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.

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We've examined before how a civilization might build undetectable stealthy spaceships,

but what if that entire civilization wish to conceal itself?

So two episodes back we were looking at stealth spaceships and the problems with them,

we saw how nigh impossible it is to hide a spaceship from a civilization, so hiding

entire civilization would presumably be much harder, especially one with many planets.

But today we're going to look at why a civilization might try to hide itself and how this might

be done, and ultimately if this might be a valid solution to the Fermi paradox, the big question

of why in the universe so ancient and immense, we don't yet see any alien civilizations.

Hiding a spaceship differs from hiding a planet in much the same way hiding a person

inside a city varies from trying to hide that entire city.

It's complicated in astronomical scales and in part because astronomy is history and the

universe is so old, a civilization hides, but the light had admitted a century ago

just reached their nearest neighbors, who may have been a mighty empire for a million years,

and knew your planet was around and peopled with clever apes for all that time. So it is essentially

hiding a city everyone already knows exists, not building a new one in secret. What would you do

...