Recently the magisterial Centauri Dreams 'blog ran a post by Paul Gilster about the "Copernican Principle" and how it conflicts with the observed facts about life in the Universe. (Short version: the Copernican Principle says Earth should be an average world, but if that's the case, why don't we see more signs of life elsewhere? If life is rare, then Earth isn't average after all.)
In the course of this discussion, Mr. Gilster touched on the famous "Zoo Hypothesis" — the notion that the reason we don't see signs of extraterrestrial life or civilizations is that someone is keeping them all away from us. He traced the idea back to Olaf Stapledon, which doesn't surprise me because almost every idea about space or life in the Universe can be traced back to Olaf Stapledon.
That started me thinking about why I've never really believed in the Zoo Hypothesis, so I've decided to share my ideas here. I want to be pretty rigorous about this, so I'm going to define my terms first and then go on to discuss the conclusions one can draw from them and why they basically refute the whole hypothesis.
Core Premise: Some active agency (hereafter "the Zookeepers") is stopping any expansionist interstellar civilizations from expanding into the region containing our Solar System.
Second Premise: The goal of the Zookeepers is protective. They want to keep infant civilizations or lifebearing worlds from being swallowed up by "grabby" aliens. If they just wanted to stop any civilizations from expanding, or even existing, that's the "Dark Forest Hypothesis" or possibly the "Berserker Hypothesis." I'm not talking about that here, although some of the topics I'm going to touch on today may be relevant to both of those.
Third Premise: The Zookeepers must have some means of preventing other civilizations from entering the "Zoo" region, which means they (or at least their agents) must surround the whole area. They may have a whole civilization enveloping the Zoo, or just a picket sphere of guard posts, possibly automated.
Fourth Premise: The entire Zoo project is not the primary purpose of the Zookeeper civilization. The majority of their resources are devoted to other things.
Fifth Premise: We will assume the Zookeepers have no magic powers. They cannot travel or communicate faster than light, or do anything else which contradicts our understanding of natural laws. We will grant them effectively infinite resources and arbitrarily advanced engineering, since presumably no civilization would undertake something like the Zoo project unless they are able to actually accomplish it.
Okay, with those premises, what can we deduce?
First, we know that we can see the Zookeepers!
Consider: the Zookeepers exist at a distance R1 from Earth. They have created a Zoo region for us with a radius R2, and R2 is less than R1. In order to surround the Zoo, the Zookeepers must send some kind of spacecraft at no more than the speed of light, to a distance of R1+R2 from their homeworld. Since R1+R2 is obviously greater than R1, and the ships can't outpace light, that means that we have to be able to see light from the Zookeeper home system which was emitted after they became capable of launching interstellar spaceships. Possibly a long time after, depending on how long the project has been in operation.
From Premises Four and Five 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.
Second, we're almost certainly alone in our Zoo.
Consider: the Zookeepers wish to protect the inhabitants (Second Premise). They can't protect individual star systems, because that would leave the Zoo inhabitants' home system surrounded by interlopers from grabby civilizations, and the Zookeepers' guard posts would be right at the edge of the protected system and thus in plain view. No, the Zoo needs to be big, incorporating multiple star systems. Its radius R2 must be on the order of tens of lightyears.
But, if a Zoo contains more than one lifebearing world, then one of those planets might produce a civilization capable of building its own starships, and therefore could become grabby and colonize the other lifebearing worlds, defeating the purpose of the Zoo. Consequently each Zoo will be centered on a lifebearing world, but will include only one. (If life is so common that nearly every star system has a good chance of harboring it, then the Zoo project is impossible.)
Third, a Zoo needs to be visible!
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.
Therefore, a Zoo has to be marked. It needs beacons around it, transmitting to anyone within range that "this volume of space is off limits." These beacons need to be visible at ranges approximating R2, so that would-be interlopers have plenty of advance warning and no shooting is required.
It may well be possible to make a particular Zoo's warning beacons invisible to any newborn civilizations inside it — but you can't hide any other Zoo spaces from them! As noted in my second conclusion, above, it's likely that each Zoo contains only one lifebearing world, so that there could be a large number of discrete Zoos (depending on how powerful the Zookeepers are). We should see clusters of beacons marking other Zoo regions.
Fourth and finally, the Zookeeper civilization, or whatever sub-unit of it is responsible for the Zoo project, needs to be incredibly stable and persistent. Even with near-lightspeed travel, they're going to be sending out starships on voyages measured in centuries, protecting planets over time scales rivaling geological periods in length. All it takes is one shift in attitude among the Zookeepers and all the worlds they've been protecting could get colonized. We know they can reach the target worlds because they can reach well beyond them, so it's only their moral fiber and devotion to the cause that keeps the Zoo in existence.
Yeah, I don't believe those are strong enough, either.
So I think the Zoo Hypothesis is kind of self-refuting. If we don't see aliens because we're in a Zoo, then . . . why don't we see the Zookeepers? Why don't we see the boundaries of the Zoo? Far from being a solution to the Fermi Paradox I think it's even more paradoxical: we don't see aliens because there are aliens all around us!
Nope. I don't buy it. I don't buy the Dark Forest or the Berserker Hypothesis either, for similar reasons. They all require large, visible alien civilizations to explain why we haven't observed any alien civilizations. There must be some other reason.
Update (January 5): While I was writing this, apparently Paul Gilster was having his own thoughts on the subject, which you can see here.
Recent Comments