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One world is not enough

On inaccessible universes and infinite planets.

Don’t worry, there are other worlds. There have to be. If there aren’t, then we haven’t yet made sense of this one. On 19 June, the celebrated science-fiction writers Terry Pratchett and Stephen Baxter will publish a new book, The Long Earth. It is based on an outlandish premise: that an infinite number of variations on our planet are hidden in inaccessible universes. If it were just science fiction, we could either welcome or dismiss it, according to our taste, as yet another example of the limitless human imagination. The trouble is, the whole thing is based in evidence. Worse, this evidence is now the bedrock of modern science.

First of all, let’s go to the roots of our physical reality. If you fire an atom at a screen containing two openings, the atom will go through both. It’s not just atoms; a molecule composed of 60 or so atoms does the same thing. Anything that follows the laws of quantum theory will do it. The only time this doesn’t happen is when someone is watching.

That we don’t get into a car using all four doors at once tells us that the weirdness disappears once you have a lot more than a few dozen atoms clumped together. No one knows why, and it doesn’t change the fact that the strange behaviour of the building blocks of matter is capable of breaking your mind.

No one knew this better than Hugh Everett, who started his career trying to solve this puzzle and ended up a chain-smoking alcoholic. Everett’s idea is now known as the “many worlds” interpretation of quantum mechanics. According to this, a new universe is created every time a subatomic particle is faced with a choice of things to be or do. The ultimate logical consequence is that the universe is composed of myriad sub-universes, each subtly different from the one that spawned it. In this cornucopia of worlds, many will be utterly different from ours. There is, according to the theory, a world where Elvis Presley is the king, not of rock’n’roll, but of Great Britain and Northern Ireland.

No one accepted the idea, and Everett responded to the pain of rejection by slowly self-destructing. Today, however, physicists take these quantum worlds seriously. And they are not the only strange fruit of modern physics.

Our best theory of how the universe began requires that it went through a ridiculous period of super-fast expansion, increasing in size by a factor of 1,000 billion billion billion in a fraction of a millisecond. If that is the case, the same “inflation” mechanism will cause other universes to blow up from tiny instabilities in the fabric of our universe. They pinch off and float away beyond our reach. And if you believe Everett’s theory, the activity of quantum particles in each of those worlds will spawn ever more worlds nested within them.

Forget reality

It might seem as if the existence of these universes would be unverifiable, but that is a supposition which ignores the ingenuity of scientists. Some have already worked out what imprint a collision with one of the inflated universes would make on the microwave background radiation that fills our universe. Having figured that out, they are now combing the universe for signs that we have touched another world.

It will be harder to verify the existence of the many quantum worlds. Some believe a better explanation for quantum phenomena is that there is no objective reality at all; nothing exists until an experimental observation brings it into being. But clearly, whatever Pratchett and Baxter have come up with, it won’t be as strange or unbelievable as the truth. 

Michael Brooks’s “The Secret Anarchy of Science” is out now in paperback (Profile Books, £8.99)