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The 21cm wavelength is also the wavelength that was proposed for potential SETI radio communication, I guess because of its distinctiveness. Of course modern SETI searches look at a wide range of frequencies.

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Calwestjobs 18 hours ago | parent [-]

if that frequency can be generated by natural phenomena then why it is so good for seti ? should not signal distinctive from natural fenomena be used instead ?

	▲	wiml 18 hours ago \| parent [-]
		SETI uses the gap between the H line and the OH line (the "water hole"). The whole region of the spectrum is kind of quiet for reasons, and the H/OH area is (perhaps parochially) thought to be a natural place for other water-chemistry life forms to look.