How will the Earth expire? John Leslie ponders how we might precipitate our demise.

Our galaxy contains many billion sun-like stars. Our telescopes see details of many billion further galaxies. Assuming that intelligent life evolves easily, why do we detect no signs of extraterrestrials? Can it be that we humans are the very first intelligent beings to evolve in this neck of the cosmic woods? That there will, in due course, be a million other technologically advanced species, but ours is the first one? Surely this would make our position altogether too extraordinary. Wouldn't it be more sensible to think that, yes, many intelligent races have evolved before us, but they have then destroyed themselves soon after developing advanced technologies?

Here is another line of reasoning that looks almost exactly like it. Can you and I really be among the first millionth of all humans who will ever have been born? Wouldn't that make our position unbelievably extraordinary too? If all humans believed they were in the first millionth, only one in a million would be right. This point occurred to Brandon Carter, a British cosmologist and mathematician, in the early 1980s. It led him to what has come to be called "the doomsday argument" for thinking that humans will quite soon be extinct.

If looking simply at the various dangers confronting the human race, without taking account of Carter's point, how high ought we to put the probability of humans surviving for millions of years more? At about the 80 per cent level, I'd answer. But what if it really were that high? And what if (as most space scientists think) a human race that survived for very much longer than it already has would spread right through its galaxy, perhaps in as little as 600,000 years? You and I would then presumably have lived among the earliest millionth of all humans. Doesn't that look too incredible?

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Incredible, at any rate, when you consider the entirely credible alternative. There has been a population explosion recently. Of all humans who have lived so far, roughly 10 per cent are alive at this very moment. If it were a moment near the end of human population history, then our position in that history would be none too extraordinary - nothing like as extraordinary, at any rate, as being in the first millionth. And aren't there plenty of threats to the continued survival of our species?

While such reasoning is controversial, it seems to me to show that "doom soon" quite probably awaits the human race. "Soon" here means soon enough for galactic colonisation to have scarcely got started. And this could well mean doom during the next few centuries.

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Let us examine just three of the ways in which our species could become extinct quickly. Two are widely discussed. They are extinction through a pollution crisis and extinction through biowarfare. The third, extinction through a vacuum metastability disaster, may come as a surprise to you. It has been considered, however, in articles in physics journals and in Before the Beginning , a recent book by British astronomer royal, Sir Martin Rees.

A pollution crisis could involve many factors. The ozone layer, Earth's shield against ultraviolet radiation, could become severely eroded. There could also be a steady accumulation of poisonous chemicals. Land could become exhausted despite ever heavier application of fertilisers. Fertilisers could themselves be a threat, bringing death to rivers, lakes and even seas. Still worse, accumulating greenhouse gases might make temperatures shoot up disastrously.

To achieve the consensus needed for influencing politicians, the Intergovernmental Panel on Climate Change has paid scant attention to worst-case scenarios. In these scenarios, harmful changes produce more changes of the same type. For instance, vegetation could die from excessive heat, and then land losing its vegetation would get hotter, leading to more death of vegetation, and therefore more heat, and so on.

Again, vast amounts of methane, a greenhouse gas already approaching carbon dioxide in importance, could be released from warming tundra and from the continental-shelf sediments of warming oceans. Eventually water vapour, at present tending to form clouds that reflect more heat than they trap, could come to be a greenhouse gas disastrous in its effects.

What would be absolutely the worst scenario for overheating produced by pollution? Famous for the hypothesis that natural checks and balances have until now kept our environment stable, James Lovelock gave us his answer in his book Gaia . Earth, he said, could heat "to a temperature near to that of boiling water".

Vacuum metastability is a danger much less known about than pollution, and it may even be illusory. The idea is that the space we inhabit, "the vacuum", is far from entirely empty. It is filled with scalar fields. These are characterised only by intensity, not by the directionality that makes a magnetic field detectable by compass needles. Fish on the ocean floor cannot know about water pressure, which is the same wherever they swim. Similarly, humans could be unaware of the scalar fields because their intensities remain identical for as far out as telescopes can probe. But if they exist, which is what most physicists think, then they determine the properties of every atom.

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Change any such field, and those properties change. Change it in a way that reduces its potential energy, and the alteration spreads with enormous violence. Reduction of potential energy is what happens to a ball after it gets knocked out of a hollow on a hillside where it had sat "metastably" - the ball gets to do what it wants to. It rolls downhill. Thanks to physicists investigating very high-energy densities, the space around us, too, might get to do what it wanted through being jolted out of its metastable condition.

We remain sadly ignorant about the physics of very high-energy densities. If vacuum metastability manifests itself here, physicists might some day create the ultimate environmental catastrophe. They might produce a tiny bubble that immediately expanded at nearly the speed of light, destroying first our planet, then the solar system, then all the stars in our galaxy, then all the nearby galaxies, and so on.

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This process could not begin tomorrow or even during the next decade. Experimenters would have to push beyond the energy densities known to be safe, which are the ones attained by colliding cosmic rays. At present our particle accelerators are below those energies by a factor of perhaps several million. But, in his Dreams of a Final Theory , physicist and Nobel laureate Steven Weinberg talks of using powerful laser beams to accelerate individual charged particles to the Planck energy - roughly equivalent, he explains, to "the chemical energy in a full automobile gasoline tank". Not even colliding cosmic rays pack anything like that amount of punch.

Maybe scalar fields are entirely fictitious. Perhaps they exist but are fully stable, like a ball at the bottom of a valley. But if they exist and are only metastable, then not even beginning to colonise the galaxy would ensure the survival of the human race. Picture those people in their spaceships, happily congratulating themselves on being safely away from an Earth laid waste by pollution. Then they see the expanding bubble. A few seconds later, it overtakes them.

The physics of this area is so difficult that perhaps only actual experiments could prove that it held no risks. In a book section headed "Premature Apocalypse?", Rees writes that "caution should surely be urged (if not enforced)". Personally, I'd be all for enforcement.

The most likely cause of premature human extinction, however, seems to me biowarfare, rather than vacuum metastability or pollution itself - although the temptation to go to war could certainly be increased by a polluted, collapsing environment. So far, the most environmentally ravaged nations have been the ones least able to resort to force because they are too impoverished. But germs could become the poor man's atomic bomb. While crops and livestock could be targets, the main risk of human extinction comes from germs designed to attack humans. A single bottle filled with tiny beads can now yield viruses in quantities that would earlier have come only from large factories. And advances in genetic engineering have made it easy to design utterly lethal organisms.

An aggressor nation's vaccination programmes or other measures to protect itself might fail, or a nation losing a war might be willing to take the terrible risk of killing everyone. A terrorist organisation might threaten to put an end to all humans, then actually carry out the threat if its demands were refused. Experiments dating from 1948 show that just a few aircraft distributing a disease such as smallpox in aerosol form could infect almost everybody in Britain.

The world's end might also come about by accident. Recently, a genetically modified mousepox killed every infected mouse without exception. It had been created by Australian researchers - by mistake.

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John Leslie is a philosopher and fellow of the Royal Society of Canada.