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It was the day after Christmas in 2004, a bright winter's day in Berkeley, California. I was outside a café at the corner of Shattuck and Cedar, waiting for Saul Perlmutter, an astrophysicist at the University of California. The campus is nestled at the base of wooded hills that rise steeply from the city's edge. About 1,000 feet up in the hills is the Lawrence Berkeley National Laboratory (LBNL). In the 1990s, the UC campus and LBNL housed several members of two teams of astronomers that simultaneously but independently discovered something that caused ripples of astonishment, even alarm. Our universe, it seems, is being blown apart.
Perlmutter was the leader of one of those teams. His enthusiastic, wide-eyed gaze, enhanced by enormous glasses, along with a forehead made larger by a receding hairline, reminded me of Woody Allen. But what he had found was no laughing matter. In fact, Perlmutter admitted that their discovery had thrown cosmology into crisis. The studies of distant supernovae by the two teams had shown that the expansion of the universe, first observed by Edwin Hubble in 1929, was accelerating - not, as many had predicted, slowing down. It was as if some mysterious energy were creating a repulsive force to counter gravity. Unsure as to its exact nature, cosmologists call it dark energy. More important, it seems to constitute nearly three-quarters of the total matter and energy in the universe.
Dark energy is the latest and most daunting puzzle to confront cosmologists, adding to another mystery that has haunted them for decades: dark matter. Nearly 90 percent of the mass of galaxies seems to be made of matter that is unknown and unseen. We know it must be there, for without its gravitational pull the galaxies would have disintegrated. Perlmutter pointed out that cosmologists in particular, and physicists in general, are now faced with the stark reality that roughly 96 percent of the universe cannot be explained with the theories at hand. All our efforts to understand the material world have illuminated only a tiny fraction of the cosmos.
And there are other mysteries. What is the origin of mass? What happened to the antimatter that should have been produced along with matter after the big bang? After almost a century of spectacular success at explaining our world using the twin pillars of modern physics - quantum mechanics and Einstein's general theory of relativity - physicists have reached a plateau of sorts. As Perlmutter put it, he and others are now looking to climb a steep stairway toward a new understanding of the universe, with only a foggy idea of what awaits them at the top.
Part of this seemingly superhuman effort will involve reconciling quantum mechanics with general relativity into a theory of quantum gravity. In situations where the two domains collide - where overwhelming gravity meets microscopic volumes, such as in black holes or in a big bang - the theories don't work well together. In fact, they fail miserably. One of the most ambitious attempts to bring them together is string theory, an edifice of incredible mathematical complexity. Its most ardent proponents hope that it will lead us not just to quantum gravity but to a theory of everything, allowing us to describe every aspect of the universe with a few elegant equations. But the discovery of dark energy and recent developments in string theory itself have conspired to confound. On yet another winter's day in the Bay Area, more than two years after meeting Perlmutter, I got a taste of just how grave things had gotten in physics.
It was a late February afternoon in 2007. A conference room on the ballroom level of the San Francisco Hilton was filled to capacity for this session at the annual meeting of the American Association for the Advancement of Science (AAAS). Three physicists were arguing about dark energy and how it relates to some of the most serious questions one can ask: Why is our universe the way it is? Is it fine-tuned for the existence of life? Dark energy, it turns out, is not merely mysterious; it seems to be at about the right value for the formation of stars and galaxies. “The great mystery is not why there is dark energy. The great mystery is why there is so little of it,” Leonard Susskind, Felix Bloch Professor of Theoretical Physics at Stanford and co-inventor of string theory, told the audience at the Hilton. He continued in a poetic vein: “The fact that we are just on the knife edge of existence, [that] if dark energy were very much bigger we wouldn't be here, that's the mystery.”
The hope until recently had been that string theory would explain this, that dark energy's value would fall out naturally as a solution to the theory's equations - as would the answers to other puzzling questions. Why does the proton weigh almost two thousand times more than the electron? Why is gravity so much weaker than the electromagnetic force? Essentially, why do the fundamental constants of nature have the values they do? The question of dark energy is emblematic of such concerns. Nothing in the laws of physics can explain why many aspects of our universe are what they are. They seem to be extraordinarily fine-tuned to produce a universe capable of supporting life - a fact that bothers physicists no end.
But string theory's hoped-for denouement is nowhere in sight. Indeed, some physicists are slowly abandoning the notion that everything about the universe can be reduced to a handful of equations. In San Francisco, Susskind rose to address this issue. His talk was titled “Why the Rats Are Fleeing the Ship.” However, abandoning reductionism hasn't meant abandoning string theory. Quite the contrary. For Susskind and many others, it has meant embracing the theory in all its mathematical glory, despite its mind-boggling consequences. One of the most outlandish implications of string theory, as it stands today, is the existence of a multiverse. The idea is that our universe is just one of a possible 10 to the five hundredth power universes, if not more. And in this extraordinary scenario lies an answer to the conundrum of why dark energy and other fundamental constants have the values they do. In a multiverse, all values of dark energy and fundamental constants are possible; in fact, the laws of physics can differ from universe to universe. To explain our universe, physicists don't have to resort to tweaking and fine-tuning. If a multiverse exists, then there is a fi- nite probability, however small, that our universe randomly emerged with the properties it has. The laws governing it give rise to stars and galaxies - and, indeed, planets and intelligent life, including physicists asking the question: Why is the universe the way it is?
This is the so-called anthropic principle, which, loosely stated, says that our universe is what it is because we are here to say so, and if it were any different we wouldn't exist to inquire. The idea is viewed by many as a cop-out, for then physicists don't have to work so hard to explain all things from first principles. Another speaker, cosmologist Andrei Linde, Susskind's colleague at Stanford, recalled his efforts to talk about the anthropic principle to physicists at Fermilab, outside Chicago, nearly twenty years ago. Linde had been warned that eggs were thrown at people who talked about such things, so he began by discussing something else entirely and switched topics midway, on the assumption that the Fermilabbers wouldn't “have enough time to go to Safeway and buy eggs.”
Given string theory's support for a multiverse, the anthropic principle is gaining traction. But string theory itself is so far from being experimentally verified that many physicists find it difficult, if not impossible, to take its implications seriously. The third participant that afternoon, cosmologist Lawrence Krauss, then of Case Western Reserve University, summed up the argument for the opposition. “I think you can imagine a theory where the multiverse would be science. If one had a theory, a real theory, a real theory that predicted lots of things we see about the universe, predicted lots of things we could test, but also predicted lots of things we couldn't test, then I think most of us would say we believe the things we cannot test [such as the existence of a multiverse],” he said.
Susskind was staring daggers at Krauss by then. But Susskind's somber tone at the end of the session suggested that it wasn't going to be easy to answer critics. “All I can say is that we worry about this,” he said. “[String theory] is the biggest question in physics right now. Can we make observational science out of it?”
One thing all three speakers agreed on: Only experiments could break this impasse.
The greatest advances in physics have come when theory has moved in near-lockstep with experiment. Sometimes the theory has come first and sometimes it's the other way around. For instance, it was an experiment performed in 1887 by Albert Michelson and Edward Morley - showing that the speed of light is independent of the motion of the observer - that influenced Einstein's 1905 formulation of the special theory of relativity. A decade later, Einstein produced the general theory of relativity, but it was only after experiments in 1919 verified its fascinating implication - the bending of starlight by the sun's gravity - that the theory gained widespread acceptance. And throughout the early to mid-1900s, theorists and experimentalists jostled and outdid each other as they shaped quantum mechanics. An equally fruitful collaboration occurred in the 1960s and 1970s, when particle physicists theorized about the fundamental particles and forces that make up the material world and experiments confirmed their startlingly accurate predictions. But this energetic interplay is now deadlocked. The discovery of dark energy and dark matter, along with the failure, so far, of experiments to find the Higgs boson (thought to give elementary particles their mass), has allowed theorists free rein. Ideas abound, adrift in a sea of speculation. Can the next generation of experiments in cosmology and particle physics help anchor the theories to reality?
This book is my attempt at an answer. It is a quest that took me from London, where I lived and worked, to the distant reaches of the Earth, from desolate deserts to the depths of derelict mines, from mountaintops to the bottom of the world, looking for cutting-edge experiments that promise to drag physics out of its theoretical morass. Many of the experiments I visited are tackling, each in its own way, the twin mysteries of dark matter and dark energy. But I also went to see the telescopes and detectors that are searching for antimatter, the Higgs boson, and neutrinos, which are elusive subatomic particles pervading the universe. Neutrinos barely interact with matter and travel unhindered through space, carrying information about the distant reaches of the cosmos in ways that no other particle can. All these experiments are building the steps of Perlmutter's metaphorical stairway. My journey, too, became a metaphor: for the forays that scientists are making to the very limits of their understanding - to the edge of physics.
The story begins with a pilgrimage to the 100-inch telescope at Mount Wilson in California, where Hubble discovered that our universe is expanding, thus laying the observational foundation for the big-bang theory and modern cosmology. The 100-inch pushed the technological boundaries of its time, but it has long been outstripped by modern telescopes now scanning the night skies. Every evening, they open their giant domes to peer more than halfway across the universe, gathering light, sometimes one photon at a time. The instruments that analyze this light are equally powerful, such as the 8.6-ton spectrograph that's helping astronomers study the universe slice by slice with incredible accuracy. In contrast are the small, hockey-puck-size silicon and germanium detectors, so exquisitely engineered that they are handled like works of art. They wait patiently, day after day, week after week, for the merest hint of dark matter.
These experiments are dwarfed by gigantic balloons that soar into the stratosphere bearing experiments that search for primordial antimatter and study the cosmic microwave background (a radiation left over from the big bang).
Experimental physics reaches its pinnacle at the Large Hadron Collider, the world's largest particle smasher. Machines weighing thousands of tons monitor the paths of subatomic particles with micrometer precision. These particles spew forth from collisions of proton beams - each beam carrying as much energy as a 400-ton train going 150 kilometers per hour. Superconducting magnets that are colder than deep space strain to keep these beams confined to their paths around a 27-kilometer-long underground tunnel. New particles that emerge from the cauldron of proton smashups may contain anything from the Higgs boson to dark matter to the first hints of extra dimensions.
These magnificent telescopes and detectors can work only in the most extreme settings. Their surreal environments are the unsung characters in this unfolding story - venues rarely appreciated and often overlooked. The cold, dry air above the Atacama Desert high in the Chilean Andes, where not a blade of grass can grow, allows starlight that has traveled for billions of years to enter a telescope without being smudged in its final approach by something as mundane as water vapor. (Space-based instruments, of course - such as the Hubble Space Telescope - don't have to contend with the atmosphere's deleterious effect on light.) The crystalline clarity of Lake Baikal in Siberia is crucial to a pioneering underwater neutrino telescope, and Russian physicists endure the piercing cold to camp on the frozen lake and work on their submerged instrument.
Descending into the Earth's crust affords similar benefits. Deep within an abandoned iron mine in Minnesota, physicists hunt for dark matter, their detectors shielded from the chaos of cosmic rays by a half mile of rock. The sweat-drenched miners who dug these mines with nothing more than drill bits and sledgehammers could hardly have imagined the role their mine now plays in deciphering the nature of our universe. Meanwhile, a vast and arid land in the interior of South Africa - a desolate expanse devoid of pollution - has been proposed as the site of the world's biggest radio telescope, its three thousand antennas capable of sweeping across vast swaths of the universe faster than any instrument ever built.
As extreme destinations go, there are few that compare with Antarctica, on average the coldest, driest, and highest continent on Earth. It's a land so frigid that a sharp intake of breath can sear one's lungs. Moist exhaled air freezes in an instant, and mortal danger, in the form of snow-covered crevasses, is only a moment of distraction away. Still, cosmologists cherish the Antarctic Plateau for its thin, dry, stable, and unpolluted air, and they are building gigantic telescopes to probe the cosmic microwave background with a precision that's impossible to emulate almost anywhere else on Earth. But it's not just the air above Antarctica that attracts the scientists. They are also turning the kilometers-thick ice at the South Pole into a neutrino detector. Nowhere else does there exist a block of material so massive, clear, and solid that it can be used to study the slipperiest particle in the universe. A frozen wasteland could lead us toward the correct theory of quantum gravity.
This book is a paean to the remote regions that are the soul of today's experimental cosmology. They astonish with their eloquence, whether it's the Milky Way strewn across a dark Chilean sky or the ethereal Hanle Valley ensconced in a secluded corner of the Tibetan Plateau, shielded from the world by the 8,000-meter peaks of the Greater Himalayas. Despite their differences, these places share a profound minimalism: There is nothing extraneous, none of the noise or distractions of modern society. A glaciologist I met in Antarctica spoke of the “absolute stillness” he felt on that continent, faced with only the elements, which were too extreme to ignore. Cosmology needs these places if it is to solve the pressing questions of our existence.