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 add-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 starfaring civilization,

there are one or more steps so unlikely that almost nothing

makes it through. Most of these aren't about alien empires falling, they're about life never

reaching the point where empires are possible at all. Somewhere, instead 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 10 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 Fermi paradox seeks to explain not why life is rare, or why 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 detectable presence.

By being big, visible, and long-lasting, they are all three.

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