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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.)
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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 ion, or OH, ion 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.
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Interfering noise (in red) is the combination of lower-frequency galactic
noise and higher-frequency noise from our atmosphere. The "water hole"
(in blue) is between the lines for hydrogen (H) and the hydroxyl (OH) ion.
There is very little noise near the "water hole."
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An important fact to note here is that the narrower a signal
(the more refined the frequency), the more efficient it is for our friends
to send. A narrower frequency signal is also easier for us to pick up above
the normal noise level. Unfortunately, very narrow frequency signals mean
that we have to look at millions of very narrow frequencies to find just
the right one. More on this later.
What kind of signals are we likely to receive? What should
we be looking for? There are really two possibilities. Either the other
civilizations are intentionally sending out a signal that is expressly meant
to get our attention, or, like us, they just happen to be doing their own
business and some of their signals are "leaking out." The earth
broadcasts lots of stuff unintentionally. Our radio and television transmitters
are very strong. Our military radars are even stronger. We've been leaking
this stuff for at least 50 years, so our signals are about 50 light-years
out. Not very far, but we are still technological infants. It seems like
we should try to search for the leakage-type signal and assume that we will
also pick up anything that is beamed at us.
What Do We Look With?
As a signal spreads out from its source, it gets weaker
and weaker. The distances between stars are so vast that any signal that
reaches the earth will be very weak (unless it's being beamed at us). To
gather up enough of this signal we need a giant "ear." In our
case, that means using a huge radio telescope.
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