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.

This episode is brought to you by Hello Fresh.

We always assume alien civilizations simply grow with time, sprawling out across the galaxy,

or else shrink and wither away, but what if they shrink in size, not in stature.

So today we are returning to the topic of the Fermi paradox, the problem of rectifying

the apparent absence of alien civilizations with the universe so ancient and enormous,

it seems like it should be teeming with a trillion interstellar empires.

And one of the key concepts to the paradox is the idea

that colonizing other worlds and star systems should be possible and practical, and something

that appeals to many species who have the same basic imperatives that we have and that life on

Earth generally has to expand and, and explore, and grow.

So the paradox always comes down to trying to account for why this has not happened for others,

which might involve explaining why life is rare, or acknowledging that it might be out there,

but we've somehow missed detecting it.

Generally the reasons why are that we would expect intelligent life to be

rare or short in duration, which falls under the great filters, or else that they are not rare,

but are hard to detect, which falls to SETI, the astronomical search for extraterrestrial

intelligence, and finally why it might hide from us or ignore us,

and regardless of their subcategories, most of our answers in this case end up being somewhere

...