Lasers could 'cloak Earth from aliens'

Astronomers use lasers now in a different way to help sharpen their images of the cosmos

We should shine lasers into space if we want to hide our presence from aliens, two US-based astronomers suggest.

The beams could compensate for the dip in light the Earth creates when it passes in front of the Sun, as viewed from far-off worlds, they contend.

A number of researchers have questioned the wisdom of advertising our existence to the galaxy.

They fear that if aliens did visit us they might not be very friendly, and could introduce disease.

The analogy is Europeans arriving in the Americas in the 15th and 16th centuries. The contact wrought havoc in the health of indigenous populations.

David Kipping and Alex Teachey from Columbia University in New York say that if we are fearful of a similar outcome from an alien encounter then lasers offer a solution.

The team has calculated what would be required to cloak the Earth and published the concept in a paper in the Monthly Notices of the Royal Astronomical Society.

It "perverts" the technique scientists already use now to look for distant planets around other stars. This method relies on staring at these suns, hoping to catch an object passing in front. When such a "transit" occurs, there is a tell-tale decrease in starlight.

The US space agency's Kepler telescope has identified more than a thousand planets this way.

If intelligent civilisations are out there, it is safe to assume they too will be looking for other worlds - like ours - using the same idea, believe Kipping and Teachey.

Unwanted attention: Some argue that Earth would regret any contact

According to the pair's calculations, emitting a continuous 30-megawatt laser for about 10 hours, once a year, would be enough to distort the characteristic dip in light when Earth transited the Sun, as viewed from an alien Kepler telescope.

"It doesn't have to be one huge laser; it could be an array positioned around the Earth. Or you could put it in space as a satellite, and we've calculated that the International Space Station already collects exactly the amount of energy we would need," Prof Kipping told BBC News.

This is true for a laser system working in visible light. Prof Kipping concedes, however, that a laser cloak that covers all wavelengths, not just the visible colours, would need a very large array of tuneable lasers with a total power of 250MW.

But an alternative might be to use a laser simply to disguise the interesting aspects about Earth - features in its atmosphere that betray the fact that life exists here. These are a suite of gases that include oxygen, ozone and methane.

"If we just cloaked out those biosignatures then another civilisation might detect our planet through a transit, everything would add up, but Earth would appear as a dead world and they'd soon lose interest," Prof Kipping added.

The search for extraterrestrial intelligence (Seti) is the collective term used to describe positive efforts to detect and contact alien life. A number of experiments are currently under way that are trying to see if aliens are actually signalling us with lasers.

But just as with the attempts to detect the radio transmissions from aliens, this "optical Seti" approach, as it is known, has also found nothing of interest among the stars… yet.