In a message dated 10/28/2003 10:38:28 PM US Mountain Standard Time, [EMAIL PROTECTED] writes:
> I thought for sure *you* would recognize the specific source of
> electromagnetic radiation with a wavelength of 21 cm, which corresponds to
> a frequency of 1420 MHz . . .
>
>
> Anyone?
>
>
>
> -- Ronn! :)
>
Hydrogen absortion line that was used by the Vegan broadcast in Contact?
A bit more general than that. Radiation with a wavelength of 21cm or a frequency of 1420 MHz corresponds to the energy difference between the two hyperfine states of the neutral hydrogen atom: i.e., to the energy difference between the states when the spins of the proton and the electron are parallel and when they are antiparallel (the spinning charged particles generate a magnetic field which in one case is aligned in the same direction around both particles, so they repel each other, and in the other case is aligned in opposite directions, so they attract each other). It has been used for several decades to map the location of clouds of neutral hydrogen (HI) in the spiral arms of our Galaxy, and so to map the spiral arms of the Galaxy. Beginning about 1980 or so, Tully and Fisher realized that the Doppler broadening of the 21-cm line from distant spiral galaxies due to the rotation of the distant galaxy (the HI regions on one side of the galaxy would be moving toward us while those on the opposite side would be moving away from us, and the more the difference in the velocities, the wider the line), and that the rotational velocity of a spiral galaxy is related to its size, hence to its overall luminosity, so measuring the broadening of the 21-cm line in distant spiral galaxies could give us a way to measure its true brightness, and then by comparing that to the apparent brightness, provide a way to measure its distance.
Many astronomers interested in SETI have suggested that, since the 21-cm line is a universal standard because hydrogen is the most common element in the Universe, it would be reasonable for ETs interested in broadcasting a signal which they wanted to be detected by other intelligent beings to broadcast a signal a bit on one side or the other of the 1420 MHZ so it might be detected accidentally by radio astronomers on other planets looking for, say, Doppler broadening of the 21-cm line from natural sources, as I described above . . .
-- Ronn! :)
Ronn Blankenship Instructor of Astronomy/Planetary Science University of Montevallo Montevallo, AL
Disclaimer: Unless specifically stated otherwise, any opinions contained herein are the personal opinions of the author and do not represent the official position of the University of Montevallo.
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