SETI public: Chances of aliens finding Earth disappearing | New Scientist

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Chances of aliens finding Earth disappearing

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15:59 09 August 04
          
NewScientist.com news service
          
A pioneer of the search for extraterrestrial intelligence (SETI) has
warned that for any intelligent aliens trying to search for us, "the Earth
is going to disappear" very soon.

Frank Drake's point, made at a SETI workshop at Harvard University on
Friday, is that television services are increasingly being delivered by
technologies that do not leak radio frequencies into space.

But he added that in some ways the observation is good news for SETI, as
it means that the failure of Earth-based observers to detect aliens so far
may be less worrisome than it would otherwise seem.

Most SETI efforts have focused on detecting radio signals that might be
emitted by intelligent beings on planets around nearby stars. For humans,
such signals "are the strongest signs of our existence", Drake said,
thanks to television.

Traditional television broadcast antennas put out one megawatt each, and
this radio-wave bubble now extends about 50 light years out from the solar
system.

Straight down

But that is changing fast, Drake says. More and more television is now
delivered by cable, with no radio-frequency leakage to space, and by
direct-broadcast satellites that put out just 20 watts per channel, all
efficiently directed straight down the intended areas on the Earth's
surface.

So from the point of view of being detected through such inadvertent
broadcasts, the longevity of humanity's detectability may be just 100
years.

And longevity may be the most important figure in Drake's famous equation
for estimating the number of detectable intelligent civilisations on other
worlds. The best estimates show that all the other crucial factors nearly
cancel out, so that the number of such civilisations in our Milky Way
galaxy is roughly equal to their average longevity of detectability in
years.

Laser beacon

Drake's insight has important implications for search strategies. It means
that eavesdropping on unintended alien transmissions is unlikely to
succeed and "argues for an emphasis on detecting beacons", i.e. signals
intentionally sent our way.

Some SETI strategies have already begun shifting toward that approach,
including efforts to find optical beacons based on high-powered lasers
deliberately aimed at nearby stars.

While optical communications across interstellar distances was initially
thought impractical, military research has led to lasers sufficiently
powerful to make such signalling much more efficient than any radio beacon.

Nuclear-powered lasers on the drawing boards could produce pulses that
would outshine the sun by a factor of 10,000, said Harvard University
physicist Paul Horowitz, who has already been searching for such pulses.
He has designed a new telescope that will soon be dedicated full-time to
that search.

And other innovative ideas keep coming along. Planet hunter Geoffrey Marcy
of the University of California, Berkeley, said someday we may learn to
use the sun itself as a gravitational-lens telescope, with a detector
parked at its focal length of 500 astronomical units.
          
David L Chandler

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