# What is the maximum electromagnetic bandwidth achievable between planets? [closed]

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.

• 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. – Carl Witthoft Aug 17 at 12:02
• @Carl: I will limit the question to electromagnetic radiation. – Exocytosis Aug 17 at 13:01
• 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. – Exocytosis Aug 17 at 13:10

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.