I’ve always heard considerations of scale used in favor of there being other intelligent life in our galaxy – There are so-and-so many suns with so-and-so many planets, many of which have the same conditions as our planet when life arose here. Surely there must be lots of other life and surely there must be intelligent life since it would be a coincidence for them to have had less evolution time than us relative to the age of the galaxy (assuming evolution has a tendency to beget intelligence).
It was interesting to hear Creon Levit mention a scale argument for the opposite conclusion on a Neo Files podcast. It goes like this: If there are many such intelligent civilizations, more advanced than us, then they probably have the ability to create self-replicating machines that can be sent out to colonize a suitable planet and prepare it for their more fragile creators. These machines could be programmed to launch identical probes to other planets. And the replicated machines in those probes could do the same. And so on. You get an exponential processs that could spread through the whole galaxy in a relatively short amount of time.
So where are they?