# Can free-space holograms exist?

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.

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:

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

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

Cross-posted on physics.stackexchange.com

• 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? Mar 23, 2015 at 6:14
• @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!) Mar 23, 2015 at 14:09
• This doesn't answer your question entirely, but it's an idea: Google plasma hologram. Jun 30, 2015 at 16:43
• @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. Jun 30, 2015 at 16:46
• Oops, sorry! <blush> Jun 30, 2015 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.

• 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. Dec 4, 2015 at 7:42
• @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... Dec 4, 2015 at 9:31
• @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... Dec 4, 2015 at 9:35
• 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. Jan 10, 2017 at 10:58
• @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. Jan 30, 2017 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.

• 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). Mar 23, 2015 at 14:17
• 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.) Mar 23, 2015 at 14:21
• 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. Mar 23, 2015 at 16:35

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.
• 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). Dec 4, 2015 at 7:52
• @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. Dec 4, 2015 at 11:28
• @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. Dec 4, 2015 at 12:05
• @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. Dec 4, 2015 at 15:07
• 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. Dec 4, 2015 at 18:31

This is a very interesting problem that seems like something that will be solved in the next hundred years. I love star trek and have always admired the holodeck. When characters wanted to leave the holodeck they had to install emitters.

I'm gonna look at this problem in terms of what a room might need to create holograms.. Well what if the emitters was a super advance set of 3d printer heads. Perhaps they could all communicate with each other and create the image from every angle. Perhaps the room could initially be filled with some sort of Nano electric molecules light enough to float on their own with many capabilities of a microchip and the properties to take commands and show specific colors. The room would have to be incredibly densely filled with them at first like thick black smoke. Then the emitters emit some sort of signals that tells the Nano molecules where to go, how many to connect with and which colors to become. Perhaps they could even be program to move in concert independently once fully formed. Once the emitters finish with their commands by way of optronics. Perhaps nano directed flashes of light to coordinate messages of 1s and 0s for the nano molecules to receive and Decipher.

Once the objects were solid, the room could vent out all the extraneous molecules so that the user count enter safely. The objects should be able to interact with the users and vice versa and do a lot of things depending on the memory and power of the collective nano molecules all working in concert.

Once the program is finished they could be shut down by being given a command to go back to the original state; ready for the next person to use their own set of instructions in the holodeck.

If this sounds plausible, please someone expand upon it with their own ideas. Maybe an actual scientist. I'm just a regular dude and computer science student. Let me know what u think guys.

• Recently on the applied materials company website they talk about a backend program where their IOT nanochips are so small that they float in the air during testing. Perhaps something like this could be used. Jan 13 at 9:40
• There's also a company called Lightmatter lightmatter.co that has created photonic processor, perhaps this could be part of the solution. Jan 13 at 9:51
• Welcome to Worldbuilding.SE! This is a bit hard to read at the moment, would you mind editing it to break it up into paragraphs? Jan 13 at 10:44
• Your idea is definitely interesting, if I understand you are suggesting that the system creates real, physical objects instead of projecting images? So if you wanted to 'video chat' someone using this system it would create a sort of android that looks like them and copies their movements, and the other person would interact with an android copy of you on their end. It's not really what I was looking for but is certainly a very cool concept in its own right! Jan 13 at 18:38
• The idea did not really have video chat in mind. If you wanted to make a representation of the person you were talking to you could program the room to create her and send a set of commands for her actions and speach and how to respond to stimuli with some sort of AI program. Anything you built in the room would look and respond as real as real life. They just might not feel right would be the only difference. I am not sure how "holographic" it is per se, but then again I could question the star trek holograms being holographic as well because you cant exactly put your hand through them either Feb 3 at 7:41

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.

• 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. Dec 4, 2015 at 5:40
• @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. Dec 4, 2015 at 5:50

It's actually quite simple:

1. Create a force field
2. Project an image on the force field (or alter the physical attributes of the force field to block / radiate certain wavelengths on a pixel scale)
3. profit

"But!", I hear you cry out in dismay, "Force fields aren't real!" To which I say:

## Teslaphoresis!

The directed motion and self-assembly of matter at a distance using the near-field energy of the Tesla coil is a phenomenon we term Teslaphoresis.

While the concept is still in it's infancy (being only able to move carbon nanotubes within 30cm of the emitter as of 2016) I can't see why a sufficiently advanced version couldn't be used to manipulate "normal" matter in a very precise way with a larger range.

This has numerous benefits

• The tech uses Tesla coils. Tesla coils are cool
• Potentially useful as an actual force field
• The tech could also be repurposed to create something akin to replicators
• Did I mention the Tesla coils?

Downsides include:

• possible electric shock?
• Tesla coils are quite noisy
• probably quite power-hungry

Here's a link to the original paper(PDF)

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.

• 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. Dec 4, 2015 at 8:10
• " 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. Dec 12, 2015 at 19:18
• 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. Dec 12, 2015 at 19:18

## Molecule quantum manipulation or alternate universe light leak

Hello,

I love holograms. So much that for college graduation I built my own holographic display for store display using fog projection (youtube.com/mauvixx).

Also, I would love if we actually achieve free space holograms someday. There are some cool ideas here.

Free space holograms are quite the hard feat to manage unless someday we come up with a cool technology that make it so easy we'll be dumbfounded.

Using the power of handwavium here are my 2 cents.

We see things because light bounces off the surface and hits our eyes. That means you have to have a "thing" for light to hit (hence the dust particles already mentioned)

Sometimes we can see heated air shimmering, along this line of thought I propose that the droid actually has devices that can for microseconds change a volume of air in front of him. How? Not sure. Laser, electromagnetic fields, you pick. The droid would manipulate the air molecules so that they bounce the ambient light in the way desired to create the image. There is no projection. The very air is being used as the medium (like a fog screen) but one that bounces ambient light.

This would of course require very advanced tech, quantum level tech, to actually use the air molecules and make them bounce light in different wavelenghts. Ok, is far fetched, but sufficiently advanced tech is very much like magic...

If we are really going to use handwavium, you can say that in a parallel universe, one that is a copy of ours (only where light doesn exist), there are huge generators installed to create projections from every angle or where the images are even made using physical rotating screens, however because of some mcguffin, only light created in that dimension can cross and appears in ours. Thus, holograms can be made using many physical techniques but for us, only the light floating in air would be visible.

• Thanks for your answer, however it is kind of begging the question... I'm looking for a specific physical mechanism, so "you pick" is not very helpful! As for 'using the air molecules,' if you look at how light scatters off of small particles you'll find that the direction of the scattered light is not controllable, which is the whole issue. Feb 3 at 18:26
• No problem, I just wanted to say something because I love holograms. However we´ll probably only be able to give you a specific answer naming a specific mechanism when we actually make a working free space hologram or at least a good theory on how to make one. As of now, anything I can imagine would have to be so advanced that it would be more like magic for a normal person of our time. Best luck! Loving the ideas so far. Feb 3 at 18:40
• Forgot to add this. I like to write and in a book about writing science-fiction, it says: you dont have to fully explain how it works, just show it working, describe how it drains power from half the US power grid, make it seem believable, but dont get bothered with too much detail. Chance is that we´re not experts and will probably say many thinks wrong. Feb 3 at 18:44

There's more than one way to skin a cat. When a human looks at something on a screen, there is a huge long system involved, with each part having to work perfectly. The elements of that system that I can think of inlude:

Computer -> LED Backplane -> TFT/LCD Screen -> Coloured light traveling through the air -> Lens in eye -> Retina in eye -> Optic nerve -> Visual cortex -> Rest of brain


By altering this system we can create the appearance of a free-space hologram. Working backwards from the end, some potentials that come to mind are:

1. Computer talks directly to the rest of the brain. It convinces the brain that "hey, there's a thing here" and the human perceives that thing as being there.
2. Compute talks directly to visual cortex. Similar to above but the computer actually has to know what things look like and render them.
3. Use nanobots to tamper with the optic nerve
4. Bend the light travelling through the air - with mirrors, magnets, local gravity distortions
5. Emit the light actually in free-space (eg LED cube)

I'm partial to #4 as it is a nice blend of "real space hologram" and "physically possible". So here's my suggestion.

1. A screen similar in size to the human pupil
2. A collimating lens
3. A big array of magnets

The big array of magnets bends the light so that the light from the screen enters the human's eye at the correct angle:

I hear you say: how does this work for multiple people? Well, it is 2202, so let's just have lots of them: an array of screens, an array of lenses and a whole gosh-darn stack of magnets. Each person get's their own viewpoint, each person has a hologram projected right into their eyeball.

Oh, and I hope you don't have any steel around - those magnets are going to be strong