How to look for something that isn’t there

A common artifact searched for in looking for ETI is Dyson spheres, or some other megastructure whose purpose is to collect energy from the star. This structure was first suggested by Dyson 1961, but later expanded on by Kardashev 1964. Kardashev suggested that civilizations could be categorized into three different types, depending on the quantity of resources collected: a type I civilization would gather resources from their planet, type II from their star, and type III from their galaxy. Kardashev 1964, and later Annis 1999, and even later (sort of) Villaroel et al. 2016 argued that these different stages of civilizations could potentially be discovered! If a civilization were to harvest all of the energy from their star, then we would no longer see them (in the visible; the heat would dissipate as IR, leading to searches for this waste heat).

Villaroel et al. looked for disappearing stars. They compared data from Sloan and from archived data, looking for sources that were present in the latter but no longer there for the former. In essence, looking for disappearing stars. In the end, they found one potential candidate.

I said earlier that they sort of argued that this was a method for detecting other civilizations. I say “sort of” because they didn’t directly mention it. The thought is there, that this is a way to look for ETI, but the reasoning behind why ETI would cause a star to disappear is not mentioned. I’m sure there are a number of explanations that sci-fi fanatics could list, and maybe the authors did not want to potentially embarrass themselves by playing sci-fi author? Nonetheless, I’m not really sure, short of a Dyson swarm, what would cause a star to vanish. Feeding it to a BH?

That being said, I am becoming a fan of “parasitic” SETI searches. There is something about only needing to pay for time (not receivers or data) that really seems great to me! The data are already there, so why not go for it?

Reaction to Davies (2010): ”Nature Plus”

Davies, in this excerpt from his popular book The Eerie Silence, examines what we consider technology and how it may not necessarily apply to an advanced intelligence a thousand or even a million years ahead of us in terms of technological progression. He gives a definition of technology as “a mind, intelligence, or purpose blended with nature, which obeys the physical laws and harnesses them.” Therefore, technology is not distinct from nature in the sense that it is physically separate from nature, but is a part of and a higher form of it. It is “Nature-Plus.”

Classically, we recognized things as being artificial technology based on the organization and structure of its constituent parts, and its utility as a system. And also, such systems are macroscopic in scale. An example of such a system would be the modern personal computer, which is made of billions of transistors that come together to perform specific functions written in software, and which has occupies a physical volume on the scale of cubic decimeters. In fiction (and sometimes in bad science), there is a tendency to anthropomorphize the technology of an alien intelligence to follow our notions of machines.  However, on the deeper level, he calls into question even the aforementioned characteristics of technology that we take for granted, and posits that an advanced technology would be like nothing we would understand. We can imagine (but not yet understand) a technology that doesn’t manifest or operate on the material level, or which is indistinct in form or topology, or which transcends our one temporal and three spatial dimensions, or which to us appears as bereft of function, or which does not appear to consist of discrete quanta. An example of such a technology in fiction would be the “sophon” of Cixin Liu’s Three Body Problem, an intelligent particle which can communicate instantaneously across interstellar distances and which is formed of a circuit system sketched onto a multi-dimensional manifold and collapsed to a point.

If such notions of technology were real and currently in operation, then our theories of how the universe works may be called into question. And so, we should ask, what exactly is it about technology that makes it distinguishable from nature? The question is loaded because we operate on the premise that the nature we observe is truly natural. That is, astrophysical phenomena are emergent properties of a universe governed by the laws of physics. This however, may not be the case. We can imagine scenarios where those phenomena that were thought to be natural turn out to be artificial. Then, any theory of how the universe or its constituents operates that was based on such an observation would be flawed. This is similar in vein to the Planetarium Hypothesis solution to the Fermi Paradox, which suggests that the universe we see is artificially constructed or designed in such a way to make us believe that it is self consistent (that is, until we can muster enough resolution to “lift the veil” so to speak, and discover that the universe is not as it seems).

I think this idea of “Nature-Plus” should be taken seriously by SETI scientists because it challenges the way we think about nature and may be an explanation for why searches based on conventional notions of technology and energy consumption have been unsuccessful. And I do indeed think that it is meaningful to search for astrophysical anomalies, because of the twofold benefit of performing novel science alongside a SETI search.

Finding life from spectra, but not your typical way

Lin et al. (2104) identify pollutants such as chlorofluorocarbons in the Earth’s atmosphere that would be detectable using JWST on other planets. In particular, they find that the time dedicated to an atmosphere with other potential biosignatures would be sufficient to constrain the levels of these CFCs, so the search can be parasitic, in a sense.

I think this is a great idea! As I come across ideas for artefact SETI, especially parasitic ones, I get more excited about the prospects for the field. Two major issues facing SETI are the lifetime of a technological civilization and funding. Both of these are, if not solved, greatly minimized by artefact SETI.

With that being said, I fear such a search for CFCs will flop. Even though the timescales of CFCs and excess carbon are large (especially compared to the age of human technology), I don’t have much faith in atmospheric measurements. I know that JWST will improve all of our current measurements, but from what I remember it will only be useful for large or close planets. All of the studies on exoplanet atmospheres have returned one result: clouds. I don’t expect this to change by much, but hopefully we’ll get a spectrum that isn’t flat! I do agree with the authors that since this data will already be gathered and analyzed for biosignatures, it might as well be analyzed for CFC absorption. The worst that happens if we find such absorption is that chemists/geologists/biologists publish ways that CFCs can be produced in the absence of intelligent life, which will further our understanding (and maybe will lead us to ways of removing the CFCs in our atmosphere!).

The one issue I have with this paper is the white dwarf thing. In the beginning, the authors say that they will only look at planets around white dwarfs, but say that their results “are generalizable to other telescopes and planetary systems.” I understand their arguments for a white dwarf, in that they provide better contrast, they could be the same temperature as the Sun, and they have very long lifetimes, but as of now (2018), we have yet to find a single planet around a white dwarf. I just feel they should have expanded their discussion to include all stars, and if they wanted the low contrast, then just M dwarfs. I personally don’t have enough background to state whether the absorption features from CFCs in an atmosphere around an Earth-like planet around an FGKM star would be visible. I wish this information had been provided by the authors in this paper.

Reaction to Schwartz & Townes 1961

Following the theme of last week’s papers which took a look at alternatives to radio searches, this week’s papers focus on laser SETI in the optical and near-infrared. The first paper to discuss this possibility was Schwarz & Townes 1961, which was published just two years after Cocconi & Morrison motivated the radio SETI search in 1959. In an act of sheer clairvoyance (probably afforded by the fact that Townes won the Nobel prize for the discovery of lasers), the authors predicted a time when “maser apparati near the optical” technology would exist and be a viable alternative method of interstellar communication.

Notably, in our timeline, the discovery of lasers followed the development of radio communications; however, it seems that there is no necessary reason why this ought to be the case. One could imagine an ETI developing proficiency with lasers first, and hence use those as the primary means to signal to other ETI. Therefore, the abilty to detect a optical beams is an important addition in the ensemble of SETI search avenues.

To detect such a beam, the authors set two criteria: 1) that it produces enough photons per unit of area on the r eceiving end to be detectable (given the design of the detector and telescope), and 2) that it is distinguishable from the background. Given those criteria, they examined the possibility of whether or not an optical beam can be used to establish interstellar communications by testing two systems: 1) one which consists of a continuous 10kW beam at 5000A with a bandwith of 1Mhz and assuming a 200in reflector telescope, and 2) an array of 25 lasers like in part (1), but with an effective aperture of 4in. They conclude that in both cases that a signal carried on such a beam ought to be detectable to a distance 10ly given c. Earth 1960 technology. Of course, the technology of today is significantly more advanced than sixty years ago, so probably this estimate is highly underrated.

This paper is important because it was one of the first to offer a novel approach to the SETI problem (I believe the second after the Dyson 1960 paper). This paper’s predictions were vindicated by papers such as the other one for this week (Wright 2014) and others which actually conducted optical and NIR SETI searches. This paper laid the groundwork on which these subsequent additions build and helped frame our thinking about how a laser search ought to be conducted. Indeed, as we move further into the 21st century (only the second century of electronic technology on Earth) we are fastly transitioning to fiberoptical communication. Could it be that other societies also inevitably reach this conclusion as well (or at least transition through such a phase on a path of development to some even more advanced communication scheme)? Only a dedicated laser SETI search can attempt to answer those questions!

Reaction to Howard et al 2004

Continuing with the theme of optical SETI from last week, this week’s Howard et al (2004) paper discussed the results of an optical SETI experiment which searched for pulsed beacons around thousands of stars. Following with the other optical SETI papers we have encountered, the authors compare the merits of searches in the optical/NIR with searches for microwave/radio signals. If one’s figure of merit for the efficiency of technique is the signal-to-noise achieved for a fixed transmitter power, then optical methods are comparable to those of radio.

They further motivated this search by presenting the “Fundamental Theorem of Optical SETI”, which is a statement of the observation that even at our early stage of technology (Earth “2000”), we can already generate artificial optical pulses could appear to outshine the brightness of the Sun by a factor of 10^4. This follows a similar line of reasoning as the Schwarz & Townes paper from last time, which plausibly suggested that some ETIs would rapidly discover some form of optical interstellar communication and use it. However, in the case of Howard’s paper, the focus is on the search for pulsed beacons, which are unambiguous detections of alien laser signals (for which there are no possible astrophysical confounders or dopplegangers).

With similar avalanche photometer instruments at Harvard and Princeton, they began their campaign which would eventually consist of some 16,000 observations totalling 2400 hours of observing time spread over a five year baseline. They searched 6176 stars in their survey, of which only a handful of signals showed any promise as plausible artificial pulses (most were explained away as being stochastic in nature). Three triggers from HD 220077 were considered the most interesting, and were allotted many follow-up observations. Upon further investigation of those candidates, they found that their photon rate was consisten with Poisson noise and thus rule out the alien hypothesis. (Remember, it’s never aliens!) Another interesting pair of triggers from HIP 107395 was considered too ambiguous because of an asynchronicity between the Princeton and Harvard clocks.
This work was performed in fulfillment of Howard’s PhD thesis in astronomy; Andrew Howard is now a prominent exoplanetologist and astronomer, and so this work is a demonstration of SETI being firmly rooted as a part of astronomy and an example of the quality that SETI papers ought to strive for (that is, when it is taken seriously by astronomers and other scientists). It is also a good example of “Forensic” SETI done right, where the candidates were scrutinized on a case-by-case basis and all natural explanations were attempted to be exhausted before jumping to unsubstantiated conclusions (which contrasts with the approach of some other papers we have read this semester *cough* faces on Mars *cough*). Although the results were null, the study still placed valuable upper limits on the occurrence of beacons around nearby stars. Therefore, this paper serves as a template for how null results ought to be reported and makes a case for them to be published.

Searching for Humans around White Dwarfs

One of the primary tasks in astrobiology is to detect biomarkers. Lin et al, in a recent paper, argue that industrial pollutants could be used in lieu of biomarkers to detect life on other planets. The unexpected twist in their hypothesis: they propose to observe old white dwarfs. They argue it is possible to look for pollutants from an industrialized society with technology comparable to ours. According to the authors, the ideal pollutant to detect would be specific chlorofluorocarbons (CFCs).

Lin et al begin by motivating their choice of host star. Following previous work by Loeb and Agol, they cite three reasons to favor these stars:

  • white dwarfs have long-lived habitable zones as they are at the end of stellar evolution,
  • the similarity in size of the white dwarf and an Earth-like planet should give the best contrast between the planet’s atmospheric transmission spectrum and the stellar background, and
  • after a few billion years, a white dwarf at the Sun’s effective surface temperature should have a spectrum similar to the Sun, creating a comparable habitable zone, albeit much closer to the star

Lin et al mention that habitable planets could plausibly form debris of the stellar remnant. In short, a white dwarf could host an Earth-like planet at ~0.01 AU. If we further assume there is an anthropogenic civilization on said planet, they could industrialize and produce pollutants. The authors argue the ideal pollutants would be CFC-11 and CFC-14. CFCs have short lifetimes in the atmosphere (at most a few thousand years) and largely produced unnaturally, at least on Earth. There may be volcanic and fumarolic CFCs, but these are appreciably lower in concentration. The estimated time for this measurement would be on the order of day with JWST, which the authors argue could be used to simultaneous detect a spectral edge, be it natural or artificial. The simulations of spectra are shown in Figure 1.

Figure 1. Above are the spectral windows used for detecting CFCs and select molecules used in this paper. The top row shows the combined transmission spectrum. The orange segments are expanded below to highlight where CFCs and other molecules have significant lines. In each zoomed segment, the black represents the contribution from the most significant components of Earth’s atmosphere. Lin et al argue the highlighted regions make good regions to observe for pollutants caused by an Earth-like civilization around a white dwarf.

This blogger is in awe at the assumptions going into this paper. The notion of white dwarfs around planets, while not new, invokes various questions of habitability. If we assume the planet existed before the stellar remnant emerged, how could it survive and retain its orbit? If we assume the planet forms from stellar ejecta, would the metallicity allow for the formation of a rocky planet? “Polluted” white dwarfs do exist and have evidence of debris disks, but as of yet no bona fide planet has been detected around a white dwarf. If we ignore that assumption, the issue of detectability persists and the choice of a cultural signal (a pollutant). CFCs are particularly heavy and would require an observation to probe beneath a significant amount of atmosphere. Furthermore, if in the outer atmosphere, UV radiation would readily destroy CFCs. The required resolution of 3000 is the estimate from JWST documentation but it may be much less at these particular windows. Lin et al assume the civilization must develop like Earth and cause an excess (more than ten times our currently level) of pollutants while refusing to control said emissions. Too many assumptions go into this argument for it to be a viable SETI experiment (i.e. a direct JWST proposal).

The nice thing about (some) pollution

In Lin et al. (2014), an interesting possibility of using biosignature detection to infer the presence of not just life, but intelligent life, is explored.

One way you can infer the presence of life on a planet is to look at the atmospheric ratios between compounds, elements, or isotopes in an atmosphere and find that they are out of equilibrium. For example, if you found molecular oxygen in combination with a reducing gas, there would be a readily available way for life to generate energy by harnessing the changing energy by combining oxygen with the reducing gas.

But what if you wanted to use atmospheric biosignatures, to find ETI instead of plain old dumb life?

So dumb

It has been suggested that we could look for signs of pollution in exoplanet atmospheres to guess at the presence of ETI. While high concentrations of molecules such as methane (CH4) and nitrous oxide (N2O) can be suggestive of polluting life,  they can also be created by unintelligent sources. While finding weird amounts of CO2 can be explained away, looking at more exotic compounds (namely specific chlorofluorocarbons) can provide much stronger evidence that life exists. Not only are they only significantly produced by unnatural processes, but some of them have short lifetimes, which could constrain how recently the ETI was on that planet.

What is extra cool about these molecules is that in high concentrations (~10x what we have here on Earth), they should be detectable with ~1 day of JWST time (RIP early 2019 launch date) for a planet around a 6000K white dwarf.

If one wanted to be silly, they could suggest using these unnatural CFCs as a form of METI beacon to announce our polluting presence to the galactic club. Not that they’d want such a self-destructive member.

Reaction to Townes (1983) and Hippke & Forgan (2017): Alternative Frequencies for SETI

The theme between the Townes (1983) and Hippke & Forgan (2017) papers is that our SETI efforts should not be solely focused on searches in microwave and radio frequencies. These papers make the case that there are in fact equally viable if not superior alternatives to radio in both the infrared (IR) and X-ray portions of the electromagnetic spectrum, respectively. SETI experiments have been influenced by the precedent set by the earliest ideas in the field, which emphasized the radio search (and often near the 1.2-1.67GHz water hole). In fact, it was Cocconi and Morrison who gave us the idea that the most important factor when imagining interstellar communication systems is their efficiency in terms of photons per watt, which led them to pursue the radio search. However, with the development of new technologies and perspectives, it is clear that this narrow viewpoint misses out on a greater variety of possibilities.

These are examples of quality SETI papers because they attempt to expand our perspective and push boundaries. They remind us that we should be ever aware of falling into narrow-minded modes of thinking, and that when dealing with the perplexity of trying to predict the motivations and strategies of an ETI, we should stoically expect that we are wrong. They are also remarkable in their approach to the question. In the case of Townes, he thinks critically about the observational challenges of moving to the infrared and quantitatively compares the pros and cons of IR methods with those of microwave/radio. He is also cognizant of the fact that there are a lot of assumptions (which he makes explicit) made about the strategy of a transmitting ETI which we can only speculate about and limit the effectiveness of the IR search. On the other hand, Hippke & Forgan are motivated by the search of the global optimum for interstellar communication, which they decide ought to be in the X-ray near 1nm. In pursuit of this grail frequency, they examine a variety of astrophysical and observational difficulties which complicate communication, such as diffractive photon loss, interstellar extinction, and atmospheric transmission. In this way, both papers are firmly rooted in taking a classically quantitative and astronomical approach to SETI. This places these papers a tier higher than those which solely offer speculation on search strategies unsubstantiated by rigorous examinations of the merits of the alternative. Overall, the field benefits when scientists take SETI seriously and improve it by contributing to it with quality papers.

ETI’s Solar Savings

The Lingam and Loeb paper “Natural and artificial spectral edges in exoplanets” contains a lot of elements that I really appreciate in a theoretical SETI paper.

The primary conceit of the paper is that, as vegetation produces a reflective spectral “red edge”, artificial materials could produce similar reflective spectral signatures. These signatures, described in the paper as a distinctive change in reflectivity over a narrow bandwidth, could be detectable if they covered enough of an exoplanet’s surface.

A figure from the paper showing the vegetation red edge and the other edges from different artificial materials

The authors find that silicon produces one of these “artificial edges” in the ultraviolet. Even 10% coverage by silicon could be detectable in certain circumstances (tidally-locked planets around M-dwarfs) with next generation telescopes like WFIRST and JWST. Given an assumption about the composition and reflectivity of the material, you can easily get the coverage fraction. From that fraction, you can guess their power usage (this is assuming that the silicon signature is produced by large-scale photovoltaic arrays).

If you think this is a lot of solar panels, check out one of Lingam and Loeb’s planets

The element I most appreciated in this paper was the authors’ clear statement of assumptions and acknowledgements of alternatives and difficulties in their methods. They make it clear that their calculations are predicated on the idea that the civilization is getting their power supply from their host star, not from geothermal energy or nuclear fusion (which they state would cause other signatures, but they don’t try to predict them in this paper). They add caveats that would affect the signature’s detectability from Earth: hazy atmosphere on the planet, strong winds, and cloud cover. They give solid reasons for choosing silicon as the element to focus on in the paper, based on nucleosynthetic abundance, but also show some spectral signatures of other plausible bases for solar panels. Finally, they considered false positives that could also cause a similar signature, in particular a natural material called enstatite*, and ways to differentiate the two (eg. looking for energy redistribution on the surface of the planet from dayside to nightside).

All in all, I found this to be a very convincing and self-aware paper, and I’m very excited to see an artifact-search like this be conducted in the next decade or so!

* Some facts about enstatite (because part of me always wanted to be a geologist)! It’s a common mineral found in igneous and metamorphic rocks and is a 5-6 on the Mohs scale. It’s essential in some Earth mantle materials and is commonly found in asteroids. It has, in fact, been found in crystalline form in some planetary nebulae.

When enstatite is gem-quality, it’s apparently called “chrome-enstatite” and looks like this (thanks Wikipedia).

Literally my first SETI paper

My opinion of this paper is completely biased by the fact that I’ve actually met David Kipping and that I read this paper back when it first went on the arxiv. This was my first exposure to SETI (beyond science fiction, if that counts) and I think it went well!

Kipping and Teachey postulate that a civilization (even the Earth) could use lasers in some interesting ways. They first suggest that a planet’s transit could be clocked, monochromatically, against a Kepler-like survey, without the need for much power (~30MW). Due to the Earth’s rotation, this would require multiple laser stations, but in the end, it would be doable. They then continue on to talk about clocking the signal at all wavelengths. This would be a bit more challenging, since many lasers at many different lasers would be required, and again these lasers would need to be placed around the planet, and the power requirement would increase by an order of magnitude, but a committed civilization could manage it. Both of these cloaking processes can be argued against since the planet would still be detectable via other detection methods (namely RV).

The last bit of cloaking they suggest involves the cloaking of biosignatures. A disequilibrium in an atmosphere (normally of oxygen) is a decent indication of life on a planet. These and other related absorption features are referred to as biosignatures. If lasers were emitted at these absorption features, then the planet would still be detected and noticed, but it would just not be studied much since it would be presumed uninhabitable. This is all, of course, under the assumption that other life out there is Earth-like, and that this Earth-like life would be looking for signatures similar to their life (Earth-like). Because of this Earth-like assumption, it is possibly that another civilization is already doing this for *their* biosignatures, we just don’t notice it though because we are looking for our biosignatures (also clouds are apparently all that we can see right now).

Lastly, the authors bring up the point that this laser method can be used not just to cloak, but also to signal existence. They briefly mention that the easiest way to get someone’s attention with this would be to cloak the transit’s ingress and egress, making the transit appear boxy and all around wrong.

Although this is a neat idea, it seems a little far fetched and specific to me. Sure, we have tons of data, so someone might as well look through for boxy transits (I think someone has already done this with Kepler data), but this seems so absurdly unlikely to happen. However, my thoughts on the likelihood of this completely come from the way I view humanity and our goals and motivations, so it’s just as possible that my thoughts of this being a waste are a minority in the galaxy.