L is for Longevity
Astronomer Frank Drake, with his eponymous equation, was probably the first to set down mathematically a way to compute the number of intelligent civilizations in the Galaxy, N. The Drake equation gives N as a product of the rate of star formation in the Galaxy (the number of stars formed in, say, a century), the fraction of those stars that have planets, the number of planets per planetary system that can support any kind of life, the fraction of planets on which life actually appears, the fraction of life-bearing planets that evolve intelligent life, the fraction of these intelligent civilizations that develop technology that creates detectable signatures (radio transmissions, for example), and the length of time L (say, centuries) the civilization releases remotely-detectable signatures. Over the past decades, we have developed a reasonable understanding of the star formation rate in the Galaxy, we have confirmed the existence of hundreds of extrasolar planets, and have tantalizing clues about organic molecules in space and planets in potential ‘habitable zones’ around some stars. Some of the remaining factors in the Drake equation will likely remain intractable to us for decades, if not centuries, such as the fraction of life-bearing planets that evolve intelligent life and the fraction of intelligent life that develop technology. But perhaps what has been the most worrying factor for astronomers is L, the longevity of a civilization. Since all the other fractions are small, if L is small as well, the value of N is not affected much. But if L happens to be extremely large — for example, of the order of billions of years — N becomes a non-zero number. N, therefore, depends somewhat crucially on L. It is perhaps the reason why Frank Drake’s vanity license plate is reported to read NEQLSL.
It is probably incorrect to use L — really, the length of time a civilization exhibits signs of life — as a proxy for longevity, that is, the length of time a civilization exists. If the human civilization is any indication — and there is no reason to think otherwise at this time — as civilizations develop, they become more efficient in terms of their energy consumption. For example, directional antennas have replaced omni-directional antennas for most radio transmissions, resulting in less energy wastage, and less ‘leakage’ that may be picked up far beyond Earth. In other words, a quiet civilization is not necessarily a dead civilization.
That is not to say that a civilization once risen will never cease to exist. Over millennia, we have witnessed various human civilizations come into being, flourish, and fade away, although small interacting civilizations on a planet is not a fair comparison to a planet-wide, monolithic civilization, such as the one we have today. We do not know for sure whether our civilization will exist for time scales of billions of years, or will cease to exist at some point in the nearer future. Astronomers who grew up during the Cold War in participant countries tend to mention ‘nuclear war’ as a reason a civilization can stop existing. Younger astronomers rarely mention nuclear annihilation, but their answer may include the term ‘climate change’, or even ‘artificial intelligence’. Fashionable fears may not provide the right answer on how a civilization might come to an end. A slow decline seems more likely than a catastrophic fall. The size of the human population is expected to plateau out by the end of the century, and there is no reason not to expect a fall in population subsequently. Moreover, if current and future efforts at lengthening the human lifespan come to fruition, and reversal of aging can indeed be achieved, there would be no biological motivation to procreate — our DNA will continue to exist without having to resort to recombination. A falling population does not need to expand to other planets for survival, as enough resources can be found on Earth. Perhaps, with basic income and AI slaves, humans of the coming centuries will cease to work, engage more in entertainment than intellectual pursuits, and — taking an admittedly large logical leap — cease to be curious and creative, and make up a long tail of slow decay far into the future, well under the threshold of detectability.
N equals L. Unfortunately, L may only be of the order of a few centuries for the human civilization. If this is true for other civilizations as well, L may not be long enough to detect them.
Not only would we die (effectively) soon, we would die alone.
Aside: Claudio Grimaldi has recently published an interesting article on the detectability of extraterrestrial intelligence, based on a statistical study of emitters in the Galaxy, taking into account signal longevity, among other things. He concludes that, even if a significant fraction of the Galaxy is covered by extra-terrestrial signals, the detectable number of civilizations is less than unity.
