The Future Of String Theory

Reposted from: http://edge.org/q2007/q07_7.html
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GINO SEGRE
Physicist, University of Pennsylvania; Author: Faust In Copenhagen: A Struggle for the Soul of Physics

The Future Of String Theory

I am optimistic about the future of our thinking regarding string theory and the early universe. Until fairly recently I did not feel this way since string theory seemed to be a community unto itself, albeit a very talented one. Controversy has created an important dialogue and strife has erupted. I think this is all to the good. The basis for the disagreement goes back 30 years.

A unified understanding or so-called "theory of everything" has long been sought. The standard model that emerged in the 1970s provided a very significant step forward but left undetermined some 20 parameters: the values of the six quark and six lepton masses, various couplings etc. Initially it was hoped that string theory, aside from a unification of forces with quantum gravity, would determine the values of these parameters. That dream has not been realized.

A very significant group of theoretical physicists has now abandoned the dream. Pointing out that even string theory supports the view that an essentially infinite number of possibilities can be realized for a universe, the so-called landscape, they maintain that we live in one of these choices, the universe where the 20 or so parameters are fixed to be the values we observe. Other universes, with other values of the parameters, are continuously emerging and dying and still others live by our side. However we are limited in the possibility of observations and measurements to our own universe so that, in a deep sense, the 20 parameters that determine our world are completely arbitrary. We would not exist if they were not what they are, but there is no further understanding of their values.

A second group maintains that abandoning the dream that set elementary particle physics on its course a century ago, that of determining the forces and parameters of the sub-atomic world, is both premature and intellectually wrong. They maintain this is not science.

There is an intermediate position that, understandably, has not been embraced vigorously by either side. Perhaps very few of the 20 or so parameters, some of the mass scales, correspond to the universe we live in, but the others are set by string theory or some future theory we have not yet discovered. This could happen if e.g. the quark and lepton masses are calculable numbers that multiply a mass given by the particular universe we happen to live in. In this case both sides would be right. The numbers would be set by the theory and the mass scale by the choice of universe. I find the notion intriguing, but it may also be that both sides are wrong and some other stunning synthesis will emerge.

So why am I optimistic? Because I believe that controversy, with clearly drawn out opposing positions, galvanizes both sides to refine their opinions, creates excitement in the field for the participants, stimulates new ideas, attracts new thinkers to the fray and finally because it provides the public at large with an entrée into the world of science at the highest level, exhibiting for them heated arguments between great minds differing on questions vital to them. What could be more exciting?