Handout from the Harvard-Smithsonian Center for Astrophysics of a light-sail powered by a radio beam (red) generated on the surface of a planet (Artist's illustration/PA)

Fast Radio Bursts (FRBs) are intense radio pulses that last no more than a millisecond. Since their discovery in 2007, fewer than two dozen have been detected by the world's largest radio telescopes - and their origin remains unknown.

Astronomers know almost nothing about them other than that they appear to occur in very remote galaxies billions of light years away.

Now a team of highly respected US scientists from Harvard University has put forward the serious suggestion that FRBs could be evidence of aliens at work.

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They believe the bursts may be leaked energy from unimaginably powerful transmitters capable of sending giant light sail ships on voyages between stars.

Professor Avi Loeb, from the Harvard-Smithsonian Centre 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."

In a new study, Prof Loeb and Harvard colleague Dr Manasvi Lingham looked at the feasibility of building a radio transmitter powerful enough to be detectable across such immense distances.

They concluded that a solar-powered system would generate the required amount of energy if it used an area twice the size of Earth to capture the sun's rays.

Water-cooling on a colossal scale would be needed to prevent the underlying structure melting.

But the question remains, why go to the trouble of constructing such an instrument in the first place? The most likely explanation is to drive interstellar light sails, the scientists argue.

A light sail uses the tiny amount of pressure exerted by light to produce a small but continuous acceleration that over time allows a space craft to attain great speeds.

The energy levels responsible for FRBs would be enough to push a payload weighing a million tons - about 20 times the mass of the largest cruise ships on Earth.

"That's big enough to carry living passengers across interstellar or even intergalactic distances," said Prof Lingam.

The transmitter would need to focus its beam on the light sail continuously. But observers on Earth would only see a brief flash because the sail and its host planet, star and galaxy are all moving relative to us. As a result, the beam sweeps across the sky and only points towards Earth for a split second.

The fact that FRBs have made repeat appearances that cannot be explained by cataclysmic natural events might provide clues about their artificial origin, said the researchers writing in the journal Astrophysical Journal Letters.