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Detecting alien spacecraft -- after this.

October 22  Detecting Alien Spacecraft

Maybe there are other civilizations out there in the galaxy.  And maybe
there aren't.  The search for extraterrestrial intelligence -- also
called SETI -- relies mainly on looking for other civilizations by
listening at radio frequencies of the electromagnetic spectrum.  Some
on-going programs utilize radio telescopes.  They hope to detect an
unmistakable pattern that would indicate a non-natural, intelligent
source.

But listening for other life forms is just one possibility.  Another
idea might be to detect the "exhaust" of an alien spacecraft.  We don't
know that other civilizations would want to travel among the stars.
But if they did, then, according to current earthly technology, they
might rely on a propulsion method that produces gamma rays.  For
example, they might use a matter/antimatter propulsion technique --
where matter and antimatter meet -- and mutually annihilate each
other.  If the resulting explosion could be properly channeled, it
could send a spacecraft on a trip among the stars.  A by-product of the
explosion would be gamma rays.

So detect gamma rays, and you've found an alien spacecraft!  Well, not
quite.  There are lots of natural sources for gamma rays.  But an alien
spacecraft would do something no natural source is known to do.  It
would move at a substantial fraction of the speed of light among the
stars of our Milky Way galaxy -- maybe a hundred times faster than
ordinary astronomical objects.  Thus we would see gamma rays from an
interstellar spacecraft with a large proper motion -- a large sideways
motion on the dome of our sky relative to the background stars.

Nobody is using this technique at the moment for the purpose of
detecting alien life.  But it's an exciting possibility -- the
detection of another lifeform by its spacecraft.

Script by Deborah Byrd and Diana Hadley.
(c) Copyright 1985, 1986 McDonald Observatory, University of Texas at Austin