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 add free, plus hours of bonus content,

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

We often worry that the reason we hear nothing in our search for extraterrestrial intelligence

is that to travel to other stars is just too hard.

What if a civilization decides it's just too dangerous to allow?

Last year we released our Formy Paradox Compendium, which was a staggering three-hour-long episode, and despite that we found that a lot of the entries in there had to get only surface

detailed in discussions, and in there and other episodes I suggested before that one of

the more disturbing solutions is that a civilization might not permit space colonization,

either from its own world or from any others that might arise nearby.

I've never heard this given a name before, except as a subtype of stay-at-home civilizations, and indeed we are

not actually thinking this would be a place where they just decide not to go settle the galaxy.

We could imagine tyrannic empires that fear letting colonies slip away from them in the distant

interstutter space, where they might escape their leash.

However, a point I sometimes raise in our discussion

of the Fermi paradox, the big question of why our vast and ancient universe seems empty

of other civilizations, is if you go out and colonize the galaxy long enough, you are going

to find alien civilizations, simply because every colony you plant is going to begin diverging so heavily from each other,

and you, that you'll eventually end up running into some unrecognizable distant cousin civilization.

Centuries of signal lag and travel time separating planets where colonists live under alien suns,

and strange environments is going to cause a lot of change, which is only

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