The Never-Never World

The Ultimate Computers

What is the difference between 20 minutes and ten million billion billion years? In the technology of the not too distant future, they describe, respectively, the amount of time a theoretical quantum computer could take to factorise a thousand-digit number and the time it could take an existing supercomputer to perform the same task. A quantum computer, which scientists are now trying to design, will be, in one respect, the most remarkable machine that humankind has ever built. For it will be the first ever that works without any consensus of how it is working or what it is doing.

As some physicists see it, a quantum computer will explore the never-never land of countless other universes simultaneously. These, they believe, are real universes in which everything that can happen, does happen. Universes, just as real as ours, in which the important meeting you've just attended never took place - or turned out differently - or in which the Tories won the 2000 election, or in which whatever could have happened did happen. In this theory, reality is constantly branching, and has been since the beginning of time. Somewhere, everything possible is true. And these branchings and sub-branchings produce states of reality that are increasingly stranger and less recognisable. Take the number 3,458,137. It takes a modern personal computer about one second to discover that it is the product of two prime numbers, 1789 and 1933. To do this, the machine does about 800 trial divisions before discovering that 1789 is the smallest prime that divides into the larger number without leaving any remainder. But it performs this task sequentially, doing one trial division after another. A quantum computer, by contrast, would make every one of those trial divisions simultaneously. It would explore every one of 800 parallel universes, and act in each of them as a separate computer. Factorising huge numbers is only one of the countless tasks that a quantum computer will be able to perform. This is of practical importance because it will enable the most intractable secret codes to be broken in minutes. But for theoretical astronomy- and perhaps even observational - it may open up huge vistas of hitherto concealed knowledge. How many branchings of reality took place immediately after the big bang? Was it inevitable, or merely an unlikely happening, that galaxies should have formed? In how many bizarre different ways could our universe have evolved? In how many ways did it evolve? And by observing countless possible solar systems, we may learn to construct new and more accurate rules about the constraints on life. Of all conceptions of the human mind, from gargoyles to black holes the idea of the quantum computer is surely the most strange. They were postulated, by Richard Feynman, as recently as 1982. It may be that two or three generations will pass before they become ubiquitous, but their coming will surely mark a watershed in human progress.