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. This episode is sponsored by Auduble.

A human floating around outside Earth's gravity is very much a fish out of water,

but our distant ancestors were literally fish out of water, and they adapted to living on land

pretty well.

Maybe some of our descendants will adapt just as well to life without gravity.

Last February we did a well-received episode on low-gravity planets and the probable effects

on human physiology and evolution, we also talked about how close to the truth science fiction

has come in its portrayals, so taking that concept all the way to zero-g or near zero-g seemed like an appropriate

follow-up. Of course, we'll be talking mostly about man-made habitats in this episode, because

to hold an atmosphere in place requires either significant gravity or an airtight enclosure,

but we will talk about some other possibilities. And while this won't be a very heavy episode, you might

still want to get a sealed drink and a non-crumbling snack and seepelt yourself in for a while.

The terms near zero-g, micro-g, and microgravity are all interchangeable terms for the same thing.

What astronauts experience in space used to popular recall zero-g, but that was a popular

misnomer, because there's a solar system and a universe around us that makes it impossible

to get away from all gravity.

If you're in a circular orbit around the Earth, centrifugal force cancels the Earth's gravity,

but the Sun still exerts about half a thousandth of a G on you and your

spacecraft, and the Moon resorts about three millionths of a G. Your craft, crew, and cargo

...