1. Why String Theory?
Johar M. Ashfaque
Abstract
String theory yields many interesting, novel results and insights not only in physics but also
mathematics.
1 Quantum Gravity
String theory offers a solution to the problem of quantum gravity. There are two established classical
gravity theories:
• Newtonian Gravity
• General Relativity
We know that nature is quantum mechanical. Thus gravity must also be quantized for consistency
with other fundamental forces. In practice, the effects of quantum gravity hardly play a role except for
considerations of the early universe and for black hole radiation.
The quantization of Einstein gravity introduces infinitely many adjustable parameters. All parameters
have 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. Electromagnetic
and weak forces combine into electroweak forces at sufficiently high energies around 102
GeV. These may
also 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.
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