Why String Theory?

Johar M. Ashfaque

Abstract

String theory yields many interesting, novel results and insights not only in physics but alsomathematics.

1 Quantum Gravity

String theory offers a solution to the problem of quantum gravity. There are two established classicalgravity theories:

• Newtonian Gravity

• General Relativity

We know that nature is quantum mechanical. Thus gravity must also be quantized for consistencywith other fundamental forces. In practice, the effects of quantum gravity hardly play a role except forconsiderations of the early universe and for black hole radiation.

The quantization of Einstein gravity introduces infinitely many adjustable parameters. All parametershave to be known in order to have a predictive description of nature. This renders the theory non-predictive. The only good prediction is at sufficiently low energies much below the Planck scale.

What does String Theory have to do with it? Quantum string theory turns out to contain particles espe-cially gravitons. Moreover, string theory does not generate divergences; it is a finite theory. Furthermore,string theory has two fundamental coupling constants.

2 Unification

String theory provides a unified description for all the fundamental forces of nature. Electromagneticand weak forces combine into electroweak forces at sufficiently high energies around 102 GeV. These mayalso combine with the string nuclear force into the so called Grand Unified Theory. There are some clues

• Charges of fermions appear to suggest larger symmetry group than the one of QCD and the elec-troweak theory

SU(3)× SU(2)× U(1)← SU(5), SO(10), E6?

• Estimated GUT scale 1015, GeV is somewhere close to the Planck scale 1019, GeV.

String theory describes gauge theories as well as gravity.

3 String Dualities

There are intricate relations between string and gauge theories.

• Some effects within gauge theory such as gluon flux tubes in QCD have a stringy nature.

• Some particular gauge theories are in fact equivalent to string theories.

1