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The Earth has been broadcasting human generated radio signals for about 100 years now. If a nearby civilization were broadcasting similar radio signals, could we detect them with our own radio observatories like Arecibo or the VLA?

It's going to depend on the type of signal: early radio/TV v. the Arecibo message. Let's focus on the daily radio emission of modern life as opposed to the directed broadcasts that are designed to be observable.

Arecibo has a sensitivity (system equivalent flux density) of about 1 Jy for 1 GHz scale frequencies (Arecibo info). What is the flux density that we send into space? Is most of our stuff aimed at the ground and/or reflected back off the ionosphere?

I always bring this question up when I get talking about SETI, and I've never gotten a good answer. How far away from Earth could Earth be and still observe Earth?


Update:

At least one paper addresses this: Sullivan, et al.,"Eavesdropping: The Radio Signature of Earth". Science, 199:4327, 377-388 (1978).

I'm sure a lot has changed in our radio signature since 1978. Sullivan, et al. say:

The problem of detecting radio leakage from Earth as a whole is thus essentially identical to the problem of detecting its single strongest transmitter.

This is due to the non-overlap of signals (different frequencies by design plus differing doppler shifts for different regions). So the question can be rephrased:

What is the Earth's most powerful isotropic radio transmitter?

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  • $\begingroup$ I assume then that we're not talking about a directed signal, but rather a similar signal to our own radio waves that broadcast everywhere? I'm afraid the sad truth is that in order to be detectable, it would have to be a sizeable strength, and if it is sizeable to us on earth, it would have to be extremely high on another planet outside our solar system, and an alien intelligence isn't likely going to need more than what we need to send and receive signals locally. $\endgroup$ – Neil Mar 25 '16 at 15:56
  • $\begingroup$ I'm asking about observing our local radio signals, which are indeed quiet, with one of our radio telescopes which is much more sensitive than a local receiver. $\endgroup$ – Paul T. Mar 25 '16 at 16:54
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    $\begingroup$ That one can detect these emissions from the distance of another star is a total myth and so is basically everything that SETI does. A rational engineer (human or alien alike) would not use radio waves, at all, to build a communication link from one star to another. If somebody is communicating with us, they are doing it with light and the spot they are focusing their transmission on is most likely not even in the inner solar system (why would it be?). $\endgroup$ – CuriousOne Mar 25 '16 at 17:21
  • $\begingroup$ @CuriousOne what do you mean by light? The visible spectrum? Lasers? $\endgroup$ – john Mar 25 '16 at 18:44
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    $\begingroup$ @john: Yes, by light. Not necessarily in the visible but fairly close to it. The interferometric baseline for viable optical communication increases with wavelength and the emissions of the parent star are of little concern once sufficient separation can be established. The second important quantity is the achievable focal point size, which, for a given interferometric order is also proportional to the wavelength and halving the wavelength therefor doubles the SNR for the same power budget. Much beyond the visible spectrum technical optics gets difficult, though. $\endgroup$ – CuriousOne Mar 25 '16 at 18:56
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Using current technology (and by that I mean experiments and telescopes that are available now) we would probably be unable to detect radio signals from Earth even if observed from a distance of a few light years. Therefore there is currently no prospect of detecting such signals from (around) another star.

If we are talking about detecting "Earth", and assuming that we are not talking about deliberate beamed attempts at communication, then we must rely on detecting random radio "chatter" and accidental signals generated by our civilisation.

The SETI Phoenix project was the most advanced search for radio signals from other intelligent life. Quoting from Cullers et al. (2000): "Typical signals, as opposed to our strongest signals fall below the detection threshold of most surveys, even if the signal were to originate from the nearest star". Quoting from Tarter (2001): "At current levels of sensitivity, targeted microwave searches could detect the equivalent power of strong TV transmitters at a distance of 1 light year (within which there are no other stars)...". The equivocation in these statements is due to the fact that we do emit stronger beamed signals in certain well-defined directions, for example to conduct metrology in the solar system using radar. Such signals have been calculated to be observable over a thousand light years or more. But these signals are brief, beamed into an extremely narrow angle and unlikely to be repeated. You would have to be very lucky to be observing in the right direction at the right time if you were performing targeted searches.

Hence my assertion that with current methods and telescopes there is not much chance of success. But of course technology advances and in the next 10-20 years there may be better opportunities.

I did read (struggling to locate it now) that SETI were targeting multiple transiting exoplanets. The idea here is that you can wait for the two planets in the transiting planets to come into line and then hope that some beamed signal from the inner planet to the outer planet "spills over" and heads towards the Earth - ingenious.

It has been suggested that new radio telescope projects and technology like the Square Kilometre Array may be capable of serendipitously detecting radio "chatter" out to distances of 50 pc ($\sim 150$ light years) - see Loeb & Zaldarriaga (2007). This array, due to begin full operation some time after 2025 could also monitor a multitude of directions at once for beamed signals. A good overview of what might be possible in the near future is given by Tarter et al. (2009).

Loeb & Zaldarriga give a table (Table 1) in their paper that lists some sources of transmission from the Earth (which are the basis for their claim). The most powerful pseudo-isotropic sigmals appear to come in the 40-850 MHz range from TV transmitters, with a summed power of $10^{9}$ W or $10^{10} $W/Hz.

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