The SETI Argument
The SETI Argument - The Radio search
The SETI search is a huge task. Where and how do we look? We have to agree on the method. Several come to mind.
First, our alien friends could travel in a space ship and visit us. That would be a great method of communication -- face to face. Unfortunately, despite all the alleged Roswells and alien abductions and UFO sightings, there is not a single shred of scientific evidence that aliens have ever visited the earth. Not one. The amount of energy and time necessary to travel between the stars is so immense that a more "economical" method is mandated. Doing some simple calculations, you can prove that even if someone could invent a ship with the most efficient type of engine ("Star Trek" fans will be happy to know that this involves matter and antimatter!) that a trip to the nearest star, Alpha Centauri, with a twenty-year round-trip travel time would use enough energy to power over a million houses for over thirty million years! And this trip would only take a few people as passengers! This is not an economical method to communicate because this amount of energy, at today's costs, would cost well over $30 quadrillion. Can you imagine any alien government allocating this kind of resource to send a few of their people for a friendly visit? Of course, they could send out thousands or millions of lightweight probes announcing their existence and the probes could report back their findings, but this would still be very costly in both time and energy. Even though we would like to believe in warp drive, interdimensional shifting, and all the other wonders of science fiction, they are just that, fiction.
How would you contact a friend or relative living in a far distant country if all you wanted to say was "hello!" Would you go there? Probably not. You would simply pick up the phone and make a call, or you might send an email, or if you have the right equipment, you might even strike up a video teleconference. All of these methods are vastly more economical and timely than traveling by jet. Likewise, any advanced civilization would probably choose to communicate via "economical" methods.
The best method that we know of is to communicate by radio waves. Transmitting information over radio waves is very cheap, uses equipment that is easy to build, and has the information-carrying capacity necessary for the task. The information also travels at the fastest speed in the universe, the speed of light. No space ship can ever match this speed. Furthermore, if you are using a radio transmitter, you can point it in millions of directions and communicate simultaneously with many different civilizations. Can't do that with a ship! Look at all the ways you use radio transmissions: radio and TV, of course, cellular phones, pagers, all satellite communication, GPS navigation, and much more. On earth, we've already decided on the most efficient method!
So if we are going to look for aliens trying to contact us by radio waves, we have another problem on our hands. When you turn on your radio, you have to decide what frequency, or channel, you are goung to listen to. You can tune through the various radio bands -- AM, FM, short-wave, ham, police, weather, etc. -- to find your desired program. Likewise, we will have to tune our extraterrestrial ear to find our alien friends. The electromagnetic spectrum is a very, very large place and there are some phenomena that we should examine that might make our tuning decision easier. It would be nice, at least, to find a reasonably small region of the electromagnetic spectrum in which to start our search.
The largest radio telescope in the world and the one used for the Berkeley SETI search is the Arecibo Radio Telescope, in northwestern Puerto Rico, near the town of Arecibo. The telescope has a fixed dish 305 meters (1000 feet) across that is built into a sinkhole in rugged Karst terrain. To imagine how big this dish is, it would take 10 billion normal bowls of cereal to fill the radio dish to the brim! Still hard to imagine? The large surface of this dish is over 20 acres with about 18 acres or 26 football fields of available aperture! The dish reflects and concentrates the weak celestial signals on the receiving antennas hung 450 feet above. Since the dish is fixed and can't swivel, the receiving antennas are mounted on a bow-shaped track that allows them to "look at" objects as far as 20 degrees from the zenith (directly overhead). This bow-shaped arm is itself mounted on a circular track to allow the antennas to follow an object as it moves across the sky due to the earth's rotation. These two motions give the telescope the ability to scan a good portion of the sky.
Nature provides an even nicer way to further refine our frequency range. The simplest "stuff" of the universe, neutral hydrogen gas in interstellar space, emits radio signals at 1.42 GHz. Another molecule in space, the hydroxyl, or OH, emits at about 1.64 GHz. Now if you look at these two, H and OH, you would see that together they make up the compound of water HOH (or more commonly H2O). Life as we know it requires water to evolve and exist. The frequency range between these two emissions, from 1.42 to 1.64 GHz, is therefore a quiet region of the spectrum called "the water hole." Where would you expect water-based intelligent civilizations to meet? Around the water hole, of course! This would be a good, and nicely limited, range of frequencies to start our search. We can always broaden our net at a later date.
The frequencies around 1420 MHz are especially interesting to look at because of their proximity to the "water hole." Also, because of the importance of this part of the spectrum to radio astronomy, by international agreement no one is allowed to produce any broadcasts between 1420 MHz and 1427 MHz. Because of this ban, it's an especially quiet part of the spectrum. Let's take a closer look at what this means.
What exactly are we looking for? As mentioned previously, the most efficient way for an alien race to get noticed would be to concentrate all the radio energy in a very narrow frequency signal. If your radio receiver is "sloppy" and can only look at broad ranges of frequencies, the narrow signal will get swamped by all the unwanted signals around it -- even if that signal is very strong.
Imagine a person with a loud whistle in a huge noisy crowd. The whistle has a very specific frequency or pitch. If you were just using your ears, which pick up a wide range of pitches, the noise from the crowd would mask out the whistle. On the other hand, what if your ears were tuned to listen only for the pitch of the whistle? You wouldn't hear much of the crowd noise because most of it wouldn't occur at the pitch you are "tuned" to. But the whistle would come through loud and clear. In the same way, the SETI@home listens in many precisely tuned channels (or frequencies) for signals that are significantly "above the noise." With your help, SETI@home does one of the most detailed and finely tuned searches ever attempted. Your computer will "listen" to the sky for signals as narrow as 0.07 Hz.
>> See SETI in a Parallel Dimension.
posted by Frank Da Silva at 12:02:00 AM
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