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1998INTERACTIONS
1998
The Search for a Theory of Everything
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Professor Rothstein received his Ph.D. from the University of Maryland in 1992. He carried out postdoctoral research at the Univ. of Michigan and the Univ. of California at San Diego until 1997 when he joined the faculty of Carnegie Mellon University.
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Research in elementary particle physics is a search for a description of the elementary constituents of matter and the forces that act upon them. Progress in this field not only helps us to understand the underlying nature of all interactions today but also explains how our universe evolved from its inception as a chaotic fire ball to the complex structure we now see. This field has seen great progress in the past 30 years, culminating in one of the great intellectual triumphs of the 20th century: the standard model of particle physics. The model agrees with all data with exceedingly good accuracy. Moreover, the theory has esthetic appeal because it unifies two forces, the electro-magnetic force, and the weak force (which is responsible for nuclear decays). Thus a great first step is reaching its culmination and we begin a new journey into the next level of sophistication.
As we have grown to appreciate the triumphs of the standard model we have also grown contemptuous of its warts. We believe that the standard model is only a low energy approximation of another theory which underlies it, just as Newtonian physics is just the small velocity approximation to the theory of relativity. The evidence for this is quite overwhelming: The model itself contains more than 20 free parameters that cannot be calculated from first principles. Examples of such free parameters are the masses of the particles and the relative strengths of the forces. The true underlying theory should relate these parameters in a calculable way, just as the discovery of the neutron and proton explains the seemingly unrelated masses in the periodic table. Furthermore, while the model unifies the weak and electromagnetic forces, it does not unify these forces with the strong force.
It is widely believed that the correct theory should unify all the forces into one underlying force, which we perceive (at our energies) to be a group of disparate forces. Perhaps most importantly, the theory does not explain why the scale of weak interactions, 102 GeV, is so small compared to the "Planck scale"1019 GeV, which is responsible for gravity. Without this hierarchy of scales, there would be no atoms or molecules and clearly, life, at least as we know it, could not have developed. Finally, the standard model does not correctly describe a quantum theory of gravity.
There are two strategies to making progress toward finding the correct underlying theory, (1) the so-called "top-down" approach where one postulates a complete theory of everything based on mathematical consistency (e.g. superstring theory), and (2) the empirically based "bottom up" approach where one uses experimental data to make smaller, incremental steps. My research is based more on the latter strategy, with an eye toward the former.
Presently my work centers on the strong interaction, one of the holy grails of modern theoretical physics. Progress via either the top down or bottom-up approaches depends upon understanding strongly interacting systems. In the bottom-up approach we need to understand strong interactions for two reasons. First, to extract the values of parameters in the standard model we need to interpret data within controlled approximations. Many observables of interest involve strong interactions, and thus our measurements of these observables are only useful if we can treat strong interaction effects. Secondly, I believe that the theory that underlies the standard model is itself a strongly interacting theory that will explain the hierarchy between the weak scale and the Planck scale as well as unify the forces.
New experiments promise to advance our quest in the coming decade. Several existing accelerators are being modified to study B mesons (B mesons are composed of bottom quarks, which only existed copiously until the universe was 10-17 seconds old). These B factories will give us a better handle on the values of many of the unknown parameters in the standard model (including those responsible for the breaking of time reversal invariance), provided we can understand the effects of the strong interaction. A few years later, the Large Hadron Collider at CERN will begin exploring higher energies in search of new physics beyond the standard model. Thus many new pieces of the puzzle will start to fall into place and particle physics will enter the next level of understanding. Nature has a history of giving us great surprises such as quantum mechanics, which completely changed our picture of the world. Who knows what the next surprise will be?
