Holograms are real. But they are not what sci-fi calls holograms.
From a simple geometric optics standpoint we can see that in order to create the illusion of an object there must be some sort of light-emitting or light-modulating surface on the rays from your eye to the points on the object.
That is, a display using any technology which can control how light is emitted from it can simulate objects behind it (like looking through a window) or directly in front of it--as long as those objects stay within the border of the screen (if you try to look at an object from the side or back, it will disappear because you are no longer looking at the display) and are not blocked by any physical object (putting your hand in front of you will block objects at any simulated depth, since again you can no longer see the display).
Is there any physical process a device can use which can circumvent this restriction, that is, emit light along a line that does not pass through the device?
- Direction of light emission must be controllable. This disqualifies the laser-plasma scanning 3d display since it can only display uniformly emitting points.
- Must work in air: no special display medium (including passive and active materials, e.g. smoke/fog or smart dust) and no hard vacuum.
- Not harmful: a hologram is useless if it bathes its viewers in gamma radiation (or constant high-intensity noise or stray laser light--I'm looking at you, laser-plasma scanning display).
(Note no requirement on color, resolution, energy required, etc.)
Again, I'm not looking for an engineering solution, just a physical mechanism.