Sean Carroll in Preposterous Universe:
The possible existence of technologically advanced extraterrestrial civilizations — not just alien microbes, but cultures as advanced (or much more) than our own — is one of the most provocative questions in modern science. So provocative that it’s difficult to talk about the idea in a rational, dispassionate way; there are those who loudly insist that the probability of advanced alien cultures existing is essentially one, even without direct evidence, and others are so exhausted by overblown claims in popular media that they want to squelch any such talk. Astronomer Avi Loeb thinks we should be taking this possibility seriously, so much so that he suggested that the recent interstellar interloper `Oumuamua might be a spaceship built by aliens. That got him in a lot of trouble. We talk about the trouble, about `Oumuamua, and the attitude scientists should take toward provocative ideas.