We usually think of the universe as being “everything there is.” But many astronomers and physicists now suspect that the universe we observe is just a small part of an unbelievably larger and richer cosmic structure, often called the “multiverse.” This mind-bending notion – that our universe may be just one of many, perhaps an infinite number, of real, physical universes – was front and center at a three-day conference entitled "A Debate in Cosmology The Multiverse," held at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, earlier this month.
The multiverse idea is not new. Physicists have been toying with it ever since Hugh Everett III came up with the “many worlds” interpretation of quantum mechanics back in the 1950s. It took on new life after 1980, when the inflationary-universe theory of the Big Bang's first moments began to suggest that our Big Bang was not a unique event but just a tiny bit of a much larger, ongoing process.
The multiverse idea has had yet another surge of interest in recent years, as a result of a newer idea: string theory. Developed as a possible “theory of everything” that would unite quantum mechanics and gravity, string theory, physicists hoped, would provide a unique description of the universe and why the laws of nature are what they are. Instead, according to some theorists, it lays out a picture of not a single universe but rather a broader “landscape” in which the laws of physics vary from one region to another. It may be that only a small fraction of these regions have conditions allowing any kind of complex matter to exist, and hence intelligent life.
Part of the appeal is that a varied multiverse like this would neatly account for the many remarkable coincidences we observe in the laws of physics that make possible any kind of complex matter, such as atoms and molecules. When we life forms arise and look around, we naturally find ourselves in one of these very rare special realms, merely because we could not have come into being anywhere else. This kind self-selection logic is called "anthropic reasoning."
Equations Meet Philosophy
The lead-off speaker at the conference was Paul Davies of Arizona State University, a prolific writer on cosmology and philosophy. He noted that the multiverse idea has been “propelled to fame” in the last decade or so by the string-theory landscape idea – the notion “that maybe the laws of physics are not absolute, fixed, universal, immutable mathematical relationships,” but instead might be “more like local bylaws.”
Several other speakers, including Laura Mersini-Houghton of the University of North Carolina, echoed that view. The idea of multiple universes “did not go down very well with scientists” when it was first put forward, she said in an interview, but now “there’s an explosion of interest in the subject… because of the discovery of the ‘landscape’ of string theory.”
Along with string theory and many-worlds quantum mechanics, a third motivation for taking the multiverse seriously comes from current ideas on Big Bang inflation. In a version known as “eternal inflation,” there are endless, ongoing big bangs breaking off from an underlying substrate of inflating space-time. Each one produces its own separate cosmos.
However, it's not at all clear how these different kinds of multiverses – grounded in quite different physical theories – may be related to one another. Still, the fact that three different lines of reasoning, all rooted in modern physics, seem to be pointing the same way makes some feel there must be a connection. “My gut feeling is that these multiverses have to be related,” said Mersini-Houghton.
David Albert, a former physicist who now teaches philosophy at Columbia Universiy, says he has more confidence in the Everett many-worlds interpretation of quantum mechanics than in the landscape of string theory. “In the Everett case, at least we have a clear formulation of what the claim is,” he said. “In these other views, the talk is still at a stage that’s much more amorphous…. This kind of meeting is a useful way to begin to sit down and think through those questions more clearly.”
With fewer than 20 scientists taking part in the conference, the talks often gave way to lively debate, with audience members challenging speakers on specific points or calling for more detail or clarification.
Albert was not the only philosopher at the meeting. One of the most interesting presentations was given by Hilary Greaves of Oxford, who discussed philosophical problems with many-worlds quantum theory. (In a nutshell: Conventional quantum theory gives the probability that each micro-event will happen, but offers no clue as to why it actually does or doesn't, leaving spooky conundrums. In the many-worlds view, every possible outcome happens with 100% probability, somewhere among the alternative universes, leaving no spookiness but giving no explanation of where these universes are. Each interpretation matches the real world equally well; you just choose which paradox to accept.)
Greaves is concerned that in the many worlds view, the probabilities have "disappeared," a notion which is hard to reconcile with traditional approaches to quantum theory. She and her colleagues have developed a particular strategy for tackling the many-worlds question, based on a field of mathematics known as “decision theory.” Her talk clearly gave the physicists in the audience much to think about.
Many scientists look down on philosophy as mere question-posing and guesswork. But philosophy seems impossible to avoid when discussing certain problems in physics, especially those dealing with fundamental aspects of reality, such as cosmology and particle physics. In his entertaining talk, Davies referred to the old Hindu story of the Earth resting on the backs of four elephants, which in turn stand on a giant turtle. One is then faced with the question of what the turtle stands on. Perhaps there is some kind of ultimate explanation down below, a Prime Cause – some kind of “super-turtle” that brings the chain of explanation to an end.
For many physicists, Davies said, the laws of physics themselves have served as such an explanation. But this view becomes problematic if the laws themselves change over time, or vary from region to region. Alternatively, there may simply be no ultimate explanation, he suggested, in which case one must accept an infinite regression of causes "turtles all the way down.”
Not everyone is ready to embrace the idea of the multiverse. At one point, Perimeter Institute physicist John Moffat – known for his work on general relativity, Einstein’s theory of gravity – told the audience that the idea of multiple universes “has come up often in science fiction – and that’s where it belongs.” Imagining unseen universes “is not the kind of science we’ve been doing since Galileo,” he said.
Several presenters addressed whether the idea of the multiverse can ever be subjected to experimental testing. Lee Smolin, also from Perimeter, said that in certain versions of the multiverse picture, one could indeed make testable predictions. (Smolin supports an intriguing model in which black holes in one universe can give rise to new universes elsewhere, an idea he describes in his book The Life of the Cosmos.)
There was one “celebrity scientist” on hand – Columbia University physicist Brian Greene, author of The Elegant Universe and The Fabric of the Cosmos. Greene did not give a scientific talk at the conference, but he did deliver an engaging public lecture based on his new book: a children’s story called Icarus at the Edge of Time.
Dan Falk is a science journalist based in Toronto.