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Space Forum / SETI / May 2004What's missing from this picture?
I've been reading the various posts to this group and it occurs to me that I haven't encountered this yet. Has it been proposed yet to study hydrogen line spectra sources and to look for missing frequencies? I know that the emmision probabilities of the various hydrogen spectra have been accurately measured, so what, if anything, would prevent detection of a possible ETI based on their use of a specific frequency of hydrogen without the attendant frequencies that are observed from stellar sources? Have I totally missed something obvious here? James > hydrogen spectra have been accurately measured, so what, if anything, > would prevent detection of a possible ETI based on their use of a > specific frequency of hydrogen without the attendant frequencies that > are observed from stellar sources? I'm not clear what you are trying to say, but the hydrogen line most associated with SETI is not due to hot stellar hydrogen, but to cold, interstellar hydrogen, and is an emission line. It is also observed at doppler shifts of up to 100s of MHz, even though the unshifted frequency is used in atomic clocks. Putting transmissions into absorption lines is normally considered as an optical SETI option and there are a lot more candidates than hydrogen. It's considered for optical SETI because the continuum spectrum from the stars is a problem at such frequencies, but not at microwave. > > hydrogen spectra have been accurately measured, so what, if anything, > > would prevent detection of a possible ETI based on their use of a [quoted text clipped - 6 lines] > at doppler shifts of up to 100s of MHz, even though the unshifted > frequency is used in atomic clocks. My thinking is that for example, the ratio of intensities for hydrogen emission spectra using the wavelengths of 102.5722nm and 97.2537nm is a pretty good constant for natural sources of excited hydrogen. What if two astronomical surveys, one at each wavelength, were compared. Could the presence of one wavelength for an object in space and not the other be interpreted as a source of intelligent origin? >>>>> "JJY" == James J Youlton <youjaes@ix.netcom.com> writes: JJY> My thinking is that for example, the ratio of intensities for JJY> hydrogen emission spectra using the wavelengths of 102.5722nm and JJY> 97.2537nm is a pretty good constant for natural sources of JJY> excited hydrogen. What if two astronomical surveys, one at each JJY> wavelength, were compared. Could the presence of one wavelength JJY> for an object in space and not the other be interpreted as a JJY> source of intelligent origin? Ah! Not necessarily. I think one would have to demonstrate that there could not be sources whose temperatures could not produce ionization ratios comparable to what would be observed. Also, for the particular example you've chosen, IIRC the scattering cross section for photons of these wavelengths is both large and somewhat different. (There's a strong wavelength dependence.) The result is that the mean free path at these wavelengths can be quite short. Thus a source seen at the longer wavelength and not at the shorter wavelength could simply be far enough away that essentially all of the short wavelength emission is absorbed but enough long wavelength emission makes it through the interstellar medium. For your particular choice of wavelengths, you might want to read about the Extreme Ultraviolet Explorer, which actually conducted observations in the EUV band.  SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html >>>>> "JJY" == James J Youlton <youjaes@ix.netcom.com> writes: JJY> I've been reading the various posts to this group and it occurs JJY> to me that I haven't encountered this yet. Has it been proposed JJY> yet to study hydrogen line spectra sources and to look for JJY> missing frequencies? I know that the emmision probabilities of JJY> the various hydrogen spectra have been accurately measured, so JJY> what, if anything, would prevent detection of a possible ETI JJY> based on their use of a specific frequency of hydrogen without JJY> the attendant frequencies that are observed from stellar sources? I'm also not entirely sure I understand what you are asking. Do you have in mind something like looking for emissions from tritium, which is not a long-lived isotope of hydrogen, like <URL: http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1986Icar...65..152V >? SignatureLt. Lazio, HTML police | e-mail: jlazio@patriot.net No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html > I've been reading the various posts to this group and it occurs to me > that I haven't encountered this yet. Has it been proposed yet to [quoted text clipped - 8 lines] > > James Perhaps you are thinking of something like (sound) noise canceling headsets? The kind which listen to incoming noise and produce an inverted (180 degrees out of phase) wave form to cancel the noise? I am not sure that our technology is up to doing something similar in the hydrogen (microwave radio) bands yet, but it might not be impossible. And it would almost certainly attract attention. Does anyone know whether such a feat might be possible and at what cost? |
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