Telescopes can tune in to alien TV
Public release date: 25-Oct-2006
Contact: Claire Bowles
New Scientist
"Radio telescopes designed to study the primordial universe could also eavesdrop on extraterrestrial civilisations similar to our own. 'By a happy accident,' says abraham Loeb of Harvard University, 'the telescopes will be sensitive to just the kind of radio emission that our civilisation is leaking into space.'
"The next generation of radio telescopes are designed to pick up radio waves emitted by neutral hydrogen molecules in the early universe. These signals originally had a wavelength of 21 centimetres, but the universe has expanded since they were emitted, stretching the waves in the process. Today, these signals have a wavelength of several metres,
corresponding to a frequency of tens or hundreds of megahertz. 'This overlaps with our civilisation's radio emissions, which are in the range 50 to 400 megahertz,' says Loeb."
http://www.eurekalert.org/pub_releas...-tct102506.php


Bob Clark

They'd be lucky if the signal retained coherence over any serious distance unless it was a sickeningly powerful transmitter.

lol, nice choice of words

Can you point me to a web-site that explains how an EM signal going a long distance (hundreds of light years) in space loses coherence? I'm not disputing it, but I've seen this argument made a few times but never understood the mechanism by which it happens.

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It seems reasonable to assume that "content corruption" would be caused by cosmic noise frequencies in the same bandwidth with destructive phase relationships and dust laden paths of thousands of light years length. Can it be that astonishing, Mr. Fermi?

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Great, we can watch reruns of I Love Vordax.

It can't be that, since the "dust laden path" has less gas and dust in it than does our atmosphere. Also, with a sufficiently large detector, we can narrow the viewing spot of the signal to a modest sized number of diameters of the sending planet, thereby reducing the interfering background AND guaranteeing sufficient photons to have a good statistical signal.

Now, if something like the SKA were on a planet 30 light years from here, and looked at the Earth, it would probably not be able to tune in specific TV or FM Radio signals, becuase we use the same frequences from many geographically separate points, thus confusing the signal. The users of that array WOULD however be able to see our unnatural use of the radio spectrum sufficiently to decide whether to build a larger array so as to try and tune us in.

--- Unless this loss of coherence is real. I am looking for a site that explains it so I don't go telling people things that aren't true about detecting alien signals.

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My personal guess is that if we can get through the bottleneck to a Type One, we will reach Type Three in a cosmological second. So a "sickening level of power" may be the equivalent of a single TV station to such a civilization.

That we could detect that a signal was artificial is fairly certain. Whether we could "watch alien TV" is a completely different thing. So much of the parameters of our own broadcasts (frequency, bandwidth, modulation rate, modulation type, scan rate, interlacing, number of scan lines per frame, etc.) are due to the limitations of early to mid twentieth century technology. Others, such as the choice of RGB color are due to our own physiological limitations. It would be amazing if we could even decode an alien transmission into a coherent picture, much less know that it was even close to "correct"!

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T. Anderson

only a perfect infinite beam retains its coherence. Since the beam had a beginning and an end it can lose that. We describe waves as combinations of other, simpler, waves. Fourier did that for us. Over time they dephase and other things happen causing the decoherence.

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http://people.deas.harvard.edu/~jone...opagation.html

Found this one, the math is probably just within my grasp, given enough paper and caffeine, but it describes the loss of signal strength in freespace.

As for the content integrity of the signal, I think I might have been inaccurate with that due to a limited grasp of the concept. What I likely should have been aiming for is the severe drop off of the signal's strength as its broadcast over distances trillions and greater times larger than intended. The signal's content might well be intact, but over interstellar distances, it would be a real neat trick to pick it up through all the natural garbage broadcast by stars and other electromagnetically active objects out there. To say nothing of distortion through gravitational lensing and the red or blue shifting involved in passing by any intervening objects with substantial gravity wells.

What I find impressive about this is how they apparently will be able to compensate for the noise our own planet puts out into space at this frequency. And expect to hear aliens over it...

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