The idea that our universe is but one of infinitely many in a multiverse, represents an extraordinary leap in our conception of the cosmos. It is often said of quantum mechanics that if you think you have understood it, you haven’t. Something similar might be true of the multiverse: if you think you can imagine it, you can’t.
Little wonder that it is easy to find cosmologists who think the hypothesis is barely scientific at all. They have a point. For if the multiverse is so wildly speculative, as even its advocates acknowledge it to be, then might the theory be a case of science being led more by metaphysics than physics? Or to put it more personally: are the religious convictions of individual cosmologists – atheistic or theistic – playing a role in determining what is presented as pure science?
Martin Rees is quite clear that metaphysics, let alone faith, should have nothing to do with physics. I sat with the astronomer royal in his 16th century, wooden panelled sitting-room, one of a suite in the Master’s Lodge at Trinity College, Cambridge. The sash windows held glass bubbled with age. The surroundings are grand, though Rees himself is the embodiment of intelligent humility in the face of big questions. ‘Fifty years ago we all thought of the big bang as very speculative,’ he suggests: ‘Now the big bang from one millisecond onwards is as well established as anything about the early history of the earth.’
So is there anything different about the multiverse? Just what is at stake?
One way of approaching the subject is to note how over the last ten years or so, there has developed a wide recognition amongst physicists that certain features of the universe appear uncanny. This observation is referred to as fine-tuning or the “anthropic principle”. The basic notion is that the existence of life appears to put limits on the values certain physical constants can take.
The anthropic idea was mocked when it was first proposed in 1973. Stephen Jay Gould penned one of the most amusing lines of ridicule. It is like declaring that ships were created so that barnacles could cling to them, he wrote. More generally, physicists shied from the principle, not just because it encouraged speculation that the universe might have a purpose, namely life, but also because they held out the hope that physics might be on the verge of deriving a “theory of everything”. One test of such a theory would be the fine-tuning were shown to be nothing but the internal consequences of such a TOE.
But the anthropic principle is mocked less frequently these days, if at all. For one thing, physicists have grown less confident that a TOE is imminent. And then, in 1998, a discovery was made which appears to give fine-tuning an astonishing boost.
In that year astronomers discovered that the universe is not just expanding, but expanding at an accelerating rate. It was a baffling finding because they expected the expansion of the universe to be slowing down, as a result of the braking effect of gravity. What the acceleration seems to imply is that there is another force at work in the cosmos, one that is pushing matter apart and can overcome everything, even gravity. What is it? Physicists don’t know, and have called it the “dark energy”.
There was one key candidate for this mysterious force, though. It came from quantum mechanics. In the strange world of the super-super-small, a vacuum is not all that it seems. In fact, it is not a vacuum. Rather, it is teeming with virtual particles that pop in and out of existence, only like the white noise on a digital radio, this mass of activity is so subtle that we don’t notice it. Physicists had known about the possibility of a quantum vacuum energy for some time. With the discovery of dark energy, they returned to it to calculate just how large it might be. Would it be right to match the acceleration of the universe?
It was not just not right, it was catastrophically too large. The calculations indicated that the quantum vacuum energy was oversized by a factor 10120. That is an extraordinary large number. There are “only” 1090 atoms in the entire known universe. If the actual dark energy were that large, the universe would be expanding so fast that light wouldn’t have time to reach our eyes from the end of our noses before they both shot apart to be amongst the stars. Only, such a large dark energy would mean there couldn’t be any stars too, and so we wouldn’t be here at all.
This new twist in the story of fine-tuning astonishes even the most atheistic of physicists. I spoke with Steven Weinberg, the Nobel-prize winner who has himself penned some witty lines mocking belief. On the telephone, from his office at the University of Texas at Austin, he said: “This is the one fine-tuning that seems to be extreme, far beyond what you could imagine just having to accept as a mere accident.”
Hence, anthropic debates have moved centre-stage in contemporary cosmology. And with them, the multiverse. The multiverse hypothesis is that our universe is just one of many. In each separate universe, the laws of nature are different. In some of those universes, the dark energy will be large – too large for stars and galaxies, rendering them cold and dead. In others, like ours, it will be about right. The notion appears to knock fine-tuning on the head, since the old rebuttal against it can be reinstated: we shouldn’t be surprised the magnitude of forces and the values of constants are as they are in our universe, since if they weren’t, we wouldn’t be here to measure them.
A handful of theories in modern physics have raised the possibility of a multiverse. For example, there is the so-called “many worlds” interpretation of quantum mechanics, championed by physicists such as David Deutsch. This view “explains” the probabilistic nature of quantum theory by postulating that the universe splits with every option that every quantum particle has. A massively splitting universe equals a multiverse.
Alternatively, the dominant model of the big bang can lead to a multiverse too. It includes what is known as inflation, a period in the early stages of the universe when it expanded rapidly. A variant on that, called eternal inflation, imagines universes continually emerging from the quantum background, and inflating to form universes in their own right. A massively inflating universe equals a multiverse.
Or again, multiverses can be part of string theory. To be precise, some string theorists have proposed that there might be 10500; universes in the multiverse.
Bernard Carr is an astronomer at Queen Mary University, London. Unlike Martin Rees, he does not enjoy wooden-panelled rooms in his day job, but inhabits an office at the top of a concrete high-rise, the windows of which hang as if on the edge of the universe. He sums up the multiverse predicament: “Everyone has their own reason why they’re keen on the multiverse. But what it comes down to is that there are these physical constants that can’t be explained. It seems clear that there is fine tuning, and you either need a tuner, who chooses the constants so that we arise, or you need a multiverse, and then we have to be in one of the universes where the constants are right for life.”
But which comes first, tuner or tuned? Who or what is leading the dance? Isn’t conjuring up a multiverse to explain already outlandish fine-tuning tantamount to leaping out of the physical frying pan and into the metaphysical fire?
Unsurprisingly, the multiverse proposal has provoked ideological opposition. In 2005, the New York Times published an opinion piece by a Roman Catholic cardinal, Christoph Schönborn, in which he called it “an abdication of human intelligence.” That comment led to a slew of letters lambasting the claim that the multiverse is a hypothesis designed to avoid “the overwhelming evidence for purpose and design found in modern science.” But even if you don’t go along with the prince of the church on that, he had another point which does resonate with many physicists, regardless of their belief. The idea that the multiverse solves the fine-tuning of the universe by effectively declaring that everything is possible is in itself not a scientific explanation at all: if you allow yourself to hypothesize any number of worlds, you can account for anything but say very little about how or why.
John Polkinghorne is a physicist who would have some sympathy with the cardinal, and not just because he is an Anglican priest. He is also a distinguished scientist, having played a key role in the discovery of quarks. “I think it’s a very remarkable fact that the universe is intelligible to us,” he says. (This time, I’m back in another 500-year-old room as Polkinghorne was the Master of Queen’s College, Cambridge: a grumpy looking Erasmus peers out at us from a gilded frame on the wall.) “I must say talking to some militantly non-believing colleagues, the intelligibility of the universe is one thing that does give them pause.” Polkinghorne himself calls the multiverse proposal a “fairly desperate measure.”
However, he adds that science only provides him with “a thin notion of God.” He is also quite clear that physics and theology are intellectual cousins, each with their own integrity that must be carefully guarded. And it is because of the integrity of physics that he primarily objects to the multiverse. It seems to be an untestable theory: astronomers can’t see to the edge of our own universe, let alone probe others. To him, that puts it beyond the realm of science. He recalls that when he was working on quarks, there existed a balance in physics between the theoretical and the empirical. Today, he believes that balance is at risk of being lost.
Countering that view is Steven Weinberg. He is an interesting voice in the debate, not just because he is a distinguished physicist, but because in the past he has been accused of favouring certain cosmological theories as they appeared to support his atheism. Another cosmologist and early advocate of the anthropic principle, Frank Tipler, made the accusation. He said that Weinberg had favoured the old steady state theory of the universe over the then new big bang, because it least resembled the account of creation in Genesis.
Weinberg is sanguine about the accusation: he changed his mind once the big bang was verified by evidence. Moreover, in relation to the multiverse hypothesis, he points out that it existed before the refinement of the anthropic principle in 1998. “The multiverse idea predates any of this thinking about fine tuning,” he says. He suggests it might be thought of as an answer waiting for a problem, and now we have both.
Also, it should be pointed out that there is no necessary correlation between being an atheist physicist and believing in the multiverse, or conversely being a believer and denying it. Again, Bernard Carr sums up: “I think if there’s a multiverse it doesn’t disprove God at all. The status of god – it could go either way if there’s a multiverse.”
What does Rees make of our central question: physics or metaphysics? “It’s not a matter for emotions and faith,” he insists. “It seems to me that it’s a scientific question and we just don’t know the answer yet. We want to know if there was one big bang or many. Obviously it affects the way we interpret the universe, just as the fact that we now know our solar system isn’t unique affects how we interpret the universe. But people’s personal prejudices are irrelevant. There is no particular reason to think our big bang was the only one, or that the volume of space we see with our telescopes is anything other than a tiny and maybe atypical fragment of reality. I think we have to be open minded about this.”
That is undoubtedly true. Evidence is what eventually settles science. But in the meantime, one should also be wary of sleights of hand. The multiverse is a hypothesis for which there is no evidence, and perhaps can never be any evidence. It is only since 1998 that it has leapt off the blackboards of a few physicists doing esoteric mathematics and lodged itself in the popular imagination. As is the way with popular science, it is easy to move from speculating that there might have been more than one big bang to proceeding on the basis that there has been more than one big bang.
So, I’d like to give the last word to another physicist, Paul Davies. He is the author of many widely read and highly readable books on cosmology and its ramifications, his latest being on the fine-tuning: The Goldilocks Enigma: Why is the Universe Just Right for Life? He is a multiverse sceptic, preferring another explanation of the fine-tuning. Very roughly, he suspects that the universe, its laws, and the presence of life somehow all emerged together. He calls it a “self-explaining universe”, that is without any external deity; though containing within it what he terms “the life principle”. Again, it is a speculative proposition: “there are many details to be worked out,” he admits.
But what is interesting about Davies is that he believes the evidence of fine-tuning is taking science in a direction that collapses the traditional distinction between physics and metaphysics: “I do take life, mind and purpose seriously, and I concede that the universe at least appears to be designed with a high level of ingenuity.”
He is quite aware that some scientists will already feel his approach is “crypto-religious”, even though he explicitly makes no appeal to non-natural processes. He retorts that all physicists are committed to some form of ideology. In another New York Times opinion piece, published in 2007, Davies argued that believing the universe is governed by laws is a form of faith too, faith in the existence and efficacy of laws. It is a faith that is well justified by evidence. But, as yet, science itself cannot account for these laws – where they come from, why they work. Davies concluded: “Until science comes up with a testable theory of the laws of the universe, its claim to be free of faith is manifestly bogus.”
Davies’ piece itself produced a slew of letters. In the present climate, it is a provocative opinion to express. And yet, the implication is that a testable theory of the laws of the universe, let alone the multiverse, isn’t going to appear any time soon. Until then at least, in some scientists’ mind, physics will inevitably rub shoulders with metaphysics.
Mark Vernon’s latest book is Wellbeing (Acumen), part of the Art of Living series, which he edits.