Stanford's Savas Dimopoulos:
New Dimensions in Theoretical Physics |
The standard model of particle physics is a self-contained picture of fundamental particles and their interactions. Physicists, on a journey from solid matter to quarks and gluons, via atoms and nuclear matter, may have reached the foundation level of fields and particles. But have we reached bedrock, or is there something deeper?
"Our new picture is that the 3-D world is embedded in extra dimensions," says Savas Dimopoulos of Stanford University. "This gives us a totally new perspective for addressing theoretical and experimental problems." |
For all its successes, the standard model is nevertheless unsatisfying. There are 18 or so free parameters, such as the electron mass, that seem arbitrary: they have to be determined experimentally. What is the origin of the masses of the fermions? How can the strong and electroweak interactions be unified? And what of the gravitational force, on which the standard model is silent?
In 1981 Savas Dimopoulos of Stanford University and Howard Georgi of Harvard University proposed the supersymmetric extension to the standard model. Revolutionary at the time, it is now accepted by many physicists. Dimopoulos has been strongly driven in his research by a desire to understand what lies beyond the standard model. His contributions have included work on grand unified theories of baryogenesis, which would provide an explanation of the origin of matter. Jointly with Stanford colleague Nima Arkani-Hamed and Gia Dvali of ICTP, Trieste, Italy, he has proposed an audacious solution to the problem of explaining the weakness of the gravitational force. The proposal invokes new large dimensions accessible to the graviton. Among the extraordinary implications of this thinking is the notion that our entire universe is a single point in space of extra dimensions, and is but one of innumerable parallel universes. Thanks to this work, Dimopoulos has recently been a mainstay of the Physics Top
Ten—one of the trio's papers on this subject has ranked among physics's most cited for more than a year (see table on
next page, paper #3).
Dimopoulos grew up in Athens, Greece, and earned his Ph.D. from the University of Chicago in 1978. He has been Professor of Physics at Stanford since 1979, and has received an Alfred P. Sloan Foundation Award.
Dimopoulos spoke to
Science Watch columnist Simon Mitton from his Stanford office.
An enduring theme of 20th century physics was the rise of geometry. Why did a century that began with a literal way of looking at time and space close with papers on multidimensional spaces collecting thousands of citations?
The theory of special relativity showed that space and time are intertwined in a very complex way, to maintain the constancy of the speed of light. It wasn’t immediately obvious how to extend special relativity, and it took Einstein a further 11 years to publish the general theory. His big idea was that the proper picture of space-time is a geometrical view with curvature arising from mass. This leads to nice analogies: if a massive rock is placed in a rubber sheet, the surface bends, and a lighter particle thrown onto the sheet will follow a path determined by the mass of the rock. Einstein’s intuitive leap reduced gravity to geometry.
At first the general theory of relativity was not taken seriously, but Einstein suggested imaginative tests. The predicted bending of light was confirmed by observing the deflection of star light during the 1919 total eclipse of the Sun. Then an explanation of the curious shape of Mercury’s orbit followed as another triumph for geometry. So Einstein reduced one of the forces of nature to geometry. Next, a number of physicists in the 1920s, notably Kaluza and Klein, asked if one could do the same thing for the other forces, particularly electromagnetism. By taking Einstein’s concept and adding a fifth dimension to space-time, they explained the existence of the photon as a consequence of higher dimensional symmetry and higher dimensional curvature. Einstein was very interested in this, but frankly not much happened for several decades. Most physicists reverted to four dimensions.
Then in the early 1950s, electromagnetism was generalized to incorporate weak interactions as well as strong interactions: the underlying theme was the role of symmetry as an underpinning of gravity. Gravity reflects the symmetries of space and time, so people asked if there are extra internal spaces, additional to those we normally experience, which have symmetries. It was shown that electromagnetism, together with the strong and the weak interactions, could be derived as a consequence of these internal symmetries. Essentially the standard model of particle physics was built up in the late 1950s and early 1960s by generalizing the idea of symmetry to internal spaces. These are the so-called gauge theories of strong, weak, and electromagnetic interactions.
The common thread, then, is symmetry: space-time symmetry gives us gravity, and its symmetry of internal spaces then leads to gauge theory and the standard model, a very successful theory.
Although the standard model works experimentally, aren’t there unsatisfactory features?
Yes, there are two niggles. First, until string theory came along, it was impossible to make a consistent theory of gravity that also incorporated quantum mechanics. Gravity differs from the other forces in physics because if you take Einstein’s formulation and extrapolate to extremely short distances you find the gravitational force becomes overwhelmingly large: the equations blow up with infinities. In contrast, gauge theory does not lead to infinities. This difficulty with gravity was largely ignored because gravity is weak at "normal" subatomic distances and the priority was to understand the three stronger forces. However, by the 1970s and 1980s the feeling was that the standard model was in good shape, and attention shifted to gravity. That’s when string theory came into play, motivated by attempts to understand the strong interaction. What string theory does is suggest that the fundamental objects are no longer hard points but softer extended objects, and this offers a way of stopping gravity from becoming enormous at extremely short distances.
String theory has many hopes! By starting with two novel features, namely that the fundamental objects are extended and that there are new dimensions of space-time, we may be able to build a geometric theory of all the forces (strong, weak, electromagnetic, and gravity) using 10 dimensions instead of 4. The extra dimensions allow a geometric description of all the forces, not just gravity.
However, the extra dimensions in string theory are vanishingly small. Why have you and your collaborators been looking at the extra dimensions from a different viewpoint?
There are two paradigms for doing particle physics: the old and the new! I will contrast them. In the old paradigm there are two fundamental mass and energy scales: weak and gravitational.The weak interaction scale is accessible experimentally at a characteristic mass 100 times the proton, that is 100 GeV. The gravitational scale is far bigger,
1018 the proton mass, way out of the reach of any present or future experiment!
What is the reason for this stupendous disparity? The strengths of the weak and gravitational interactions are inversely proportional to the square of the corresponding mass scales. Now we have a difference of
1016 in the mass scales, and when we apply the inverse square law this blows up to
1032. So the extreme weakness of gravity arises from this staggering discrepancy in mass scales. In this picture all gravitational and string phenomena are happening at an energy far removed from anything we can directly access. Arguably that is questionable physics!
continued
Science
Watch®, May/June 2001, Vol. 12, No. 3
Citing URL: http://www.sciencewatch.com/may-june2001/sw_may-june2001_page3.htm |
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