Behind every man alive stand thirty ghosts, for that is the ratio by which the dead outnumber the living. Since the dawn of time, a hundred billion human beings have walked the planet Earth   Now this is an interesting number, for by a curious coincidence there are approximately a hundred billion stars in our local universe, the Milky Way. So for every man and woman who has ever lived, in this universe there shines a star. But every one of those stars is a sun, often far more brilliant and glorious than the small, nearby star we call the Sun. And many-perhaps most-of those alien suns have planets circling them. So almost certainly there is enough land in the sky to give every member of the human species, back to the first apeman, his own private world-sized heaven-or hell.   How many of those potential heavens and hells are inhabited, and by what manner of creatures, we have