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.

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

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

enter image description here

Do note the tag: I would like, if possible, to use real-world physics.

Cross-posted on physics.stackexchange.com

  • $\begingroup$ I take it augmented reality is out of the question, as you need a screen or something like a screen putting light on your eyes? $\endgroup$ – PipperChip Mar 23 '15 at 6:14
  • $\begingroup$ @PipperChip yes, I'd prefer if all my characters didn't have to wear Google Glass or get retinal implants. (As a side issue, you wouldn't be able to take a picture of an AR 'hologram' unless your camera had AR too!) $\endgroup$ – 2012rcampion Mar 23 '15 at 14:09
  • $\begingroup$ This doesn't answer your question entirely, but it's an idea: Google plasma hologram. $\endgroup$ – MackTuesday Jun 30 '15 at 16:43
  • $\begingroup$ @Mack I specifically mentioned the laser-scanning plasma display in my question... It's very loud and there's lots of stray laser light. Think of it as inducing ball lightning tens of thousands of times per second. $\endgroup$ – 2012rcampion Jun 30 '15 at 16:46
  • $\begingroup$ Oops, sorry! <blush> $\endgroup$ – MackTuesday Jun 30 '15 at 16:46

Just use "optical dust" and lasers.

An RGB laser system scans the volume to locate every dust particle in the volume. As a side effect a diffuse lighting and enough localized heat to keep the dust suspended is generated. Infrared lasers can be used if haze effect is not wanted and the dust particles are larger.

When you want to create an image, you simply hit the correct dust particles at correct locations with lasers of correct colors and energies. The dust particles will scatter the light and light up in precisely the correct color in almost correct location. The deviations in location can be compensated in the rendering algorithm.

The difficult part here is getting the properties of the particles right as they presumably need to be safe to breathe and swallow. Should not cause adverse reactions on skin or eyes either. But unless your story is about the invention or manufacture of the system you can just assume somebody solved all those annoying issues and got all the proper licenses and permissions to scatter artificial dust into the air.

  • 2
    $\begingroup$ I had thought about using particles before, but I hadn't considered the idea of using the dust already present in the environment. That mostly negates the effect of air movement... and even in a relatively clean environment (100,000 particles per cubic foot), you should be able to get a resolution of around a centimeter. $\endgroup$ – 2012rcampion Dec 4 '15 at 7:42
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    $\begingroup$ @2012rcampion I didn't consider using the dust already present either. But it might actually work well enough for the "star wars quality", now that you mention it. There probably is enough dust capable of scattering blue light present. Adding artificial dust with better optical properties is still what I would do. Unless I need something to mount on a droid, of course... $\endgroup$ – Ville Niemi Dec 4 '15 at 9:31
  • $\begingroup$ @2012rcampion Actually it would look totally like in Star Wars, wouldn't it? And an astromech droid totally would have close range sensors capable of tracking dust particles... $\endgroup$ – Ville Niemi Dec 4 '15 at 9:35
  • $\begingroup$ How does this solution handle fully 3D projections? Unless you have a 360 degree array laser source, a single laser source wouldn't be able to hit two different particles that are in front of each other and be able to handle 3D projections that are viewable from points of view other than the projector. 360 degree array laser sources would be able to handle fully 3D projections but would be completely isolated. $\endgroup$ – Shiri Jan 10 '17 at 10:58
  • $\begingroup$ @Shiri Sorry for delay. // Dust suspended in the air is not opaque. The dust is simply not dense enough for particles to block each other to significant extent. If it was all you'd see would be a light grey blob. And it probably would be too dense for air suspension as well. $\endgroup$ – Ville Niemi Jan 30 '17 at 9:18

It is possible that they could exist as some kind of augmented reality that is either beamed directly onto the viewer's retina or inserted into the visual cortex by some other means such as ubiquitous nanotech augmentation. This makes a lot of sense if your setting is one where this could exist - given that it seems quite conceivable in the next few decades from there, that is not unreasonable.

If you are determined that they appear as objects in three dimensional space rather than being purely in the eye of the beholder then my guess would be that you did something very clever with a couple of different ray sources. These excite the air molecules at their point of intersection to emit light in a given wavelength or to reflect light in a given wavelength. Now I am not a physicist and I'm sure this is laced with problems, but it sounds plausible and unless you are using it as a cornerstone of your narrative, that is probably good enough.

  • 1
    $\begingroup$ The "seeing objects behind you" is not a philosophical quandary, merely a failure mode of classical holography. Light which is being simulated as emitted from the right side of the cube is actually being emitted from the display surface on the left, and your eyes intercept it before it reaches the point it's supposed to be emitted from! You would see something, although your eyes would be unable to focus on the converging light (like looking in a magnifying glass too close to your face, everything is blurry). $\endgroup$ – 2012rcampion Mar 23 '15 at 14:17
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    $\begingroup$ Also, the solution of using 'rays' to excite air molecules is exactly what the laser-plasma scanning 3d display does. If you stood behind the object (like in the first two pictures) you would be bathed in the stray laser light. Also, "excite the air molecules" really means "create plasma." This is like having constant lighting in the room with you. (The system in the linked video can only display a few points, the heat and noise from a simulated solid object would be immense.) $\endgroup$ – 2012rcampion Mar 23 '15 at 14:21
  • $\begingroup$ Oh, I see, so you're saying you can see objects behind your head, not that you want to be able to. Holography is wack but also awesome. As for using plasma, I don't know if that is necessary- what about using something closer to a closely controlled mirage by affecting the air temperature at a very fine level? Or a system that creates a fine particulate cloud and then projects into that? Noise and heat are the kind of things that we can usually eliminate over time with technological refinement given a strong enough requirement. $\endgroup$ – glenatron Mar 23 '15 at 16:35

You are intentionally restricting the definition of "hologram" to exclude some well known examples of things that might have satisfied your question, so I presume you intend to actually using the proper definition of a hologram. As such, it is a light-field recording of a scene. There is no way for an object to appear outside of the viewing angle due to physics: light travels in straight lines. There is no way for the hologram to produce a light ray that arrives at your eye at an incident angle outside the field of view of the hologram itself. Thus holograms cannot be free standing.

You might be able to play some clever games with a black hole between you and the hologram, bending spacetime, but I think that would fail your "must be safe" clause.

If you want a "free standing hologram," you will need to rely on one of the technologies you explicitly excluded from the question, such as the plasma display or augmented reality glasses.

  • $\begingroup$ I came to the same conclusion as you: my characters will probably have to wear AR devices. Plasma displays are getting better though; so if someone can convince me they can be made quiet enough, that could be a viable answer. Plus I'm still holding out hope that there may be some nonlinear-optics or exotic-particle based solution. $\endgroup$ – 2012rcampion Dec 4 '15 at 5:40
  • $\begingroup$ @2012rcampion I think you could get something like that, except the nonlinear part has to be done using the medium (forbidden by the rules of the question), and an exotic particle based solution would be hard to pull off without creating the world's coolest plasma display. What about shared hallucinations? Perhaps you could adjust the environment in a way such that all humans become suggestible to hallucinated 3d objects defined by a data pattern (a. la. Neil Stephenson's Snowcrash). It's certainly not a hologram, but it could be interesting. $\endgroup$ – Cort Ammon Dec 4 '15 at 5:50

Yes they do!
We are really close to having this using femtosecond lasers.

One cool thing about these projections is that you can actually feel them. Who knows, in a century it could turn into something like Voyagers doctor.

As 2012rcampion points out, this is early technology, and there are some shortcomings to overcome.

  1. If a 3D object is projected, the back is visible from the front. This is a side effect of being transparent, and is a problem for any hologram.
    One solution is to not project the full object.
    Another might be to increase the density to where the front would drown out the back.
  2. It's kind of noisy. This is because it's using lasers to create plasma. It could be possible to modulate the sound to be used as a speaker, or perhaps shorter bursts would be quieter.
  • $\begingroup$ I actually already shared that link in a comment on Cort's answer =) The reasons I don't like the laser-plasma scanning displays is that they emit isotropically (so you can see the "back side" of virtual objects), and they're loud (the arXiv paper indicates 75-80 dB for a pinhead-size virtual object). And, in the end this is a fundamentally 'additive' projection system: it can't block light from objects to achieve a solid hologram (not a problem for me, but it does mean no EMH, sadly). $\endgroup$ – 2012rcampion Dec 4 '15 at 7:52
  • $\begingroup$ @2012rcampion Well, op says "the hologram can be transparent", so I think it still fits, but this is really early Tech. Projecting forward to where the resolution is denser and more solid could remove some of that transparency. It may also be possible to fix the noise issue by going smaller than a femtosecond. Most Tech had issues in the beginning. Like computers being as large as buildings. $\endgroup$ – AndyD273 Dec 4 '15 at 11:28
  • $\begingroup$ @2012rcampion I never saw your comment by the way. I just checked the answers to see if anyone else had it, and used the first search result. The noise is the big one in my mind, but it may be possible to modulate the noise in order to use it as the speaker. In the beginning it would sound like chip tunes, but given time it would improve. $\endgroup$ – AndyD273 Dec 4 '15 at 12:05
  • $\begingroup$ @2012rcampion So I looked over your other comments, and the video you put up from CES 11 is in fact like lightning going off; light and heat and noise would be intense. You wouldn't want to touch it as it would burn you. The Gizmodo link says that the femtosecond version is updated from the original nanosecond version as it is safe to touch (much less heat). I haven't been able to find a video with actual audio, so I don't know if the shorter pulses had any effect on that part of the problem. $\endgroup$ – AndyD273 Dec 4 '15 at 15:07
  • $\begingroup$ Here's a link to the paper if you're interested. Scaling from their < 1 mm display to, say, a 10 cm figure would increase the sound by 40 dB (roughly). And since the plasma-generation mechanism is laser-induced breakdown (i.e. "exploding the air molecules apart") I don't think it's going to get orders of magnitude quieter. Apart from the noise and lack of self-occlusion though, it's very close to what I want. $\endgroup$ – 2012rcampion Dec 4 '15 at 18:31

When you say there's "no requirements" on computational requirements or energy consumption, how far does that go?

It'd be inefficient to a silly degree, when you can just program your magnetically-guided mist to part out of the way of people passing through, but lets say the person calling the shots doesn't actually understand the tech, but is paranoid about poison gas, so you have non-technical reasons to do things the hard way. In that case, with enough computational resources (and this would take a lot of real-time computation), and enough emitters and sensors everywhere (hidden in every wall and other stationary surface in the room), theoretically, you could track every eyeball, camera, and other image-perceiving mechanism in the room, track every ambient surface as they move through the room, and scatter light off the latter and into the former. get enough of the right photons, with the right wavelengths, bouncing off the right surfaces, at the right angles of incidence, and hitting the right parts of the eyes and cameras, at the right angles, and they all add up to an image.

Frankly, though, any civilization technologically advanced enough to pull this off, let alone be practical with it, has probably long since outgrown any of the objectives such an image would be useful for conveying, unless it's specifically intended to be used to communicate to less advanced species in a way less threatening to them than having them all just suddenly know everything you want to convey to them. I'm not sure current technology would even be enough to allow us to calculate the amount of real-time computation pulling off such a stunt would require. It really is a silly large amount.

  • $\begingroup$ The real reason I'm avoiding the "smart dust" solution is that introducing nanotech in my setting creates more problems than it solves (or rather, it solves problems too easily). And regardless of the computational requirement, I feel like "emitters and sensors hidden in every surface" is straying too close to the territory of "ubiquitous smart dust coating every surface." The other problem is getting light in the right places without getting tons of stray light everywhere else, which I don't think is possible even after just one scattering. $\endgroup$ – 2012rcampion Dec 4 '15 at 8:10
  • $\begingroup$ " The other problem is getting light in the right places without getting tons of stray light everywhere else, which I don't think is possible even after just one scattering." Which is why it takes so much computation, so much sensing, such precise projection from so many places, and so very very much computation: you can't afford to scatter. You have to track every surface you bounce off of at high enough resolution to always have a smooth surface to bounce off of cleanly, with angle of reflection equalling angle of incidence. $\endgroup$ – Matthew Najmon Dec 12 '15 at 19:18
  • $\begingroup$ As for "emitters and sensors hidden in every surface", I'm not sure how that's necessarily too close to the smart dust problem. Simply specify that it's not a smart-dust coating on the surfaces doing it; rather, every panel is manufactured with the completely-non-nanite sensors and emitters imbedded in it. Also, it doesn't necessarily have to be absolutely all surfaces; missed ones simply increase the chances of "gaps" in the image where it couldn't find a photon path. Crowding the room with people and other non-emitter/sensor-embedded objects will likewise reduce image quality. $\endgroup$ – Matthew Najmon Dec 12 '15 at 19:18

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