Where are all the aliens? 

This question, often called the Fermi Paradox, has been the subject of decades of thought. The sheer scale of the Universe heavily favours a Milky Way teeming with life—so why haven’t humans seen even the slightest convincing sign of any of it?

Are humans not advanced enough yet? Are the aliens too far away? Can humans merely not see them—or, if they can see them, can they just not comprehend them? Or are humans genuinely alone?

The speculation for answers ranges from hard science to pure philosophy to seemingly everything in between. Without much useful information to work from, the possible answers are frustratingly broad, often not actually resolving anything at all.

Attempting to sum all of them would take up at least a brick-like book, but it’s possible to look at individual ideas. Indeed, there is one particularly bizarre-sounding idea that often flies under the radar: the so-called “zoo hypothesis.”

You probably already have some idea what this entails—the clue is quite literally in the name. The reason Earth has never seen any signs of alien life is because aliens have determined to keep Earth in a zoo—a wildlife sanctuary in space, or some other form of artificial preserve. Some aliens have determined that it is best to leave Earth alone, to keep it in the dark from outside contact.

I searched for a bit to find the origins of the zoo hypothesis. The earliest serious assertion of the zoo hypothesis seems to be this 1973 article in the planetary science journal Icarus by an MIT-based astronomer named John A. Ball. In his words, “[humans] shall never find [aliens] because they do not want to be found and they have the technological ability to insure this.” Ball draws from existing discussion involving others including Carl Sagan, but he seems to have been the one to put the zoo hypothesis to paper.

Why would aliens bother with the effort? The name “zoo” indicates that Earth is nothing more than a curated display for the amusement of eldritch beings whose identities and prhumans cannot understand; after all, can an elephant in a wildlife sanctuary comprehend the politics and people behind conservation and tourism? Ball himself called the zoo hypothesis “pessimistic and psychologically unpleasant,” and went as far as to wildly spectaulate that Earth might be nothing more than a laboratory.

Then again, there is nothing that is inherently cynical or pessimistic about the zoo hypothesis. On Earth, wildlife sanctuaries may far too often become entangled in political meddling, but they are still idealistically doing something to protect habitats. The zoo hypothesis is similar to Star Trek’s Prime Directive, which prevents meddling on planets that haven’t developed interstellar travel, in order to prevent interference with those planets’ natural evolution. There isn’t any reason to assume otherwise, or to assume that aliens are toying with Earth for some sinister scheme. Perhaps Earth is isolated because the alternative is worse.

Ultimately, without knowing how aliens think—and without assuming aliens’ thought processes operate anything like human minds, and without assuming one alien thinks and operates like a second alien, and so forth—it is impossible to assign the zoo hypothesis a motive or a karmic purpose.

That brings the whole discussion full circle: there is little useful information to use, and the whole thing is at best speculation. John A. Ball called his own idea “probably flawed and imcomplete,” hoping little more than “that it can provide some sort of inspiration for further work.”

Perhaps, with the rapid advances in Earth telescopes, soon humans may find out where all the aliens really are.