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.

This episode is sponsored by Brilliant.

Joffrey Chaucer said that all good things must come to an end, but this channel is not the

place for that kind of pessimism.

So today we'll be discussing ways we might postpone the heat death of the universe, and possibly

even delayed entirely. To do that, we must first discuss what the heat death of the universe, and possibly even delay it entirely.

To do that, we must first discuss what the heat death is and why we think it's going to

happen. The best place to begin is to note that there's a bit of a difference between what we

normally think of as heat and what the term means in physics. Heat, to us, in a casual and day-to-day sense, is how warm it is,

but in a physics and thermodynamics sense, it refers more to how entropic a system is,

essentially what portion of its total energy is heat energy. Heat is random motion of particles,

and we can only get work done using such a system if it's got

other energy still stored in it in other ways, or if we connect it to something colder.

As an example, the sun is very hot, so we can easily get work done off it by connecting

to something cooler, which we obviously have plenty of choices for.

It is making that heat by converting hydrogen

into helium, photons, and neutrinos, converting mass energy into heat energy in the process.

A system is at maximum entropy when all those options have been used up and everything is

converted to heat that can be, hence heat death. It's a bit confusing though since we'd expect this to happen nowadays when it's rather cold,

...