The vast distance between solar systems may be a form of divine quarantine: they prevent the spiritual infection of a fallen species from spreading; they block  it from playing the role of the serpent in the Garden of Eden. C.S. Lewis

Our galaxy, the Milky Way, may have as many as 40 billion, habitable, earth-sized planets, so say scientists analyzing data from NASA’s Kepler space telescope.

So if life often originates where conditions exist to support it, and if intelligence often emerges where life occurs, then even in this tiny corner of the universe, our home galaxy, intelligent extraterrestrials must be abundant.

But if so, where are they?

There are many possible answerssome of them quite weird. But the correct explanation may be mundane. It may just be a matter of distance.

Used as we are to instant information gratification via telephone, radio, television and the internet, the sheer slowness of communicating, let alone traveling, across the galaxy is not immediately obvious. But,  in the case of galactic neighbors, the difficulty of making a courtesy call, even by iPhone, is remarkably difficult. Assume, for example, that:

  1. life emerged on one in ten of the estimated eight billion possibly habitable worlds in the Milky Way,
  2. intelligence at least equal to our own, evolved on one in ten planets where life emerged, and
  3. of intelligent life forms that have evolved in our galaxy, only one in ten survives to the present day.

Then of the eight billion possibly habitable worlds in our galaxy only eight million have intelligent life now.  And that seems a generous estimate when you consider that human intelligence has existed for less than a million years, or just one thirteen thousandth of the past life of the universe, and only one five thousandth of the life of the earth.

Still, even eight million planets with intelligent life seems a lot, until you begin thinking about the distances between them. The galaxy has a volume of 8 trillion cubic light years, so each home to intelligent life will have, on average, a million cubic light years of galactic space to itself, which makes the average distance between intelligently inhabited worlds about 100 light years.

At that distance the difficulty of social contact  becomes evident. A telephone conversation would take as long as human civilization has existed. A visit in person, assuming travel at some significant fraction of the speed of light, would take as long. So what possible purpose would be served by any attempt to communicate?

One obvious objective of extraterrestrial contact might be conquest, but would George W. Bush really have bother to “take Saddam out,” if the conquest of Iraq had meant a campaign of at least ten thousand years?

the answer to that question is possibly yes if the objective is to find a new world where humanity, faced at home by the danger of extra-terrestrial colonization, destruction by a local astronomical disaster, or self-destruction through internecine strife, might continue to exist.

But what chance is there of those evolved on one world reaching a another where they might survive? On each inhabited world life will have evolved in a way that adapts it to the local conditions. But how many worlds have conditions exactly like another. Humans, for example, need a wet world, but with land as well as ocean; an oxygen rich atmosphere, but not too rich or everything goes up in flames; a global mean temperature close to 14 degrees C; enough sunlight to support photosynthesis, but not enough to fry us; a daylenth very close to 24 hours; a gravitational force close to one g; and many other factors beside.

Such worlds must be rare indeed,  and the voyage to discover such a new world could take forever. A better bet, therefore, might be to seed the universe, or at least the galaxy, with a primitive but adaptable life form, in the hope that new forms of intelligent life, sharing our early ancestry, will emerge adapted to the different local conditions on other  worlds.

According to that line of thinking, it seems possible that we are the progeny of organisms dispersed by some remote and perhaps long-extinct intelligent civilization.

 The Pandora virus has more than 2000 genes. Image source.

The Pandora virus has more than 2000 genes. Image source.

A better approach, though, to the distribution of terrestrial life across the galaxy, might be to bioengineer the preexisting advanced life forms of remote worlds to endow them with the intellectual characteristics of the parent species. In that case, the answer to the question, “where are they?” may be that they are us.

How an advanced civilization might go about seeding sterile planets or engineering existing life forms on remote worlds is not difficult to envisage.  Robot space capsules fired off in all different directions would seek out livable worlds and inject into their atmospheres hardy spores of unicellular organisms, bacteria, archaea, fungi, etc., together with retroviruses carrying a payload of dozens, hundreds or even thousands of genes able to infect higher life forms and kick-start the evolution of nervous systems and intelligence.