Beamed propulsion continues to be a particular fascination of mine, which is why I want to start a discussion tomorrow of Jim Benford’s latest paper on beamed sails and how they might be optimized for both performance and cost. Reading through Benford’s work, however, I also came across Chris Wilson’s recent articles in Slate, which discuss Jim and Gregory Benford’s work on interstellar beacons and the SETI ramifications. I want to be sure to point to Wilson’s How to Build a Beacon because I don’t see ‘Benford beacons,’ as they’re increasingly called, discussed much in the media, and Wilson does a fine job at setting the concept in context.

Messages into Deep Time

The two part Slate series (the first article is The Great Silence) considers humanity’s legacy and relates it to the issues raised by SETI. The Arecibo message sent in the direction of the globular cluster M13 in November of 1974 is Wilson’s point of departure. Carl Sagan and Frank Drake set up the famous message in the form of 1679 binary digits that could be decoded into a set of simple pictures showing the image of a human being and other aspects of our existence including a double helix and a graphic of the Solar System. There is rudimentary content here, but the idea that such a fleeting signal would be received is dubious, given the odds on its happening to have a receptive civilization in its path in the first place. As Wilson writes:

Even if we left the Arecibo telescope squealing out its signal until its power ran out and its hardware rusted, there’s virtually no chance that the emanations would get anywhere in particular, and hang around long enough to be seen or heard. The only way we’ll make contact is if we can make a beacon that keeps going for millions or billions of years after we’re gone.

Two of the signals sent to nearby star systems by Alexander Zaitsev from Evpatoria show up as part of the same argument, the point being that for an interstellar beacon to be noticed, it has to transmit for a long period and be energy-efficient as well. By ‘a long time’ I mean potentially eons, because such a beacon might be set up as the final gift to the universe from a dying civilization, and it might not be found for millions of years. Enter the Benford brothers, whose work on cost-optimized beacons we discussed here in articles like A Beacon-Oriented Strategy for SETI (and you can use the search function to pull up other Centauri Dreams stories on this work).

Signature of a Cost-Optimized Beacon

If you’re building such a beacon, you’re naturally going to think in terms of efficiency, as the cost of delivering a powerful signal continuously over a vast period of time would be mind-boggling. An efficient beacon is one that would choose its coverage area carefully to optimize the chances of being heard, and one that would offer short pulses that recur over regular periods. Note what a departure such a signal would be from the kind of signal conventional SETI is optimized to find, its searches running quick sweeps past stars to find continually broadcasting beacons. A Benford beacon’s signature would be intermittent, a brief pulse that would eventually recur.

No technology available in the near-term will allow us to deliver powerful signals every minute of the day over a span of multiple epochs… But we might be able to make a beacon that works more efficiently, by targeting only those star systems where life seems most likely, and then pinging them each in turn, repeating the cycle every few months or so. Presumably, if a curious civilization caught one transmission, it would train its telescopes on that exact spot until the next part of the beacon’s message arrived. This more sensible approach—a sort of Energy Star specification for SETI—would save enough power to keep the beacon running for millions of years.

If we’re trying to receive such a signal, slow and steady scans of the galactic plane might turn it up in the form of a short narrowband burst that would eventually repeat, which would call for longer ‘dwell times’ — the time devoted to looking at a particular target — and a good deal of patience. Gregory Benford notes that a civilization building such a beacon in our system might locate it at roughly 0.5 AU, allowing for plenty of energy for the beacon’s solar cells. Such a beacon would also face the threat of space debris, given that we’re talking about an artifact that will need to survive for hundreds of millions of years — deep time — and remain functional. Advanced robotics are one way Benford sees to repair damage and keep systems running.

Monument to a Lost Civilization

Benford beacons get around the synchronicity problem that bedevils those obsessed with communication with extraterrestrial civilizations. We have no way of knowing the average lifespan of a technological culture, and it’s possible that such lifetimes are measured in mere hundreds of years or millennia. A long-term beacon keeps sending detectable signals long after the civilization that created it is gone, perhaps as a monument in the fashion of the pyramids. Wilson writes about the Star Trek episode called ‘The Inner Light,’ in which Captain Picard lives out an entire existence on an unknown planet as he experiences an alien mind-probe, only to come to the end of the message and awake to find that a mere 25 minutes have passed.

Would our own species build a Benford beacon if catastrophe loomed? It’s an interesting notion, and it gets around the concerns that some have expressed about METI (Messaging to Extraterrestrial Intelligence), in that a society that builds a ‘funeral pyre’ beacon isn’t even thinking about a response, whether dangerous or otherwise. Wilson suggests a kind of beacon insurance system — build an interstellar beacon that is programmed not to function unless it loses all contact from its creators for an extended period. If it determines the creating culture is extinct, it switches on to send out the valedictory. A civilization dies, but in the remote future, perhaps another detects and decodes its final thoughts. There’s a bleakness in the concept, and yet at the same time a certain degree of grandeur.

To explore Benford beacons at the source, see James Benford et al., “Cost Optimized Interstellar Beacons: METI,” available here, and Gregory Benford et al., “Cost Optimized Interstellar Beacons: SETI,” available here.