Time may be fuzzy. If so, the idea of causality may be in trouble

THE thing about Gedankenexperimente—or thought experiments, for those who find Albert Einstein’s native tongue too twisting—is that you never know where they might lead. For Einstein, they led to the theory of relativity. For James Clerk Maxwell, they conjured an imaginary demon who could violate the second law of thermodynamics. For Erwin Schrödinger, they created an existentially confused cat that was simultaneously dead and alive.

Physicists like to devise Gedankenexperimente because they are a way to consider ideas that cannot be tested for real, usually because the technology needed is not yet available or even envisaged. Though not a substitute for true experimentation, a good Gedankenexperiment may point to conclusions that proper experiments can indeed test. And, though the famous Gedankenexperimente mentioned above are all quite old now, the idea of conducting them has neither...