What is nothing? Sounds like a simple question—nothing is simply the absence of something, of course—until you begin to think about it. The other night the American Museum of Natural History hosted its 14th annual Asimov Memorial Debate, which featured five leading thinkers opining (and sparring, sometimes testily, but more on that later) about the nature of nothing.

“Nothing is the most important part of the universe,” said Lawrence Krauss, a physicist at Arizona State University and author of the recent “A Universe from Nothing: Why There is Something Rather than Nothing.” Of course we can imagine the (mostly) empty space between galaxies as being a sort of nothing. But we should also remember that most of the space around us is empty—even an atom is mostly empty space between the nucleus and electrons.

This sort of nothing—the absence of matter—we might consider to be the first level of nothing, clarified J. Richard Gott, a physicist and cosmologist at Princeton University and the author of “Sizing Up the Universe: The Cosmos in Perspective.” It’s what scientists call a quantum vacuum state. It’s a box with everything taken out of it—all the stuff, all the air, all the light. “It even has a color—it’s black,” deadpanned Gott, who frequently demonstrated the best comedic timing of the bunch. Yet even in this nothing, something remains. Virtual quantum particles pop in and out of being, and the empty box still contains the basic scaffolding of existence: space, time and quantum fields.

But where did these come from? Was this something always there? We can trace the history of the universe back to the first instant after the Big Bang, when the cosmos was unimaginably hot and dense and expanding rapidly. But here the laws of physics break down, and with them our ability to reconstruct what came before—indeed, if its even proper to speak of a “before.” This space outside of the universe (though it is certainly misleading to call it a “space”) is the second kind of nothing—the complete lack of space and time and quantum fields. The absence not just of matter and energy, but of the conditions necessary for being.

Much of the evening was consumed with debate over how the cosmos went from this state—the state of complete nothingness—to the universe we know today. The physicists seemed to be of two minds. Gott argued that it’s possible that there was no beginning. Just as we can continuously travel east without reaching Earth’s easternmost point, the universe may have a loop of time at its origin, a place where you can forever travel into the past but always loop back upon yourself as you do. (The idea springs from his decades-old work showing how cosmic strings can allow for time travel into the past, which Paul Davies discussed in his 2002 Scientific American article “How to Build a Time Machine” [subscription required].)

The other, more popular idea was that of the multiverse. Rather than ask how the universe came to exist from nothing, the multiversers assert that being is the natural state. Perhaps a near-infinite number of universes exist, each with slightly different sets of physical laws. We find ourselves in the universe that has physical laws conducive to advanced life-forms for one simple reason: in order for us to exist, the laws of the universe must allow it.

Krauss presented these anthropic arguments as “cosmic natural selection,” and a solution to the problem of where the universe comes from. But Jim Holt, author of “Why Does the World Exist? An Existential Detective Story,” pointed out that this line of thinking has a long and not-so illustrious history. What physicists today call the multiverse is known by philosophers as the “principle of plentitude” or “principle of fecundity”: every possible universe exists, and of all these possible worlds, the one we happen to live in is the known world.

This is something of a magic trick, said Gott—an explanation without explanatory power. It appears to answer the question of why there is something rather than nothing, but instead it shifts the blame down the line. It answers the question why are we here? with a tautology: because we are.

Here Krauss and Holt showed their sharpest differences. Krauss claims that we know exactly why the universe exists—indeed, much of his book is given over to the argument that there’s no mystery to existence at all. The universe exists because the laws of physics demand it. Once we have quantum fields, and a Big Bang, the universe had to take the shape that it now has. “What would be the characteristics of the universe built from natural law and nothing else?” he asked. “It would be our universe.”

Holt certainly agrees that quantum field theory is the best available description of our known universe, but he thinks that Krauss’s explanation is incomplete. It answers the question: why does the universe look the way it does? with another equally mysterious explanation: because quantum fields make it so. To Holt, the obvious next question is: so where do these quantum fields come from?

This line of inquiry exasperated Krauss. “The endless why? question is stupid—anyone with kids knows that. Why? Why? Why? Why? Why? The only answer is: go to bed,” he said. “The real question is: how?” Later in the program, he tried to explain the creation of the universe from nothing as being like a photon shooting out of a light bulb—it didn’t exist a second ago, but now here it is. (At which point the night’s moderator, Neil Degrasse Tyson, director of the Hayden Planetarium, interrupted him with “but that was energy, it wasn’t just nothing.”)

To Krauss, the endless cycles of why? are beside the point. “Science doesn’t need a first cause, religion does,” said Krauss, a vocal atheist who made his distaste for both religion and philosophy known from the get-go. Krauss’s evasions didn’t quite ring true to Holt. “You’re still in thrall to Christian metaphysics,” he charged. “You see the laws of quantum field theory as divine commands. It used to be that nothing plus God equals universe. You replaced God with the laws of nature. You are insufficiently enlightened.”

Holt later clarified that he considers laws of nature to be “effective descriptions of what’s out there,” not pre-existing entities, and so somewhat useless if we’re seeking enlightenment about why things are they way they are. But he also offered hope to those of us trying to experience the concept of nothing: just go to sleep. Every night we all enter into a brief period of dreamless sleep, with our minds free of all thought. “A little taste of nothing,” he called it. Until death or the end of the universe—whichever comes first—this is as close as we can get to truly understanding nothing.

Image of the Helix nebula courtesy NASA/JPL-Caltech