Theoretical physicists used to dream of producing a “theory of everything” that would relate the two principal breakthroughs of 20th-century physics: quantum mechanics and Einstein’s general theory of relativity. They hoped such a vision would show that the workings of the universe can be explained by a few fundamental parameters and laws of nature.

 

That goal is increasingly thought to be chimerical, observes Alan Lightman, a physicist at MIT. Advances in new areas of investigation have cast doubt on a linchpin for a “theory of everything”: the assumption that our universe is the only one out there. More and more physicists are open to the idea that we may be part of a “multiverse” that contains “many different self-consistent universes, with many different properties,” he writes.

 

One of the new areas of investigation, string theory, holds that the smallest units of energy are “extremely tiny one-dimensional ‘strings’ ” that operate in extra dimensions. Adherents now believe that the “string landscape” predicts a practically infinite number of possible universes, each with different sets of properties. Good luck coming up with a theory that unites all of them. Physicists who study eternal inflation—one of the possible consequences of the big bang—have come to think that the “original, rapidly expanding universe spawns a multitude of new universes, in a never-ending process,” Lightman says.

 

Mind bending as it is, the multiverse theory clears up some confusion. It provides physicists with a plausible answer to a conundrum that has always vexed the field: how our universe, with its precise conditions for supporting life, came into existence without the guiding hand of an intelligent designer. If, however, there is an abundance of universes, as the multiverse theory holds, it is more probable that one (or even more than one) could possess the conditions necessary to foster life.

 

The multiverse theory hasn’t won over all physicists, but it is the theory of choice for some of the field’s leading thinkers. If the idea wins more adherents, the mandate of theoretical physics may have to be substantially revised, Lightman says. Consensus on the multiverse would suggest that the highly sought “theory of everything” is “futile, a beautiful philosophical dream that simply isn’t true.”