Alien transit systems may be a telltale sign in the search for ET

Avi Loeb has an unorthodox new idea on how to search for a̳l̳i̳e̳n̳ c̳i̳v̳i̳l̳i̳z̳a̳t̳i̳o̳n̳s — and it\’s no 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 has become intrigued with the search for ex̳t̳r̳a̳t̳e̳r̳r̳e̳s̳t̳r̳i̳a̳l̳ intelligence, or SETI, and tends to get at it in unusual ways.
In recent years, for example, Loeb has suggested searching for a̳l̳i̳e̳n̳s by looking for artificial lighting on Pluto, in the admittedly unlikely event that ex̳t̳r̳a̳t̳e̳r̳r̳e̳s̳t̳r̳i̳a̳l̳s (ET) have established an outpost there. He has also proposed trying to detect industrial pollution on distant exoplanets. His latest idea, laid out in a paper he and a co-author just put online: We must search for microwave beams that ETs could use to send light sails floating between planets in their home solar systems. “I don\’t think it\’s crazy,” says Seth Shostak, a senior astronomer at the SETI Institute in California. “It\’s a smart idea.” Candles of Light are a real thing, at least in theory; They use huge layers of ultra-thin Mylar to trap the solar wind, allowing them to carry a payload through interplanetary space without rockets. A prototype is now in the works sponsored by the Planetary Society, which has already flown a test mission and hopes to make a full demonstration flight next year.
“Unfortunately,” Loeb says, “there isn\’t enough pressure from sunlight to provide strong acceleration, so one can imagine using artificial radiation instead.” Loeb and his co-author, James Guillochon, an Einstein Fellow postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics, decided that microwaves would be the best candidate, based on efficiency and other factors. To move quickly between planets in an extrasolar system, they figured, you\’d need a microwave beam worth about a terawatt of energy. “That\’s about a tenth of all of Earth\’s production,” says Loeb – quite a bit. But these are the a̳l̳i̳e̳n̳s being talked about, so they could plausibly pull it off, using a powerful ground-based microwave transmitter aimed at the dinghy.
Most of that power would be trapped by the light sails. Something, however, would inevitably leak around the edges, so the two astrophysicists did some calculations to see if the leak could be detected from Earth. His equations said yes. “It would be easily detectable hundreds of light-years away with existing antennas,” says Loeb. The signal would arrive as a burst of energy caused by leakage from one side of the candle, followed by a pause and then a comparable pulse from the other side—a pattern, the authors say, that would be distinguishable from natural sources of microwaves.
The only time we could see microwaves would be when the beam was pointed more or less directly at us. And since the a̳l̳i̳e̳n̳s would presumably be using them to travel between planets, the two worlds in question would have to be aligned along line of sight to Earth. That would only happen in a solar system oriented from the edge to our perspective, just the kind of solar system that the Kepler spacecraft has been discovering along its trajectory. So it\’s already clear, says Loeb, where to point our antennae.

As to whether it is worth doing, however, it is not so clear. “It\’s not absurd,” says Freeman Dyson of the Institute for Advanced Study, who came up with his own unorthodox SETI strategy, in a Science 1960 article: Looking for infrared leaks from c̳i̳v̳i̳l̳i̳z̳a̳t̳i̳o̳n̳s that had completely closed off to its stars into hollow man-made “Dyson spheres” to trap every ounce 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.”
Ever since astronomer Frank Drake made the world\’s first SETI search, however, astronomers have looked mostly for ex̳t̳r̳a̳t̳e̳r̳r̳e̳s̳t̳r̳i̳a̳l̳ radio transmissions and, more recently, for a̳l̳i̳e̳n̳ laser beacons, thinking we should be looking for technologies that we\’ve really perfected. Light candles aren\’t particularly close at hand, however, let alone Dyson spheres, and there are only so many telescopes, radios and other gadgets to go around.
Still, says Shostak, any SETI search we can think of is based on our assumptions about the behavior of a̳l̳i̳e̳n̳s, about which we know literally nothing. Most likely, any advanced c̳i̳v̳i̳l̳i̳z̳a̳t̳i̳o̳n̳ out there is more advanced than ours, since we only discovered radio a century ago and digital computing much more recently. “A̳l̳i̳e̳n̳s may have gone beyond biological intelligence, and don\’t really know what machines might choose to do.” Finding a̳l̳i̳e̳n̳s, he says, could well happen by accident, the result of an observation or experiment that had nothing to do with SETI in the first place. That being the case, he tells him, “I\’m aw̳a̳r̳e that [Loeb and Guillochon] are thinking outside the box.”
Ed Turner, an astrophysicist at Princeton, Loeb\’s mastermind co-author on the experiment to search for artificial lighting on Pluto (if, by some incredibly remote chance, a̳l̳i̳e̳n̳s had chosen to build a city there), thinks the same. “Collaborating with Avi on SETI and other speculative issues,” he says, “is a bit like buying a lottery ticket. It\’s very likely he won\’t deliver, but if you\’re very lucky, it could turn out to be the most important work of his career.”
Which is very much in the original spirit of SETI, expounded in a 1959 Nature paper that inspired Frank Drake to launch the first radio search the following year. “The probability of success is difficult to calculate,” co-authors Giuseppe Cocconi and Philip Morrison wrote, “but if we never search, the chance of success is zero.”