Spoke too soon

I should have read on in the article before writing:

Deutsch found that, almost all the time, information flows only within small pieces of the quantum calculation, and not in between those pieces. These pieces, he says, are separate universes. They feel separate and autonomous because all the information we receive through our senses has come from within one universe. As Oxford philosopher Michael Lockwood put it, "We cannot look sideways, through the multiverse, any more than we can look into the future."

Sometimes universes in Deutsch's model peel apart only locally and fleetingly, and then slap back together again. This is the cause of quantum interference, which is at the root of everything from the two-slit experiment to the basic structure of atoms.

More from the New Scientist article:

"One day, a quantum computer will be built which does more simultaneous calculations than there are particles in the Universe," says Deutsch. "Since the Universe as we see it lacks the computational resources to do the calculations, where are they being done?" It can only be in other universes, he says. "Quantum computers share information with huge numbers of versions of themselves throughout the multiverse."

Imagine that you have a quantum PC and you set it a problem. What happens is that a huge number of versions of your PC split off from this Universe into their own separate, local universes, and work on parallel strands of the problem. A split second later, the pocket universes recombine into one, and those strands are pulled together to provide the answer that pops up on your screen. "Quantum computers are the first machines humans have ever built to exploit the multiverse directly," says Deutsch.

And:

What would it mean for you and me to know there are inconceivably many yous and mes living out all possible histories? Surely, there is no point in making any choices for the better if all possible outcomes happen? We might as well stay in bed or commit suicide.

Deutsch does not agree. In fact, he thinks it could make real choice possible. In classical physics, he says, there is no such thing as "if"; the future is determined absolutely by the past. So there can be no free will. In the multiverse, however, there are alternatives; the quantum possibilities really happen. Free will might have a sensible definition, Deutsch thinks, because the alternatives don't have to occur within equally large slices of the multiverse. "By making good choices, doing the right thing, we thicken the stack of universes in which versions of us live reasonable lives," he says. "When you succeed, all the copies of you who made the same decision succeed too. What you do for the better increases the portion of the multiverse where good things happen."

David Deutsch's article "The structure of the multiverse" is here