The Drake equation, developed in the 1960s by the astronomer Frank Drake, laid the foundation for the scientific search for extraterrestrial life. This equation provides a way to estimate the number of intelligent, communicating civilizations in the Milky Way galaxy by combining all the prerequisites for the existence of such civilizations.

The remarkable thing about the Drake equation is that even seemingly conservative values for its various factors tend to predict a galaxy overflowing with life and intelligence, with tens, hundreds or even thousands of extraterrestrial civilizations. (Try it out and see for yourself.) Yet, indisputably, the observational evidence does not match that expectation. We know of no convincing evidence that there are any other intelligent species in the cosmos.

This is Fermi’s paradox, named after the physicist Enrico Fermi who is said to have first posed it, and it cries out for an explanation. Some people have proposed that intelligent life is much harder to develop than we think, and we are indeed the only sentient species in the galaxy. Others hypothesize that intelligent life arises frequently, but tends to destroy itself through resource exhaustion or planetary war just as quickly.

Personally, I doubt that either of these are the case. We now know that planetary systems are abundant in our galaxy. The geologically rapid appearance of life on Earth, dating back to within a few hundred million years after the world had cooled enough to make it possible, suggests that on a planetary scale life is not so hard to come by. And once life exists, intelligence seems like a fairly obvious adaptation – a “Good Trick”, as Daniel Dennett puts it, one that has utility in a wide variety of contexts and would be likely for evolution to stumble upon eventually. Perhaps some intelligent species destroy themselves, but surely not all of them do; there must be at least a few who are intelligent enough to recognize their danger and avert it. Both these hypotheses seem unlikely to me on statistical grounds. But then, why have we not detected signals from other intelligent life? I’ll survey a few explanations that seem at least possible:

The “we came first” hypothesis. No matter how many intelligent species there are, one must come first. It may be that, by some astonishing stroke of luck, humanity is the first. Or, a slightly weaker alternative: we are not the first but one of the first, and other civilizations have not existed long enough for their signals to traverse the vast distances of interstellar space to reach us.

I am well aware that many other hypotheses which granted humanity a special or privileged place in the grand scheme of things have been disproven. This explanation might seem to be another suspiciously anthropocentric speculation. Especially, given the vast periods of cosmic time that elapsed prior to our appearance and the exponentially rapid development of high technology and culture in our own species as compared to geological time, it might seem that there have been ample opportunities for others to precede us. However, that need not be the case. There could, for instance, be some previously undiscovered property of the universe that only makes the existence of complex, intelligent life feasible after a certain point in time, and so our appearance as one of the first sentient species would be no coincidence.

The smoke-signal hypothesis. Imagine that at some point in the distant past, before European contact and the age of colonialism, a native civilization in the Americas or the Pacific islands wondered if they were the only human society in the world, or if there might be other, more advanced civilizations across the vast and uncharted oceans that surrounded them. Imagine that this society was accustomed to communicating over great distances with smoke signals, and assumed that a more advanced society would do the same thing, only on a larger scale. Accordingly, they dispatch watchers to the peak of a nearby mountain to search the horizon for vast columns of smoke that these hypothetical societies might be using to signal across oceans. Finding none, the watchers return and report that as far as they can tell, their civilization is alone in the cosmos.

This analogy may apply to our own situation. We are searching the sky for radio waves and other forms of electromagnetic radiation, assuming that this is how a more advanced species would communicate. However, it may be that there are advanced civilizations using something else far superior, something which we do not even know about yet, and that these civilizations have no pressing interest in making contact with a species at such a primitive stage of development that they do not know about this other means.

The zoo hypothesis. One suggestion I find particularly ironic is that there are advanced civilizations that are well aware of our existence, but are deliberately choosing to conceal themselves from us so as not to panic a species at an insufficient level of intellectual development to handle such a discovery wisely. We could imagine this as something like the “Prime Directive” of Star Trek, the rule that forbids advanced spacefaring civilizations from disrupting the normal course of development of those that have not yet attained this technological level. Less charitably, we could interpret it in the way the term “zoo hypothesis” implies – to them, we are like animals in a zoo, living out our lives placidly with no clue that we are being gawked at by watchers on the other side of the one-way glass.

The infeasibility hypothesis. Futurists often predict that technology will increase without limit. The astronomer Nikolai Kardashev, for example, proposed a three-point scale for species able to harness all the power available in a single planet, star, or galaxy respectively. A Type III Kardashev civilization would be advanced beyond imagining, and even a Type II would be vastly superior to anything humanity can currently hope to match. However, perhaps the error is in assuming that this is an inevitable or even possible progression. Perhaps it is just too hard to become the kind of planet-shaping society many futurists envision. The laws of physics may not allow it: perhaps the materials that can possibly be created are too weak, the energy we can usefully collect too small. If that is the case, technology levels equal to or slightly above that which is currently possessed by humanity may be the limit dictated by the cosmos, and interstellar communication and travel may never occur because it is impossible to engage in the kind of engineering that such an endeavor would require.

The missed signal hypothesis. Intelligent aliens do exist, they are trying to make contact, and they are transmitting the right kind of signal – but we’re not listening. Programs such as SETI have been mocked by politicians as frivolous wastes of money (meanwhile, those same politicians almost always vote for the development of powerful new destructive weapons systems), and at the moment depend largely on private sources of funding. As a result, there are large sections of the sky whose radio coverage is poor or nonexistent, and there is no way of knowing until we look if there are signals arriving from those regions that we are missing. The cosmos could be awash in intelligent signals that we, in our inward-looking myopia, have completely failed to see.