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.

This episode is sponsored by Audub.

It's possible that the reason we appear to be alone in this galaxy is because we're the

first life forms to emerge here, but it's also possible that we're the first to re-emerge here.

So today we are returning to our Fermi Paradox series to consider the idea of galactic disasters,

natural or artificial events that might prevent life from developing or wipe out civilizations.

If you are new to the Fermi paradox, it is essentially the notion that the universe is a vast and ancient place,

probably containing billions of billions of planets with conditions

parallel to early Earth, and thus we would expect to see a ton of intelligent life out

there, but we don't seem to see any.

This apparent paradox has many proposed solutions, and the one we'll examine is that something

or someone might have cured off life in our galaxy at some point in the past, and that Earth is

the first or among the first places where life has returned. So we'll be considering a number

of potential galactic disasters today, and also some extra-galactic, universal, and even multiverse

disasters. Let's discuss first, what would it take to sterilize just a planet of our life?

The most heat-resistant known life forms on Earth are microbes that can survive at 130

centigrade for no more than a couple of hours, but it's still rather tricky because there

are microbes living down the deep ocean or caves where they might survive our whole planet's

surface being scorched sterile.

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