The Great Filter is the idea that, in the development of life from the earliest stages of abiogenesis to reaching the highest levels of development on the Kardashev scale, there is a barrier to development that makes detectable extraterrestrial life exceedingly rare. The Great Filter is one possible resolution of the Fermi paradox.
https://en.wikipedia.org/wiki/Great_Filter
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high likelihood of its existence. As a 2015 article put it, “If life is so easy, someone from somewhere must have come calling by now.”
https://en.wikipedia.org/wiki/Fermi_paradox
Personally I think it’s photosynthesis. Life itself developed and spread but photosynthesis started an inevitable chain of ever-greater and more-efficient life. I think a random chain of mutations that turns carbon-based proto-life into something that can harvest light energy is wildly unlikely, even after the wildly unlikely event of life beginning in the first place.
I have no data to back that up, just a guess.
This is quickly becoming beyond my knowledge pool, but does this assume that all life is intrinsically linked to oxygen?
It’s not specifically oxygen that’s linked to life, it’s chemical disequilibrium. Oxygen is highly reactive, there are lots of minerals that will bind it up and there aren’t any natural geological processes that unbind it again in significant quantities. If you put an oxygen atmosphere on a lifeless planet then pretty soon all of the oxygen will be bound up in other compounds - carbon dioxide, silicon oxides, ferric oxides, and so forth. There has to be some process that’s constantly producing oxygen in vast quantities to keep Earth’s atmosphere in the state that it’s in.
There are other chemicals that could also be taken as signs of life, depending on the conditions on a planet. Methane, for example, also has a short lifespan under Earthlike conditions. You may have seen headlines a little while back about the detection of “life signs” on Venus, in that case it was phosphine gas (PH3) that they thought they’d spotted (turns out it may have been a false alarm). These sorts of gasses can be detected in planetary atmospheres at interstellar distances, especially in the case of something like Earth where it’s quite flagrant.
Even if these are sometimes false alarms, in a “Dark Forest” scenario it’d still be worth sending a probe to go and kill whatever planets exhibit signs like that. It’s a lot cheaper and quieter than trying to fight an actual civilization. That’s why I can’t see why we wouldn’t have already been wiped out aeons ago in this scenario.
Thanks! That’s a different way of looking at the problem that I hadn’t considered.