After numerous posts, in our speculations about the Fermi paradox, we’ve now reached an analysis of the final term in the Drake equation, L, the average lifespan of high-tech civilizations. If you thought putting a value on f_c, the likelihood of an intelligent civilization developing a high enough technology to make its existence known across interstellar distances, was a prime avenue for political and ideological axe grinding, this one is even more.
Still, we have to try. First, we’ll look at some factors supporting a large value for L, then, some factors supporting a small value. If you’re playing along at home, pick the one that most appeals to you.
Factors supporting a large value for L
Sustainability is one of those teeth-grating buzzwords of contemporary life, but it’s still the case that a civilization not much more advanced than ours could sustain contemporary US levels of per capita energy consumption for billions of years. Capturing energy from hydrogen fusion would consume just 0.1% of the oceans’ volume by the time the sun turns into a red giant and swallows our planet. Even nuclear fission, making use of relatively rare isotopes, could fuel a contemporary level of civilization for 5 billion years. Covering a tiny fraction of the Earth’s surface with solar panels would do the same.
(We don’t have the technology for hydrogen fusion or long-range transmission of solar power, you ask? True, we don’t. Yet. But today’s nuclear fission would give us more than enough time to develop those technologies, if we need to).
With enough energy, of course, human beings can solve all the other sustainability concerns–growing enough food per acre of farmland, extracting enough fresh water from seawater, etc. Given this, powering interstellar communications or sending out interstellar probes is trivial.
Free markets mean that some actor — a company, a state, or another form of social organization — will always have an incentive to pursue technological change that others do not. E.g., the Chinese will keep building nuclear reactors, even if greens in the Western world lobby their home governments to decommission them. It would take a world government to regulate or suppress technological advance. The separatist movements and civil wars in our world today suggest world government would be difficult to establish and an even more difficult to sustain. So no one is going to be able to stop technological innovation.
Adaptation of people to high-tech civilization may mean, depending on your political proclivities, recent and copious human evolution
If the life of a planet’s sun is a limiting factor, then M-type stars could have high-tech civilizations lasting tens or even hundreds of billions of years. Of course, at this writing, the universe is too young for this to have much impact on L.
Factors supporting a small value for L
The most benevolent is a high-tech civilization switching to tightbeam and/or fully wired internal communications and giving up on space travel because it can’t pay. We won’t pick up their stray radio communications, we won’t see their probes, and if they get bored with SETI, they won’t hear us either.
Collapse could come from environmental degradation (whatever your beliefs about global warming, at some point the atmosphere can have too much carbon dioxide, methane, water vapor, etc., the oceans can be too acidic, or the like), from diminishing marginal returns of complexity
The flip side of free markets is violent competition. Carl Sagan estimated high values for the early terms in the Drake Equation, and coincidentally came up with the hypothesis of nuclear war leading to nuclear winter as a megaextinctionary event. Needless to say, megaextinction through nuclear war would reduce L. So would a war of electromagnetic pulses, bioengineered plagues, gray goo….
A number of scenarios emerge if people cannot be adapted to high-tech civilization. Billions of people lack the brainpower to be knowledge workers. And with strong enough AI, today’s knowledge workers would be out of a job too
2023 Update: We’re about a year into the ChatGPT/Midjourney era. Book cover artists and audiobook narrators are being replaced by generative AI. And that’s with relatively weak, million-monkeys-with-typewriters AI. Is your job next?
Nanotechnology and/or robot factories and farms might be able to feed, clothe, and entertain 10 billion welfare recipients, especially if virtual reality is too cheap to meter. I’ve referred to this as “virtual fugue” in deep backstory in the first two Confederated Worlds novels, Take the Shilling (The Confederated Worlds) (Volume 1)
The old saying the devil finds work for idle hands comes to mind. How much trouble can 10 billion unemployed people cause? Perhaps this is nothing to worry about. The robots could rebel and turn ten billion human bodies into a paperclip basilisk?
Another scenario is barbarians toppling effete city dwellers. The highest technology in the world is no defense if no one’s willing to pull the trigger. Related is civilizations losing the will to live. I must confess that Oswald Spengler’s The Decline of the West 
What’s the value for L? What’s the value for N?
Remember, our previous posts gave us N = [5e-9 to 8e-8] * L. So even a highly optimistic value of L, 5 billion years, gives us at most 400 high-tech civilizations in the galaxy today. On average, their homeworlds are about 5000 light-years apart.
Can a radio astronomer comment on whether we could detect the contemporary Earth’s TV and radio transmissions from 5000 light years away? My guess would be “not likely,” but it’s not my area of expertise.
Personally, although I’m optimistic about the sustainability of high-tech civilization, something like virtual fugue, coupled with tightbeam communications, along with large chunks of the galaxy being dangerous, leads me to lower L to, let’s say, 100 million years. Combine that with the lower value from the prior terms of the equation, and N = 0.5 high tech civs in the galaxy today. (For the sake of human ego, we’ll round that up to 1).
Therefore, we conclude the human race is essentially alone in the galaxy. There might be dozens, scores, even hundreds of distant worlds that once bore high-tech civilizations whose radio waves bloomed across the galaxy. Now, though, those worlds may be filled with the ruins of civilizations rendered extinct by cosmic cataclysms. May be peopled with the no-longer-sapient descendants of intelligent life. May be forgotten by inhabitants long since succumbed to the infinite sterile pleasures of virtual reality.
Yet on these last worlds, perhaps a handful of inhabitants remain aware of the world on which they live. And, perhaps, on clear nights, they look up and wonder what other life might live, and thoughts be thought, in orbit around some distant star like ours.
We’ll be back soon with one more post recapping the series. See you then.