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For about a century, humanity has broadcasted noise to space in the form of TV broadcasts, radio, etc...

The nearest star is 4.242 light-years away, so for about 90 years, aliens -- if they exist in the Alpha Centauri system -- have been able to listen to this noise.

Is humanity capable of eavesdropping on similar noise from aliens? I mean, low quality noise from a civilization similar to ours.

As an example, some civilization, on the back side of the Milky Way, could probably catch some information after a couple of hundreds centuries with very sensitive receiver. Can we?

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Because radio waves in space follow the inverse square law, it would require an exceptionally large antenna to pull in enough of an ordinary radio signal to actually notice it, let alone get any useful data.

Consider that a typical radio signal is designed for distances measured in miles. TV broadcasts disappear into the noise within about 100 miles from the transmitter. One light year is about 5.9 trillion miles, or about 59 billion times farther than that, so the power of the a TV station at that distance is 59 billion squared times as weak.

Now a signal to a geostationary relay satellite is designed to be received at a distance of about 30,000 miles. One light year is about 200 million times that distance, so the signal would be 40 quadrillion times weaker at one light year, or 640 quadrillion times weaker at 4 light years distance.

It might be possible to use techniques such as autocorrelation to turn the problem from a near impossibility into something merely very difficult; at the present time, Earth does not possess any capability for receiving anything short of deliberate attempts to contact us.

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Short answer - it's theoretically possible but extremely improbable. I found this very well referenced article explaining some of the challenges involved. Basically, we would need a REALLY huge receiver and we would need to know exactly where to point it. And we would need the extraordinary luck of pointing it during a time exactly like this one when a civilization has radio technology, but has not refined it enough to stop wasting huge amounts of energy pouring their signals into space.

But, this perspective is also taken from our current vantage point in a civilization that is barely advanced enough to start turning down the noise a bit. Fractal antennas, for example, were a huge game-changer in this area in some ways. Cell phones are much smaller now AND have better range, and we haven't even been using this tech for 20 years yet. So what might be possible in 100 years?

There are more and more antennas on this planet every day. I haven't found any data on the exact exponential growth rate, but I would venture a guess that the quantity of similar quality antennas has been doubling every 5 or 10 years recently. They are not directional, but who's to say that some new breakthrough in 50 years won't allow the NSA SETI-at-home to tap into every device with an antenna and use the whole planet as if it was one big ear and with precise timing circuits, quantum computing, intelligent noise cancellation, and crazy new signal processing technologies, be able to hear the entire sky at once with sufficient sensitivity to hear the birth of a new technologically active civilization of cuddle-fish people.

Until some of these sci-fi ideas become a reality, no, I don't think there's any real chance of hearing anything. But let's just wait and see what somebody comes up with next. I think it might become possible one day. And if so, maybe someone will hear us as our signals continue to dissipate out into deep space over the next several centuries.

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