IOP Institute of Physics

# String Theory: Ideology or Tool Box?

26 February 2013

Dr Berman clearly and succinctly tackled the concept of String Theory, its implications and its utility.

Dr David Berman and Prof. Peter Kalmus

String Theory is a versatile theory which is currently aiming at providing explanation to many natural phenomena; but as a with any scientific theory, there are instances in which String Theory is less able to do so.

The basic concept stems from the fact that the fundamental constituents of Nature aren't point-like structures. They have extension. These 'ultra microscopical filamentous' particles vibrate.

The different forces and particles observed are associated to the string's internal degrees of freedom being different.

In conclusion, there aren't different  particles or forces, they are the same phenomena which are simply vibrating differently.

All matter are forces, including gravity, and are strings. String Theory can thus be conceptualised as a Unifying Theory. But there are some constraints with this idea. It requires supersymmetry and the existence of 10 dimensions.

Additional symmetries between forces and matter have not yet been found.  We don't have supersymmetry and we only have 4 dimensions. It appears to be the case that the extra dimensions are small, hidden à la Kaluza-Klein theory (1921) and that supersymmetry (SUSY) is broken.

Nevertheless, String theory has been successful:1- in Gravity, where Einstein's equations emerge and are quantised efficiently and  2- in Quantum Mechanics, this attesting to its credibility.

String theory also works well in properties of duality. Duality in Physics is when two seemingly different physical systems can communicate with each other by means of interchanging either components and/or magnitudes.

For example, swapping the electric and magnetic fields and invert the charge. So, by virtue of this duality, we can infer the meaning of system “A” by knowing system “B” and vice versa.

Having the right duality frame can make calculations easier and String Theory provides this 'dictionary.'

There are closed and open strings, and both can  simultaneously be he same, but also be different. The world sheet of a closed string is a cylinder.

The world sheet of an open string at one loop is also a cylinder. But whereas closed strings are simple and straightforward with particles propagating  regularly, open strings are far more complex.

The open strings split into two, then recombine and eventually annihilate each other. Particles are created and then destroyed. But both properties of openness and closeness simultaneously and inherently exist. Their characteristics differ depending on the observer's point of view.
Furthermore, closed strings relate to gravity and open strings to gauge theory. Having the duality in mind will help us 'translate' between the two. Gauge theory interactions are ubiquitous in nature and have helped explain and understand convoluted and mathematically cumbersome challenges. For example: Gauge Theory Thermodynamics such as in the RHIC (Relativistic Heavy Ion Collider), and Black Holes. String Theory has provided the best theoretical fit so far.

String theory has  applications in condensed matter, superconductors and Hydrodynamics. It is also useful in mathematics, where practically every area is connected to String Theory.

Unfortunately, String Theory allows an excessive amount of possibilities, is formulated as a perturbation theory and is not unique, creating problems and lacunae in knowledge. M-Theory seems to avoid there problems. M-Theory is unique, strings aren't presents, only membranes and fivebranes. It has no dimensionless parameters, is non-perturbative and is effective at solving quantum field theories at low energies (all loops and instantons) although the branes' interaction modelling is still in its embryonic phase.

It all looks very promising and further refinements (such as string field theory) are underway. Hopefully, these theories will be instrumental in assisting our existing physical models of understanding.