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 ad-free, plus hours of bonus content,

check out go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

Gravity is a fascinating topic, and one vital to life, but you should never put your

trust in gravity because it will always let you down.

Earth is huge and we are but tiny dots on it, and yet it is but a pale blue dot compared to our galaxy, which in turn is but a speck compared to the universe, which likewise

might be only a small corner of the wider cosmos and reality.

So it tends to be almost inconceivable that this universe is empty of life besides us.

Whether as a product of evolution or some grand design, it just feels like our solitary

existence would be improbable.

This is the core concept behind the Fermi paradox, the big question of where

all the aliens are. But when we say that, what we really mean is intelligent aliens.

There could be alien life as close as Mars or Europa. Yet without a clever brain and the

will to do so, that life can send us no radio signals,

not launch any spaceship to escape from their homeworld's gravity well.

And this will be our topic for today, gravity, and it's one that factors in at so many levels

of the Fermi paradox.

From the formation of those galaxies to the lifetime of stars, to if a

planet can keep an atmosphere, to avoid keeping so much sea and sky that complex life can't

emerge. If there is no land for photosynthetic organisms to evolve on, as it's all buried under

water and the air is too thick for enough light to get through, then no

...