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

we're asked at 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.

One day our civilization may settle our whole galaxy, with mighty star empires consisting of millions,

if not billions of worlds.

But what would the centers of such empires be like?

No. Way back in Isaac Asma's Epic Foundation series, we encounter the planet of Trantor, capital of the Galactic Empire of 25 million planets, and home itself to a planet-wide metropolis

of 40 billion people.

Ever since I first read about it, the idea was captivating to me, but so was some of the problems.

Today, as we look at what the capitals of great cosmos spanning nations might be like,

I thought we might start by considering Asma's classic example.

It is an immense discussion appropriate for such immense empires, so we'll be here for a bit,

and you might want to grab a drink and a snack, and don't forget to hit those like and

subscribe buttons if you enjoy learning science, along with a fun dose of science fiction

and imagination fuel. Trantor's 40 billion people are on a planet described as being all towers and buildings across the whole planet.

As we note in other episodes like Hive Worlds, it is a very lowball figure for such a planet-wide city or Yucaminopolis.

And we see better but still low estimates in other fiction like Star Wars, where they pushed

up to more like a trillion, but the density of the population isn't the key issue there.

Let's imagine a population of 40 billion implies 25 billion people in the workforce, which

is a bit high, for instance the US runs at 50% of the

population in the workforce, give or take. Many others being children, retired, stay-at-home

...