Your Turn, Round 2-3: A Problem: The Drake Equation In This Setting
Frank Drake’s formula for estimating the number of intelligent civilizations in the universe that could be located by SETI efforts has some limitations. But it does provide a convenient framework for thinking about the real search for extraterrestrial intelligence. And it can also be used to see the implications for the Fermi Problems setting of having two intelligent species within 15 parsecs of Earth.
The equation: Number of civilizations in the galaxy N = (number of stars)*(number of planets per star)*(fraction of planets where life evolves)*(fraction of biospheres where intelligence evolves)*(how long intelligence lasts)/(age of the universe)
There are some modifications here from the usual form of the equation. As I mentioned earlier, the usual definition of ‘intelligent’ for SETI is ‘having built a radio transmitter or other beacon readily detectable over interstellar distances’. Here I’m using a more comprehensive definition, which includes humans anytime since behavioral modernity has been around. And any one occurrence of intelligence lasts a long time. As a starting point, call it 1 million years.
Several of the numbers in the equation are known. The universe is 13.77 ± 0.06 billion years old, although I can round that to 10 billion for this because it takes a while for nucleosynthesis to work up to building enough non-hydrogen and non-helium material to make planets. There are ~300 billion stars in the galaxy – although there could be somewhat more depending on how we count brown dwarfs. The Kepler Mission and micro-lensing surveys show that there at least as many planets as there are stars. Again, the accounting here is difficult: the surveys are generally not sensitive to planets smaller than the Earth (to say nothing of asteroids – although they should be massively down-weighted compared to larger objects). Call the number of planets per star 3, to make the order-of-magnitude calculation easier.
So: In the Fermi Problems setting, N = 10^8 * (fraction of planets with life) * (fraction of biospheres that evolve intelligence). There are ~1900 stars in ~1400 stellar systems within 15 parsecs of Earth right now. If civilizations are uniformly distributed, which they may not be, for there to be two civilizations extant in that volume, there must be one civilization per ~700 stellar systems on average. I can make it 1 per 1000 stars without straining the odds. But N = 10^8 is 1 civilization per 3000 stars. This poses a problem.
What Should I Do?
There are five different variables I can adjust in the setting. The fraction of planets with life can’t be greater than one, and based on the evidence of our solar system it is most likely far less than 0.1. I could say that biospheres evolve intelligence many times, making that fraction greater than 1. Counting the apes, the dolphins, the elephants, the canines, the cats, the parrots, the corvids, and some of the cephalopods as separate evolutions of intelligence on Earth gives 8, but we are obviously by far the most extensive of the lot. It is pretty much impossible to get the product of those two fractions above 1, so I am left with three possible changes:
1. Intelligence lasts a long time. I have had the ursians be trapped at the bottom of a hole in high-tech social stasis for hundreds of thousands of years. Can I make it several million instead, to push the number up to 10^7 years? This starts to get problematic for the neari, who can spread across 15 parsecs in that time even with limited technology; and technological stasis does not apply for them. I don’t think I can push the lifetime up much further than that without the setting breaking.
2. There are more ‘planets’ per star. In addition to terrestrials and gas giants, I count large satellites like Europa and Titan and some appropriately-down-weighted number of asteroids. But, like the lifetime number, this can only go up so much. Maybe we count 15 places in the solar system rather than 8, but most of those are less appealing for biologically interesting chemistry.
This leaves the last option, which is the easiest in scientific terms, but much harder in terms of plot:
3. Move the neari and the ursians further away from each other and from Earth. Putting them each 150 pc away rather than 15 gives 1000 times as many stars. Then, with civilizations lasting a million years and 3 planets per star, (fraction of planets with life)*(fraction of life with intelligence) need only be 0.001 or so. In this version of the setting, one out of every 30 stars will have a biosphere near it somewhere – there will be strange microbes all over the place, and the nearest macroscopic ET organisms will be something like 30 lightyears away. And in places with fossil records, there will be ruins/remains of billion-years-dead cultures; far far more of those than the extant cultures.
If I do increase the distances so much, then there is no way for a neari comet boat to travel to Big Bear and find the ursians. And the timeline will need to be extended; with things involving humans and either group of aliens happening thousands of years in the future rather than only 500 years from now.
So, shall I change the setting to avoid the improbability of having not just two, but three civilizations so close together? The price is much less of characters meeting aliens in person, much longer time lags when talking to aliens remotely, and much more far-future exoplanet geobiology and archaeology. It seems that’s what the universe is like.