Last month, Russian billionaire Yuri Milner, joined by Stephen Hawking and other esteemed scientists, announced a plan that still sounds impossibly brazen: They want to send a fleet of tiny probes to Alpha Centauri, the nearest star to our own. The $100 million Breakthrough Starshot initiative would beam lasers from Earth onto a few hundred metal sails, accelerating them to one-fifth the speed of light. If all went well, the probes would only need two decades to reach Alpha Centauri, and another 4 years to beam back images of any planets orbiting the system.
The plan is audacious, to say the least. But if it worked, we might be able to do more than send wisps of metal to another star. The laser that sent them packing would also be the biggest lighthouse in the cosmos—that we know of—and we could use it to let other civilizations know we’re here. Its beam could travel well beyond Alpha Centauri, to any planet orbiting any star in the observable universe, says Philip Lubin of the University of California, Santa Barbara.
“We’ve reached the point that we can project our presence throughout the universe. We’ve never been able to make that statement before,” Lubin says. “We’ve evolved technologically to the point that another civilization like us, anywhere in the universe, could detect us. And vice versa is, of course, much more interesting.”
We already broadcast radio waves constantly, but we haven't intentionally sent out messages of our own, apart from a few short-lived, quixotic projects. Astronomers have proposed building optical or radio-wave lighthouses for some time, however—and in recent years, they have studied how we might search the cosmos for someone else’s beacon. Recent leaps forward in laser technology and optics, which are advancing at roughly the breakneck pace of semiconductors, mean these signals could soon become powerful enough to reach the whole observable cosmos, Lubin says.
He studies directed energy for propulsion, and his research, which is funded by NASA, is the driving force behind Breakthrough Starshot. Like using a garden hose to roll a soccer ball, directed energy from a laser pushes and steers a spacecraft. Lubin says he realized that this same intense energy source could also be used as a lighthouse, or rather, an interstellar flare gun of sorts.
“If you take the same device for sending things out at relativistic speeds, and just point it, what do they see when the light finally gets there?” he says. He ran some calculations for a few planets around stars the Kepler Space Telescope has scrutinized, which are an average 1,000 light years from Earth. A hypothetical alien on a hypothetically habitable Kepler world might look up, notice a new flash brighter than anything else in the sky, and wonder what it was, he says. If we pointed the laser array at the Andromeda Galaxy, it would take the light 2.5 million years to arrive. But if, circling one of Andromeda’s stars, there were a being looking through a modest telescope, the signal would be visible even at that distance, according to Lubin’s calculations. Given the right combination of light, direction, timing, and luck, our flare gun would be visible from anywhere, Lubin says.
We would need to decide what the flare would look like, how long it would last and where it should point, although the most obvious choice would be rocky planets around sun-like stars. There could also be problems with intervening interstellar dust, or bright stars masking the signal.
Lubin insists that he’s not proposing we send laser messages to extraterrestrial intelligence. Rather, he wants to understand how to find someone else’s beacon. Lubin is careful not to advocate for messaging extraterrestrial intelligence, because messaging is controversial. Along with questions about what to say, who gets to say it and why, some in the SETI community argue that beckoning intelligent extraterrestrials would be inviting our own demise. Hawking himself famously invoked European colonists massacring native Americans, noting that we might not want to roll out a cosmic welcome mat. “We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet,” he noted in a 2010 documentary.
Other scientists are less cynical. Jill Tarter, the former director of the SETI Institute, has said any civilization that lasts long enough to figure out interstellar travel would have had to work out its internal problems first. “If aliens were able to visit Earth, that would mean they would have technological capabilities sophisticated enough not to need slaves, food, or other planets,” she said in response. “If aliens were to come here, it would be simply to explore.”
As Lubin points out, it doesn’t really matter whether we want to beckon E.T., because Earth is loud, electromagnetically speaking. There is nothing anyone can do to stop the noise from Game of Thrones and cell phone conversations and Donald Trump on CNN. “Every time you send out an email that says we should not broadcast, you are broadcasting,” Lubin says. “Every time you turn on your cell phone and order a pizza, you are ordering an interstellar pizza.”
The Milky Way alone has countless other planets, many of which are rocky and may be nestled in the “Goldilocks zone” of their stars, where conditions are just right for water to exist in liquid form (and maybe life). Many of these planets are much older than Earth, allowing plenty of time for another civilization to mature enough to build a lighthouse like the one Lubin envisions. “Hopefully they are vegetarians and have gotten beyond the point of eating each other," he says. Maybe their beacon is already sweeping the galaxy.
Lubin suggests a targeted search of planets in the putative habitable zones of Sun-like stars. He outlines how this would work in a new paper accepted in the aptly named journal REACH, Reviews in Human Space Exploration. The upshot is that it would actually be very easy. Thanks to rapid advances in optics and computers, that includes most people with a good digital camera. Ultimately, finding extraterrestrial life might be as simple as seeing a light in the dark.
How would we know—and how would aliens know—that a newly bright spot was a signal, not just a supernova? Lubin says it would not be hard to produce a signal with obvious unnatural forms, like radio modulation or a non-random structure — think Morse code, or a message along similar lines. The structure of such signals is an active area of debate in the SETI community.
“We might think, look at that bright thing, in a very narrow band in the optical or infrared. We’ve never seen anything like that. Is that thing made by nature, or made by nurture?” he says. “Is it an unusual kind of light? Are they pointing at me? Do they flash on and off? Then you start looking for patterns. You look for signatures of things that are not natural.”
None of this answers the really big question, of course. That is, if we located someone else’s beacon, what would we expect to find? And for that matter, what would we say in return?
Lubin thinks it might be very simple.
“We are here,” he says. “Or, ‘we were once here.’ That’s my hope, that there is some sort of intelligent civilization that has adopted that philosophy. SETI is based on things being not like us. They want people to find them; they want things to find them. They are not skittish like we are.”