How Many Extra-Solar Civilizations?

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10 Spacing Between Civilizations

If civilizations are distributed uniformly within the Galaxy, then estimates of their numbers have important implications for the spacing between planetary civilizations and for the possibility of communication between them.  A simple calculation of spacing can be made by assuming the Galaxy is a disk 100,000 light-years across and 1,000 light-years thick, and that N civilizations are uniformly spaced within a cubic lattice with sides of length d. If this is the case, then each civilization sits in an average volume of d cubed. N multiplied by d cubed equals the volume of the Galaxy, from which the separation d can be calculated.

The approximation breaks down when the spacing exceeds the thickness of the Galaxy, 1,000 light-years. This occurs when the number of civilizations becomes 7,850 or less. 1,000 light-years is also likely to be the limit of potential communications in the near term. The calculation can be continued for fewer civilizations by assuming  that the lattice becomes limited to 1,000 light years in one dimension and that the civilization continues to sit in the middle of the resulting cuboid volume.

The result is shown in the tables and graphs below.


Table 1: Civilization Spacing versus Number of Civilizations

Civilizations            0           3          33         333         3,317
Spacing, light-years            - 51,166   15,427      4,864         1,539
Civilizations           1         33        332      3,317       33,170
Spacing, light-years           -  15,427     4,864      1,539            619
Civilizations         10       332     3,317    33,170     331,695
Spacing, light-years    9,025    4,864     1,539         619            287
Civilizations         30    1,000 10,000 100,000 1,000,000
Spacing, light-years 16,180    2,802       923         428            199


These simple estimates can be improved by using a more accurate description of the distribution of stars in the Galaxy, but the results are illustrative of the significance of lifetime on the ability to travel between civilizations, exchange communications, or receive signals indicating intelligence. Success in contacting extraterrestrial consciousness becomes much more likely if the  average lifetime of galactic civilizations is in the millions of years.

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