Looking for Neo-Tokyo in the Kuiper Belt

In Loeb & Turner (2012), a new Solar System SETI method is described. If Kuiper Belt objects (KBOs) are artificially illuminated, we should be able to detect that based on how their brightness changes with distance (both from us and the sun).

If a KBO has artificial illumination on its surface, then its brightness should only decrease with distance (from us on Earth) squared (a geometric effect of the intensity of the light diluting as it gets further from the source, see Wikipedia’s explanation). But, if a KBO is illuminated solely by the Sun (as we expect them to be), the light is coming from the Sun, so the light gets diluted twice and we would expect it to decrease with distance to the fourth power. The distance from the Sun to the KBO and from the Earth to the KBO are essentially the same because the Earth is relatively close to the Sun. KBOs are 30-50 AU away from the Sun while Earth orbits snuggly at 1 AU. So the distances can only be different by at most at most ~3%, a subtlety I feel should have been made explicit in the paper. Presumably this power law identification could be performed (at least in a rudimentary sense) by putting the data into log space and identifying the linear trend of brightness as a function of distance (hopefully with a slope of -2).

With the completion of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) and the Large Synoptic Survey Telescope (LSST), there will be an explosion of discoveries of new KBOs (finding ~10-100x more than we know about now). This should open up a window for this new type of proposed search.

Another interesting tidbit was the use of a unit defined as 1% of the solar daylight illumination of Earth, ~ 1.4 · 10^4 erg/(s cm^2). They state that this corresponds roughly to the illumination in a brightly lit office or to that provided by the Sun just as it rises or sets in a clear sky on Earth. I spent an inordinate amount of time wrestling with this fact, as it is repeatedly used as a baseline in the paper and is not immediately obvious to me what this statement even means. It doesn’t feel like outside my office is 100x brighter on a sunny day, but who knows.

Scrapyard SETI (Get an Undergrad to Do It)

In Davies & Wagner (2013), the authors describe and motivate an ongoing (circa 2013) search of Lunar Reconnaissance Orbiter imagery for unusual features at the LRO Laboratory at Arizona State University.

The first point of note that I found in the paper was the initial statement of assumptions. They state that anything we could find (outside of communication SETI) will most likely be something “post-biological” due to the long timescales we would have expected it to last. Then they directly state that we have no reasonable way to extrapolate our own technology to guess what type of artifacts we should be looking for. An openness I found refreshing.

What follows is an intriguing and frank basis of motivation for the search of the LRO database (or any database). In the case that we don’t know what we are looking for, they propose that the best way to make meaningful progress with limited resources is to simply search all existing databases for “artificiality” and that choosing which databases to search should be tied only to cost rather than the likelihood of results. This was an interesting thought. Cost is obviously a good thing to keep in mind when deciding on what to do with resources, but in every other field, missions and grants are proposed and evaluated with heavy basis placed on their merit. But what is it is nearly impossible to quantify the merit of an experiment?  We can estimate the number of planets TESS will find (>20,000) or the amount of stars GAIA will get parallax for (>1 billion), but there’s no way for us to know the amount of ETI signatures will be found by performing any given search (although, one could cheekily say 0 based on the results of all other searches). While this seems to make sense at first glance, I think it makes some false equivalencies. Just because we don’t know the utility of two different searches does not mean their utility is equal, as this line of thinking implies. If cost is the only thing that matters, I should just submit two half-cost proposals that each cover half of a database. That’s two half-cost searches for the price of one! In the same vein, some databases are clearly more valuable to search than others. Imagine two equivalent cameras take pictures of the Martian surface, the only difference being one of the cameras can take pictures of much higher resolution. It is clear that while it would be more expensive, looking at the higher resolution data would be much more useful. While it is difficult to quantify the utility of SETI searches, they can be viewed as setting limits on the parameter space (a la Jill Tarter’s Needle In A Cosmic Haystack) that SETI artifacts can inhabit (see Appendix A Wright & Oman-Reagan (2017) for a motivation of this type of quantification).

Besides these points, the authors discuss how automation is ill-suited towards artifact SETI as we have to program in exactly what signatures we want to look for. Currently, they have some students and faculty searching the Narrow Angle Camera images for interesting features by eye. They suggest that the best strategy may be to utilize the time of enthusiastic volunteers to perform the image analysis.

Could ETI be in the Solar System?

In Freitas (1983), the author does a deep dive into where we should look and what ETI objects we should look for in our Solar System.

The paper takes a series of interesting assumptions in order to start quantifying the search it eventually describes. From the beginning, all ETI objects are categorized into 3 categories. “(I) Objects intended to be found, (II) objects intended not to be found, and (III) objects for which detection by us is irrelevant or unimportant”. He argues that objects in classes I don’t exist because the alien technology is so advanced, if they wanted it to be found, it would be. I don’t really agree with this assertion as most all communication methods rely at least somewhat on the technology level of the receiving civilization. Unless they had probes looking for population centers and landing directly next to groups of people, there is no guarantee that we find anything, especially if we aren’t spending a lot of effort looking. He also argues that objects of class II are impossible to observe. I will accept that class II objects are not worth looking for as if they do exist, I could easily see them having some advanced stealth technology that makes them nearly impossible to detect (at least at our current tech level).

Once he decides that we are looking for “objects for which detection by us is irrelevant or unimportant”, he places these objects in geocentric or selenocentric (moon-centric) orbit, likely at the L4 or L5 Lagrange points, as they are the only stable ones. Then, it is decided that an optical ground resolution of <10 m “is required for unambiguous visual detection from orbit of intelligent activity on the surface of the Earth”. I have no idea how he came up with this number, but if we can see totally unambiguous evidence of intelligent meddling on Mar’s surface with only 50m/pixel, I don’t see why we need this resolution.

After these assumptions, he compares his proposal to other proposed search spaces in a reasonable fashion.

Fitting Megastructures Into Lightcurve Holes

In Arnold (2005), the possibility of finding evidence of ETI by looking for transits of unnatural shapes is proposed (a new search method!). Several possible examples are discussed and their transit depths and shapes are calculated. In addition, the effectiveness of using these structures as a way to communicate is evaluated and compared to other proposed, direct communication methods.

Arnold singles out equilateral triangles and a louver-like screen (repeated, aligned rectangles, like a window screen) as his examples of choice to analyze. He simulates the stellar flux during the transits of these objects by taking a simulated image of the star and setting the pixel values to be equal to zero when the object being tested in the intermediate line-of-sight.

Actually identifying the signatures of these shapes in transit signals, is surprisingly difficult. Not only are the differences between the transits caused by the investigated shapes and best fit sphere only on the order of 10^-4, but the differences you can see may be indistinguishable from a totally natural cause like rings around a normal planet.

It’s like this, but you can’t see the blocks and all you know about the holes is how big they are

This paper was important as it was one of the first to popularize the idea of searching transit studies for possible signals imparted by ETI megastructures. Since this paper was published, the Kepler space observatory was launched and the ideas from this paper were used in a Kepler data search (Wright et. al. 2016), where some interesting (aka weird) transit candidates were found!

“Interesting” Statistical Techniques for Archival SETI

In Annis (1999), an archival data based search is conducted to see if any nearby galaxies display the characteristics of hosting an extremely advanced ETI. In particular, the paper looks for Kardashev type III civilization (one that can control energy on the scale of its entire host galaxy) that is converting most of the starlight in its galaxy into usable energy. Deflecting large amounts of starlight should noticeably dim the surface brightness of the galaxy and would show up clearly as an outlier in the well-documented galaxy relations. The author searches for these outliers in the brightness and temperature data from spiral galaxies in clusters and nearby elliptical galaxies. None are found.

One question I had while reading this paper was how would we know a dim galaxy were there? I mean, if almost all of the starlight is being blocked, then how would we even know to look at that place in the sky? Presumably, we would see a dim galaxy’s gravitational effects on its neighbors, which may have been why the study focusses somewhat on galaxies in nearby spiral galaxy clusters (Virgo and Ursa Major).

The author chooses a pretty arbitrary floor for what he thinks is an outlier. He decides that any galaxy that is 75% dimmer than its respective (spiral or elliptical galaxy) relation is an outlier. His methodology seems pretty circular though. He visually finds what he thinks is where, if one assumes there is Gaussian scatter about the relations, a cutoff is that is “at some high sigma enough to avoid statistical outliers”. There are certainly more robust ways to calculate outliers (including finding the Mahalanobis distances from each data point to the sample means).

An interesting calculation that is performed at the end of the article constrains the emergence time of a star-fed Kardashev type III civilization. Assuming that there is a constant chance of a KIII civ occurring throughout time, you can use Poissonian statistics and the fact that since it appears that all of the surveyed galaxies (including our own) have no KIII civilizations to put a lower limit on the occurrence time for these civilizations. Each new galaxy that they aren’t detected in adds ~10 Gyr of non-detection time. The paper arrives at a lower limit of ~6.5 billion years at 99% confidence. Neat.


Dyson and the Beginnings of Artifact SETI

In Dyson (1960), the beginnings of a new type of SETI is proposed that has significantly fewer behavioral and technological assumptions about ETI (Wright 2014). If ETI does exist and use a large amount of energy, they will likely be utilizing starlight as an easily accessible energy source and be indirectly converting it into IR radiation. The search for this IR radiation is proposed to complement the existing searches for communication.

Dyson argues that if exponential population and technology growth is sustained for a couple millennia, a civilization could use on the order of a Jupiter mass’s worth of material to construct a spherical shell around the Sun (using only 800 years worth of solar energy!). This is followed by the bold statement that “One should expect that, within a few thousand years of its entering the stage of industrial development, any intelligent species should be found occupying an artificial biosphere which completely surrounds its parent star.” Finally, he notes that the thermal emission of these shells will be in the IR range and that ground telescopes will be able to detect them through the atmosphere.

A popular idea that resulted from this paper was that of a Dyson sphere, a hypothetical megastructure that completely encloses a host star, capturing all of its energy. It is sometimes postulated that the sphere is not only used to capture energy, but to also serve as a space for habitation, where life can live on the inner surface of the sphere. If one was constructed with a radius of 1 AU around out Sun, there would be more livable space than half a billion Earths! There are several problems with this implementation, namely, there is no material we know of that could withstand thepressure imposed by the star’s gravity. Additionally, there would be no effective gravity imparted by the shell on anything in its interior, so there would be nothing holding anything to the interior surface and the shell could drift into the star over time.

The Dyson Sphere has remained a prevalent idea in popular culture to this day, with appearances in many forms of science fiction media (including the television show Star Trek: The Next Generation and the grand strategy videogame Stellaris)

In Stellaris, you can spend an inordinate amount of resources constructing this mega structure. Realistically, it turns all rocky planets and moons in the system into frozen or barren worlds. Unrealistically, the amount of energy provided does not vary between different star types. Tsk tsk developers.


Would you like some ETea?

How do you welcome a stranger into your home? It can be an awkward experience, and how you greet them depends wholly on the cultural assumptions you make about them. In this conceptually unique (as far as I am aware) column by Oman-Reagan, we are invited to think about how the inevitable cultural differences between ourselves and visiting ETI would complicate relations and communications.

This paper is not directly related to SETI, but a reminder of how our human customs flavor how we think about possible future interactions with ETI. There are many assumptions we might inadvertently make about our visitors and these are absolutely critical to how well we will be able to communicate.

The column proposes several example scenarios that would render casual communication basically impossible. There are already many many dissimilarities between human cultures and one can have a lot of fun thinking about possible differences. Maybe ET’s way of greeting new friends is offering them a piece of their own appendage to be consumed or blowing some nice gasses that its biology produces at you.

One limitation of the column is its assumption that aliens have similar senses to ours. It seems reasonable that at the minimum, ETI will be able to sense at least some of the EM spectrum, but who knows if or how they directly sense nearby molecules (smell) or if they will have specialized organs for interpreting pressure waves in their surrounding media (hearing).

What SETI at Home thinks of METI

A couple weeks ago, I asked a friend what the point of SETI was if we were not sending messages ourselves. How can we expect other civilizations to be sending things we can find if we aren’t willing to do it?

I think my initial conjecture has several reasonable responses that were not initially clear to me, including that if ETI does exist, I highly doubt they will have motivations we can sufficiently understand to reliably predict their behavior, especially based on our own.

In a Statement Regarding METI (Messaging Extra Terrestrial Intelligence),  scientists at Berkeley running the world’s leading SETI program declare a stern condemnation of METI efforts, mostly by employing logical arguments directly against several common METI sentiments. Sharing sentiments similar to other leading minds like Carl Sagan, Stephen Hawking, and Sean Carroll*, they argue that we cannot understand how ETI (who are statistically certain to be much more technologically advanced) will react to our message.

Apparently, a common argument for METI is that SETI obviously hasn’t found anything yet, so we need to try other methods. Maybe we are so used to seeing rapid (decade-timescale) progress in our scientific fields that it feels like SETI not finding anything means that they are doing something inherently wrong. SETI is peculiar as a field in that, at some level, no matter how many resources we throw at it (not that we have been throwing much at it), we could still find absolutely nothing. And that would be progress! It feels like a fundamental misunderstanding to say that we haven’t been listening well, so have to try talking.

Also, the argument that sufficiently advanced ETI would have already seen our early radio emissions, or atmospheric so METI wouldn’t make anything worse is a peculiar point to try to stand behind, considering it seems to invalidate the need for METI at all.

Interestingly their final argument is very political in that they do not want METI to cause dissension in the ranks of SETI leading to a future lack of funding. It is illustrative to see such a point spelled out so directly and publicly.

*Serious side note, no matter how many famous people believe something, that does not mean that you should blindly agree with them #bandwagon. It is important to remember that your thoughts and conclusions should be your own, especially when trying to convince others.

Such Strong. Much SNR. Wow!

Kraus (1979) is about telling the story of the discovery of the Wow! Signal, arguably the strongest candidate for an ETI transmission detected.

The Ohio State University Radio Observatory holds the record for the longest ET search ever performed. Non-SETI whole sky radio surveys in started in 1965, but funding was cut abruptly in 1972 leaving a world-class telescope unable to perform large-scale operations. At this point, it was decided to use a previously-built detector to try to look for ETI signals in the “waterhole”. A couple upgrades and a lot of unpaid student and volunteer work later, the radio telescope was running ~full-time on a routine SETI program.

In August of 1977, the search paid off with the detection of a transient, celestial signal that caused discoverer Jerry Ehman to exclaim “Wow!” in the margins of the data print-out. Even though the same region of the sky was searched for weeks afterward, no new signals were found.

The article reads weirdly like an underdog sports story. I’m sure the back of the book would read something like “No funding, no precedent, and no excuses. Can these scrappy buckeyes go against all odds and find the interstellar connection that many believe impossible?”. This does make sense though as this is a magazine article, not a research paper.

Some obvious next steps (for the follow-up article, of course) would be to see if this region of the sky had been observed since then and report back either on the tragic lack of signal or the tragic lack of observation.

Fermi’s Paradox. Neither Fermi’s nor a Paradox

Gray (2015) is about discussing why the Fermi paradox is ill-named. The author firmly believes that the term is not only misleading in an attributive sense, but also in its conviction value. This paper falls squarely into the “explaining/dissolving/sharpening the Fermi Paradox” category.

As cited by Gray, the Fermi Paradox is described as “If technologically advanced civilizations have inhabited our Galaxy for timescales of approximately a billion years, and if some of these have engaged in interstellar travel and colonization, then why have we not seen physical evidence of their visits?” by Paul Horowitz.

The main points that he pulls out are that the doubtful ETI argument laid out in the “Fermi Paradox” actually originates in early papers by Michael Hart and Frank Tipler, and is only loosely connected to Enrico Fermi by an out of context quote from a dinner party. Apparently, he said, “Where is everybody?”, but, instead of doubting the existence of ETI, he meant that he thought the difficulty of interstellar travel was the reason for not seeing extraterrestrials. Ergo, use of Fermi’s name and reputation lends false credence to the arguments.

In response to the “paradox part of the phrase”, Gray states that the “Fermi Paradox” presents a fact (i.e. we haven’t found physical evidence of being visited by ETI) as evidence for the conclusion that advanced ETI doesn’t exist. This is not a paradox, just a leading question. One that has many possible solutions that aren’t the obvious one (there isn’t any ETI). That conclusion relies on several assumptions itself that may or may not be true (e.g. “interstellar travel is feasible, the Galaxy would be filled quickly” etc.).

The whole purpose of writing a paper about this is to help disentangle SETI from its murky public reputation. It seeks to strengthen the justification for SETI by weakening the power of a phrase (and set of ideas) commonly wielded against SETI supporters. I don’t think this type of paper would be very important in fields with steadier support. It “is important, because the Hart-Tipler argument (proposed renaming of the Fermi Paradox ideas) was cited as a reason for killing NASA’s SETI program on one occasion in the U.S. Congress, and under the guise of Fermi’s name and the claim of a logical paradox, it may continue to inhibit funding and research in that area of astrobiology.” The last sentence of the conclusion puts it all out in the open.

Along with Garber (1999), this paper shows the political climate surrounding SETI funding, which is not that optimistic (circa early 2000’s).