Technology is the world where everyone in today’s world can be related to something exciting.. We also hadn’t really locked on the Age of the Universe by then, the Big Bang model had only been proposed a generation before, and I’m not even sure if we had switched to calling it the Big Bang yet.
It was originally used as a derogatory reference to the theory - we may have still been calling it the Primeval Atom Hypothesis, the original name proposed for the Big Bang. Even with the original variables, we can apply some knowledge from today: very few of the stars which have formed, and which could plausibly have had a life like ours evolve around them, are likely to have expired by the present day.
The big giant stars that represent the lion share of visibly dying stars don’t stick around long enough for the likely evolution of complex life, and very nearly every star has planets, so we can probably safely just assume there are at least 100 billion stars in this galaxy with at least one planet, probably more. There are likely trillions of planets.
The rate of formation has more to do with the assumption that civilizations pop up on a life-bearing planet after 4 billion years or so, then disappear at some point. I really do think that suffers from both assuming that life arrives at complex intelligence at the same rate as Earth and that civilizations inevitably wipe themselves out.
This seemed to be a very common view of scientists and academics during the days of the Cold War. I’m sure it will come as no surprise that I disagree with both of those assumptions.
The first one is overly simplistic and overlooks all of the variables involved in the story of life and civilization, as though it should be exactly the same in every case, regardless of factors such as elemental abundance, ecological relationships, and important events.
As for the second, I’d personally caution folks about being overly fatalistic or pessimistic about their fellow humans and have never thought it very likely you could have a doomsday scenario that got everybody, without replacing them with other folks of a different type. In considering the Drake equation, we have to imagine this as being almost insignificant, since it would only generate a pause in detectability.
Even if you nuke yourselves, some folks will generally survive, and nobody in the Cold War was aiming to ‘wipe out humanity as their end-goal, nor were any of our weapons capable of doing so at the time. Fallout does not last forever, and humans breed pretty quickly on galactic timelines.
And that’s a good example too, because we really didn’t replace the Neanderthals, and any artificial intelligence or other civilization replacing us is likely to be at least part-human in some fashion, even if it’s a metaphorical kinship as in the case of AI or uplifted animals, like super-intelligent humanoid cats, dogs, or dolphins.
A lot of times the future doesn’t belong to A or B, but some hybrid of both. An individual or group which replaces another generally has some superior array of traits, though that might mean they are more coordinated and generous or more hyper-aggressive or very good at being sneaky and stabbing folks in the back.
Most of these options actually make a replacement species more likely to be detected, not because they are necessarily more likely to be broadcasting hello signals into space, but because we generally see signatures of civilizations.
In general, the more successful something is, the more noticeable it is. It is often suggested that they might hide, and we’ll come back to that point later too, but I would point out for now that civilizations hiding are not a tactic that seems common in our own history.
Cities and nations don’t hide, but are far more likely to spend tons of money on various monuments and attractions to advertise themselves, and building roads and infrastructure to connect with other peoples and places. While there are numerous reasons a group may or may not advertise itself in some form or another, either directly or indirectly so, it seems fair to say that we observe this behavior as a general rule.
But with a bit of reflection, it seems like this group might be the minority. Let’s take an example: still assuming that at least 100 billion stars in our galaxy have planets, we’re left with five remaining factors to consider.
If we assume that only 10% of stars have planets, and that of those only a further 10% are potentially able to support life, and only 10% of those actually developed life at some point, and only 10% of those developed intelligent life, and finally, that only a tenth of all such civilizations ever released detectable signature into space, we would still have had 10 million such detectable civilizations throughout our galactic history.