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.

If the machines inherit the stars, will they even notice the silence left behind by those who created them?

It has been over half a century since the last moon landing, and the vast majority of people alive today were not around to witness it. Only the Apollo missions ever took humans

beyond low Earth orbit, a mere few hundred kilometers above the planet. The record for

the farthest distance traveled by humans still belongs to the fateful Apollo 13 mission, which reached 400,171

kilometers from Earth, just a fraction of the distance to even nearest planet.

Many humans venture to low orbit now, though not even a thousand souls have gone there,

and none beyond since Apollo, let alone further.

Yet while humans have not ventured further,

we have set many robotic missions beyond this range. Even beyond the many satellites in

medium or higher orbits, we have set probes and landers to distant worlds across the solar

system. I remain confident that one day we will send humans these far-off places as well.

However, many believe otherwise that those worlds will forever remain untouched by human footsteps,

explored only by the metal wheels and limbs of machines.

There are many things we do not yet know, we do not know if humanity will venture beyond

this world, if aliens have arisen on other worlds, or having arisen, if they manage to

travel to strange new worlds themselves.

This uncertainty is deeply tied to the Fermi paradox, the profound question of whether aliens

exist, where they might be, and why we haven't

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