by DAN FALK
Thought experiments played a crucial role in the history of science. But do they tell us anything about the real world?
In Galileo’s Dialogue Concerning the Two Chief World Systems (1632), three Italian gentlemen – one philosopher and two laymen – debate the structure of the Universe. The philosopher, Salviati, argues in support of the Copernican theory, even though it requires a moving Earth – something that strikes his interlocutors as problematic, if not absurd. After all, we don’t feel the ground moving beneath our feet; clouds and birds are not swept backwards as the planet whooshes through space; a ball dropped from a tower does not land far away from the base of that tower.
But Salviati, standing in for Galileo, asks his companions, Sagredo and Simplicio, to reconsider their intuitions. Suppose one were to drop an object from the mast of a tall ship. Does it make any difference if the ship is moving? No, Salviati insists; it lands at the base of the mast regardless, and therefore one cannot conclude anything at all about the ship’s motion from such an experiment. If the ship can be in motion, then why not the whole planet? Simplicio objects: Salviati has not actually carried out this shipboard experiment, so how can he be sure of the result?
‘Without experiment, I am sure that the effect will happen as I tell you,’ he replies. After some further cajoling, Simplicio is won over.
Today, most scientists and philosophers believe that there is only one reliable way to learn about the world, namely, to poke and prod at it – the view that philosophers call empiricism. When a child does the poking and prodding, the activity is called play. When a scientist does it, it’s called observation and experiment. In either case, though, we learn by seeing and doing.
But as Galileo has shown, there seem to be exceptions to this rule. There are – allegedly – occasions when we come to understand something about the world via a peculiar kind of experiment that takes place only in the mind. Thought experiments, as they’re known, are an exercise of pure imagination. We think about some particular arrangement of things in the world, and then work out what the consequences would be. In doing so, we seem to learn something about the laws of nature.
Thought experiments have played a crucial role in the history of physics. Galileo was the first great master of the thought experiment; Albert Einstein was another. In one of his most celebrated thought experiments, Galileo shows that heavy objects and small objects must fall at the same rate. On another occasion – building on the ship’s mast argument – he deduces the equivalence of reference frames moving at a constant speed with respect to one another (what we now call Galilean relativity), a cornerstone of classical physics.
Einstein, too, was adept at performing such imaginative feats in his head. As a young man, he imagined what it would be like to run alongside a beam of light, and it led him to special relativity. Later, he imagined a falling man, and realised that in freefall one doesn’t feel one’s own weight; from this insight, he concluded that acceleration was indistinguishable from the tug of gravity. This second breakthrough became known as the ‘principle of equivalence’, and led Einstein to his greatest triumph, the general theory of relativity.
What these examples have in common is that knowledge seems to arise from within the mind, rather than from some external source. They require no laboratory, no grant proposal, no actual doing of … anything. When we perform a thought experiment, we learn, it would seem, by pure introspection. ‘Seem’ is perhaps the key word. Whether thought experiments actually do present a challenge to empiricism is hotly contested.
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