as the agitation of the atoms and molecules of matter in motion. Also, it was thought prior to the work of Gregor Mendel (1822-1884) that when parents produced a child, the characteristics of the parents smoothly blended together in the offspring. This theory was replaced by Mendel's particulate theory which proposes that parents' individual characteristics independently mix and sort as they pass from one generation to the next.
The work of Isaac Newton (1643-1727) revolutionised our understanding of how the universe works. Newton postulated a universal force of gravitation that interacts between all bodies in the universe. In his Principia Mathematica he outlined his three laws of motion and his law of universal gravitation. The latter states that two bodies are attracted to each other by the force of gravity whose strength is proportional to the product of the masses of the objects and inversely proportional to the square of the distance between them. Newton threw his explanatory cloak over the entire universe by showing that his law of universal gravitation explained Kepler's laws of planetary motion. Newton's law demonstrated its awesome power by correctly predicting the return of Halley's comet in 1758. Newton's eminence was captured in Alexander Pope's epitaph:
"Nature and Nature's Laws lay hid in night / God said, let Newton be and all was light".
Newton's theory assumed "action at a distance" between distant bodies that instantaneously influence each other through gravitation. This contradicted the mechanical philosophy of Boyle, Descartes and Huygens, the dominant scientific metaphysics of the day which explained all interactions as mediated by local contact between impenetrable particles of matter. Leibniz described action-at-a-distance as "occult". But, the mechanical philosophy could not account for planetary motion and action at a distance gradually replaced the mechanical philosophy in the metaphysics of science.
Newton's theory of gravitation, like all scientific theories, must contend with incoming new evidence and data. It had another spectacular success in the 1840s. It was long known that the motion of the planet Uranus deviated from what was predicted by Newton's law. Did this mean that the law was wrong, or needed adjustment? No. John Couch Adams and Joseph Le Verrier showed that the anomalies in the motion of Uranus could be explained by retaining Newton's law but positing the existence of an eighth planet. This prediction was confirmed in 1846 by Johann Gottfried Galle in Berlin. The new planet was later called Neptune.
But Newton's law did not fully survive the next attack of new evidence. In 1859 Le Verrier pointed to a discrepancy between the observed and the predicted motion of Mercury around the sun. Many explanations were proposed, eg Mercury had a moon, there was another planet within Mercury's orbit, the sun was more oblate than observed, and so on. It was also proposed that Newton's theory required modification and it gradually became apparent that this was the only explanation not ruled out by observation or experiment.
Einstein solved the problem in 1915 with his new theory of gravity called general relativity, which reinterpreted gravitation as the curvature of space-time. Until then it was assumed that space and matter were independent of each other and that space has a flat geometry. Einstein showed that we live in a curved space-time and that the curvature changes as matter moves around. Einstein's theory perfectly explained the motion of Mercury. It also predicted that light would bend around matter and this was experimentally confirmed during a solar eclipse in 1919.
It would be incorrect to say that Newton's theory of gravitation was wrong, rather it is incomplete.
Scientific theories are abandoned only in the face of compelling and unimpeachable evidence, and before abandoning it, it is reasonable to attempt to modify the theory minimally to see if it can accommodate the new awkward data. Metaphysical difficulty with a theory is not sufficient for its abandonment - there must be an alternative that is empirically compelling. Relativity replaced Newton's mechanics because of compelling positive supporting evidence. In fact, strange metaphysics can be accepted so long as a theory is empirically successful. Action at a distance was accepted so long as Newton's law faced no anomaly.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC understandingscience.ucc.ie