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Searching for Alien Beacons
by Gregory Benford (email gbenfored @
Professor of Astronomy and Physics, University of California, Irvine

Project Phoenix, the SETI Institute's sensitive targeted SETI effort, regularly cycles among 1000 stars, those nearer than 155 light years. So far, no luck.

Not surprising, really, in a galaxy spanning 100,000 light years, when even the optimists estimate there might be only a million alien societies capable of sending waves our way right now.With those numbers (guesses, really) the nearest similar society would be more like a thousand light years away.

But maybe it's time to admit an obvious fact--we live in the 'burbs. Our sun circles the galactic core about four times in a billion years, two-thirds out from the compact center. There are not a lot of nearby stars (next door neighbor Alpha Centauri is 4.2 light years away) and many are small, dull, red suns probably incapable of ever warming an Earthlike world to life.

We eavesdrop on our neighbors which seem promising, sifting through a third of a million candidates, cupping an electromagnetic ear at the skies... It's lonely work, as the SETI teams have discovered.

Though SETI listeners can check on a suspiciously narrow-band signal in real time, using the microwave telescope dishes at their command, much data processed is left for later inspection. The trouble is that there are many narrow signals, seemingly artificial, which turn out to be our own satellites radiating in the field of view, or natural astronomical sources of subtle signatures.

Is there a different strategy? The SETI Institute targets single stars; how about looking for the glaring, wealthy societies sending beacons?

This, too, is an old idea. New astronomical discoveries and the steady advance of radio astronomy have opened new reasons to look for beacons, though, following on ideas that go back 30 years, and have been updated by artist Jon Lomberg and others. Lomberg helped design the famous messages that fly now on Voyager, bound out of the solar system.

A bright beacon would by definition be so obvious, so powerful that it would light up the galaxy for a short while. The power would have to be a major fraction of the star's output. Beacons attract interest, then direct attention to a weaker signal that carries the larger message.

Where to look, though? Certainly, we should direct some of our radio ears into the plane of the spiral disk, where most stars dwell. In all that welter, one direction stands out : the center, the Times Square of the galaxy. Near it, a million stars lie within a single light year. Think of a sky with dozens of stars brighter than the full moon.

Of course, the dense center is a dangerous place for life like us. Proton sleet slams onto worlds, and stars swoop near each other every 100,000 years or so, scrambling up planetary orbits and raining down comets upon them.

But a bit further out, weather gets better. We now know that star formation started at the center, 10 billion years ago. That means that metals built up from the early supernovas there, so planets capable of sustaining interesting organic life like us could have begun their slow winding path upward toward life and intelligence within the first billion or so years.

Think of it--a world like ours that took 4.5 billion years to produce smart chimpanzees like us would have done so about four billion years ago!

In that much time, intelligence might have died out, arisen again, and gotten inconceivably rich. The beyond-all-reckoning wealthy aliens near the center could afford to lavish a pittance blaring their presence out to all those crouched out here in the 'burbs, just getting started in the interstellar game.

Whatever forms might dwell further in toward the center, they must know the basic symmetry of the spiral. This suggests the natural corridor for communication is along the spiral's radius, a simple direction known to everyone which maximizes the number of stars within a telescope's view. (A radius is better than aiming along a spiral arm, since the arm curves away from any straight-line view of view.) So a beacon should broadcast outward in both directions from near the center, and we 'burbanites should look inward.

Presto!--point your 'scope in a narrow angle (a few degrees) toward the constellation Saggitarius. Listen for the big spenders to shout at us.

But how often should we cup an ear? Here I prefer to invoke a Principle of Mediocrity--that we're mediocre, near the middle, in planetary properties. We follow a daily cycle atop an annual sway of climate.

If aliens are anything like us, they might then broadcast for a day, once a year. But which day?

No way to tell -- so listen every day, for maybe an hour. Watch for narrow-band signals that stand out even against the bright glare of that Times Square glow.

Radio astronomers have to know what frequency to listen to, as well. The universe is full of electromagnetic noise at all wavelengths from the size of atoms to those of planets. Quite a din. There's an old argument that water-based life like us might pick the "water hole" -- a band near 1.5 billion cycles/second where both hydrogen and hydroxyl molecules radiate strongly. Maybe not right on top of those signals, but nearby, because that's also in the minimum of all the galaxy's background noise.

Conventional SETI spends a lot of effort looking for nearby sources, shifting to their rest frame, and then eavesdropping on certain frequencies in that frame. But a beacon strategy could plausibly presume that the rest frame of the galactic center was the obvious gathering spot, so anyone broadcasting would be near there.

Luckily, several radio telescopes could easily carry out a beacon-seeking strategy. Piggy-backing on existing observing agendas demands little time and effort. Looking inward, such a 'scope listens to a billion or more stars at once. Of course the power demanded of a broadcaster is high, rising with the square of the distance away, because the signal spreads out over a larger area. But the number of stars listened to rises almost as fast as the cube of the distance (since it's proportional to the volume of a sphere).

We would be listening the the oldest stars. Here we can dream a little...

Suppose ancient societies, feeling their energies ebb, yet treasuring their trove of accumulated art, wisdom and insight, wanted to pass this on? Not just by leaving it in a building somewhere, hoping some younger species comes calling someday. Instead, build a robotic "funeral pyre" that blares out your greatness:

My name is Ozymandias, King of Kings,
Look on my works, ye Mighty, and despair!
as the poet Percy Bisse Shelley put it, witnessing the ruins of ancient Egypt.

We cannot know how often some civilization in the galaxy gets really rich, and lasts long enough to send its tidings across thousands of light years. The galactic center is about 25,000 light years away, and between its densely packed stars radio communication may be common. After all, they don't have to wait a long time to hear from neighbors that may lie quite near. We live in cities for similar reasons.

Finally, we should consider beacons because people here are already pondering it. It gets cheaper every year to call out to the stars electromagnetically. Already a firm promises to broadcast your very own Message to the Galaxy for a small fee. I'd rather that our society not send out personal messages, probably more like graffiti than the Psalm of Psalms. But there's no way to stop people from doing it. So far no radio telescope team has responded to questions about using their dishes to shout at the stars. But it will come.

A way of focusing on this issue is to listen first, establishing a calm, orderly way of thinking about this issue, for the discussions that will surely come--as soon as somebody begins broadcasting.

Then too, the prospect of beacons might well bring a pulse-quickening sense back into the SETI search. We may look back on this era and remember the old joke about the guy searching for his keys under a street lamp. He had lost them elsewhere, but the light was so good for searching right here, near the lamp...

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