In their recent book The Grand Design, Stephen Hawking and Leonard Mlodinow describe a way of looking at the world called “model-dependent realism.” The idea is that one model of the world is as good as another if it describes the results of experiments just as well as that other model. We use whatever model is most useful. From the book:

“According to model-dependent realism, it is pointless to ask whether a model is real, only whether it agrees with observation. If there are two models that both agree with observation … then one cannot say that one is more real than another. One can use whichever model is more convenient in the situation under consideration.”

Colin Bruce says something similar in Schrodinger’s Rabbits. Bruce describes a game played by advanced aliens (“gods” in his wording) that turns out to have three equivalent descriptions. The simplest, at least for the way our brains work, is tic-tac-toe. Bruce argues that many worlds is like the tic-tac-toe version of the aliens’ game. It is the simplest description for our human minds. Whether many worlds is true is irrelevant.

David Deutsch will have none of this. For Deutsch, many worlds is reality, because many worlds appears in our best explanations. Here’s how Deutsch describes “realism”:

“The commonsense, and true, doctrine that the physical world really exists, and is accessible to rational inquiry.” (BoI, page 23)

However, Deutsch also emphasizes that our knowledge is fallible, and will always be imperfect (infinitely imperfect, in fact). We are always at the beginning of infinity. For Deutsch, many worlds (the multiverse in his words) is real because it is the consequence of our best explanation of single-particle interference. “We should conclude,” Deutsch writes, “that a particular thing is real if and only if it figures in our best explanations of something.” (BoI, page 30)

But is that “really” real? What happens when a better explanation comes around (as it must, if we’re at the beginning of infinity)? Here’s what Deutsch says:

“Sweeping away the entities through which a theory makes its explanation is not the same as sweeping away the whole of the explanation. Although there is no force of gravity, it is true that something real (the curvature of spacetime), caused by the sun, has a strength that varies approximately according to Newton’s inverse-square law, and affects the motion of objects, seen and unseen.” (BoI, page 108)

So what of many worlds. Are they really there? Something real causes single-particle interference. That something behaves an awful lot like photons.Could that something have some other explanation? Of course it could. Many worlds could be the wrong explanation. Our knowledge is always fallible. And whether it is right or wrong will have real consequences for what is to come. But Deutsch has something to say about that, as well. It isn’t so much what we know, it’s how we use what we know.

“The ability to create and use explanatory knowledge gives people a power to transform nature which is ultimately not limited by parochial factors, as all other adaptations are, but only by universal laws. This is the cosmic significance of explanatory knowledge – and hence of people . . .” (BoI, page 60)

This passage affected me more than almost any other in the book, because it helped me see that knowledge (what one might see as pure science) and transformation (what one might see as technology) are intimately linked. There is no separating them. The path to knowledge and the path to transformation are one and the same – the only way to improve our explanations is to use them to transform (and through that transformation learn yet more about) the world.

And that leads me directly into one of the most exciting ideas of many worlds – the quantum computer. Next time.