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 ad-free, plus hours of bonus content,

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

One of the greatest cities of tomorrow had no streets, no floors, and no sky, only in the

space in every direction. Picture a home where up and down don't exist, where farms float

in the air, and rivers curl through the void. When we think of space habitats, the first image that comes to mind is usually a great

rotating wheel or cylinder, massive structures like O'Neo-cylinders or Stanford-Torai, designed

to spin and give their inhabitants the comfort of gravity underfoot.

These are iconic for good reason.

Humanity is a gravity-bound species, and zero-g has proved to be less than kind to our bodies.

Yet not all visions of the future rely on spin.

Some engineers and dreamers are managed habitats that embrace the absence of gravity, rather

than working so hard to recreate it.

The idea of a bubble habitat, a vast, pressurized

enclosure, floating free in space, goes back nearly a century. John Desmond Bernal's

1929 essay, The World, the Flesh, and the Devil, gave us by the earliest visions, a spherical

space habitat intended not just as a stationary waypoint, who's a genuine

community adrift among the stars. Unlike the rotating designs that followed, including

updates of the Bernal's sphere, to add spin gravity, but originally envisioned people adapting

to microgravity itself, enjoying homes and fields and environment, free from weather,

...