**Joseph Polchinski**

The search for a theory of quantum gravity faces two great challenges: the incredibly small scales of the Planck length and time, and the possibility that the observed constants of nature are in part the result of random processes. A priori, one might have expected these to be insuperable obstacles. However, clues from observed physics, and the discovery of string theory, raise the hope that the unification of quantum mechanics and general relativity is within reach.

… Read more at http://arxiv.org/pdf/1512.02477v1.pdf

# Tag Archives: String Theory

# Strings and the Magic of Extra Dimensions

**Cumrun Vafa**

Harvard University, MA

05 Jun 2013

Faculty Hall, IISc, Bangalore

The extra dimensions of string theory which were originally viewed as a source of embarrassment for the theory, have proven to be instrumental in resolving a number of puzzles associated with 3+1 dimensional physics. I discuss examples of this in the context of black holes, gauge theory and particle phenomenology.

# Life at the Interface of Particle Physics and String Theory

**A.N. Schellekens**

If the results of the first LHC run are not betraying us, many decades of particle physics are culminating in a complete and consistent theory for all non-gravitational physics: the Standard Model. But despite this monumental achievement there is a clear sense of disappointment: many questions remain unanswered. Remarkably, most unanswered questions could just be environmental, and disturbingly (to some) the existence of life may depend on that environment. Meanwhile there has been increasing evidence that the seemingly ideal candidatefor answering these questions, String Theory, gives an answer few people initially expected: a huge “landscape” of possibilities, that can be realized in a multiverse and populated by eternal inflation. At the interface of “bottom-up” and “top-down” physics, a discussion of anthropic arguments becomes unavoidable. We review developments in this area, focusing especially on the last decade….

Read more: http://arxiv.org/pdf/1306.5083v1.pdf

# Multidimensional black holes get electric when flexed

Bending a black hole can juice it up. In extra dimensions, a black hole behaves like a fluid and a solid at the same time, and flexing the solid form may generate an electric field.

Although these effects exist only in the theoretical realm, the underlying equations could help us puzzle out some of the real-world properties of the hot, superdense matter that existed right after the big bang.

In our four-dimensional universe – three of space and one of time – black holes occupy single points in space-time. String theory says that if you add a fifth dimension, the black hole becomes a black string. Adding a sixth yields a sheet, or a “black brane”.

This multidimensional universe has a boundary, which when described mathematically looks a lot like the equations for quark-gluon plasma, a primordial form of matter that can be created fleetingly in particle accelerators but which can be too chaotic to study directly. Effects at this boundary also apply to black brane behaviour, which means branes can be used to glean the properties of quark-gluon plasma.

**Brane bender**

But describing black branes requires Einstein’s equations, which are complex and unwieldy, says Joan Camps at the University of Cambridge, who was not involved in the new work. So one trick is to try to describe them as ordinary materials.

Previously, physicists showed that black branes follow the mathematics of fluid dynamics, which in turn allowed them to accurately predict the viscosity of quark-gluon plasma.

Now Jay Armas of Copenhagen University in Denmark and colleagues have shown that black branes can behave like solids as well. If the black brane has an electric charge, bending it converts mechanical stress into an electric field, as in piezoelectric materials. Armas hopes the results will yield further insights into quark-gluon plasma.

“This kind of black hole can be approximated by materials,” Camps says. “This is new progress, and it’s progress that you can formulate in terms of ordinary concepts.”

Journal reference: Physical Review Letters, doi.org/j2c

Read more: http://www.newscientist.com

# The Cosmological Constant Problem, Dark Energy, and the Landscape of String Theory

**Raphael Bousso**

In this colloquium-level account, I describe the cosmological constant problem: why is the energy of empty space at least 60 orders of magnitude smaller than several known contributions to it from the Standard Model of particle physics?

I explain why the “dark energy” responsible for the accelerated expansion of the universe is almost certainly vacuum energy.

The second half of the paper explores a more speculative subject.

The vacuum landscape of string theory leads to a multiverse in which many different three-dimensional vacua coexist, albeit in widely separated regions.

This can explain both the smallness of the observed vacuum energy and the coincidence that its magnitude is comparable to the present matter density….

Read more: http://arxiv.org/pdf/1203.0307v1.pdf

# The Early History of String Theory and Supersymmetry

**John H. Schwarz**

This lecture presents a brief overview of the early history of string theory and supersymmetry.

It describes how the S-matrix theory program for understanding the strong nuclear force evolved into superstring theory, which is a promising framework for constructing a unified quantum theory of all forces including gravity.

The period covered begins with S-matrix theory in the mid 1960s and ends with the widespread acceptance of superstring theory in the mid 1980s.

Further details and additional references can be found in** Schwarz (2007)**…..

Read more: **arxiv.org/pdf**

# String-theory calculations describe ‘birth of the universe’

Researchers in Japan have developed what may be the first string-theory model with a natural mechanism for explaining why our universe would seem to exist in three spatial dimensions if it actually has six more. According to their model, only three of the nine dimensions started to grow at the beginning of the universe, accounting both for the universe’s continuing expansion and for its apparently three-dimensional nature.

Expanding universe as a classical solution in the Lorentzian matrix model for nonperturbative superstring theory

Sang-Woo Kim, Jun Nishimura, Asato Tsuchiya

Recently we have shown by Monte Carlo simulation that expanding (3+1)-dimensional universe appears dynamically from a Lorentzian matrix model for type IIB superstring theory in (9+1)-dimensions. The mechanism for the spontaneous breaking of rotational symmetry relies crucially on the noncommutative nature of the space. Here we study the classical equations of motion as a complementary approach. In particular, we find a unique class of SO(3) symmetric solutions, which exhibits the time-dependence compatible with the expanding universe. The space-space noncommutativity is exactly zero, whereas the space-time noncommutativity becomes significant only towards the end of the expansion. We interpret the Monte Carlo results and the classical solution as describing the behavior of the model at earlier time and at later time, respectively…… http://arxiv.org/pdf

String theory is a potential “theory of everything”, uniting all matter and forces in a single theoretical framework, which describes the fundamental level of the universe in terms of vibrating strings rather than particles. Although the framework can naturally incorporate gravity even on the subatomic level, it implies that the universe has some strange properties, such as nine or ten spatial dimensions. String theorists have approached this problem by finding ways to “compactify” six or seven of these dimensions, or shrink them down so that we wouldn’t notice them. Unfortunately, Jun Nishimura of the High Energy Accelerator Research Organization (KEK) in Tsukuba says “There are many ways to get four-dimensional space–time, and the different ways lead to different physics.” The solution is not unique enough to produce useful predictions.

These compactification schemes are studied through perturbation theory, in which all the possible ways that strings could interact are added up to describe the interaction. However, this only works if the interaction is relatively weak, with a distinct hierarchy in the likelihood of each possible interaction. If the interactions between the strings are stronger, with multiple outcomes equally likely, perturbation theory no longer works……

Read more: http://physicsworld.com