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.

What if the rarest thing in the universe isn't life, but a mind that can wonder where it came from?

The great filter hypothesis suggests that somewhere along the road from lifeless rock to star-faring civilization,

there are one or more steps so unlikely that almost nothing makes it through.

Most of these aren't about alien empires falling,

about life never reaching the point where empires are possible at all.

Somewhere, a set of necessary conditions or a chain of evolutionary hurdles,

from a stable, habitable planet or the origin of cells to the rise of intelligence, quietly

stops billions of worlds in their tracks. Today, we'll be counting down the ten best candidates

for those barriers, looking not for doomsday events, but for the improbable twists of biology,

chemistry, and planetary history, then make a thinking mind one of the universe's

rarest treasures.

The Frimmy paradox asks a haunting question, if the galaxy is old and vast, and the ingredients

for life are everywhere.

Why don't we see anyone else anywhere?

And this is the key piece.

The Frumi paradox seeks to explain not why life is rare,

or while intelligence is rare, but specifically why we, at our current level of technological

ability, do not see or hear from what we call loud aliens. Loud aliens leave a very

...