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In a few sci-fi and fantasy books I've read (IIRC, In the Year 2889 by Jules Verne was one of them), people had global communications through mirrors that are lined up exactly with each other. ( obviously, visual only; no audio )
Would this be possible to set up, at all? ....assuming it is possible, would it be possible to set this up with a society that has ~1500 CE technology?
Impossible given 1500CE technology
First issue: mirror quality
I'm going to assume that squinting and looking really far away is not an option, so you want to see some curved mirrors redirecting light. I'm assuming something akin to the much larger parabolic reflectors used for whisper dishes.
The key to any long distance transfer is that normal light radiates in all directions and you really need to columnate it into a highly parallel beam of light. You then have to catch that parallel beam of light, and display it. A pair of telescopes pointed at each other would form the basic structure.
First key limit: there would be no depth to the image. It would not appear as a hologram, where you can view it from any angle. The columnating process would pick one viewpoint, and that is the only viewpoint you could show.
Next major limit: optical quality. The telescope was not invented until around 1600CE, which gives suggestions as to their comprehension of optics. But lets advance the timeline of your question to at least 1617CE, which was the first mirror telescope. In fact, lets advance it to 1672, with Laurent Cassegrain and his telescope which used mirrors. Its style is used today.
If you think about the Cassegrain telescope in reverse, it becomes a projector. In effect, the light from any object at the "eyepiece" will be mapped to a block of parallel beams emitted from the telescope the size of the reflector. On the other side, one gathers that light, and recreates the image. We can use this to define the angular accuracies needed for the image.
A Jules Verne style communication device would at least be accurate enough to read lips. Let's give a best case scenario where the communicators are focused only on the area of the lips used in lip reading, as opposed to showing an entire face (which would be desirable for detecting emotion). It has been shown that an 80x60 box of pixels centered on the face is sufficient for lip reading, so our receiving telescope needs to at least be able to identify that many pixels across the surface of the mirror.
Now at this point we're going to start having to break down and cheat. Global communication would involve a hopelessly difficult to manage network of mirrors with many losses, so lets target a smaller goal: project and receive the image over 25km.
"Heat haze" is a major issue for snipers, because the turbulent air of different temperatures refracts the light around unpredictably. They have trouble over distance of merely a mile, and they don't need to have magnification levels sufficient for reading lips. Over longer distance, heat haze would demolish any hope of seeing a meaningful image. (This effect is the primary limiting factor of ground based telescopes until the turn of the millennium, with adaptive optics).
So lets pretend you can put a shroud on the air column. Make a large cylinder with a telescope on each end. Let's say this stabilizes the air, so that we are only limited by the quality of mirrors available in 1672. Now let's go big or go home: a 8m telescope like a modern telescope would give us a much better chance of this whole thing working than if we limited ourselves to the sizes seen in that era. The diffraction limit for telescopes is $\alpha_R = \frac{138}{D}$ for visible light ($alpha_R$ in arcseconds, $D$ in millimeters). Applying our diameter, we see a diffraction limit of $\alpha_R = .01725 arcseconds = 8.363036\cdot 10^{-8} \text{radians}$
On the sending side, our 8m projector can be diced up into 80 pixels on each side. This means each pixel is .1m tall. At 25km away, that is $4\cdot 10^{-6} \text{radians}$
What does this say? If we built a 25km long shroud between the mirrors to cut down on the turbulence of the air, we are still within 2 order of magnitude of the diffraction limit. The diffraction limit is something we worry about with modern lenses and modern processes. The processes of 1500CE would not get anywhere near that limit, so the images would be too hopelessly blurred, even in that case.
Why did I choose 25km?
There's another reason this wouldn't work well. Unless you plan on digging massive tunnels to go with your giant shrouds, you can only shine light to the edge of a horizon. 25km is roughly the limit for how far you can see from a 100m tall tower (its actually 35km, but the point stands).
To go any further, you would need to redirect the beam, but that is not easy. Bending light beams calls for waveguides, well beyond 1500CE technology. Passing them through a lens won't work because that would easily demolish the image quality (mirrors are much better for this sort of thing due to a lack of chromatic aberrations). You would either have to set up a giant horizontal mirror, perfectly aligned so that you can skip the light-beam off of it, or you would need to aim at an angle to your target, so that you can bounce off of a smaller mirror which is further from the ideal transmission direction.
It's definitely possible to set up medium-to-long range systems like this - but a truely global system would be difficult/unwieldy enough to be effectively impossible.
Setting up two mirrors so that standing in front of one and looking into it lets you see whatever is in front of the other mirror is straightforward enough. Scaling it up from there is just an engineering problem - but a difficult one.
First, you need very high precision in both making and positioning your mirrors*. You'll need the mirrors machined to an incredibly high level of smoothness - probably similar to that used for high-end modern telescopes. Secondly, you need to put it in exactly the right position - if you're trying to spot a metre-wide spot 100km away, then you need to get the angle right to within half of one-thousandth of a degree. As mentioned above, this is 'merely' an engineering problem ... but probably one solidly out of reach of 1500CE technology.
Secondly, there is a fundamental limit on the resolution of an optical system based on the size of the apertures involved. A 10mm resolution for the above-mentioned 100km system would need a mirror almost 5m across - and that resolution would be barely useable. Certainly not good enough to recognise faces, so you'd have no way of knowing who you were talking to. Trying to improve the resolution causes the mirror size to go up fast - 1-2mm resolution (which we would still consider pixellated/blurry...) needs a mirror 50 to 100m in diameter. Once again, a 'mere' engineering problem... and once again, one that would make the project impossible to the target tech level.
*Lenses are theoretically useful as well, but refractive variations between different wavelengths will cause problems ... and we're going to have enough problems as it is. They're just not worth the trouble.
Err... Fiber optics?
(And why can't a meaningful answer be less than 30 characters?)
But to elaborate, fiber optics sends light waves down precisely-aligned transparent media which eliminates distortions such as heat haze, fog, &c that you find in air, and also allows the light to traverse a non-straight path. Granted that current technology mostly uses them to carry digital information, but they can be used for viewing, e.g. endoscopes.
(And why can't a meaningful answer be less than 30 characters?) You could ask on meta.
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Dec 26, 2014 at 20:21
I'm not sure it is that different from semaphores. A few mirrors along the way + light source + morse code = telegraph.
If you have very rough, mountainous landscape without much snow, and dark nights, it becomes viable.
Only if your vision is good enough to detect things at extreme range.
If you were willing to use telescopes, maybe you could look at a mirror and see around a corner within visual distance. Not past the Earth's curvature.
But making a telescope lens/mirror big enough to display something that could be looked at by another telescope would be... problematic.
While not possible with non-magical/steampunk 1500's technology, it should be viable with near future technology. We're working on flexible displays. From there it becomes a small step to put such a plastic display overtop a mirror and hook it up to a webcam. There would be technical hurdles to make the film not interfere with normal mirror use, and to not cause the mirror to reflect the display in odd ways, but that seems not insurmountable, even in our lifetimes.
Or, of course, you could just have a normal flat panel that displays what it sees when not active. It "mirrors", even if it is not a mirror itself.
