Galileo’s Thought Experiment Rethought

David Atkinson & Jeanne Peijnenburg (University of Groningen)

We argue that thought experiments should be looked upon with a critical, if not a distrustful eye. Even the most famous thought experiment in history, viz. the argument by which Galileo claims
to refute Aristotle’s theory of falling bodies, does not attain the impeccable standard that is generally assumed. Impressive as this thought experiment may be, it is in fact flawed, both in its
attempted refutation of the old, as in its attempted demonstration of the new ideas about falling bodies. It is irrelevant here that Galileo lacked the knowledge that we possess; the point is that situations can be envisaged in which Galileo’s claims are, and other situations in which they are not correct.

We consider four ways in which two bodies fall the same distance:
1. in vacuo in a uniform gravitational field;
2. slowly in viscous fluids;
3. in the inhomogeneous field of the earth;
4. rapidly in viscous fluids.
In case 1, Galileo is right in claiming that the times of fall for both bodies are the same. In case 2, Aristotle is right in claiming that the times are inversely proportional to the weights. In case 3, Galileo’s thought experiment applies point by point, but his grand conclusion that the times of fall are equal does not follow, nor is it generally true. In case 4, neither Galileo’s nor Aristotle’s claims are true.

Our purpose in this talk is not to reinstate Aristotelian physics at the expense of Galileo, but rather to show that even such an awe-inspiring thought experiment as that of Galileo is not as decisive as it might seem, and that we need real experiments to teach us how bodies fall in
various situations.

Reference

David Atkinson and Jeanne Peijnenburg, Galileo and Prior Philosophy, Studies In History and Philosophy of Science (Part A) 35, 2004, 115-136