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Author : Claire Bowles
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GAMMA-RAY bursts -- incredibly powerful explosions that may be causedby collisions between collapsed stars -- could solve one of theoldest riddles about extraterrestrial civilisations: why haven't theyreached Earth already? After studying the effects of gamma-ray burstson life, an astrophysicist has concluded that aliens may have juststarted to explore their galaxies.

Enthusiasts for the existence of extraterrestrials have long beenhaunted by a simple question supposedly posed by the Nobelprizewinning physicist Enrico Fermi around 1950. Fermi pointed outthat the Galaxy is about 100 000 light years across. So even if aspacefaring race could explore the Galaxy at only a thousandth of thespeed of light, it would take them just 100 million years to spreadacross the entire Galaxy. This is far less than the Galaxy's age ofabout 10 billion years.

So if ETs exist in the Milky Way, where are they? Maybe they don'tshare the human urge to explore. Or perhaps there's another reason,says James Annis, an astrophysicist at Fermilab near Chicago. Hethinks cataclysmic gamma-ray bursts often sterilise galaxies, wipingout life forms before they have evolved sufficiently to leave theirplanet (Journal of the British Interplanetary Society, vol 52, p 19).GRBs are thought to be the most powerful explosions in the Universe,releasing as much energy as a supernova in seconds. Many scientiststhink the bursts occur when the remnants of dead stars such asneutron stars or black holes collide.

Annis points out that each GRB unleashes devastating amounts ofradiation. "If one went off in the Galactic centre, we heretwo-thirds of the way out on the Galactic disc would be exposed overa few seconds to a wave of powerful gamma rays." He believes thiswould be lethal to life on land.

The rate of GRBs is about one burst per galaxy every few hundredmillion years. But Annis says theories of GRBs suggest the rate wasmuch higher in the past, with galaxies suffering one strike every fewmillion years -- far shorter than any plausible time scale for theemergence of intelligent life capable of space travel. That, saysAnnis, may be the answer to Fermi's question. "They just haven't hadenough time to get here yet," he says. "The GRB model essentiallyresets the available time for the rise of intelligent life to zeroeach time a burst occurs."

Paul Davies, a visiting physicist at Imperial College, London, saysthe basic idea for resolving the paradox makes sense. "AnyGalaxy-wide sterilising event would do," he says. However, he addsthat GRBs may be too brief: "If the drama is all over in seconds, youonly zap half a planet. The planet's mass shields the shadowed side."Annis counters that GRBs are likely to have many indirect effects,such as wrecking ozone layers that protect planets from deadly levelsof ultraviolet radiation.

Annis also highlights an intriguing implication of the theory: thecurrent rate of GRBs allows intelligent life to evolve for a fewhundred million years before being zapped, possibly giving it enoughtime to reach the spacefaring stage. "It may be thatintelligent life has recently sprouted up at many places in theGalaxy and that at least a few groups are busily engaged inspreading."