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What is the the cosmological footprint of humanity? By this I mean, how might a cosmological observer detect humanity or its products on earth?

  • How far away can humanity or it's products (not mere landscape changes that might have happened naturally) be detected by humanly visible Light? (The story of the Great Wall of China visible from moon is a legend, but the shadows of the Great Pyramids and other huge buildings can be seen from outer orbits.)
  • When was humanity first detectable in space by something else than visible light? (EM-Emission, other waves?)
  • What products of humanity other than EM-Waves are there in outer space and how far are they away? (Voyager, Lunar and Mars Rovers)
  • In what parts of the EM-spectrum is humanity clearly detectable against the Sun's background emission and how far away is that possible? (How far away can a cosmological observer watch human television or listen to human radio?)
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    $\begingroup$ How far can we detected depends on the precision of the measurements. $\endgroup$ Jun 17 '14 at 8:55
  • $\begingroup$ @AntonioRagagnin Does it really? I though it won't be detectable due to background noise. $\endgroup$
    – Toscho
    Jun 17 '14 at 14:48
  • $\begingroup$ No, it has very special spectrum (for example, the quanting noise of the digital signals or the line synhron of the analogous tv). The different senders couldn't be differentiated, but the radio wave spectrum of the whole earth is very significant and nothing like this is coming from natural sources. $\endgroup$
    – peterh
    Jun 17 '14 at 18:33
  • $\begingroup$ +1 for the good question. But regarding pt. 1, how do you know which part of the EM spectrum has the observer evolved in, so that it is ''visible light'' for him/her ? Strictly, on the cosmological scale, there is nothing special about $400-700 \ {\rm nm}$, or is it? $\endgroup$
    – 299792458
    Jun 18 '14 at 7:58
  • $\begingroup$ @New_new_newbie I mean't humanly visible light. The other wavelengths were covered in another part. But I will change it to make it clearer. $\endgroup$
    – Toscho
    Jun 18 '14 at 10:28
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Important note: I don't looked for references a lot only because I gave mostly wellknown information. Please don't downvote because of this - I am ready to provide them on need.

I assume you are thinking about our measurement possibilities. Thus your question is to know, from how far could be the humanity detected with devices equal precise as our current ones.

I think, the answer is the following.

1) How far away can humanity or it's products (not mere landscape changes that might have happened naturally) be detected by visible Light?

The far most visible light what the humanity produces is the city lights of the dark side of the Earth. It makes the dark side significantly lighter. I think, it were surely visible from where there is possible to get a dissolved (i.e. not point-like) picture. In the case of the farthest Kuiper-belt objects is it yet nearly possible, but in the case of even the nearest exoplanets is it totally hopeless. This is because I estimated this as a 1000 A.U. (0.015 light years) or so. This is the distance from which the Hubble could detect the city lights of the dark side of the Earth.

2) When was humanity first detectable in space by something else than visible light? (EM-Emission, other waves?) I think it was the first radio waves. The first happened in the 1890s, but large scale radio waves, with their very special characteristic began only around 1930. This is 80 years. I want to mention, that our radio wave spectrum has significant changes with the decades (TV spectrum added, digital radio & television, wifi, etc). AFAIK in our technology - radio wave observatories - it could be easily detectable in this 80 light year across sphere around the earth.

3) What products of humanity other than EM-Waves are there in outer space and how far are they away?

The most distant object is the Voyager 1. It is currently around 125 A.U. (0.002 light year) distant. Their radio signals are currently good detectable with a satellite cluster - but only because we know, for what we need to looking for.

4) In what parts of the EM-spectrum is humanity clearly detectable against the Sun's background emission and how far away is that possible?

See 2). For far (i.e. from AUs) away is theoretically impossible to differentiate the senders giving on the same frequency, but it is quite possible to get the "sum" of their signals. This doesn't get a comprehensible data, but has very special characteristics [frequency spectrum], which we never get from any natural sources.


For your not said question, I am sorry to worry you. There is total radio silence around us for apparently non-natural sources.

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