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.

I was going to start the episode with a time-travel joke, but nobody liked it.

Science tells us that every action has a cause and an effect, but could something happen

before whatever caused it? This is the core concept

behind retro causality, the notion that the cause might come after an effect, and while this

is an idea most common in sci-fi, we have some indications it can happen, at least at the

quantum level, which seems to leave the door open to it potentially

occurring at a wider scale of space and time. We recently looked at a parallel notion,

a causal events in our episode Roco's Basilisk, events that don't require time travel,

but have causes that almost seem to imply it, though it's a different scenario

than retro causality.

Amusingly, that episode is actually a sequel to today's episode, which is also not the result

of time travel hijinks.

This episode originally aired on Nebula last year, and like a lot of our Nebula originals,

I'm tweaking a bit before airing it on YouTube a year year, and like a lot of our Nebula originals, I'm tweaking a bit

before airing it on YouTube a year later, so you might notice my speech impediment is a bit

stronger after this intro, being about a year of speech therapy behind.

As with all our episodes, close captions are available.

I want to emphasize from the outset that this is not an episode that's going to answer

...