Holograms *are* a real thing. But what science-fiction TV shows and movies call holograms are something different entirely.

From a classical geometric optics standpoint, in order to create the illusion of an object there must be some sort of light-emitting or light-modulating surface along the line from your eye to the points on the simulated object.

![Real-life hologram: Although light appears to be emitted from a virtual object, in reality light is emitted from the display surface.  If you try to look at the virtual object from the side, without the display behind the object, you won't see anything.  Sci-fi "hologram": In fiction, "holographic projectors" actually do cause light to be emitted from the location of the virtual object.  This allows you to view the object from any angle, without the need for a room-encompassing display.](https://i.stack.imgur.com/xbFyp.png)

So, my question is: **is there any physical process that I can use to justify sci-fi-style holgrams?** In order to qualify, your "hologram" should meet the following requirements:

* The display volume must be air at atmospheric pressure (and it should work across a reasonable range of temperature and humidity) so that people can walk through the display volume. In particular, your solution can't rely upon a specific medium in the display volume (like smoke/fog or "smart dust").
* The display volume should be open on at least three sides (floor, ceiling, and one wall) so that you can walk around the hologram. Note that it is perfectly permissible to require devices around the display volume, as long as three or more sides are open enough to walk into.
* The hologram must appear "correct" to anyone standing outside the display volume, including occluding the view of other holographic objects behind one another. (For example, if a holographic person is holding something behind their back, you would need to walk around them to see it.)
* The hologram must be safe enough that a person can walk into the display volume. A hologram is useless if it bathes its viewers in gamma radiation, or constant high-intensity noise and stray laser light (I'm looking at you, laser-plasma scanning display!).

There are no requirements on:

* Color reproduction. The hologram can be monochromatic, or the color can depend on the viewing angle, and the objects it displays need not be textured.
* Computational requirements or energy consumption.
* Occlusion of real objects (the hologram can be transparent).
* Appearance of holograms from *inside* the display volume.
* Interaction with physical objects inside the display volume.
* Presence of visual artifacts (like beams of light extending from the projectors to the virtual object).

That is, I don't need holograms as realistic as in Star Trek:

![enter image description here](https://i.stack.imgur.com/2GVTm.jpg)

Something as basic as Star Wars's holograms would fit the bill:

![enter image description here](https://i.stack.imgur.com/GjCVb.jpg)

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Do note the [science-based](/questions/tagged/science-based "show questions tagged 'science-based'") tag: I would like, if possible, to use real-world physics.

[Cross-posted on physics.stackexchange.com](https://physics.stackexchange.com/q/158868/70557)