Physicists create 'Star Wars'-style 3D projections

Daniel Smalley has long dreamed of building the kind of 3D holograms that pepper science fiction films. But watching inventor Tony Stark thrust his hands through ghostly 3D body armour in the 2008 film Iron Man, Daniel realised that he could never achieve that using holography, the current standard for high-tech 3D display, because Stark's hand would block the hologram's light source. "That irritated me," says Daniel, a physicist at Brigham Young University, USA. He immediately tried to work out how to get around that.

Daniel's team has taken a different approach - using a technique known as volumetric display - to create moving 3D images that viewers can see from any angle. Some physicists say that the technology comes closer than any other to recreating the 3D projection of Princess Leia calling for help in the 1977 film Star Wars. "This is doing something that a hologram can never do - giving you an all-around view, a Princess Leia-style display - because it's not a hologram," says Miles Padgett, University of Glasgow, UK.

The technique, described in the journal Nature last month, works more like a high-speed Etch a Sketch: it uses forces conveyed by a set of near-invisible laser beams to trap a single particle - of a plant fibre called cellulose - and heat it unevenly. That allows researchers to push and pull the cellulose around. A second set of lasers projects visible light - red, green and blue - onto the particle, illuminating it as it moves through space. Humans cannot discern images at rates faster than around 10 per second, so if the particle is moved fast enough, its trajectory appears as a solid line - like a sparkler moving in the dark. And if the image changes quickly enough, it seems to move. The display can be overlaid on real objects and viewers can walk around it in real space.

The images created so far are tiny - just millimetres across. And only simple line drawings can be created at the speeds needed to fashion moving images. The team managed to depict a moving spiral line drawing and the static outline of a butterfly. The technique needs substantial development but is a simple design with huge potential for improvement, says William Wilson, a researcher in nanotechnology at Harvard University, USA.

Ghostly remnants

The approach has many advantages over existing 3D-display techniques. Hologram technology creates 3D images by sending light through a 2D screen that contains a diffraction grating. The grating manipulates the light rays' paths such that they interfere to create the perception that an image has depth. State-of-the art holograms can be full colour and life-size but, because the light must always emerge from a 2D surface, the viewing angle is limited. And because changing a diffraction grating at speed is challenging, holograms are also generally static.

Volumetric displays physically recreate an image in 3D space. Most existing systems project images onto a rapidly spinning 2D screen. More sophisticated displays use balls of superheated plasma in 3D space. But these can currently use only a single colour. Other approaches use augmented-reality hardware, such as Microsoft's HoloLens, that can create the illusion of a real-world 3D image. But these need specialised headgear and are data intensive, says Daniel.

The latest system can create images in higher resolution than a conventional computer screen - up to 1,600 dots per inch. But to create realistic pictures, with complex moving images and larger visualisations, physicists will need to find ways to speed up the movement of the particles and to control several of them at once. Daniel says he has ideas about how to address both these issues. "If we make as much progress in the next four years as we made in the last, I think we will be successful making a display of useful size," he says.

One drawback of the technique is that it will be difficult to get rid of the ghostly, see-through quality of the projections, says Nasser Peyghambarian, an optical physicist at the University of Arizona, USA. That's because the eye will receive light from a particle at the 'back' of an image as much as from the 'front'.

A final problem is that because the forces used to control the particles are so tiny, the system is easily destabilised. That could hamper military applications, such as simulating a 3D battle scene to train soldiers, because any strong winds would knock the particles off their trajectories. To get around that, Daniel says, the system could be made to scatter light off mists of particles that appear only temporarily. "You're never going to do it in a hurricane," he says. "But it's not beyond the realm of imagination that it could happen outside."