I don’t suppose that Frank Drake intended his famous Drake Equation to be anything more than a pedagogical device, or rather, an illustrative tool to explain what he viewed as the most significant things we would need to know to figure out how many other civilizations might be out there in the galaxy. This was back in 1961, and naturally the equation was all about probabilities, because we didn’t have hard information on most of the factors in the equation. Drake was already searching for radio signals at Green Bank, in the process inventing SETI as practiced through radio telescopes.
The factors here should look familiar to most Centauri Dreams readers, but let’s run through them, because among the old hands here we also get an encouraging number of students and people new to the field. N is the number of civilizations with communications potential in the galaxy, with R* the rate of star formation, fp the fraction of stars with planets, ne the number of planets that can support life per system, fl the fraction of planets that actually develop life, fi the fraction that develop intelligent life, fc the fraction that go on to communicate and L the life time of a technological civilization.
The idea of course is that you can multiply all these things together to derive some idea of how many civilizations are out there whose signals might be detected on Earth. Multiplying all those factors obviously ratchets up the uncertainties, but given that we have been proceeding with our investigations, and fruitfully so, for decades since Drake addressed the 1961 meeting at Green Bank, it’s interesting to see just where we stand today. Bringing us up to date is what Pascal Lee did at the recent symposium of the Interstellar Research Group in Montreal, where he gave the talk you can access here.
Image: M31, the Andromeda galaxy. Are civilizations common in spirals like this one? The Drake Equation is one way of probing the question, with ever-changing results. Credit: Space Telescope Science Institute.
Lee (SETI Institute, NASA Ames) is all too aware that with Earth as our only datapoint, we run the risk of being eaten alive by our own assumptions. So he takes a conservative approach on each of the factors involved in the Drake Equation. I think some of the most interesting factors here relate to the nature of intelligence, which didn’t pop up until halfway through the life of our star. That’s assuming you posit, as Lee does, that the appearance of homo erectus signifies this development, and this datapoint indicates that intelligence is not necessarily a common thing.
After all, the creatures that ran the show here on Earth for well over 200 million years do not seem to have developed intelligence, if we take Lee’s definition and say that this trait involves making technologies that did not exist earlier. A beaver dam is not a mark of intelligence because over the course of time it remains the same basic structure. Whereas true intelligence produces evolving and improving technologies. Thus primitive humans learn how to control fire. Then they make it portable. They start using tools. That this occurs so late causes Lee to give the fi factor a value of 0.0002, derived by dividing the median duration homo erectus has been around, roughly one million years, by the age of our planet. That’s but a sliver in Earth’s geology. We can reasonably argue that intelligence is circumstantial and fortuitous.
And what about intelligence developing into a technological civilization? This one is likewise interesting, and I like Lee’s idea of pegging 1865 as the time when humans became capable of electromagnetic communications because of James Clerk Maxwell’s equations. Thus we go from intelligence to communicative technologies and SETI possibilities in roughly a million years, from homo erectus to Maxwell.
But that’s just Earth’s datapoint. What about ocean worlds, or life forms in gas giant atmospheres or in heavy gravity surface environments where attaining orbit is itself problematic and perhaps the stars are rarely visible? Intelligence may develop without producing advanced civilizations that can become SETI targets. Here we have to get arbitrary, but Lee’s choice for the fraction of intelligent life turning into communicating civilizations is 0.1. It happens, in other words, just one time in ten. He’s being deliberately conservative about this, and the result has a lot of play in the equation.
Take a look at the rest of Lee’s values, based on all the work on these matters since the Drake Equation was conceived. Here’s his summary slide of the current outlook. I won’t run through each of the factors because we’re making progress on refining our numbers for those on the left side of the equation, whereas for these last two points we’re still extrapolating from a serious shortage of data. And that is certainly true in the last factor, which is L.
Obviously how we evaluate the longevity of a civilization determines the outcome, for if it is common for advanced technological cultures to destroy themselves, then we could be looking at a galaxy full of ruins rather than one with a flourishing network of intelligence. Our own threats are obvious enough: nuclear war, pandemics, runaway AI or nanotech, the ‘democratizing’ of potentially lethal technologies and more. Speculation on this matter runs the gamut, from a lifetime of 100 years to over a million, but historically human cultures last somewhere between 500 and 5000 years.
Is the anthropocene to be little more than a thin layer of rust in the geological strata? Lee sees 10,000 years for the lifetime of a civilization as a generous estimate, given that we are global and have more than the capability of self-annihilation. If this estimate is correct, we arrive at the conclusion shown in the slide above: The number of civilizations in our galaxy most likely equals one. And that would be us.
I was once asked in the question session after a talk how many civilizations I thought were in the Milky Way right now. I remember hedging my bets by referring to everybody else’s estimates – the 1961 estimate from the Green Bank meeting had ranged from 1,000 to 100,000,000, whereas one recent paper pegged the number at 30. But my interlocutor pressed me: What was my estimate? My conservative nature came to the fore, and I heard myself answering: “Between 1 and 10.” That’s still my estimate, but as I told my audience then, it’s the hunch of a writer, not the conclusion of a scientist, so take it for what it is.
Now I find Lee reaching the same conclusion, but two things about this stick out. If a single civilization did somehow get past what Robin Hanson calls ‘the great filter’ to emerge as a star-faring species living on many worlds, then their presence could still make all our talk of an Encyclopedia Galactica relevant. We might one day find that there is indeed a thriving network of intelligence, but one based around the work of a single Ur-civilization whose works we have better learn about and emulate. It’s a pleasing thought that such a civilization might be biological as well as machine-based, but all bets are off.
The other point, and this is one Lee makes in his talk: If life is common but technological civilizations are rare, that still leaves room for a value for N that takes in not just the Milky Way but the entire universe. N=1 means that the visible universe should contain 1011 civilizations, a satisfyingly large number and one that keeps our SETI hopes alive. We had better, in this case, concentrate our attention on nearby galaxies to have the greatest chance of success. There are 60 of them within 2 million light years, and over a thousand within 33 million light years. M31, the Andromeda galaxy, may deserve more SETI attention than it has been getting.