archive-1: SETI [ASTRO] Cataclysmic Explosions May Have Held Up Alien Visitors
SETI [ASTRO] Cataclysmic Explosions May Have Held Up Alien Visitors
Larry Klaes ( lklaes@bbn.com )
Thu, 21 Jan 1999 09:19:21 -0500
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>Date: Thu, 21 Jan 1999 6:17:29 GMT
>From: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>To: astro@lists.mindspring.com
>Subject: [ASTRO] Cataclysmic Explosions May Have Held Up Alien Visitors
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>Reply-To: Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
>
>New Scientist
>
>Contact:
>Claire Bowles, claire.bowles@rbi.co.uk, 44-171-331-2751
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>EMBARGOED FOR RELEASE: January 20, 1999, 2 p.m. EST
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>Cataclysmic Explosions May Have Held Up Alien Visitors
>
>GAMMA-RAY bursts -- incredibly powerful explosions that may be
>caused by collisions between collapsed stars -- could solve one of
>the oldest riddles about extraterrestrial civilisations: 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 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."
>
> ###
>
>Author: Robert Matthews
>New Scientist magazine issue 23rd Jan 99
>
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