Gedankenexperiment (Thought Experiment)

by : Professor Gino Segre

       The notion of a gedankenexperiment, or thought experiment, has been integral to the theoretical physies toolkit ever since that discipline came into existence. It involves setting up an imagined piece of apparatus and running a simple experiment with it in your mind, for the purpose of proving or disproving a hypothesis. In many cases, a gedankenexperiment is the only approach. An actual experiment to examine retrieval of information falling into a black hole cannot be carried out. 

       The notion was particularly important during the development of quantum mechanics, when legendary gedankenexperiments were conducted by the likes of Niels Bohr and Albert Einstein to test such novel ideas as the uncertainty principle and wave-particle duality. Examples, like that of "Schrödinger's cat," have even come into the popular lexicon. Is the cat simultaneously dead and alive ? Others, particularly the classic double slit through which a particle/wave passes, were part of the first attempt to understand quantum mechanics and have remained as tools for understanding its meaning.

       However, the subject need not be an esoteric one for a gedankenexperiment to be fruitful. My own favorite is Galileo's proot that, contrary to Aristotle's view, objects of different mass fall in a vacuum with the same acceleration. One might think that a real experiment needs to be conducted to test that hypothesis, but Galileo simply asked us to consider a large and a small stone  tied together by a very light string. If Aristotle was right, the large stone should speed up the smaller one, and the smaller one retard the larger one, if they fall at different rates. However, if you let the string length approach zero, you have a single object with a mass equal to the sum of their masses, and hence it should fall at a higher rate than either. This is nonsensical. The conclusion is that all objects fall in a vacuum at the same rate. 

    Consciously or unconsciously, we carry out gedankenexperiments of one sort or another in our everyday life and are even trained to perform them in a variety of disciplines, but it would be useful to have a greater awareness of how they are conducted and how they can be positively applied. We could ask;, when confronted with a puzzling situation, "How can I set up a gedankenexperiment to resolve the issue ?" Perhaps our financial, political, and military experts would benefit from following such a tactic and arrive at happier outcomes.