Multiuniverse

Parallel universes are no longer a figment of our imagination. They're so real that we can reach out and touch them, and even use them to change our world. Until now, the multiverse was a hazy, ill-defined concept-little more than a philosophical trick. But in a paper yet to be published, Deutsch has worked out the structure of the multivers.

Taming the multiverse
Parallel universes are no longer a figment of our imagination. They’re so real that we can reach out and touch them, and even use them to change our world.

Until now, the multiverse was a hazy, ill-defined concept-little more than a philosophical trick. But in a paper yet to be published, Deutsch has worked out the structure of the multiverse. With it, he claims, he has answered the last criticism of the sceptics. “For 70 years physicists have been hiding from it, but they can hide no longer.” If he’s right, the multiverse is no trick. It is real. So real that we can mould the fate of the universes and exploit them.

The theory that describes atoms and their constituents is quantum mechanics. It is hugely successful. It has led to computers, lasers and nuclear reactors, and it tells us why the Sun shines and why the ground beneath our feet is solid. But quantum theory also tells us something very disturbing about atoms and their like: they can be in many places at once. This isn’t just a crazy theory-it has observable consequences .

But how is it that atoms can be in many places at once whereas big things made out of atoms-tables, trees and pencils-apparently cannot? Reconciling the difference between the microscopic and the macroscopic is the central problem in quantum theory.

The many worlds interpretation is one way to do it. This idea was proposed by Princeton graduate student Hugh Everett III in 1957. According to many worlds, quantum theory doesn’t just apply to atoms, says Deutsch. “The world of tables is exactly the same as the world of atoms.”
But surely this means tables can be in many places at once. Right. But nobody has ever seen such a schizophrenic table. So what gives?

The idea is that if you observe a table that is in two places at once, there are also two versions of you-one that sees the table in one place and one that sees it in another place.

The consequences are remarkable. A universe must exist for every physical possibility. There are Earths where the Nazis prevailed in the Second World War, where Marilyn Monroe married Einstein, and where the dinosaurs survived and evolved into intelligent beings.

However, many worlds is not the only interpretation of quantum theory. Physicists can choose between half a dozen interpretations, all of which predict identical outcomes for all conceivable experiments.
Observation forces an atom to make up its mind, and plump for being in only one place out of all the possible places it could be.

Why are physicists reluctant to accept many worlds? Deutsch blames logical positivism, the idea that science should concern itself only with objects that can be observed. In the early 20th century, some logical positivists even denied the existence of atoms-until the evidence became overwhelming. The evidence for the multiverse, according to Deutsch, is equally overwhelming. “Admittedly, it’s indirect,” he says. “But then, we can detect pterodactyls and quarks only indirectly too. The evidence that other universes exist is at least as strong as the evidence for pterodactyls or quarks.”

Perhaps the sceptics will be convinced by a practical demonstration of the multiverse. And Deutsch thinks he knows how. By building a quantum computer, he says, we can reach out and mould the multiverse.

“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.

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.”