He drives home, zigging and zagging through the grid. He often likes to take the first left, then the first right, then the first left, doing it as long as the grid allows, and see where he ends up. Though by now he generally knows. His choice is constrained once he reaches the bottom of East Hill, so at that point he finds his street and ascends. People think gravity = death, when in fact gravity = life. The exorbitant demands of gravity may well defeat space exploration, but then, space = death. Life was made for planets.
He pulls into his driveway, cuts the engine. Gets out, circles the car. His house has once again not burned down in his absence. Enters through the kitchen door, disposes of keys and laptop in their proper places, keeps on his coat and shoes, grabs a beer and two towels, goes out onto his back deck (unsagging and, like the house, not in flames). When the trees are bare, as now, he has a good view of the sky. The temperature has risen into the forties and the clouds have been blown east. The gibbous moon is just past the zenith. Mark has always thought, for some reason he can’t articulate, that when the moon is like this, about 85 percent illuminated, it looks most like a face. A mother’s face, to be specific, looking in the direction of its shadowed edge. No discernible features, just something about the shape of the “head,” maybe the shadowed hint of an offside cheek. Actually, and he wouldn’t want to admit this to anyone, it seems so terribly sentimental, but he always thinks of a mother looking down into a crib. With love, actually. God knows why.
He spreads one towel over the metal-mesh chair and the other over the patio table, sits with his legs up, drinks his beer. 9:05 p.m. No spring birds yet, no crickets; the sound of water trickling downhill. He finds himself thinking again about the day’s lecture, all the things he wishes he could add. The only piece of evidence astronomers and exobiologists have to support the hypothesis that life arises easily on a habitable planet is that it seems to have arisen on Earth shortly after the planet’s surface cooled enough to make life possible, perhaps within 200 million years. But how robust is this argument? If you take the four billion years between the beginning of habitability and the present time and divide them into 200-million-year intervals, you only get twenty. Therefore there’s a 1 in 20 likelihood, which is not insignificant, that life arose purely by chance in the first interval. And the argument is even weaker than that. Because the Sun is getting hotter, the Earth will be habitable for only about another billion years. It’s reasonable to assume that what we call habitable planets—Earth-like planets orbiting Sun-like stars—have a similar window of approximately five billion years in which life might develop. On Earth, it took 3.5 billion years for prokaryotic life to figure out how to become eukaryotic and then multicelled. Then it took another 600 million to 800 million years for multicelled life to figure out how to develop enough intelligence to sit on back decks, drink beer, and mull over these questions. If those time frames are typical, then any planet that’s going to eventually host intelligent life has to get started somewhere within the first billion years of its habitable window. There’s a one in five probability that it will accomplish this within the first 200 million years.
He drains his beer, goes in, checks his email, talks Gerhardt off a ledge. Watches Fox News for an hour. Then sits at the piano and plays the middle ten variations of Goldberg.
Almost midnight. He meticulously brushes and flosses his teeth (his dentist heaps praise on him twice yearly), crawls into bed. This room, like his childhood bedroom, occasionally admits moonlight when the trees are bare. A rhomboid of light creeps just perceptibly across the floor. My discarded socks are bathed in moonlight . . .
Since there does exist one small piece of evidence suggesting that broadcasting and/or exploring civilizations are not common in the universe (i.e., Where is everybody?), then somewhere along the developmental line from habitable world to bustling spaceports lies what astronomers like to call the Great Filter. Of course he has gut feelings. Even he is human. (He blames Mette’s mother for the fact that this phrase intrudes on his thoughts once or twice a week.) Despite his reasoning earlier, if he were forced to guess he would say that analogs of prokaryotic life are probably common in the universe. He finds it compelling that bacterial life is so resourceful and tenacious, thriving at extreme temperatures, surviving for millions of years within rocks and meteoroids. Not that tenaciousness once alive really says anything about the probability of coming alive in the first place—but hey, that’s what gut feelings are for.
He remembers a charming anecdote—he thinks it was about the physicist Freeman Dyson, or maybe Niels Bohr, he’s not sure. Anyway, a visitor to (let’s say) Dyson’s office, noticing that he had a horseshoe mounted on the wall, exclaimed, “Surely, Mr. Dyson, you don’t believe that a horseshoe brings good luck.” Whereupon Dyson replied, “Of course not. But they tell me it brings good luck whether you believe in it or not.”
As for the Great Filter, there are two periods of time on Earth that give Mark “pause,” as they say. The first is that 3.5 billion years during which the prokaryotes seem to have invaded every conceivable niche
