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PHVSICS OF THE IMPOSSIBLE
video projector that lights up the front of the cloak, so it appears as if
light has passed through the person.
Prototypes of the optical camouflage cloak actually exist in the lab.
If you look directly at a person wearing this screenlike cloak, it appears
as if the person has disappeared, because all you see is the image behind the person. But if you move your eyes a bit, the image on the cloak
does not change, which tells you that it is a fake. A more realistic optical camouflage would need to create the illusion of a 3-D image. For
this, one would need holograms.
A hologram is a 3-D image created by lasers (like the 3-D image of
Princess Leia in Star Wars). A person could be rendered invisible if the
background scenery was photographed with a special holographic
camera and the holographic image was then projected out through a
special holographic screen placed in front of the person. A viewer
standing in front of that person would see the holographic screen, containing the 3-D image of the background scenery, minus the person. It
would appear as if the person had disappeared. In that person's place
would be a precise 3-D image of the background scenery. Even if you
moved your eyes, you would not be able to tell that what you were seeing was fake.
These 3-D images are made possible because laser light is "coherent," that is, all the waves are vibrating in perfect unison. Holograms
are produced by making a coherent laser beam split in two pieces. Half
of the beam shines on a photographic film. The other half illuminates
an object, bounces off, and then shines on the same photographic film.
When these two beams interfere on the film, an interference pattern is
created that encodes all the information of the original 3-D wave. The
film, when developed, doesn't look like much, just an intricate spiderweb pattern of whirls and lines. But when a laser beam is allowed to
shine on this film, an exact 3-D replica of the original object suddenly
appears as if by magic.
The technical problems with holographic invisibility are formidable, however. One challenge is to create a holographic camera that is
capable of taking at least 30 frames per second. Another problem is
PHVSICS OF THE IMPOSSIBLE
video projector that lights up the front of the cloak, so it appears as if
light has passed through the person.
Prototypes of the optical camouflage cloak actually exist in the lab.
If you look directly at a person wearing this screenlike cloak, it appears
as if the person has disappeared, because all you see is the image behind the person. But if you move your eyes a bit, the image on the cloak
does not change, which tells you that it is a fake. A more realistic optical camouflage would need to create the illusion of a 3-D image. For
this, one would need holograms.
A hologram is a 3-D image created by lasers (like the 3-D image of
Princess Leia in Star Wars). A person could be rendered invisible if the
background scenery was photographed with a special holographic
camera and the holographic image was then projected out through a
special holographic screen placed in front of the person. A viewer
standing in front of that person would see the holographic screen, containing the 3-D image of the background scenery, minus the person. It
would appear as if the person had disappeared. In that person's place
would be a precise 3-D image of the background scenery. Even if you
moved your eyes, you would not be able to tell that what you were seeing was fake.
These 3-D images are made possible because laser light is "coherent," that is, all the waves are vibrating in perfect unison. Holograms
are produced by making a coherent laser beam split in two pieces. Half
of the beam shines on a photographic film. The other half illuminates
an object, bounces off, and then shines on the same photographic film.
When these two beams interfere on the film, an interference pattern is
created that encodes all the information of the original 3-D wave. The
film, when developed, doesn't look like much, just an intricate spiderweb pattern of whirls and lines. But when a laser beam is allowed to
shine on this film, an exact 3-D replica of the original object suddenly
appears as if by magic.
The technical problems with holographic invisibility are formidable, however. One challenge is to create a holographic camera that is
capable of taking at least 30 frames per second. Another problem is