杏吧原创

THE ALIENS are millions of years more advanced than us. Long ago they
explored the limits of their own solar system, and now, imprisoned by vast
interstellar distances, they are bored. Rather than sit around twiddling their
tentacles, they鈥檝e decided to find a younger race and bestow the wisdom of their
great age on these primitives. So they make a list of likely stars and transmit
their legacy into the unknown.

Some vision like this is probably in the back of the mind of most people
involved in SETI, the search for extraterrestrial intelligence. Their hope is
that the human race will some day pick up messages sent by creatures elsewhere
in the Galaxy. Even if the message doesn鈥檛 impart ancient wisdom, the simple
existence of other intelligences would be profoundly important鈥攔eassuring
to some, frightening to others. Humans have already begun to send greetings to the stars
(New 杏吧原创, 18 September 1999, p 36),
so surely other civilisations would try to communicate too.

Then why, after 40 years of listening, have we not heard a message? Perhaps,
some astronomers say, we have simply been tuning into the wrong channel.

The electromagnetic spectrum is broad. A transmission could be hiding at any
wavelength, and no one has an alien TV guide to hand. But the laws of physics
allow us to make some educated guesses about which channel ET would choose. In
particular, there is one band of radio waves, between about 1 and 10 gigahertz,
that can penetrate Earth鈥檚 atmosphere.

Even that leaves millions of channels to pick from. So in 1959, Philip
Morrison and Giuseppe Cocconi of Cornell University in Ithaca, New York,
proposed listening at 1.5 gigahertz, the radio frequency emitted by hydrogen
gas. They reasoned that this frequency might suggest itself to any intelligence
who wanted to be heard, because hydrogen is the most common element in the
Universe.

The first real attempt to search for extraterrestrial intelligence came just
a year later. In Project Ozma, Frank Drake of the National Radio Astronomy
Observatory in West Virginia aimed a radio telescope at the stars Tau Ceti and
Epsilon Eridani, listening at Morrison鈥檚 suggested frequency.

In the four decades since then, radio searches have expanded in number and
scope. The telescopes have grown, and powerful computers now scan many
frequencies at once, listening for the repeated pattern of an artificial signal.
Project Phoenix, for instance, listens to millions of channels through the giant
radio telescope at Arecibo in Puerto Rico.

Despite all this effort, no obviously intelligent radio signals have been
picked up. Is that because our galaxy is devoid of other technological
civilisations? Not necessarily. The same year that Drake pointed his radio
telescope at the sky, scientists created a new device鈥攖he laser. One man
who had a hand in the invention was Charles Townes, then at Columbia University
in New York. By 1961, Townes was arguing that light, the other band of
wavelengths that penetrates our atmosphere, would be a good way for aliens to
talk to us.

And yet astronomers continued to concentrate on radio. That might just have
been an accident of history. Dan Werthimer, a SETI astronomer at the University
of California, Berkeley, points out that radar and radio were already well
developed technologies by the early 1960s, whereas the first lasers emitted only
weak beams. 鈥淎ll of us were aware of these big powerful radio transmitters, but
none of us were thinking that lasers would ever get that big,鈥� says
Werthimer.

Stuart Kingsley, an amateur astronomer and optical engineer based in
Columbus, Ohio, blames the Cyclops Report. In this 1971 鈥渂ible鈥� of SETI, Barney
Oliver of Stanford University argued that optical signals need too much power.
Photons of visible light have a higher frequency, and therefore higher energy,
than photons in the radio band. Any receiver has to gather quite a few photons
to be sure there鈥檚 a signal, so you鈥檇 have to put a lot of energy into light
waves to broadcast to the Galaxy.

But as it turns out, you can send more of that energy where you want it.
Light is easier to focus into a tight beam than radio waves, so a higher
percentage of its photons reach the target. An alien laser beam could be
concentrated on the habitable zone around our Sun, a small region out to the
orbit of Mars. 鈥淭hose photons may be expensive, but it鈥檚 relatively
straightforward to aim them at a nearby star,鈥� says Seth Shostak, an astronomer
at the SETI Institute in Mountain View, California, who works on Project
Phoenix. 鈥淵ou get a better searchlight with the optical.鈥�

To aim that searchlight, the aliens would have to know how our star is moving
through the galaxy to predict where the Sun would be when the beam got here,
much as a clay-pigeon shooter aims ahead of a moving target. Oliver argued that
it would be difficult to get that accurate, but SETI proponents think it would
be a piece of cake for advanced beings.

Sending a laser signal is so simple we could almost do it now, says Paul
Horowitz, a physics professor at Harvard University. The NOVA laser at the
Lawrence Livermore National Laboratory in California, which is used in nuclear
fusion experiments, can produce more than a million billion watts of laser light
for a tiny fraction of a second. Focused with a 10-metre mirror like those in
Hawaii鈥檚 Keck telescopes, such a beam would be 5000 times brighter than our
Sun.

Loud hailer

How would an alien civilisation choose which star systems to hail? It could
be that they鈥檙e merely aiming at stars like our Sun, on the assumption that it
takes a Sun-like star to support life. Or they may know that Earth exists. A
civilisation slightly more advanced than ours could have measured the wobble of
the Sun, and seen that our planet鈥檚 gravity was tugging on it. Better still,
they could set up space-based spectrographs to measure the light emitted by
Earth. NASA is planning such a mission: the orbiting telescopes of the
Terrestrial Planet Finder, due for launch in 2011, will put the faint light from
distant planets through a spectrograph and look for the spectral signature of
free oxygen or chlorophyll鈥攕igns of life. An alien instrument might even
have measured the hydrocarbon pollutants in our air and deduced the existence of
an industrial society. And if they live around any of the many thousands of
stars within 64 light years of Earth, they could now be watching the first TV
broadcasts from the BBC.

The tight, bright beam of a laser is also a better carrier of information.
Waves of higher frequency can carry more bits per second, and optical
frequencies are huge鈥攁round a hundred thousand times higher than the radio
SETI band. 鈥淵ou can convey so much information in a laser signal,鈥� says
Werthimer, 鈥測ou can send a whole encyclopedia in a second.鈥�

鈥淭hat鈥檚 why an optical fibre coming into your house would be a much better
Internet connection than that phone line,鈥� adds Shostak. To the aliens, waiting
centuries for our reply to cross interstellar space might not make sense. Better
to include their knowledge in the first greeting.

Astronomers have finally begun to see the laser鈥檚 potential as an
interstellar communicator. Horowitz was won over after listening to Townes at a
1998 SETI Institute workshop, and within a few months started a search using an
old 1.5-metre telescope on a wooded hill in Harvard, Massachusetts. The
telescope鈥檚 main job is to collect spectrographic information on about 13,000
Sun-like stars, looking for those that might have brown dwarfs in orbit. But the
experiment does not need all the light that enters the telescope, so about a
third is siphoned off to Horowitz鈥檚 detectors. A sudden spike in the number of
photons could mean that something beyond the usual starlight is striking the
detectors鈥攑erhaps a nanosecond flash from an alien laser, calling out to
humanity.

At Princeton University in New Jersey, David Wilkinson is setting up a
telescope to run in parallel with Horowitz鈥檚 project. At the Lick Observatory on
Mount Hamilton in California, director Remington Stone will team up with
Werthimer and Drake for another search. And in Australia, Ragbir Bhathal of the
University of Western Sydney is watching the southern skies.

Instead of the pulsed beam these astronomers are seeking, aliens might use a
continuous beam at lower energy, which could contain even more information. It
would be drowned out by the light from a star under normal conditions, but
spreading the light through a spectrograph would make the laser
beam鈥攕hining at a single wavelength鈥攕tand out as a bright line.
Planet-hunter Geoffrey Marcy of San Francisco State University has a computer
combing through his spectrographic data for such beacons. Andrew Howard, a
graduate student working with Horowitz, suggests that the Terrestrial Planet
Finder could add such a search to its main mission. And the TPF will look at
infrared light, which many astronomers believe is better for communication than
visible light because it penetrates interstellar gas and dust.

There is now widespread agreement that optical SETI is worthwhile, but many
astronomers are cautious. 鈥淚鈥檓 a little pessimistic about it because it seems to
me it鈥檚 restricted to relatively small distances,鈥� says Morrison. Horowitz
calculates that optical SETI could easily work out to about 1000 light years,
encompassing about a million Sun-like stars. Morrison thinks a search should be
able to reach about 65,000 light years鈥攖aking in most of the Galaxy and
maybe 10 billion Sun-like stars鈥攖o have reasonable odds for success.

鈥淚f you said, `Dan, you鈥檙e only allowed to do one search,鈥� then I would pick
radio,鈥� says Werthimer. Radio might pick up not only a deliberate signal, but
also a stray beam from the alien version of television or radar. 鈥淏ut the best
strategy is a multiple strategy,鈥� he adds. Drake agrees: 鈥淵ou don鈥檛 know the
expertise of critters that are a million years ahead of us.鈥�

So should we be looking at other wavebands too? Few look promising. X-ray and
gamma-ray photons have even higher energy than light rays, so Werthimer thinks
aliens would avoid these. Neutrinos could travel through almost any obstacle,
but they鈥檙e almost impossible to detect. 鈥淣obody鈥檚 had the guts to propose
gravity waves,鈥� says Drake. 杏吧原创s have found indirect evidence of these,
but we have not yet detected them directly. And it鈥檚 hard to see how to generate
a gravity-wave signal, other than by smashing a pair of neutron stars together,
which might be a little difficult to arrange and wouldn鈥檛 send a lot of
information anyway.

Horowitz prefers simply to search for anything we鈥檙e able to look for, rather
than argue over the aliens鈥� preferred channel. 鈥淢aybe they鈥檙e using `zeta
rays鈥�,鈥� he says, 鈥渁nd those won鈥檛 be discovered until the year 2400.鈥�