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.

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plus hours of bonus content,

check out go.nebola.tv slash Isaac Arthur,

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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?

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 Fermi 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

...