Adam Frank: Alien Civilizations and the Search for Extraterrestrial Life | Lex Fridman Podcast #455

... if we don't ask how long they last, but instead ask what's the probability that there have been any civilizations at all, no matter how long they lasted. And I'm not asking whether they exist now or not, I'm just asking in general, um, about probabilities to make a technological civilization anywhere and at any time in the history of the universe. And that we were able to constrain. And so what we found was basically, uh, that the, there have been 10 billion trillion habitable zone planets in the universe. And what that means is, that are, those are 10 billion trillion experiments that have been run. Um, and the only way that we're the only time that this is, you know, this whole process from bi-, you know, uh, abiogenesis to a civilization has occurred, is if every one of those experiments failed, right? So therefore, you could put a, a probability... You could just... We called it the pessimism line, right? We don't really know what nature sets for the probability of making intelligent civilizations, right? But we could set a limit to using this. We could say, look, as... If the probability per habitable zone planet is less than 10 to the minus 22, one in 10 billion trillion, then yeah, we're alone. If it's anywhere larger than that, then they're, we're not the first. It's happened somewhere else. And to me that was an anno-, that was mind-blowing. Doesn't tell me there's anybody nearby, the galaxy could be sterile. It just told me that, like, you know, uh, unless nature's really against, has some bias against civilizations, we're not the first time this has happened. This has happened elsewhere over the course of cosmic history.

The following is a conversation with Adam Frank, an astrophysicist interested in the evolution of star systems and the search for alien civilizations in our universe. This is the Lex Fridman Podcast. To support it, please check out our sponsors in the description. And now, dear friends, here's Adam Frank. You wrote a book about aliens. So the big question, how many alien civilizations are out there?

Yeah, that's the question, right? The amazing thing is that after two and a half millennia of, you know, people yelling at each other or setting each other on fire occasionally over the answer, we now actually have the capacity to answer that question. So in the next 10, 20, 30 years, we're gonna have data relevant to the answer to that question. We're gonna have hard data finally that will one way or the other, you know, even if we don't find anything immediately, we will have gone through a number of planets, we'll be able to start putting limits on how common life is. Uh, the one answer I can tell you, uh, which is, was an important part of the problem is how many planets are there, right? And just like people have been arguing about the, uh, existence of life elsewhere for 2,500 years, people have been arguing about planets for the exact same amount of time, right? You can see Aristotle yelling at Democritus about this. You know, you can see they had very wildly different opinions about how common planets were gonna be and how unique Earth was. And that question got answered, right? Which is pretty remarkable that, uh, in a lifetime you can have a 2,500-year-old question. The answer is they're everywhere. There are planets everywhere. And it was possible that, uh, planets were really rare. We didn't really understand how planets formed, and so if you go back to say the turn of the 20th century, uh, there was a theory that said planets formed when two stars passed by each other closely, and then material was gravitationally squeezed out. In which case, those kinds of, uh, collisions are so rare that you would expect one in a trillion stars to have planets. Instead, every star in the night sky has planets.