I read a long time ago an essay by Asimov that described what elements could support life as a building block and why most elements cannot. I didn’t fully understand that at the time but it was interesting to see why carbon based makes sense and some other random element does not work.
Similarly, I wonder if under different planetary conditions, smart alien life (so not single cell life) can be very tiny or very large, or if there are biological constraints that would restrict that size range regardless of basic setup.
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I’m too lazy to do the math properly but the theoretical minimum size is potentially roughly whatever the [Bekenstein bound](https://en.wikipedia.org/wiki/Bekenstein_bound) for the roughly 2.5 petabytes of data in the human brain would be – that’s just the memory, not the processing, so the real size will be way greater, but should probably be somewhere around the same order of magnitude. Apparently it’s about [10^69 bits per square meter](https://www.pbs.org/wgbh/nova/article/is-information-fundamental/) so for 2.5 petabytes it would be about 10^-27 meters.
That’s really small – it is small enough that if you were the size of an atom, it would still be the size of an atom to you, except you repeat that one more time. It’s so small that we don’t know anything about what’s possible at those sizes and there might be something unknown that makes it completely impossible for computation to take place there for some reason or way weirder things we don’t know about.
You would probably not be looking at life that has evolved out of atoms but something way, way, way smaller, and as we don’t know anything about whatever particles might exist at that scale. It’s impossible to say if it would support life (presumably all you need for life is a particle system of some kind, ours just happens to be one that does support life), but if it can, there’s no fundamental reason it wouldn’t be capable of evolving to the same cognitive level as us.
It’s a weird intuition that many people seem to have that planets and stars might be analogous to atoms, while galaxies may be like small cells in some larger organism, and as such that atoms might be like small solar systems – or some variation on that same theme – it’s probably a misinterpretation of the Bohr model of atoms, but who knows, there might be something to it. There is no proof for any of this, but the atom got its name from being unsplittable, so it’s not like there are limits to our ignorance.
Something very big looking at our solar system by launching planets at it might not notice how one of the “electrons” has a moon or people on it. They just notice how, on average, stuff going in tends to come out. When 5 billion years is less than the blink of an eye and the size of the universe is microscopic, the deaths of stars might look more like chemical reactions. Somewhere in that infinitely huge world somebody might be doubting whether we could exist even in theory.
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