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 add free, plus hours of bonus content,

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

We sometimes consider that life might be so rare that we have the whole galaxy of trillions

of worlds all to ourselves, and yet the cosmos is made of billions and billions of galaxies,

so aliens would surely still be common on the grander scale.

But what if humanity is truly alone in the universe?

From the earliest days of the show, we've talked about humanity's future in a way that

tend to assume we have the galaxy to ourselves, and we've often discussed a future in which

aliens are a rarity, and maybe so rare we might claim this whole galaxy and built telescopes

fueled by entire stars before we even spotted the signs of a distant civilization

a billion light years away.

This is a concept that, like colonizing the moon or Mars, is so critical to the show's discussions

that it probably needs updated, but beyond that, it's a giant topic no single episode could ever

cover with lots of implications.

Indeed, I never have covered the case where humanity is genuinely the first civilization

in the universe, period, not just our local sector.

That is a very different case than aliens just being rare in terms of how civilization is

going to act and view things if it gets out on

the galactic scene and just never sees anyone else.

So I thought today we would explore the reasoning for us being ultra rare or even unique, along

...