An updated (and serious) estimate on the Drake equation, which estimates the number of alien civilizations in the Milky Way, says it’s complicated, and uncertain, and highly sensitive to input assumptions…and a possible answer is 36. This kind of sounds like a lot to me, but the article says that given the size of the galaxy, and how far the 36 would be from each other on average, it could explain why we haven’t been able to detect anyone else so far. An important variable in the Drake equation is how long advanced civilizations tend to last. “Advanced” is defined as having invented the radio, because this should make them possible to detect by other civilizations that have also invented the radio. But the theory goes that once the radio is invented, the ball also starts rolling on potentially civilization-ending and ecosystem-killing technologies. There could be many civilizations that come and go, but only a few around at any given time.