Hello, SFIA audio listeners. In this month's Nebula exclusive, big alien theory,

we're asked at 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. It can be hard to hear a signal 100 miles away,

so what does it take to make a beacon loud enough to hear 100,000 light years away?

One of the big problems we have with SETI, the search for extraterrestrial intelligence,

is that based on the civilizations off earth, it's the

kind of search that should be virtually impossible not to rapidly succeed at if there's anything

to find. The great civilizations of the world are hardly a secret. We have yet to find a civilization

that hides its existence, which we might joke means they presumably succeeded, but there weren't a bunch

tried and failing either, and you would expect if folks made such efforts, many would fail

and leave evidence their intent and existence behind, or later change their mind and become public.

However, in practice, what we see with Earth-based civilizations is the opposite of hiding,

in favor of active advertising

and announcing of their presence, frequently with giant neon signs and beacon fires.

But the dynamics do change the galactic scale, since we don't know how any given alien

would behave, and we also don't know that they have any motivation to be heard 10,000 light

years away, especially when it takes an armidirectional signal a hundred have any motivation to be heard 10,000 light years away, especially

when it takes an armory directional signal a hundred times as powerful to be heard 10 times

further away, and a million times more powerful to be heard a thousand times further away.

So you would tend to assume the signal is a civilization sends, either are low-powered local ones

or high-powered ones that are more directional,

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