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Home , Leonard Susskind, The Cosmic Landscape

NEWS FEATURE NATURE|Vol 439|5 January 2006 J. MAGEE J. OUTRAGEOUS FORTUNE

A growing number of cosmologists and string theorists suspect the form of our Universe is little more than a coincidence. Are these harmless thought experiments, or a challenge to science itself? Geoff Brumfielinvestigates.

hy are we here? It’s a question way by chance are infinitesimal — one in a stand up, it would mean the constants of nature that has troubled philosophers, very large number. “It’s like you’re throwing are meaningless. “In the past, many people theologians and those who’ve darts, and the bullseye is just one part in 10120 were almost violently opposed to that idea Whad one drink too many. But of the dart board,” says Leonard Susskind, a because it wasn’t seen as proper science,” Carr theoretical physicists have a more essentialist string theorist based at Stanford University in says. “But there’s been a change of attitude.” way of asking the question: why is there any- California. “It’s just stupid.” Much of that change stems from work thing here at all? showing that our Universe may not be unique. For two decades now, theorists in the think- One in a zillion Since the early 1980s, some cosmologists have big field of cosmology have been stymied by a Physicists have historically approached this argued that multiple universes could have mathematical quirk in their equations. If the predicament with the attitude that it’s not just formed during a period of cosmic inflation number controlling the growth of the Uni- dumb luck. In their view, there must be some- that preceded the Big Bang. More recently, verse since the Big Bang is just slightly too thing underlying and yet-to-be-discovered string theorists have calculated that there high, the Universe expands so rapidly that pro- setting the value of these variables. “The idea is could be 10500universes, which is more than tons and neutrons never come close enough to that we have got to work harder because some the number of atoms in our observable Uni- bond into atoms. If it is just ever-so-slightly principle is missing,” says David Gross, a verse. Under these circumstances, it becomes too small, it never expands enough, and every- Nobel-prizewinning theorist and director of more reasonable to assume that several would thing remains too hot for even a single nucleus the Kavli Institute for Theoretical Physics in turn out like ours. It’s like getting zillions and to form. Similar problems afflict the observed Santa Barbara, California. zillions of darts to throw at the dart board, masses of elementary particles and the But things have changed in the past few Susskind says. “Surely, a large number of them strengths of fundamental forces. years, says astronomer Bernard Carr of Queen are going to wind up in the target zone.” And of In other words, if you believe the equations Mary, University of London, UK. String theo- course, we exist in our particular Universe of the world’s leading cosmologists, the proba- rists and cosmologists are increasingly turning because we couldn’t exist anywhere else. bility that the Universe would turn out this to dumb luck as an explanation. If their ideas It’s an intriguing idea with just one problem,

says Gross: “It’s impossible to disprove.” come to completely incorrect Because our Universe is, almost by definition, conclusions about the Uni- everything we can observe, there are no verse,” argues Max Tegmark, a apparent measurements that would confirm cosmologist at the Massachu- whether we exist within a cosmic landscape of setts Institute of Technology, multiple universes, or if ours is the only one. Cambridge. “For example, you And because we can’t falsify the idea, Gross might assume our Solar System says, it isn’t science. Or at least, it isn’t science is typical, but a typical point in

in any conventional sense of the word. “I think space is some intergalactic void SERVICE NEWS CICERO/STANFORD L. Gross sees this as science taking on some of the where you can’t see a single traits of religion,” says Carr. “In a sense he’s star.” correct, because things like faith and beauty Failing to consider our obser- are becoming a component of the discussion.” vational location has burned And yet in the overlapping circles of cosmol- scientists in the past. The six- “People in string ogy and string theory, the concept of a land- teenth-century German astron- theory are very scape of universes is becoming the dominant omer Johannes Kepler spent frustrated , view. “I really hope we have a better idea in the years trying to understand what future,” says Juan Maldacena, a string theorist seemed to be the even, geomet- as am I, by our at the Institute for Advanced Study in Prince- rical spacing of our planets inability to be more ton, New Jersey, summing up the views of from the Sun. Kepler searched predictive after all many in the field. “But this idea of a landscape for meaning in the planets is the best we have today.” The stakes are high: because he thought our Solar these years.” string theorists know that pursuing an unveri- System was unique; today’s scientists under- DAVIDGROSS fiable theory could look like desperation, but stand that our Solar System is but one of proba- they fear that looking for meaning in a mean- bly billions in the Galaxy. Under such ingless set of numbers may be equally fruitless. circumstances it seems reasonable to assume life had to evolve on a planet with water, he says, the planets are spaced according to little more perhaps we also had to evolve in a Universe Kepler’s error than random chance. where atoms could form. At the core of this dilemma is a concept known In much the same way as Kepler worried Until recently, Susskind was in the minority. as the anthropic principle: the idea that things about planetary orbits, cosmologists now Hints of multiple universes, however, were appear the way they do because we live at a puzzle over numbers such as the cosmological given by a cosmological theory known as infla- certain spot in the Universe. It’s not a new con- constant, which describes how quickly the Uni- tion. Inflation is the leading theory of the early cept, and has previously been regarded more verse expands. The observed value is so much Universe; it postulates that a period of rapid as philosophy than science. smaller than existing theories suggest, and yet early expansion created the flat and uniform But some scientists say that it offers a useful so precisely constrained by observations, that cosmos we see today. One version of inflation change of perspective. “It’s very important to theorists are left trying to figure out a deeper theory, devised in the early 1980s, suggests that take into account stuff like this, or you can meaning for why the cosmological constant inflation occurred even before the Big Bang. In has the value it does. this version, the expanding cosmos was foamy Many are still searching and energetic, says Steven Weinberg, a for some great unifying researcher at the University of Texas, Austin. theory that would explain “Every once in a while, one part of the Universe these variables. But others would expand and become a Big Bang,” he says.

P. KLEIN/REUTERS P. have started to believe “And these Big Bangs would all have different that, like Kepler, today’s values for their fundamental constants.” physicists are looking for meaning where there is Strings attached “It would be very none. “In recent years, it In 1987, Weinberg made a prediction that was looking more and turned out to support the idea of an anthropic foolish to throw more to me like the laws of Universe. Preliminary observations indicated away the right nature were environmen- that the cosmological constant was zero, but answer on the tal,” says Susskind, who has Weinberg reasoned that if the constant was just written a book making constrained by our anthropic perspective then basis that it this argument (L. Susskind it would be small, so as not to interfere with the doesn’t conform The Cosmic Landscape: formation of galaxies, stars and planets, but to some criteria String Theory and the Illu- non-zero, because it would be essentially sion of Intelligent Design. random. “That prediction has since been con- for what is or Little Brown, 2005). He firmed by observations of supernovae and the isn’t science.” suspects that there are microwave background,” says Weinberg, who LEONARDSUSSKIND many universes, all with admits he was a reluctant convert to the idea. different values for these The latest circumstantial arguments for variables. Just as human multiple universes come from string theory.

Landscape of possibilities: the geometry of string theory predicts as many as 10500universes.

String theory posits that tiny strings vibrating from our own, it seems impossible to tell perfectly tuned number would only exacerbate in the fabric of space-time give rise to the mul- whether our cosmos is the only one, or one of the debate, but Arkani-Hamed argues that it titude of particles and forces in the macro- many. Most scientists find this disturbing. will have the opposite effect. Unlike the cos- scopic Universe. Although string theory lacks Talk of a Universe fine-tuned for life has mological constant, which has had a contro- experimental support, it attracts broad interest already attracted supporters of intelligent versial history even in cosmology, this because it seems to offer a route to a grand the- design, who claim that an intelligent force fine-tuning would appear in the standard ory of everything — a way to unify relativity shaped evolution. If there’s no way to tell model, which most physicists consider to be with quantum mechanics. whether the values of scientific constants are the most complete physical theory ever devel- But as theorists developed string theory, a coincidence, the movement’s followers oped and tested. It would strengthen the they discovered that the equations gave rise to argue, then why not also consider them case for the arbitrary nature of certain funda- multiple solutions, each of which represented evidence of God’s handiwork? mental constants, Arkani-Hamed contends: a universe with different physical properties. The anthropic reasoning behind the land- “These measurements wouldn’t directly prove “The hope always was that we would under- scape of universes is disturbing on another or disprove the landscape, but they would be a stand why one solution was picked out,” says level, says Gross. Most theories grow stronger very big push in that direction.” Joe Polchinski, a string theorist at the Kavli with each observation that matches their pre- Institute. But despite their best efforts, after dictions. However, for the anthropic principle, Leap of faith two decades theorists are still stuck with a mil- random chance is the main factor. Patterns Still, many scientists distrust the concept and lion different solutions for the equations, and and correlations, the stones from which sci- continue to seek alternative explanations. therefore a million potential universes. entific theories are built, weaken it. In other Among them is Lisa Randall, also at Harvard. This landscape of solutions, as it became words, he says: “The power of the principle is Randall suspects that multiple universes are a known in the community, was both troubling strongest where you have ignorance.” mirage resulting from the unrefined equations and intriguing. On the one hand, the theory That may be, but measurements that could of string theory. “You really need to explore stubbornly refused to yield a single solution support anthropic reasoning are in the works. alternatives before taking such radical leaps of resembling our own cosmos, but then, some In 2007, researchers at Europe’s CERN particle faith,” she contends. And with no foreseeable argued, that might also explain the cosmologi- physics centre in Geneva, Switzerland, will way to detect other universes, Gross feels that cal constant’s apparent randomness. If these turn on the Large Hadron Collider, a massive such leaps of faith should not be taken. “I feel many solutions actually represent millions of accelerator that will probe particle energies that it’s a rather extreme conclusion to reach at universes, then the idea that one had worked never before seen by researchers. The acceler- this point,” he says. out just right for us wasn’t so far-fetched. ator might detect so-called supersymmetric Susskind, too, finds it “deeply, deeply trou- particles, predicted by some as a way of unify- bling” that there’s no way to test the principle. Ignorance is bliss ing the strong and weak nuclear forces with But he is not yet ready to rule it out completely. The snag was that one million universes wasn’t the electromagnetic force, an important step in “It would be very foolish to throw away the enough. To explain the perfectly adjusted cos- uniting all the forces of physics within a single right answer on the basis that it doesn’t con- mological constant one would need at least 1060 theory. form to some criteria for what is or isn’t universes, says Polchinski. Then, in 2000, he These particles could also hint at whether science,” he says. and Raphael Bousso at the Lawrence Berkeley we live in one of many universes, says Nima Gross believes that the emergence of multi- National Laboratory in Berkeley, California, Arkani-Hamed, a string theorist at Harvard ple universes in science has its origins in theo- calculated that there could be a lot more than a University in Cambridge, Massachusetts. If rists’ 20-year struggle to explain the finely million solutions. “The calculation had such the collider detects certain types of super- tuned numbers of the cosmos. “People in topological complexity that you could poten- symmetric particles, he says, it will indicate string theory are very frustrated, as am I, by tially get 10500universes,” Polchinski says. With another fine-tuning in the cosmos — the ratio our inability to be more predictive after all so many solutions, says Weinberg, it becomes of the weak nuclear force to the strength of these years,” he says. But that’s no excuse for easier to imagine that we happen to live in a gravity. The anthropic argument is the same: using such “bizarre science”, he warns. “It is Universe that seems tailored for our existence. if the number was off by as little as one part in a dangerous business.” ■ Easy to imagine, hard to prove. Because 1030, then we would not be here to discuss it. Geoff Brumfiel is Nature’s physical sciences other universes would be causally separated It might seem that the detection of a second, Washington correspondent.