Mysterious flashes observed in deep space may be evidence of alien spacecraft flying between the stars in distant galaxies, according to a team of astrophysicists. 

The astonishing theory is being proposed to explain brief but powerful flashes of energy detected by radio telescopes and so far unexplained.

Known as Fast Radio Bursts, the phenomena typically last just a millisecond. Fewer than two dozen have been detected since the first was spotted in 2007.

Some of the biggest telescopes in the world, including the Parkes Observatory in Australia and the Arecibo Observatory in Puerto Rico, have detected the flashes.

Astronomers have assumed that they have a natural cause and originate from some of the most remote galaxies, made up of billions of stars, billions of light-years away. Possible explanations previously proposed include supramassive neutron stars, gamma-ray bursts and stellar flares.

Now two leading astrophysicists at top American university Harvard say the radio bursts could be evidence of advanced alien technology.

They suggest specifically that they might be leakage from planet-sized transmitters that are powering alien interstellar probes, Star Trek-style, across faraway galaxies.

But instead of warp-drive engines, they believe the most likely use of the alien technology would be to drive spacecraft fitted with vast light sails. The energy from the transmitters would accelerate the spaceships just as wind pushes yachts’ sails on Earth.

A star-powered radio transmitter would need to be twice the size of Earth to generate enough energy, say the astronomers. Although beyond what is possible with human technology, such a device is physically possible, they say.

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One of the team, theorist Avi Loeb, of the Harvard-Smithsonian Center for Astrophysics, said: “Fast radio bursts are exceedingly bright given their short duration and origin at great distances, and we haven’t identified a possible natural source with any confidence. An artificial origin is worth contemplating and checking.”

The amount of power produced by the transmitters would be enough to push a spacecraft about 20 times the size of the largest cruise liners on Earth, says Loeb and his colleague Manasvi Lingam.

Lingam said: “That’s big enough to carry living passengers across interstellar or even intergalactic distances.”

In their paper, the two astrophysicists say their study of the energy and engineering requirements for such a beam both suggest that a transmitter would need to be the size of a large rocky planet. They assume that starlight will provide the energy for the beams and that the transmitter will be cooled by vast quantities  of water.

They also considered the possibility that the beams are beacons being used to tell the Universe of the presence of alien civilisations.  Reasons might include a call of help, a desire to proclaim their technological achievements, or similar sociological motives, they say.

However, they say in their research paper: “It seems rather implausible that this power would be expended on merely broadcasting a civilization’s existence,” and so consider that they might be powering light sails instead.

To power a light sail, the transmitter would need to focus a beam on it continuously. The reason telescopes observe only brief flashes would be because the sail and its host planet, star and galaxy are all moving relative to us, and so the beam only points towards us for an instant.

The astrophysicists accept that their theory, which is published online here, is highly speculative. But Loeb says: “Science isn’t a matter of belief, it’s a matter of evidence. Deciding what’s likely ahead of time limits the possibilities. It’s worth putting ideas out there and letting the data be the judge.”