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 add free, plus hours of bonus content,

check out go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

One of the most terrifying things about our universe is the idea that it is 14 billion years old,

home to billions and billions of planets, and that, as best as we can tell, every single one of them is dead.

One of our favorite topics on this show is the Fermi Paradox, the big question of where

all the aliens are, and to discuss those hypothetical alien civilizations. By default, I work from the assumption

that those clever aliens simply have yet to come into existence, that life itself may or may

not be rare in this galaxy, but if it is not, then it's probably relatively simple creatures,

or even a whole galaxy of lichen-covered rocks

and oceanic algae.

The alternative is that intelligent aliens either never get past where we are now with their

space technology, or blow themselves as smithereens, and some way that allows no continuity

of intelligence, such as if we got murdered off by AI,

which then went on to colonize the galaxy in our stead.

While the amount of heavier elements and their proportion in protoplanetary disks has grown

with time, we believe rocky planets could have been forming at least as far back as 10 billion years

ago, meaning we should have seen alien life arising since then.

If we assume we are a fairly mediocre example of evolutionary timelines and that perhaps

one in 1,000 star systems holds a planet on which complex life could arise, and that such

life tends to move

...