Seeing time as the fourth dimension made sense of Einstein鈥檚 special relativity. The German mathematician Theodor Kaluza had even grander designs. In 1919, he sent a paper to Einstein in which he argued that by adding a fifth dimension to space-time, it was possible to show that gravity and electromagnetism were two aspects of one and the same force.
A few years later a Swedish mathematician, Oskar Klein, took Kaluza鈥檚 idea and . He countered the obvious objection to the existence of a fifth dimension 鈥� that it is not immediately apparent 鈥� by showing it could be tiny and curled up at every point in our four-dimensional space-time. Thus began the trend of searching for the unification of forces in the hidden dimensions of hyperspace that continues today in string theory.
Perhaps, however, the fifth dimension is not as tiny as Klein suggested. Using string theory, Harvard physicist and of Johns Hopkins University in Baltimore, Maryland, that a fifth dimension could explain a vexing mystery: why gravity seems so much weaker than nature鈥檚 other forces. Their model has our familiar four dimensions floating in an infinitely large, negatively curved fifth dimension. While the electromagnetic and nuclear forces are stuck inside a 鈥渂rane鈥� made of four dimensions, gravity leaks out into the fifth.
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Meanwhile, of the University of Waterloo in Ontario, Canada, has argued that reality in fact has five dimensions that can be broken down into our four familiar dimensions plus the mass that populates our world. This theory not only rids physics of the problem of why things have mass 鈥� it becomes an incarnation of geometry 鈥� but also of the big bang singularity, the vexing state of infinite temperature and density from which our universe sprang and where all current physical theories break down. Seen from the perspective of the full 5D universe, the big bang beginning is no more than an illusion.
The possibility of five dimensions is also raising more subtle questions. In one of the most remarkable results of string theory, theoretical physicist posited in 1997 that some string theories in five large dimensions that include gravity are equivalent to ordinary quantum-field theories in four dimensions without gravity. The former is a holographic projection of the latter 鈥� potentially making our everyday world as ethereal as a hologram projected from the boundary of the universe.
That might sound esoteric, but the correspondence has recently been successfully applied to computationally difficult problems in many areas, including the physics of high-temperature superconductors. In Maldacena鈥檚 picture, the 4D theory is no 鈥渢ruer鈥� a description of the world than the 5D one. Seen in that light, the question, 鈥渉ow many dimensions does the universe have?鈥� does not have a unique answer after all.
鈥淭he question 鈥榟ow many dimensions does the universe have?鈥� may not have a unique answer鈥�
Read more: Beyond space and time