Avoiding “Sagan Syndrome.” Why Astronomers and Journalists should pay heed to Biologists about ET.


A new paper using data from NASA’s Kepler telescope came out recently, estimating that 22% of Sun-like stars harbor Earth-sized planets. This is a big increase over previous estimates. It’s very cool work. Love it. But the news spin was predictable:

  • New York TimesThe known odds of something — or someone — living far, far away from Earth improved beyond astronomers’ boldest dreams on Monday.
  • USA TodayWe are not alone.

You get the idea. Aliens under every rock. The existence of extraterrestrial intelligence (henceforth ETIs, or just ETs) is normally discussed in the context of the Fermi Paradox, which Wikipedia describes as “the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity’s lack of contact with, or evidence for, such civilizations.” Now I’m a strong advocate for there being no ETs in our galaxy, as explained in this recent post. In fact I’ve gotten so tired of hearing about ETs I’ve started thinking of it as “Carl Sagan Syndrome.” Name checking the deservedly well regarded astronomer and advocate for the Search for Extraterrestrial Intelligence (SETI). With this latest news cycle I got to wondering. Why so much Sagan Syndrome? What am I missing?

A good starting point is Stephen Webb’s book “If the Universe Is Teeming with Aliens … WHERE IS EVERYBODY?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life.” It’s a fun romp through the history of the Fermi Paradox. From page 23: “it was a 1975 paper by Michael Hart in the Quarterly Journal of the Royal Astronomical Society that sparked an explosion of interest in the paradox. Hart demanded an explanation for one key fact: there are no intelligent beings from outer space on Earth at the present time.” Hart’s explanation was “we are the first civilization in our Galaxy.

Hart’s 1975 paper is short and clear, and worth a quick read. Hart runs various scenarios, but for me the key insight is one of time scale. It takes (only) millions of years for intelligent life to completely fill the galaxy, but billions of years for it to evolve. So first out the gate should be everywhere before second out the gate. Logically if ETs exist they should be here. And they aren’t. So case closed. The Fermi Paradox literature since Hart could arguably be characterized as nonstop special pleading to avoid a common sense conclusion. Besides my recent posts, you can find similar views from Robin Hanson, Ian Crawford, Leonard Ornstein. And in particular I want to cite Stephen Ashworth, both for his article “Alien Civilisations: Two Competing Models“, plus an email exchange where he was generous enough to spend time answering questions. Finally of course we have Stephen Webb himself (spoiler alert) finishing his book of 50 explanations by concluding ETs aren’t there. So while this is a minority view, it’s not uncommon. Back to discussing Hart’s impact, Webb goes on:

Hart’s paper led to a vigorous debate, much of it appearing in the pages of the Quarterly journal. It was a debate that anyone could enter — one of the earliest contributions came from the House of Lords at Westminsterlz. Perhaps the most controversial offering came from Frank Tipler, in a paper with the uncompromising title “Extraterrestrial Intelligent Beings Do Not Exist.” Tipler reasoned that advanced ETCs could use self-replicating probes to explore or colonize the Galaxy cheaply and in a relatively short time. The Abstract to Tipler’s paper sums it up: “It is argued that if extraterrestrial intelligent beings exist, then their spaceships must already be present in our Solar System.” Tipler contended that the SETI program had no chance of success, and was therefore a waste of time and money.

Now unfortunately later on Tipler became a crackpot. But at the time he wrote his 1980 paper he was doing good work. In particular Tipler was an early and forceful advocate for self-replicating robotic probes as the best way for humans or ETs to explore the galaxy. This scenario makes exploration and expansion faster, cheaper and far more plausible. But by directly attacking SETI, Tipler naturally annoyed Carl Sagan.

In response, Sagan co-wrote a paper with William Newman “The Solipsist Approach to Extraterrestrial Intelligence” which right from the title attacks Tipler for believing Earth to be unique. Sagan is of course citing the Copernican Principle, which roughly states the Earth is NOT the center of the heavens. The Copernican Principle is the modern foundation for Astronomy, Cosmology and Relativistic Physics. Sagan thought anyone claiming the Earth to be special must be doing bad science. Here’s a typical quote: Despite the utter mediocrity of our position in space and time, it is occasionally asserted, with no sense of irony, that our intelligence and technology are unparalleled in the history of the cosmos. It seems to us more likely that this is merely the latest in the long series of anthropocentric and self-congratulatory pronouncements on scientific issues that dates back to well before the time of Claudius Ptolemy.

So how did Sagan and Newman make Earth typical? By concluding ETs were common in galaxy. Then they resolved the Fermi Paradox by assuming every single ET civilization decided to expand slowly. Very, very slowly. So none have reached Earth yet. As Webb points out in his bookNewman and Sagan assumed very low population growth rates – rates that many people find too conservative…… A final criticism: even if advanced ETCs are not driven to expansion by population pressure, would they not explore the Galaxy out of curiosity?

Now unlike Astronomy, the discipline of Biology takes a highly favorable view of uniqueness. Evolution constantly discovers quirky and highly contingent historical paths. Biology takes it for granted that everybody is a special snowflake. In fact the third sentence of Tipler’s 1980 paper calls this out:

The contemporary advocates for the existence of such [extraterrestrial intelligent] beings seem to be primarily astronomers and physicists, such as Sagan (2), Drake (3), and Morrison (4), while most leading experts in evolutionary biology, such as Dobzhansky (5), Simpson (6) Francois (7), Ayala et al. (8) and Mayr (9) contend that the Earth is probably unique in harbouring intelligence, at least amongst the planets of our Galaxy.

And as quoted in Mark A. Sheirdan’s book, we have eminent Evolutionary Biologist Theodosius Dobzhansky (“Nothing in Biology Makes Sense Except in the Light of Evolution“) joining the fray:

In his article Dobzhanksy turned Sagan’s argument on its head. Dobzhansky cited the fact that of the more than two million species living on Earth only one had evolved language, extragenetically transmitted culture, and awareness of self and death, as proof that it is “fatuous” to hold “the opinion that if life exists anywhere else it must eventually give rise to rational beings.” 

Now we’ve nailed it. It’s Evolutionary Biologists versus Astronomers. In table format:


Obviously I’m harshing on Astronomers to make a point. And while I’m at it, I want to harsh on science journalists as well, for narrowly talking to Astronomers about ETs, without paying enough heed to Biologists. But full disclosure, I took Astronomy 101 at Cornell University in the 1980’s when Carl Sagan was there. Now of course Sagan obviously never taught Astronomy 101, but it turns out I was at Cornell in the year he wrote his paper with Newman that trashed Tipler. Furthermore I love Cosmos, and plan to watch the reboot series with my kids. So I have a genuine fondness for Sagan’s work. And when I attempt to summon my inner Carl Sagan, it does feel extremely discomforting to claim the Earth is unique. Sagan Syndrome is contagious.

But perhaps some perspective shifts will allow us to restore the Copernican Principle to its rightful place. Recall from my previous post how we have three wildly disparate time scales in play: millions, billions and trillions. Rounding to the nearest 20, we have:

  1. Time for intelligent life to fill a galaxy: super short 20 million years
  2. Time for intelligent life to evolve in a galaxy: moderate 20 billion years
  3. Time of universe to keep having stars: super long 20 trillion years

The first perspective shift is to step back in time, and realize the universe is very young. With 20 trillion years of star generation ahead, the universe has only covered 13.7 billion years or roughly .07% of its life span. Compare this to a person who expects to live 70 years, and you’d get  .07% * 70 years = roughly 18 days. So in human terms the universe is a three week old baby. No wonder there’s not too much life out there yet.

The second perspective shift is to step back in space. Our Milky Way galaxy is part of the Local Group, which contains 54 galaxies. In turn this is part of the Virgo Supercluster. And on we go until we hit 100 billion galaxies in the universe. Now nearly all the Fermi Paradox literature I’ve come across limits itself to just our Milky Way galaxy. Though if you read closely you’ll see Hart and many others after him considered larger scales, as we’re doing now. Why limit to our local galaxy? Because communicating between distant galaxies would take millions or even billions of years. Not much fun. But what happens if assume intelligent life is so rare, and the universe is so young, we only have a few examples within the Local Group? Or worst case the Virgo Supercluster? At this scale our Fermi Paradox problem goes away. We’re now talking billions of years, if it’s possible at all, to travel and expand between galaxies. Sagan’s solution of ETs needing more time to arrive can be made to work. What does all this mean? Well, nobody for millions of years will meet ETs face to face. But we can expect our billion year distant cyborg descendants to do so. There’s lots of time, trillions of years in fact, for this to happen.

I’d like to think Carl Sagan would prefer this approach, which recovers the Copernican Principle, to Tipler’s idea of defunding SETI. Our pale blue dot can be mundane at the scale of galactic clusters, while simultaneously being the crown jewel of the Milky Way. SETI has shown it’s unlikely there’s anyone else here in our galaxy to talk to, so perhaps it’s time for SETI to look outward. Much, much farther outward.

Hubble Ultra Deep Field 2014

Image credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team

By Nathan Taylor

I blog at http://praxtime.com on tech trends and the near future. I'm on twitter as @ntaylor963.


  1. The problem with this is the assumption that it would take “only” millions of years for clever aliens to populate the galaxy.

    Interstellar travel may simply require too much energy and resources for anyone to master it.

    That is, assuming many civilizations survive technological adolescence. Or, if they do, they may wipe each other out when they encounter each other- perhaps by accidental transfer of pathogens the other has no way to counter.

    Or, radio silence may make a lot of sense.

  2. > “Dobzhansky cited the fact that of the more than two million species living on Earth only one had evolved language, extragenetically transmitted culture, and awareness of self and death, as proof that it is “fatuous” to hold “the opinion that if life exists anywhere else it must eventually give rise to rational beings.””

    This is increasingly revealed to be anthropocentric hubris. Other species have language, awareness of self, and of death. You’d have to live without electronic communications not to be pummeled by the evidence on a weekly basis. And “extragenetically” transmitted culture is obvious, with the history of salt-water food-washing monkeys as the most obvious example to spring to mind.

    Frankly with lines like “extragenetically transmitted culture”, it sounds like Dobzhansky buys into “race memory”.

  3. Maybe they came and we were just monkeys swinging in trees. They left and haven’t had a chance to get back yet. Given the fact that ETs would only have made sense to our civilization for the past couple hundred years, that’s less than a rounding error on the time scales we are considering. Maybe we just have to wait a while and they’ll be back to visit again. Of course that “little while” might be 20,000 years. Perhaps not long for them, but impossibly long for us.

    1. If one civilization had made it here, we would see repeated visits from other civilizations accomplishing the same goal. Either they haven’t made it here, or they have good reason to visit covertly.

  4. There could be a lot of explanations. Not the least of which is a technological wall that pops up on the timeline, a tendency to self-extinction among intelligent life, or just the explicit desire not to make direct contact with us. At this point we have no direct data for even a semi-reliable hypothesis. It’s all just speculation right now.

  5. My bet as to the absence of ET is that post-singularity civilizations find much more to interest themselves in the cybernetic heavens they can create than in the comparably boring and deserted real. They might well send out probes, but there would be no need to colonize anything.

  6. Perhaps it is quite simply distance and the horrible thought that the speed of light is an absolute speed limit everywhere. Now time to evolve is compounded by time to travel. As stated elsewhere, great distances can be spanned with artificial life but what is the guarantee that a returning probe after millions of years travel will find a civilization left to report to ? I believe that intelligent life will eventually always evolve given enough time so there are probably thousands of intelligent galactic planets; or will be; or has been and the great isolationists of distance and a cosmic speed limit make contact almost impossible.

  7. Enter your comment here…Yeah given the most likely low probability (but certainly not zero) of evolving an “advanced” type of civilization/beings together with the difficulties of space travel the “aliens” that would be able to contact us would have had a much longer evolution span so long in fact that we should revise our notions of reasons for making contact.

  8. We have evidence of exactly one species as intelligent as ours, a species that evolutionary biology tells us arose after a few billion year process of a highly intricate interplay of chance and opportunity. To put forth that there is something else like it anywhere close enough in time and space and character to allow mutually recognizable existence of the other is so highly speculative as to be mere folly.

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