Activate cloaking device: Hiding Earth from unfriendly aliens using lasers

A 22 W laser used for adaptive optics on the Very Large Telescope in Chile(Credit: RAS/ESO/G. Hüdepoh)

In recent years, mankind has become very good at finding other planets. Using instruments like the Kepler Space Telescope, scientists have, to date, discovered over 2,000 planets outside our Solar System, but what if some of those planets are inhabited by beings we'd rather not talk to, much less have drop in? Just in case any potential visitors are less ET and more Aliens, a pair of Columbia University scientists have figured out how to use lasers to hide the Earth from prying eyes by camouflaging its light signature.

If science fiction and human history have taught us anything, it's that sometimes it's a good idea to keep your head down. While the search for extraterrestrial civilisations has gone on for over a century, there are some thinkers, such as Stephen Hawking, who believe that advertising our own presence isn't such a good idea and that beaming messages into space could lead to our being on the receiving end of an Independence Day scenario without the Hollywood ending.

The trouble is, keeping quiet isn't enough. Professor David Kipping and graduate student Alex Teachey of Columbia contend that if we can find potentially habitable extrasolar planets, there's no reason why we can't be found in the same way. Therefore, they suggest camouflaging the Earth using lasers as a sort of cloaking device.

The Columbia proposal is based on one of the main techniques used in exoplanet hunting known as the transit method. In this, a telescope focuses on a star and records the light intensity. A dip in the brightness of the star indicates that a planet is passing in front of it. By studying the curve of this dip, astronomers can calculate the size of the planet, its mass, its distance from the star, its orbit, and even something of its composition.

If the exoplanet is the right size and is in the area around the star where liquid water can exist, called the habitable zone, this suggests that life could exist there. The good news is that this is a very powerful tool for scientists seeking evidence of life beyond our world. The bad news is that since Earth does have life, its parameters are likely to set off alarm bells in any alien SETI lab.

To prevent any awkward meetings, Kipping and Teachey propose shining a continuous 30 MW laser once a year for about 10 hours as the Earth passes between the Sun and any extrasolar planet that might be looking for us. This laser would cancel out the dip in the visible light curve and render the Earth essentially invisible. To cover all light wavelengths, the 30 MW laser would need to be replaced with a large array of tunable lasers with total power of 250 MW.

However, the astronomers say that it may not be necessary to completely cloak the Earth.

"Alternatively, we could cloak only the atmospheric signatures associated with biological activity, such as oxygen, which is achievable with a peak laser power of just 160 kW per transit," says Teachey. "To another civilisation, this should make the Earth appear as if life never took hold on our world."

On the other hand, the cloaking technology could have friendlier applications. For example, it could be used to augment our presence with a blatantly artificial signal that could also be used to transmit data to any listeners. In addition, knowing that such technology exists means that astronomers could look out for obviously artificial transits – provided they aren't hiding from us.

The Columbia research was published in the Monthly Notices of the Royal Astronomical Society.

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