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For one of my (many) science-fiction projects, I am designing immense rotating space-habitats. These titanic structures, (similar to Bishop rings; see here https://en.wikipedia.org/wiki/Bishop_Ring_(habitat) have an area comparable to that of Germany, and are large enough to retain an atmosphere, with just high walls that prevent the air from spilling out into space.

However, awesome as these things are, they need a source of illumination. My first thought was some kind of mirror arrangement, but I kind of want there to be something visible as a sun in the habitat’s “sky”, so I came up with another idea. If there were some kind of projectors on the walls of the habitat, they could project the holographic image of a sun, which would provide both lighting and something nice to look at. I thought that, rather than use an electricity-powered laser, these projectors have a lens that focuses light from the actual sun (which the habitat orbits) into a beam that is then used to generate the hologram.

But reality check: would this work? I am no expert on holography and am kind of expecting there to be a problem with this system, but I want to be sure. So, to recap, is there a way to create a holographic projector which uses a focused beam if sunlight instead of a laser, and therefore can operate whenever sunlight happens to fall on it?

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    $\begingroup$ So long as the ring has some axial tilt relative to the ecliptic, the real sun will be visible in the sky already - sunlight will directly shine on nearly half of the inside of the ring, yielding a completely typical day-night cycle if the rotational period is on the scale of a day. Do you really need to illuminate the night side of the ring as well? $\endgroup$
    – Nuclear Hoagie
    Commented Jun 1, 2023 at 20:34
  • $\begingroup$ I have no idea what you mean by a "holographic projector". Can you elaborate what this wondrous device is supposed to do? $\endgroup$
    – AlexP
    Commented Jun 2, 2023 at 11:25
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    $\begingroup$ Something that "projects a holographic image of the Sun" without using any power is called a "Mirror". $\endgroup$
    – RBarryYoung
    Commented Jun 2, 2023 at 11:54
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    $\begingroup$ @NuclearHoagie - To produce 1 gee of gravity by rotating a 1000 km radius ring requires a full rotation every ~33.5 minutes, so no nice day-night cycle. So it would be better to have the rotational axis face the sun and use a conical mirror to reflect it, and arrange some sort of moving shade if a day-night cycle is desired. (It may not be: spend some time in the far north in the summer, and you'll realize you can adapt to it never getting dark.) $\endgroup$
    – Paul Sinclair
    Commented Jun 2, 2023 at 14:35

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Mirrors work fine

...as the Norwegian town of Rjukan has already demonstrated.

Read about it here.

The villages of Rjukan, Norway, and Viganella, Italy, are both situated in deep valleys where mountains block the sun's rays for up to six months every year. To illuminate those darker winter months, the two towns have built gigantic mirrors that track the sun and reflect daylight downwards. Viganella completed its huge computer-controlled mirror in 2006, and Rjukan followed suit just this month, mounting a mirror that will reflect a 600 square meter (6,500 square foot) beam of sunshine into the town square below.

enter image description here

(image credit: Reuters/Tore Meek/NTB Scanpix)

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    $\begingroup$ It's not quite powerless, when you need a computer to track the sun and actuators to orient the mirror, but it's pretty good bang for your buck for sure. $\endgroup$
    – Matthieu M.
    Commented Jun 2, 2023 at 14:30
  • $\begingroup$ @MatthieuM. I am applying the assumption that OP meant that the main source of the light was not something that needs power. That the infrastructure of the light source is powered and controlled, I would assume being quite acceptable, since that power requirement would be minuscule in comparison to trying to power the light itself. And in any case, it would be a sinch to make this into a self-powering/self-regulating system, using the incoming sunlight to power the movement of the mirror array, either through photovoltaics or sealed pneumatics (such systems exist today). $\endgroup$
    – MichaelK
    Commented Jun 2, 2023 at 14:39
  • $\begingroup$ I agree with your analysis (minuscule and self-powered). I'm just not sure why the OP specified "powerless" -- if they have any odd reasons. $\endgroup$
    – Matthieu M.
    Commented Jun 2, 2023 at 14:52
  • $\begingroup$ @MatthieuM. Well, as I said: providing the primary power to the light source would demand some pretty high capacity power generation. Just turning some mirrors would require a lot less. C.f. SmartFlower, that powers its own motors to stay aligned with the sun. $\endgroup$
    – MichaelK
    Commented Jun 2, 2023 at 14:58
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    $\begingroup$ I mean, technically you could have it powerless if you built it like a wide-brim receiver dish. $\endgroup$
    – lilHar
    Commented Jun 3, 2023 at 4:54
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Take a credit card, or something else with an hologram in it, and put it under the sun.

You see the hologram without needing a power supply for the object, because the hologram has already been realised on the item.

Similarly, if you have the hologram already realised in your station, you will not need to power supply it to have displaying the sun.

However, I don't get why you want to go through the complications of an hologram when you can simply place a mirror, which will give less diffraction.

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