What is something that definitely should not exist?

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squints at the Fermi paradox
Umm life?
- "Paradox" is a strong word when you sit on a speck of dust middle of nowhere and can barely see.
  
  There should be life ahead of us and life behind us.
  We can see about 10,000 other solar systems with our naked eyes.
  We can see 217,000 with a pair of binoculars
  5.3 million with a 3-in telescope
  380 million with a 15-in telescope
  I can't find the number for a 64 node earthwide radio telescope but If we were going to find life it would probably be with that.
  
  You're right, but that's the 'paradox'. Given our most pessimistic estimates for the chances of life we should have seen at least something that was a huge give away by now. Maybe better telescopes and observation methods will find them, we can get a spectrum from exoplanets. That's incredible; but so far all we see with our telescopes is more lifeless space. That doesn't mean they're not out there, it means our estimates are wrong. It probably means that we just don't understand what factors are required to create life very well and advanced life is incredibly rare.
  
  And yet. We wouldn't see ourselves from Alpha Centari unless there was deliberate directed transmission.
  
  Yeah, but we have like a billion years of "observation time" minimum, since there's every reason to think one of the alien species will be expansionistic. They're not here yet, blurry photos aside.
  
  There is a very good chance that this is the first truly habitable era in milkyways history and species now active are truly the first in sensible distance.
  
  Source? Like, sure, there probably wasn't enough heavy elements in the first few billion, but it only takes one planet to grow aliens, and aliens could colonise the whole galaxy in just a few million years, so you have to constrain things pretty tightly for us to be early at however many billion years in we are (the exact count is uncertain these days).
  
  Well you can find this material usually with "we are the first"-solutions to the Fermi Paradox.
  If dust and gas is too hot it can't form new stars. So star formation has it's own cycles. Too much new big stars and star formation halts for some time till things cool down. There are plenty of collisions in Milkyways early history that caused a star birth eras when there was very little heavy elements present. There is also probability that milkyway had an active center in early days that kept things nice and sterile.
  
  Okay, yeah, I'm familiar with the argument. I'm not alone in being unconvinced, though. There's a lot of exoplanets, including rocky ones around very old stars. Honestly, I felt assuming just a billion years of potential alien arrival was conservative.
  There is also probability that milkyway had an active center in early days that kept things nice and sterile.
  Fairly unrelated to this discussion, but I'll link it because it's cool: there's a detectable echo of radiation from our galaxy being more active just a couple centuries ago, at least momentarily.
  I don't know enough about the radiation one of those galaxies produce to comment on whether it could be sterilising. A thick enough atmosphere can block pretty much anything, though.

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