We like the idea that someone else is out there. Someone intelligible to us. The crowded, fictive universes of Star Wars and Star Trek are fun to imagine. They are also easy to imagine, possibly in part be because of the influence of the mediocrity principle, which has been rattling about in modern cosmology for some time. A simple way to think about mediocrity is that if you are on a walk and pick up a random rock, it is apt to be of a common type. For extra-terrestrial life this means that because we happen to be here, complex life is likely to exist on lots of other earth-like planets.
Not so fast. A recent Discover Magazine article suggests something different:
A model of the universe predicts the universe holds some 700 quintillion planets, but none like Earth.
Source: Earth May Be a 1-in-700-Quintillion Kind of Place – Discover Magazine
The idea that our particular type of world might be rare is not new. As Peter Ward and Donald Brownlee suggested over a decade ago, there really is quite a laundry list of things that have to happen to make a place suitable for complex life. Microorganisms can live in some really nasty places (including possibly the oceans of Saturn’s icy moons). But not much else.
And if we do share the cosmos with someone else they might be too far away to ever know about. It is really hard to grasp just how far away celestial objects actually are. The roughly ten thousand years of human history (settlement and agriculture) is a statistical blip when distances are measured in thousands, tens of thousands, and hundreds of thousands of light years. By the time an electromagnetic signal from another civilization is received and responded to our own may be long dead.
So far we have not heard from anyone, which was commented on in an Atlantic Monthly article about SETI in 1988. Nothing has changed since then. SETI has been listening to silence for quite a few years now. It’s as if we are the only flea on an elephant. How is that possible?
This idea that we might actually be alone really bothers us. Having killed off belief in God and the immaterial (or at least made it irrelevant) we seem to feel compelled to populate our empty spaces with something. Granted, most of the sci-fi is just good fun — Guardians of the Galaxy was a hoot. But in the last couple decades it seems like there has been rather a lot of it.
And it’s been apocalyptic. The blockbusters all seem to be about someone or some thing trying to end everything and someone else trying to prevent it. And one side (or both) having extraordinary abilities. The perseverance of ordinary people against long odds no longer speaks to us. We want heroes and demi-gods, and we them to ride in and save us from being alone in an anxious world saturated with bad news.
And then some scientist comes along and tells us that no one will be coming. That we really are the only flea on the elephant.
Looking into the vastness of the cosmos by ourselves unsettles us. It gnaws away inside us in a way that has been spoken of before by the writer of Ecclesiastes:
He has made everything appropriate in its time. He has also set eternity in their heart, yet so that man will not find out the work which God has done from the beginning even to the end (Ecc. 3:11 NASB).
The Hebrew word translated as “eternity” seems to have the sense of darkness or obscurity. This gnawing inside appears to have been bothering us for a very long time.
 The Mediocrity Principle is essentially that the chance selection of an item is more likely to come from more numerous classes than from less numerous ones. The caveat is that the selection must actually be random.
 Peter D Ward and Donald Brownlee. Rare Earth: Why Complex Life Is Uncommon in the Universe. New York: Copernicus, 2000. Print.
 I don’t claim to read Hebrew. This is what I’ve teased out of an interlinear which also referred to it as “eon.” It has also been translated as “ignorance” (NET). I would welcome comment from someone knowledgeable.