Section One: Dimensions

The irresistible, mind-boggling fantasy comes to just about everyone, sooner or later: Do other universes exist?

Astronomers believe the Big Bang first produced atomic nuclei in the first three minutes of the universe. 300,000 years later, atoms formed and light was released. Today we can still observe evidence of these primordial reactions. 
One second, according to theory, there was nothingness. The next, our cosmos sprang into existence. Nature seems to have pulled off the feat of getting something in fact, everything for nothing. 

As unimaginable as that sounds, it comes straight out of the theory of quantum mechanics, a set of mathematical rules that describe how the universe works on the smallest scales, inside atoms. Quantum mechanics says that matter and energy can appear spontaneously out of the vacuum of space, thanks to something called a quantum fluctuation, a sort of hiccup in the energy field thought to pervade the cosmos. Cosmologists say that a quantum fluctuation gave rise to the Big Bang. And the thing about quantum fluctuations is that they can happen anywhere, any time. And if our universe was born out of a quantum fluctuation, say theorists, then it's possible that other quantum fluctuations could have spawned other universes.

There's a reason some theorists want other universes to exist: They believe it's the only way to explain why our own universe, whose physical laws are just right to allow life, happens to exist. According to the anthropic principle, there are perhaps an infinite number of universes, each with its own set of physical laws. And one of them happens to be ours. That's much easier to believe, say the anthropic advocates, than a single universe fine-tuned for our existence. But there's a problem. If these other universes exist, there's no way for us to detect them. But if these other universes do exist, are we really destined never to detect them? Some theorists have speculated that gravitational energy from other universes might leak into ours, and that someday we might figure out how to detect it. But even the most open-minded cosmologists say that's a long shot at best.

It wouldn't be the first time that a wild idea turned out to be right. A bit more than 100 years ago, in the second half of the 19th century, most scientists didn't accept the idea that matter was composed of atoms an idea supported not by direct observation, but by inferences based on theories of temperature, heat, and viscosity. Like quantum mechanics, atomic theory was a construction that went way beyond what anyone could see 100 years ago...