The closest physics has ever come to producing a bona fide ghost is probably the hologram, that eerily unreal object made entirely of light. Floating in space with no visible means of support, a holographic star ship can hover, a disembodied holographic face can turn to follow you, a holographic hand can wave in greeting.
Yet reach out to touch it, and it melts away like a cloud.
Once the exotica of science museums, holograms are now everyday objects, adorning books, credit cards, toys.
If physicist Leonard Susskind of Stanford University is right, holograms may be even more a part of our everyday existence than we think. In fact, the universe itself may be a kind of hologram.
That means that everything in it, including particles, stars, planets and people, is also a hologram. Holograms, if you like, are us.
How can the universe be a hologram?
In essence, a hologram is simply a three-dimensional image reconstructed in space from the information encoded on a two-dimensional surface. The hologram on a credit card, for example, appears to pop out like a solid object even though your fingers confirm that the plastic card is flat.
The way it works, roughly, is simple: Just as the grooves on an old-time phonograph record encode all the information needed to reproduce a sound, so all the information needed to produce the image in a hologram is encoded on a two-dimensional surface. When you put a needle on the record and hook it up to speakers, the original sound is reproduced. When you shine a light on or through a holographic plate, a three-dimensional image is produced.
In Susskind's view, the universe is a hologram that is created from information (that is to say, the laws of nature) encoded entirely on a lower-dimensional boundary. For example, if the universe were a bowl of goldfish, the glass surface of the bowl would contain all the information needed to produce the fish, the sand, the water, the plants. In fact, the "real" universe would be the glass; the fish would be merely a holographic projection.
Susskind and his colleague Gerald t'Hooft came to this discovery by an unexpected route--exploring the treacherous territory around black holes. But now he thinks that through the holographic principle, they have discovered an entirely new way of looking at the laws of physics. Perhaps the only way. And at least a handful of other top physicists agree with him. Susskind and t'Hooft are two of most respected minds in the field.
