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Take for instance a phenomenon called frustrated spontaneous emission. It sounds like an embarrassing sexual complaint that psychotherapy might help with. In fact, it involves the decay of radioactive particles, which ordinarily takes place at a predictably random rate. The exception, however, is when radioactive material is placed in an environment that cannot absorb the photons that are emitted by decay. In that case, decay ceases—the atoms become “frustrated.” How do these atoms “know” to stop decaying until conditions are suitable? According to Wharton, the unpredictable decay of radioactive particles may be determined in part by whatever receives their emitted photons in the future.20 Decay may not really be random at all, in other words. Another quantum mystery that arguably becomes less mysterious in a retrocausal world is the quantum Zeno effect. Usually, the results of measurements are unpredictable—again according to the famous uncertainty believed to govern the quantum kingdom—but there is a loophole. Persistent, rapid probing of reality by repeating the same measurement over and over produces repetition of the same “answer” from the physical world, almost as if it is “stopping time” in some sense (hence the name of the effect, which refers to Zeno’s paradoxes like an arrow that must first get halfway to its target, and then halfway from there, and so on, and thus is never able to reach the target at all).21 If the measurement itself is somehow influencing a particle retrocausally, then repeating the same measurement in the same conditions may effectively be influencing the measured particles the same way in their past, thereby producing the consistent behavior. Retrocausation may also be at the basis of a long-known but, again, hitherto unsatisfyingly explained quirk of light’s behavior: Fermat’s principle of least time. Light always takes the fastest possible path to its destination, which means taking the shortest available path through different media like water or glass. It is the rule that accounts for the refraction of light through lenses, and the reason why an object underwater appears displaced from its true location.22 It is yet another example of a creature in the quantum bestiary that makes little sense unless photons somehow “know” where they are going in order to take the most efficient possible route to get there. If the photon’s angle of deflection when entering a refractive medium is somehow determined by its destination, Fermat’s principle would make much more sense. (We will return to Fermat’s principle later in this book; it plays an important role in Ted Chiang’s short story, “Story of Your Life,” the basis for the wonderful precognition movie Arrival.) And retrocausation could also offer new ways of looking at the double-slit experiment and its myriad variants.
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