Our universe is not alone

Dr Brian Greene spent a good part of the last decade extolling the virtues of what is called string theory.

He dreamed that one day it would provide physicists with a theory of everything that would describe our universe – ours and ours alone. His bestselling book, The Elegant Universe, eloquently captured the quest for this ultimate theory.

“But the fly in the ointment was that string theory allowed for, in principle, many universes,” says Dr Greene, who is a theoretical physicist at Columbia University in New York. In other words, string theory seems equally capable of describing universes very different from ours.

Dr Greene hoped that something in the theory would eventually rule out most of the possibilities and single out one of these universes as the real one: ours. So far, it hasn’t – though not for any lack of trying.

As a result, string theorists are beginning to accept that their ambitions for the theory may have been misguided. Perhaps our universe is not the only one after all. Maybe string theory has been right all along.

Dr Greene, certainly, has had a change of heart. “You walk along a number of pathways in physics far enough and you bang into the possibility that we are one universe of many,” he says.

“So what do you do? You smack yourself in the head and say, ‘Ah, maybe the universe is trying to tell me something.’

I have personally undergone a sort of transformation, where I am very warm to this possibility of there being many universes, and that we are in the one where we can survive.”

Dr Greene’s transformation is emblematic of a profound change among the majority of physicists. Until recently, many were reluctant to accept this idea of the “multiverse”, or were even belligerent towards it.

However, recent progress is bringing about a major shift in thinking. Gone is the grudging acceptance or outright loathing of the multiverse. Instead, physicists are starting to look at ways of working with it and maybe even trying to prove its existence.


If such ventures succeed, our universe will go the way of Earth – from seeming to be the centre of everything to being exposed as just a backwater in a far vaster cosmos. We will have to accept that some things about our universe are a random accident, inexplicable except in the context of the multiverse.

One of the first to argue for a multiverse was the Russian physicist Dr Andrei Linde, now at Stanford University in California, back in the 1980s. It wasn’t until 1998, however, that the multiverse gained any traction, when astronomers studying distant supernovae announced that the expansion of the universe is accelerating.

They put this down to the vacuum of space having a small energy density, which exerts a repulsive force to counteract gravity as the universe ages. This became known as dark energy, or the cosmological constant.

Its discovery was a huge blow. Up till then, physicists had hoped that some ultimate theory would deduce the values of fundamental constants of nature from first principles, including the cosmological constant, and explain why the laws of physics are as they are, just right for the formation of stars and galaxies and possibly the emergence of life. This seems not to be the case. Nothing in string theory, or indeed any other theory, can predict the observed value of the constant.

However, if our universe is part of a multiverse then we can ascribe the value of the cosmological constant to an accident. The same goes for other aspects of our universe, such as the mass of the electron.

The idea is simply that each universe’s laws of physics and fundamental constants are randomly determined, and we just happen to live in one where these are suited for life. “If not for the multiverse, you would have these unsolved problems at every corner,” says Dr Linde.

The other compelling argument for a multiverse comes from string theory. This maintains that all fundamental particles of matter and forces of nature arise from the vibration of tiny strings in 10 dimensions.

For us not to notice the extra six dimensions of space, they must be curled up so small as to be undetectable. For decades, mathematicians toiled over what different forms this compaction could take, and they found myriad ways of scrunching up space-time — a staggering 10 to the power of 500 or more.

Each form gives rise to a different universe with its own fundamental particles and laws of physics.

The hope, nurtured by Dr Greene and others, was that there was some kind of uniqueness principle that would pick out the particular form that produces our universe.

That hope receded dramatically a few years ago when Dr Michael Douglas of the State University of New York in Stony Brook, and Dr Leonard Susskind of Stanford University surveyed the developments in string theory and concluded that nothing suggests that any one of these universes is preferred over others.

“Just about everybody is convinced that the idea of uniqueness has gone down the drain,” says Susskind.

So what are we to do? Throw up our hands and admit that we will never be able to explain why our universe is the way it is? Not a bit of it.

Dr Susskind argues that we can still ask meaningful questions within the context of the multiverse, just not the ones we’d ask if ours were the only universe. Investigating a collection of 10 to the power of 500 universes is not a matter of enumerating the properties of each of them, however.

“We just can’t make a list of 10 to the power of 500 things,” says the Nobel laureate Dr Steven Weinberg of the University of Texas at Austin. “That’s more than the number of atoms in the observable universe.”

Experiments will be crucial. The European Space Agency’s Planck satellite, due to launch soon, could lend support to some ideas.

Other tests could come because string theory requires that the universe has a property known as supersymmetry, which posits that every particle has a heavier and as yet unseen superpartner.

Physicists will be looking for some of these superpartners at the Large Hadron Collider, the new particle accelerator at Cern, near Geneva, Switzerland.

Until findings undermine the idea, the smart money will remain with the multiverse and string theory.

“It has the best chance of anything we know to be right,” Dr Weinberg says of string theory.

“There’s an old joke about a gambler playing a game of poker,” he adds. “His friend says, ‘Don’t you know this game is crooked and you are bound to lose?’ The gambler says, ‘Yes, but what can I do, it’s the only game in town.’ We don’t know if we are bound to lose, but even if we suspect we may, it is the only game in town.”


* www.newscientist.com