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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?

Requirements are:

  • 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.

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  • $\begingroup$ Physical mechanism for what, exactly? Getting photons paths to bend at large angles at some time after they've been emitted? Or getting air molecules (or some other type of particle of a similar size or smaller) to emit light in some controllable direction (but still in a straight line from their position) rather than energizing them to emit light in all directions? If the device shot up some light-emitter much larger than an air molecule, but still microscopic, and that was what emitted the light in a straight line from its location, would this count? $\endgroup$ – Hypnosifl Jan 11 '15 at 22:54
  • $\begingroup$ @Hypnosifl Either deflecting photons emitted from the device or creating them in mid-air work. I don't want to exclude either solution so I didn't specify. However having emitters distributed throughout the display volume counts as a special display medium (and maybe as harmful depending on the effect of inhaling them!). $\endgroup$ – 2012rcampion Jan 11 '15 at 23:42
  • $\begingroup$ What would count as an emitter? If there were a way to get air molecules to preferentially emit photons in a particular direction (though I don't think there is), presumably you wouldn't count the air molecules themselves as the kind of "emitters" that would disqualify a solution, but if it was nanotechnology I guess you would? If there were some complex molecule that is natural and harmless to ingest, but isn't normally found in air, and it could be induced to emit photons in a preferential direction, would spraying this molecule into the display volume count as a solution? $\endgroup$ – Hypnosifl Jan 12 '15 at 0:16
  • $\begingroup$ @Hypnosifl, You are correct: since the air is already there, it doesn't count as a special medium. Anything that you add to the air is disallowed. (For the reason that such a solution wouldn't work if there was wind, or be able to display a room-size hologram from a handheld emitter). $\endgroup$ – 2012rcampion Jan 12 '15 at 0:20
  • $\begingroup$ OK, with those requirements it seems the only two solution types you'd allow would be to either significantly bend the paths of photons that have already been emitted from the device as they travel through the air, or somehow induce air molecules to emit photons in a preferred direction (which seems to be a rare property, see this article on 'hyperbolic metamaterials' which touts their ability to 'create the conditions in which the normally isotropic spontaneous emission of photons occurs primarily in one direction'). Would you agree? $\endgroup$ – Hypnosifl Jan 12 '15 at 0:25

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