Warped Passages - Lisa Randall [146]
So, even if string theory is correct, we are unlikely to find the many additional heavy particles it predicts. The energy of current experiments is sixteen orders of magnitude too low. Because the extra particles are so extraordinarily heavy, the prospects for discovering evidence of strings experimentally is very poor, with the possible exception of the extra-dimensional models I’ll discuss later on.
In most string theory scenarios however, because the string length is so tiny and the string tension is so high, we won’t see any evidence to support string theory at the energies achievable in accelerators, even if the string description is correct. Particle physicists who are interested in predicting experimental results can safely apply conventional four-dimensional quantum field theory, ignore string theory, and still get the correct results. As long as you look only at sizes greater than 10-33 cm, (or, equivalently, energies below 1019 GeV), nothing we have considered earlier about the low-energy consequences of particle physics would change. Given that the size of a proton is about 10-13 cm and that the maximum energy reach of current accelerators is about a thousand GeV, it’s a pretty safe bet that particle-theory predictions will suffice.
Even so, particle physicists who concentrate on low-energy phenomena have good reasons to pay attention to string theory. String theory introduces new ideas, both mathematical and physical, that no one would otherwise have considered, such as branes and other extra-dimensional notions. Even in four dimensions, string theory has paved the way to an improved understanding of supersymmetry, quantum field theory, and the forces a quantum field theory model might contain. And of course, if string theory does give a fully consistent quantum mechanical description of gravity, that would be a formidable achievement. These benefits make string theory very worthwhile, even to those exclusively concerned with experimentally accessible phenomena. Although it will be very difficult (if not impossible) to detect strings, the theoretical ideas illuminated by string theory might be pertinent to our world. We’ll soon see what some of these might be.
Aftermath of the Revolution
In 1984, at the height of the “superstring revolution,” I was a graduate student at Harvard. It rapidly became apparent that in research, a beginning physicist had two choices. She could adopt string theory, following in the footsteps of Ed Witten and David Gross, who were then both at Princeton. Or she could remain a particle physicist with more immediate contact with experimental results, in the school of Howard Georgi and Sheldon Glashow, both then at Harvard. It might seem incredible that physicists interested in the same problems could have been so divided, but the notions of how to make progress were very different in the two camps.
The excitement at Harvard remained with particle physics, and many physicists there largely dismissed string theory. A number of problems in particle physics and cosmology remained unsolved—why not answer these questions before delving into the mathematical minefield that string theory was threatening to become? Was it acceptable for physics to extend into unmeasurable domains? With the many brilliant people and many exciting ideas about how to go beyond the Standard Model of particle physics using more traditional methods, there