Speculating about the diffusion of intelligent species through the galaxy, as we’ve been doing these past few posts, is always jarring. I go back to the concept of ‘deep time,’ which is forced on us when we confront years in their billions. I can’t speak for anyone else, but for me thinking on this level is closer to mathematics than philosophy. I can accept a number like 13.4 × 10⁹ years (the estimate for the age of globular cluster NGC 6397 and a pointer to the Milky Way’s age) without truly comprehending how vast it is. As biological beings, a century pushes us to the limit. What exactly is an aeon?

NGC 6397 and other globular clusters are relevant because these ancient stellar metropolises are the oldest large-scale populations in the Milky Way. But I’m reminded that even talking about the Milky Way can peg me as insufferably parochial. David Kipping takes me entirely out of this comparatively ‘short-term’ mindset by pushing the limits of chronological speculation into a future so remote that elementary particles themselves have begun to break down. Not only that – the Columbia University astrophysicist finds a way for human intelligence to witness this.

You absolutely have to see how he does this in Outlasting the Universe, a presentation on his Cool Worlds YouTube channel. Now Cool Worlds is a regular stop here because Kipping is a natural at rendering high-level science into thoughtful explanations that even the mathematically challenged like me can understand. Outlasting the Universe begins with Kipping the narrator saying “We are in what you would call the future…the deep future” and takes human evolution through the end of its biological era and into a computer-borne existence in which a consciousness can long outlive a galaxy.

Image: Astrophysicist, author and indeed philosopher David Kipping. Credit: Columbia University.

Along the way we remember (and visit in simulation) Freeman Dyson, who once speculated that to become (almost) immortal, a culture could slow down the perceived rate of time. “Like Zeno’s arrow,” says Kipping, “we keep dialing down the speed.” The visuals here are cannily chosen, the script crisp and elegant, imbued with the ‘sense of wonder’ that brought so many of us to science fiction. Outlasting the Universe is indeed science fiction of the ‘hard SF’ variety as Kipping draws out the consequences of deep time and human consciousness in ways that make raw physics ravishing. I envy this man’s students.

With scenarios like this to play with, where do we stand with the ‘zoo hypothesis?’ It must, after all, reckon with years by the billions and the spread of intelligence. Science fiction writer James Cambias responded to my Life Elsewhere? Relaxing the Copernican Principle post with a tight analysis of the notion that we may be under observation from a civilization whose principles forbid contact with species they study. This is of course Star Trek’s Prime Directive exemplified (although the lineage of the hypothesis dates back decades), and it brings up Jim’s work because he has been so persistent a critic of the idea of shielding a population from ETI contact.

Jim’s doubts about the zoo hypothesis go back to his first novel. A Darkling Sea posits an Europa-like exoplanet being studied by a star-faring species called the Sholen, who are employing a hands-off policy toward local intelligence even as they demand that human scientists on the world’s sea bottom do the same. Not long after publication of the novel (Tor, 2014), he told John Scalzi that he saw Prime Directives and such as “ …a mix of outrageous arrogance and equally overblown self-loathing, a toxic brew masked by pure and noble rhetoric.” The arrogance comes from ignoring the desires of the species under study and denying them a choice in the matter.

In a current blog post called The Zoo Hypothesis: Objections, Jim lays this out in rousing fashion:

…we deduce that you can’t hide a star system which contains a civilization capable of large-scale interstellar operations, which the Zookeepers are by definition. They’re going to be emitting heat, EM radiation, laser light, all the spoor of a Kardashev Type I or higher civilization. And the farther away they are, the more they’re going to be emitting because they need to be bigger and more energy-rich in order to have greater reach.

This gives us one important lesson: if the goal of a Zoo is to keep the civilizations inside from even knowing of the existence of other civilizations, the whole thing is impossible. You can’t have a Zoo without Zookeepers, and the inhabitants of the Zoo will detect them.

Jim’s points are well-taken, and he extends the visibility issue by noting that we need to address time, which must be deep indeed. For a civilization maintaining all the apparatus of a protected area around a given star has to do so on time frames that are practically geological in length. Here we can argue a bit, for a ‘zoo’ set up for reasons we don’t understand in the first place might well come into existence only when the species being studied has reached the capability of detecting its observers.

I referenced Amri Wandel (Hebrew University of Jerusalem) on this the other day. Wandel argues that our own industrial lifespan is currently on the order of a few centuries, and who knows what level of technological sophistication a ‘zoo-keeping’ observer culture might want us to reach before it decides it can initiate contact? That would drop the geological timeframe down to a more manageable span, although the detectability problem still remains. So does the issue of interaction with other star-faring species who might conceivably need to be warned off entering the zoo. Cambias again:

If Captain Kirk or whoever shows up on your planet and says “I’m from another planet. Let’s talk and maybe exchange genetic material — or not, if you want me to leave just say so,” that’s an infinitely more reasonable and moral act than for Captain Kirk to sneak around watching you without revealing his own existence. The first is an interaction between equals, the second is the attitude of a scientist watching bacteria. Is that really a moral thing to do? Why does having cooler toys than someone else give you the right to treat them like bacteria?

This is lively stuff, and speculation of this order is why many people begin reading and writing science fiction in the first place. A hard SF writer, a ‘world builder,’ will make sure that he or she has thought through implications for every action he attributes not only to his characters but the non-human intelligences they may interact with. One thing that had never occurred to me was the issue of visibility when translated to the broader galaxy. Because a zoo needs to be clearly marked. Here’s Jim’s view:

If you’re going to exclude other civilizations from a particular region of the Galaxy, you have to let them know. Shooting relativistic projectiles or giant laser beams at incoming starships is a very ham-fisted way of communicating “keep out!” — and it runs the risk of convincing the grabby civilization that you’re shooting at to start shooting back. And if they’re grabby and control a lot of star systems, that’s going to be a lot of shooting.

Jim’s points are telling, and the comments on my recent Centauri Dreams posts also reflect readers’ issues with the zoo hypothesis. My partiality to it takes these issues into account. If the zoo hypothesis is the best of the solutions to the Fermi question, then the likelihood that other intelligent species are in our neighborhood is vanishingly small. Which lets me circle back to the paper by Ian Crawford and Dirk Schulze-Makuch that set off this entire discussion. It asked, you’ll recall, whether the zoo hypothesis wasn’t the last standing alternative to the idea that technological civilizations are, at the least, rare. It’s not a good alternative, but there it is.

In other words, I’d like the zoo hypothesis to have some traction, because it’s the only way I can find to imagine a galaxy in which intelligent civilizations are common.

Consider the thinking of Crawford and Schulze-Makuch on other hypotheses. Interstellar flight might be impossible for reasons of distance and energy, but this seems a non-starter given that we know of ways within known physics to send a payload to another star even in this century. A slow exploration front moving at Voyager speeds could do the trick in a fraction of the time available given the age of the Milky Way. The lack of SETI detections likewise points to technologies that are physically feasible (various kinds of technosignatures) but are not yet observed.

Is the answer that civilizations don’t live very long, and the chances of any two existing at the same brief time in the galaxy are remote? The nagging issue here is that we would have to assume that all civilizations are temporally limited. It takes only one to find a way through whatever ‘great filter’ is out there and survive into a star-faring maturity to get the galaxy effectively visited and perhaps colonized by now. Crawford and Schulze-Makuch reject models that result in volumes of the galactic disk being unvisited during the four billion years of Earth’s existence, considering them valid mathematically but implausible as solutions to the larger Fermi puzzle.

Many of the hypotheses to explain the Great Silence go even further into the unknowable. What, for example, do we make of attempts to parse out an alien psychology, which inevitably is seen, wittingly or not, as reflecting our own human instincts and passions? Monkish cultures that choose not to expand for philosophical reasons will remain unknowable to us, for example, as will societies that self-destruct before they achieve interstellar flight. We can still draw a few conclusions, though, as Crawford and Schulze-Makuch do, all pointing at least to intelligence being rare.

Although we know nothing of alien sociology, it seems inevitable that the propensity for self-destruction, interstellar colonization and so on must be governed by probability distributions of some kind. The greater the number of ETIs that have existed over the history of the Galaxy, the more populated will be the non-self-destructed and/or pro-colonization wings of these distributions, and it is these ETIs that we do not observe. On the other hand, if the numbers of ETIs have always been small, these distributions will have been sparsely populated and the non-observation of ETIs in their expansionist wings follows naturally.

Image: Are ancient ruins the only thing we may expect to find if we reach other star systems? Are civilizations always going to destroy themselves? The imposing remains of Angkor Wat. Credit: @viajerosaladeriva.

Likewise, we still face the problem that, as Stapledon long ago noted, different cultures will choose different priorities. Why assume that in a galaxy perhaps stuffed with aliens adopting Trappist-like vows of silence there will not be a few societies that do want to broadcast to the universe, a METI-prone minority perhaps, but observable in theory. We have no paradox in the Fermi question if we assume that aliens are rare, but if they are as common as early science fiction implied, the paradox is only reinforced.

So Crawford and Schulze-Makuch have boiled this down to the zoo hypothesis or nothing, with the strong implication that technological life must indeed be rare. I rather like my “one to ten” answer to the question of how many technological species are in the galaxy, because I think it squares with their conclusions. And while we can currently only speculate on reasons for this, it’s clear that we’re on a path to draw conclusions about the prevalence of abiogenesis probably in this century. How often technologies emerge after unicellular life covers a planet is a question that may have to wait for the detection of a technosignature. And as is all too clear, it’s possible this will never come.

The paper is Crawford & Schulze-Makuch, “Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” Published online in Nature Astronomy 28 December 2023 (abstract). James Cambias’ fine A Darkling Sea (Tor, 2014) is only the first of his novels, the most recent of which is The Scarab Mission (Baen, 2023), part of his ‘billion worlds’ series. Modesty almost, but not quite, forbids me from mentioning my essay “Ancient Ruins” which ran in Aeon a few years back.