In 1977 the Voyager probes were launched toward the outer Solar System, each carrying a Golden Record containing hundreds of sounds and images, from the cry of a newborn baby to the music of Beethoven. In the emptiness of space, they could last for millions of years. By that time, will they be the sole representatives of human culture in the cosmos? Or primitive relics of a civilization that has since bloomed to the galactic scale?
The Drake equation estimates the number of currently communicative civilizations in the Milky Way, by multiplying a series of terms, such as the fraction of stars with planets and the fraction of inhabited planets on which intelligence evolves. The final term in the equation does not get much attention. Yet it is crucial not just for the question of intelligent life, but for the question of how to live intelligently. This is L, the “Longevity Factor,” and it represents the average lifespan of a technological civilization.
What determines this average? Surely the intelligence of the civilizations. The list of existential threats to humanity includes climate change, nuclear war, pandemics, asteroid collisions and perhaps AI. And all of these can be avoided. Some can be addressed here on Earth. Others require activity in space, but with the ultimate aim of protecting the planet.
In 1974, Princeton physicist Gerard K. O’Neill published a paper "The Colonization of Space," which led to the first conference on space manufacturing, sponsored by Stewart Brand’s Point Foundation, and to O’Neill’s highly influential 1976 book, The High Frontier. That has been an inspiration for the current generation of visionaries who are advocating steps such as the transfer of heavy industry into orbit, where it can run on solar energy and direct its heat and waste away from Earth, and the colonization of Mars.
However powerful our local solutions, betting everything on one planet would be imprudent. Stephen Hawking has estimated that “although the chance of a disaster to planet Earth in a given year may be quite low, it adds up over time, and becomes a near certainty in the next thousand or ten thousand years. By that time we should have spread out into space.”
In the long term, Mars must be a stepping-stone to more distant destinations, because two adjacent planets could be simultaneously affected by the Universe’s more violent events, such as a nearby supernova. That means we need to start thinking at the galactic level. The first target might be the Earth-size planet Proxima b, recently discovered around the nearest star to the Sun, 4.2 light years away. Sooner rather than later, we will have to master propulsion fast enough to make interstellar journeys practical. Perhaps by that time we will have developed beyond our organic origins. It has been estimated that Von Neumann probes—robots that could land on a planet, mine local materials and replicate themselves—could colonize the entire galaxy within ten million years.
But even a galactic civilization might face existential threats. According to our current understanding of the laws of physics, in any region of space there is a chance that a “death bubble” forms, and then expands at speeds approaching the speed of light. Because the physics inside the bubble would differ from that of ordinary space, as it expanded it would destroy all matter, including life. The chances of this happening in a given year may seem very low—perhaps less than one in ten billion. But as Hawking reminded us, if you wait long enough the improbable is inevitable.
Yet the renowned physicist Ashoke Sen has recently suggested that even in the face of death bubbles, there might be an escape route. The loophole is the accelerating expansion of the Universe. In 1998, astronomers discovered that all galaxies that are not strongly bound together by gravity are moving apart ever faster. This accelerating expansion will eventually carry them beyond each other’s “cosmic horizon”—so far apart that even light from one can never reach the other. That means they can no longer communicate; but on the bright side, they can also never be swallowed by the same death bubble.
So by splitting into daughter civilizations and putting as much distance between them as possible, a civilization could “ride” the expansion of the Universe to relative safety. Of course, another death bubble will eventually pop up within any cosmic horizon, so the remaining civilizations need to keep replicating and parting ways. Their chances of survival depend on how far they can travel: if they were able to move at a substantial fraction of the speed of light, they could raise their chances of survival dramatically. But even those that dispersed only far enough that their galaxies were not bound together by gravity—about 5 million light years apart—might significantly improve their odds.
Such problems may seem remote. But they will be real to our descendants—if we can survive long enough to allow those descendants to exist. Our responsibility as a civilization is to keep going for as long as the laws of physics allow.
The longevity factor is a measure of intelligence: the ability to predict potential problems and solve them in advance. The question is, how intelligent are we?