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An electric field, light, a movement of molecules; anything at all. What is the maximum distance they can go before they are 'noise' to an ideal measurement instrument?

So if the distance is infinite, and you can reverse time, can you pull all information back to it's original origin, then back again; or does quantum superposition stop you from pushing the information out again exactly as it was before?

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  • $\begingroup$ If you have enough energy, you can propagate light across the entire visible universe. We are seeing an electromagnetic signal, the cosmic microwave background, that originated just 300,000 light years from the longest possible distance from us (at least in the current distance scale, I think one still has to throw the expansion since the big bang in there to get the real scale, but I am not completely sure). $\endgroup$
    – CuriousOne
    Commented May 18, 2015 at 23:47
  • $\begingroup$ Cosmic background radiation is a collection of signals though. Once a signal 'becomes' part of it, its information is permanently lost. So how is it a signal? It's just noise, right? You cannot pick out the first SETI broadcasts (or whatever signal you want) from it. $\endgroup$
    – ARMATAV
    Commented May 19, 2015 at 0:02
  • $\begingroup$ Define signal @ARMATAV. Cosmic microwave (not general) background radiation came from one point source and hence I would consider that it came from one source. $\endgroup$
    – Cicero
    Commented May 19, 2015 at 0:47
  • $\begingroup$ Anything coherent carrying information. Hawking radiation propagates away from a black hole and into cosmic background radiation, that's not one source. $\endgroup$
    – ARMATAV
    Commented May 19, 2015 at 1:04
  • $\begingroup$ The CMB has a thermodynamic temperature and a certain spectrum. As long as the signal you are trying to detect has a different spectrum, it can be discerned from the CMB, even if is is weak. If the signal carries an actual modulation that distinguishes it from noise, virtually any level of SNR can be overcome with integration over a long enough time period. At this point I am not sure what your real question is. $\endgroup$
    – CuriousOne
    Commented May 19, 2015 at 1:31

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If the light was emitted after the recombination, it can't have traveled over 13.7 billion years. The EGS-zs8-1 galaxy is located 13.1 billion light years away , which is close to the maximum for a plausible picture. More details on the wiki http://en.wikipedia.org/wiki/EGS-zs8-1 .

If the light is not absorbed by an obstacle, it will probably travel to the end of time.

I don't understand what do you mean exactly by "you can reverse time". In the common sense of recording a signal and sending it back, it's impossible to record it completely. Even if it was possible, at such distances, the path covered stretched more , growing faster than the speed of light. At 13 billion and more , the return trip is impossible.

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  • $\begingroup$ Theoretically, if you COULD reverse time, could you follow your signal all the way back home, or is it permanently violated. $\endgroup$
    – ARMATAV
    Commented May 19, 2015 at 2:07
  • $\begingroup$ What do you mean exactly by reversing the time ? reversing the expansion too ? $\endgroup$
    – user46925
    Commented May 19, 2015 at 2:19
  • $\begingroup$ Yeah, reversing EVERYTHING in theory. Is there a point where the information becomes unrecoverable, similar to how the double-slit experiment changes the state permanently? Is the state of my information changed when it becomes part of the noise permanently, and I can never see it again even if I reversed time? $\endgroup$
    – ARMATAV
    Commented May 19, 2015 at 2:22
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    $\begingroup$ If you consider that you can get only a small part of the signal, the original message is lost. But, at the photon level, if one avoids collisions, expansion will redshift it without limit. $\endgroup$
    – user46925
    Commented May 19, 2015 at 2:31
  • $\begingroup$ So if you can't avoid collisions, your information is getting permanently messed up. And if you could, you could just shift it the other way and get the message. But you can't. Kind of sad when you think about it. $\endgroup$
    – ARMATAV
    Commented May 19, 2015 at 2:38

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