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Thread: radio waves detectable from another galaxy?

  1. #1
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    radio waves detectable from another galaxy?

    i have another question. this is in connection with my previous question...
    if a civilization had (has) formed on a nearby galaxy as early in the universe as possible and developed the ability to send radio waves into space as quick as we did, would it be possible that we could detect those radio waves by now?

  2. #2
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    Quote Originally Posted by paulel View Post
    i have another question. this is in connection with my previous question...
    if a civilization had (has) formed on a nearby galaxy as early in the universe as possible and developed the ability to send radio waves into space as quick as we did, would it be possible that we could detect those radio waves by now?
    There are probably two parts to the question. There are about 150 galaxies within 12 million light years, so the civilisation needed to have arisen any time prior to 12 million years ago for that to be possible. In terms of the age of the universe that's extremely recently, so yes.

    The second part is probably trickier - the size of the equipment required to detect (say) interstellar communication from a neighbouring galaxy and distinguish them as such from the background radio noise would be mind-boggling.

  3. #3
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    Turn the question around, how detectable are Earth radio signals detectable from elsewhere.

    Emily Lakdawalla touches on it.
    Humans have been broadcasting radio waves into deep space for about a hundred years now, since the days of Marconi. That, of course, means there is an ever-expanding bubble announcing Humanity's presence to anyone listening in the Milky Way. This bubble is astronomically large (literally), and currently spans approximately 200 light years. But how big is this, really, compared to the size of the Galaxy in which we live (which is, itself, just one of countless billions of galaxies in the observable universe)?

    ...

    A special note to the pedants: yes, I do realize that the signal from our radio and TV broadcasts is so attenuated by that 100-light-year boundary as to be undetectable except by some kind of magical alien technology.
    From a BBC News story
    But television and radio broadcasts are omni-directional - albeit focused as much as possible towards the horizon - and that means a lot of diffusion.

    Assuming the energy spread out equally in a sphere, and that the receiver on Gliese C was as big as the planned Square Kilometre Array of antennas on Earth, the television signals reaching the planet would be a billion, billion, billion times smaller than the original signal generated on Earth, says Dr Maggie Aderin, a space scientist at technology firm Astrium.

    "Detecting a signal like this with lots of background noise would be incredibly hard, but what they would look for is a pattern in the signals to show that they were not naturally occurring."

    ...

    "Some of our radars are easily detectable quite far, hundreds of light-years, into space, if the aliens wish to try, and if they're in the beam," says Seth Shostak, an astronomer at Seti.

    ...

    Shostak calculates that Nasa's recent broadcast of Beatles music towards Polaris, the North Star, using a 210ft antenna and 20kW of power, would require any potential aliens to have an antenna seven miles across to be aware of it. To actually receive it as music, this would need to be increased to a 500-mile wide antenna. Polaris is 430 light years away.
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  4. #4
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    From Table 1 here, it looks like something along the sensitivity of the Arecibo dish could receive a signal from a comparable dish transmitting 720 light years away. So even if that's a low estimate, or if we had significantly more power or sensitivity, I'd think intergalactic communication would not be possible.
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  5. #5
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    thanks all for replies and input. I was just curious.

  6. #6
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    A few things to add:

    First, the frequency and bandwidth are important, of both transmitter and receiver: apart from considerations of noise suppression or minimization, some choices would make detection distances much greater ... if only technology we know and love were to be deployed.

    Second, the calculations etc posted above seem not to consider more modern (shall we say) radio "receivers" such as LOFAR and the SKA.

    Third, while we do not have the ability to produce a radio transmission detectable over inter-galactic distances (with our current receivers/telescopes) - especially a highly beamed one - nature/the Universe manages just fine ... AGNs can produce jets which produce lobes which are easily detectable with current radio telescopes, even out to z ~5+. If we tried really hard, could we make a directed radio burst powerful enough to be picked up in M31, say?

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