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Here, I outline the nature of the electromagnetic spectrum. I note that various living organisms can detect ultraviolet, visible, and infrared wavelengths, but that none can detect the longer wavelengths of radio. This may be partly due to the fact that there is little to be gained from evolving such an ability and partly because unrealistically large eyes would be required to ‘see’ these long wavelengths. I then turn to human use of radio, which began in the early twentieth century. I consider the question of when the ‘radio age’ started, from the perspective of our transmissions into space – both accidental and deliberate. Leakage of broadcasts could have occurred since the 1920s; messages specifically aimed into space began in the 1960s. When the radio age will end is hard to predict; guessing how long other broadcasting civilizations will last is even harder. However, using both optimistic and pessimistic figures, I use the Drake equation to guestimate how many broadcasting civilizations there are in the Milky Way right now. The result: anything from just one (us) to about a quarter of a million. I end by stressing the difference between radio waves and radio signals.
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