Centauri Dreams takes an optimistic view of the human future, one in which interstellar flight becomes a reality at some point in this millennium. My impression is that we’d all better be optimists. Think about the Drake Equation. Perhaps its most significant variable is the lifetime of a technological civilization, a figure that has implications for any creatures who have developed the tools to go into space. If the lifetime of such a civilization averages a million years, then the ‘where are they’ question Fermi asked becomes more charged. Shouldn’t we be detecting them?

But if the average lifetime of a technological culture is, say, five hundred years, then we may be confronted with a galaxy filled with wreckage, planets where life persists in evolving and forming intelligent beings who bring about their own destruction. Like I say, I’d rather be an optimist, but none of us knows the real answer. I note that Jan Zalasiewicz (University of Leicester) has offered up a new book that assumes at least long-range limits for our own species. Thus this comment:

“From the perspective of 100 million years in the future-a geologist’s view-the reign of humans on Earth would seem very short: we would almost certainly have died out long before then. What footprint will we leave in the rocks? What would have become of our great cities, our roads and tunnels, our cars, our plastic cups in the far distant future? What fossils would we leave behind?”

On the other hand, Zalasiewicz is a geologist. He’s used to working with time measured in eons, and even a million year run for humans would be dwarfed from that perspective. The book, called The Earth After Us (Oxford University Press, 2008), asks what alien explorers might discover if they arrived on Earth one hundred million years from now. Their scientists would find evidence of vast tectonic movements, ice ages and the movement of oceans, a geological history sprinkled with life and its occasional catastrophic collapse. They might also find, in a single layer of rock, signs of cities and the creatures who built them.

The concept of ‘deep time’ was originally developed to encompass the perspective changes induced by geological study (thus 18th Century mathematician John Playfair’s comment upon studying a particular site, “the mind seemed to grow giddy by looking so far into the abyss of time.” We might also consider it in terms of communication, as Gregory Benford does in his book Deep Time: How Humanity Communicates Across the Millennia (Bard, 2001). Or maybe we should think about deep time in the sense of placing human awareness in the broadest possible context.

Those of you who have read Greg Laughlin and Fred Adams’ The Five Ages of the Universe (Free Press, 1999) will readily identify with the concept. Laughlin and Adams look at the history of the universe in terms of ‘cosmological decades,’ each decade being ten times as long as the one before. When I say that the end of stellar burning 100 trillion years from now is only the early part of the story, you’ll understand how interesting is the possibility of intelligent life surviving the entire cosmic history.

As to Zalasiewicz (whose book I much look forward to reading), another question posed by alien archaeologists sifting through the traces of our cities in layered rock is this: If a technological civilization does indeed have a sharply limited lifetime, then how long is enough to ensure survival elsewhere? In other words, if 10,000 years is the technological average, is this long enough for that culture to have spread to space habitats or the nearest stars? An optimist’s answer to that one comes easy: Yes, we’ll have that chance to move outward before the big collapse, but given the uncertainties, we won’t want to wait too long.