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JUST don鈥檛 say they made it up. 鈥淥ne of the most common misconceptions is that the multiverse is a hypothesis,鈥� says at the California Institute of Technology in Pasadena. In fact, it is forced upon us. 鈥淚t is a prediction of theories we have good reason to think are correct.鈥�

The idea that the already vast universe we can see is just one of perhaps infinitely many we can鈥檛 is certainly a lot to swallow. And it doesn鈥檛 stop there. The multiverse itself comes in many guises. Take the cosmological multiverses. This concept sprouts from eternal inflation, our best explanation for why the universe looks as it does. In the split second after the big bang, the idea goes, space-time expanded exponentially. Random quantum effects brought this inflation to an end in small regions, and these became more sedately expanding bubble universes 鈥� like ours 鈥� inside a continually ballooning container, budding off more and more bubbles.

Even if we travelled at close to the speed of light, we couldn鈥檛 reach the boundary of our expanding bubble, says , a theorist at Tufts University in Massachusetts 鈥� let alone cross over into another part of the multiverse. If we could, we probably wouldn鈥檛 find a place friendly to life like us. 鈥淭he so-called constants of nature, like the mass of the electron or Newton鈥檚 gravitational constant, will have different values in different bubbles,鈥� says Vilenkin. Or at least so says string theory, our best stab at a theory of everything, which predicts a vast landscape of at least 10⁵⁰⁰ different configurations of physics.

Then there is the quantum multiverse, predicted by the 鈥渕any worlds鈥� interpretation of quantum theory (see 鈥�How to think about鈥� Schr枚dinger鈥檚 cat鈥�). It says that when we make a measurement of the quantum world, forcing it to snap out of its accustomed fuzzy state, the other possible measurement outcomes persist in worlds parallel to our own.

Things get really hairy when you ask where these worlds are. 鈥淚f the bubble universes exist in the same physical space, the many worlds are truly parallel universes, completely separate from one another,鈥� says Vilenkin. 鈥淭here is a bigger mathematical structure that they鈥檙e all inside, but in no sense does it look like space,鈥� says Carroll.

What鈥檚 more, copies of ourselves would exist in all these worlds, although their universes would have absolutely no physical connection with our own. So we needn鈥檛 worry about the fate of our doppelg盲ngers. 鈥淢any worlds has no impact whatsoever on my decisions,鈥� says Carroll. 鈥淵ou鈥檙e talking about a person I can never talk to. You might as well worry about the people who live a million years from now, or those who lived a million years ago.鈥�

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This article appeared in print under the headline 鈥淗ow to think about鈥� The multiverse鈥�