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 add-free, plus hours of bonus content,

check out go.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

If we want to outlast the stars, we will scatter like seeds on the cosmic wind,

and so our survival lies not in one lone arc, but entire fleets to settle the billions and

billions of worlds awaiting us.

For all our technology, for all our history, humanity remains tied to a single world.

Earth is our cradle, our garden, our fortress, our pale blue dot, but it is also our cage.

So as we remain bound to one planet, we are only a single catastrophe away from extinction,

and the goal is not just to survive, but to thrive across cosmic timescales, then no single colony, no single

arc will ever be enough. True survival means dispersal. It means not alone ship carrying

all our hopes, but fleets, swarms, branching out across the stars.

Why arc swarms, the survival imperative?

Every civilization, every species, eventually faces its doomsday clock.

Some threats are of own making, nuclear wars, engineered pandemics, runaway artificial intelligence,

ecological collapse. Others are written in the universe itself. Asteroid strikes,

supervolcanoes, nearby supernovae, gamma-ray bursts, or simply the slow wearing away of

habitability as the Earth changes on the march of geological and cosmic time.

It doesn't take imagination to see how fragile our position is. A rock only a few kilometers

wide could undo centuries of progress in a single strike. A starquake or rogue planet

could warp through our neighborhood with no warning. and if we dodge every bullet, Earth itself

...