The new alien water hole: How aliens could use lasers for both communication and cloaking

First published on 6th August 2016. Last updated on 5th June 2017 by Dr Helen Klus

In a paper published in MNRAS in June 2016, astronomers David Kipping and Alex Teachey showed how we could cloak the Earth using lasers. We could also do the opposite, and use lasers to broadcast our presence. If we can do this, then presumably any other intelligent, technologically advanced species can do this too. Kipping and Teachey suggest that although it's unlikely, we should look through data from the Kepler satellite to see if we already have evidence for intelligent alien life[1].

1. Clocking a planet

The Kepler satellite is used to detect planets by looking at the change in brightness that occurs when a planet crosses the path of a star. This blocks part of the star's light and causes the brightness to decrease. This is known as the transit method.

Plot of brightness against time, showing how brightness decreases when a planet passes in front of a star, creating a dip.

Light curve produced by a planet crossing the path of a star. Image credit: Helen Klus/CC-NC-SA.

Kepler has been used to find over 85% of the 5400 planets discovered so far[2][3]. The transit method is currently the most efficient method for discovering planets, and any technologically advanced species may know this.

In order to hide a planet from the Kepler satellite, or satellites like it, the inhabitants could simply shine a light from their planet that fills in this gap. A similar idea to this was used to cloak planes during and after the Second World War, when lights, known as Yehudi lights, were used to prevent planes from creating a silhouette against the sky.

Plot of brightness against time, showing how light from a laser could create a peak, the same size as the dip created by a planet.

Light curve produced by a laser.
Image credit: Helen Klus/CC-NC-SA.

Plot showing both the dip from the planet and the peak from the laser. These cancel out so the overall signal is flat.

When both light curves are superimposed, the dip in brightness is filled in by the laser. Image credit: Helen Klus/CC-NC-SA.

A light can be shone from the dark side of a planet using mirrors, which could deflect the star's light around the planet, however this would be extremely difficult and expensive.

Kipping and Teachey show that we could achieve the same effect at very little cost using lasers. They suggest that if we just want to cloak the wavelengths of light that Kepler uses, it would only require about 30 MW of power for about 10 hours per year.

A more advanced system covering all wavelengths would use about 250 MW of power. This could be produced by solar powered cells, like those used on the International Space Station, which currently generate about 100 kW.

The problem with this approach is that it would only cloak planets from satellites that use the transit method of planet detection. The planet would still be visible using other methods, like the radial velocity method, which detects changes in the rotational velocity of a star due to the mass of orbiting planets, or using microlensing, an effect where spacetime is curved by a planet's mass.

If a planet were detected using one of these methods, but not by the transit method, then this would seem very unusual, and so would have the opposite effect to a cloak.

2. Clocking biosignatures

A better method may be to just cloak the signs of life on a planet. That way, all the methods for detecting planets would give the same result but the planet would appear uninhabited, and a ‘dead' planet would presumably be of little interest to a species looking for life.

This could be achieved by blocking evidence of molecules created naturally by life, like molecular oxygen, and molecules produced by industrial pollution like CFCs[4]. It would even be possible to block evidence of the whole atmosphere.

Elements and molecules are detected using transit spectroscopy. This involves splitting the light into a spectrum and looking for dark lines, where specific colours are missing. The colours correspond to specific wavelengths of light, which have specific energies.

The light is missing because it has been absorbed by molecules in the planet's atmosphere. Every molecule absorbs light of a specific energy, and so each missing piece of light corresponds to a specific molecule.

In order to hide evidence of oxygen in a planet's atmosphere, for example, the inhabitants could fill this gap with light from a laser. This method uses less power than that needed to cloak a whole planet because only very specific wavelengths need to be emitted.

Diagram showing that a continuous spectrum is created if light comes directly from a star. Absorption lines are created if the light travels from the star and through a cloud. This is because matter in the cloud absorbs some of the light. An emission spectrum is created from light directly emanating from the cloud, where light is only produced at specific wavelengths.

Image credit: modified by Helen Klus, original image by Magnus Manske/Jhausauer/Public domain.

Image of absorption, emission, and continuous spectra. Absorption spectra show spectral lines. Continuous spectra have no lines, and emission spectra are dark, with lines of colour.

Image credit: Magnus Manske/Jhausauer/Public domain.

One problem with this is that it wouldn't hide signs of life from direct imaging, which involves taking photographs of planets, and we have already discovered over 30 planets this way[5].

Another problem is that life on our planet has already been visible for billions of years, and any sudden disappearance, of either the planet of its atmosphere, would seem highly suspicious.

Both cloaking methods may seem futile since we have been broadcasting radio and television signals into space for about 80 years. It's unlikely that any aliens will ever hear them, however, as they were sent with a very low power. They would be almost impossible to detect after travelling more than a few light years, and the closest stars to the Earth are about 4 light years away[6].

3. Broadcasting evidence of life

Lasers could also be used to change the shape of a planet's transit curve in ways that would broadcast the existence of technologically advanced life forms.

Artificial megastructures, like Dyson spheres, may be difficult to detect because many natural phenomena, like clouds of comets, may produce the same effect. In order to broadcast their existence, the inhabitants of a planet would want to change the shape of their light curve to make it appear obviously artificial.

The shape of a planet's transit curve could be changed mechanically, by putting large artificially shaped objects into orbit, such as triangles[7]. These would be most efficient if they were placed close to the host star, where they would orbit more quickly – this is Kepler's 3rd law - although this is a far beyond our current ability.

Kipping and Teachey show that a similar effect can be achieved using lasers to distort the light curve, and suggest emitting light at the beginning and end of each planetary transit, creating peaks before and after the dip.

Information could be beamed along the laser, and Kipping and Teachey suggest that planetary transits may provide a universal way for life forms to commutate using optical light, providing an ‘optical waterhole', analogous to the ‘radio waterhole'.

The Project Cyclops team coined the term ‘radio waterhole' in 1971. The Project Cyclops team helped design SETI, and was headed by engineer, and vice-president of Hewlett Packard, Bernard Oliver.

Oliver and his team suggested that the most obvious part of the spectrum for aliens to communicate in would be between 1400 and 1700 MHz, corresponding to wavelengths of about 21 and 18 cm, which are in the radio spectrum. This is the part of the spectrum where spectral lines for hydroxyls - OH – and hydrogen - H - can be found. Together, these make water – H2O.

The Project Cyclops team suggested that:

"different galactic species might meet there just as different terrestrial species have always met at more mundane water holes"[8].

Photograph of different species, including giraffes and zebra sharing a water hole.

Different species sharing a waterhole. Image credit: Heyheyuwb at English Wikipedia/CC-SA.

There are many things to consider before we decide whether to cloak signs of life on Earth, or whether to advertise them. In the meantime, Kipping and Teachey suggest that we should search Kepler's data for light curves that show evidence of artificial signals.

4. References

  1. Kipping, D. M. and Teachey, A., 2016, 'A cloaking device for transiting planets', Monthly Notices of the Royal Astronomical Society, 459, pp.1233-1241.

  2., 'The Exoplanet Orbit Database', last accessed 01-06-17.

  3. NASA, 'Kepler', last accessed 01-06-17.

  4. Lin, H. W., Abad, G. G., and Loeb, A., 2014, 'Detecting industrial pollution in the atmospheres of earth-like exoplanets', The Astrophysical Journal Letters, 792, pp.7-10.

  5. NASA, '5 Ways to Find a Planet', last accessed 01-06-17.

  6. BBC News, 'Can our TV signals be picked up on other planets?', last accessed 01-06-17.

  7. Arnold, L. F., 2005, 'Transit light-curve signatures of artificial objects', The Astrophysical Journal, 627, pp.534-539.

  8. Oliver, B. M., 1979, 'Rationale for the water hole', Acta Astronautica, 6, pp.71-79.

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