If extra-terrestrials were looking our way, would you rather hide in the vastness of space with a Romulan-style cloaking device, or announce our presence with a burst of light?
Scientists at Columbia University have come up with a way to do both, either letting the aliens know we’re here, or fooling them into looking elsewhere.
Their idea could provide hope to those, like Professor Stephen Hawking, who fear contact with technologically advanced civilisations would be as devastating for humanity now as the arrival of Europeans was for native societies around the world half a millennia ago.
An artist's impression of lasers being fired from the European Extremely Large Telescope/RAS
But it could also encourage those who are actively looking for signs of aliens through programmes such as Seti, the Search for Extra-Terrestrial Intelligence.
The Columbia astronomers theorised in a paper for the Royal Astronomical Society that a 30MW laser could mask the visual signal from a planet, or transmit huge amounts of data between the stars. A 250MW tuneable laser could create a "chromatic cloak", hiding us at every wavelength.
The technique could also explain the Fermi Paradox, first proposed by Italian physicist Enrico Fermi, who asked his colleagues, during a conversation about alien visitors: “Where is everyone?”
One answer is that if they had such a laser system, they could be hiding right beneath our noses, cosmologically speaking.
Prof David Kipping and his graduate student Alex Teachey came up with the idea after wondering how hard it would be for aliens to conceal their existence, and realising it would be remarkably easy.
“What we did is find a way to cloak planets in general,” Prof Kipping told Forbes.com. “Another civilisation might be hiding right now, waiting until they’re comfortable with us. If we can do it, surely they could.”
Our current method for detecting exoplanets is to look for a dip in the brightness of a star as a planet passes in front of it.
So far, 1,963 such worlds have been found, half of them thanks to Nasa’s Kepler space telescope.
But by using a laser it would be possible to replace the missing light, keeping a star’s brightness steady as a planet passed in front of it.
Because only 1 per cent of stars are in a position to see an Earth transit, and just 2 per cent of those are thought to have Earth-like planets (the kind most likely to be inhabited), the laser would only need to operate for ten hours a year, the pair calculated.
It would be even easier to hide the fact that the Earth holds life, they said. This would involve replacing the light absorbed by oxygen in our atmosphere.
Oxygen is considered a sure sign of life as it is produced biologically and is highly reactive with other elements, so it quickly disappears from the air if it is not continuously renewed.
Humans are currently unable to detect such a subtle sign, but Nasa’s next-generation space telescope, the James Webb, due to be launched in 2018, would be able to do so.
Knowing that an exoplanet has life would make it far more interesting than just knowing it exists as a lifeless ball.
Transits could also be used to transmit information, since it’s more likely someone would be looking at us then.
Prof Kipping is eager to get funding to put a graduate student to work sifting through the data collected by Kepler in case we’ve already received a message from our cosmic neighbours.
