joshua wilson's List: Fermi Paradox Research

• Another closely related question is the Great Silence^[3]—even if travel is hard, if life is common, why don't we detect their radio transmissions?
• Counterarguments suggest that intelligent extraterrestrial life does not exist or occurs so rarely that humans will never make contact with it.
• Starting with Hart, a great deal of effort has gone into developing scientific theories about, and possible models of, extraterrestrial life, and the Fermi paradox has become a theoretical reference point in much of this work.

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• Major questions in astronomy

   

  Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground and space-based instruments, and possibly new developments in theoretical and experimental physics. 

   
  • Are there Earth-like planets around other stars? Astronomers have found massive stars and disks of debris around other stars. So the existence of smaller, terrestrial planets seems likely.^[44] 
  • Is there other life in the universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications.^[45][46] 
  • What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet we are still uncertain about their true nature.^[47] 
  • Why did the universe come to be? Why, for example, are the physical constants so finely tuned that they permit the existence of life?  
  • What caused the cosmic inflation that produced our homogeneous universe?^[48]

• Main argument

   

  We have not yet observed evidence of intelligent extraterrestrial life, though we have observed a great number of stars. Therefore, the whole process of starting with a star, and ending with communicating intelligent life, must be unlikely (the Great Filter). This implies that at least one step in this process must be improbable. Robin Hanson listed the likely steps as:

   
   
  1. The right star system (including organics & potentially habitable planets)
   
  2. Reproductive molecules (e.g. RNA)
   
  3. Simple (prokaryotic) single-cell life
   
  4. Complex (archaeatic & eukaryotic) single-cell life
   
  5. Sexual reproduction
   
  6. Multi-cell life
   
  7. Tool-using animals with big brains
   
  8. Colonization explosion
   
   

  Hanson considers this list to be incomplete.

   

  At least one of these steps must be improbable. If it's not 1-7, then the implication is that our future (step 8) is bleak. If 1-7 are likely, then many civilizations would have developed to the current level of the human race. However, none appear to have made it to step 8, or the Milky Way would be full of colonies. So perhaps step 8 is the unlikely one, and the only thing that appears likely to keep us from step 8 is some sort of catastrophe. So by this argument, finding multicellular life on Mars (provided it evolved independently) would be bad news, since it would imply steps 2-6 are easy, and hence only 1, 7 or 8 could be the big problem.

   

  Although steps 1-7 have occurred on Earth any one of these may be unlikely. If the first seven steps are necessary preconditions to calculating the likelihood (using the local environment) then an anthropically biased observer can infer nothing about the general probabilities from its (pre-determined) surroundings.