PARIS -- Is the universe -- correction: "our" universe -- no more than a speck of cosmic dust amid an infinite number of parallel worlds?
A staple of mind-bending science fiction, the possibility of multiple universes has long intrigued hard-nosed physicists, mathematicians and cosmologists too.
We may not be able -- as least not yet -- to prove they exist, many serious scientists say, but there are plenty of reasons to think that parallel dimensions are more than figments of eggheaded imagination.
The specter of shadow worlds has been thrown into relief by the December release of "The Golden Compass," a Hollywood blockbuster adapted from the first volume of Philip Pullman's classic sci-fi trilogy, "His Dark Materials".
In the film, an orphaned girl living in an alternate universe goes on a quest, accompanied by an animal manifestation of her soul, to rescue kidnapped children and discover the secret of a contaminating dust said to be leaking from a parallel realm.
Talking bears and magic dust aside, the basic premise of Pullman's fantasy is not beyond the scientific pale.
"The idea of multiple universes is more than a fantastic invention -- it appears naturally within several scientific theories, and deserves to be taken seriously," said Aurelien Barrau, a French particle physicist at the European Organization for Nuclear Research (CERN), hardly a hotbed of flaky science.
"The multiverse is no longer a model, it is a consequence of our models," explained Barrau, who recently published an essay for CERN defending the concept.
There are several competing and overlapping theories about parallel universes, but the most basic is based on the simple, if mind-boggling, idea that if the universe is infinite then logically everything that could possible occur has happened or will happen.
Try this on for size: a copy of you living on a planet and in a solar system like ours is reading these words just as you are. Your lives have been carbon copies up to now, but maybe he or she will keep reading even if you don't, says Max Tegmark, a cosmologist at MIT in Boston, Massachusetts.
The existence of such a doppleganger "does not even assume speculative modern physics, merely that space is infinite and rather uniformly filled with matter as indicated by recent astronomical observations," Tegmark concluded in a study of parallel universes published by Cambridge University.
"Your alter ego is simply a prediction of the so-called concordance model of cosmology," he said.
Another type of multiverse arises with the theory of chaotic inflation, which tells us that all these parallel worlds are expanding so rapidly -- stretching further and further in to space -- that they remain out of reach even if one could travel at the speed of light forever.
Things get even stranger when one brings the often counter-intuitive laws of quantum physics into the picture, these experts say.
In a landmark paper published in 1957 while he was still a graduate student at Princeton University, mathematician Hugh Everett showed how quantum theory predicts that a single classical reality should gradually split into separate but simultaneously existing realms.
"This is simply a way of trusting strictly the fundamental equations of quantum mechanics," says Barrau. "The worlds are not spatially separated, but exist as kinds of 'parallel' universes."
The borderline between physics and metaphysics is not defined by whether an entity can be observed, but whether it is testable, pointed out Tegmark.
There are many phenomena -- black holes, curved space, the slowing of time at high speeds, even a round and rotating Earth -- that were once rejected as scientific heresy before being proven through experimentation, even if some remain beyond the grasp of observation, he said.
He concluded that it was becoming increasingly clear that multiverse models grounded in modern physics could be empirically testable, predictive and disprovable.