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What happens to a billiard ball, say, if you shoot it through a wormhole at its slightly younger self, trying to deflect it off course? A physicist at the Russian Space Institute in Moscow named Igor Novikov worked out the math that would govern a trans-temporal, suicidal (or at least self-inhibiting) billiards game (a sort of cross between billiards and Russian roulette), and he discovered something remarkably reassuring: physical law would actually prevent the billiard ball from inhibiting its past self. In fact, a principle of self-consistency would govern a wormhole-riddled universe. Even if an object could enter a wormhole at some time point B and emerge earlier, at some time point A, it could never actually interfere with its own entry into the wormhole at that later time point B.7 Two of Thorne’s students checked and found that Novikov was right: a time-traveling billiard ball cannot take the place of its younger self.8 (According to physicist Nick Herbert, it is analogous to the exclusion principle discovered by Wolfgang Pauli, which prevents any two electrons from occupying the same states simultaneously—a principle that ultimately makes the world built of tiny probabilistic particles solid.9) More recently, the physicist Seth Lloyd designed and actually conducted such an experiment using a photon and what he called a quantum gun—essentially shooting the photon a few billionths of a second back in time to interfere with its past self. He discovered he couldn’t. “No matter how hard the time-traveler tries, she finds her grandfather is a tough guy to kill.”10 This does not mean that time travel is impossible. Quite the contrary. It means that the time-traveling object encounters and interacts with its earlier self in precisely such a way that its later entry into the wormhole is facilitated rather than impeded. In other words, all possible paths of a billiard ball entering a wormhole would, upon exiting the wormhole earlier, nudge itself into the mouth of the wormhole later, thus completing the causal tautology, or what physicists call the closed-timelike curve. These days, quantum physicists like Lloyd use the idiom of postselection, a kind of informational-causal Darwinism that ensures that the only information that survives its journey into the past is information that does not foreclose its origins in the future. It’s not like there’s a Causality Police stepping in now and again to prevent grandfather paradoxes from occurring, or that time travelers need to step gingerly in the past to avoid disturbing things (a common trope in time-travel stories)—although they may in fact find that funny paranormal experiences impede them in ways they hadn’t expected. Guns might misfire at a crucial moment, for instance. (There’s nothing keeping you from trying to kill your grandfather.) But mainly, it is that time travelers from the future who survive their journey into the past are the ones whose actions somehow lead to the identical future from which they will have been sent back. Time loops, in other words.
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Eric Wargo (Precognitive Dreamwork and the Long Self: Interpreting Messages from Your Future (A Sacred Planet Book))