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.

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In some of our greatest wars on this world, we have seen entire regions covered in fortifications,

but in great wars across whole regions of our galaxy, could we see entire planets fortified?

One of the most common themes we have on this show is that a future out in the galaxy

isn't one where most people live on planets, but rather in space habitats or deep inside

asteroids or small moon's.

While I was pondering the topic for today of Fortress Worlds, which is an informal sequel

to our Hive Worlds and Forge Worlds episodes, I found myself

confronting some of classic flaws of science fiction. We have two key ones to begin with, and the

first is that in sci-fi, we have these big galaxy-wide wars, in which a planet is a mere

minor battlefield, and is taken by a few thousand troops in a few days.

In reality, it would seem like a battle for a planet should require millions of troops,

potentially billions, and might take years or even decades.

The two World Wars of last century give us some context.

World War I was mostly confined to Europe and the neighboring regions of Africa and Asia,

...