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There are two important kinds of signals:
(click to hear)
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Narrow-band
Contains a single frequency.
Spectrum is narrow.
Doesn't occur in nature.
Sources: TV/Radio stations, cell phones
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Broad-band
Contains many frequencies.
Spectrum is broad.
Occurs in nature.
Sources: stars, planets, nebulae
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(click and drag to change frequency)
So if we hear a narrow-band signal,
it's either an ET,
or it's a man-made signal coming from Earth.
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