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.

The universe is vast, ancient, and full of mysteries, but is it also home to creatures the size of mountains,

drifting through the void, or lurking in alien seas?

Science fiction has long imagined titanic beings, from Star Wars space slugs to Dune's mighty sandworms, and even legendary kaiju like Godzilla. While these stories often

stretch the limits of biology, they raise an intriguing question. Could truly enormous organisms

exist somewhere in the cosmos? Not just in fiction,

but in fact. And if so, what environments might allow such giants to thrive? It's a fascinating

topic in science, not just science fiction, and one we've explored before in episodes

like space whales, void ecology, and Giant Space Monsters.

But today we're going even bigger, because when it comes to life's potential in the universe,

size might be limited only by imagination or physics, and even those limits might fall away

in other universes with different physical constants, where creatures could exist with stars for hearts,

or entire galaxies as living, breathing organisms. So grab a drink and a snack, give the

like and subscribe button a smack, and let's dive head first into the colossal world of

cosmic megafauna and megaflora. Before we can imagine massive life forms

swimming through alien oceans or drifting between the stars, we need to confront one of biology's

most stubborn challenges, the so-called cube square law. As creatures grow larger, their volume,

and thus their mass, increases far faster than the strength of their bones

or muscles. That's because volume scales with a cube of size, while cross-sectional strength only

...