Linde’s recent research has helped solidify the connection between string theory and the multiverse. Some physicists have long embraced the notion that the extra dimensions of string theory play a key role in shaping the properties of new universes spawned during eternal chaotic inflation. When a new universe sprouts from its parent, the concept goes, only three of the dimensions of space predicted by string theory will inflate into large, full-blown, inhabitable spaces. The other dimensions of space will remain essentially invisible—but nonetheless will influence the form the universe takes. Linde and his colleagues figured out how the invisible dimensions stayed compact and went on to propose billions of permutations, each giving rise to a unique universe.
Linde’s ideas may make the notion of a multiverse more plausible, but they do not prove that other universes are really out there. The staggering challenge is to think of a way to confirm the existence of other universes when every conceivable experiment or observation must be confined to our own. Does it make sense to talk about other universes if they can never be detected?
I put that question to Cambridge University astrophysicist Martin Rees, the United Kingdom’s Astronomer Royal. We meet at his residence at Trinity College, in rooms on the west side of a meticulously groomed courtyard, directly across from an office once occupied by Isaac Newton.
Rees, an early supporter of Linde’s ideas, agrees that it may never be possible to observe other universes directly, but he argues that scientists may still be able to make a convincing case for their existence. To do that, he says, physicists will need a theory of the multiverse that makes new but testable predictions about properties of our own universe. If experiments confirmed such a theory’s predictions about the universe we can see, Rees believes, they would also make a strong case for the reality of those we cannot. String theory is still very much a work in progress, but it could form the basis for the sort of theory that Rees has in mind.
“If a theory did gain credibility by explaining previously unexplained features of the physical world, then we should take seriously its further predictions, even if those predictions aren’t directly testable,” he says. “Fifty years ago we all thought of the Big Bang as very speculative. Now the Big Bang from one millisecond onward is as well established as anything about the early history of Earth.”
The credibility of string theory and the multiverse may get a boost within the next year or two, once physicists start analyzing results from the Large Hadron Collider, the new, $8 billion particle accelerator built on the Swiss-French border. If string theory is right, the collider should produce a host of new particles. There is even a small chance that it may find evidence for the mysterious extra dimensions of string theory. “If you measure something which confirms certain elaborations of string theory, then you’ve got indirect evidence for the multiverse,” says Bernard Carr, a cosmologist at Queen Mary University of London.
Support for the multiverse might also come from some upcoming space missions. Susskind says there is a chance that the European Space Agency’s Planck satellite, scheduled for launch early next year, could lend a hand. Some multiverse models predict that our universe must have a specific geometry that would bend the path of light rays in specific ways that might be detectable by Planck, which will analyze radiation left from the Big Bang. If Planck’s observations match the predictions, it would suggest the existence of the multiverse.
When I ask Linde whether physicists will ever be able to prove that the multiverse is real, he has a simple answer. “Nothing else fits the data,” he tells me. “We don’t have any alternative explanation for the dark energy; we don’t have any alternative explanation for the smallness of the mass of the electron; we don’t have any alternative explanation for many properties of particles.
“What I am saying is, look at it with open eyes. These are experimental facts, and these facts fit one theory: the multiverse theory. They do not fit any other theory so far. I’m not saying these properties necessarily imply the multiverse theory is right, but you asked me if there is any experimental evidence, and the answer is yes. It was Arthur Conan Doyle who said, ‘When you have eliminated the impossible, whatever remains, however improbable, must be the truth.’”
What About God?
For many physicists, the multiverse remains a desperate measure, ruled out by the impossibility of confirmation. Critics see the anthropic principle as a step backward, a return to a human-centered way of looking at the universe that Copernicus discredited five centuries ago. They complain that using the anthropic principle to explain the properties of the universe is like saying that ships were created so that barnacles could stick to them.
“If you allow yourself to hypothesize an almost unlimited portfolio of different worlds, you can explain anything,” says John Polkinghorne, formerly a theoretical particle physicist at Cambridge University and, for the past 26 years, an ordained Anglican priest. If a theory allows anything to be possible, it explains nothing; a theory of anything is not the same as a theory of everything, he adds.
Supporters of the multiverse theory say that critics are on the wrong side of history. “Throughout the history of science, the universe has always gotten bigger,” Carr says. “We’ve gone from geocentric to heliocentric to galactocentric. Then in the 1920s there was this huge shift when we realized that our galaxy wasn’t the universe. I just see this as one more step in the progression. Every time this expansion has occurred, the more conservative scientists have said, ‘This isn’t science.’ This is just the same process repeating itself.”
If the multiverse is the final stage of the Copernican revolution, with our universe but a speck in an infinite megacosmos, where does humanity fit in? If the life-friendly fine-tuning of our universe is just a chance occurrence, something that inevitably arises in an endless array of universes, is there any need for a fine-tuner—for a god?
“I don’t think that the multiverse idea destroys the possibility of an intelligent, benevolent creator,” Weinberg says. “What it does is remove one of the arguments for it, just as Darwin’s theory of evolution made it unnecessary to appeal to a benevolent designer to understand how life developed with such remarkable abilities to survive and breed.”
On the other hand, if there is no multiverse, where does that leave physicists? “If there is only one universe,” Carr says, “you might have to have a fine-tuner. If you don’t want God, you’d better have a multiverse.”
As for Linde, he is especially interested in the mystery of consciousness and has speculated that consciousness may be a fundamental component of the universe, much like space and time. He wonders whether the physical universe, its laws, and conscious observers might form an integrated whole. A complete description of reality, he says, could require all three of those components, which he posits emerged simultaneously. “Without someone observing the universe,” he says, “the universe is actually dead.”
Yet for all of his boldness, Linde hesitates when I ask whether he truly believes that the multiverse idea will one day be as well established as Newton’s law of gravity and the Big Bang. “I do not want to predict the future,” he answers. “I once predicted my own future. I had a very firm prediction. I knew that I was going to die in the hospital at the Academy of Sciences in Moscow near where I worked. I would go there for all my physical examinations. Once, when I had an ulcer, I was lying there in bed, thinking I knew this was the place where I was going to die. Why? Because I knew I would always be living in Russia. Moscow was the only place in Russia where I could do physics. This was the only hospital for the Academy of Sciences, and so on. It was quite completely predictable.
“Then I ended up in the United States. On one of my returns to Moscow, I looked at this hospital at the Academy of Sciences, and it was in ruins. There was a tree growing from the roof. And I looked at it and I thought, What can you predict? What can you know about the future?”
If these cosmic traits were just slightly altered, life as we know it would be impossible. A few examples:
• Stars like the sun produce energy by fusing two hydrogen atoms into a single helium atom. During that reaction, 0.007 percent of the mass of the hydrogen atoms is converted into energy, via Einstein’s famous e = mc2 equation. But if that percentage were, say, 0.006 or 0.008, the universe would be far more hostile to life. The lower number would result in a universe filled only with hydrogen; the higher number would leave a universe with no hydrogen (and therefore no water) and no stars like the sun.
• The early universe was delicately poised between runaway expansion and terminal collapse. Had the universe contained much more matter, additional gravity would have made it implode. If it contained less, the universe would have expanded too quickly for galaxies to form.
• Had matter in the universe been more evenly distributed, it would not have clumped together to form galaxies. Had matter been clumpier, it would have condensed into black holes.
• Atomic nuclei are bound together by the so-called strong force. If that force were slightly more powerful, all the protons in the early universe would have paired off and there would be no hydrogen, which fuels long-lived stars. Water would not exist, nor would any known form of life.