Philosophy of Physics: A Very Short Introduction (Very Short Introductions)

by David Wallace

According to falsificationism, that should be the end for Newtonian gravity. It made a prediction; that prediction was false; time to move on to the next theory! But that isn’t what happened, and it isn’t what should have happened. For one thing, Newtonian gravity had been highly successful for hundreds of years, with a huge number of successful predictions and informative explanations to its name: simply discarding it and starting afresh, in the absence of any concrete ideas of how to do better, would have paralysed astronomy. Even more importantly, it wasn’t strictly true that Newtonian gravity was falsified by the discrepancy. For Newtonian gravity—like any theory in physics—only makes predictions with the aid of what philosophers call auxiliary hypotheses: about which planets there are, where they are, how massive they are, what other moonlets and asteroids and dust-clouds might be around, what non-gravitational effects might come into play, and even about how our telescopes and timepieces function. The anomalous precession might be because of a failure in the theory of gravity—but it equally well might be because of another distant planet that we didn’t know about. Indeed, we can turn this logic around: assuming that Newtonian gravity is correct, where would another planet have to be in order for it to resolve the anomaly? When mathematicians asked and answered that question for Uranus, and then astronomers looked at that point in the night sky, they found the planet ...

auxiliary hypotheses Newtonian gravity Uranus - Neptune