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.

In the ancient past, civilization arose around long rivers that took weeks to travel down,

but in the future we might build civilizations around rivers that were billions of miles long.

Ancient Egypt is a fascinating period in place in human history, covering a vast stretch

of time and containing immense structures like the pyramids, and yet as big as the modern

nation of Egypt looks on a map, and often might have on an ancient map too, it was a region

of civilization almost entirely dominated by the Nile

River, and as a result was a skinny realm 500 miles long, but only 5 miles wide. It is not alone in that

regard either. We see that with other rivers or coastlines, and indeed even with modern roads

and freeways in the

U.S., where people will often feel like they're driving by endless houses and commercial

centers in some eternal suburban sprawl without realizing that a thousand feet from that road,

the buildings and concrete and landscaping shift into miles of forest or swamp.

In today's Nebula exclusive, we're going to be exploring the Topopolis megastructure,

one of the biggest buildable under known science and technology, and which has an interesting

feature in that it can only be so wide in diameter, but can be any length you wish, indeed

billions of miles long if you desire,

as a result, you get these long, thin habitats, and it would seem almost inevitable you would

...