[escepticos] RV: (eS) - Why haven't we met ET?

[planetario@...>]

Un universo esterilizado???


>exoScience - http://www.exosci.com/
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>GAMMA RAY BURSTS MAY STERLISE THE UNIVERSE
>  from New Scientist
>
>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." 
>
>
>David Watanabe
>davew en exosci.com
>
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>Portions Copyright (c) 1998 David Watanabe
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