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Originally Posted by August
Thing is the universe is so big that its not unreasonable to assume that life likely exists out there somewhere and it's also obvious that any alien race so advanced that it can travel between the stars could also keep their existence completely hidden from us.
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Quote:
The Great Filter
With no evidence of intelligent life other than ourselves, it appears that the process of starting with a star and ending with "advanced explosive lasting life" must be unlikely. This implies that at least one step in this process must be improbable. Hanson's list, while incomplete, describes the following nine steps in an "evolutionary path" that results in the colonization of the observable universe:
The right star system (including organics and potentially habitable planets)
Reproductive molecules (e.g. RNA)
Simple (prokaryotic) single-cell life
Complex (eukaryotic) single-cell life
Sexual reproduction
Multi-cell life
Tool-using animals with big brains
Where we are now
Colonization explosion
According to the Great Filter hypothesis at least one of these steps—if the list were complete—must be improbable. If it's not an early step (i.e., in our past), then the implication is that the improbable step lies in our future and our prospects of reaching step 9 (interstellar colonization) are still bleak. If the past steps are likely, then many civilizations would have developed to the current level of the human species. However, none appear to have made it to step 9, or the Milky Way would be full of colonies. So perhaps step 9 is the unlikely one ....
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https://en.wikipedia.org/wiki/Great_Filter
Another problem is the time line. Since the universe is billions of years old highly advance life forms may have evolved, traveled the cosmos and then disappeared. It's possible that they existed hundreds of millions of years in the past.
See also the
'Fermi paradox'.
https://en.wikipedia.org/wiki/Fermi_paradox