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.

Our galaxy is beyond immense, containing more stars than every human who has ever lived,

and even more planets than that, and even more planets than that,

and yet it is but a tiny dot compared to the universe,

and to reach that vastly larger realm, we will need to

contemplate voyages between the galaxies themselves.

Prior to about a century ago, we didn't really know what the difference between the galaxy

and the universe was, that our observable universe is something like a million times wider

and a billion

billion times greater in value.

It is the difference between a room and a building and our entire planet.

However, in terms of space and civilizations, we often still tend to use the two interchangeably,

talking about getting out into the galaxy and colonizing it.

I think that in the past it was

just assumed the galaxy all its own was so huge we would never colonize at all, and that it

probably had at least a handful of other civilizations we would need to share it with.

So even greater journeys to other galaxies that probably already were populated by aliens

didn't seem worthwhile to contemplate.

Nonetheless, there's a very good chance this galaxy contains no other intelligent life,

...