bewareofdogmas said:
I see those "hologram" thingies in movies like Star Wars, X2 ect.
I've thought about it and can't see how they would be possible.
Are they?
The closest thing that I've seen to a real application of this was a few years ago at Siggraph. A woman had put together an apparatus using two lasers of different wavelengths projected into a block of glass, which was doped with a material. Where the lasers crosed, electrons were promoted in the doping agent to an energy state two orbitals above the rest state (with each of the two energy states given by the energy of each of the two lasers). The electrons occasionally dropped both energy states at once, giving rise to visible photons.
It was way cool, but the block of glass was only about a centimeter squared.
Practically, the best technologies seem to be rear-screen projection technologies. The best I've seen is the CAVE, produced by the University of Illinois. (I generated and exhibited some scientific visualizations for the CAVE at SIGGRAPH a few year ago. Although we only showed thunderstorm data in the public exhibition, we actually modified a general-purpose visualization package of ours to work, so in a later private showing, we were able to do some gel electrophoresis simulations and some molecular code as well. But I digress.)
In the CAVE, flicker-stereo images are projected onto three walls and a floor. One person wears stereo flip glasses, and the computer tracks that position and generates different images. Other viewers have to wear pasive flip glasses, which of course generate a distorted image. Nevertheless, it's a pretty damn good illusion.
To make a complete system like this, you'd have to have walls where every pixel presented a 2-D image, with the different pixels within the subimage pointing out at a different angle. This is not hard for one pixel; just a lens and a screen. However, packing them all together would be a trick, and of course generating even a static image would be a 4-D problem, which would be a trick.
Some of the people writing books about Star Trek gave this some thought and suggested such a mechanism, but only for far objects. A virtual treadmill mechanism gave the illusion of being able to walk long distances. This is all posssible.
Near objects were real, made of matter beamed there with transporter technology, manipulated with tractor beams. Of course, this is, as yet, just pure fiction.
All of these technologies require something behind the image that can emit the photons. Free-standing 3-D images, such as the R2D2 Princess Leia recording, well, there doesn't seem to be a mechanism to do that.
