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.

We often imagine a future of vast gladdic empires,

civilizations stretching across the stars,

uniting thousands of worlds under a common banner.

What do they rise and then fall?

And what does that mean for the question

of whether we are alone in the universe?

Thank you. And what does that mean for the question of whether we are alone in the universe? As you might guess from today's title, Twilight Imperium, we will be exploring the idea of civilizations in decline,

not just human empires of the past, but far futuregalactic societies collapsing to ruin. The theme shows up everywhere

in science fiction, in Asmos Foundation, in Dune, in Warhammer 40,000, and in dozens of other

franchises like Babylon 5, Stargate, Mass Effect, and the Culture Series by E&M Banks, or the board

game Twilight Imperium, that's a personal favorite

and inspire the episode's name.

We'll be examining many of these in detail today, using them as case studies, not to critique

their stories. They didn't make it out on my list without being good ones, but to ask whether

this idea of Civilizational ebb actually

makes sense as an explanation for the great silence of the cosmos, the so-called Fermi

paradox, that is, could galactic empires be common, but all pass their prime or dead?

Before we explore those fictional worlds, we need to take a step back and look at our own.

...