Alien Transit Systems May Be a Giveaway in the Search for ET

Avi Loeb has an unorthodox new idea about how to search for alien civilizations—and it is hardly a surprise. Loeb, who chairs the astronomy department at Harvard University, has spent much of his career thinking about how the first stars came to life after the big bang, and how galaxies were born. But lately he’s become intrigued with the search for extraterrestrial intelligence, or SETI, and he tends to come at it in unusual ways.

Over the past few years, for example, Loeb has suggested searching for aliens by looking for artificial lighting on Pluto, in the admittedly unlikely event that extraterrestrials (ET) have set up an outpost there. He also has proposed trying to detect industrial pollution on distant exoplanets. His latest notion, laid out in he and a co-author just put online: We should look for the microwave beams ETs might use to send light sails wafting between the planets in their home solar systems. “I don’t think it’s nuts,” says Seth Shostak, senior astronomer at the SETI Institute in California. “It’s a clever idea.” Light sails themselves are an actual thing, at least in theory; they use huge sheets of ultrathin Mylar to catch the solar wind, allowing them to carry a payload across interplanetary space without rockets. A prototype is sponsored by the Planetary Society, which has already flown a test mission and hopes to do a full-fledged demonstration flight next year..

“Unfortunately,” Loeb says, “there’s not enough push in sunlight to provide a very strong acceleration, so one can imagine using artificial radiation instead.” Loeb and his co-author, James Guillochon, a postdoctoral Einstein Fellow at the Harvard–Smithsonian Center for Astrophysics decided that microwaves would be the best candidate, based on efficiency and other factors. To move briskly between planets in an extrasolar system, they figured, you’d need a microwave beam with about a terawatt’s worth of power. “That’s about a tenth of Earth’s entire output,” says Loeb—kind of a lot. But these are aliens he’s talking about, so they could plausibly pull it off, using a powerful ground-based microwave transmitter aimed at the light sail.

Most of that power would be trapped by the light sails. Some, however, would inevitably leak around the edges, so the two astrophysicists did some calculations to see if the leakage could be detected from Earth. Their equations said yes. “It would be easily detectable out to hundreds of light-years away with existing antennas,” Loeb says. The signal would arrive as a burst of energy caused by leakage from one side of the sail, followed by a pause and then a comparable pulse from the other side—a pattern, the authors say, that would distinguish it from natural sources of microwaves.

The only time we could see the microwaves would be when the beam was pointed more or less straight at us. And since the aliens would presumably be using them to travel between planets, the two worlds in question would have to be lined up along the line of sight to Earth. That would only happen in a solar system oriented edge-on from our perspective—just the kind of solar system the Kepler space probe has been discovering by the score. It’s therefore already clear, Loeb says, where to point our antennas.

Whether it’s worth doing so, however, isn’t quite as clear. “It’s not absurd,” says Freeman Dyson, of the Institute for Advanced Study, who came up with his own outside-the-box SETI strategy in a : looking for infrared leakage from civilizations that had completely enclosed their stars in artificial, hollow “Dyson spheres” to catch every last drop of solar energy. “But it’s not enough by itself,” he says. “Any practical search program should aim to cover a multitude of possibilities, not just one.”

Since did the world’s very first SETI search, however, astronomers have looked mostly for extraterrestrial radio transmissions and, more recently, for alien laser beacons, figuring that we should look for technologies we ourselves have actually perfected. Light sails aren’t quite there yet, to say nothing of Dyson spheres, and there are only so many telescopes, radio and otherwise, to go around.

Still, Shostak says, any SETI search we can think of is based on our assumptions about the behavior of aliens, which we know literally nothing about. Odds are that any advanced civilization out there is more advanced than ours, given that we only discovered radio a century ago and digital computing much more recently than that. “The aliens may well have gone beyond biological intelligence, and we really don’t know what machines would choose to do.” Finding ET’s, he says, might well happen by accident, the result of some observation or experiment that had nothing to do with SETI in the first place. That being the case, he says, “I appreciate that [Loeb and Guillochon] are thinking outside the box.”

Ed Turner, a senior Princeton astrophysicist, Loeb’s co-author on the thought experiment about looking for artificial lighting on Pluto (if, by some insanely remote chance, aliens had chosen to build a city there), feels the same. “Collaborating with Avi on SETI and similarly speculative topics,” he says, “is a bit like buying a lottery ticket. It’s extremely likely to yield nothing but if you happen to be very lucky, it could end up being the most important work of your career.”

Which is very much in the original spirit of SETI, laid out in a that inspired Frank Drake to launch the very first radio search the following year. “The probability of success is difficult to estimate,” wrote co-authors and , “but if we never search, the chance of success is zero." ( is part of Nature Publishing Group.)

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