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.

Amid the swirling vastness of galaxies, each a cosmic city of stars and planets, the silence

raises a compelling question.

Could the very nature of galaxies themselves hold the key to why intelligent life remains unseen.

The Fermi paradox explores a profound question.

If the universe is so vast and old, filled with countless stars and potentially habitable planets, why haven't

we encountered any signs of intelligent extraterrestrial life?

Or as Enrico Fermi famously wondered, where is everybody?

This was back in 1950 and spoked debates and scientific inquiries into the appearance

of the cosmos.

Given the sheer probability of other civilizations arising in our galaxy alone, the lack of contact

or observable evidence presents a puzzling mystery that challenges our understanding of life's

prevalence and development in the universe. And we've considered a number of explanations in various episodes over the years.

One possible explanation lies in the concept of filters, significant barriers that prevent life

from reaching advanced stages of development, or in some cases, from arising at all.

These filters could act on a planetary, solar system, or even galactic scale, wiping out,

or severely limiting life's chances long before it reaches the capacity for interstellar communication

with us.

Filters may include natural cataclysms, evolutionary constraints,

...