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As an example, if communication was done using visible light, communication would be limited by the respective atmospheres, dust and obstacles along the line of sight. At the same time, multiple channels could propagate in parallel, just as we can image the lunar surface from Earth as a plane of pixels instead of solving only a single dot. So if the entire surface of both planets are used for communication, bandwidth raises, as long as resolution is sufficient. There are many limiting factors in interplanetary communication, and my intuition is that the highest the carrier frequency of one channel (one EM pixel), the more difficult it is to propagate it reliably in such a point to point access. I may be wrong about that, although it is well known cellular networks, reaching higher frequencies do require a tighter grid of antennas. Lastly, I know there is a different answer for every couple of planets one chooses. So here are two examples that cover what I am interested in: Earth to Neptune, and Earth to the closest known planet from a different stellar system (4 light years, Proxima Centauri b or c). My guess is we cannot achieve reliable communication with planets far away, just a few bits per second top, since we almost cannot even image exoplanets. My question is about the physical limits of such communication, not the technical barrier.

Edit: communication only through electromagnetic radiation.

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  • $\begingroup$ As your question stands, it's really too wide-open to have meaning. Consider the famous example of maximum comms bandwidth being a semi-trailer full of flash drives. You seem to have missed the option (such as we have currently in fiberoptic links) for multiple parallel channels as well as QAM, etc. $\endgroup$ – Carl Witthoft Aug 17 at 12:02
  • $\begingroup$ @Carl: I will limit the question to electromagnetic radiation. $\endgroup$ – Exocytosis Aug 17 at 13:01
  • $\begingroup$ Regarding the second part, I did not miss anything, I am asking for information. I did not exclude using multiple frequencies. Yet there are still limits regarding frequency quality factor, spatial and temporal resolution, signal noise ratio over extremely large distances in space, etc. This is a simple question that requires a complex answer, or at least someone willing to try. $\endgroup$ – Exocytosis Aug 17 at 13:10
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My personal opinion is that whatever signal you will pass onto distant planet- most limiting factor will be signal divergence. Using laser communications, maximum bandwidth due to laser beam divergence can be defined as : $$ \Delta \lambda = \pi~ w_0 ~\Delta \theta$$ Where $w_0$ is laser beam radius near emitter, $\Delta \theta$ - maximum allowed divergence for beam. Now lets say we try to establish communication with Neptune planet. If we could build laser emitter array with that of Earth size radius, and for maximum beam divergence substituting Neptune angular diameter which is just $2.3 ~\text{arcseconds}$ or about $1.12 \times 10^{-5} ~\text{radians}$, we estimate maximum bandwidth as $\Delta \lambda = 224 ~\text{meters}$ or about $8.4 ~\text{MHz}$. And this is just for Neptune. For Proxima Centauri situation is far more awful. Probably for communication with Centauri system you would need to build laser emitter array with size of our solar system. Otherwise you will only pass just a few bits into Centrauri with no good. Distances are huge, and so it is a signal divergence effect.

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  • $\begingroup$ Thank you, this is very informative. There is something I don't understand though. In your equation, maximum bandwidth wavelength is proportional to the laser emitter radius. Does that mean the larger the laser, the smaller the maximum bandwidth (in frequency)? If so why use the Earth radius and not something smaller? $\endgroup$ – Exocytosis Aug 17 at 13:40
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    $\begingroup$ I found on Wikipedia "Neglecting divergence due to poor beam quality, the divergence of a laser beam is proportional to its wavelength and inversely proportional to the diameter of the beam at its narrowest point." Now I understand decreasing the radius would limit maximum bandwidth even further. $\endgroup$ – Exocytosis Aug 17 at 13:57
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    $\begingroup$ Yes. Decreasing beam diameter will expand beam divergence angle. But we are targeting very far planets, so beam divergence must be very well controlled. That's why in usual optical setups there's a bunch of collimating lens. So either we send a broad signal to Neptune or we should have signal re-translating stations in between Neptune, which catches signal, colimates and amplifies it, and then passses further onto Neptune. In this case we could have a narrow start beam and good bandwidth. But we need a bunch of intermediate power stations then. $\endgroup$ – Agnius Vasiliauskas Aug 18 at 6:25
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    $\begingroup$ Yes, I thought about these relays, which are clearly a good solution if we were to develop an "Internet of space", but I was curious to know about the limits of point to point transmission. Especially I wanted to know if it really made sense to pretend hypothetical aliens could receive Earth communications and decide they found life. My opinion is that if they are located far enough, they will not receive any meaningful signal, because of noise, cross-talk and dissipation. $\endgroup$ – Exocytosis Aug 18 at 8:46
  • $\begingroup$ Well, then that is the true question you should be asking. Because it's totally different problem. Despite the fact that our TV (or other signal sources) can not be truly decoded to view "X-Files" series in Proxima Centauri,- one may be able to extract signal modulation info, to recover the fact that signal is highly periodic. Periodicity in signal is enough to conclude that it was send by humans, if one can eliminate natural periodicity causes, such as quasars, spinning neutron stars, etc. SETI is based on that. $\endgroup$ – Agnius Vasiliauskas Aug 18 at 13:22

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