Quantum Theory Cannot Hurt You - Marcus Chown (2007)
Part II. BIG THINGS
Chapter 10. THE ULTIMATE RABBIT OUT OF A HAT
HOW WE LEARNED THAT THE UNIVERSE HAS NOT EXISTED FOREVER BUT WAS BORN IN A TITANIC EXPLOSION 13.7 BILLION YEARS AGO
A white rabbit is pulled out of a top hat. Because it is an extremely large rabbit, the trick takes billions of years.
They are high-tech glasses. Merely by twiddling a knob on the frame, you can “tune” them to see all kinds of light normally invisible to the human eye. You take them outside on a cold, starry night and start twiddling.
The first thing you see is the sky in ultraviolet, light pumped out by stars much hotter than the Sun. Some familiar stars have vanished, and some new ones have swum into view, shrouded in misty nebulosity. The most striking feature of the sky, however, is the same as it was for the naked-eye sky. It’s mostly black.
You twiddle on.
Now you’re seeing X-rays, high-energy light radiated by gas heated to hundreds of thousands of degrees as it swirls down onto exotic objects like black holes. Once again, the most striking feature of the sky is that it is mostly black.
You twiddle back the other way, zipping back through ultraviolet light and visible light to infrared light, given out by objects much colder than the Sun. Now the sky is peppered by stellar embers—stars so recently born they are still swathed in shimmering placental gas and bloated red giants in their death throes. But despite the fact that the sky is lit by a new population of stars, its most striking thing remains the same. It is mostly black.
You twiddle on. Now you are seeing microwaves—the kind of light used for radar, mobile phones, and microwave ovens. But something odd is happening. The sky is getting brighter. Not just bits of it—all of it!
You take off the glasses, rub your eyes, and put them back on. But nothing has changed. Now the whole sky, from horizon to horizon, is glowing a uniform, pearly white. You twiddle further, but the sky just gets brighter and brighter. The whole of space seems to be glowing. It’s like being inside a giant lightbulb.
Are the glasses malfunctioning? No, they are working perfectly. What you are seeing is the cosmic background radiation, the relic of the fireball in which the Universe was born 13.7 billion years ago. Incredibly, it still permeates every pore of space, greatly cooled by the expansion of the Universe so that it now appears as low-energy microwaves rather than visible light. Believe it or not, the cosmic background radiation accounts for an astonishing 99 per cent of the light in today’s Universe. It is incontrovertible proof that the Universe began in a titanic explosion—the Big Bang.
The cosmic background radiation was discovered in 1965. But the realisation that there had been a Big Bang actually came earlier. In fact, the first step was taken by Einstein.
THE ULTIMATE SCIENCE
Einstein’s theory of gravity—the general theory of relativity—describes how every chunk of matter pulls on every other chunk of matter. The biggest collection of matter we know of is the Universe. Never one to shy away from the really big problems in science, Einstein in 1916 applied his theory of gravity to the whole of creation. In doing so he created cosmology—the ultimate science—which deals with the origin, evolution, and ultimate fate of the Universe.
Although the ideas behind Einstein’s theory of gravity are deceptively simple, the mathematical apparatus is not. Working out exactly how a particular distribution of matter warps space-time is very hard indeed. It was not until 1962, for instance—almost half a century after Einstein published his general theory of relativity—that New Zealand physicist Roy Kerr calculated the distortion of space-time caused by a realistic, spinning, black hole.
Figuring out how the whole Universe warps space-time would have been impossible without making some simplifying assumptions about how its matter is spread throughout space. Einstein assumed that it makes no difference where in the Universe an observer happens to be. In other words, he assumed that the Universe has the same gross properties wherever you are located and, from wherever you are located, it looks roughly the same in every direction.
Astronomical observations since 1916 have actually shown these assumptions to be well founded. The Universe’s building blocks—which Einstein and everyone else were unaware of at the time—are galaxies, great islands of stars like our own Milky Way. And modern telescopes do indeed show them to be scattered pretty evenly around the Universe, so the view from one galaxy is much the same as the view from any other.
Einstein’s conclusion, after applying his theory to the Universe as a whole, was that its overall space-time must be warped. Warped space-time, however, causes matter to move. This is the central mantra of general relativity. Consequently, the Universe could not possibly be still. This dismayed Einstein. Like Newton before him, he fervently believed the Universe to be static, its constituent bodies—now known to be galaxies—suspended essentially motionless in the void.
A static universe was appealing because it remained the same for all time. There was no need to address sticky questions about where the Universe came from or where it was going. It had no beginning. It had no end. The reason the Universe was the way it was was because that was the way it had always been.
According to Newton, for the Universe to be static, one condition had to be satisfied: matter had to extend infinitely in all directions. In such a neverending cosmos, each body has just as many bodies on one side, pulling it one way with their gravity, as on the opposite side, pulling it the other way. Like a rope being pulled by two equally strong tug-of-war teams, it therefore remains motionless.
However, according to Einstein’s theory of gravity, the Universe was finite, not infinite. Its space-time curved back on itself—the four-dimensional equivalent of the two-dimensional surface of a basketball. In such a Universe the gravitational tug-of-war is at no point perfectly balanced. Because every body tries to pull every other body toward it, the Universe shrinks uncontrollably.
To salvage the idea of a static Universe, Einstein had to resort to mutilating his elegant theory. He added a mysterious force of cosmic repulsion, which pushed apart the objects in the Universe. He hypothesised that it had a significant effect only on bodies that were enormously far apart, explaining why it had not been noticed before in Earth’s neighbourhood. By precisely counteracting the force of gravity that was perpetually trying to drag bodies together, the cosmic repulsion kept the Universe forever static.
THE EXPANDING UNIVERSE
Einstein’s instincts turned out to be wrong. In 1929, Edwin Hubble—the American astronomer responsible for discovering that the Universe’s building blocks were galaxies—announced a dramatic new discovery. The galaxies were flying apart from each other like pieces of cosmic shrapnel. Far from being static, the Universe was growing in size. As soon as Einstein learned of Hubble’s discovery of the expanding universe, he renounced his cosmic repulsion, calling it the biggest blunder he ever made in his life.1 Einstein’s mysterious repulsive force could never have kept the galaxies hanging motionless in space. As Arthur Eddington pointed out in 1930, a static cosmos is inherently unstable, like a knife balanced on its point. The merest nudge would be enough to set it expanding or contracting.
Others did not make the same mistake as Einstein. In 1922 the Russian physicist Aleksandr Friedmann applied Einstein’s theory of gravity to the Universe and correctly concluded that it must either be contracting or expanding. Five years later the same conclusion was reached independently by the Belgian Catholic priest Georges-Henri Lemaître.
As John Wheeler has said: “Einstein’s description of gravity as curvature of space-time led directly to that greatest of all predictions: The Universe itself is in motion.” It is ironic that Einstein himself missed the message in his own theory.
THE BIG BANG UNIVERSE
Since the Universe is expanding, one conclusion is inescapable: it must have been smaller in the past. By imagining the expansion running backwards, like a movie in reverse, astronomers deduce that 13.7 billion years ago all of Creation was squeezed into the tiniest of tiny volumes. The lesson of the receding galaxies is that the Universe, though old, has not existed forever. There was a beginning to time. A mere 13.7 billion years ago, all matter, energy, space, and time fountained into existence in a titanic explosion—the Big Bang.
The cosmic expansion turns out to obey a remarkably simple law: Every galaxy is rushing away from the Milky Way with a speed that is in direct proportion to its distance. So a galaxy that is twice as far away as another is receding twice as fast, one 10 times as far away 10 times as fast, and so on. This relation, known as Hubble’s law, turns out to be unavoidable in any universe that grows in size while continuing to look the same from every galaxy.
Imagine a cake with raisins in it. If you could shrink in size and sit on any raisin, the view will always be the same. Furthermore, if the cake is put in an oven and expands, or rises, not only will you see all the other raisins recede from you but you will see them recede with speeds in direct proportion to their distance from you. It matters not at all what raisin you sit on. The view will always be the same. (The tacit assumption here is that it is a big cake, so that you are always far from the edge.) Galaxies in an expanding universe are like raisins in a rising cake.
It follows that, just because we see all the galaxies flying away from us, we should not assume that we are at the centre of the Universe and that the Big Bang happened in our cosmic backyard. Were we to be in any galaxy other than the Milky Way, we would see the same thing—all the other galaxies fleeing from us. The Big Bang did not happen here, or over there, or at any one point in the Universe. It happened in all places simultaneously. “In the universe, no centre or circumference exists, but the centre is everywhere,” said the 16thcentury philosopher Giordano Bruno.
The Big Bang is a bit of a misnomer. It was totally unlike any explosion with which we are familiar. When a stick of dynamite detonates, for instance, it explodes outwards from a localised point and the debris expands into preexisting space. The Big Bang did not happen at a single point and there was no preexisting void! Everything—space, time, energy, and matter—came into being in the Big Bang and began expanding everywhere at once.
THE HOT BIG BANG
Whenever you squeeze something into a smaller volume—for in-stance, air into a bicycle pump—it gets hot. The Big Bang was there-fore a hot Big Bang. The first person to realise this was the Ukrainian-American physicist George Gamow. In the first few moments after the Big Bang, he reasoned, the Universe was reminiscent of the blisteringly hot fireball of a nuclear explosion.2
But whereas the heat and light of a nuclear fireball dissipate into the atmosphere so that, hours or days after the explosion, they are all gone, this was not true of the heat and light of the Big Bang fireball. Since the Universe, by definition, is all there is, there was simply nowhere for it to go. The “afterglow” of the Big Bang was instead bottled up in the Universe forever. This means it should still be around today, not as visible light—since it would have been greatly cooled by the expansion of the Universe since the Big Bang—but as microwaves, an invisible form of light characteristic of very cold bodies.3
Gamow did not believe it would be possible to distinguish this microwave afterglow from other sources of light in today’s Universe. However, he was mistaken. As his research students Ralph Alpher and Robert Herman realised, the relic heat of the Big Bang would have two unique features that would make it stand out. First, because it came from the Big Bang, and the Big Bang happened everywhere simultaneously, the light should be coming equally from every direction in the sky. And, second, its spectrum—the way the brightness of the light changed with the light’s energy—would be that of a “black body.” It’s not necessary to know what a black body is, only that a black body spectrum is a unique “fingerprint.”
Although Alpher and Herman predicted the existence of the afterglow of the Big Bang—the cosmic microwave background radiation—in 1948, it was not discovered until 1965 and then totally by accident. Arno Penzias and Robert Wilson, two young astronomers at Bell Labs at Holmdel in New Jersey, were using a horn-shaped microwave antenna formerly used for communicating with Telstar, the first modern communications satellite, when they picked up a mysterious hiss of microwave “static” coming equally from every direction in the sky. Over the following months as they puzzled over the signal, they variously thought that it might be radio static from nearby New York City, atmospheric nuclear tests, or even pigeon droppings coating the interior of their microwave horn. In fact, they had made the most important cosmological discovery since Hubble found that the Universe was expanding. The afterglow of creation was powerful evidence that our Universe had indeed begun in a hot, dense state—a Big Bang—and had been growing in size and cooling ever since.
Penzias and Wilson did not accept the Big Bang origin of their mysterious static for at least two years. Nevertheless, for the discovery of the afterglow of creation, they carried off the 1978 Nobel Prize for Physics.
The cosmic background radiation is the oldest “fossil” in creation. It comes to us directly from the Big Bang, carrying with it precious information about the state of the Universe in its infancy, almost 13.7 billion years ago. The cosmic background is also the coldest thing in nature—only 2.7 degrees above absolute zero, the lowest possible temperature (–270 degrees Celsius).
The cosmic background radiation is actually one of the most striking features of our Universe. When we look up at the night sky, its most obvious feature is that it is mostly black. However, if our eyes were sensitive to microwave light rather than visible light, we would see something very different. Far from being black, the entire sky, from horizon to horizon, would be white, like the inside of a lightbulb. Even billions of years after the event, all of space is still glowing softly with relic heat of the Big Bang fireball.
In fact, every sugarcube-sized region of empty space contains 300 photons of the cosmic background radiation. Ninety-nine per cent of all the photons in the Universe are tied up in it, with a mere 1 per cent in starlight. The cosmic background radiation is truly ubiquitous. If you tune your TV between stations, 1 per cent of the “snow” on the screen is the relic static of the Big Bang.
DARKNESS AT NIGHT
The fact that the Universe began in a Big Bang explains another great mystery—why the night sky is dark. The German astronomer Johannes Kepler, in 1610, was the first to realise this was a puzzle.
Think of a forest of regularly spaced pine trees going on forever. If you ran into the forest in a straight line, sooner or later you would bump into a tree. Similarly, if the Universe is filled with regularly spaced stars and goes on forever, your gaze will alight on a star no matter which direction you look out from Earth. Some of those stars will be distant and faint. However, there will be more distant stars than nearby ones. In fact—and this is the crucial point—the number of stars will increase in such a way that it exactly compensates for their faintness. In other words, the stars at a certain distance from Earth will contribute just as much light in total as the ones twice as far away, three times away, four times away, and so on. When all the light arriving at Earth is added up, the result will therefore be an infinite amount of light!
This is clearly nonsensical. Stars are not pointlike; they are tiny discs. So nearby stars blot out some of the light of more distant stars just as nearby pine trees block out more distant pine trees. But even taking this effect into account, the conclusion seems inescapable that the entire sky should be “papered” with stars, with no gaps in between. Far from being dark at night, the night sky should be as bright as the surface of a typical star. A typical star is a red dwarf, a star glowing like a dying ember. Consequently, the sky at midnight should be glowing blood red. The puzzle of why it isn’t was popularised in the early 19th century by the German astronomer Heinrich Olbers and is known as Olbers’ paradox in his honour.
The way out of Olbers’ paradox is the realisation that the Universe has not in fact existed forever but was born in a Big Bang. Since the moment of creation, there has been only 13.7 billion years for the light of distant stars to reach us. So the only stars and galaxies we see are those that are near enough that their light has taken less than 13.7 billion years to get to us. Most of the stars and galaxies in the Universe are so far away that their light will take more than 13.7 billion years to reach us. The light of these objects is still on its way to Earth.
Therefore, the main reason the sky at night is dark is that the light from most of the objects in the Universe has yet to reach us. Ever since the dawn of human history, the fact that the Universe had a beginning has been staring us in the face in the darkness of the night sky. We have simply been too stupid to realise it.
Of course, if we could wait another billion years, we would see stars and galaxies so far away that their light has taken 14.7 billion years to get here. The question therefore arises of whether, if we lived many trillions of years in the future when the light from many more stars and galaxies had time to reach us, the sky at night would be red. The answer turns out to be no. The reasoning of Kepler and Olbers is based on an incorrect assumption—that stars live forever. In fact, even the longest-lived stars will use up all their fuel and burn out after about 100 billion years. This is long before enough light has arrived at Earth to make the sky red.
The Big Bang has enormous explanatory power. Nevertheless, it has serious problems. For one it is difficult to understand where galaxies like our Milky Way came from.
The fireball of the Big Bang was a mix of particles of matter and light. The matter would have affected the light. For instance, if the matter had curdled into clumps, this would be reflected in the afterglow of the Big Bang—it would not be uniform all over the sky today but would be brighter in some places than others. The fact that the afterglow is even all around the sky means that matter in the fireball of the Big Bang must have been spread about extremely smoothly. But we know that it could not be spread completely smoothly. After all, today’s Universe is clumpy, with galaxies of stars and clusters of galaxies and great voids of empty space in between. At some point, therefore, the matter in the Universe must have gone from being smoothly distributed throughout space to being clumpy. And the start of this process should be visible in the cosmic background radiation.
Sure enough, in 1992, very slight variations in the brightness of the afterglow of the Big Bang were discovered by NASA’s Cosmic Background Explorer Satellite, COBE. These cosmic ripples—one of the scientists involved was more picturesque in likening them to “the face of God”—showed that, about 450,000 years after the Big Bang, some parts of the Universe were a few thousandths of a per cent denser than others. Somehow, these barely noticeable clumps of matter—the “seeds” of structure—had to grow to form the great clusters of galaxies we see in today’s Universe. But there is a problem.
Clumps of matter grow to become bigger clumps because of gravity. Basically, if a region has slightly more matter than a neighbouring region, its stronger gravity will ensure that it will steal yet more matter from its neighbour. Just as the richer get richer and the poor get poorer, the denser regions of the Universe will get ever denser until, eventually, they become the galaxies we see around us today. The problem the theorists noticed was that 13.7 billion years was not enough time for gravity to make today’s galaxies out of the tiny clumps of matter seen by the COBE satellite. The only way they could do it was if there was much more matter in the Universe than was tied up in visible stars.
Actually, there was strong evidence for missing matter close to home. Spiral galaxies like our own Milky Way are like giant whirlpools of stars, only their stars turn out to be whirling about their centres far too fast. By rights, they should fly off into intergalactic space just as you would be flung off a merry-go-round that someone had spun too fast. The extraordinary explanation that the world’s astronomers have come up with is that galaxies like our Milky Way actually contain about 10 times as much matter as is visible in stars. They call the invisible matter dark matter. Nobody knows what it is. However, the extra gravity of the dark matter holds the stars in their orbits and stops them from flying off into intergalactic space.
If the Universe as a whole contains 10 times as much dark matter as ordinary matter, the extra gravity is just enough to turn the clumps of matter seen by COBE into today’s galaxy clusters in the 13.7 billion years since the Universe was born. The Big Bang picture is saved.4 The price is the addition of a lot of dark matter, whose identity nobody knows—well, almost, nobody. In the words of Douglas Adams in Mostly Harmless: “For a long period of time there was much speculation and controversy about where the so-called ‘missing matter’ of the Universe had gotten to. All over the Galaxy the science departments of all the major universities were acquiring more and elaborate equipment to probe and search the hearts of distant galaxies, and then the very centre and the very edges of the whole Universe, but when eventually it was tracked down it turned out in fact to be all the stuff which the equipment had been packed in!”
The fact that the standard Big Bang picture does not provide enough time for matter to clump into galaxies is not the only problem with the scenario. There is another, arguably more serious, one. It concerns the smoothness of the cosmic background radiation.
Things reach the same temperature when heat travels from a hot body to a cold body. For instance, if you put your cold hand on a hot water bottle, heat will flow from the bottle until your hand reaches the same temperature. The cosmic background radiation is basically all at the same temperature. This means that, as the early Universe grew in size, and some bits lagged behind others in temperature, heat always flowed into them from a warmer bit, equalising the temperature.
The problem arises if you imagine the expansion of the Universe running backwards like a movie in reverse. At the time that the cosmic background radiation last had any contact with matter—about 450,000 years after the Big Bang—bits of the Universe that today are on opposite sides of the sky were too far apart for heat to flow from one to the other. The maximum speed it could flow is the speed of light, and the 450,000 years the Universe had been in-existence was simply not long enough. So how is it that the cosmic background radiation is the same temperature everywhere today?
Physicists have come up with an extraordinary answer. Heat could have flowed back and forth throughout the Universe, equalising the temperature, only if the early Universe was much smaller than our backward-running movie would imply. If regions were much closer together than expected, there would have been plenty of time for heat to flow from hot to cold regions and equalise the temperature. But if the Universe was much smaller earlier on, it must have put on a big spurt of growth to get to its present size.
According to the theory of inflation, the Universe “inflated” during its first split-second of existence, undergoing a phenomenally violent expansion. What drove the expansion was a peculiar property of the vacuum of empty space, although that’s still hazy to physicists. The point is that there was this enormously fast expansion, which very quickly ran out of steam, and then the more sedate expansion that we see today took over. If the normal Big Bang expansion is likened to the explosion of a stick of dynamite, inflation can be likened to a nuclear explosion. “The standard Big Bang theory says nothing about what banged, why it banged or what happened before it banged,” says inflation pioneer Alan Guth. Inflation is at least an at-tempt to address such questions.
With inflation plus dark matter tagged on, the Big Bang scenario can be rescued. In fact, when astronomers talk of the Big Bang these days, they often mean the Big Bang plus inflation plus dark matter. However, inflation and dark matter are not as well-founded ideas as the Big Bang. Beyond any doubt, we know that the Universe began in a hot dense state and has been expanding and cooling ever since—the Big Bang scenario. That inflation happened is still not certain, and nobody has yet discovered the identity of dark matter.
One of the pluses of inflation is that it provides a possible explanation of the origins of structures such as galaxies in today’s Universe. For such structures to have formed, there must have been some kind of unevenness in the Universe at a very early stage. That primordial roughness could have been caused by so-called quantum fluctuations. Basically, the laws of microscopic physics cause extremely small regions of space and matter to jiggle about restlessly like water in a boiling saucepan. Such fluctuations in the density of matter were minuscule—smaller even than present-day atoms. However, the phenomenal expansion of space caused by inflation would actually have enhanced them, blowing them up to noticeable size. Bizarrely, the largest structures in today’s Universe—great clusters of galaxies—may have been spawned by “seeds” smaller than atoms!
Inflation, however, predicts something about our Universe that does not seem to accord with the facts. Currently, the Universe is expanding. However, the gravity of all the matter in the Universe is acting to brake the expansion. There are two main possibilities. One is that the Universe contains sufficient matter to eventually slow and reverse its expansion, causing the Universe to collapse back down to a Big Crunch, a sort of mirror image of the Big Bang in which the Universe was born. The other is that it contains insufficient matter and goes on expanding forever. Inflation predicts that the Universe should be balanced on the knife edge between these two possibilities. It will continue expanding, but slowing down all the time, and finally running out of steam only in the infinite future. For this to happen, the Universe must have what is known as the critical mass. The problem is that, even when all the matter in the Universe—visible matter and dark matter—is added up, it amounts to only about a third of the critical mass. Inflation, it would seem, is a nonstarter. Well, that’s how it seemed—until a sensational discovery was made in 1998.
Two teams were observing “supernovas”—exploding stars—in distant galaxies. One team was led by American Saul Perlmutter and the other by Australians Nick Suntzeff and Brian Schmidt. Supernovas are exploding stars that often outshine their parent galaxy and so can be seen at great distances out into the Universe. The kind the two teams of astronomers were looking at were known as Type Ia supernovas. They have the property that, when they detonate, they always shine with the same peak luminosity. So if you see one that is fainter than another, you know it is farther away.
What the astronomers saw, however, was that the ones that were farther away were fainter than they ought to be, taking into account their distance from Earth. The only way to explain what they were seeing was that the Universe’s expansion had speeded up since the stars exploded, pushing them farther away than expected and making them appear fainter.
It was a bombshell dropped into the world of science. The sole force affecting the galaxies ought to be their mutual gravitational pull. That should be braking the expansion, not speeding it up.
The only thing that could be accelerating things was space itself. Contrary to all expectations, it could not be empty. It must contain some kind of weird stuff unknown to science— “dark energy”—that was exerting a kind of cosmic repulsion on the Universe, countering gravity and driving the galaxies apart.
Physicists are totally at sea when it comes to understanding dark energy. Their best theory—quantum mechanics—predicts an energy associated with empty space that is 1 followed by 123 zeroes bigger than Perlmutter observed! Nobel laureate Steven Weinberg has described this as “the worst failure of an order-of-magnitude estimate in the history of science.”
Despite this embarrassment, the dark energy has at least one desirable consequence. Recall that inflation requires the Universe to have the critical mass but that all the matter in the Universe adds up to only about a third of the critical mass. Well all forms of energy, as Einstein discovered, have an effective mass. And that includes the dark energy. In fact, it turns out to account for about two-thirds of the critical mass, so that the Universe has exactly the critical mass—just what is predicted by inflation.
Although nobody knows what the dark energy is, one possibility is that it is associated with the repulsive force of empty space proposed by Einstein. In science, it seems, all things begin and end with Einstein. His biggest mistake may yet turn out to be his biggest success.
It is worth stressing, however, that the Big Bang, for all its successes, is still basically a description of how our Universe has evolved from a superdense, superhot state to its present state, with galaxies, stars, and planets. How it all began is still shrouded in mystery.
TO THE SINGULARITY AND BEYOND
Imagine the expansion of the Universe running backwards again like a movie in reverse. As the Universe shrinks down to a speck, its matter content becomes ever more compressed and ever hotter. In fact, there is no limit to this process. At the instant the Universe’s expansion began—the moment of its birth—it was infinitely dense and infinitely hot. Physicists call the point when something skyrockets to infinity a singularity. According to the standard Big Bang picture, the Universe was therefore born in a singularity.
The other place where Einstein’s theory of gravity predicts a singularity is at the heart of a black hole. In this case the matter of a catastrophically shrinking star eventually becomes compressed into zero volume and therefore becomes infinitely dense and infinitely hot. “Black holes,” as someone once said, “are where God divided by zero.”5
A singularity is a nonsense. When such a monstrous entity pops up in a theory of physics, it is telling us that the theory—in this case, Einstein’s theory of gravity—is faulty. We are stretching it beyond the domain where it has anything sensible to say about the world. This is not surprising. General relativity is a theory of the very large. In its earliest stages, however, the Universe was smaller than an atom. And the theory of the atomic realm is quantum theory.
Normally, there is no overlap between these two towering monuments of 20th-century physics. However, they come into conflict at the heart of black holes and at the birth of the Universe. If we are ever going to understand how the Universe came into being, we are going to have to find a better description of reality than Einstein’s theory of gravity. We need a quantum theory of gravity.
The task of finding such a theory is formidable because of the fundamental incompatibility between general relativity and quantum theory. General relativity, like every theory of physics before it, is a recipe for predicting the future. If a planet is here now, in a day’s time it will have moved over there, by following this path. All these things are predictable with 100 per cent certainty. Quantum theory, however, is a recipe for predicting probabilities. If an atom is flying through space, all we can predict is its probable final position, its probable path. Quantum theory therefore undermines the very foundation stones of general relativity.
Currently, physicists are trying to discover the elusive quantum theory of gravity by a number of routes. Undoubtedly, the one getting the most publicity is superstring theory, which views the fundamental building blocks of matter not as pointlike particles but as ultra-tiny pieces of “string.” The string—superconcentrated mass-energy—can vibrate just like a violin string, and each distinct vibration “mode” corresponds to a fundamental particle such as an electron or a photon.
What excites string theorists is that some form of gravity—although not necessarily general relativity—is automatically contained within string theory. One slight complication is that the strings of string theory vibrate in a 10-dimensional world, which means there have to exist an additional six space dimensions too small for us to have noticed. Another problem is that string theory involves such horrendously complicated mathematics that it has so far proved impossible to make a prediction with it that can be tested against reality.
No one knows how close or how far away we are to possessing a quantum theory of gravity. But without it there is no hope of travelling those last tantalising steps back to the beginning of the Universe. However, some of the things that must happen along the route are clear.
Think of the expansion of the Universe in reverse again. At first the Universe will shrink at the same rate in all directions. This is because the Universe is pretty much the same in all directions. But pretty much the same is not the same as exactly the same. Undoubtedly, there will be slightly more galaxies in one direction than another. In the early stages of the contraction this imbalance will have no noticeable effect. However, as the Universe shrinks down to zero volume, such matter irregularities will become ever more magnified. So when the body shrinks to zero volume, the final stages of the collapse will be wildly chaotic. Gravity—warped space-time—will vary wildly depending on the direction from which the singularity is approached by an in-falling body.
Very close to the singularity, the warpage of space-time will become so violent and chaotic that space and time will actually shatter, splitting into myriad droplets. Concepts like “before” and “after” now lose all meaning. So too do concepts like “distance” and “direction.” An impenetrable fog blocks the view ahead. It shrouds the mysterious domain of quantum gravity, where no theory yet exists to act as our guide.
But deep in that fog lie the answers to science’s most pressing questions. Where did the Universe come from? Why did it burst into being in a Big Bang 13.7 billion years ago? What, if anything, existed before the Big Bang?
The fervent hope is that, when at last we manage to mesh together our theory of the very small with our theory of the very large, we will find the answers to these questions. Then we will come face to face with the ultimate question: How could something have come from nothing? “It is enough to hold a stone in your hand,” wrote Jostein Gaarder in Sophie’s World. “The universe would have been equally incomprehensible if it had only consisted of that one stone the size of an orange. The question would be just as impenetrable: Where did this stone come from?”
1 See My World Line by George Gamow (New York, 1970), in which the author writes of Einstein: “He remarked [to me] that the introduction of the cosmological term was the biggest blunder he ever made in his life.”
2 The Big Bang was named by the English astronomer Fred Hoyle during a BBC radio programme in 1949. The great irony is that Hoyle, to the day he died, never believed in the Big Bang.
3 And of magnetrons, which power microwave ovens and radar trans-mitters.
4 Actually, there is thought to be between 6 and 7 times as much dark matter as ordinary matter. This is because the stars account for only about half the ordinary matter. The rest, which may be in the form of dim gas clouds between the galaxies, has not yet been identified.
5 Actually, there is a subtle distinction between the singularities at the heart of a black hole and the Big Bang. The former is a singularity in time and the latter a singularity in space.