Remember, while light is made of particles called photons, those photons actually behave like waves. Waves have high parts and low parts, peaks and troughs. When two waves collide, like they would when the reference beams and object beams hit a holographic plate, if the peaks meet each other they amplify, or make each other stronger. This is called constructive interference. If a peak meets a trough, however, they cancel each other out. This is called, you guessed it, destructive interference. What you’re left with is a kind of code called “interference fringes,” and to the naked eye, they don’t look like much. But hit them with the right kind of light and you’ll get your hologram.
The reflective holograms we talked about earlier, the ones that are a staple of credit card security, produce images with just plain old, ordinary white light. Ever notice how the hologram disappears if the card is held at the wrong angle? That’s because without properly reflected white light, all we see are those interference fringes. The second kind, transmission holograms, work the same way except light—usually a laser of the same color used to create the recording—is passed through the holographic plate instead of bounced off it. Both kinds give you a characteristic three-dimensional representation of the recorded object.
Aside from true three dimensions, there’s another aspect present in all true holograms: redundancy. Thanks to the interference between the reference beam and the object beam and the physical laws that govern the way light is reflected and scattered, every part of the holographic film contains information about the entire recorded object. That means if you cut a hologram in half, you could still see the whole hologram in each half. In fact, you could break the holographic plate into a dozen pieces, and you’d still see the entire hologram in each piece. This might sound like a by-product, but think of it this way. If you have a damaged holographic disc, it’s easy to recover the data because every part of it contains the whole. Entire organizational concepts have been based on this idea. Just type “holonomic organization” into Google and you’ll see what I’m talking about.
Despite its high-tech applications, holography is actually quite an old science. In 1971, Dennis Gabor was awarded the Nobel Prize in Physics “for his invention and development of the holographic method,” described above, based on work he’d done in the 1940s. We’ve come a ways since then. Today, his process, a very systematic analog recording, is done almost completely by computer. Thanks to sophisticated 3D modeling software, we can create holograms from objects that only exist in a computer file, or would be too large to record with actual lasers. Software algorithms calculate the interference fringes and we print them out to holographic plates. The military uses applications like this a lot already. Satellite images are turned into holographic sheets that they can lay out on tables to better visualize on-the-ground troop movements and combat situations. Instead of looking at flat maps, commanders get a three-dimensional, full-parallax viewpoint of what soldiers face in the field.
How this gets us to a holodeck is still a question. We can’t even be sure that what Star Trek calls holograms actually have much if anything to do with current holographic science. The similarities are slim, but that’s not to say we’re on the wrong track. Our holograms could work in a device like a holodeck, but there’s still a lot of work to do if we ever want to touch them.
The force is not with us
Today’s holograms are three dimensional, but they’re mostly still contained in a surface, like a piece of holographic film or glass. Projecting 3D images, or any image, hologram or not, into the air is a challenge we’ve just started to overcome. One company in Japan has figured out how to use lasers to cause a reaction in the oxygen and nitrogen in the air that results in a dot of light. Do this at a high frame rate and with enough dots-per-inch, and you get a realistic 3D image that can even move. Combine red, blue and green lasers and you have the full spectrum of color at your command, all out of thin air. This isn’t necessarily holography, at least in the traditional sense we’ve already discussed, but it’s our closest step yet toward the kinds of projections the holodeck probably uses. They’re not photo-realistic, though and even if they were, the illusion would fade as soon as you tried to touch them.
Star Trek solved this problem by wrapping their holodeck holograms in force fields. These not only provided structure and physical presence, but also tactile sensations. A rock in a holodeck not only looks like a rock, but if you touch it, an electromagnetic field makes it feel like a rock, too. Force fields also added a sense of movement. According to the Technical Manual, holodeck users are trapped in a kind of perpetual treadmill. As the user thinks he’s walking, he’s actually held in place by force fields while images on the walls scroll by and “omnidirectional holo diode clusters” project the appropriate images into his field of view.
Today’s force field technologies, though, are limited, as in almost non-existent. We haven’t yet developed force fields that can provide any safe tactile feedback, let alone the nuances of seemingly unlimited materials and surfaces. Some current research is focused on charged plasma clouds to protect ships and satellites in space, but this approach still requires a physical wire mesh to contain the energy. It’s probably not an adaptable technique for a holodeck. A few years ago, the British Ministry of Defence announced a method to use super capacitors in tank armor to generate a brief, but powerful magnetic field that could deflect grenades and bullets. While work is ongoing, most of it is geared toward these kinds of defensive applications. We obviously have a good deal of work to do before we can manipulate force fields with the kind of sophistication and complexity that a holodeck would require, and sadly, it might not be on anyone’s list of priorities. Even on Star Trek, humans had both transporters and warp drive before they invented force fields. The Enterprise NX-01 didn’t even have shields. Instead, they relied on polarized hull plates to protect them from weapons fire.
But all Holodeck hope isn’t lost
While there’s probably no substitute for a real holodeck, we do have some technology that allows us to immerse ourselves in virtual environments. Think about the last time you saw a true IMAX movie, the kind that fills your field of vision. If you take that concept but project the images a bit differently, say with deep depth of field so the pictures come out as crisp as they would in real life, you could actually fool the brain into thinking it’s someplace it isn’t. Several companies in Europe and the United States have developed environment simulators that operate this way, mostly used for training. While these virtual environments work well as flight simulators or as road tests for cars, they fall short when there’s no barrier separating the user from the image. There’s also obviously no physical interaction, either—you can’t touch the images, so the tech falls short of the holodeck.