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.nebula.tv slash Isaac Arthur and use my code, Isaac Arthur.

One day you wake up and every channel a newsfeed is a buzz with the news.

We've received an alien signal beyond any reasonable doubt.

What do we do next?

All of our discussions on the Fermi paradox on this show and by others

is predicated on the

notion that there's been no unambiguous extraterrestrial signal sent to us or overheard.

There's a lot of proposed reasons for why this should be the case, ranging from aliens

simply not existing, to them landing here regularly and us just not seeing and believing it.

One of the more popular

reasons is just that we haven't really gotten good enough at looking and haven't been looking

long enough to detect any of them by their own regular transmission, which might not even use

radio waves, or that they are signaling us but we haven't gotten good enough to recognize it,

or that they aren't signaling

us because they haven't seen us yet.

We'll be examining a few different scenarios today for what we might do if we either receive

or spot an unambiguously intelligent signal from space, but let's start with that one.

Our own radio signals have only penetrated about a light century out from Earth,

which is not even a millionth of the volume of this galaxy, and that itself would be a

...