Drake equation (attempt to quantitively estimate seti"s potential success: n = r* x fp x ne x fl x fi x fc x l, frank drake"s estimate: n ~ l. Optimism about l implies optimism about n and about seti. L ~ 100years (so far) as transmitting and receiving species: estimate and not a true scientific derivation. Fermi paradox: so where is everybody? : other civilizations are likely far more advanced that we are. Possible solutions to fermi paradox : we are alone, civilizations are common, but colonization is not due to: technical difficulties. Self-destruction on timescales shorter than colonization timescale lack of interest. Assume 1 in 10,000 stars have eti, assume 10,000 year survival, can establish contact with 10 civilizations: civilizations are common, but are not making their presence known. Distance between civilizations: v = r2 x t. R = 50,000 ly, t = 1000 ly so v ~1013 ly3. If there are n~10,000 civ. , d ~1,000 ly.