Physics of the Impossible Part 10

"But do you really mean, sir," said Peter, "that there could be other worlds-all over the place, just around the corner-like that?"

"Nothing is more probable," said the Professor...while he muttered to himself, "I wonder what they do teach them at these schools."

-C. S. LEWIS, THE LION, THE WITCH AND THE WARDROBE.

listen: there's a h.e.l.l of a good universe next door; let's go.

-E. E. c.u.mMINGS.

Are alternate universes really possible? They are a favorite device for Hollywood scriptwriters, as in the Star Trek episode called "Mirror, Mirror." Captain Kirk is accidentally transported to a bizarre parallel universe in which the Federation of Planets is an evil empire held together by brutal conquest, greed, and plunder. In that universe Spock wears a menacing beard and Captain Kirk is the leader of a band of ravenous pirates, advancing by enslaving their rivals and a.s.sa.s.sinating their superiors.

Alternate universes enable us to explore the world of "what if" and its delicious, intriguing possibilities. In the Superman comics, for example, there have been several alternate universes in which Superman's home planet, Krypton, never blew up, or Superman finally reveals his true ident.i.ty as mild-mannered Clark Kent, or he marries Lois Lane and has superkids. But are parallel universes just the domain of Twilight Zone reruns, or do they have a basis in modern physics?

Throughout history going back to almost all ancient societies, people have believed in other planes of existence, the homes of the G.o.ds or ghosts. The Church believes in heaven, h.e.l.l, and purgatory. The Buddhists have Nirvana and different states of consciousness. And the Hindus have thousands of planes of existence.

Christian theologians, at a loss to explain where heaven might be located, have often speculated that perhaps G.o.d lives in a higher dimensional plane. Surprisingly, if higher dimensions did exist, many of the properties ascribed to the G.o.ds might become possible. A being in a higher dimension might be able to disappear and reappear at will or walk through walls-powers usually ascribed to deities.

Recently the idea of parallel universes has become one of the most hotly debated topics in theoretical physics. There are, in fact, several types of parallel universes that force us to reconsider what we mean by what is "real." What is at stake in this debate about various parallel universes is nothing less than the meaning of reality itself.

There are at least three types of parallel universes that are intensely discussed in the scientific literature: a. hypers.p.a.ce, or higher dimensions, b. the multiverse, and.

c. quantum parallel universes.

HYPERs.p.a.cE.

The parallel universe that has been the subject of the longest historical debate is one of higher dimensions. The fact that we live in three dimensions (length, width, height) is common sense. No matter how we move an object in s.p.a.ce, all positions can be described by these three coordinates. In fact, with these three numbers we can locate any object in the universe, from the tip of our noses to the most distant of all galaxies.

A fourth spatial dimension seems to violate common sense. If smoke, for example, is allowed to fill up a room, we do not see the smoke disappearing into another dimension. Nowhere in our universe do we see objects suddenly disappearing or drifting off into another universe. This means that any higher dimensions, if they exist at all, must be smaller than an atom.

Three spatial dimensions form the fundamental basis of Greek geometry. Aristotle, for example, in his essay "On Heaven," wrote, "The line has magnitude in one way, the plane in two ways, and the solid in three ways, and beyond these there is no other magnitude because the three are all." In AD 150 Ptolemy of Alexandria offered first "proof" that higher dimensions were "impossible." In his essay "On Distance," he reasoned as follows. Draw three lines that are mutually perpendicular (like the lines forming the corner of a room). Clearly, he said, a fourth line perpendicular to the other three cannot be drawn, hence a fourth dimension must be impossible. (What he actually proved was that our brains are incapable of visualizing the fourth dimension. The PC on your desk calculates in hypers.p.a.ce all the time.) For two thousand years, any mathematician who dared to speak of the fourth dimension potentially suffered ridicule. In 1685 mathematician John Wallis polemicized against the fourth dimension, calling it a "Monster in Nature, less possible than a Chimera or Centaure." In the nineteenth century Karl Gauss, the "prince of mathematicians," worked out much of the mathematics of the fourth dimension but was afraid to publish because of the backlash it would cause. But privately Gauss conducted experiments to test whether flat, three-dimensional Greek geometry really described the universe. In one experiment he placed his a.s.sistants on three mountaintops. Each one had a lantern, thereby forming a huge triangle. Gauss then measured the angles of each corner of the triangle. To his disappointment, he found that the interior angles all summed up to 180 degrees. He concluded that if there were deviations to standard Greek geometry, they must be so small that they could not be detected with his lanterns.

Gauss left it to his student, Georg Bernhard Riemann, to write down the fundamental mathematics of higher dimensions (which were then imported wholesale decades later into Einstein's theory of general relativity). In one powerful sweep, in a celebrated lecture Riemann delivered in 1854, he overthrew two thousand years of Greek geometry and established the basic mathematics of the higher, curved dimensions that we use even today.

After Riemann's remarkable discovery was popularized in Europe in the late 1800s, the "fourth dimension" became quite a sensation among artists, musicians, writers, philosophers, and painters. Pica.s.so's cubism, in fact, was partly inspired by the fourth dimension, according to art historian Linda Dalrymple Henderson. (Pica.s.so's paintings of women with eyes facing forward and nose to the side was an attempt to visualize a fourth-dimensional perspective, since one looking down from the fourth dimension could see a woman's face, nose, and the back of her head simultaneously.) Henderson writes, "Like a Black Hole, the 'fourth dimension' possessed mysterious qualities that could not be completely understood, even by the scientists themselves. Yet, the impact of 'the fourth dimension' was far more comprehensive than that of Black Holes or any other more recent scientific hypothesis except Relativity Theory after 1919."

Other painters drew from the fourth dimension, as well. In Salvador Dali's Christus Hypercubius, Christ is crucified in front of a strange, floating three-dimensional cross, which is actually a "tesseract," an unraveled four-dimensional cube. In his famous Persistence of Memory, he attempted to represent time as the fourth dimension, and hence the metaphor of melted clocks. Marcel Duchamps's Nude Descending a Staircase was an attempt to represent time as the fourth dimension by capturing the time-lapse motion of a nude walking down a staircase. The fourth dimension even pops up in a story by Oscar Wilde, "The Canterville Ghost," in which a ghost haunting a house lives in the fourth dimension.

The fourth dimension also appears in several of H. G. Wells's works, including The Invisible Man, The Plattner Story, and The Wonderful Visit. (In the latter, which has since been the basis of scores of Hollywood movies and science fiction novels, our universe somehow collides with a parallel universe. A poor angel from the other universe falls into our universe after being accidentally shot by a hunter. Horrified by all the greed, pettiness, and selfishness of our universe, the angel eventually commits suicide.) The idea of parallel universes was also explored, tongue-in-cheek, by Robert Heinlein in The Number of the Beast. Heinlein imagines a group of four brave individuals who romp across parallel universes in a mad professor's interdimensional sports car.

In the TV series Sliders, a young boy reads a book and gets the inspiration to build a machine that would allow him to "slide" between parallel universes. (The book that the young boy was reading was actually my book, Hypers.p.a.ce.) But historically the fourth dimension has been considered a mere curiosity by physicists. No evidence has ever been found for higher dimensions. This began to change in 1919 when physicist Theodor Kaluza wrote a highly controversial paper that hinted at the presence of higher dimensions. He started with Einstein's theory of general relativity, but placed it in five dimensions (one dimension of time and four dimensions of s.p.a.ce; since time is the fourth s.p.a.ce-time dimension, physicists now refer to the fourth spatial dimension as the fifth dimension). If the fifth dimension were made smaller and smaller, the equations magically split into two pieces. One piece describes Einstein's standard theory of relativity, but the other piece becomes Maxwell's theory of light!

This was a stunning revelation. Perhaps the secret of light lies in the fifth dimension! Einstein himself was shocked by this solution, which seemed to provide an elegant unification of light and gravity. (Einstein was so shaken by Kaluza's proposal that he mulled it over for two years before finally agreeing to have this paper published.) Einstein wrote to Kaluza, "The idea of achieving [a unified theory] by means of a five-dimensional cylinder world never dawned on me...At first glance, I like your idea enormously...The formal unity of your theory is startling."

For years physicists had asked the question: if light is a wave, then what is waving? Light can pa.s.s through billions of light-years of empty s.p.a.ce, but empty s.p.a.ce is a vacuum, devoid of any material. So what is waving in the vacuum? With Kaluza's theory we had a concrete proposal to answer this problem: light is ripples in the fifth dimension. Maxwell's equations, which accurately describe all the properties of light, emerge simply as the equations for waves traveling in the fifth dimension.

Imagine fish swimming in a shallow pond. They might never suspect the presence of a third dimension, because their eyes point to the side, and they can only swim forward and backward, left and right. A third dimension to them might appear impossible. But then imagine it rains on the pond. Although they cannot see the third dimension, they can clearly see the shadows of the ripples on the surface of the pond. In the same way, Kaluza's theory explained light as ripples traveling on the fifth dimension.

Kaluza also gave an answer as to where the fifth dimension was. Since we see no evidence of a fifth dimension, it must have "curled up" so small that it cannot be observed. (Imagine taking a two-dimensional sheet of paper and rolling it up tightly into a cylinder. From a distance, the cylinder looks like a one-dimensional line. In this way, a two-dimensional object has been turned into a one-dimensional object by curling it up.) Kaluza's paper initially created a sensation. But in the coming years, objections were found to his theory. What was the size of this new fifth dimension? How did it curl up? No answers could be found.

For decades Einstein would work on this theory in fits and starts. After he pa.s.sed away in 1955, the theory was soon forgotten, becoming just a strange footnote to the evolution of physics.

STRING THEORY.

All this has changed with the coming of a startling new theory, called the superstring theory. By the 1980s physicists were drowning in a sea of subatomic particles. Every time they smashed an atom apart with powerful particle accelerators, they found scores of new particles spitting out. It was so frustrating that J. Robert Oppenheimer declared that the n.o.bel Prize in Physics should go to the physicist who did not discover a new particle that year! (Enrico Fermi, horrified at the proliferation of subatomic particles with Greek-sounding names, said, "If I could remember the names of all these particles, I would have become a botanist.") After decades of hard work, this zoo of particles could be arranged into something called the Standard Model. Billions of dollars, the sweat of thousands of engineers and physicists, and twenty n.o.bel Prizes have gone into painfully a.s.sembling, piece by piece, the Standard Model. It is a truly remarkable theory, which seems to fit all the experimental data concerning subatomic physics.

But the Standard Model, for all its experimental successes, suffered from one serious defect. As Stephen Hawking says, "It is ugly and ad hoc." It contains at least nineteen free parameters (including the particle ma.s.ses and the strength of their interactions with other particles), thirty-six quarks and antiquarks, three exact and redundant copies of sub-particles, and a host of strange-sounding subatomic particles, such as tau neutrinos, Yang-Mills gluons, Higgs bosons, W bosons, and Z particles. Worse, the Standard Model makes no mention of gravity. It seemed hard to believe that nature, at its most supreme, fundamental level, could be so haphazard and supremely inelegant. Here was a theory only a mother could love. The sheer inelegance of the Standard Model forced physicists to rea.n.a.lyze all their a.s.sumptions about nature. Something was terribly wrong.

If one a.n.a.lyzes the last few centuries in physics, one of the most important achievements of the last century was to summarize all fundamental physics into two great theories: the quantum theory (represented by the Standard Model) and Einstein's theory of general relativity (describing gravity). Remarkably, together they represent the sum total of all physical knowledge at a fundamental level. The first theory describes the world of the very small, the subatomic quantum world where particles perform a fantastic dance, darting in and out of existence and appearing two places at the same time. The second theory describes the world of the very large, such as black holes and the big bang, and uses the language of smooth surfaces, stretched fabrics, and warped surfaces. The theories are opposites in every way, using different mathematics, different a.s.sumptions, and different physical pictures. It's as if nature had two hands, neither of which communicated with the other. Furthermore, any attempt to join these two theories has led to meaningless answers. For half a century any physicist who tried to mediate a shotgun wedding between the quantum theory and general relativity found that the theory blew up in their faces, producing infinite answers that made no sense.

All of this changed with the advent of the superstring theory, which posits that the electron and the other subatomic particles are nothing but different vibrations of a string, acting like a tiny rubber band. If one strikes the rubber band, it vibrates in different modes, with each note corresponding to a different subatomic particle. In this way, superstring theory explains the hundreds of subatomic particles that have been discovered so far in our particle accelerators. Einstein's theory, in fact, emerges as just one of the lowest vibrations of the string.

String theory has been hailed as a "theory of everything," the fabled theory that eluded Einstein for the last thirty years of his life. Einstein wanted a single, comprehensive theory that would summarize all physical law, that would allow him to "read the Mind of G.o.d." If string theory is correct in unifying gravity with the quantum theory, then it might represent the crowning achievement of science going back two thousand years ago to when the Greeks asked what matter was made of.

But the bizarre feature of superstring theory is that these strings can only vibrate in a specific dimension of s.p.a.ce-time; they can only vibrate in ten dimensions. If one tries to create a string theory in other dimensions, the theory breaks down mathematically.

Our universe, of course, is four-dimensional (with three dimensions of s.p.a.ce and one of time). This means that the other six dimensions must have collapsed somehow, or curled up, like Kaluza's fifth dimension.

Recently physicists have given serious thought to proving or disproving the existence of these higher dimensions. Perhaps the simplest way to prove the existence of higher dimensions would be to find deviations from Newton's law of gravity. In high school we learn that the gravity of the Earth diminishes as we go into outer s.p.a.ce. More precisely, gravity diminishes with the square of the distance of separation. But this is only because we live in a three-dimensional world. (Think of a sphere surrounding the Earth. The gravity of the Earth spreads out evenly across the surface of the sphere, so the larger the sphere, the weaker the gravity. But since the surface of the sphere grows as the square of its radius, the strength of gravity, spread out over the surface of the sphere, must diminish as the square of the radius.) But if the universe had four spatial dimensions, then gravity should diminish as the cube of the distance of separation. If the universe had n spatial dimensions, then gravity should diminish as the n-1-th power. Newton's famous inverse-square law has been tested with great accuracy over astronomical distances; that is why we can send s.p.a.ce probes soaring past the rings of Saturn with breathtaking accuracy. But until recently Newton's inverse-square law had never been tested at small distances in the laboratory.

The first experiment to test the inverse-square law at small distances was performed at the University of Colorado in 2003 with negative results. Apparently there is no parallel universe, at least not in Colorado. But this negative result has only whetted the appet.i.te of other physicists, who hope to duplicate this experiment with even greater accuracy.

Furthermore, the Large Hadron Collider, which will become operational in 2008 outside Geneva, Switzerland, will be looking for a new type of particle called the "sparticle," or superparticle, which is a higher vibration of the superstring (everything you see around you is but the lowest vibration of the superstring). If sparticles are found by the LHC, it could signal a revolution in the way we view the universe. In this picture of the universe, the Standard Model simply represents the lowest vibration of the superstring.

Kip Thorne says, "By 2020, physicists will understand the laws of quantum gravity, which will be found to be a variant of string theory."

In addition to higher dimensions, there is another parallel universe predicted by string theory, and this is the "multiverse."

THE MULTIVERSE.

There is still one nagging question about string theory: why should there be five different versions of string theory? String theory could successfully unify the quantum theory with gravity, but there were five ways in which this could be done. This was rather embarra.s.sing, since most physicists wanted a unique "theory of everything." Einstein, for example, wanted to know if "G.o.d had any choice in making the universe." His belief was that the unified field theory of everything should be unique. So why should there be five string theories?

In 1994 another bombsh.e.l.l was dropped. Edward Witten of Princeton's Inst.i.tute for Advanced Study and Paul Townsend of Cambridge University speculated that all five string theories were in fact the same theory-but only if we add an eleventh dimension. From the vantage point of the eleventh dimension, all five different theories collapsed into one! The theory was unique after all, but only if we ascended to the mountaintop of the eleventh dimension.

In the eleventh dimension a new mathematical object can exist, called the membrane (e.g., like the surface of a sphere). Here was the amazing observation: if one dropped from eleven dimensions down to ten dimensions, all five string theories would emerge, starting from a single membrane. Hence all five string theories were just different ways of moving a membrane down from eleven to ten dimensions.

(To visualize this, imagine a beach ball with a rubber band stretched around the equator. Imagine taking a pair of scissors and cutting the beach ball twice, once above and once below the rubber band, thereby lopping off the top and bottom of the beach ball. All that is left is the rubber band, a string. In the same way, if we curl up the eleventh dimension, all that is left of a membrane is its equator, which is the string. In fact, mathematically there are five ways in which this slicing can occur, leaving us with five different string theories in ten dimensions.) The eleventh dimension gave us a new picture. It also meant that perhaps the universe itself was a membrane, floating in an eleven-dimensional s.p.a.ce-time. Moreover, not all these dimensions had to be small. In fact, some of these dimensions might actually be infinite.

This raises the possibility that our universe exists in a multiverse of other universes. Think of a vast collection of floating soap bubbles or membranes. Each soap bubble represents an entire universe floating in a larger arena of eleven-dimensional hypers.p.a.ce. These bubbles can join with other bubbles, or split apart, and even pop into existence and disappear. We might live on the skin of just one of these bubble universes.

Max Tegmark of MIT believes that in fifty years "the existence of these 'parallel universes' will be no more controversial than the existence of other galaxies-then called 'island universes'-was 100 years ago."

How many universes does string theory predict? One embarra.s.sing feature of string theory is that there are trillions upon trillions of possible universes, each one compatible with relativity and the quantum theory. One estimate claims that there might be a googol of such universes. (A googol is 1 followed by 100 zeros.) Normally communication between these universes is impossible. The atoms of our body are like flies trapped on flypaper. We can move freely about in three dimensions along our membrane universe, but we cannot leap off the universe into hypers.p.a.ce, because we are glued onto our universe. But gravity, being the warping of s.p.a.ce-time, can freely float into the s.p.a.ce between universes.

In fact, there is one theory that states that dark matter, an invisible form of matter that surrounds the galaxy, might be ordinary matter floating in a parallel universe. As in H. G. Wells's novel The Invisible Man, a person would become invisible if he floated just above us in the fourth dimension. Imagine two parallel sheets of paper, with someone floating on one sheet, just above the other.

In the same way there is speculation that dark matter might be an ordinary galaxy hovering above us in another membrane universe. We could feel the gravity of this galaxy, since gravity can ooze its way between universes, but the other galaxy would be invisible to us because light moves underneath the galaxy. In this way, the galaxy would have gravity but would be invisible, which fits the description of dark matter. (Yet another possibility is that dark matter might consist of the next vibration of the superstring. Everything we see around us, such as atoms and light, is nothing but the lowest vibration of the superstring. Dark matter might be the next higher set of vibrations.) To be sure, most of these parallel universes are probably dead ones, consisting of a formless gas of subatomic particles, such as electrons and neutrinos. In these universes the proton might be unstable, so all matter as we know it would slowly decay and dissolve. Complex matter, consisting of atoms and molecules, probably would not be possible in many of these universes.

Other parallel universes might be just the opposite, with complex forms of matter far beyond anything we can conceive of. Instead of just one type of atom consisting of protons, neutrons, and electrons, they might have a dazzling array of other types of stable matter.

These membrane universes might also collide, creating cosmic fireworks. Some physicists at Princeton believe that perhaps our universe started out as two gigantic membranes that collided 13.7 billion years ago. The shock waves from that cataclysmic collision created our universe, they believe. Remarkably, when the experimental consequences of this strange idea are explored they apparently match the results from the WMAP satellite currently orbiting the Earth. (This is called the "Big Splat" theory.) The theory of the multiverse has one fact in its favor. When we a.n.a.lyze the constants of nature, we find that they are "tuned" very precisely to allow for life. If we increase the strength of the nuclear force, then the stars burn out too quickly to give rise to life. If we decrease the strength of the nuclear force, then stars never ignite at all and life cannot exist. If we increase the force of gravity, then our universe dies quickly in a Big Crunch. If we decrease the strength of gravity, then the universe expands rapidly into a Big Freeze. In fact, there are scores of "accidents" involving the constants of nature that allow for life. Apparently, our universe lives in a "Goldilocks zone" of many parameters, all of which are "fine-tuned" to allow for life. So either we are left with the conclusion that there is a G.o.d of some sort who has chosen our universe to be "just right" to allow for life, or there are billions of parallel universes, many of them dead. As Freeman Dyson has said, "The universe seemed to know we were coming."

Sir Martin Rees of Cambridge University has written that this fine tuning is, in fact, convincing evidence for the multiverse. There are five physical constants (such as the strength of the various forces) that are fine-tuned to allow for life, and he believes that there are also an infinite number of universes in which the constants of nature are not compatible with life.

This is the so-called "anthropic principle." The weak version simply states that our universe is fine-tuned to allow for life (because we are here to make this statement in the first place). The strong version says that perhaps our existence was a by-product of design or purpose. Most cosmologists would agree to the weak version of the anthropic principle, but there is considerable debate over whether the anthropic principle is a new principle of science that could lead to new discoveries and results, or whether it is simply a statement of the obvious.

QUANTUM THEORY.

In addition to higher dimensions and the multiverse, there is yet another type of parallel universe, one that gave Einstein headaches and one that continues to bedevil physicists today. This is the quantum universe predicted by ordinary quantum mechanics. The paradoxes within quantum physics seem so intractable that n.o.bel laureate Richard Feynman was fond of saying that no one really understands the quantum theory.

Ironically, although the quantum theory is the most successful theory ever proposed by the human mind (often accurate to within one part in 10 billion), it is built on a sand of chance, luck, and probabilities. Unlike Newtonian theory, which gave definite, hard answers to the motion of objects, the quantum theory can give only probabilities. The wonders of the modern age, such as lasers, the Internet, computers, TV, cell phones, radar, microwave ovens, and so forth, are all based on the shifting sands of probabilities.

The sharpest example of this conundrum is the famous "Schrodinger's cat" problem (formulated by one of the founders of the quantum theory, who paradoxically proposed the problem in order to smash this probabilistic interpretation). Schrodinger railed against this interpretation of his theory, stating, "If one has to stick to this d.a.m.ned quantum jumping, then I regret having ever been involved in this thing."

The Schrodinger's cat paradox is as follows: a cat is placed in a sealed box. Inside a gun is pointed at the cat (and the trigger is then connected to a Geiger counter next to a piece of uranium). Normally when the uranium atom decays it sets off the Geiger counter and then the gun and the cat is killed. The uranium atom can either decay or not. The cat is either dead or alive. This is just common sense.

But in the quantum theory, we don't know for sure if the uranium has decayed. So we have to add the two possibilities, adding the wave function of a decayed atom with the wave function of an intact atom. But this means that, in order to describe the cat, we have to add the two states of the cat. So the cat is neither dead nor alive. It is represented as the sum of a dead cat and a live cat!

As Feynman once wrote, quantum mechanics "describes nature as absurd from the point of view of common sense. And it fully agrees with experiment. So I hope you can accept nature as She is-absurd."

To Einstein and Schrodinger, this was preposterous. Einstein believed in "objective reality," a commonsense, Newtonian view in which objects existed in definite states, not as the sum of many possible states. And yet this bizarre interpretation lies at the heart of modern civilization. Without it modern electronics (and the very atoms of our body) would cease to exist. (In our ordinary world we sometimes joke that it's impossible to be "a little bit pregnant." But in the quantum world, it's even worse. We exist simultaneously as the sum of all possible bodily states: unpregnant, pregnant, a child, an elderly woman, a teenager, a career woman, etc.) There are several ways to resolve this sticky paradox. The founders of the quantum theory believed in the Copenhagen School, which said that once you open the box, you make a measurement and can determine if the cat is dead or alive. The wave function has "collapsed" into a single state and common sense takes over. The waves have disappeared, leaving only particles. This means that the cat now enters a definite state (either dead or alive) and is no longer described by a wave function.

Thus there is an invisible barrier separating the bizarre world of the atom and the macroscopic world of humans. For the atomic world, everything is described by waves of probability, in which atoms can be in many places at the same time. The larger the wave at some location, the greater the probability of finding the particle at that point. But for large objects these waves have collapsed and objects exist in definite states, and hence common sense prevails.

(When guests would come to Einstein's house, he would point to the moon and ask, "Does the moon exist because a mouse looks at it?" In some sense, the answer of the Copenhagen School might be yes.) Most Ph.D. physics textbooks religiously adhere to the original Copenhagen School, but many research physicists have abandoned it. We now have nanotechnology and can manipulate individual atoms, so atoms that dart in and out of existence can be manipulated at will, using our scanning tunneling microscopes. There is no invisible "wall" separating the microscopic and macroscopic world. There is a continuum.

At present there is no consensus on how to resolve this issue, which strikes at the very heart of modern physics. At conferences, many theories heatedly compete with others. One minority point of view is that there must be a "cosmic consciousness" pervading the universe. Objects spring into being when measurements are made, and measurements are made by conscious beings. Hence there must be cosmic consciousness that pervades the universe determining which state we are in. Some, like n.o.bel laureate Eugene Wigner, have argued that this proves the existence of G.o.d or some cosmic consciousness. (Wigner wrote, "It was not possible to formulate the laws [of the quantum theory] in a fully consistent way without reference to consciousness." In fact, he even expressed an interest in the Vedanta philosophy of Hinduism, in which the universe is pervaded by an all-embracing consciousness.) Another viewpoint on the paradox is the "many worlds" idea, proposed by Hugh Everett in 1957, which states that the universe simply splits in half, with a live cat in one half and a dead cat in the other. This means that there is a vast proliferation or branching of parallel universes each time a quantum event occurs. Any universe that can exist, does. The more bizarre the universe, the less likely it is, but nonetheless these universes exist. This means there is a parallel world in which the n.a.z.is won World War II, or a world where the Spanish Armada was never defeated and everyone is speaking in Spanish. In other words, the wave function never collapses. It simply continues on its way, merrily splitting off into countless universes.

As MIT physicist Alan Guth has said, "There is a universe where Elvis is still alive, and Al Gore is President." n.o.bel laureate Frank Wilczek says, "We are haunted by the awareness that infinitely many slightly variant copies of ourselves are living out their parallel lives and that every moment more duplicates spring into existence and take up our many alternative futures."

One point of view that is gaining in popularity among physicists is something called "decoherence." This theory states that all these parallel universes are possibilities, but our wave function has decohered from them (i.e., it no longer vibrates in unison with them) and hence no longer interacts with them. This means that inside your living room you coexist simultaneously with the wave function of dinosaurs, aliens, pirates, unicorns, all of them believing firmly that their universe is the "real" one, but we are no longer "in tune" with them.

According to n.o.bel laureate Steve Weinberg, this is like tuning into a radio station in your living room. You know that your living room is flooded with signals from scores of radio stations from around the country and the world. But your radio tunes into only one station. It has "decohered" from all the other stations. (In summing up, Weinberg notes that the "many worlds" idea is "a miserable idea, except for all the other ideas.") So does there exist the wave function of an evil Federation of Planets that plunders weaker planets and slaughters its enemies? Perhaps, but if so, we have decohered from that universe.

QUANTUM UNIVERSES.

When Hugh Everett discussed his "many worlds" theory with other physicists, he received puzzled or indifferent reactions. One physicist, Bryce DeWitt of the University of Texas, objected to the theory because "I just can't feel myself split." But this, Everett said, is similar to the way Galileo answered his critics who said that they could not feel the Earth moving. (Eventually DeWitt was won over to Everett's side and became a leading proponent of the theory.) For decades the "many worlds" theory languished in obscurity. It was simply too fantastic to be true. John Wheeler, Everett's adviser at Princeton, finally concluded that there was too much "excess baggage" a.s.sociated with the theory. But one reason that Everett's theory is suddenly in vogue right now is because physicists are attempting to apply the quantum theory to the last domain that has resisted being quantized: the universe itself. Applying the uncertainty principle to the entire universe naturally leads to a multiverse.

The concept of "quantum cosmology" at first seems like a contradiction in terms: the quantum theory refers to the infinitesimally tiny world of the atom, while cosmology refers to the entire universe. But consider this: at the instant of the big bang, the universe was much smaller than an electron. Every physicist agrees that electrons must be quantized; that is, they are described by a probabilistic wave equation (the Dirac equation) and can exist in parallel states. Hence if electrons must be quantized, and if the universe was once smaller than an electron, then the universe must also exist in parallel states-a theory that naturally leads to a "many worlds" approach.

The Copenhagen interpretation of Niels Bohr, however, encounters problems when applied to the entire universe. The Copenhagen interpretation, although it is taught in every Ph.D.-level quantum mechanics course on Earth, depends on an "observer" making an observation and collapsing the wave function. The observation process is absolutely essential in defining the macroscopic world. But how can one be "outside" the universe while observing the entire universe? If a wave function describes the universe, then how can an "outside" observer collapse the wave function of the universe? In fact, some see the inability to observe the universe from "outside" the universe as a fatal flaw of the Copenhagen interpretation.

In the "many worlds" approach the solution to this problem is simple: the universe simply exists in many parallel states, all defined by a master wave function, called the "wave function of the universe." In quantum cosmology the universe started out as a quantum fluctuation of the vacuum, that is, as a tiny bubble in the s.p.a.ce-time foam. Most baby universes in the s.p.a.ce-time foam have a big bang and then immediately have a Big Crunch afterward. That is why we never see them, because they are extremely small and short-lived, dancing in and out of the vacuum. This means that even "nothing" is boiling with baby universes popping in and out of existence, but on a scale that is too small to detect with our instruments. But for some reason, one of the bubbles in the s.p.a.ce-time foam did not recollapse into a Big Crunch, but kept on expanding. This is our universe. According to Alan Guth, this means that the entire universe is a free lunch.

In quantum cosmology, physicists start with an a.n.a.logue of the Schrodinger equation, which governs the wave function of electrons and atoms. They use the DeWitt-Wheeler equation, which acts on the "wave function of the universe." Usually the Schrodinger wave function is defined at every point in s.p.a.ce and time, and hence you can calculate the chances of finding an electron at that point in s.p.a.ce and time. But the "wave function of the universe" is defined over all possible universes. If the wave function of the universe happens to be large when defined for a specific universe, it means that there is a good chance that the universe will be in that particular state.

Hawking has been pushing this point of view. Our universe, he claims, is special among other universes. The wave function of the universe is large for our universe and is nearly zero for most other universes. Thus there is a small but finite probability that other universes can exist in the multiverse, but ours has the largest probability. Hawking, in fact, tries to derive inflation in this way. In this picture a universe that inflates is simply more likely than a universe that does not, and hence our universe has inflated.

The theory that our universe came from the "nothingness" of the s.p.a.ce-time foam might seem to be totally untestable, but it is consistent with several simple observations. First, many physicists have pointed out that it is astonishing that the total amount of positive charges and negative charges in our universe comes out to exactly zero, at least to within experimental accuracy. We take it for granted that in outer s.p.a.ce gravity is the dominant force, yet this is only because the positive and negative charges cancel out precisely. If there was the slightest imbalance between positive and negative charges on the Earth, it might be sufficient to rip the Earth apart, overcoming the gravitational force that holds the Earth together. One simple way to explain why there is this balance between positive and negative charges is to a.s.sume that our universe came from "nothing," and "nothing" has zero charge.

Second, our universe has zero spin. Although for years Kurt G.o.del tried to show that the universe was spinning by adding up the spins of the various galaxies, astronomers today believe that the total spin of the universe is zero. The phenomenon would be easily explained if the universe came from "nothing," since "nothing" has zero spin.

Third, our universe's coming from nothing would help to explain why the total matter-energy content of the universe is so small, perhaps even zero. When we add up the positive energy of matter and the negative energy a.s.sociated with gravity, the two seem to cancel each other out. According to general relativity, if the universe is closed and finite, then the total amount of matter-energy in the universe should be exactly zero. (If our universe is open and infinite, this does not have to be true, but inflationary theory does seem to indicate that the total amount of matter-energy in our universe is remarkably small.) CONTACT BETWEEN UNIVERSES?.

This leaves open some tantalizing questions: If physicists can't rule out the possibility of several types of parallel universes, would it be possible to make contact with them? To visit them? Or is it possible that perhaps beings from other universes have visited us?

Contact with other quantum universes that have decohered from us seems highly unlikely. The reason that we have decohered from these other universes is that our atoms have b.u.mped into countless other atoms in the surrounding environment. Each time a collision occurs, the wave function of that atom appears to "collapse" a bit; that is, the number of parallel universes decreases. Each collision narrows the number of possibilities. The sum total of all these trillions of atomic "mini-collapses" gives the illusion that the atoms of our body are totally collapsed in a definite state. The "objective reality" of Einstein is an illusion created by the fact that we have so many atoms in our body, each one b.u.mping into others, each time narrowing the number of possible universes.

It's like looking at an out-of-focus image through a camera. This would correspond to the microworld, where everything seems fuzzy and indefinite. But each time you adjust the focus of the camera, the image gets sharper and sharper. This corresponds to trillions of tiny collisions with neighboring atoms, each of which reduces the number of possible universes. In this way, we smoothly make the transition from the fuzzy microworld to the macroworld.

So the probability of interacting with another quantum universe similar to ours is not zero, but it decreases rapidly with the number of atoms in your body. Since there are trillions upon trillions of atoms in your body, the chance that you will interact with another universe consisting of dinosaurs or aliens is infinitesimally small. You can calculate that you would have to wait much longer than the lifetime of the universe for such an event to happen.

So contact with a quantum parallel universe cannot be ruled out, but it would be an exceedingly rare event since we have decohered from them. But in cosmology, we encounter a different type of parallel universe: a multiverse of universes that coexist with each other, like soap bubbles floating in a bubble bath. Contact with another universe in the multiverse is a different question. It would undoubtedly be a difficult feat, but one that might be possible for a Type III civilization.

As we discussed before, the energy necessary to open a hole in s.p.a.ce or to magnify the s.p.a.ce-time foam is on the order of the Planck energy, where all known physics breaks down. s.p.a.ce and time are not stable at that energy, and this opens the possibility of leaving our universe (a.s.suming that other universes exist and we are not killed in the process).

This is not a purely academic question, since all intelligent life in the universe will one day have to confront the end of the universe. Ultimately, the theory of the multiverse may be the salvation for all intelligent life in our universe. Recent data from the WMAP satellite currently orbiting the Earth confirms that the universe is expanding at an accelerating rate. One day we may all perish in what physicists refer to as the Big Freeze. Eventually, the entire universe will go black; all the stars in the heavens will blink out and the universe will consist of dead stars, neutron stars, and black holes. Even the very atoms of their bodies may begin to decay. Temperatures may plunge to near absolute zero, making life impossible.

As the universe approaches that point, an advanced civilization facing the ultimate death of the universe could contemplate taking the ultimate journey to another universe. For these beings the choice would be to freeze to death or leave. The laws of physics are a death warrant for all intelligent life, but there is an escape clause in those laws.

Such a civilization would have to harness the power of huge atom smashers and laser beams as large as a solar system or star cl.u.s.ter to concentrate enormous power at a single point in order to attain the fabled Planck energy. It is possible that doing so would be sufficient to open up a wormhole or gateway to another universe. A Type III civilization may use the colossal energy at their disposal to open a wormhole as it makes a journey to another universe, leaving our dying universe and starting over again.

A BABY UNIVERSE IN THE LABORATORY?.

As far-fetched as some of these ideas appear, they have been seriously considered by physicists. For example, when trying to understand how the big bang got started, we have to a.n.a.lyze the conditions that may have led to that original explosion. In other words, we have to ask: how do you make a baby universe in the laboratory? Andrei Linde of Stanford University, one of the cocreators of the inflationary universe idea, says that if we can create baby universes, then "maybe it's time we redefine G.o.d as something more sophisticated than just the creator of the universe."

The idea is not new. Years ago when physicists calculated the energy necessary to ignite the big bang "people immediately started to wonder what would happen if you put lots of energy in one s.p.a.ce in the lab-shot lots of cannons together. Could you concentrate enough energy to set off a mini big bang?" asks Linde.

If you concentrated enough energy at a single point all you would get would be a collapse of s.p.a.ce-time into a black hole, nothing more. But in 1981 Alan Guth of MIT and Linde proposed the "inflationary universe" theory, which has since generated enormous interest among cosmologists. According to this idea, the big bang started off with a turbocharged expansion, much faster than previously believed. (The inflationary universe idea solved many stubborn problems in cosmology, such as why the universe should be so uniform. Everywhere we look, from one part of the night sky to the opposite side, we see a uniform universe, even though there has not been enough time since the big bang for these vastly separated regions to be in contact. The answer to this puzzle, according to the inflationary universe theory, is that a tiny piece of s.p.a.ce-time that was relatively uniform blew up to become the entire visible universe.) In order to jump-start inflation, Guth a.s.sumed that at the beginning of time there were tiny bubbles of s.p.a.ce-time, one of which inflated enormously to become the universe of today.

In one swoop the inflationary universe theory answered a host of cosmological questions. Moreover, it is consistent with all the data pouring in today from outer s.p.a.ce from the WMAP and COBE satellites. It is, in fact, unquestionably the leading candidate for a theory of the big bang.

Yet the inflationary universe theory raises a series of embarra.s.sing questions. Why did this bubble start to inflate? What turned off the expansion, resulting in the present-day universe? If inflation happened once, could it happen again? Ironically, although the inflation scenario is the leading theory in cosmology, almost nothing is known about what set the inflation into motion and why it stopped.

In order to answer these nagging questions, in 1987 Alan Guth and Edward Fahri of MIT asked another hypothetical question: how might an advanced civilization inflate its own universe? They believed that if they could answer this question, they might be able to answer the deeper question of why the universe inflated to begin with.

They found that if you concentrated enough energy at a single point, tiny bubbles of s.p.a.ce-time would form spontaneously. But if the bubbles were too small, they would disappear back into the s.p.a.ce-time foam. Only if the bubbles were big enough could they expand into an entire universe.

On the outside the birth of this new universe would not look like much, perhaps no more than the detonation of a 500-kiloton nuclear bomb. It would appear as if a small bubble had disappeared from the universe, leaving a small nuclear explosion. But inside the bubble an entirely new universe might expand out. Think of a soap bubble that splits or buds a smaller bubble, creating a baby soap bubble. The tiny soap bubble might expand rapidly into an entirely new soap bubble. Likewise, inside the universe you would see an enormous explosion of s.p.a.ce-time and the creation of an entire universe.

Since 1987 many theories have been proposed to see if the introduction of energy can make a large bubble expand into an entire universe. The most commonly accepted theory is that a new particle, called the "inflaton," destabilized s.p.a.ce-time, causing these bubbles to form and expand.

The latest controversy erupted in 2006 when physicists began to look seriously at a new proposal to ignite a baby universe with a monopole. Although monopoles-particles that carry only a single north or south pole-have never been seen, it is believed that they dominated the original early universe. They are so ma.s.sive that they are extremely hard to create in the laboratory, but precisely because they are so ma.s.sive, if we injected even more energy into a monopole we might be able to ignite a baby universe into expanding into a real universe.

Why would physicists want to create a universe? Linde says, "In this perspective, each of us can become a G.o.d." But there is a more practical reason for wanting to create a new universe: ultimately, to escape the eventual death of our universe.

THE EVOLUTION OF UNIVERSES?.