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Where Are the Aliens? (1) (2) | |
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As yet, no one has detected extraterrestrial intelligence – or even extraterrestrial life. While people have wondered about its existence for thousands of years, this question has probably only been seriously examined – scientifically – for about fifty. [Some cynics would say "quasi-scientifically."] In 1961, while working at the National Radio Astronomy Observatory in Green Bank, West Virginia, Frank Drake introduced the equation named after him: |
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| C = N * fP * nE * fL * fI * fT | ||||||||||||||||||||
where |
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| C = | the number of spacefaring civilizations in the galaxy; | |||||||||||||||||||
| N = | the number of stars in the galaxy; | |||||||||||||||||||
| fP = | the fraction of stars with planetary systems; | |||||||||||||||||||
| nE = | the number of planets per solar system; | |||||||||||||||||||
| fL = | the fraction of planets which have life; | |||||||||||||||||||
| fI = | the fraction of planets, with life, which have intelligent life; | |||||||||||||||||||
| fT = | the fraction of planets, with intelligent life, having a technological civilization | |||||||||||||||||||
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I remember reading about this "equation", in LIFE magazine, when I was a kid. The article assumed about 100 billion (1011) stars in the galaxy (the actual number is now believed to be about twice that, but that doesn't change the argument). Even if only one in a hundred stars could have planets, and only one in a hundred of those with planets have life, that would still provide 10 million (107) planets with life. And even if only one in a thousand had intelligent life, that would still mean ten thousand planets, in the galaxy, with intelligent life. (The actual article tried to calculate the number of stars similar to our Sun, and changed some of the other numbers a bit – I remember a figure of about 16,000 civilizations.) The problem with this "equation" is you're pulling numbers out of the air. If you say one in ten stars has planets, and one in ten of those have life, you get a billion stars with life. If one in ten has intelligent life, the number of civilizations could be a hundred million. If, on the other hand, only one in a thousand stars has planets, and one in a thousand of those has life, and one in a thousand of those has intelligent life, and one in a thousand of those has developed technology, the number of technological civilizations is – ONE-TENTH. (Which means you'd have to search, on average, ten galaxies to find ONE technological civilization.) Who knows if the numbers are one in two, or one in a million? Until we do, these kinds of "calculations" are pointless. The late Michael Crichton once posted on his website (the link is apparently no longer valid) some rather blunt assessments of the scientific validity of SETI. Let's just say SETI didn't come off looking too good. The period from 1960 till 1980 or 1990 could probably be called the Drake-Sagan era. Drake appeared to be (and still appears to be) optimistic about the prospects for intelligent life in the galaxy. Carl Sagan, the late and famous science popularizer, was also optimistic. At some time in the past decade or two, however, more and more researchers have expressed pessimism. One widely-publicized book, Rare Earth, by Peter Ward and Don Brownlee of the University of Washington, does a good job of summarizing the case for pessimism: |
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For a planet to have life, it has to be in a very narrow zone. Too close to the sun, and you have a runaway greenhouse effect (Venus). Too far, and you freeze (Mars). The sun gets warmer over billions of years; a planet that's now just right might have been too cold in the past; just right in the past and it might be too hot now. If the planet is too small (Mars), it won't hold onto its atmosphere. If it's too large, it becomes a home to massive storms which might prevent life from ever evolving (Jupiter). There has to be a large planet in the outer solar system (Jupiter) to clear out comets and asteroids that would otherwise cause mass extinctions. But if the large planet is in the inner solar system, other planets cannot form. If the orbit (of the potential life-endowed planet) is too elliptical, the planet will be too hot during part of the year, or too cold during the other (or both). In parts of the galaxy (near the center) stars are too close together, and radiation may prevent life from ever evolving. The Earth's moon (and its creation was a very rare event) may be necessary to stabilize the Earth's axis, and allow stable seasons. Plate tectonics may be necessary for life to evolve. Even on the Earth, which seems well-suited to life, complex life has existed for only about ten percent of the planet's lifetime. Intelligent life has only existed here for two or three million years – less than one tenth of one percent of the planet's lifetime. And, after billions of years of evolution, the human race was almost wiped out by a volcanic calamity. And the earth's climate may not be all that stable. Civilization has only existed for about eight thousand years. Space travel has only occurred within a human lifetime (since 1957, if you include Sputnik; 1961, if you include humans in space) |
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Put all of the above together, and things look pretty bleak. The controversy – between the Drake-Sagan types and the Ward-Brownlee types – will doubtless continue, perhaps switching from optimism to pessimism and back, until we can actually examine a large fraction of the nearby stars for signs of life. The problem with the Drake "equation" is – it's not really an equation. It's an estimate – an extremely crude one. And we have no idea of what any of the numbers should be, except for the number of stars in the galaxy. Drake thought a lot of the numbers in the equation were relatively large (one in a hundred or maybe one in a thousand); Ward and Brownlee suggest they may be more like one in a million. In 1995, researchers discovered the first exoplanet (planet outside our solar system). This was quite remarkable. Since the beginning of civilization (and probably much longer), people had wondered if we were alone in the universe. And even after discoveries of other stars, other galaxies, the expansion of the universe, the big bang, we still did not know if our solar system was unique. A star surrounded by planets could be very common, or very rare. Knowing this would give just one number – fP above – but it's a start. And apparently planetary systems are quite common; fP is close to one. But that's just one number. Going back to things I remember as a kid – only a few years after reading about the Drake equation, I read somewhere (don't remember where) the following argument: |
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Intelligent life on Earth has taken about five billion years to evolve. The lifetime of the Sun (at least the time that temperatures on Earth allow life to exist) is about ten billion years. These numbers are of the same order of magnitude. So if conditions in another solar system were just a little less favorable than on Earth, it's entirely possible that intelligent life would not be able to evolve in the lifetime of that system's star. If conditions on Earth are only slightly atypical (in terms of being favorable to the evolution of intelligent life), such life might be very rare. |
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The Ward-Brownlee types get a lot more specific regarding favorable conditions, but this argument is a pretty good one. If intelligent life takes a long time to evolve, relative to the lifetime of a star, it's far from certain that it will get a chance to. The flip side of this argument is that stars with very long lifetimes (such as red dwarfs, with a lifetime of roughly 100 billion or 1011 years) would be very likely to contain planets with intelligent life. (As long as the other factors were amenable to it. And the problem with red dwarfs is that their habitable zone is so close to the star that coronal mass ejections and other such phenomena could make it difficult for life to evolve near them.)
And there are other problems. I've seen documentaries which claim that the earth will remain habitable for only another 500 million years or so. In other words, it has taken 4.5 billion years for intelligent life to evolve, and after five billion the earth becomes uninhabitable. Space Daily, at a link that's no longer active, had an article (January 1999) about Gamma Ray Bursts. Such bursts, which release as much energy as a supernova, and which could destroy all life on land – even if they occur thousands of light years away – occur every few hundred million years. But earlier in the galaxy's history, they occurred more often. Perhaps they kept intelligent life from evolving until now. A more recent article expresses a similar sentiment, except that the explosions it considers would cause mass extinctions, but not necessarily destroy all life on land. Scientific American had a very informative article on extraterrestrial intelligence in its July 2000 issue. Many people have asked "If there are so many intelligent civilizations out there, why haven't they contacted us?" This is called the Fermi Paradox. Just do a web search on "Fermi Paradox" and you will see all kinds of information and speculation. To me, there is no paradox. If you think about it for a few minutes, the answer (or at least, one POSSIBLE answer) is obvious. |
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