The Holographic Universe

The Holographic Universe

by : Aria Ratmandanu

        There is a rather mysterious prediction of M-theory that is still not understood but may have deep physical and philosophical consequences. This result forces us to ask the question: is the universe a hologram ? Is there a “shadow universe” in which our bodies exist in a compressed two-dimensional form ? This also raises another, equally disturbing question: is the universe a computer program ? Can the universe be placed on a CD, to be played at our leisure ? 

         Holograms are now found on credit cards, in children’s museums, and in amusement parks. They are remarkable because they can capture a complete three-dimensional image on a two-dimensional surface. Normally, if you glance at a photograph and then move your head, the image on the photograph does not change. But a hologram is different. When you glance at a holographic picture and then move your head, you find the picture changing, as if you were look- ing at the image through a window or a keyhole. (Holograms may eventually lead to three-dimensional TV and movies. In the future, perhaps we will relax in our living room and glance at a wall screen that gives us the complete three-dimensional image of distant locations, as if the TV wall screen were actually a window peering out over a new landscape. Furthermore, if the wall screen were shaped like a large cylinder with our living room placed in the center, it would appear as if we were transported to a new world. Everywhere we looked, we would see the three-dimensional image of a new reality, indistinguishable from the real thing.) 

        The essence of the hologram is that the two-dimensional surface of the hologram encodes all the information necessary to reproduce a three-dimensional image. (Holograms are made in the laboratory by shining laser light onto a sensitive photographic plate and allowing the light to interfere with laser light from the original source. The interference of the two light sources creates an interference pat- tern that “freezes” the image onto the two-dimensional plate.) 

         Some cosmologists have conjectured that this may also apply to the universe itself—that perhaps we live in a hologram. The origin of this strange speculation arises from black hole physics. Bekenstein and Hawking conjecture that the total amount of information contained in a black hole is proportional to the surface area of its event horizon (which is a sphere). This is a strange result, be- cause usually the information stored in an object is proportional to its volume. For example, the amount of information stored in a book is proportional to its size, not to the surface area of its cover. We know this instinctively, when we say that we cannot judge a book by its cover. But this intuition fails for black holes: we can completely judge a black hole by its cover. 

          We may dismiss this curious hypothesis because black holes are strange oddities in themselves, where normal intuition breaks down. However, this result also applies to M-theory, which may give us the best description of the entire universe. In 1997, Juan Maldacena, at the Institute for Advanced Study at Princeton, created quite a sensation when he showed that string theory leads to a new type of holographic universe. 

         He started with a five-dimensional “anti–de Sitter universe” which often appears in string theory and supergravity theory. A de Sitter universe is one with a positive cosmological constant that creates an accelerating universe. (We recall that our universe is currently best represented as a de Sitter universe, with a cosmological constant pushing the galaxies away at faster and faster velocities. An anti–de Sitter universe has a negative cosmological constant and hence can implode.) Maldacena showed that there is a duality be- tween this five-dimensional universe and its “boundary,” which is a four-dimensional universe. Strangely enough, any beings living in this five-dimensional space would be mathematically equivalent to beings living in this four-dimensional space. There is no way to tell them apart. 

          By crude analogy, think of fish swimming inside a goldfish bowl. These fish think that their fish bowl corresponds to reality. Now imagine a two-dimensional holographic image of these fish that is projected onto the surface of the fish bowl. This image contains an exact replica of the original fish, except they are flattened. Any movement the fish make in the fish bowl is mirrored by the flat image on the surface of the fish bowl. Both the fish swimming in the bowl and the flattened fish living on the surface of the bowl think that they are the real fish, that the other is an illusion. Both fish are alive and act as if they are the true fish. Which description is correct? Actually, both are, since they are mathematically equivalent and indistinguishable. 

          What excited string theorists is the fact that five-dimensional anti–de Sitter space is relatively easy to calculate with, while four- dimensional field theories are notoriously difficult to handle. (Even today, after decades of hard work, our most powerful computers can- not solve the four-dimensional quark model and derive the masses of the proton and neutron. The equations for the quarks themselves are fairly well understood, but solving them in four dimensions to obtain the properties of protons and neutrons has proved to be more difficult than previously thought.) One goal is to calculate the masses and properties of the proton and neutron, using this strange duality. 

         This holographic duality may also have practical applications, such as solving the information problem in black hole physics. In four dimensions, it is extremely difficult to prove that information isn’t lost when we throw objects through a black hole. But such a space is dual to a five-dimensional world, in which information is perhaps never lost. The hope is that problems that are intractable in four dimensions (such as the information problem, calculating the masses of the quark model, and so forth) may eventually be solved in five dimensions, where the mathematics is simpler. And it is al- ways possible that this analogy is actually a reflection of the real world—that we really exist as holograms.