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I've always wondered about the concept of listening to radio signals from space to try and look for alien life - how accurately can we distinguish "man"made radio signals from other forms of radiation over such long distances?

To ask a specific question: If, ten thousand years from now, a civilization that is an exact copy of today's humanity, but lives ten thousand light years away from Earth, turned their telescopes or receivers or whatever devices you use for this purpose towards the Solar system, would they be able to pick up signals from today's Earth that gave away the presence of life? What can be picked up? Just vague spikes on a chart that indicates something unnatural is going on, or could they listen the evening radio broadcast? Or would it merely be shrugged over as some astronomical phenomena or natural variances in background radiation?

What if that distance was only 10 light years? Or a million light years? Would we be able to pick up signals from the other side of the Milky Way, through the gas and dust and the natural radiation? How about visible light or thermal emissions?

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    $\begingroup$ One problem for picking up alien radio signals: There is only a short window (on the scale of perhaps 50-100 years) of easily recognizable analogue radio/tv transmission. Error corrected digital channels are indistinguishable from noise (you can only find them if you know exactly what you are looking for). After that the most easily trackable signals might be periodic beacon signals or a strong OTH radar may leak periodic signals (even though most radiation is reflected by the ionosphere) with recognizable coding. $\endgroup$ – Sebastian Riese Aug 28 '15 at 23:00
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    $\begingroup$ Radio-astronomers pick up radio waves from space all the time, mind you. Now, tuning in to an alien concert special is a little different. But, in the right frequencies, and assuming something out there trying hard, well, maybe... $\endgroup$ – Jon Custer Aug 28 '15 at 23:23

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