The Mystery of the Multiverse

Article excerpt

Byline: Brian Greene

"What really interests me is whether God had any choice in creating the world." That's how Albert Einstein, in his characteristically poetic way, asked whether our universe is the only possible universe.

The reference to God is easily misread, as Einstein's question wasn't theological. Instead, Einstein wanted to know whether the laws of physics necessarily yield a unique universe--ours--filled with galaxies, stars, and planets. Or instead, like each year's assortment of new cars on the dealer's lot, could the laws allow for universes with a wide range of different features? And if so, is the majestic reality we've come to know--through powerful telescopes and mammoth particle colliders--the product of some random process, a cosmic roll of the dice that selected our features from a menu of possibilities? Or is there a deeper explanation for why things are the way they are?

In Einstein's day, the possibility that our universe could have turned out differently was a mind-bender that physicists might have bandied about long after the day's more serious research was done. But recently, the question has shifted from the outskirts of physics to the mainstream. And rather than merely imagining that our universe might have had different properties, proponents of three independent developments now suggest that there are other universes, separate from ours, most made from different kinds of particles and governed by different forces, populating an astoundingly vast cosmos.

The multiverse, as this vast cosmos is called, is one of the most polarizing concepts to have emerged from physics in decades, inspiring heated arguments between those who propose that it is the next phase in our understanding of reality, and those who claim that it is utter nonsense, a travesty born of theoreticians letting their imaginations run wild.

So which is it? And why should we care? Grasping the answer requires that we first come to grips with the big bang.

In Search of the Bang

In 1915, Einstein published the most important of all his works, the general theory of relativity, which was the culmination of a 10-year search to understand the force of gravity. The theory was a marvel of mathematical beauty, providing equations that could explain everything from the motion of planets to the trajectory of starlight with stupendous accuracy.

Within a few short years, additional mathematical analyses concluded that space itself is expanding, dragging each galaxy away from every other. Though Einstein at first strongly resisted this startling implication of his own theory, observations of deep space made by the great American astronomer Edwin Hubble in 1929 confirmed it. And before long, scientists reasoned that if space is now expanding, then at ever earlier times the universe must have been ever smaller. At some moment in the distant past, everything we now see--the ingredients responsible for every planet, every star, every galaxy, even space itself--must have been compressed to an infinitesimal speck that then swelled outward, evolving into the universe as we know it.

The big-bang theory was born. During the decades that followed, the theory would receive overwhelming observational support. Yet scientists were aware that the big-bang theory suffered from a significant shortcoming. Of all things, it leaves out the bang. Einstein's equations do a wonderful job of describing how the universe evolved from a split second after the bang, but the equations break down (similar to the error message returned by a calculator when you try to divide 1 by 0?) when applied to the extreme environment of the universe's earliest moment. The big bang thus provides no insight into what might have powered the bang itself.

Fuel for the Fire

In the 1980s, physicist Alan Guth offered an enhanced version of the big-bang theory, called inflationary cosmology, which promised to fill this critical gap. …