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Is there some law of physics that strictly prohibits the projection of 2D or 3D images into thin air (such as holograms in movies) or is a solution to achieve this still up for grabs by an eventual discoverer?

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This question may be already answered here. –  Andrestand Sep 14 '14 at 9:41

5 Answers 5

In principle you could do it using two carefully tuned laser beams that scan a volume of air with invisible to the human eye light but feeding a two photon transition to a state that falls back to the ground state with a visible light transition.

There would, however, be the practical problem of having two fairly intense laser beams being shot out into the general environment...probably not eye safe without googles (and maybe protective clothing) to adsorb the (invisible) laser beams.

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Holograms and holographic projections... physics.stackexchange.com/a/135435/30123 –  Andrestand Sep 14 '14 at 9:41

The most common way used to "project images on thin air" is by optically creating a virtual image. That's what 3D TVs and holograms do. (Note that 3D TV create a only a "static" virtual image : the point of view of the scene does not change if you move around ; unlike holograms). But one can tell none of those technologies can be used to reproduce what you see in (for example) StarWars : there is a need for a screen or a glass behind (every single pixel of) the 3D image for it to appear : the light rays have to come from somewhere, and we cannot yet curve them.. so.. in StarWars they are using a totally different technology : it litterally projects the image on thin air either by changing the state of the matter at the point of the projection or.. by curving light rays.. or.. by using some software like 3D Studio Max or Adobe's AfterEffect :P

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To project an image you need it to be real!! There are many 3D TV systems, and the ones I know... don't even create an image, they are just an array of point real sources of light, an object in optics words! Te Star Wars holograms could be accomplished with a cylindrical glass around the projected figures. It's easy to do that with static images, and it is possible with videos, though you need a cylindrical screen with an incredible resolution, and a powerful system to deliver there a great amount of information. –  Andrestand Sep 14 '14 at 9:31
"..a cylindrical glass around the projected figures.." -> There is no cylindrical glass around the projected figures in Star Wars. –  franssu Sep 15 '14 at 15:52
Errrr... yes... maybe you cannot see it due to anti-reflective coatings and a proper camera location(?) –  Andrestand Sep 16 '14 at 11:32
We were talking about projections as they seem to be working the fictionnal world, not special effects that could be used to simulate them. –  franssu Sep 16 '14 at 13:14
We were actually talking about physics, not Sci-Fi. And I'm not talking about special effects to simulate anything, but about a real system which can explain what you actually see on screen. –  Andrestand Sep 17 '14 at 9:33

This imaging seems about state of the art right now http://www.gizmag.com/burton-true-3d-laser-plasma-display/20499/

Basically, a laser is focused at a certain point in air, and ionizes the air at that point, causing it to glow (and crackle.. not too safe)

Other approaches, use a similar principle, but with water or steam to require less power - not needing to ionize the air at the focal point.

An interesting paper that I have come across (can't find the reference just now) is not to use thin air but to use a clear aerogel, with quantum dots embedded in it.

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Yes. http://www.youtube.com/watch?v=lFqlQiTTHRs

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Hi Chris, and welcome to Physics Stack Exchange! Although that's a good link to have, it would make a much better answer if you describe what's going on in the video. (Rule of thumb: a good answer should make sense and be useful without having to click on any links.) –  David Z Nov 9 '11 at 7:16

Depends what you mean by project (and image!)

You can create a real image anywhere in space it's just that you can only see it if there is a screen or something to reflect it into your eyes - the image is obviously still there if you remove the screen.

You can also create a virtual image where the path of rays into your eye is the same as if the light was coming from an image at a particular position - even though there is nothing at that position. This is essentially what you are doing when you use a magnifying glass

So if you wanted to create the illusion of an image floating in space - you could do this by projecting the correct image directly into your eyes. If you want to do this and be able to move your head around then it gets trickier!

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You don't need a screen to see a real image! And this is a sadly common misconception :( A nice hologram would fit as answer to the question, but you're going to see the image through or in front of some glass... I would recommend to read my answer to this SE question. –  Andrestand Sep 14 '14 at 9:40

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