


        
                      From the SETI notebook of James Brown
        
        
                                Drakes  Equation
        
        One of the first things asked when I say that I am evolved in the 
        search  for extraterrestrial life is "what makes you think  there 
        is any one else in the universe ?".   That's a very good question 
        and  one  that deserves an answer.   It is also one that has  not 
        been successfully answered over the whole of human experience.
        
        Carl  Sagan participated in a conference of people interested  in 
        the  question of Communication with Extraterrestrial Intelligence 
        (CETI)  that  was  held  in  the  USSR  back  in  1971.   In  his 
        introduction address he said something that gets the whole  thing 
        in   perspective.    He   said  "..the  cetaceans  (whales)   are 
        undoubtedly  another intelligent species inhabiting  our  planet, 
        and it has been argued that if we cannot communicate with them we 
        should   not   be  able  to  communicate  with   extraterrestrial 
        civilizations".   Without  much reflection it would seem that  it 
        should   be  much  easier to communicate with whales  than  ET's.  
        After  all  we share the same planet,  both  breath  oxygen,  eat 
        plants that depend on the sun for there energy source.  But since 
        that  meeting,  15 years ago,  we are no closer to  talking  with 
        whales than we ever were.
        
        The  conference  had  as its agenda the task  of  estimating  the 
        possibility  of communication with extraterrestrial  intelligence 
        and  since  they were all capable scientific people they felt  it 
        necessary  to  express there doubts and optimism in the  form  of 
        mathematics.   Dr. Frank Drake was at the same conference and had 
        sometime  earlier  created  a  mathematical  expression  of   the 
        possibility that could be used as a framework for this and future 
        discussions.  
        
        Drakes  equation  became a way for experts to tackle the  problem 
        piecemeal, concentration in there own specialty without trying to 
        understand the question as a whole.  For example the evolution of 
        technical civilizations was taken by an anthropologist,  the rate 
        of star formation in the universe by a cosmologist, etc.
        
        The equation is:
        
             N = R Fp Ne F1 Fi Fc L
               Where:
                  N = Number of civilizations to look for
                  R = Rate of star formation
                  Fp= Fraction of stars with planetary systems
                  Ne= Ecologically suitable planets.
                  F1= Fraction of planets on which life occurs.
                  Fi= Fraction of planets with life AND intelligent life
                  Fc= Fraction with communication capabilities like ours
                  L = Life time of such a civilization
        

        It  was generally agreed on that the terms most easily  estimated 
        were  the  rate  of star formation and the number of  stars  with 
        planets.   The  most  difficult term was "L" the life time of  an 
        intelligent civilization because we only have experience with one 
        such civilization and it appears intent on terminating itself  at 
        the earliest possible moment.
        
        AS  you  would expect none of these things can be estimated  with 
        anymore than a good guess but here is what the results seem to be 
        based on everything we now know about the problem.   An optimist, 
        like   Dr Sagan  would  say  that  there  is  at  least   300,000 
        civilizations  that  are capable of sending signals and  that  we 
        could  detect in our galaxy alone.   A pessimist,  and there were 
        several even in that conference,  would say that there can be  as 
        few as 100 possible civilizations down to and including zero.  
        
        So what is the true number of civilizations?  We shall never know 
        because even when we find the first,  we shall never know when we 
        have found the last.  


        
	So what is the true number of civilizations?
 
 
 


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