From mib@area.51 Sat May 03 04:17:12 1997 Newsgroups: alt.paranet.ufo Subject: UFO FILES: The Drake Equation From: earl grey Date: Sat, 03 May 1997 01:17:12 -0700 --------------------------------------------------------------------------- Extraterrestrial Life: The Drake Equation There is a famous formula called the Drake Equation, which estimates the number of technical civilizations, N, that exist in the Milky Way Galaxy. In this article, I present a value for N, based on my own estimate of the Drake equation coefficients. --------------------------------------------------------------------------- The Drake Equation takes the form N = ng * fp * ne * fl * fi * fc * ft where ng is the number of stars in the Galaxy fp is the fraction of stars that have planets ne is the mean number of planets in a planetary system that could support an ecology fl is the fraction of suitable planets on which life actually arises fi is the fraction of inhabited worlds on which intelligent life arises fc is the fraction of worlds inhabited by intelligent life on which a technical civilization arises ft is the fraction of a planetary lifetime graced by a technical civilization --------------------------------------------------------------------------- ng Only the first coefficient in the Drake equation, ng, is known with any degree of accuracy at this time, and has been measured to be approximately 400,000,000,000. The remaining coefficients are all subject to educated guesswork, and depending on your point of view, give a value for N somewhere near 1 (that's us on our own) and many billions. --------------------------------------------------------------------------- fp It is now widely accepted that stellar and planetary formation is due to a process of nebular accretion, and where one finds a star, there is a reasonable probability - some would say a high probability - of finding an associated planetary system. An accretion disc (which arises because of the conservation of angular momentum) is an ideal place for planets to form, and is a likely counterpart to a protostar. This view is reinforced by the sheer abundance of planets and moons that we observe in the solar system. There is also compelling observational evidence to suggest that some nearby stars have planetary systems. This being the case, it would not be unreasonable to ascribe a value of 0.1 to fp. --------------------------------------------------------------------------- ne Given a planetary system, how many planets on average could be expected to be capable of supporting an ecology? For the purposes of this argument, I shall assume that an abundance of liquid water (an ocean) is the essential defining feature of such a planet, and that an ecological system cannot exist without it (this may in fact be a pessimistic assumption, as life may prevail in very different circumstances to those found on Earth). For an ocean to exist, the planetary environment must be constrained in terms of temperature and pressure. An outer gas giant (like Jupiter) would not be a likely candidate (though the moon of a gas giant might); nor would a rocky planet that is very close to its parent star. This implies a relatively small range of possible orbits for a suitable planet around a given star. In our own system, we seems to have the uncomfortably small value of 1. Let us be highly pessimistic, and assume that only 1% of planetary systems have a single planet with water oceans, and assign a value of 0.01 to ne. ---------------------------------------------------------------------- fl The emergence of life on an ecologically suitable planet is not assured. However, in the case of the Earth, life started to form remarkably early in the planet's history. It seems common sense to think that once life has started in a place, it will aggressively grow and flourish, and the main difficulty seems to be that of becoming started in the first place. With a planet full of chemicals to experiment with, and geological time-scales to do it in, it may well be that under the right conditions, life is almost bound to arise. Evolution can have powerful consequences for even the simplest of self-replicating molecules, and the possible scenarios for the origins of life in the form of a primitive chemical soup have been replicated with limited success in the laboratory. However, let us again be cautious, and assume that only a tiny proportion of suitable planets will ever give rise to any form of life. Let us say that fl has a value of 0.001. --------------------------------------------------------------------------- fi Given an inhabited world (of bacteria, say), what are the chances that intelligent life will form? In the case of the Earth, there are numerous instances of intelligent species, and since intelligence seems to be a universal weapon that can be employed in the struggle for survival, there are powerful reasons for believing that evolution will assuredly lead to the formation of intelligence in an emerging ecology. I have no difficulty in assigning a value of 0.5 to fi, even in the cases where cosmic impacts intermittently wipe out much of a planet's ecology (as has happened on Earth, for instance). --------------------------------------------------------------------------- fc It is one thing to have intelligent life on a planet, and another matter entirely to have intelligent life that is capable of producing a technological civilization. I have already argued that intelligence is a useful aid to survival, and should be expected to arise in most ecologies. However, the evidence on this planet is that conscious thought (as we understand it) is a very rare thing. In fact, many believe humans to be the only species capable of such a way of being. Nevertheless, the fact that we ourselves can think, and the common sense point of view that animals are capable of a similar, but limited, mode of existence, leads me to think that fc, while it may be small, is not necessarily a vanishingly small number. I would therefore, give a value of 1%, or 0.01, to fc. The question of technological advancement is surely a certainty given a species capable of rational thought (and experiment), though it may take some species very much longer than others to reach the 'same' level (it would be difficult to conduct scientific studies in an aquatic environment, for instance). --------------------------------------------------------------------------- ft If a planet has the lucky status of being classed as that of a technological civilization, for what proportion of its history is it likely to be classed as such? This is no easy question to answer, since it requires knowledge of how long the planet exists, how long it takes for technology to emerge, and how long a civilization can stay intact before it is destroyed by disease, war, or cosmic disaster. The only experiment we can draw experience from is that of our own civilization, coupled with an assumption that there is nothing 'special' about our own particular environment (given the Drake coefficients, of course). It has taken about 4.5 billion years for our technological civilization to emerge, which is probably about half the anticipated lifetime of the Earth. If this is to be taken as nothing out of the ordinary, then ft has an upper value of 0.5 straight away. Further, it seems exceedingly unlikely that mankind will last the course. It seems to me quite conceivable that plague or war could end the human species, and at this point in time it is difficult to say whether technology is increasing our chances of survival or decreasing them! There is also the distinct possibility that mass extinctions due to cometary impacts occur on a periodic basis (30 million years or so) as the Earth wanders in and out of the Galactic plane. This does not bode well for humankind, but perhaps I am being unduly pessimistic. It may be that we are on a 'cusp' of technological development, and technology will rapidly advance to such an extent that such problems are readily overcome. Indeed, it is a possibility that the dominant form of intelligent life on Earth may one day take the form of an Immortal Superconsciousness, in which case disease, at least, could be ignored as a threat. But again, to be pessimistic, I shall assume that most species are self-destructive, and guess that technological civilizations, on the whole, could be expected to last for about one million years (ft being approximately 0.0001). --------------------------------------------------------------------------- N N = 400,000,000,000 x 0.1 x 0.01 x 0.001 x 0.5 x 0.01 x 0.0001 N = 0.2 According to my calculations, which are based on what I consider to be pessimistic assumptions, there are 0.2 technological civilizations to be found in our Galaxy at any one time. This is at first disappointing - the implication is that we shall never encounter another civilization - but at the same time, there is the distinct possibility that I have been massively pessimistic (in which case the Galaxy is teeming with life). Even a value as low as 0.2 implies that many civilizations do arise but for short periods, because you will recall that I put little faith in technology's ability to preserve a civilization. Remember, the Milky Way Galaxy is but one of many. It is estimated that there are at least as many galaxies in the universe as there are stars in our own Galaxy. This means, according to my estimates, we are sharing the universe with another 80,000,000,000 technological civilizations. On the other hand, my figures may have been wildly optimistic in other respects. We shan't know, for some time yet... It may be worth pointing out that Carl Sagan comes up with N = 10 in his book, Cosmos. I computed my answer without consulting Sagan's estimate, and this shows me to be fifty times as pessimistic as he is. But, as he points out, much of the pessimism is down to economics and politics - if self-destruction is not the fate of most civilizations, we are not too far away from some very interesting neighbours! ---------------------------------------------------------------------------