> assumption that life is rare in the universe

Great filters start with the observation that we have detected no signs of alien technological civilization. The assumption is this means they’re rare.

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withinboredom 2 days ago | parent | next [-]

That doesn’t mean much. If it is actually common and the great filter is passing technological adolescence, then at least one civilisation would need to be around at least a hundred thousand years ago to be detected by us. Then, they would need to survive the great filter. We’ve only been broadcasting for 80 years or so, and any modern technology is probably indistinguishable from noise...

In other words, even if the average technological civilisation lasts 1000 years, the odds of those civilisations overlapping are nearly zero if the great filter is ahead of us. Unless civilisations manage to last much, much longer than 1,000 years (millions of years), the chance that two blips overlap in time and space closely enough to detect each other is basically negligible.

That is why the “life on Mars” point feels ominous:

- If abiogenesis is easy, the filter isn’t there.

- If the filter is later (like surviving technological adolescence), then most civilisations blink out quickly.

Which means overlapping, detectable civilisations would be vanishingly rare, explaining the silence, but also suggesting our future may be short.

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pantalaimon 2 days ago | parent [-]

Life is pretty robust, civilization is not. I'd say it's pretty intuitive that the filter is ahead of us - civilizations collapsing has happened multiple times already in the last few millennia. Earth becoming sterile has never happened as far as we know.

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withinboredom 2 days ago | parent [-]

Yes, but a civilization collapsing doesn’t kill off an entire species nor (usually) wipe out knowledge completely. Thus it wouldn’t be a great filter. However, perhaps it is pretty common to create a grey goo, or nuke yourself, or whatever. That would be a life ending event.

	▲	pantalaimon a day ago \| parent [-]
		It's still about what we are able to observe. If there is a bronze age civilization in the vicinity of betelgeuse, how would we tell? You'd need a civilization advanced enough to do radio communication - and that has a lot of fragile interdependencies.

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harshalizee 2 days ago | parent | prev [-]

Or just really hard to detect with existing technology. Combine that with the vastness of the universe, it's not a unreasonable take.