IN 2001 a team of computer scientists at an IBM research centre in California performed a historic experiment. They showed that 15 equals five times three. Hard to see why John Gribbin should compare this "landmark event" to the Wright brothers' firs
t flight, until you appreciate how they did it, and what their success implied. The computer was a liquid, its billions of molecules serving as countless tiny processors.
Their speedy calculation demonstrated the existence of parallel universes.
The idea is a staple of science fiction and popular culture: other worlds where history took a different course, from Hitler winning the war to the twists of Gwyneth Paltrow's love life in the movie Sliding Doors. Lump together all the possibilities and you get the "multiverse", a word coined in 1895 but only recently embraced by physicists.
Numerous books on the subject have appeared in the past few years, but Gribbin's stands out for succinctness and readability. Long renowned as one of Britain's finest popular science writers – his 1985 book In Search Of Schrödinger's Cat remains a classic of the genre – Gribbin combines expert knowledge with straightforward, no-frills exposition.
The infamous cat in a box – simultaneously alive and dead, according to the rules of quantum theory – prompted PhD student Hugh Everett to suggest in 1957 that really there were two universes; it was only by opening the box that we could discover which one we happened to live in.
A "cold individual who loved computers", Everett did little to advertise his own theory, and it was not until the 1970s that he gained widespread recognition, partly helped by an article about his work in a science-fiction magazine. Chain-smoking and alcoholism took him to an early grave in 1982, but his reputation continued to rise.
The Oxford physicist David Deutsch, inspired by Everett's work, proposed the idea of a "quantum computer", where instead of storing information as ones or zeroes, the components would be in both states at once, enabling calculations to be done far more quickly. Peter Shor wrote a program that could run on such a device, and what the IBM team created in 2001 was a working version. "Looking at what happened to aviation in the century following the Wright brothers' success," says Gribbin, "gives the merest hint of where quantum computing might be by the year 2100."
But does it really prove multiple universes exist? This is what Deutsch has claimed, and Gribbin agrees. Deutsch, he says, "knows more about computation than anyone I have met"; the IBM experiment "proved that quantum computing works, proved that Shor's algorithm works, and makes it very difficult to doubt the existence of the multiverse".
However, not every specialist in the field considers the case closed; the clarity of Gribbin's argument comes at a cost of suppressing opposing views. This should not put off prospective readers, who will learn a great deal from this excellent book, but it does mean they should exercise caution. "Evolution is a fact," Gribbin says at one point, "like the fact that apples fall off trees." Yet are they really the same sort of fact?
He defines the multiverse as "everything there is", then says: "We still don't know what electrons (or other quantum entities) are, nor how they do the things they do." It's hard to add up the sum of all existence when you can't say what exists. "Either there is a multiverse, or there isn't. That is clearly a scientific question." Only if philosophical questions are brushed under the carpet.
What Gribbin very ably explains is that really there are lots of multiverse theories; the oldest, around for centuries, is the idea that in an infinite universe everything is possible, so if you could fly far enough you would meet another you, an identical arrangement of atoms. Everett's parallel worlds, on the other hand, are physically unreachable, while superstring theory implies ones where not only history but physics itself is different. In fact, there is now such a multitude of multiverses that cosmologist Max Tegmark has found it necessary to introduce a classification system for them.
Like Tegmark, Gribbin assumes any unification of these diverse ideas must itself be a scientific theory. Perhaps, though, it should belong to some other discipline. Could it be that the reason why so many physicists have produced so many multiverse models in the past half century is that they all grew up on the same science-fiction stories? v
