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 brought to you by Raycon.

In the future, a technologically advanced humanity might find it has sucked its world dry

and turned his eyes to other tempting worlds.

But can there be any world more tempting than a past incarnation of our own planet?

And if so, might we invade and conquer our own past?

Probably one of the most common themes in science fiction is time travel, and even outside

that genre it is pretty common in fiction.

Folks asking what if, like what if I'd done things differently, or what if this hadn't

happened or this had, isn't just a popular storyline but a habit we all engage in both after

a situation when we ruminate on alternative outcomes and when trying to make

decisions in the first place, where we seek to predict plausible outcomes.

Now for my part I'm very dubious about concepts like time travel and alternate timelines.

They've been popularized because they're great for fiction rather than any strong evidence

they can or do occur.

However, our knowledge of time is still almost entirely theoretical, so today we will

be discussing both why the possibility of time travel or alternate timelines is dubious,

but also how we might use time travel or alternate timelines if we could make it work.

There's also the question of how wars conducted through time might go.

...