Hello, SFIA audio listeners. In this month's Nebula exclusive, big alien theory,

we're asked at 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.

It is amazing what we've accomplished with rockets, but to bring people to space cheaply,

we need a highway to the heavens, and recent progress on space elevators may finally be bringing them to a future near you.

Space elevators are a topic that needs no introduction.

As we all know, they're an immensely strong rope down from orbit that you can climb up to

space on, but there's a lot of details that tend not to get covered, like how you actually

build one, how it can get used for traveling to other planets, or even

moving around ours faster, does it actually have to be built at the equator?

Do we have materials besides graphene to build it from?

How big is the elevator?

How massive is it?

What happens if it falls down?

How big are the climber cars that go up it?

How many people can it carry at once? And of course, the answer depends not only on a lot of

the technical specifics, we won't be able to narrow down till we build some prototypes,

but also on how big an elevator you want to build, one twice as thick can carry twice the

cargo, assuming you're made of the same substance. And we'll be talking about those possible

substances, particularly carbon nanotubes, graphene, and hexagonal boron nitride as we go,

including the idea that we might make a tether that was graphing on the inside and boron

...