This forty-page paper is a doozy, but so, so important. I only wish that we had an equivalent paper written in 2018 (Caleb and Will were brave enough to tackle this project – check out their work!)
[Warning: this post is also a doozy]
Tarter (2001) is a comprehensive review of the state of SETI as a field: where it came from, where it is, and where it’s going. This is important in a field with so many proposed projects and so few performed projects. I think it’s safe to say that most people assume that we’ve searched far more than we actually have, such that the Fermi Paradox seems very sharp. But unless you’re a proponent of very, very obvious, ubiquitous von Neumann machines in the Solar System, this paper makes it pretty obvious that we’ve barely scratched the surface of searching.
At that point in time, only 99 projects had ever been documented. I tried to see the comprehensive list, but unfortunately, the link to the supplementary material (it seems to be via seti.org) is broken. Hence, I can’t actually back up this statement, but I would guess that many of these were single/few target observations like Project Ozma.
It’s absolutely amazing to me that a paper written within my conscious memory can look so different from the scientific lessons and landscape that I’ve experienced less than two decades later (not that I was aware of the publication of this paper at six years old, but my point stands). Kepler was competing for selection, Huygens hadn’t happened yet, and Tarter’s plot of known exoplanets looks frighteningly sparse compared to the plots I generated effortlessly on exoplanets.org for Dr. Bekki Dawson’s graduate class on the subject. Tarter’s discussion of exoplanet finding techniques, however, and her thoughts on how the revolution would progress were incredibly prescient.
Things I Learned: I liked her explanation of the uncertainty principle argument to justify the search for signals that were unnaturally short in time or frequency. I had always thought of narrowband searches as the most obvious SETI search method (with much bias), but this is the best argument I’ve read for pulsed optical SETI having equal footing. Interesting sidenote: Charles Townes himself talked about how his invention could be used for interstellar communication.
I also did not know that there were distance-dependent effects that happen in both the optical (dust extinction) and the radio (minimum frequency resolution you care about because of broadening through the ionized ISM). I did not appreciate how interstellar scintillation could, frequency-dependently, affect the signal amplitude. This is a good reason to search for combs – I’d be curious to see if anyone has ever run a comb search, because if so, I’ve never heard about it.
There’s so much to say about this content-dense paper, but I’ll leave it off on a final point starting with the following mediocre metaphor I just came up with.
Mediocre metaphor: the search space is a beach of multi-colored rocks. We know we’re looking for something that looks out of place, but that’s all the information we have. If we only look for, say, paths in the black rocks, we might walk right by a conspicuous grouping of green ones, or an artificial stack of large ones. But add two more pieces of information: 1) there’s a particular kind of basalt on this beach which isn’t found elsewhere 2) geologists love to study it. Now there’s a very obvious reason for our unknown signalers to add something weird into the basalt – maybe they shape a column of basalt into a triskaidecagon (it usually forms hexagonal columns). Even if we (as the searchers) don’t know where to look and don’t see anything, the granite experts will notice that something is up.
This is the rationale behind looking for “impossible objects” (an intentional variant of the idea of “nature plus”): making a signal that almost looks like an astronomical object that would be studied in the normal course of a civilization’s scientific advancement… but with something weird about it. Pulsars that don’t follow physical rules, stars with weird emission lines, supernovae going off in the Fibonacci sequence. This eliminates a lot of the Schelling Point philosophizing (frequency/target/bandwidth/time/etc./etc./etc.) involved in guessing what a transmitting civilization would do, which I am very much in favour of, personally.