How Many Extra-Solar Civilizations?

star field

11 Two-Way Radio Communication

For two-way communication between civilizations in different star systems, the spacing in light years should not be greater than half of the lifetime of the civilization, and should preferably be much less. In addition, exchanges with round-trip times greater than 1,000 years appear unlikely, on the basis that it is difficult to keep up a two-way conversation with such large gaps. Only with lifetimes substantially greater than ten million years is there likely to be an attempt at two-way communication.


Table 1:  Likelihood of Two-way Communication

Number of Civilizations       33,170      3,317      332      33
Spacing, light-years            619      1,539   4,864 6,187
Communication Likelihood          Low        Zero    Zero   Zero
Number of Civilizations 1,000,000 100,000 10,000 1,000
Spacing, light-years            199         428       923 2,802
Communication Likelihood           Fair         Low      Zero    Zero 


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