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.

Earth is immense, but we only live on a thin shell of its surface. So what if we built a planet

with only that thin shell, or with several layers of them? So today we'll be discussing

why and how you would build hollow planets, what we often

call shell worlds, as well as multi-layer concentric designs, the Matriuska shell world, and

we'll also touch on hollow moons and hollow suns.

I want to emphasize that much like when we discuss building flat-earth megastructures,

these are serious designs with a lot of logic behind them, for man-made planets humanity

might one day build, or which other civilizations that might exist elsewhere in the universe

might have built.

We are not really interested in conspiracy theories claiming our

own planet might be a hollow Earth, but as we'll see today, you could build a hollow planet

that would, convincing the full, even modern science into thinking it wasn't hollow, when it

actually was. So we can't exactly say hollow Earth is debunked, especially since, as we'll

also see today, there is very good motivation to construct hollow planets and shells, as opposed

to classically terrifying Earth-like planets. We probably should start by noting that there is

no single unified hollow Earth theory, as it describes

a lot of theories, ranging from the scientifically coherent to pseudoscientific to simply

fun plot devices for fictional stories. Those last are equally important to us, because

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