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'Neill cylinders or Stanford-Torri, 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 imagined habitats 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 Bernoules' 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 was a genuine community adrift among the stars. Unlike the rotating designs

that followed, including updates of the Burner's sphere, to add spin gravity,

but originally envisioned people adapting to microgravity itself, enjoying homes and fields

and environment free from and environment, free from

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