From a casual glance about us, the physical world appears to be completely incomprehensible. The universe is so complicated, its structures and processes so diverse and fitful, there seems to be no reason why human beings should ever come to understand it. Yet the entire scientific enterprise is founded on the audacious assumption—accepted as an act of faith by scientists—that beneath the baffling kaleidoscope of phenomena that confront our inspection lies a hidden mathematical order. More than this. Science proceeds on the basis that the underlying order in nature can, at least in part, be grasped by the human intellect.
Following three centuries of spectacular progress, the scientific frontier may conveniently be divided into three broad categories: the very large, the very small and the very complex. The first category deals with cosmology, the overall structure and evolution of the universe. The second is the realm of subatomic particle physics and the search for the fundamental building blocks of matter. In recent years these two disciplines have begun to merge, with the realization that the big bang that started the universe off about 14 billion years ago would have released enormous energy, fleetingly exposing the ultimate constituents of matter. Cosmologists suppose that the large scale structure of the universe owes its origin to super-energetic sub-nuclear processes in the first split-second of its existence. In this way, subatomic physics helps shape the overall properties of the universe. Conversely, the manner in which the universe came to exist served to determine the number and properties of the fundamental particles of matter that were coughed out of the big bang. Thus the large determines the small even as the small determines the large.
By contrast, the third great frontier of research—the very complex— remains in its infancy. Complexity is, by its very nature, complicated, and so hard to understand. But what is becoming clear is that complexity does not always amount to messy, idiosyncratic complication. In many cases, beneath the surface of chaotic or complicated behaviour simple mathematical principles are at work. The advent of ever-greater computational power has led to an increasing understanding of the different types of complexity found in nature, and a growing belief in the existence of distinct laws of complexity that complement, but do not violate, the familiar laws of physics.
The first edition of this book was published in the 1980s, when chaos theory had received wide publicity. Although the roots of chaos theory date back a century or more, it came to prominence with the realization that chaos is a general feature of dynamical systems, so that randomness and unpredictability afflict not just the weather and biodiversity, but even such everyday systems as the stock market. Today, scientists accept that chaos theory describes just one among a diverse range of complex behaviours found in nature, and that a full understanding of complexity involves far more than simply identifying the difference between regular and irregular behaviour.
Just as the sciences of the large and small have begun to merge, so has the study of the very complex begun to overlap with that of the microworld. The most exciting developments are taking place at the interface of biological, chemical and computational systems. The acronym BINS has been coined for 'bio-info-nano systems'. These refer to the realm of molecular machines (so-called nanotechnology, on the scale of one-billionth of a metre) and information-processing systems, of which the living cell is a classic natural example. In the last decade, a central goal of this field has been the attempt to build a quantum computer. This is a device designed to exploit quantum weirdness to process information. The power of quantum systems is that they may exist in many different configurations simultaneously. An atom, for example, might be both excited and unexcited at the same time. By attaching information to certain special quantum states, physicists hope to process it exponentially faster than in a conventional computer. If this quest succeeds—and the research is still in its infancy—it will transform not only the investigation of complexity, but our very understanding of what is meant by the term.
In Chapter 12 I toy with the idea that quantum mechanics may hold the key to a better appreciation of biological complexity—the thing that distinguishes life from complex inanimate systems. Since formulating these early ideas in the original edition of this book, I have developed the subject in greater depth, and readers are referred to my book The Fifth Miracle
(re-titled The Origin of Life in the UK) for more on quantum biology. It is my belief that quantum nano-machines will soon blur the distinction between the living and the nonliving, and that the secret of life will lie with the extraordinary information processing capabilities of living systems. The impending merger of the subjects of information, computation, quantum mechanics and nanotechnology will lead to a revolution in our understanding of bio-systems.
Many of the puzzles I wrote about in 1988, such as the origin of life, remain deeply problematic, and there is little I wish to add. The one field that has advanced spectacularly in the intervening years, however, is cosmology. Advances in the last decade have transformed the subject from a speculative backwater to a mainstream scientific discipline. Consider, for example, the data from a satellite called the Wilkinson Microwave Anisotropy Probe, or WMAP, published in 2003. Newspapers across the world carried a picture showing a thermal map of the sky built up painstakingly from high Earth orbit. In effect, it is a snapshot of what the universe looked like 380,000 years after its birth in a hot big bang. The searing heat that accompanied the origin of the universe has now faded to a gentle afterglow that bathes the whole universe. WMAP was designed to map that dwindling primordial heat, which has been travelling almost undisturbed for over 13 billion years. Enfolded in the blobs and splodges of the map are the answers to key cosmic questions, such as how old the universe is, what it is made of and how it will die. By mining the map for data, scientists have been able to reconstruct an accurate account of the universe in unprecedented detail.
Perhaps the most significant fact to emerge from the results of WMAP, and many ground-based observations, is the existence of a type of cosmic antigravity, now dubbed 'dark energy'. The story goes back to 1915, when Einstein published his general theory of relativity. This work of pure genius offered a totally new description of gravity, the force that keeps our feet on the ground, and acts between all bodies in the universe, trying to pull them together. But this universal attraction presented Einstein with a headache. Why, he asked, doesn't the universe just collapse into a big heap, dragged inward by its own colossal weight? Was there something fundamentally wrong with his new theory?
Today we know the answer. The universe hasn't collapsed (at least yet) because the galaxies are flying apart, impelled by the power of the big bang. But in 1915 nobody knew the universe was expanding. So Einstein set out to describe a static universe. To achieve this he dreamed up the idea of anti-
gravity. This hitherto unknown force would serve to oppose the weight of the universe, shoring it up and averting collapse. To incorporate antigrav-ity into his theory of relativity, Einstein tinkered with his original equations, adding an extra term that has been cynically called 'Einstein's fudge factor'.
It was immediately obvious that antigravity is like no other known force. For a start, it had the peculiar property of increasing with distance. This means we would never notice its effects on Earth or even in the solar system. But over cosmic dimensions it builds in strength to a point where, if the numbers are juggled right, it could exactly balance the attractive force of gravity between all the galaxies.
It was a neat idea, but short-lived. It crumbled in the 1920s when Edwin Hubble found that the universe is expanding. When Einstein met Hubble in 1931 he immediately realised that antigravity is unnecessary, and abandoned it, called it 'the biggest blunder of my life'. After this debacle, anti-gravity was firmly off the cosmological agenda. When I was a student in the 1960s it was dismissed as repulsive in both senses of the word. But as so often in science, events took an unexpected turn. Just because antigravity wasn't needed for its original purpose didn't logically mean it was nonexistent, and in the 1970s the idea popped up again in an entirely different context. For forty years physicists had been puzzling over the nature of empty space. Quantum mechanics, which deals with processes on a subatomic scale, predicted that even in the total absence of matter, space should be seething with unseen, or dark, energy. Einstein's famous formula E=mc2 implies that this dark energy should possess mass, and as a result it should exert a gravitational pull. Put simply, quantum mechanics implies that even totally empty space has weight.
At first sight this seems absurd. How can space itself—a vacuum—weigh anything? But since it's impossible to grab a bit of empty space and put it on a pair of scales, the claim isn't easy to test. Only by weighing the universe as a whole can the weight of its (very considerable) space be measured. Weighing the universe is no mean feat, but as I shall shortly discuss, it can be done.
Before getting into the question of how much a given volume of space weighs, a tricky aspect of dark energy needs to be explained. Space doesn't just have weight, it exerts a pressure too. In Einstein's theory, pressure as well as mass generates a gravitational pull. For example, the Earth's internal pressure contributes a small amount to your body weight. This is confusing, because pressure pushes outward, yet it creates a gravitational force that pulls inwards. When it comes to dark energy, the situation is reversed—its pressure turns out to be negative. Put simply, space sucks. And just as pressure creates gravity, so sucking creates antigravity. When the sums are done, the conclusion is startling: space sucks so hard, its anti-gravity wins out. The upshot is that dark energy precisely mimics Einstein's fudge factor!
In spite of this amazing coincidence, few scientists took up the cause of dark energy. Theorists hoped it would somehow go away. Then in 1998 came a true bombshell. Astronomers in Australia and elsewhere were doing a head count of exploding stars. From the light of these so-called supernovae they could work out the distances the explosions occurred. It soon became clear that these violent events were situated too far away to fit into the standard model of a universe that started out with a big bang and then progressively slowed its expansion over time. The only explanation seemed to be that, some time in the past, the pace of expansion had begun to pick up again, as if driven by a mysterious cosmic repulsion. Suddenly dark energy was back in vogue.
The results from such surveys, together with those of WMAP, indicate that only about 5 percent of the universe is made of normal matter such as atoms. About a quarter consists of some sort of dark matter yet to be identified, but widely believed to be exotic subatomic particles coughed out of the big bang. The lion's share of the universe is in the form of dark energy. To put a figure to it, the empty space of the observable universe weighs in at about a hundred trillion trillion trillion trillion tonnes, far more than all the stars combined. Large this may be, but to place it in context, the weight of the space inside a car is a few trillion-trillionths of a gram.
Theorists have no idea why the amount of dark energy weighs in at just the value it does. Indeed, they remain divided whether the dark energy is just Einstein's antigravity or some more complicated and exotic phenomenon. Whatever its explanation, dark energy probably seals the fate of the cosmos. As time goes on and the pace of cosmic expansion accelerates, so the galaxies will be drawn farther and farther apart, speeding up all the time. Eventually, even the galaxies near our own Milky Way (or what's left of it) will be receding faster than light, and so will be invisible. If nothing acts to change this trend, the ultimate state of the universe will be dark, near-empty space for all eternity. It is a depressing thought.
There is a glimmer of hope, however. The same physical processes that triggered the inflationary burst at the birth of the universe could, in principle, be re-created. With trillions of years to worry about it, our descendants in the far future might figure out a way to produce a new big bang in the laboratory, in effect creating a baby universe. Theory suggests that this new universe will balloon out, generating its own space and time as it goes, and will eventually disconnect itself from the mother universe. For a while, mother and baby will be joined by an umbilical cord of space, offering a bridge between the old universe and the new. Our descendants might be able to scramble into the new universe, and embark on a new cycle of cosmic evolution and development. This would be the ultimate in emigration: decamping to a brand-new cosmos, hopefully customised for bio-friendliness!
The dark energy idea has drifted in and out of favour for over seven decades. If the astronomical evidence is to be believed, it is now on again for good. Though dark energy predicts the demise of the universe, it might also contain the basis for cosmic salvation. If so, Einstein's greatest mistake could yet turn out to be his greatest triumph. And if the laws of the universe really are a sort of cosmic blueprint, as I suggest, they may also be a blueprint for survival.
Paul Davies Sydney, January 2004
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