can someone explaining to me how does "Lifetime of communicationg civilization" make sense in this equation? i dont get it,(reply>), @T'Lan Imass thank you, thats a very good explanation,(reply>), Can you talk to your dead ancestors? There’s your answer.,(reply>), To meet, 2 civilization's lifetimes must overlap each other. For example, if each civilization's lifetime is 10k years, and there are a total of 1k civilizations spread over 1 billion years without overlap, they will never meet each other, since they existed in different times. Thus, if you include this variable, the final result will be civilizations that are potentially currently around during our time.