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 ad-free, plus hours of bonus content,

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

Perhaps one day our descendants will awaken beneath another sun, not as ghosts of a frozen age,

but as dreamers who carried humanity's dawn into the next.

Sleep has always been one of humanity's great mysteries, a daily surrender to stillness that

somehow restores us, a voyage through dreams that resets our minds for the waking world.

One wonders why evolution permitted such a protracted vulnerable period every day, and I

could imagine seeking technological genetic workarounds to cut down on sleep without other losses,

and yet in the far future, it may become the key to crossing the gulf between the stars.

If we cannot outrun time, perhaps we can sleep through it.

Space, for all its wonders, is not friendly to the impatient.

Even our fastest robotic probe crawls along at a fraction of percent of light speed.

For a human crew, that means months or even years just to reach Mars,

decades to reach the outer planets, and centuries to reach even the nearest stars.

You can build bigger engines, burn harder fuel, or try to stretch physics to its limits.

But the numbers are merciless.

When you start measuring journeys and lifetimes, you begin to ask whether the travelers themselves

should wait out the voyage in another way entirely. That's where the dream of the cryogenic arc

begins, a ship that doesn't so much travel through space as it drifts through time, carrying its crew

...