The Mother of All Beacons

There's asmall difficulty with SETI that reminds me of a situation I often encounter inEuropean restaurants. I call it the "synchronicity problem."

Imaginethat I've spent two hours savoring the restaurant's house special (usually somespecies of fish known only to ichthyologists), and want to get the bill and getout. What is the probability that when I stare at the waiter to signal myintentions, he'll be looking at me?

Not high.

This is aSETI problem too. Since we observe a star system, at any particular frequency,for only a few seconds or minutes, can we reasonably expect that during thatbrief scrutiny the extraterrestrial transmitters are "looking" at us?After all, an alien society will surely be too distant to have picked up ourtelevision or radar and thereby know we're here, scanning the skies for signsof life. So why would they be sending messages to us now?

An obviousresponse is that the extraterrestrials could be beaming a non-stop signal tothe entire galaxy. Their strategy for pinging unknown neighbors maysimply be "all alien, all the time."

The obviousappeal of a continuous, pan-galactic broadcast has prompted some scientists toinvent unconventional schemes for omnidirectional signaling. For example,fifteen years ago G. M. Beskin and A. V. Sannikov, of the Russian Academy ofSciences, figured that advanced societies might co-opt big stars as billboardsthat would grab the notice of just about anyone in the Milky Way.

Theirsuggestion was this: Aliens with a yen to yak could set up powerful generatorsto fire a beam of hard X-rays at so-called super-supergiant stars. These bulkysuns, also called hypergiants, are the galaxy's heftiest stellar inhabitants:millions of times brighter than Sol, and considerably larger. Indeed, if thesun were suddenly swapped for the most voluminous of these hypergiants, YVCanis Majoris, the solar system would be stuffed with star all the way toSaturn.

Everyastronomically literate society will know of hypergiants, and would undoubtedlyburden grad students with their study. Consequently, sooner or later someonewould notice if the beam from an alien race lit up the outer layers of ahypergiant, causing it to develop an unnatural X-ray glow. The observer couldquickly deduce that advanced aliens were trying to get his attention.

This ideais like the bat signal used by the Gotham City police whenever they need Batmanbackup. The desperate gendarmes use a powerful searchlight to project a batlogo onto a cloud layer, producing a simple call sign that would be noticed,one presumes, everywhere in the city (including suburban and stately WayneManor).

The BatSignal is a one-bit signaling device, but the X-ray generator lighting up ahypergiant could pulse on and off to send — Morse-code style — a message. Ofcourse, the bandwidth is low because scattered rays from the edges of the starwill arrive at the receiver later than those from the center, blurring thepulses. The maximum data rate, even for a star only ten times the diameter of thesun, would be about one bit per minute. That's worse than dial-up.

The limitedbandwidth of this scheme is a problem, but there's more bad news. Lighting up astar for signaling purposes takes a lot of power. Beskin and Sannikov estimatedthat their scheme would require an X-ray generator that was at least 0.01% ofthe star's light output, which even for a fairly dim hypergiant is 1025watts. For comparison, that's about a million times more power than requiredfor an all-galaxy radio beacon that could be easily detected by today's SETIexperiments! X-raying stars seems like a hard way to get out your message.

Starlight,star bright

Recently,astronomers in Hawaii and France have come up with other ideas for turningstars into beacons — schemes that, once again, advanced aliens might use to getthe attention of galactic inhabitants.

JohnLearned (University of Hawaii) and his colleagues propose something reminiscentof the Russian approach. They suggest that aliens could fire neutrinos into theincandescent innards of a Cepheid variable star, thereby inducing it toslightly change the rate at which it brightens and dims.

Cepheidvariables are massive stars (albeit not hypergiants) that pulsate regularly,and are routinely used to calibrate the distances to galaxies. They're usefulto astronomers the way lab rats are to medical researchers, and will be noticedand studied by scientists on any world. In addition, they can be seen atintergalactic distances. So causing a Cepheid to change its pulse period isakin to projecting a bat signal, not on some low-lying stratocumulus, but onthe moon!

Lamentably,once again the bit rate will be low, and the utility bills high. The formerwon't exceed one bit per second, and the latter will spin your electric meterat the not-inconsiderable burn rate of 1023 watts. That's somewhatless pricey than the hypergiant scheme, but still nowhere near the much lowercost of an all-galaxy radio beacon.

Finally,there's the scenario proposed by French astronomer Luc Arnold, who suggeststhat the aliens will build giant, odd-shaped shadow puppets to orbit their ownsun, occasionally blocking some of its light. In our search for planets thattransit their suns, we might notice one of these artificial occulters. Thedimming would be different from that of a round planet if the object is, forexample, a giant triangle, or a louvered screen. If we saw one of these strangemini-eclipses, we'd know that someone was at home in that solar system.

But again,this is really slow signaling — the transit would occur for a few hours everyyear (whatever their year might be), and would only convey a single bit ofinformation: "Aliens are here." And while there's no operatingexpense (the occulter needn't fire either X-rays or neutrinos. It merely orbitsits star), there's the not-inconsiderable expense of building a Christmas treeornament roughly the size of Earth.

Whileadmirably clever, these schemes to take advantage of stars for signaling strikeme as impractical. They're all cursed by high costs and extremely lowbandwidth. Radio transmitters or pulsed lasers do enormously better on bothcounts.

So sure,it's possible that the aliens are tickling stars to get our attention. Butlooking at it from a cost-benefit point of view, I think these ideas are akinto using smoke signals instead of cell phones. Slow and inefficient, not tomention being a severe annoyance in restaurants.

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Seth Shostak
Senior Astronomer, SETI Institute

Seth Shostak is an astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, who places a high priority on communicating science to the public. In addition to his many academic papers, Seth has published hundreds of popular science articles, and not just for; he makes regular contributions to NBC News MACH, for example. Seth has also co-authored a college textbook on astrobiology and written three popular science books on SETI, including "Confessions of an Alien Hunter" (National Geographic, 2009). In addition, Seth ahosts the SETI Institute's weekly radio show, "Big Picture Science."