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CellBioGuy comments on The Octopus, the Dolphin and Us: a Great Filter tale - Less Wrong
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I don't think filters have to be sequential - some could be alternatives to each other, and they might interact. Consider the following.
Each supernova sterilizes everything for several lightyears around them. This galaxy has three supernovas per century, and it used to have more. Earth has gone unsterilized for 3.6 billion years, i.e. each of the last (very roughly) 100 million supernovas was far enough away to not kill it.
That's easy to do for a planet somewhere on the outer rim, but the ones out there seem to lack heavy elements. If single-celled, mullti-celled, even intelligent life was easy given a couple billion years of evolution, you still couldn't go to space on a periodic table that didn't contain any metals.
So planets in areas with lots of supernova activity (i.e. high density of stars) could simply never have enough time between sterilizations to achieve spacefaring civilization, while planets in areas with low density of stars/supernovas haven't accumulated enough heavy elements to build industry and spaceships. Neither effect prohibits everything, but together they're a great filter.
There could be other combinations of prohibitive factors, where passing one makes passing the other more difficult. Maybe you need to be a carnivore in order to evolve theory of mind, but you also need to be a herbivore in order to evolve agriculture and exponential food surplus. Or maybe you need tectonic plates to avoid stratification of elements, but you also need a very stable orbit around your star, and those two conditions usually rule our each other. I don't know. It just seems that a practically linear model of sequential filters, where filters basically don't interact with each other, is entirely too simplistic to merit confidence.
In a few years, we'll have a much clearer picture of the chemical makeup of the closest few hundred exoplanets, and that'll cut down the number of possible explanations of Fermi's Paradox to a maybe sort of manageable size. Until then, this discussion is unlikely to lead anywhere.
Really-quick-and-dirty calculation time!
Let's say 3 supernovas per century and each sterilizing 10 light years in radius.
That produces an average sterilization volume of about ten cubic light years per year. Total volume of the galactic thin disc is on the order of 2*10^13 cubic light years. That produces a half life of sterilization on the order of trillions of years, though you can bring it down to billions if you increase the supernova rate by a factor of a thousand or increase sterilization radius out to 100+ light years.
We can probably discount the galactic core for any purposes though - I've seen fun papers proposing evidence that it undergoes periodic starbursts every few tens of millions of years and the galactic supernova rate then briefly goes up to something like one per year with most of them in the core.
Thanks, but it appears we're both wrong. Here is a nice intro article that gives proper numbers on this very subject and concludes supernovae aren't a life-forbidding problem even in the galactic center.
But high density of stars might lead to planetary orbit perturbations which could be. It appears the galaxy is a bit complicated.