Hello, SFIA audio listeners. In this month's Nebula exclusive, Big Alien Theory,

we're asking the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and ad-free, plus hours of bonus content,

check out go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

Welcome to the SFIA Alien Civilization series.

This one began as a spin-off of our looks at the Fermi Paradox, the big question of where all

the aliens seem to be hiding, as I've often noted that where Fermi Paradox solutions go,

you basically always have two fundamental camps.

The first being reasons why intelligent life might be so rare or short existing that we

just hadn't seen them yet, is more the focus of our Fermi Paradox Great Filter series.

This essentially places the reasoning on more physical causes like biology and physics,

and asks what might limit

how many worlds could have life or what barriers the evolutionary pathway to intelligence

might face, or what sort of weapons or space engines might cause us to blow ourselves up

or be unable to colonize space.

But the second, that alien life might be plentiful but we just don't see or recognizing

them for some reason, generally revolves more around the psychological, and we often turn

to examples of behaviors from aliens in science fiction define answers.

I'd observed before that most of these motivations really didn't make much sense on inspection, and that

many honestly seemed downright stupid, hence our first episode, stupid aliens, which was also

our first episode of 2017.

I followed that up the next week with an episode looking at first contact scenarios with aliens,

...