The thing that I love about holographics is non-locality. The human brain processes information in discrete bits best. If the information is scattered randomly, then we have to stop and add up everything to come at the Resolution.
For example, if you got a shopping list with all the items in various orientations and sizes all over the paper, you'd have much more trouble understanding what was on there, even though it indicates the same information as a neatly organized list. By the same token, as you cross off the things on your list, it becomes harder and harder to read because you have to ignore some things and pay attention to others. Our brains are pattern-finders, though, so if the scattering is non-random, the brain can pick it up. If you crossed out every second thing on your shopping list, it wouldn't be nearly as hard to read as if you randomly crossed out half the things as you came to them in the store.
So for most humans, external information is stored "classically" or on a one-to-one basis. One bit of information is discrete, it's one point on a graph, one part of a picture, one item on a list. Any object can be reduced to its components and listed out. Then the reverse of the reduction listing process is employed and those components to are converted to reality. In the home, absent products are listed in English characters on a piece of paper. The reverse of that is reading the piece of paper in the Supermarket and getting the products. With classical information, information bits are grouped in locations that give them meaning.
Holography records information totally different than a shopping list. A hologram is a very special mishmash of light that is entirely specific to the image it generates.
Holograms are made using laser light as opposed to normal light. This is because the light waves from a laser are perfectly uniform and planar, while light from most other places is a scrambled mess:
Another thing to note is that when one light wave hits another, they interact like ripples in a pool, and holography takes advantage of this.
Ordinary pictures are taken by shining light (like a flashbulb) on to your subject, then exposing film to the light. Holograms are also recorded by shining light on the subject, but the illuminated subject is not recorded directly by the film. Instead, a plane laser is shone in the path of the reflected light, and they both strike the film at the same time:
Imagine the beam on the right just bounced off of an apple, and the beam from the left is directly from the laser. The apple-beam will be a mess of waves bonking against each other, but still recognizable as reflecting off of an apple. But when the apple-beam hits the plane beam, they bonk into each other and make a mess on the film.
Here's where it gets cool. So you have this piece of film that has the pattern of waves that emerge when a planar laser beam and a laser reflected off of an apple smack into each other, and it looks like a picture of static. So what? Well, if you shine the original planar straight laser onto the film, the apple appears above it in three dimensions. That blob of static is still light from the apple, just mixed up. By shining the same light on there, the light from the apple goes through the reverse process of becoming mixed up and becomes visible.
Another thing that is fascinating about this is that if you cut a hologram in half and shine the reference beam on it, you get a dimmer image of the apple, not half of one. This is because, unlike a photograph, each point on the hologram holds information from all points of the image, because the information was scattered all over the place before recording.
And that's not all. You don't have to use a planar laser to record and retrieve the image, you can use any laser to record that can be reproduced later to retreive. Imagine the two beams you are using to record are reflected off of an apple and an orange, instead of an apple and a planar beam. Then you would have to shine the light from the orange to see the apple, not the planar beam because that's not what you used to record.
Also, each image is only recorded at one angle at a time. So if you record many different objects, each at a different angle to the film, each one will only reappear if you shine the same light at the same angle.
What's freaky about holography is how well it works as a metaphor for a variety of concepts, most notably memory and quantum physics.
Images above from
Practical Holography by
Christopher Outwater & Van Hamersveld
http://www.holo.com/holo/book/book.html
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