SCIENCE / Countdown to Doomsday

How will it end? How did it begin? Where does humankind fit in? Over the next three weeks, some of the leading scientific thinkers of today will address the big questions. This week, Paul Davies, Professor of Theoretical Physics at the University of Adelaide and renowned author and philosopher, describes the end of the world, of the universe and of matter itself. The articles are based on a new series of books which offers scientists - from astrophysicists to zoologists - a platform to communicate their ideas to the general public.

THE DATE: 21 August 2126. Doomsday. The place: Earth. Across the planet a despairing population attempts to hide. For billions there is nowhere to go. Some people flee deep underground, desperately seeking out caves and disused mine shafts, or take to the sea in submarines. Others go on the rampage, murderous and uncaring. Most just sit, sullen and bemused, waiting for the end.

High in the sky, a huge shaft of light is etched into the fabric of the heavens. What began as a slender pencil of softly radiating nebulosity has swollen day by day to form a maelstrom of gas boiling into the vacuum of space. At the apex of a vapour trail lies a dark, misshapen, menacing lump. The diminutive head of the comet belies its enormous destructive power. It is closing on planet Earth at 40,000 miles per hour - 11 miles every second - a trillion tons of ice and rock, destined to strike at 70 times the speed of sound.

Mankind can only watch and wait. The scientists, who have long since abandoned their telescopes in the face of the inevitable, quietly shut down the computers. The endless simulations of disaster are still too uncertain, and their conclusions are too alarming to release to the public anyway. Some plan to observe the cataclysm as carefully as possible, maintaining their role as true scientists to the very end, transmitting data to time capsules buried deep in the Earth. For posterity . . .

The moment of impact approaches. All over the world, millions of people nervously check their watches. The last three minutes.

Directly above ground zero, the sky splits open. A thousand cubic miles of air are blasted aside. A finger of searing flame wider than a city arcs groundwards and 15 seconds later lances the Earth. The planet shudders with the force of 10,000 earthquakes. A shock wave of displaced air sweeps over the surface of the globe, flattening all structures, pulverising everything in its path. The flat terrain around the impact site rises in a ring of liquid mountains several miles high, exposing the bowels of the Earth in a crater 100 miles across. The wall of molten rock ripples outwards, tossing the landscape about like a blanket flicked in slow motion.

Within the crater itself, trillions of tons of rock are vaporised. Much more is splashed aloft, some of it flung out into space. Still more is pitched across half a continent to rain down hundreds or even thousands of miles away, wreaking massive destruction on all beneath. Some of the molten ejecta falls into the ocean, raising giant waves that add to the spreading turmoil. A vast column of dusty debris fans out into the atmosphere, blotting out the Sun across the whole planet. Now the sunlight is replaced by the sinister flickering glare of a billion meteors, roasting the ground below with their searing heat, as displaced material plunges back from space into the atmosphere.

THE preceding scenario is based on the prediction that comet Swift-Tuttle will hit the Earth on 21 August 2126. If it did, global devastation would undoubtedly follow, destroying human civilisation. When this comet paid us a visit in 1993, early calculations suggested that a collision in 2126 was a distinct possibility. Since then, revised calculations indicate that the comet will in fact miss Earth by a close shave, so we can breathe easily. But the danger won't go away entirely. Sooner or later Swift-Tuttle, or an object like it, will hit the Earth. Estimates suggest that 10,000 objects half a kilometre or more in diameter move on Earth-intersecting orbits.

These astronomical interlopers originate in the frigid outer reaches of the solar system. Some are comets (large, dirty 'snowballs' of ice, dust and gas), or the remains of comets, that have become trapped by the gravitational fields of the planets. Others are asteroids, large lumps of rock that come from the 'asteroid belt' between Mars and Jupiter.

Many of these objects are capable of causing more damage than all the world's nuclear weapons put together. It is only a matter of time before one strikes. When it does, there will be an abrupt and unprecedented interruption in the history of our species. But for the Earth such an event is more or less routine. Cometary or asteroid impacts of this magnitude occur, on average, every few million years. It is widely believed that one or more such events caused the extinction of the dinosaurs 65 million years ago. It could be us next time.

Belief in Armageddon is deep-rooted in most religions and cultures. The biblical book of Revelation gives a vivid account of the destruction that may lie in store for us: 'Then there came flashes of lightning, rumblings, peals of thunder, and a severe earthquake. No earthquake like it has ever occurred since man has been on Earth, so tremendous was the quake . . . The cities of the nations collapsed . . . Every island fled away and the mountains could not be found. From the sky huge hailstones of about a hundred pounds each fell upon men. And they cursed God on account of the plague of hail, because the plague was so terrible.'

There are certainly lots of nasty things that could happen to Earth, a puny object in a universe pervaded by violent forces, yet our planet has remained hospitable to life for at least three-and-a-half billion years. The secret of our success on planet Earth is space. Lots of it. Our solar system is a tiny island of activity in an ocean of emptiness. The nearest star (after the Sun) lies more than four light-years away. To get some idea of how far that is, consider that light traverses the 93 million miles from the Sun in only eight-and-a-half minutes. In four years, it travels more than 20 trillion miles.

All this space means cosmic collisions are rare. The greatest threat to Earth is from our own backyard. Asteroids do not normally orbit close to Earth; they are largely confined to the belt between Mars and Jupiter. But the huge mass of Jupiter can disturb the asteroids' orbits, occasionally sending one of them plunging towards the Sun, and thus menacing Earth.

Comets pose another threat. These spectacular bodies are believed to originate in the Oort Cloud, which is situated about a light-year from the Sun. In this case the threat comes not from the planet Jupiter, but from passing stars. The galaxy is not static; it rotates slowly as its stars orbit the galactic nucleus. One such star, the Sun - along with its little retinue of planets - takes about 200 million years to complete one circuit of the galaxy.

When voyaging stars brush close enough to the Oort Cloud, their gravity may displace some of the comets Sunwards. The Sun evaporates some of their volatile material of ice and dust, and the solar wind blows it out in a long streamer - the famous cometary tail. Very rarely, a comet will collide with the Earth during its sojourn in the inner solar system. The comet does the damage, but the passing star must bear the responsibility. Fortunately, the huge distances between the stars insulate us against too many such encounters.

Other objects can also pass our way on their journey around the galaxy. Giant clouds of gas drift slowly by, and though they constitute an even more perfect vacuum than exists in a laboratory, they can drastically alter the solar wind and may affect the heat flow from the Sun. Other, more sinister objects may lurk in the inky depths of space: rogue planets, neutron stars, brown dwarfs, black holes - all these and more could come upon us without warning, and wreak havoc with the solar system.

Or the threat could be more subtle. Some astronomers believe that the Sun and its solar system, including the planet Earth, belong to a 'double-star system'. If it exists, our Sun will have a companion star - dubbed Nemesis, or the Death Star - which is too dim and far away for astronomers to have detected it. In its slow orbit around our Sun, this star could still make its presence felt gravitationally by disturbing distant comets, sending some plunging Earthwards. Geologists have found that wholesale ecological destruction on Earth does indeed occur about every 30 million years.

MOST people are fascinated by the prospect of Doomsday. But, looking further afield, astronomers have observed entire galaxies - not just planets and comets - in apparent collision. What chance is there that the whole Milky Way will be smashed by another galaxy? There is some evidence, in the very rapid movement of certain stars, that the Milky Way may have already been disrupted by collisions with small nearby galaxies. However, the collision of two galaxies does not necessarily spell disaster for their constituent stars. Galaxies are so sparsely populated that they can merge into one another without individual stellar collisions.

But the violent death of our planet or our galaxy is less of a threat than slow decay. Over the eons there will be many insidious physical processes that will change the shape of the universe: the gradual dimming of stars, the mayhem of marauding black holes - even the decay of matter itself. To survive in the face of this universal degeneration, our descendants will have to harness cosmic forces and extend their technology into the galaxy and beyond. Ultimately, however, the entire universe may become unstable and implode to a big crunch, inexorably destroying everything that exists, both natural and artificial. Thus the fate of our descendants is inextricably bound up with the fate of the stars.

What will happen to the stars and the wider universe long after the death of our own planet is the subject of some debate by scientists. We are used to the idea of the universe being around forever, but forever is a long time. The important thing about infinity is that it is not just a very big number. Infinity is qualitatively different from something that is merely stupendously, unimaginably huge.

Suppose the universe were to have no end. For it to endure for all eternity means that it would have an infinite lifetime. If this were the case, any physical process, however slow or improbable, would have to happen sometime. Given enough time, processes that are negligible on a human time-scale, but are nevertheless persistent, may eventually come to predominate and thus serve to determine the ultimate fate of physical systems.

Let us imagine the state of the universe a very, very long time in the future - say, in a trillion, trillion years. The stars have long since burned out; the universe is dark. But it is not empty. Amid the black vastness of space lurk spinning black holes, stray neutron stars, and black dwarfs - even a few planetary bodies. At this epoch, the density of such objects is exceedingly low: the universe has expanded to 10,000 trillion times its present size.

Gravity would play out a strange battle. The expanding universe attempts to pull every object farther apart from its neighbours, but the mutual gravitational attractions oppose this and try to bring bodies together. As a result, certain collections of bodies - for example, clusters of galaxies, or what pass for galaxies after eons of structural degeneration - remain gravitationally bound, but these collections drift ever farther from neighbouring collections. The ultimate outcome of this tug-of-war depends on how fast the rate of expansion decelerates. The lower the density of matter in the universe, the more 'encouragement' these collections of bodies get to disengage from their neighbours and move apart independently.

Not everything in the universe continues to move apart, however. What is left of the galaxies and other objects in 'gravitationally bound' systems are still subject to the slow but inexorable processes of gravity. Gravitational radiation, feeble though it is at any one moment, insidiously saps the system's energy, causing a slow spiral of death. Ever so gradually, dead stars creep closer to other dead stars or black holes, and coalesce in an orgy of cannibalism.

Astronomers have good evidence that there already exist monster black holes at the centres of some galaxies, greedily gobbling up swirling gases and releasing huge amounts of energy as a result. Such a feeding frenzy will await most galaxies in time, and will continue until the material surrounding the black hole has been either sucked up or ejected, perhaps to fall back again eventually or to join the dwindling intergalactic gases. The bloated black hole will then remain quiescent, with only the occasional rogue neutron star or small black hole plunging in.

Eventually all black holes - even the supermassive ones - will probably disappear too, their death pangs creating momentary flashes of light in the inky blackness of eternal cosmic night, a fleeting epitaph to the erstwhile existence of a billion blazing suns.

IF STARS and galaxies end in this way, what will be left? What will happen to matter itself? Certainly, not all of it falls into black holes. We need to think about the neutron stars and black dwarfs and planets that wander off alone into the vast intergalactic spaces, not to mention the rarefied gas and dust that never got itself together into stars, and the asteroids, comets, meteoroids, and odd chunks of rock that clutter star systems. Do these things survive forever? Ultimately we need to know whether ordinary matter - the stuff of you and me and the Earth - is absolutely stable.

This is where quantum mechanics - the branch of physics that decribes the microworld - comes into play. Although quantum processes are normally associated with atomic and subatomic systems, the laws of quantum physics should apply to everything, including planets. This means that, given enough time, even these exceptionally small effects, when they occur in large objects, will bring about major changes in the universe.

The hallmarks of quantum physics are uncertainty and probability. This means that if a process is at all possible, given enough time it will occur. We can observe this rule at work in the case of radioactivity. A nucleus of uranium-238 is almost completely stable. There is, however, a minute chance that it will eject an alpha particle (two protons and two neutrons) and transmute into the element thorium. To be precise, there is a certain very small probability that a given uranium nucleus will decay. On average, it takes about four-and-a-half billion years to happen. The laws of physics therefore state that any given uranium nucleus is certain to decay eventually.

Some physicists believe that all nuclei, including individual protons, will decay after an immense duration. If so, the consequences for the far future of the universe are profound. All matter would be unstable, and would eventually disappear. Solid objects, like planets, that had avoided falling into a black hole would not last forever. Instead, they would very gradually evaporate.

The universe of the far future would thus be an inconceivably dilute soup of sub-atomic particles. As far as we know, no further basic physical processes would happen. No significant event would occur to interrupt the sterility of a universe that has run its course yet faces eternal life - perhaps eternal death would be a better description. This dismal image of near-nothingness is the closest modern cosmology comes to the 'heat death' of 19th-century physics, the idea that the universe is sliding toward a state of complete degeneration, thermodynamic equilibrium. The time taken for the universe to degenerate to this state is so long that it defies imagination. Yet it is but an infinitessimal portion of the infinite time available. Forever is a long time.

Although the decay of the universe occupies a duration so in excess of human time-scales it is virtually meaningless to us, people still ask, 'What will happen to our descendants? Are they doomed by a universe that will shut down around them?' Given the unpromising state that science predicts for the universe, it seems that all life must ultimately be doomed. But death is not that simple.

1994 by Paul Davies. Taken from 'The Last Three Minutes', published on 3 October in the 'Science Masters' series by Weidenfeld and Nicolson, pounds 9.99