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SETI: The Radio Search Story by Ron Hipschman

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 visit9 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.

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 going 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.

If you point your radio telescope at the sky there are all kinds of signals. Some come from the galaxy itself. Some come from our atmosphere. If you made a diagram of this unavoidable noise, you would notice that it's really noisy at low frequencies because of the galaxy. It also becomes noisy at higher frequencies due to atmospheric noise. Between the two noisy regions you would find a relatively quiet region - from about 1 GHz to about 10 GHz. (1 GHZ or 1 GigaHertz is 1 billion Hertz or 1 billion vibrations per second This part of the radio spectrum is just above the part used for your electronic pagers and many wireless phones.)

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 H20).'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 meet9 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.