One proposal, to be published in Astrophysical Journal Letters, may be the wildest yet: Sailing aliens.
“An artificial origin is worth contemplating and checking,” said Avi Loeb, a theorist and author of the paper at the Harvard-Smithsonian Center for Astrophysics, in a statement on Thursday.
A decade ago, Lorimer and his mentor, Matthew Bailes, described the phenomenon as a fast radio burst, or FRB. “Duncan Lorimer and I were just completely gobsmacked,” said Bailes, a professor at Swinburne University of Technology in Melbourne, to The Washington Post. “The day we discovered the first FRB we couldn’t sleep.” Astrophysicists have detected only 25 other FRBs since Bailes and four other astronomers published their groundbreaking report in 2007, he said.
But the origin of FRBs remained an open question. The problem proved to be at once formidable, resilient and brain twisting. Some scientists proposed that FRBs were the fault of massive neutron stars, suns that had collapsed into dense cores. Perhaps there existed stellar flares capable of spitting out a radio wave that traveled across half of the known universe. Or maybe vanishing black holes spewed the FRBs our way.
“I am not exaggerating when I say there are more models for what FRBs could be than there are FRBs,” Cornell University astronomer Shami Chatterjee told The Washington Post in January.
“We don’t have a convincing model for FRBs at the moment,” Bailes said. “The leading model is some form of very exotic neutron star.”
The new hypothesis put forth by a pair of theorists at the Center for Astrophysics was even more exotic. Loeb and his co-author, Manasvi Lingam, ditched natural sources entirely. They speculated that FRBs could, in theory, be traced back to extragalactic civilizations. Specifically, aliens who flashed superpowered beacons or cruised through space on the wings of giant light-sail technology.
“It’s a delightful thought experiment,” said Bailes, who was not involved with the paper. (Bailes told The Post he considered himself more of an FRB hunter than a theorist.) But, he said, the new hypotheses amounted to an “incredible long shot.”
The two Harvard theorists recognized that their FRB origin story dealt with possibility, not probability. “Deciding what’s likely ahead of time limits the possibilities,” Loeb said. “It’s worth putting ideas out there and letting the data be the judge.”
FRBs could represent artificial beams, Loeb and Lingam wrote, created by a far-off civilization either as giant beacons or “for driving light sails.” Given the luminosity of the FRBs detected on Earth, an intelligent civilization would need to harness energy from a sun and cool the machinery with planet-sized amounts of water. Although alien traffic controllers would need to keep this beam aimed at the sail, distant observers would see only a flash as the beam pivoted across the universe.
When sailing by light, the sun’s radiation provides propulsion. If caught by a large enough sail and given adequate time, solar photons bouncing off a reflective surface could push a spacecraft with ever-increasing speeds.
Solar sailing has been a science-fiction concept for decades. Fiction writer Arthur C. Clarke described solar ships in his short story “Sunjammer,” published in 1964. Perhaps the most famous sci-fi solar sail appeared briefly in the 2002 movie “Star Wars: Attack of the Clones,” when villain Count Dooku traveled by a sail in his ship (a “heavily-modified Punworcca 116-class interstellar sloop,” per Wookieepedia). Solar sails are poised to jump into real life, too, in 2018. Two years ago, NASA announced its Near-Earth Asteroid Scout, which will use a reflective sail to travel toward a lump of space rock.
By Loeb and Lingam’s calculations, the hypothetical alien solar ship would be gigantic. They calculated that a solar-powered radio transmitter capable of producing FRBs would beam sunlight at an area roughly twice the diameter of Earth. If a solar sail were massive enough to catch these rays, it would propel a million-ton payload — about equal to three Empire State Buildings glued together, on par with what the theorists called an “interstellar ark” or “world ship.”
“That’s big enough to carry living passengers across interstellar or even intergalactic distances,” Lingam said in the news release.
The billions of galaxies in the universe may be the saving grace of Loeb and Lingam’s exotic speculation. “It only takes one galaxy,” Bailes said, “to develop some awesome technology.”
Still, from our planet, surrounded by hundreds of thousands of structurally deficient bridges, an infrastructure project of this magnitude may be difficult to imagine. Although Bailes said he would encourage his fellow scientists to take creative approaches toward modeling FRBs, his gut feeling was that the signals came from some unknown natural phenomenon.
The unusual nature of FRBs will require unusual models. (The area of FRB research is young but growing. A new radio telescope under construction in Canada has the potential to find dozens of FRBs a day.) Scientists recently detected one burst, FRB 121102, which was not a single flare but an irregular sequence — Bailes called it “the repeater.” The repeater seemed to rule out a singular catastrophic event as the FRB cause, but offered little in the way of answers.