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starship-design: GRB's

Galactic Explosions Inhibit Life

by Robert Matthews at New Scientist

London - January 21, 1999 - Gamma-Ray bursts -- incredibly powerful
explosions that may be caused by collisions between collapsed stars -- could
solve one of the oldest riddles about extraterrestrial civilizations: why
haven't 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'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 sterilize 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 center, 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
sterilizing 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