L/L_MW = Lifetime of civilization with ability and desire to communicate compared to the lifetime of the Milky Way galaxy = L (years) / 10 billion years

The value of L is completely unknown. We have reached a communicative state only some 70 years ago; we think the Earth will remain in a habitable zone for at least another billion years. However, humanity faces extreme dangers in the next few centuries (especially from nuclear warfare and environmental damage). I adopt the optimistic view: once we overcome the teething problems associated with technology, we stand a good chance of long-term survival.

(or N = L /40 in textbook). The number of currently active civilizations in the Galaxy is APPROXIMATELY equal to the lifetime (in years) of civilizations in the communicative state.