Everyone has been in a situation where they need to make themselves conspicuous. Proponents of SETI have often provided novel solutions to ensure an observer would readily identify their planet as one hosting life. The answer can be condensed to a basic principle: do something unnatural at the exact moment someone is observing you. David Kipping, an astronomer at Columbia University, who searches for planets and moons beyond our solar system, believes lasers can be used by ETI to serve as a beacon or mask a planet entirely. In a recent paper, Kipping and a graduate student argue that artificial transit profiles can be feasibly generated using laser emission. Unlike optical SETI, which focuses on pulses of light, Kipping believes the transit can be a useful signal to or cloak from Earth (see Movie 1).
|One of the co-authors of this paper sets out to describe how lasers could be used to cloak a transit. The timing of this video showed poor foresight (April Fool’s Day….). A secondary video by Alex provides answers to some common questions from YouTubers.|
The use of transits in SETI goes back to the pre-Kepler days, when Luc Arnold first proposed distinguishing a transiting mega-structure from a natural body. Cloaking a planet requires many assumptions. Kipping ask us to consider an arbitrarily advanced civilization that discover all “nearby” habitable planets along their ecliptic plane. Kipping assumes the inhabitants would know which of these planets could observe their transits and, through some machinations privy only to ETI, such civilization would decide to prevent detection by these planets using the transit method. Kipping et. al. dismiss a previous suggestion of a mega-structure, arguing a powerful laser would be “technologically more feasible”. After performing a few calculations, Kipping et. al. argue a ~60 MW laser would serve as an optical, “broad-band” cloak and prevent detection from a mission such as Kepler. A laser, while monochromatic, could in theory serve to effectively mask a transit, as shown in Figure 1. Kipping et. al. argue that a laser array on the surface of a planet would be difficult and that instead ETI could place an array of lasers in space (colloquially known as a weapon). The authors aptly refuse to compare either solution. A similar and energetically cheaper alternative would be to use lasers to block out the absorption lines of biosignatures.
|Both images are from Kipping et. al. 2016. On the left: Cloaking of a Transit Signal. The top panel shows the unaltered transit for various missions. The middle panel is the power profile of a 600 nm laser array designed to cloak the Earth. The bottom panel shows what an observer would detect. On the right: Using Transits as a Beacon. The top panel shows the power profile of a laser array designed to broadcast the Earth. The bottom panel shows the transit signature an observer would detect. The laser makes for very unnatural signatures that distinguish it from orbiting planets.|
In addition to cloaking, Kipping et. al. briefly discuss signaling via lasers. Broadcasting would be much cheaper, as it would not have to be broadband. The ingress and egress could be altered with lasers as shown in Figure 1. Another possibility, is to use lasers to etch intriguing patterns during the light curve. Kipping has stated:
You can make your transit look strange, have bumps and wiggles, maybe even the New York City skyline—whatever you want.
Savvy extraterrestrial scientists could use a deformed transit as a beacon to announce their existence (see Figure 2). By Kipping’s hypothesis, ETI no longer required planet-size megastructures, such as a rotating triangle or louvres, to produce unnatural transit signatures.
|Going from top to bottom: (i) An unperturbed transit showing how a star dims slightly when an orbiting planet passes in front of it. (ii) A transit showing different shapes due to a laser array aimed toward an observer. This example shows the New York City skyline. (iii) The ideal beacon would be a square. This is a simple shape that would never occur naturally (yay limb darkening) and would require a laser only at ingress and egress. Source: David Kipping|
The reader is left with many questions and a sense of unease given all the assumptions. The ETI in question is apparently aware of all habitable planets in its ecliptic plane and capable of generating an array of lasers to block its transit. This is an act in vain if said planets use other techniques (i.e. direct imaging or radial velocity) to detect said planet. Kipping et. al. acknowledge this:
Transits are not the only method to discover planets and thus a truly xenophobic civilization may conclude that even a perfect and chromatic transit cloak would be ultimately defeated by observation of the planet using radial velocities. In this sense, the biocloak is perhaps the most effective strategy since certainly the transit and radial velocity measurements would appear compatible. However, even here, direct imaging would reveal a strong discrepancy in terms of the atmospheric interpretation and thus overcome the cloak.
A large part of this paper was to discuss how a transit could be cloaked, only to have that entire hypothesis appear to be an act in vain. The discussion on broadcasting with a strange transit signature is not fundamentally new. This blogger is left pondering the purpose of this paper. The authors themselves have dismissed the efficacy of cloaking and suggest we search for strange transits, something proposed by Arnold a decade earlier. Even if one were to assume cloaking to be efficient, SETI has predominantly concerned itself with civilizations indifferent to outside observers. After all, one could always invoke any arbitrary set of conditions or technology that would make a civilization impossible to detect. While the method of using lasers is novel, the rest of the paper reminds astronomers to search for strange transit signatures. Believe this requires strong priors and an indifference to all the assumptions. Kipping himself expects detections “on the order of a few dozen” and this blogger wishes him the best in his future endeavors.