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.

One of the worst things about becoming a physicist after growing up on science fiction

was finding out that traveling faster than light speed is impossible, and not from

lack of imagination or by casual hyperbole, just not allowed under unknown physics and at

best not technically banned in certain theoretical math.

To make it worse, though, is when you find out that even going close to light speed is

very nearly impossible, so it's not just that getting to someplace like Alpha Centauri,

or nearest neighboring star, might take four years instead of four days, or even four hours

like it might in sci-fi, but that getting

there even in 40 years requires optimistic outcomes for technologies we don't have yet, like fusion.

Chemical rockets combined with gravitational assist can offer us almost 0.1% of light speed,

making a journey in 4,000 years instead of 4. And we have some options

for ion drive or nuclear proportion that could push that to 1%, maybe in a few percent

percent for something like the Orion Drive that drives itself along by blowing nuclear bombs

out behind it. Fusion might get you to 10% or even a bit higher, but nowhere near 50%.

Last month I did an impromptu episode on Bustod Ram's Spaceship Drives,

and it got me thinking we should discuss what options actually let you get close to light speed,

at least better than 80%, and some that can potentially get to 99% or better.

So I thought we would cover them briefly in today's Nebula exclusive.

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