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.

We ask whether stars are silent, most answers reach for the exotic, alien AIs,

great filters or civilizations too advanced to care about us.

But maybe the reason is much simpler.

Maybe they just ran out of gas.

When it comes to figure out why we don't seem to see any aliens in a universe so vast and ancient,

the answer is usually fall into three broad camps.

First, the intelligent life is rare for some reason.

Second, that it is not rare, but it is simply hard for us to see or hear.

And third, that neither is true, that intelligent life is

common, and some of it is even visiting Earth. There's no strong consensus or evidence

locking into any one of these particular possibilities, but Rarity tends to lead the pack,

and the various reasons for that usually far into what we call filters, things that thin out the

number of civilizations

along the path from being a microbe to someone who builds microscopes to study microbes.

That perspective is a humbling one. We are, after all, descended from those same microbes,

which remain the dominant form of life on Earth, and yet we, the most intelligent species

our world has ever produced, spent most of our history

unaware that microbes even existed. That's a useful bit of humility to bring to any discussion

...