Wittgenstein's Beetle And Other Classic Thought Experiments

WITTGENSTEIN’S BEETLE AND OTHER CLASSIC THOUGHT EXPERIMENTS I do not want to give the impression that the use of large machines or of elaborate techniques is always justified; sometimes it contributes merely to the sense of self-importance of the investigator, and it is always salutary to remember Rutherford’s ‘We haven’t got the the money, so we’ve got to think!’ R. V. Jones in the Bulletin of the Institute of Physics (1962) recalling the dictum of Ernest Rutherford MARTIN COHEN Wittgenstein’s Beetle and Other Classic Thought Experiments Contents List of Figures Forward! Introduction: Deep Thought – a brief history of thought experiments vii viii 1 The A–Z A is for Alice and Astronomers Arguing about Acceleration B is for Bernard’s Body-Exchange Machine C is for the Catholic Cannibal D is for Maxwell’s Demon E is for Evolution (and an Embarrassing Problem with it) F is for the Forms Lost Forever to the Prisoners of the Cave G is for Galileo’s Gravitational Balls H is for Hume’s Shades I is for the Identity of Indiscernibles J is for Henri Poincaré and Alternative Geometries K is for the Kritik and Kant’s Kind of Thought Experiments L is for Lucretius’ Spear M is for Mach’s Motionless Chain N is for Newton’s Bucket O is for Olbers’ Paradox P is for Parfit’s Person Q is for the Questions Raised by Thought Experiments Quotidiennes R is for the Rule-Ruled Room S is for Salvatius’ Ship, Sailing along its own Space-Time Line T is for the Time-Travelling Twins U is for the Universe, and Einstein’s Attempts to Understand it V is for the Vexed Case of the Violinist 15 18 20 25 28 30 33 37 41 45 48 52 55 58 62 65 67 70 74 78 81 84 v Contents W is for Wittgenstein’s Beetle X is for Xenophanes and Thinking by Examples Y is for Counterfactuals and a Backwards Approach to History Z is for Zeno and the Mysteries of Infinity 87 90 93 97 Notes for Experimenters How to Experiment Notes and Cuttings Who’s Who of Experimenting 103 117 126 Acknowledgements 131 Index 132 vi Figures 1 2 3 4 5 6 7a 7b 8 9 10 11 12 13 Alice and the hole through the Earth Cannibal and cooking pot Maxwell’s demon Galileo’s balls Poincaré’s ladder Lucretius’ spear Mach worried about motions Mach mightily reassured Newton’s bucket Olbers’ astronomical paradox Tweeking the curtain . . . Einstein unravelling light waves Wittgenstein’s menagerie Zeno counting up to infinity 16 23 26 35 46 54 56 57 60 63 68 82 88 98 vii Forward! This book is a collection of the 26 most interesting, if not the most useful, thought experiments (although some have indeed been very useful). It is no exaggeration to say that the whole of modern science is built upon the surprisingly modest foundations of half a dozen of the thought experiments included here. They are no more elementary, say, than Einstein’s Theory of Relativity, or more complicated than, say, Sherlock Holmes at his most lucid. And in fact, in this collection, science, or natural philosophy, features more prominently than other sorts of philosophy. I make no apologies for t