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ChickCorea
2011-Apr-28, 12:07 PM
Dear all,

as far as I understood in the SETI project people look for radio waves and signals on the frequency corresponding to the hyperfine structure transition in hydrogen. However all the stars one can look at, have a certain Doppler shift due to the expansion of the universe.
Is this taken into account in every single measurement individually or do people look at a band gap which is wide enough around that frequency that Doppler effects have no influence?

Thanks for the answere.

Best

ChickCorea

WayneFrancis
2011-Apr-28, 02:59 PM
The distance that radio based SETI observations are made are not greatly effected by the expansion of the universe. In actuality the proper motion of the objects is a bigger effect and that is factored in.

IE I'd be surprised to find any SETI observations that occur outside our local group of galaxies for a few reasons. 1) VERY difficult to pick up radio signal from that range amongst the back ground noise. 2) The further away the object the further back in time = less time for life to get to the technology producing period.

...hmmm
I just thought of something I guess one ways of broadcasting to the rest of the universe that you are there could be to set up a dyson ring around a pulsar that can be used to block the beam of the pulsar in a controlled manner. Still a huge effort but you've got one heck of a broadcast tower :P

Amber Robot
2011-Apr-28, 06:12 PM
as far as I understood in the SETI project people look for radio waves and signals on the frequency corresponding to the hyperfine structure transition in hydrogen.

Is that true? I would imagine considering how much hydrogen there is in space that this would be a bad place to look.

EigenState
2011-Apr-28, 07:17 PM
Greetings,


as far as I understood in the SETI project people look for radio waves and signals on the frequency corresponding to the hyperfine structure transition in hydrogen.


That is correct. The SETI@Home data was limited to a 2.5MHz band centered on the HI ground state hyperfine structure transition frequency.

With SERENDIP V, the band will be increased to 300 MHz.

Reference: The Planetary Society: Projects: SETI@home SERENDIP Takes a Great Leap Forward (http://www.planetary.org/programs/projects/setiathome/setiathome_20090526.html), May 2009.

Best regards,
EigenState

Shaula
2011-Apr-28, 09:27 PM
Is that true? I would imagine considering how much hydrogen there is in space that this would be a bad place to look.
Au contraire. The idea being that people would be looking there (radio astronomy) and a non-natural signal should stand out a mile. Picking some obscure band no one ever looks at and you can pretty much guarantee that no one will get your signal!

Amber Robot
2011-Apr-28, 09:40 PM
Fair enough.

Jeff Root
2011-Apr-29, 05:35 PM
My technical knowledge of this is terribly limited. My most
intimate contact with SETI was when my microwave output
was detected and recorded by the Big Ear radiotelescope in
Ohio (no longer in existence).

My understanding is that in addition to the problems you
have in trying to tune a radio with an analogue frequency
control, SETI has the problems of not knowing the carrier
waveform, the modulation technique, or the bandwidth of
an individual "channel". So many different combinations
of values have to be tried, with enough overlap between
the different "channels" that a signal won't be missed in
the gap. But a signal might have a bandwidth that is wider
than any being looked for, making it undetectible in that
particular search.

Cosmic redshift has no relevance to SETI. The motions
of nearby stars do have significant Doppler shift, though,
so it is a factor in detecting signals. Since stars in the Milky
Way are moving both toward us and away from us equally,
redshifts and blueshifts are found equally often. A transmitter
orbiting a star and/or on a rotating planet may have a Doppler
shift due to those motions as large as or larger than that
caused by the star's motion. That Doppler shift could make
the signal harder to detect, but once detected, the signal
should be fairly easy to track, and the variation will tell us
something about where the signal originates.

-- Jeff, in Minneapolis

EigenState
2011-Apr-30, 01:04 AM
Greetings,


My understanding is that in addition to the problems you
have in trying to tune a radio with an analogue frequency
control, SETI has the problems of not knowing the carrier
waveform, the modulation technique, or the bandwidth of
an individual "channel". So many different combinations
of values have to be tried, with enough overlap between
the different "channels" that a signal won't be missed in
the gap. But a signal might have a bandwidth that is wider
than any being looked for, making it undetectible in that
particular search.

They utilize multichannel spectrum analysis.

Best regards,
EigenState