Collider Part 6

The following year, Bulgarian-born physicist Fritz Zwicky, working at Caltech, completed an independent investigation of the gravitational "glue" needed to keep a ma.s.sive group of galaxies called the Coma Cl.u.s.ter from drifting apart. Because the galaxies within this formation are widely separated, Zwicky estimated a very high figure for the gravity needed. Calculating the amount of ma.s.s needed to furnish such a large force, he was astounded to discover that it was hundreds of times that of the luminous matter. Some invisible scaffolding seemed to be providing the support required to hold such a far-flung structure together.

Scientists in the 1930s knew little about the cosmos, aside from Hubble's discovery of its expansion. Even the concept of galaxies as "island universes" beyond the Milky Way was relatively new. With physical cosmology in such an early stage of development, the astonis.h.i.+ng findings of Oort and Zwicky were largely ignored. Decades pa.s.sed before astronomers acknowledged their importance.

We owe current interest in dark matter to a courageous young astronomer, Vera Cooper Rubin, who entered the field when women were often discouraged from pursuing it. Rubin was born in Was.h.i.+ngton, D.C., and her childhood hobbies included stargazing from her bedroom window and reading astronomy books-particularly a biography of comet discoverer Maria Mitch.e.l.l. Much to her frustration, the vocation then seemed a clubhouse with a No Girls Allowed sign prominently displayed.

As Rubin later recalled: "When I was in school, I was continually told to go find something else to study, or told I wouldn't get a job as an astronomer. But I just didn't listen. If it's something you really want to do, you just have to do it-and maybe have the courage to do it a little differently."1 After earning a B.A. at Va.s.sar, where Mitch.e.l.l once taught, and an M.A. at Cornell, Rubin returned to her native city to pursue graduate studies in astronomy at Georgetown University. Though not on Georgetown's faculty, George Gamow, with whom she shared an interest in the behavior of galaxies, was permitted to serve as her thesis adviser. Under his valuable supervision, she received her Ph.D. in 1954.

While raising four children with her husband, mathematician Robert Rubin, it took some time for her to find a permanent position that offered her suitable flexibility. In 1965, the Department of Terrestrial Magnetism of Carnegie Inst.i.tution in Was.h.i.+ngton appointed her to its research staff. She soon teamed up with a colleague, Kent Ford, who had built his own telescope. Together they began an extensive study of the outer reaches of galaxies.

Focusing on the Milky Way's nearest spiral neighbor, the Andromeda galaxy, Rubin and Ford used a spectrograph to record the Doppler s.h.i.+fts of stars on its periphery. A Doppler s.h.i.+ft is an increase (or decrease) in frequency of something moving toward (or away) from an observer. The amount of the s.h.i.+ft depends on the moving object's relative velocity. It occurs for all kinds of wave phenomena-light as well as sound. We notice the Doppler effect when fire engines wail at higher and higher pitch when racing closer, and lower and lower when speeding away. For light, moving closer means a s.h.i.+ft toward the bluer end of the spectrum (a blue s.h.i.+ft, for short), and moving away, a s.h.i.+ft toward the redder end (a red s.h.i.+ft). Hubble used galactic red s.h.i.+fts in his proof that remote galaxies are receding from us. Doppler spectroscopy has continued to serve as a vital tool in astronomy.

Mapping out the s.h.i.+fts in light spectra of Andromeda's outermost stars, Rubin and Ford were able to calculate their velocities. They determined how quickly these outliers...o...b..ted the galaxy's center. Plotting stars' orbital speeds versus their radial distances, the Carnegie researchers produced an impressive graph, called a galactic rotation curve, displaying how Andromeda steered its remotest material.

As Kepler discovered centuries ago, for astronomical situations, such as the solar system, for which the bulk of the material is in the center, objects take much longer to orbit the farther they are from the middle. The outer planets...o...b..t much slower than the inner ones. While Neptune orbits at a tortoiselike 3.4 miles per second, Mercury whizzes around the Sun at an average pace of 30 miles per second. The reason is that the gravitational influence of the Sun drops off sharply at large radial distances and there is not enough ma.s.s in the outer solar system to affect planetary speeds to a large degree.

Spiral galaxies such as the Milky Way were once thought to have a similar central concentration of material. Visibly, the densest concentration of stars lies in bulges around their middles. The outer spiral arms and haloes surrounding the main disks seem, in contrast, to be wispy and dilute. But appearances can deceive.

The Carnegie researchers connected the dots on Andromeda's rotation curve. Fully expecting to see the velocities drop off with radial distances, as in the solar system, they were baffled when their points, even in the outermost reaches, continued along a flat line. Rather than a mountain slope, the curve resembled a level plateau. A flat-velocity profile meant a sprawling out in ma.s.s beyond the frontiers of the observed. Something unseen was lending gravitational strength to places where gravity should be puny.

To see if their results were peculiar to Andromeda or more general, Rubin and Ford teamed up with two of their colleagues, Norbert Thonnard and David Burstein, to investigate sixty additional spiral galaxies. Although not all galaxies are spiral-some are elliptical and others are irregular-they chose that pinwheel-like shape because of its simplicity. Unlike other galactic types, the outer stars in spirals generally revolve in the same direction. For that reason, their speeds are easier to plot and a.n.a.lyze.

Relying on data collected by telescopes at Kitt Peak Observatory in Arizona and Cerro Tololo Observatory in Chile, the team members plotted out rotation curves for all sixty galaxies. Amazingly, each exhibited the same leveling off in velocities observed for Andromeda. Rubin and her coworkers concluded that most of the material in spirals is spread out and invisible-revealing nothing about its content except its weighty influence. The mystery that had so troubled Oort and Zwicky was back in full force!

What lies behind the mask? Could dark matter be something ordinary that's simply very hard to see? Could our telescopes just not be powerful enough to reveal the bulk of material in s.p.a.ce?

A one-time leading dark matter candidate carries a name that matches its supposed gravitational brawn: MACHOs (Ma.s.sive Compact Halo Objects). These are ma.s.sive bodies in the haloes of galaxies that radiate very little. Examples include large planets (the size of Jupiter or greater), brown dwarfs (stars that never ignited), red dwarfs (weakly s.h.i.+ning stars), neutron stars (collapsed stellar cores composed of nuclear material), and black holes. Each of these was formed from baryonic matter: the stuff of atomic nuclei and the like, such as hydrogen gas.

To hunt for MACHOs and other hard-to-see gravitating objects, astronomers developed a powerful technique called gravitational microlensing. A gravitational lens is a ma.s.sive object that bends light like a prism. It relies on Einstein's general relativity theory that heavy objects curve s.p.a.ce-time, which in turn distorts the paths of light rays in their vicinity. This was verified in the 1919 observations of the bending of starlight by the Sun during a solar eclipse.

Microlensing is a way of using the gravitational distortion of light to weigh potential MACHOs when they pa.s.s between distant stars and Earth. If an unseen MACHO happened to move in front of a visible star (from a neighboring galaxy in the background, for instance), the starlight would brighten due to the MACHO's gravitational focusing. After the MACHO moves on, the light would dim again, back to its original intensity. From this brightness curve, astronomers could determine the MACHO's ma.s.s.

During the 1990s, the MACHO Project, an international group of astronomers based at Mt. Stromlo Observatory in Australia, catalogued thirteen to seventeen candidate microlensing events. The team discovered these characteristic brightness variations during an extensive search of the galactic halo using the Large Magellanic Cloud (a smaller neighboring galaxy) to provide the stellar background. From their data, the astronomers estimated that 20 percent of the matter in the galactic halo is due to MACHOs, ranging from 15 percent to 90 percent of the ma.s.s of the Sun. These results point to a population of lighter, dimmer stars in the Milky Way's periphery that cannot directly be seen but only weighed. Though these objects might add some heft to the galactic suburbs, the MACHO Project has shown that they could account for only a fraction of the missing ma.s.s.

There are other reasons to believe that MACHOs could help resolve only part of the dark matter mystery. Using nucleosynthesis (element-building) models that estimate how many protons must have been present in the moments after the Big Bang to produce the elements we see today, astrophysicists have been able to estimate the percentage of baryonic matter in the universe. Unfortunately, these estimates show that only a small fraction of dark matter could be baryonic in nature; the rest must be something else. Made of conventional baryonic matter, MACHOs thereby could not provide the full explanation. Consequently, researchers have turned to other candidates.

The beefy acronym MACHO was chosen to contrast it with another cla.s.s of dark-matter candidates, the ethereal WIMPs (Weakly Interacting Ma.s.sive Particles). Unlike MACHOs, WIMPs would not be astronomical objects but rather new types of ma.s.sive particles that interact exclusively through weak and gravitational forces. Because of their heaviness, they'd be slow moving-enabling their gravitational "glue" to help cement together the large structures in s.p.a.ce we observe, such as galaxies and cl.u.s.ters.

Neutrinos would fit the bill if they were heavier and more lethargic, given that as leptons, they ignore the strong force, and as neutral particles, they pay no heed to electromagnetism. The lightness and swiftness of neutrinos, however, seems to rule them out as significant components. This fleeting nature is akin to a featherweight, constantly traveling politician trying to draw support for a local council race. Without setting down robust roots in his community, how could he bring people together? Similarly, neutrinos never hang around long enough or make enough of an impact to serve as uniters.

Particles such as neutrinos that would be too light and quick to create structure are sometimes referred to as "hot dark matter." Although they might compose a portion of the missing ma.s.s in the universe, they could not explain how galaxies maintain such tight holds on their outermost stars nor how they clump together into cl.u.s.ters. Slower, bulkier substances, such as MACHOs and WIMPs, are grouped together into "cold dark matter." These would offer suitable scaffolding-if we could only find enough of them.

If not neutrinos, then which other neutral, nonhadronic particles could carry enough ma.s.s and move at a slow enough pace to steer stars and gravitate galaxies? Unfortunately, the standard model doesn't call any suitable candidates to mind. Apart from neutrinos, MACHOs, and WIMPs, another option, a hypothetical ma.s.sive particle called the axion, postulated to play a role in quantum chromodynamics (the theory of the strong force) and tagged by some theorists as a leading dark-matter contender, has yet to be found. The search for the universe's missing ma.s.s has been at an impa.s.se.

Enter the LHC to the rescue. Perhaps somewhere in its collision debris the secret key ingredients of cold dark matter will be revealed. Prime contenders would be the lightest supersymmetric companion particles, such as neutralinos, charginos, gluinos, photinos, squarks, and sleptons. Presuming they have energies on the TeV scale, each would present itself through characteristic decay profiles that would show up in tracking and calorimetry.

If dark matter were the main cosmic mystery, physicists would simply be clenching their teeth, crossing their fingers, and waiting expectantly for results at the LHC or elsewhere to turn up a suitable prospect. It would be like posting a reasonable job description and hoping that the right person will eventually apply. However, a much more nebulous search-the quest for dark energy-has turned out to be far more unnerving. Not only is something seriously missing, but scientists have little idea where to look.

Dark energy first jolted the scientific community in 1998, when two teams of astronomers-a group from Lawrence Berkeley National Laboratory led by Saul Perlmutter and a collaboration based at Mt. Stromlo Observatory that included Adam Riess, Robert Kirschner, and Brian Schmidt-announced startling results about the expansion of the universe. Each used supernovas in remote galaxies as distance gauges to trace the cosmic expansion far back in time. By plotting the distances to these galaxies versus their velocities as found by Doppler red-s.h.i.+fts in their spectral lines, the teams could determine how Hubble's law of galactic recession has changed over billions of years.

The type of exploding stars examined, called Supernova Ia, has the special property that their energy produced follows a regular progression. Because of this predictability, the teams were able to compare their actual with their observed light outputs and calculate how far away they are. This offered a yardstick to galaxies billions of light years away-recording their distances at the time of the stellar burst.

Astronomical objects with known energy output are called standard candles. Like distant street lamps on a dark road, you can judge their remoteness by how bright or dim they seem-a.s.suming that they put out roughly the same wattage. If, when walking down a street at night, your eyes were dazzled by an intense glare, you would likely deem its light source much closer than if it were so faint that you could barely see it. You would thereby be able to use its relative brightness to estimate its distance. Similarly, astronomers rely on standard candles such as Supernova Ia to gauge distances for which there would be no other measure.

The team led by Perlmutter, called the Supernova Cosmology Project (SCP), has deep connections with the world of particle physics. First of all, along with George Smoot's n.o.bel Prize- winning exploration of the cosmic microwave background using the Cosmic Background Explorer satellite, it represents an expansion of the mission of Lawrence's lab. Given that Lawrence was always looking for connections and applications, such a broad perspective perfectly suits the former Rad Lab. Also, one of the SCP's founding members is Gerson Goldhaber, who won acclaim for his role in the Stanford Linear Accelerator Center- led group that jointly discovered the J/psi particle. His older brother Maurice Goldhaber worked at Cavendish during the Rutherford/Chadwick era and was the longtime director of Brookhaven National Laboratory. So you could say that cosmology and high energy physics-the sciences of the extremely large and extraordinarily small-have become part of the same family.

When the SCP began its explorations, its researchers hoped to use supernova standard candles as means of pinning down the deceleration deceleration of the universe. The attractive nature of gravity means that any set of ma.s.sive objects moving apart must reduce its outward rate of expansion over time. Simply put, what goes up must come down-or at least slow down. Cosmologists therefore expected that the dynamics of the cosmos would follow one of three different paths, depending on the universe's density relative to a critical value: brake rapidly enough to reverse course, brake gradually enough not to reverse course, or brake at the precise rate required to remain perpetually on the cusp. of the universe. The attractive nature of gravity means that any set of ma.s.sive objects moving apart must reduce its outward rate of expansion over time. Simply put, what goes up must come down-or at least slow down. Cosmologists therefore expected that the dynamics of the cosmos would follow one of three different paths, depending on the universe's density relative to a critical value: brake rapidly enough to reverse course, brake gradually enough not to reverse course, or brake at the precise rate required to remain perpetually on the cusp.

All three scenarios would begin with the standard Big Bang. If the density were high enough, the universe would slow down enough over time that after many billions of years its expansion would transform into a contraction. Eventually, everything would compress back together in a Big Crunch. If the density were lower than the critical value, on the other hand, the cosmic expansion would forever continue to slow down-like a tired runner sluggishly pus.h.i.+ng himself forward. Though galaxies would move apart at an increasingly lethargic pace, they'd never muster the will to reunite. This possibility is called the Big Whimper. A third option-the density exactly matching the critical value-would involve a universe slowing down so much that it threatened recollapse but never quite did, like an acrobat carefully poised on a tightrope.

Perlmutter and his team fully expected to encounter one of these three possibilities. Their supernova data surprised them with a different story. Plots of velocity versus distance showed that the cosmic rate of expansion was speeding up, not slowing down. Something was pressing the gas pedal instead of the brakes-and it couldn't be any of the known forces. University of Chicago theorist Michael Turner dubbed this unknown agent "dark energy."

While both have mysterious ident.i.ties, dark energy could not be the same as dark matter. In contrast to dark matter, which would gravitate in the same way as ordinary matter, dark energy would serve as a kind of "antigravity," causing outward acceleration. If dark matter walked into a party, it would serve as a graceful host introducing people and bringing them together, but if dark energy intruded, it would act like the riot police dispersing the crowd. Indeed, too much dark energy in the cosmos would be no fun at all-the universe would eventually tear itself apart in a catastrophic scenario called the Big Rip.

Some physicists have represented dark energy by restoring Einstein's once-discarded cosmological constant term to general relativity. Although adding such a constant antigravity term would be a simple move, it could use some physical motivation. Physicists would be loath to add anything to a well-established theory without understanding the need for the new term on a fundamental level. That would mean interpreting the field theory behind it. Current field theories, however, support a much larger value of the vacuum energy that would need to be almost, but not exactly, canceled out to yield a reasonable cosmological constant. Thus, matching experimental bounds for cosmic acceleration has proven a daunting task.

Moreover, if dark energy were a constant throughout s.p.a.ce and time, it would never lose its effect. With gravity ceding more and more ground over the eons to dark energy, the Big Rip would be an absolute certainty. Before accepting such an outcome as inevitable, most theorists would like to mull over the alternatives.

Princeton physicist Paul Steinhardt, along with theorists Robert Caldwell and Rahul Dave, has suggested a different way of modeling dark energy, through a wholly new type of substance called quintessence. Quintessence is a hypothetical material with negative pressure that pushes things apart (like an elemental Samson on the Philistines' columns) rather than pulling them together (like ordinary, gravitating matter). Its name harks back to the four cla.s.sical elements of Empedocles-with quintessence representing the fifth. The distinction between a cosmological constant and quintessence is that while the former would be as stable as granite, the latter could vary from place to place and time to time like moldable putty.

Findings of the Wilkinson Microwave Anisotropy Probe of the cosmic microwave background support the idea that the cosmos is a mixture of dark energy, dark matter, and visible matter-in that order. The satellite picture has not been able to tell us, however, what specific ingredients const.i.tute the duet of dark substances.

Physicists hope that further clues as to the nature of dark energy, as well as dark matter, will turn up at the LHC. The discovery of quintessence at the LHC, for example, would revolutionize the field of cosmology and transform our understanding of matter, energy, and the universe. Indeed, based upon what is found, the fate of everything in s.p.a.ce could be in the balance.

Adding a cosmological constant or postulating a novel kind of material are not the only alternatives. Some theorists see a need to rethink the nature of gravity completely. Could gravitation behave distinctly on different scales-acting one way in the planetary arena and another in the galactic realm? Might Einstein's equations of general relativity, accurate as far as we can judge, be superseded in the grandest domain by another theory? As Rubin has said, "I suspect we won't know what dark matter is until we know what gravity is."2 Radical new gravitational theories propose a fundamental change in its mechanism and scope. They imagine that some of gravity's properties could be explained through its ability to penetrate unseen extra dimensions impervious to other forms of matter and energy. Conceivably, the dark substances in the universe could be shadows of a higher reality.

Remarkably, some of these novel theories, as strange as they seem, could be tested at the LHC. The extraordinary power of high-energy transformations may well reveal new dimensions in addition to novel particles. Who knows which of nature's long-held secrets the LHC's unprecedented energies will divulge?

10.

The Brane Drain Looking for Portals to Higher Dimensions If this is the best of all possible worlds, what are the others like?-VOLTAIRE, CANDIDE (1759)

The LHC, with its matter-changing properties, could be said to offer a modern-day "philosopher's stone." Interestingly, its region's stone has already been used to build a philosopher's house. That philosopher was Francois-Marie Arouet, better known as Voltaire.

The Chateau de Ferney, where the witty writer lived from 1758 almost until his death in 1778, is situated within a mile of the ring traced by the LHC. Within that mansion he completed his most famous work, Candide Candide, a cutting satire of the optimism of the German thinker Gottfried Leibniz. At first glance, the LHC and Leibniz (and Voltaire's parody thereof) might seem to have little in common. However, they are profoundly connected through the concept of parallel universes and alternative realities.

A rival of Newton in developing calculus, Leibniz believed that our world represents the optimum among the set of all possibilities. Leibniz drew his conclusions from the calculus of variations-the method he developed for finding shortest paths along a surface and related problems. An example of such a situation considers the ideal way to cross a hill-among the myriad routes there is one that minimizes the length. Leibniz pondered that G.o.d, in designing the universe, would choose the optimal solution whenever there are alternatives.

Voltaire's farcical character Dr. Pangloss, a teacher of "metaphysico-theologico-cosmolonigology," takes this concept to the extreme by concocting a convoluted rationale for anything that happens in this "best of all possible worlds."

"Observe that noses were made to support spectacles," remarks Pangloss. "Hence we have spectacles. Legs were obviously inst.i.tuted to be clad in breeches, and we have breeches."1 Even after Pangloss, along with his pupil Candide, suffers through the most horrific series of events imaginable, including the destruction wrought by the Lisbon earthquake and the terror of the Inquisition, he continues to rationalize his experiences. He concludes that if a solitary link were broken in the cosmic chain of events, no matter how dreadful the occurrence seemed at the time, ultimate good would never ensue-in their case, the possibility to cultivate a small garden. No one following the dismal adventure could miss Voltaire's irony.

Could we be living in the "best of all possible worlds?" The concept implies the existence of alternative realities-perhaps even universes parallel to our own. Until recently the concept of parallel universes lay exclusively in the realm of speculation. However, remarkably, one of the projects planned for the LHC is to test a new type of parallel universe idea, called the "braneworld" hypothesis, positing that everything we observe resides within a three-dimensional island surrounded by a sea of higher dimensions. "Brane" is short for "membrane," a description of structures such as the one theorized to support the observable cosmos. According to this hypothesis, the only particles able to leave our brane are gravitons, which are the carriers of gravity. Consequently, researchers plan to use the LHC to search for gravitons leaking into higher dimensions. If such extra dimensions are found, perhaps there are other branes parallel to our own. If these other branes turn out to be lifeless structures, perhaps we do indeed live in the optimum world.

The concept of parallel universes first entered physics in a kind of abstract, mathematical way through Richard Feynman's diagrammatic method of calculating the likelihoods of certain types of exchanges between charged particles. Each possibility is a.s.signed a certain weight and added up. One way of expressing this "sum over histories" makes use of Leibniz's calculus of variations through what is called the path integral formulation. According to this approach, if you know the starting and ending states of any quantum interaction, what happens in between is like a "hill" of multiple trajectories. You never know exactly which way the players in the interaction breached the hill; in fact, they traversed it many ways at once. All you can calculate is the most probable means of crossing, which works out to be the shortest distance.

Feynman did not intend his method, which he began to develop in the early 1940s under the supervision of his thesis adviser John Wheeler, to represent paths through a labyrinth of actual parallel universes. The math worked out splendidly and the predictions turned out perfectly accurate, that's all. However, in 1957, another of Wheeler's students, Hugh Everett, took matters a step further through his "Many Worlds Interpretation" of quantum mechanics.

According to Everett's hypothesis, each time particles interact on the microscopic level, the universe responds by bifurcating into a maze of alternatives, each slightly different. When an experimenter measures the result, he or she replicates into versions corresponding to each alternate reality. Each copy records a different result of the measurement, attributing the outcome to chance. But in reality, there is no chance involved, because every possibility is actually realized by a replica researcher-unable to communicate with other versions and compare results. Over time, the number of parallel universes-and occupants within-grows into a staggering figure, dwarfing even the number of atoms in all of visible s.p.a.ce.

Despite this shocking conclusion, in the 1970s noted theorist Bryce DeWitt became convinced of the importance of Everett's conjecture. DeWitt named and popularized the concept, arguing that it was the only reasonable way to make quantum mechanics objective, rather than dependent on the subjective act of measurement. After all, who could step outside of the universe itself, take readings, and cause its wave function to collapse into various possibilities? As crazy as the Many World Interpretation sounds, he argued, isn't it crazier to think of humans influencing the cosmos through their sensory perceptions? Although by then Everett had left theoretical physics (and would die in 1982 at the age of fifty-one), DeWitt was very effective in promoting the idea that we live in an ever-expanding web of parallel universes.

Along with Wheeler, DeWitt had already made inroads into the question of applying quantum principles to gravity. Wheeler was very interested in developing a sum-over-histories method for encapsulating the solutions of Einsteinian general relativity. In quantum mechanics, states can have different positions, momenta, spins, and so forth. These are like the distinct musical notes that make up a composition. What would be the equivalent keyboard for general relativity? Eventually, it occurred to him that the range of possible three-dimensional geometries would offer the medley of tones needed to compose his symphony. Exuberantly, he prodded DeWitt to help him develop the mathematical notation for this idea. As DeWitt recalled: Wheeler used to bug everybody. I got a telephone call from him one day around 1964 saying he would be pa.s.sing through the Raleigh-Durham airport-that's when I was in North Carolina-between planes for two hours. Would I please come out there and we would discuss physics? I knew that he was bugging everybody with the question "What is the domain s.p.a.ce for quantum gravity?" And I guess he had it finally figured out in his mind that it was the s.p.a.ce of three-geometries. This was not the direction I was really concentrating my efforts, but it was an interesting problem so . . . I wrote down this equation. I just found a piece of paper out there in the airport. Wheeler got very excited about this.2 The result was the Wheeler-DeWitt equation: a way of a.s.signing weights to three-dimensional geometries and summing them up to determine the most probable evolution of the universe. In theory, it was supposed to help researchers understand how reality as we know it emerged from the chaotic jumble of possibilities. In practice, however, the equation would become unwieldy if applied to complex situations.

In 1973, C. B. Collins and Stephen Hawking considered this question cla.s.sically in their influential paper "Why Is the Universe Isotropic?" Pondering the myriad possible general relativistic solutions-including isotropic as well as anisotropic cosmologies-they wondered which could evolve into the familiar present-day universe. The difference between isotropic and anisotropic cosmologies is that while the former expands evenly in all directions, like a spherical balloon being filled with air, the latter blows up at unequal rates depending on which way you look, more like a hotdog-shaped balloon becoming longer and longer as it is inflated but not much wider.

Not surprisingly, according to astronomers, the present-day universe on the largest scales is close to isotropic. s.p.a.ce seems to be expanding close to the same rate in all directions. The cosmic microwave background, a snapshot of the "era of recombination" three hundred thousand years after the birth of the universe, is similarly very close to being isotropic. (As we discussed, the COBE [Cosmic Background Explorer] and WMAP [Wilkinson Microwave Anisotropy Probe] satellites mapped out minute anisotropies.) Collins and Hawking wondered whether the very early universe, instants after the Big Bang, needed to have been isotropic as well. Why couldn't it have been arbitrarily chaotic like the hodgepodge of sand dunes on a rugged beach?

To examine the possibility of cosmic evolution from chaos to order, they considered what is now called the multiverse: a kind of universe of universes embodying the range of all geometric possibilities. Of this cosmic zoo, they wondered, which kind of creatures could evolve into the tame ent.i.ty with which we are familiar: isotropic s.p.a.ce as we see it today. Surprisingly, according to their calculations, only an infinitesimally minute percentage could make the leap. Only universes that were extraordinarily isotropic to begin with could end up with ordinary present-day conditions. Any deviation from perfection in the beginning would blow up over time into a cosmic monstrosity. How then to justify the improbable normality of today?

In lieu of an explanation based exclusively on physical laws, Collins and Hawking decided to invoke what Australian physicist Brandon Carter dubbed the anthropic principle: the concept that the existence of humans constrains the nature of the universe. If the universe were sufficiently different, anthropic reasoning a.s.serts, stars like the Sun wouldn't have formed, planets like Earth would be absent, beings like humans would not exist, and there would be n.o.body to experience reality. Therefore the fact that we, as intelligent ent.i.ties, are around implies that the universe must have been close enough to its present form to guarantee the emergence of such cognizant observers. Collins and Hawking applied the anthropic principle as follows to explain why the universe is isotropic: Suppose there are an infinite number of universes with all possible different initial conditions. Only those universes which are expanding just fast enough to avoid recollapsing would contain galaxies, and hence intelligent life. [These] would in general approach isotropy. On this view, the fact that we observe the universe to be isotropic would be simply a reflection of our own existence.3 The use of anthropic reasoning is akin to compiling news clippings about lottery results around the world and realizing that the reason for all of the success stories is that coverage is biased in favor of winners. Although there are millions of "parallel histories" of people who buy lottery tickets, only those who hit the jackpot generally make the news. If you perused all lottery stories without knowing this, you might wonder if lotteries almost always pay off handsomely. Not only would this seem unprofitable for those running such contests, it would also appear to violate the laws of chance. However, the selection principle of newsworthiness strongly favors the minute subset of parallel histories that end up in success. Similarly, the selection principle of conscious observation strongly favors the minute subset of parallel universes that end up producing intelligent life.

Throughout the final decades of the twentieth century-with respected physicists such as DeWitt, Collins, and Hawking referring in their research to a large or even infinite tally of universes-the speculative concept of alternative realities became a serious scientific talking point. Theorists grew bolder in their allusions to parallel realms beyond the reach of telescopic surveys. If a physical parameter couldn't be nailed down through an a.n.a.lysis of the observed universe, many researchers began to rely on a toolbox of effects based on the supposition of a largely unseen multiverse.

In 1980, American physicist Alan Guth proposed cosmic inflation as a potential solution of a number of issues in modern cosmology, including the question of why the present-day universe is so uniform. Instead of invoking anthropic reasoning, he suggested that the very early universe went through a stage of ultrarapid expansion that stretched out all abnormalities beyond the point of observability-similar to pulling on a bed sheet to smooth out the wrinkles. Guth's initial theory, though promising, presented a number of quandaries, including a prediction of observable transition zones between sectors of the universe possessing different conditions. Because astronomy doesn't record such barriers, the theory required modification.

Three years later, Russian cosmologist Andrei Linde linked the inflationary concept with the multiverse idea through a novel proposal called chaotic inflation. In Linde's variation, the multiverse is a nursery harboring the seeds of myriad baby universes. These seeds are sown through a randomly fluctuating scalar field (something like the Higgs but more variable) that sets the value of the vacuum energy for each region. Through the general relativistic principle that ma.s.s and energy govern geometry, the places where this energy is highest stimulate the fastest-growing areas-such as the abundance of jobs triggering growth in certain communities. As in sprawling suburbs plowing over fallow farms, the most rapidly expanding parts of the universe-the inflationary regions-quickly dominate all of the others. Linde's conclusion was that we live in one of these hyperexpanded megalopolises-with any others long since nudged away beyond possible detection.

Inflation has become a popular way of understanding the overall uniformity of the observable cosmos. One of its key advantages over pure anthropic argumentation is that it doesn't rely on the existence of humans to explain how tapioca pudding-like blandness emerged from the bubbling chaos of the primordial universe. Yet, by literally pus.h.i.+ng alternative versions of our universe beyond measurement, inflationary cosmology removes a potential means of verification. Fortunately, it offers predictions about the distribution of matter and energy in the stages of the universe after inflation. These characteristic patterns manifest themselves in the cosmic background radiation, which has been a.n.a.lyzed by WMAP and other surveys. The consensus of astrophysicists today is that cosmic inflation in some form remains a viable explanation of how the early universe developed. What form of inflation might have occurred and what could have caused such an era remain open questions.

The latest breed of parallel universe theory, the braneworld hypothesis, relies not on unseen realms of our own s.p.a.ce but rather on dimensions beyond the familiar three. The far-reaching idea proposes that ordinary s.p.a.ce comprises a three-dimensional membrane-or "brane," for short-floating in a higher dimensional reality called the bulk. According to this notion, the bulk would be impervious to all particles except gravitons. Because the carriers of the electroweak and strong interactions cannot penetrate its depths, its existence would affect only gravitational interactions. Hence, without photons being able to enter the bulk, we could not see it. The dilution of gravity by means of gravitons leaving our brane and infusing the bulk would explain why gravity is so much weaker than the other forces.

The concept of branes is a variation of string theory that generalizes the jump rope-like vibrations of string into pulsating objects of two, three, or higher dimensions-akin to bouncy trampolines or s.h.i.+mmering raindrops. These could have an enormous span of sizes-ranging from minute enough to represent elementary particles to grand enough to encompa.s.s all of observable s.p.a.ce. From the latter stems the idea that everything around us, except for gravitons, lives on a brane.

Branes have been under discussion as particle models for several decades. Dirac conceived the idea in the 1960s that particles are extended rather than pointlike. He didn't push the concept very far, however, and it was scarcely noticed by the physics community. In 1986, Texas researchers James Hughes, Jun Liu, and Joseph Polchinski constructed the first supersymmetric theory of membranes, demonstrating how they could represent various types of particles. The following year, Cambridge physicist Paul Townshend coined the term p-branes p-branes to denote higher-dimensional extended objects dwelling in an eleven-dimensional reality-like curiously shaped peas living in an exceptionally s.p.a.cious and intricate pod. (The "p" takes on values representing the number of dimensions of the membrane.) to denote higher-dimensional extended objects dwelling in an eleven-dimensional reality-like curiously shaped peas living in an exceptionally s.p.a.cious and intricate pod. (The "p" takes on values representing the number of dimensions of the membrane.) Around the same time, Townshend, his colleague Michael Duff, and other theorists revealed deep connections between membranes and strings called dualities. A duality is a kind of mathematical equivalence that allows swaps between the extreme cases of certain variables-for example, exchanging a microscopically small radius for an enormous one-while preserving other physical properties. It is like a card game in which the numbers "1" and "11" are both represented by aces, allowing players with aces to switch their value strategically from low to high to wield the best hand. Similarly, there are cases in membrane theory in which flipping certain variables from small to large serve well in proving certain equivalences.

Membrane theory was little noticed by the mainstream physics community until the mid-1990s, when a combination of dualities developed by researchers in that area served to unite the five kinds of string theory. When string theory first came into prominence in the early 1980s as a potential "theory of everything," various theorists proposed an embarra.s.sing a.s.sortment of types-technically known as Type I, Type IIA, Type IIB, Heterotic-O, and Heterotic-E-each of which seemed suitable. How to distinguish which was the real deal? Surely a theory of everything must be unique.

It would be like different witnesses to a crime scene relating clas.h.i.+ng descriptions to a detective-with one saying, "He had a long gray coat," another indicating, "He was wearing a short blue vest," and so forth-until the sleuth figured out that shadows and lighting altered the culprit's appearance. An awning blocking the Sun from a certain angle darkened his jacket and made it seem longer. Similarly, dualities gleaned from membrane theory showed that by changing perspectives all five varieties of string theory can be transformed into one another.

At a 1995 conference in Southern California, the leading string theorist Ed Witten dramatically announced the discovery of the "duality of dualities" uniting all of the string theory brands into a single approach, which he called "M-theory." Rather than defining the term, he left its meaning open to interpretation, a.s.serting that the "M" could stand for "magic," "mystery," or "matrix." Others thought immediately of "membranes" and "mother of all theories." The excitement generated by that announcement and the realization that string theory could be unified heralded what became known as the second string revolution (the first revolution being the 1980s discovery that string theory doesn't have mathematical anomalies).

In the unification of string theory, one of the parameters found to be adjustable is the size of what is called the large extra dimension. This nomenclature distinguishes several distinct kinds of dimensions. First of all, there are the three dimensions of s.p.a.ce, length, width, and height, which, along with the dimension of time, make up four-dimensional s.p.a.ce-time. Second, following an approach first suggested by Swedish physicist Oskar Klein, there are the small "compactified" dimensions-those curled up into tight knots too minuscule ever to observe. According to the ideas of Witten and others, these form various types of six-dimensional cl.u.s.ters named Calabi-Yau s.p.a.ces after mathematicians Eugenio Calabi and s.h.i.+ng-Tung Yau. Finally, there is an eleventh dimension of adjustable size-with dualities enabling it to thicken like dough mixed with yeast. This large extra dimension could conceivably be of detectable proportions.

How can we envision an extra dimension perpendicular to those we normally experience? It's like describing a hot air balloon ride to people who have never left the ground. Before the age of ballooning, n.o.body ever experienced Earth from an aerial perspective. Balloons-and later airplanes and s.p.a.ce-s.h.i.+ps-permitted far greater exploration of the dimension of height. If the eleventh dimension exists, and it is not curled up, what prevents us from experiencing that, too? According to some theorists, the answer may lie in the stickiness of the strings that make up matter and luminous energy.

A critical aspect of M-theory is the concept of the Dirichlet brane, or "D-brane" for short, developed by UC Santa Barbara researcher Joseph Polchinski, along with J. Dai and R. G. Leigh. Polchinski defined these as extended objects to which the end points of open strings can be attached. Open strings are those not connected with themselves, rather hanging loose like strands of spaghetti. The opposite of these are closed strings, which form complete loops like onion rings. Polchinski and his colleagues showed that open strings naturally cling to D-branes as if their ends were made of glue, but closed strings have no such constraint.

String theory represents quarks, leptons, photons, and most other particles as open strings. The exception is gravitons, modeled by closed strings. Therefore, aside from gravitons, all particles would naturally stick to a D-brane. Gravitons, on the other hand, would be free to wander away from one D-brane and head, like migrating birds, toward another.

The dichotomy between the stringy behavior of gravitons and other particles suggested a way to model the relative weakness of gravity using M-theory and resolve the hierarchy problem described earlier. In 1998, Stanford physicists Nima Arkani-Hamed, Savas Dimopoulos, and Gia Dvali (joined for one paper by Ignatius Antoniadas) suggested a scenario called the ADD model (after their initials) involving two D-branes separated by a large extra dimension on the order of 25 of an inch in size. The second D-brane would represent a parallel universe, or another section of our own universe, right in front of our eyes but completely invisible. Because all standard model fields would remain confined to our own brane, photons would never be able to make the leap and illuminate the parallel brane. The strong and weak forces would similarly keep mum about the close but hidden realm. Instead, the only means of discerning its existence would be through the unseen tugs of gravity.

Due to its ability to fill the bulk in between our brane and the parallel brane, gravity would become diluted-making it much less powerful than the other interactions. It would be similar to four boilers in the bas.e.m.e.nt of a ten-story apartment building, with the first three used to provide steam for a spa and sauna in an adjacent room, and the fourth pumping out heat for the other floors. While those in the spa might enjoy the full force of the steam, those on the highest floor might be huddled under blankets in chilly rooms. The strength of the boilers might be the same, but the dilution of the steam the fourth one produced would make it much less effective. Similarly, the leakage of gravitons from our brane would allow them to have the same interactive strength in principle as other exchange particles-weakened only because of their seepage into the bulk.

Unlike the MSSM (Minimal Supersymmetric Standard Model) method of augmenting the Standard Model with supersymmetry, the large extra dimensions approach offers the advantage of a single unification energy rather than several. Everything would be united on the TeV scale, which is, conveniently, the energy of the LHC. Gravity would just appear weaker because of its secret excursions to a nether realm. The tremendous energy of the Planck scale, which is much higher than the TeV scale, would never have to be approached to test unification. If that's true, it would sure save a lot of money.

Large extra dimensions also offer the enticing possibility of helping astronomers understand dark matter. In a variation of the scheme, called the Manyfold Universe, the ADD team, along with Stanford physicist Nemanja Kaloper, pondered what would happen if our brane were folded up like an accordion. Stars that are distant along our brane-their light taking millions of years to reach us-could be close together by way of a shortcut through the eleventh dimension. It would be the same as standing at the end of a serpentine line winding around a zigzag chain-link fence and then having someone lift a chain that allows you suddenly to be right next to the person formerly well ahead of you.

Because gravitons could breach the shortcut through the bulk, two otherwise distant stars could gravitationally influence each other. This influence would be felt but not seen, offering a possible explanation for at least one form of dark matter. In other words, some types of dark matter would actually be luminous matter pulling on other luminous matter through the curtain of the bulk.

One of the appealing aspects of the ADD scenario is that it offers testable predictions. It presages modifications to the law of gravity on scales less than 25of an inch. In contrast to the conclusions of both Newton and Einstein, it hypothesizes that gravitational attraction, for those short distances, would no longer follow an inverse-squared behavior but would rather be modified by an additional factor. For large distances, such as the radius of the moon's...o...b..t around the Earth, this factor would be insignificant, explaining why the discrepancy has never been noticed. Unfortunately for the success of the theory, despite numerous tests using sensitive instruments, the discrepancy has yet to be detected on the smallest scales, either. For example, experiments by a group led by Eric Adelberger at the University of Was.h.i.+ngton using a delicate kind of twisted pendulum called a torsion balance have measured gravitational attraction to be an inverse-squared relations.h.i.+p for distances much smaller than 25 inch. This has cast doubt upon at least the simplest form of the theory.

In 1999, physicists Lisa Randall and Raman Sundrum proposed a different kind of braneworld scheme that doesn't require the same stark modifications of the law of gravity. Although, like the ADD model, the Randall-Sundrum model posits two three-dimensional branes-one representing the observable universe where the Standard Model lives; and the other, a kind of forbidden zone where only gravitons dare venture-it doesn't require the distance between the branes to be measurably large. Rather, the parallel branes could be so close together to elude even the most sensitive instruments.

To achieve this feat, Randall and Sundrum found a clever way to dilute gravity without the need for a large extradimensional arena. They proposed a warping of the bulk that would concentrate the greatest part of the gravitons' wave function away from our brane. This warping would be a function of the distance from our brane in the extra dimension-growing deeper like the ocean away from the sh.o.r.e. Consequently, gravitons would have a much higher probability of being in the region near the other brane than touching ours. They would have minimal interaction with the particles on our brane-rendering gravity much weaker than the other forces.

We can envision the distinction between the ADD and Randall-Sundrum models in terms of choices an urban planner might make about accommodations for parking to keep cars away from the main street of a town. One option, a.n.a.logous to the ADD approach, would be a s.p.a.cious, flat parking lot nearby. Most cars would be scattered around its interior, far from the street. If there isn't much s.p.a.ce to spare, however, the planner might choose instead to dig deep and build an underground garage. Cars entering the garage would follow a ramp downward. As in the flat case, the cars would be well off the street-with the advantage of minimizing the impact on the cityscape. That second option would be more akin to the Randall-Sundrum approach.

To complete the a.n.a.logy, we can think of Main Street as corresponding to the three-dimensional s.p.a.ce in which we live, the number of parked cars on the street as representing the measured strength of gravity, and satellite photos as signifying scientific observation. The situation with no off-street parking opportunities and cars jammed on Main Street would represent gravity as being much stronger in our three-dimensional s.p.a.ce than what we actually detect in nature. The case with the flat parking lot would represent weak gravity and a large extra dimension that could easily be spotted by scientists. Finally, the case with the underground garage would similarly allow for weak gravity, but with the bonus of keeping the extra dimension concealed from direct scientific detection. Someone perusing an aerial shot might mistake the community for a quiet town with few cars-like a physicist mistaking our cosmos for one with a paltry number of dimensions and weak gravity.

If the bulk is scooping up gravitons like a plethora of zealous dustpans, might there be a way of detecting the extradimensional leakage with the LHC? One method, already attempted at the Tevatron, would be to look for events in which the detected particles spray in one direction but not another. This imbalance would indicate that an unseen particle (or set of particles) carried away a portion of the momentum and energy. Although this could represent an escaping graviton, more likely possibilities would need to be ruled out, such as the commonplace production of neutrinos. Unfortunately, even a hermetic detector such as ATLAS can't account for the streams of lost neutrinos that pa.s.s unhindered through almost everything in nature-except by estimating the missing momentum and a.s.suming it is all being transferred to neutrinos. Some physicists hope that statistical models of neutrino production would eventually prove sharp enough to indicate significant differences between the expected and actual pictures. Such discrepancies could prove that gravitons fled from collisions and ducked into regions beyond.

Another potential means of establis.h.i.+ng the existence of extra dimensions would be to look for the hypothetical phenomena called Kaluza-Klein excitations (named for Klein and an earlier unification pioneer, German mathematician Theodor Kaluza). These would reveal themselves as shadows in our brane of particles traveling though the bulk. We'd observe these as particles with the same charge, spin, and other properties as familiar particles except for curiously higher ma.s.ses.

Plato wrote a famous allegory about prisoners shackled to the interior wall of a cave since childhood, unable to observe the outside world directly. They watch the shadows on the opposite wall and mistake these images for real things. For example, they think that the shadows of individuals carrying vessels as they walk by are actual people. Eventually, one of the prisoners escapes, explores the world outside the cave, and informs the others about their delusion.

Similarly, it is possible that the LHC results (from ATLAS or CMS) could serve as a "cave wall" by which we could observe shadows of particles moving in a greater reality. These particles would have an extra component of their momentum corresponding to their ability to travel along an extra dimension. Because of the un.o.bservability of the extra dimension, we couldn't actually see the particles moving in that direction. Rather, their unseen motion would manifest itself through an additional amount of ma.s.s a.s.sociated with their extra energy and momentum. Researchers hope that the energies of some of the lightest Kaluza-Klein excitations are at the low end of the TeV scale, which could enable them to be observed by LHC researchers.

A host of research articles have offered predictions for potential signals of Kaluza-Klein gravitons and other particles beefed up by extra dimensions. These could decay into electron-positron pairs, muon-antimuon pairs, or other products at energies indicating their possible origin. Studying such excitations would yield valuable information about the size, shape, and other properties of the bulk.

Finding evidence of extra dimensions isn't one of the primary goals of the LHC. However, discovering unseen romping grounds beyond the view stands of our familiar arena would make particle physics a whole new game. Like Plato's cave dwellers, we'd have to face the possibility that everything around us is a shadow of a greater reality. Yet if, on the other hand, visible s.p.a.ce plus time make up all that there is, the quest for extra dimensions would ultimately prove futile. Theorists would need to concoct other explanations for why all the other forces are so much more potent than gravity.

One can imagine Voltaire's spirit hovering over the LHC, stirred by the whirlwind of particles circulating beneath his former village of Ferney, and smiling at the search for other possible worlds. Would he have considered it valid science or an exercise in Panglossian "metaphysico-theologico-cosmolonigology"? Perhaps he'd simply be pleased that his haunting grounds remain un jardin ouvert sur le monde un jardin ouvert sur le monde, cultivated above and below by motivated gardeners to sustain the body and the mind.

11.

Microscopic Black Holes A Boon to Science or a Boom for the World?

Human minds weren't meant to picture something that was smaller than an atom, and yet weighed megatons. . . . Something ineffably but insatiably hungry, and which grew ever hungrier the more it ate.-DAVID BRIN, EARTH (1990)

Now I am become Death, the destroyer of worlds.-ROBERT OPPENHEIMER, QUOTING THE BHAGAVAD GITA FOLLOWING THE TRINITY NUCLEAR TEST.

Not all scientists are lunatics. On the contrary, despite cinematic depictions ranging from Dr. Caligari to Dr. Evil, and aside from a smattering of harmless eccentrics, genuinely mad scientists are few and far between. Yet the cultural stereotypes persist, and drive particularly nervous members of the public to a.s.sume that the average laboratory researcher would think nothing of taking chances with the fate of the world.