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.

"That is also pure speculation," says Impey. "It’s maybe reasonable speculation, but it’s speculation in a very similar vein to the speculation of someone like Kip Thorne about wormholes and time travel and white holes and black holes. It’s very careful speculation by a highly trained theoretical physicist who knows what the boundary of the current theory is."

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 19^th century, Albrecht says, 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, Albrecht ponts out, atomic theory was a construction that went way beyond what anyone could see 100 years ago. And if it's a challenge for scientists now to embrace wild ideas like other universes, he says, that just comes with the territory.

"So far, everything we've done to try to understand the universe has pulled us out of our shell, so to speak, and made us think about things that are way beyond what we see, and way beyond what we'll see in the foreseeable future. So we're just stuck with that… Unfortunately, it's part of the nature of always being at the frontier of what we understand."