katic@sri-unix (10/26/82)
I don't know about Tipler's ideas, but maybe I can add something to this discussion. In the book "Intelligent Life in the Universe" (Carl Sagan & I S Shklovski, Delta Books) there is an attempt at a mathmetical formula for the number of currently existing, technically advanced sentient races in the galaxy. This attempt is in Chapter 29, "Distribution of technical civilizations in the galaxy." The following is an extract/paraphrase of that attempt. The formula is: N = Rst * Fp * Ne * Fl * Fi * Fc * L Where: Rst Mean rate of star formation, averaged over the lifetime of the Galaxy Fp Fraction of stars with planetary formations Ne Mean number of planets in each planetary system with environments favorable for the origin of life Fl Fraction of such favorable planets on which life develops Fi Fraction of such inhabited planets on which intelligent life with manipulative abilities arises during the lifetime of the local sun Fc Fraction of planets populated by intelligent beings on which an advanced technical civilization arises during the lifetime of the local sun L Lifetime of the technical civilization ======== Lets look at S&S's estimates of the numbers: Rst ~10 stars per year. This is an educated guess. Fp ~1 S&S essentially assume that all stars will have planetary systems Ne ~1 Ditto for inhabitable planets. Fl ~1 Ditto for life arising. Fi A quote: "The question of the evolution of intelligent life is a difficult one. This is not a field which lends itselt to labratory experiementation, and the number of intelligent species available for study on Earth is limited." S&S have adopted the value of 0.1 for this factor. This is close to a WAG (Wild Ass Guess!). Fc "The developement of a technical civilization has a high survival value at least up to a point; but in any given case it depends on the concat- enation of many improbable events, and it has occurred only recently in terrestrial history." S&S have again adopted a value of 0.1, another close WAG. L This one is the kicker! "We can imagine two extreme alternatives for the evaluation of L: (a) a technical civilization destroys itself soon after reaching the communicative phase (L less then 10e2 years); or (b) a technical civilization learns to live with itself soon after reaching the communicative phase. If it survives more than 10e2 years, it will be unlikely to to destroy itself afterwards. In the latter case, its lifetime may be measured on a stellar evolutionary timescale (L much greater than 10e8 years)." They go on to use two values: L < 10e2 years < 10 communicative civilizations! L >> 10e8 years > 10e7 comm. civ's In case one, we may be the only ones, in the later case, civilizations maybe everywhere. Sagan adopts ~10e7 for an average figure leading to: N ~ 10e6 Assuming that this is accurate, they then figure that the average steller distance between technical civilizations is several hundred light years. If we (the net) use this, then we can easily deduce that we haven't been around long enough to notice or be noticed! S&S go through some more stuff and get some more conclusions, but you can easily get the gist of thier ideas. ======== Let me add to this a few factors which I will NOT try to quantify: F!p Fraction of those technical civilizations whose outlook on life is not directed toward a pastoral existance. Ft Fraction of those technical civilizations whose outlook is sufficiently outwardly directed to make intersteller communication attractive Fs Fraction of those technical civilizations who are sane enough to not fear the dangers of intersteller communication S Speed ratio of life, assign mankind=1, those who 'think faster' than us get >1, slower >0 & <1. ======== Philosophically, we can state two facts: 1) We have not proved that intelligent life exists on other planets 2) We have not proved that intelligent life does not exist elsewhere All we can say is that we haven't found anything YET. katic (....!nsc!katic)