Tag: New Methods

I feel that it is quite appropriate for me to review this paper by David Kipping two days after we conducted an observation of 12 transiting Kepler planets from Green Bank Telescope in association with Breakthrough Listen, based on the principle outlined in this paper.

The paper talks about using lasers to cloak the presence of a planet during its transit. However, in this blog I shall not talk about a civilization trying to mask its presence but its attempts to broadcast itself. The paper proposes the principle of a temporal Schelling point in our search for ETI. The question often arises of the best time to search. Since there is no real special time, this paper suggests that the transit of an exoplanet around its host star could be one. If there is a beacon on the night side of the planet, then it would sweep out an arc as the planet revolves around its star. This beacon would be visible from our line of sight when the planet transits the star, if it is directly aimed at its sub – stellar point. This beacon if broadcast continuously would be visible to observers periodically with every transit. Doing this during a transit is an interesting proposition since transits allow for us to also measure the atmospheric composition of planets using spectroscopy. Further, in the near future we should be able to map the longitudinal heat profile as well as atmospheric composition of planets using phase curve spectroscopy. This would provide for definite clues of bio-signatures.

However, the beacon might not necessarily be on a planet which is inhabited by the ETI. The beacon can be on the closest planet, since that would have the highest probability for transiting in a randomly oriented system.

I think this paper is important in acknowledging the special place transits occupy in the optical astronomy, and subsequently extending it to SETI. Its ideas about a civilization using this phenomenon to hide its presence or beam out and advertise itself are novel, and can be one of the anomalies being considered in Wright et al. 2015 (GHat 4).

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).

Movie 1. Alex Teachey on Cloaking Planets

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.

Figure 1. The Strange World of David Kipping

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.

Figure 2. Laser Doodles

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.

Since the conception of communications with extraterrestrial civilizations in the late fifties (Cocconi & Morrison 1959), the overwhelming majority of SETI endeavors have centered on electromagnetic communication systems, often in one narrow fraction of the entire spectrum. Hippke is aware of the potential shortcomings of such an approach and presents the possibility of alternatives, not just to microwave emission as in his previous work (Hippke 2017), but to electromagnetism as a medium for information carrying in general. In particular, he examines the merits and shortcomings of a variety of non-EM carriers such as electrons, protons, neutrinos, gravity waves, and occulting megastructures. Vetting based on energy efficiency and data rates, Hippke places these alternative channels in competition with EM-based communications. For transiting megastructures, Hippke fails to find a way for this method to be competitive when it comes to target communication with high data rates, and so tepidly dismisses them. He also quickly rules out charged particles, particles with short lifetimes, and heavy particles due to interstellar magnetism, longevity, and energy requirements, respectively. He is also critical of gravitational waves as a medium for signal carrying as their artificial production is extremely resource intensive and wasteful. Lastly he examines neutrino based communication, which fails due to issues with focusing when compared to photons and size requirements of detectors. All of his conclusions are based on current knowledge of physics, and so the possibility is open that with an improvement in knowledge, some of these avenues may potentially become viable again. He has framed this investigation to work within the confines of what is currently understood. With these limitations, he concludes that the best medium for point-to-point communications is still electromagnetic radiation, at around the 1nm scale. If the assumption of preference for speed is relaxed, then the best alternative would be inscribed matter, or probes carrying vast databases of information. This paper was a novel contribution to SETI because it is one of the first attempts at an exhaustive analysis of alternative modes of communication. Scientists can often times get caught up in the present paradigm, and so it is beneficial to get a fresh perspective on the issue from someone who is not formally scientifically trained and thus potentially not subject to the same prior perceptions. His conclusions also vindicate the thinking behind the Pioneer and Voyager plaques and records, since physical media transported on long timescales is shown to be one of the preferred methods of communication. The potential this paper had to to retroactively dismiss all of our previous SETI efforts as foolishly narrow-minded or misguided should not be discounted. While we will continue to perform SETI in the radio and microwave, we should always be open to the possibility of alternative means of communication, and at the very least entertain a more expanded search of the electromagnetic spectrum when designing future SETI surveys.

This 1961 paper by R.N. Schwartz and Charles Townes, discusses using Optical Masers (Lasers) for communication across interstellar distances. I feel that it is worth noting that this falls closely on the Cocconi and Morrison paper of 1959 which first suggested the water-hole in the radio as the ideal place to look for, for intelligent extra terrestrial (ETI) civilization.

The authors talk about the recent discovery of ruby optical Masers  by Townes. Since the M in Masers is for Microwave, optical Masers, were soon called Lasers or Light Amplification by Simulated Emission of Radiation.

The authors consider using Optical Masers (Lasers) on two different systems and compare the two. One is a laser on a 200 inch telescope (like the 200 inch Hale Telescope), whereas the other is 25 individual 4 inch  telescopes with Lasers pointed in the same direction. They consider atmospheric seeing as a limiting factor and hence consider that the 25 individual small telescopes might be a better option. I think this paper was really advanced for its time, since 4 years after the launch of Sputnik (1957) it considers the use of Adaptive optics and space telescopes.

It also considers the detectability of Lasers using 1961 technology levels for laser power and detectability. The paper also talks about high resolution spectrometers which could spectrally resolve the laser and hence detect that this artificial beacon outshines the host star. This would be a hallmark of its artificial origins.

The paper concludes by noting that the water hole in the radio should not be the only region where we look for interstellar communication. It also mentions that an advanced ETI might develop capabilities that we have ruled out and consider impractical.

Optical SETI is not exactly a novel approach, but one that has not yet been pursued in earnest. There have been recent efforts by Andrew Howard, Shelley Wright, Nathaniel Tellis in this direction. We must take advantage of the vast resources that are plowed by the astronomical community in this direction and utilize the instruments, development and data sets that exist as a product of this.

 

 

This paper contains an argument for X-ray SETI. I will admit that I was skeptical when I read the abstract – after all, X-ray photons are much higher energy than radio photons and thus the typical logic of energy efficiency (of the transmitter) does not apply.

The paper speaks about the “streetlight effect” – an observational bias that causes you to “look where the light is good” aka. to search where it’s easiest (cheapest, already available data, good quality data given your technology, etc. etc. etc.). So, to try to preempt the gradual growing of our “streetlight” and cut to the chase, so to speak, the authors wanted to derive a physical optimum instead of just looking at the current technological “sweet spot”.

In this paper, the authors only considered photons, which is a choice that I’ll comment on at the end of this post.

They decide that a minimum wavelength for communication is probably about an atomic width; they adopt 1 nm as their order of magnitude value. The choice of this wavelength is dictated by how smooth we could possibly make a physical receiver surface.

The actual focusing of wavelengths of this scale is typically done with X-ray grazing mirrors that involve multiple mirrors in the design, but they are expensive to build. Because of this, the authors also discuss as-of-yet unknown alloys that could be used in a single mirror design and focusing with EM fields (not possible now, needs too much energy, but maybe in the future?).

The authors make the point that the advantage of X-rays is the amount of data that you can send and the tightness of the beam that you can create (both functions of the shorter wavelength). With a tighter beam, the pointings that you choose have to be proportionally more precise, even down to having to account for a planet’s position around a star.

Another benefit of the 1 nm wavelength choice is that it works at all distances, even when extinction is considered. Gamma rays are even better in this regard (they are barely extinct by anything), but they also would require instrument precisions that exceed the physical limitations described in the early part of the paper.

Finally, if you assume that each photon carries 1 bit of information, the authors find that you can get reasonable data rates in the megabit per second-year range, which would be sufficient for substantial communications. They propose searching for intentional communications in the existing Newton-XMM X-ray data. They note, however, that we have the technology to create pulses that are orders of magnitude shorter than we can detect with current technology, and that the time domain constraint might make us miss potential signals.

A Few Final Thoughts

• “Ephemerides sharing is likely to be a small but significant component of all interstellar communications” – how best to share ephemerides in a general way, with the least assumptions possible (Schelling Points?) might be an interesting topic in CETI.
• Choosing to look at only photons is a fair choice, but I can’t help but wonder if this is analogous to placing a physical constraint on the giant wheat triangle proposed by Gauss by setting it at an Earth diameter. It’s a physical limit for the method, but it means nothing if the method itself is (in hindsight) quaint/silly/outdated. Maybe photons will be a quaint/silly/outdated mode three centuries from now, and this is just a pointless thought experiment. I have a little bit more faith in EM communication than that, but it is something to consider.

Wright (2017) explains that that searches for ETI should include searches within the solar system. He argues that it is a good place to look (either for extant or extinct life), because it is near the only place we for sure know harbours life, Earth. He argues that life could have arisen on other bodies in the solar system (namely Venus, Mars, maybe the moon, the icy moons, and possibly asteroids or KBOs), and mentions that it is even possible that a prior species on Earth arose before humans that was intelligent.

I agree with many points in this paper. I believe that SETI should include solar system searches, especially because of the data we already have for many of the bodies. This links back to the argument by Davies and Wagner (2013) that SETI should be done not based on the possibility of discovery, but based on the time and effort the search would take. Solar system searches take remarkably less time since the objects are closer to us.

Wright points out that Venus could have been a good host of life before it lost all of its water and went runaway greenhouse. Unfortunately, Venus has such a young surface from its volcanic flows that any evidence older than a couple hundred million years would be completely erased, and any evidence prior would likely be erased too. Mars is a decent target since it was probably wet once and capable of hosting life before it lost almost all of its atmosphere, but weathering on its surface is decent and any artifacts would probably be covered by now. Other objects in the solar system and their associated datasets should be searched for technosignatures though.

That being said, there is a point in Wright’s paper that I very much disagree with, and that is the possibility of an intelligent species before humans. While I will grant that the fossil record is horrifically incomplete, one flaw I find in this suggestion is the need for oxygen. Our planet was very anoxic before about 500 million years ago, and the rise of life is highly correlated with rising oxygen levels. While any life could have been anoxic, most astrobiologists search for life that requires oxygen since it is the only life we know. Wright is calling for an analysis of ancient facies in search for technosignatures; I think this exceeds the “low cost” criteria, especially given that a successful search requires the assumption that any life inferred did not require oxygen. On top of that, although the fossil record is lacking, there is no indication of any line of evolution that was anoxic, nor evidence of a rise in intelligence that went extinct (unless we include intelligent dinosaurs?). This suggestion just seems fanciful to me; although it would be kind of neat, we have absolutely no reason at all to believe that such a species arose, no evidence of anything that could destroy such a species (except maybe KT?), and any intelligent life would need to be anoxic and would defy our current understanding of life.