If we are going to "listen" for electromagnetic radiation at 21 cm from extraterrestrial sources,then we had better not generate too many waves at this wavelength from terrestrial sources.The standard FM broadcast band (as you can see by looking at your radio dial)spans frequencies from 88 MHz (million cycles per second)to 108 MHz.How does the frequency of the 21-cm signal compare with the FM band? The frequency of the 21-cm line is:

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The Answer of If we are going to "listen" for electromagnetic radiation at 21 cm from extraterrestrial sources,then we had better not generate too many waves at this wavelength from terrestrial sources.The standard FM broadcast band (as you can see by looking at your radio dial)spans frequencies from 88 MHz (million cycles per second)to 108 MHz.How does the frequency of the 21-cm signal compare with the FM band? The frequency of the 21-cm line is:

If We Are Going to "Listen" for Electromagnetic Radiation at 21