SETI@home is a scientific experiment that uses Internet-connected computers in the Search for Extraterrestrial Intelligence (SETI). One approach, known as radio SETI, uses radio telescopes to listen for narrow-bandwidth radio signals from space. Such signals are not known to occur naturally, so a detection would provide evidence of extraterrestrial technology.
SETI@home project URL; http://setiathome.berkeley.edu/
About SETI@home: Searching for Life
Story by Ron Hipschman
"Contact," "Star Trek," "Babylon 5," "Star Wars," "Alien," and the rest of the lot have one thing in common. They all deal with alien civilizations and their relationships to humans. Some of these futuristic fictions portray alien life as friendly, some as hostile. Most of the aliens are curiously "humanoid." Many of us dream of one day meeting up with a (friendly) alien race. Much could be learned and discovered about each other. What are we doing right now to make this happen?
If we assume that our alien neighbors are trying to contact us, we should be looking for them. We are currently engaged in several programs that are now looking for the evidence of life elsewhere in the cosmos. Collectively, these programs are called SETI (the Search for Extra-Terrestrial Intelligence.)
Our sun is only a single star in a collection of over 400 billion we call the Milky Way galaxy. The Milky Way is only 1 of billions of galaxies in the universe. Seems like there should be lots of life out there! Can we make an initial estimate? The first to do so was the astronomer Frank Drake. He came up with a simple equation, now called the Drake Equation, that maps out the possibilities. The equation is quite easy to understand, so don't tune out, even if arithmetic isn't your strong suit! Here it is:
N = R * f(p) * n(e) * f(l) * f(i) * f(c) * L
"N" here represents the number of communicating civilizations in our Milky Way galaxy. This number depends on several factors. "R" is the rate of "suitable" star formation in the galaxy. "f(p)" is the fraction of stars that have planets. "n(e)" is the number of these planets around any star within the suitable ecosphere of the star. An "ecosphere" is a shell that surrounds a star within which the conditions are suitable for life to form. Too close and it's too hot; too far and it's too cold. "f(l)" is the fraction of those planets within the ecosphere on which life actually evolves. "f(i)" is the fraction of those planets on which intelligent life evolves. "f(c)" is the fraction of those planets where intelligent life develops a technology and attempts communication. The last factor, "L," is the length of time that an intelligent, communicating civilization lasts. Let's briefly look at each of these factors separately and try to put some reasonable numbers to them.
Although the rate of suitable star formation was undoubtedly much higher when our galaxy formed, one can still see where stars are being born today. Take a look at the beautiful pictures of stellar nurseries taken by the Hubble Telescope in the Eagle nebula and the Orion nebula. Here, huge clouds of gas collapse to form stars. A good guess at the rate of this star formation is about 20 stars per year. R=20.
Many of these clouds have a little bit of rotation. As they collapse, the cloud spins faster and faster, like an ice skater pulling in her arms. This causes the cloud to form a flattened disk of gas. At the center, the main star forms. Out further, smaller eddies can form planets. Until recently, we did not have any evidence of planets outside our solar system. In the last couple of years, several teams of astronomers have announced the discovery of planets surrounding nearby stars (see interview with Geoff Marcy and Didier Queloz). This exciting discovery increases the likelihood of other planets around many stars. Let's estimate conservatively that one-half of the stars form planetary systems; the other half form binary star systems, so f(p) = 0.5.
Geoffrey Marcy is a professor at San Francisco State University. There's more on Marcy and his search for extraterrestrial planets from the Exploratorium's "What's New" December 1996.
The n(e) factor is a little tricky. Small stars are cool and red. Planets would have to orbit very close to be in the ecosphere. Also, this ecosphere would be very narrow; like the skin on an orange. Not much room for planets. Planets that orbit very close to their parent star are often tidally locked and present one face to the star at all times. The atmosphere of such a planet would freeze on the cool side that faces away from the star; this does not promote life. On the other hand, huge hot blue stars have a farther and wider ecosphere. Of course, judging from our solar system, planets are spaced further apart the farther they are from the star, so the wider ecosphere is cancelled by this effect. These larger stars also burn their fuel faster and don't last very long. They are usually so short-lived that life does not even get a chance to start before the star goes nova or supernova and destroys everything in the system. In our solar system, with our average-sized yellow sun, we have two (Earth and Mars) or maybe three (Venus) planets within the ecosphere. A conservative guess for the number of planets within the "life zone" or ecosphere is one. n(e) = 1.
The next factor, f(l), is where things become a little sticky. The problem is that we only have a few examples of planets where conditions are right for life to evolve. As stated above, Venus, Earth, and Mars all could have had, at one time, proper conditions. We know life evolved on Earth, and there is now tantalizing evidence for primitive life existing on Mars billions of years ago. A conservative guess for this number is 0.2, or one in five planets with proper conditions will evolve life. f(l) = 0.2.
How many of these planets will evolve intelligent life? Tough question, but if we really believe the evidence for natural selection and survival of the fittest, most scientists would put this number at 100 percent -- that intelligent life is a natural outcome of evolution. Of course, here we have only one example, earth. f(i) = 1.
How many of these intelligent species will develop technology and use it to communicate? If we look at the earth, we see humans doing it, but we also see whales and dolphins, who may also possess a moderate level of intelligence but never developed technology. We'll set this number to .5 as a first guess. f(c) = 0.5.
Now we get to the hardest number to determine. "L" is the number of years that a technologically adept and communicative civilization lasts. We've only been in this phase of our evolution for about 50 years. Do advanced civilizations blow themselves up after discovering the technology to do so? Or do they get together and solve their problems before this happens? For now, let's not assign a number to L. Let's plug in the other numbers and see what we get.
N = R * f(p) * n(e) * f(l) * f(i) * f(c) * L
N = 20 * 0.5 * 1 * 0.2 * 1 * 0.5 * L
Do advanced civilizations use their technology to solve their problems or do they destroy themselves? On earth we've survived the first 50 years.
Multiplying all the numbers gives us N = L. In other words, the number of intelligent communicating civilizations in the galaxy equals the number of years such a civilization lasts! The figure about which we know the least bears a great significance in our calculations. Most scientists hope that if a civilization can overcome its initial tendency to destroy itself with its own technology, then that civilization is likely to last for a very long time. Let's hope those scientists are right. In any case, there should be at least 50 (the number of years WE'VE been around communicating) and if a communicative civilization lasts for millions of years, there may possibly be millions of civilizations we can look for.
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