Cause and Effect in a Probabilistic Multiverse
September 16, 2011 § 1 Comment
Larry Niven’s “All the Myriad Ways” is set in a world which, shortly after the “many worlds” hypothesis has been verified through the work of a company called Crosstime, begins to experience an “epidemic” of random suicides and crimes from people who had previously exhibited no signs of instability. In terms of the many worlds interpretation of quantum mechanics, the obvious explanation for this epidemic is that the particular timeline “All the Myriad Ways” follows just happens to be the one in which a large number of people decided to commit “casual murder, casual suicide, [and] casual crime.” At every moment, there was a possibility that everyone could spontaneously decide to kill themselves; in this timeline, quite a few of them did.
This doesn’t seem to mesh with our understanding of human behavior, though. Aren’t people’s actions affected, at least to some degree, by their pasts? How can we take a theory seriously if that theory would have us believe that, one second from now, any of us is just as likely to go on a killing spree as to go get a cup of coffee?
The key phrase in this objection, of course, is “just as likely”. Neither quantum mechanics in general nor the many worlds hypothesis in particular holds that all things that are possible are equally probable; on the contrary, a key element of quantum mechanics (at least according to my admittedly limited understanding of it) is the idea that every particle can be represented as a “probability wave” describing the likelihood that the corresponding particle is at any particular position. The many worlds interpretation, meanwhile, incorporates the analogous idea that the wave form of a particle represents the proportion of worlds in which that particle has a given position. These probability waves are affected by events that occur; as I move a ball from one side of a room to another, the likelihood that the particles of that ball are in any particular place shifts rapidly, and they are always more likely to be wherever I have moved the ball than anywhere else. In terms of the many world hypothesis, in some worlds, the ball will have spontaneously traveled out of the room; however, in the vast majority of worlds, it will have simply followed the path of my hand.
Taking this into account, people’s actions in “All the Myriad Ways” start to make much more sense. The fact that the epidemic of casual suicide and casual crime only occurred after the success of Crosstime’s project implies that, in effect, the discovery of parallel universes shifted the probability waves of the particles in people’s brains in much the same way that opening a door shifts the probability waves of the particles in that door. Perhaps the story, by following a timeline in which so many people commit suicide after thinking about the meaning of Crosstime’s discovery, overstates the effect that proving the many worlds hypothesis would have; however, the story does not imply that past events have no effect on people’s present actions. It simply implies that this effect is a probabilistic one rather than a deterministic one.