High above the earth's atmosphere, in one of Nasa's most expensive spacecraft, sit eight crystal discs of sodium iodide. Now and then they are lit with the faintest of glows. The cause is a sudden surge of gamma rays-electromagnetic radiation like that which makes up visible light, but of shorter wavelength-sleeting through the spacecraft. Within seconds, if all works according to plan, news of the gamma rays' passage bounces back down to Earth and on to the internet, so that other telescopes can seek out the rays' source. To astrophysicists, the source of such gamma-ray bursts is one of the most enticing questions around.
For most of the earth's other inhabitants, gamma-ray bursts are a subject of only passing interest. As far as astronomy goes, the public finds the search for new abodes of life far more interesting than the peculiar emanations of distant stars and galaxies. Following the inconclusive suggestion of fossils in a Martian meteorite and the discovery of ever more, ever stranger, planetary systems around nearby stars, the possibility of life elsewhere is reasserting itself as a central theme in the exploration of space, and gamma-ray astronomy seems remote from these "astrobiological" concerns. But, as we shall see, there is a disturbing link between these two fields.
Gamma-ray bursts were discovered in the 1960s, when satellites looking for nuclear tests started to see them regularly. They were a surprise; unrelated to any objects or events that could be seen in any other wavelengths. Some thought they came from inside our galaxy, the result of disturbances on the surfaces of the dense neutron stars into which some old massive stars collapse. Others thought that they occurred further afield.
The Compton Observatory-the spacecraft which carries those gently glowing crystals-has ended this debate. Its detectors pick up almost one a day, and it is estimated that they miss one third of all gamma-ray bursts because of the way the earth blocks the sky. All told, there are some 500-600 gamma-ray bursts a year. With this much data, it has been possible to look for any patterns that they make on the sky. If they came from old stars in the galaxy it would be reasonable to expect them to cluster in the galaxy's central plane. They don't. The sources are scattered evenly across the sky, and it is now accepted that they can be found in distant galaxies.
The extreme distance-many millions of light years-thus imputed to the bursts means that their sources must be phenomenally bright, briefly outshining all the hundreds of billions of stars with which they share their galaxies. Only by converting a star's worth of mass into pure energy in a very short time can such fireworks be produced. Today's theories suggest that gamma-ray bursts represent not the hiccuping of neutron stars, but their near total destruction. Expert opinion is split as to the precise nature of these catastrophes; they could be collisions between neutron stars, or the destruction of neutron stars by black holes, or the creation of black holes through the collapse of massive normal stars.
Such extravagance is best seen from afar. Anything in the same solar system, or its near neighbours, would be singed to a crisp. Even many hundreds or thousands of light years away, a gamma-ray burst could have a profound effect on a planet. It would destroy any ozone layer which the planet might have, and convert hitherto stable atomic nuclei into radioactive ones. Cosmic radiation which came in its wake would add to the atmospheric damage and might produce secondary showers of radiation which could kill organisms living at the surface-although those living deep below the surface and in the oceans might survive.
Is the earth safe?
We don't know at what range such effects might prove capable of wiping life from a planet's surface. One hundred light years seems hazardous; 10,000 light years seems reasonably safe. Suggestions that gamma-ray bursts might destroy earth-like biospheres throughout an entire galaxy-that is to say, across ranges of 100,000 light years-seem unduly pessimistic. From the point of view of the earth, none of this is a great cause for concern. With only 500-600 detectable gamma-ray bursts a year in the observable universe, the rate of bursts in a typical galaxy such as ours must be low (perhaps once every 10m or 100m years). None the less, the earth has, in all likelihood, shared its galaxy with a fair few such bursts in the half a billion years since multicellular life evolved-and it is still here to tell (or, rather, discover) the tale.
The fossil record shows that there have been five mass extinctions during the past half a billion years. While one of these is clearly linked to the impact of a medium-sized asteroid or comet, a nearby gamma-ray burst might have caused one or more of the others.
Even if there has never been a gamma-ray burst close enough to harm the earth, this does not mean that there will never be one. Pairs of neutron stars in orbit around each other have been observed in our vicinity, and these orbits are not stable. In the end the pirouetting partners will meet; such meetings are thought to produce black holes and give off gamma-ray bursts in the process. In the case of the nearest known pair of neutron stars, the pulsar PSR B1534+12 and its companion, this coalescence will take place in more than a billion years. If it were to take place today, with the neutron stars at their current distance from the earth of a few thousand light years, the gamma rays from this cataclysm would deliver an energy equivalent to the yield of all the earth's nuclear arsenals into our upper atmosphere. While the sun delivers a similar amount of energy every half hour or so, such an influx of rays could still have a serious effect. By the time PSR B1534+12 and its companion meet, they will probably be much more distant from the sun than they are today-stars move a long way in a billion years. But there is no guarantee that another such pair may not pull into view in the meantime.
At some point in the planet's future, a gamma-ray burst will have some effect on the earth. Since the chances of that point arriving in the near future are very low, this need not concern us all that much (a civilisation-destroying meteor is much more likely). But while planets may, on average, be spared from the gamma-ray bursts for long periods of time, every day some of them do fall victim to the fires. If a gamma-ray burst's deadly effects have a range of a few hundred light years, millions of planets could be affected by each burst. The bursts might thus represent spasms of destruction far greater in their reach than single planet mass-extinctions. Thus one of the first truths of astrobiology: if life is common in the universe, so is its catastrophic disruption on huge scales.
The extinction of something you have never seen might not matter much. Few weep for all the species wiped out at the end of the Permian period, 245m years ago-why worry about extinctions even further removed from us in space than the Permian is in time? One difference might be if the extinctions involved intelligent life. To astrobiologists, any life beyond the earth would be of huge excitement; but for most people it is the possibility of intelligent life, not interesting lichens, that feeds our curiosity. It is a possibility about which almost nothing can be said for certain. We have a limited understanding of how intelligent life developed on earth, but not of how likely that happening was-or of whether the three and a half billion years it took should be considered laggardly or precocious.
The great silence
Intelligent life from elsewhere appears not to have visited the earth while we have been in a position to take note. This causes some of us to worry. Enrico Fermi, the great physicist, calculated the time it would take for life capable of travelling between stars to fill up a galaxy and found that the answer was surprisingly short: 100m years or so. "Where are they?" he asked. This question is still unanswered. Not only have aliens-as far as we can tell-failed to visit the earth; they aren't even trying to attract our attention from a distance. Attempts to pick up intelligent signals have so far failed to turn up anything at all, although to be fair, we have not looked terribly hard. Many explanations for this "Fermi Paradox," sometimes known as "The Great Silence," have been suggested, both in the academic literature and in science fiction. It may be that most intelligent lifeforms find more interesting things to do than to travel between stars or expand their populations. It may be that they frown on contact with the newly technological. It may be that most intelligent species are sea-dwellers, not land-dwellers, and thus do not develop technology based on combustion, without which an interest in space travel seems unlikely. (This may, indeed, be the case on earth, say those with a soft spot for whales.) It may be that there simply is no one else out there.
Gamma-ray bursts have been called into the great-silence debate by James Annis, a physicist at the Fermi National Accelerator Laboratory. He suggests that a gamma-ray burst may be capable of wiping out land-based higher life forms on all the planets of all the stars of the galaxy in which it occurs. If the frequency of gamma-ray bursts across the universe used to be higher than it is today-a reasonable but unconfirmed assumption-and if it takes about 300m years to go from complex land-living organisms to technological intelligence (which it did on earth)-then, up until now, most land-based biospheres would have been wiped out by gamma-ray bursts before intelligence could emerge. Gamma-ray-burst-free opportunities long enough to allow the evolution of intelligence are only now becoming available as the rate of bursting drops. This, Annis suggests, would explain the great silence: humanity may simply be one of the first species to benefit from enough un-irradiated time to get smart. Within a short while, cosmically speaking, more creatures like us will have evolved all over the now more peaceable and more permissive universe. The era of intelligence would finally dawn, with humanity in the vanguard.
This is not wholly convincing. The earth seems to shrug off mass extinctions reasonably well, bouncing back, in a few tens of millions of years at most, with living creatures just as complex as their predecessors. Mass extinctions have never set the clock back to zero here, so why should they elsewhere? Indeed, by allowing fresh starts on a regular basis, they might open up new opportunities for the evolution of intelligence. For Annis's idea to convince, gamma-ray-burst mass extinctions would have to be much more complete than any of the extinctions observed in the fossil record.
But by linking gamma-ray bursts to extraterrestrial intelligence, Annis does bring to mind a sobering thought. When we profess an interest in extraterrestrial civilisations, we are looking for one nearby. A message from a civilisation in another galaxy, or on the other side of this one, would be of great interest, but the fact that information cannot travel faster than the speed of light would mean that it could not be expected to dev-elop into a conversation. Others reaching out to us could not be answered or questioned if separated from us by thousands or millions of light years-distances equivalent to the lifespans of whole civilisations, even species.
"Conversational" contact with extraterrestrial intelligence requires that the interlocutors be within 100 light years or so of each other. Even that may be stretching things. Anything further away than the old age of our grandchildren seems far off to most of us. Scientific questions are rendered moot more quickly than that; basic human questions we think of as timeless may well be unanswerable or even unintelligible to basically inhuman interlocutors. But, still, an answer received today to a question posed a couple of centuries ago might be interesting. And institutions for talking and listening across the gulf could survive for hundreds, even thousands, of years. The Vatican has.
But if conversations are to occur even at these slow speeds, we will need to find intelligence within a few hundred light years of the sun. There is no reason to think that this is impossible. However, if there is another civilisation within that sort of range of the solar system, then it is a fair assumption-based on simple probability-that there is such a civilisation within a few hundred light years of any other solar system in any other galaxy. So if there is an extraterrestrial civilisation close enough to us to be interesting, we have to expect that, on average, there will also be such a civilisation dangerously close to every one of the gamma-ray bursts which the Compton Observatory detects. At a range of 100 light years or so, we have to assume that these civilisations are severely disrupted and often wiped out by the cataclysms. Some civilisations may span many star systems and thus weather the storm; some may avail themselves of early warning systems, burrow deep, store things well and survive. Ours, though, would be in no position to do either, and we must assume that at least some of the others are just as ill-prepared.
In his classic short story, The Star (see p42), Arthur C Clarke describes the crisis of faith experienced by a Jesuit astrophysicist examining the remains of a civilisation a bit more than 2,000 light years from earth, a civilisation destroyed when its sun exploded. The more he sees of the ruins, the graver his doubts become. The picture of gamma-ray bursts that the Compton Observatory is producing is not as affecting as the situation in Clarke's story. But it does present us with a melancholy choice. We must either acknowledge that there is little likelihood of ever contacting other thinking, feeling beings close enough to converse with, or accept that every day, when the Compton detectors glow, there is a good chance that a far-off civilisation has been consumed by fire. The universe is savage.
