Some scenarios for the Great Filter

Graphs demonstrating the Great Filter and the sinister conclusion

A reminder: the Great Filter is whatever has so-far prevented any planet in our galaxy from evolving a galaxy-colonising civilisation.

Scenario A: The Filter is the evolution of life, and so is in our past:

ooooooooooooooooooooooooooooooooooo  - planets capable of supporting life
o                                    - planets where life evolves
o                                    - planets where animal life evolves
o                                    - planets where intelligent life evolves (we are here)
                                     - planets where galaxy-colonising life evolves (none yet)

Scenario B: The Filter is the evolution of intelligent life, and so is in our past:

ooooooooooooooooooooooooooooooooooo  - planets capable of supporting life
ooooooooooooooooooooooooooooo        - planets where life evolves
oooooooooooooooooooooooooo           - planets where animal life evolves
o                                    - planets where intelligent life evolves (we are here)
                                     - planets where galaxy-colonising life evolves (none yet)

Scenario C: The Filter is the transition to a galaxy-colonising civilisation, and is in our future:

ooooooooooooooooooooooooooooooooooo  - planets capable of supporting life
ooooooooooooooooooooooooooooo        - planets where life evolves
oooooooooooooooooooooooooo           - planets where animal life evolves
oooooooooooo                         - planets where intelligent life evolves (we are here)
                                     - planets where galaxy-colonising life evolves (none yet)

(These are not the only scenarios.)

An important insight made by Hanson and Bostrom is that evidence of life on other planets can rule out some of these scenarios. For example, if we find simple life on Mars (which evolved independently of Earth's), then Scenario A can effectively be ruled out (because it's extremely unlikely that life on two neighbouring planets would both just happen to get past the Filter).

One can also see that certain scenarios are bleaker than others. In Scenario C, the outlook is dire, because even though there are lots of intelligent species in the galaxy, no species has ever advanced to the galaxy-colonising stage, which surely means our own chances of doing so are low.

Updated: 2008-08-12
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