FAR, far away, in a galaxy with a remarkable resemblance to the Milky Way, is a star that looks remarkably like the sun. And on the star’s third planet, which looks like a twin of the Earth, lives someone who, for all the world, is you. Not only do they look the same as you and lead an identical life, they are reading this exact same article – in fact, they are focused on this very line. Weird? I’ve hardly started. In fact, there are an infinite number of galaxies that look just like our own, containing infinite copies of you and your loved ones leading lives, up until this moment, that are absolutely identical to yours. The existence of these parallel worlds is not just idle speculation. It does not depend on exotic theories such as the multiverse or the “many worlds” interpretation of quantum mechanics, in which the universe constantly bifurcates. It is an unavoidable consequence of the standard theory of our universe. All this needs some explanation. The furthest we can see is the distance light has been able to travel since the universe was born 13.7 billion years ago. Light from objects further away has not arrived yet. They are beyond our cosmic horizon. Yet we know there is more to the universe. Radiation left over from the big bang appears to confirm that the cosmos went through a fleeting phase of superfast expansion known as inflation. And, according to inflation, there is effectively an infinite amount of universe out there. So our observable universe is akin to a bubble and beyond it lies an infinite number of other bubbles that have a similarly restricted view. Each one experienced the same big bang we did and has the same laws of physics. Yet the initial conditions were slightly different, so different stars and galaxies congealed out of the cooling debris. Despite this, finding another universe just like ours seems unlikely. Yet quantum mechanics tells a different story. Zoom in and you’ll find that the universe is grainy, with space resembling a chessboard. Immediately after the big bang, our observable universe – our bubble – contained only a few “squares”. So there were only a few places for the matter that seeded the formation of today’s galaxies. The neighbouring bubbles contained a slightly different arrangement of matter. So did their neighbours. And so on. But eventually you run out of possible ways to arrange the matter in the bubbles. Eventually you come across an identical bubble to ours. As a result, there are a finite number of ways history can play out. Given that the universe is infinite, there must be a infinite number of histories just like ours, plus an infinite number of different ones. If you could travel far enough in any direction today, you would eventually come across a universe identical to our observable universe right down to the last detail, including you. Max Tegmark at the Massachusetts Institute of Technology has worked out that to find your closest identical copy you would have to travel 101028 metres. That corresponds to 1 followed by 10 billion billion billion zeros. Sadly that means you will never be able to meet your other you. With each passing moment, more of the universe appears over the horizon. Yet by the time our observable universe had expanded to encompass your nearest doppelganger, all the stars will have long burned out. Remarkably, the only way to evade this bizarre conclusion is if our standard pictures of cosmology and quantum theory are wrong. Unsettled? You’re not alone. Cosmologist Alexander Vilenkin of Tufts University in Medford, Massachusetts, has been working on such ideas for more than 25 years. “I have never been happy with the idea that there are an infinite number of Alexander Vilenkins out there,” he says. “Unfortunately, I think it is likely to be true.” It is worth reiterating that this is the most basic and uncontroversial of all conceptions of multiple universes. There are many other “multiverse” theories. For instance, string theory, which views the fundamental building blocks of matter as ultra-tiny, vibrating strings of mass-energy, predicts the existence of other universes. The fact that the universe is apparently fine-tuned for us may be telling us of the existence of other universes with different laws of physics . And then there is the many-worlds interpretation of quantum mechanics in which all possible histories and futures – including yours – play out in separate universes. Many-worlds is a minority view, but if it is true there is a universe somewhere where you are Wimbledon champion. Tegmark has classified such multiverses into a hierarchy of ever-bigger versions, but nobody yet knows if or how all these versions mesh together. The multiverse is an emerging idea; science in the making. The dust has yet to settle and give us – and our infinite doppelgangers – a consistent and clear picture. Marcus Chown ■
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