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.

Life is incredibly complicated, but for most of Earth's history, it was much simpler.

Is it possible the universe is full of planets with very simple life, and complex organisms

are rare?

Yeah. There are billions and billions of stars in this galaxy alone, and billions of galaxies with their own seas of stars,

and likely most of those stars have planets. Even if only one percent of those had a planet

reasonably akin to Earth, and in a warm orbit around its own star, there should be at least

a billion places like Earth out there where life could have arisen.

And this is the crux of the Fermi paradox, the big question about where all the alien civilizations

are, because the sheer volume of worlds out there life might have arisen on seems to demand

that examples be so numerous that surely some made it to our level or higher and went on

to settle the galaxy.

This gives us what we call the filters of the Furby Paradox, or the Great Filters.

These are hurdles on the way to that goal of being visible galaxy wide, and specifically

by us, that presumably exists between a solar system forming and the day it sends some

Kari ships to claim new worlds.

Some of those we call late filters, problems civilizations like our own might still need

to face on our road ahead, like finding out if interstellar colonization is even practical,

or avoiding accidental crises like artificial intelligence.

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