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...are summarized in this diagram. The black area shows which civilizations could have eluded our radio searches, either because they are too far away or because their transmitters are too weak. To make sense of this diagram, choose a transmitter strength (vertical axis), read across to the edge of the black area and go down to find the distance from Earth (horizontal axis). For example, an Arecibo-class transmitter of 1014 watts must be farther away than about 4,000 light-years to have eluded the searches altogether. The color code provides more detailed information--namely, the estimated percentage of all star systems that have been examined for transmitters of a given power or greater.
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