In 1950, physicist Enrico Fermi argued that, given a number of assumptions, the Earth should already have been visited by aliens.

Fermi’s Paradox is the apparent contradiction between the lack of evidence and the seemingly high probability estimates for the existence of extraterrestrial civilisations.

Many attempts have been made to explain the Fermi paradox.  Primarily, these suggested either that intelligent extraterrestrial life is extremely rare, or proposed reasons why such civilisations have not contacted or visited Earth:

• Extra-terrestrial life is rare or non-existent

  
  

• No other intelligent species have arisen elsewhere in the Universe

  
  

• Intelligent alien species lack advanced technology

  
  

• It is the nature of intelligent life to destroy itself

  
  

• It is the nature of intelligent life to destroy others

  
  

• Periodic extinction by natural events

  
  

• Resource depletion and climate change

  
  

• Inflation hypothesis and the youngness argument

  
  

• Intelligent civilisations are too far apart in space or time

  
  

• It is too expensive to spread physically throughout the Galaxy

  
  

• Human beings have not existed long enough

  
  

• Civilisations broadcast detectable radio signals only for a brief period of time

  
  

• They tend to isolate themselves

  
  

• They are too alien

  
  

• Everyone is listening, no one is transmitting

  
  

• Earth is deliberately not contacted

  
  

• Earth is purposely isolated – the Planetarium Hypothesis

  
  

• It is dangerous to communicate

  
  

• They are already here undetected

  
  

• They are here unacknowledged

  
  

• Human beings are NOT listening properly