It`s true stars emit trillions of watts of electromagnetic radiation. It`s also true that radio waves have very long wavelengths which mostly range from less than a meter to 10 kilometers. Thus, they`re only a part of that electromagnetic energy at the long end, (in terms of wavelengths), of the electromagnetic spectrum. But only certain radio wavelengths would be useful for this type of communication. Wavelengths longer than about 30 cm can`t be used since the signal would be lost in the background radio noise from our galaxy. We also can`t use wavelengths much smaller than 1 cm because they`d be absorbed by our atmosphere. Therefore, only a certain range of wavelengths can be used. Within this communication range lies the 21-cm line of neutral hydrogen and the 18-cm line of OH. This interval between these two lines has been dubbed the 'water hole' because the combo of H and OH yields water. Water is the fundamental solvent in our life form so it only seems natural for similar water creatures to call out to each other at wavelengths in the water hole. But even a silicon creature would be familiar with the 21-cm line of hydrogen. Thus, they too might select wavelengths near the water hole. But such messages would have to be sent consisting of a pattern of pulses obviously designed by intelligent beings to distinguish it from natural radio sources emitted by nebulae and pulsars and so on. For ex., pulses counting off the 1st 12 prime nos. would do. Hope this helps.

• Re: Radio Telescopes - charles/">charles - February 25, 2000 - 19:07 UTC