Enthusiasts for the existence of extraterrestrials have long been haunted by a simple question supposedly posed by the Nobel prizewinning physicist Enrico Fermi around 1950. Fermi pointed out that the Galaxy is about 100 000 light years across. So even if a spacefaring race could explore the Galaxy at only a thousandth of the speed of light, it would take them just 100 million years to spread across the entire Galaxy. This is far less than the Galaxy's age of about 10 billion years.

So if ETs exist in the Milky Way, where are they? Maybe they don't share the human urge to explore. Or perhaps there's another reason, says James Annis, an astrophysicist at Fermilab near Chicago. He thinks cataclysmic gamma-ray bursts often sterilise galaxies, wiping out life forms before they have evolved sufficiently to leave their planet (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 scientists think the bursts occur when the remnants of dead stars such as neutron stars or black holes collide.

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

The rate of GRBs is about one burst per galaxy every few hundred million years. But Annis says theories of GRBs suggest the rate was much higher in the past, with galaxies suffering one strike every few million years -- far shorter than any plausible time scale for the emergence of intelligent life capable of space travel. That, says Annis, may be the answer to Fermi's question. "They just haven't had enough time to get here yet," he says. "The GRB model essentially resets the available time for the rise of intelligent life to zero each time a burst occurs."

Paul Davies, a visiting physicist at Imperial College, London, says the basic idea for resolving the paradox makes sense. "Any Galaxy-wide sterilising event would do," he says. However, he adds that GRBs may be too brief: "If the drama is all over in seconds, you only 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 levels of ultraviolet radiation.

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

New Scientist Magazine
Issue 23rd Jan 99