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.

The universe is beyond immense, and yet it might be nothing more than a tiny dot beside the rest of reality.

I'm not sure how long it was after the first telescope was invented that we began really realizing that the stars numbered not in the

thousands, but in almost uncountable quantities, billions and billions.

I can't imagine it took long before they at least wondered, though.

Certainly by Isaac Newton's time, they were already thinking it must stretch out infinitely

far in every direction, including time,

for everything not to collapse in on itself from gravity.

This does not appear to be the tale of our own universe, which is stupefying the ancient and large,

but not infinite in either regard.

It seems to have begun about 14 billion years ago, and the part we can see stretches out 47 billion

light years in every direction.

We don't think it stops there, since we can see that far in every direction out from Earth,

and we assume Earth is not the center of the universe.

But we don't know how much further it goes and have no way we can think of to

look beyond that horizon that wouldn't seem to violate some law of physics. Indeed, it might

not go any further at all. Earth might really be the center of everything. That is what the

current evidence actually shows. But we tend to assume not, and often did even before modern astronomy or Copernicus.

The Greeks were actively debating the enormity of the universe, at least as early as the 6th century

...