gino_segre's picture
Professor of Physics & Astronomy, University of Pennsylvania; Author, The Pope of Physics: Enrico Fermi and the Birth of the Atomic Age

Einstein’s Theory of General Relativity, first presented in the fall of 1915, and his earlier Special Theory of Relativity have changed very little of our day to day world, but they have radically altered the way we think about both space and time and have also launched the modern theory of cosmology. If in the near future we discover additional space-time dimensions we will undergo a shift in our perceptions every bit as radical as the one experienced almost a hundred years ago.

Though proof of their existence would necessarily alter our view of the Universe, there is also a way in which our psyches would be changed. I believe we would gain a new confidence that great almost unimaginable phenomena are yet to be discovered. It would also make us realize once again the power that lies in a few simple equations, in the tools we can build to test them and in the human imagination.

At the November 6, 1919 joint meeting of the Royal Society and the Royal Astronomical Society, Sir Frank Watson Dyson reported on the observations of starlight made during the previous May’s solar eclipse. “After a careful study of the plates I am prepared to say that they confirm Einstein’s prediction. A very definite result had been obtained, that light is deflected in accordance with Einstein’s law of gravitation.” Sir John Joseph Thomson, presiding, afterwards called the result “one of the highest achievements of human thought.” It was a triumphant moment for both theoretical physics and observational astronomy.

A few years after the momentous Royal Society meeting a German and a Swedish physicist, Theodor Kaluza and Oskar Klein, reached a striking conclusion. They noticed that the equations of general relativity, when solved in five rather than four dimensions, led to additional solutions that were identical to the well-known Maxwell equations of electromagnetism. Since the apparent fifth dimension had not, and still has not been observed, a necessary additional postulate for this theory to correspond to possible reality was that the fifth dimension was curled up so tightly that any motion in its direction had not been detected.

Einstein, finding this extension of his General Theory of Relativity extraordinarily attractive, tried more than once, without success, to make it part of his lifelong dream of a unified field theory of interactions. But this direction of research fell into relative disfavor during the first post World War II decades during which theoretical physics turned its attention to other matters. It returned with a vengeance during the late 1970s, gaining momentum in the 1980s as physicists began to seriously examine theories that could unite all fundamental interactions into one comprehensive scheme. The rising popularity of superstring theory, mathematically consistent only if additional space-time dimensions are present, has provided the decisive impetus for such considerations.

There are striking differences from the 1915 situation, most particularly the lack of a clear test for the detection of extra dimensions. The novel theories now in fashion do predict that additional particles must be present in nature because of these extensions of space and time, but since the mass of these particles is related to the unknown scale of the extra dimensions, it also remains unknown. Roughly speaking, the smaller the one, the larger the other. Nevertheless the hunt has begun; we are beginning to see in the literature publications from major laboratories with titles such as “ Search for Gamma Rays from the Lightest Kaluza-Klein Particle”, that being the name frequently given to the as of yet undiscovered particles associated with extra dimensions.

These searches are largely motivated by the desire to identify Dark Matter, estimated to be several times more plentiful in our Universe’s makeup than all known species of matter. Kaluza-Klein particles are one possible candidate, perhaps hard to distinguish from other candidates even if found. Challenges abound, but the stakes are very high as well.