How to rewrite history
Ordinary language affirms the claim “If the ball hadn’t been caught, it would have fallen.” In fact the ball was caught. Therefore, seriously evaluating the claim requires rewriting history. In my last post, I argued that the rewriting must go back forever. It would be unserious to quit rewriting just when we find it convenient.
Endless rewriting doesn’t mean spinning endless fantasies. The only serious way to rewrite the past is to invoke well-confirmed laws of motion. The laws governing deterministic systems like our universe exhibit sensitive dependence: the tiniest difference now makes huge differences later. The laws are also time-symmetric: they work both forward and backward in time. Therefore, the tiniest difference now, worked backward in time, makes huge differences in the past.
Remove the catching of the ball, evolve that change backward, and eventually you get a past so different that, almost certainly, the ball never comes to exist. I use “almost certainly” in exactly the sense of “Almost certainly, that ice cube will melt at room temperature”: the odds against it are literally negligible.
On p. 323 of Until the End of Time, physicist Brian Greene writes that “our existence is astonishing. Rerun the big bang but slightly shift this particle’s position or that field’s value, and for virtually any fiddling the new cosmic unfolding will not include you or me or the human species or planet earth or anything else we value deeply.” (I construe Greene’s “virtually” just like my “almost,” above.) All the more so if the changes to the remote past are huge rather than slight.
