Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality

by Anil Ananthaswamy

In one talk, he asked his audience to consider two theories, A and B, which have different takes on the nature of reality but which are mathematically equivalent, make the same empirical predictions, and are impossible to tell apart experimentally (he could have been talking about the Copenhagen interpretation and Bohmian mechanics, but he wasn’t—he was making a general point). Feynman argued that it’s important to understand that the philosophies behind A and B can lead us in different directions even if they are indistinguishable at some stage of the scientific process. “In order to get new theories, these two things are very far from equivalent. Because one gives a man different ideas than the other,” said Feynman. For example, it might be possible to make a tiny tweak to A that isn’t possible with B. In which case, A can lead to a very different theory after the change. “In other words, although they are identical before they are changed, there are certain ways of changing one which look natural, which don’t look natural in the other. Therefore, psychologically we must keep all the theories in our head,” said Feynman. “And every theoretical physicist that’s any good knows six or seven different theoretical representations for exactly the same physics and knows that they are all equivalent and that nobody is ever going to be able to decide which one is right at that level . . . but he keeps them in his head, hoping that they’ll give him different ideas.”