GAMMA-RAY bursts鈥攊ncredibly powerful explosions that may be caused by
collisions between collapsed stars鈥攃ould solve one of the oldest riddles
about extraterrestrial civilisations: why haven鈥檛 they reached Earth already?
After studying the effects of gamma-ray bursts on life, an astrophysicist has
concluded that aliens may have just started to explore their galaxies.
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鈥檚 age of about
10 billion years.
So if ETs exist in the Milky Way, where are they? Maybe they don鈥檛 share the
human urge to explore. Or perhaps there鈥檚 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.
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Annis points out that each GRB unleashes devastating amounts of radiation.
鈥淚f 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.鈥� 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鈥攆ar
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鈥檚
question. 鈥淭hey just haven鈥檛 had enough time to get here yet,鈥� he says. 鈥淭he GRB
model essentially resets the available time for the rise of intelligent life to
zero each time a burst occurs.鈥�
Paul Davies, a visiting physicist at Imperial College, London, says the basic
idea for resolving the paradox makes sense. 鈥淎ny Galaxy-wide sterilising event
would do,鈥� he says. However, he adds that GRBs may be too brief: 鈥淚f the drama
is all over in seconds, you only zap half a planet. The planet鈥檚 mass shields
the shadowed side.鈥� 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. 鈥淚t 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.鈥�