A decade ago, scientists discovered intense pulses of radio waves lasting just milliseconds that gave off more energy in a fraction of a second than the sun does in hours, days or weeks. These fast radio bursts, or FRBs, likely originate from distant galaxies billions of light-years away, and researchers have estimated that they might occur as often as 10,000 times a day across the entire sky.
Scientists have proposed more than a dozen possible explanations for what might cause FRBs. They range from evaporating black holes to colliding neutron stars to starquakes on highly magnetized kinds of neutron stars known as magnetars. Still, the origins of FRBs remain unknown because the brief nature of these flashes makes it difficult to track down where they come from.
Now, scientists at Harvard suggest one potential explanation for these bursts might be planet-size transmitters emitting radio waves in order to propel spaceships across interstellar distances.
Theoretical astrophysicist Abraham Loeb at the Harvard-Smithsonian Center for Astrophysics is chair of the advisory committee for the $100 million Breakthrough Starshot project to visit Alpha Centauri, the nearest star system to Earth. The initiative’s plan is to get there using “light sailing,” a strategy that involves equipping chip-sized spacecraft with thin, highly reflective sails and employing powerful lasers to launch thousands of these probes outward per year. The project aims to build a ground-based 100-gigawatt laser array to propel swarms of probes at 20 percent the speed of light for 20-year journeys to Alpha Centauri.
“In the same way that a sailboat is pushed by wind, a light sail is pushed by light and can reach up to the speed of light,” Loeb said.
The aim of the Starshot project is to send a tiny spacecraft propelled by an enormous rectangular photon sail to the Alpha Centauri star system, where it would fly past the Earth-like planet Proxima Centauri b. The four red beams emitted from the corners of the sail depict laser pulses for communication with the Earth. Credit: Planetary Habitability Laboratory, Univesity of Puerto Rico at Arecibo
Loeb and study lead-author Manasvi Lingam wondered how well a truly giant laser might propel interstellar spaceships. Their calculations investigated a solar-powered transmitter array covering a planet roughly twice the diameter of Earth, about the size of either a super-Earth or a mini-Neptune. Their work suggested that cooling such a megastructure with water could help keep the incredible energies involved with such an undertaking from melting any underlying structure. The researchers calculated that such an enormous laser could help push a payload a million tons in size, roughly 20 times the largest cruise ships on Earth. This is big enough to carry living passengers across interstellar or even intergalactic distances, Lingam said.
Moreover, assuming that extraterrestrial passengers of these immense ships can tolerate accelerations of about one gee — the strength of the pull of Earth’s gravity — Lingam and Loeb calculated that radio waves with a frequency similar that that of FRBs could help propel those ships at one gee. “This is an exciting coincidence,” Loeb said.
To propel a light sail, a transmitter would need to focus a beam on a ship continuously. From the point of view of Earth, such a beam would look like a brief flash because the sail and transmitter are moving relative to us — for instance, because of “the spin of the host planet or the motion of the host star or galaxy,” Loeb said.
The researchers also estimated the number of civilizations that might develop all this technology “in our own Milky Way galaxy,” Loeb said. “We should be able to detect a gigantic burst from the Milky Way within a timescale of centuries. Such a signal will be detectable by cell phone.”
Although Loeb admitted that an artificial origin for FRBs was speculative, “we need more data to rule it out. The exciting aspect of doing science is that one rules out possibilities with better data. The history of science shows that it would be unwise to rule out possibilities just based on prejudice.”
One way to distinguish between FRBs of natural and artificial origin would involve analyzing the spectrum of radio waves in these bursts, as a light sail would cast a moving shadow on the pulses. “A peculiar radio spectrum would be suggestive of an artificial origin,” Loeb said.
The scientists detailed their findings online March 10 in the journal Astrophysical Journal Letters.