**Paul Townsend (University of Cambridge, Department of Applied Mathematics and Theoretical Physics, UK)**

Abstract of the memorial lecture “A Brief History of Branes”: Joe Polchinski made many groundbreaking discoveries in theoretical physics. This talk will focus on his contributions to the circle of ideas that led to M-theory in the late 1990s, especially his work of the 1980s on supermembranes (’86) and D-branes and T-duality (’89). This will be part of a survey of the changing role of branes in physics, with personal commentary on various related topics (such as M-branes, U-dualities, black branes) in supergravity and string theory.

Polchinski was a professor at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. His great contributions to theoretical physics, including the discovery of D-branes –– a type of membrane in string theory –– have led to advances in the understanding of string theory and quantum gravity. In 2008, he shared ICTP’s Dirac Medal with Juan Maldacena and Cumrun Vafa for their fundamental contributions to superstring theory. The three scientists’ profound achievements have helped to address outstanding questions like confinement of quarks and QCD mass spectrum from a new perspective and have found applications in practical calculations. In addition to the Dirac Medal, Polchinski was awarded the American Physical Society’s 2007 Dannie Heineman Prize for Mathematical Physics, the Milner Foundation’s Physics Frontiers Prize in 2013 and 2014, as well as the 2017 Breakthrough Prize in Fundamental Physics. His work touched the lives of many ICTP scientists, from the hundreds who attended his lectures to those who worked directly with him.

# Category Archives: STRING THEORY

# Oxford Scientist Explains the Physics of Playing Electric Guitar Solos

You’ve heard it before. A power ballad from the 1970s or 1980s is playing and there, smack in the middle, is a face-melting guitar solo that seems to go all over the place before blowing your mind with sheer awesomeness. Think Jimi Hendrix. Think Eric Clapton. And especially think Eddie Van Halen. Unlike the piano, which can only play discrete notes, the guitar can, in the hands of someone like Sir Eddie, bend notes. It’s a quality that recalls the human voice, and it’s most likely what has made the electric guitar the go-to instrument for popular music over the past 50 years.

Enter Dr. David Grimes of Oxford University. While by day he might be working out mathematical models of oxygen distribution to help improve cancer treatment, by night he, too, likes to shred on his electric guitar. So, at some point along the line, he decided to apply a little scientific rigor to the instrument he loves. “I wanted to understand what it was about these guitar techniques that allows you to manipulate pitch,” he said in an interview.

In the name of science, Grimes was forced to make some pretty brutal sacrifices. “I took one of my oldest guitars down to the engineering lab at Dublin City University to one of the people I knew there and explained that I wanted to strip it down to do this experiment. We had to accurately bend the strings to different extents and measure the frequency produced. He was a musician too and looked at me with abject horror. But we both knew it needed to be done – We put some nails into my guitar for science.’

Grimes ended up writing an academic paper on the topic called “String Theory – The Physics of String-Bending and Other Electric Guitar Techniques.” “It turns out it’s actually reasonably straightforward,’ said Grimes. “It’s an experiment a decent physics undergraduate could do, and a cool way of studying some basic physics principles. It’s also potentially useful to string manufacturers and digital instrument modellers.”

You can read Grime’s paper here or, if your idea of fun does not include wading through a lot of complex equations, you can watch the brief video presentation above on his research. And below is a ridiculously sweet guitar solo from Van Halen. While you watch ponder the totally awesome physics involved.

Read more at www.openculture.com

# Tensile Strength and the Mining of Black Holes

**Adam R. Brown**

There are a number of important thought experiments that involve raising and lowering boxes full of radiation in the vicinity of black hole horizons. This Letter looks at the limitations placed on these thought experiments by the null energy condition, which imposes a fundamental bound on the tensile-strength-to-weight ratio of the materials involved, makes it impossible to build a box near the horizon that is wider than a single wavelength of the Hawking quanta, and puts a severe constraint on the operation of “space elevators” near black holes. In particular, it is shown that proposals for mining black holes by lowering boxes near the horizon, collecting some Hawking radiation, and dragging it out to infinity cannot proceed nearly as rapidly as has previously been claimed. As a consequence of this limitation, the boxes and all the moving parts are superfluous and black holes can be destroyed equally rapidly by threading the horizon with strings.

Read more at http://prl.aps.org/abstract/PRL/v111/i21/e211301

Read also: “**To kill a black hole, recruit a cosmic string army**“

# 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

# Higher Dimensions from String Theory

String Theory predicts the existence of more than the 3 space dimensions and 1 time dimension we are all familiar with. According to string theory, there are additional dimensions that we are unfamiliar with because they are curled up into tiny complicated shapes that can only be seen on tiny scales. If we could shrink to this tiny, Planck-sized scale we could see that at every 3D point in space, we can also explore 6 additional dimensions. This animation shows a Calabi-Yau surface which is a projection of these higher dimensions into the more familiar dimensions we are aware of….

Read more: http://members.wolfram.com

# Where are we in extra dimensions?

**Basic scenarios of string theory**

Gordon has assured me that (almost) no non-expert has understood advanced basics of string phenomenology, despite dozens if not hundreds of blog entries about these topics that have been written on this blog during the years.

So I would like to be a little bit (but not too much) more comprehensible and address this text to some of the readers who have never studied any string theory at a technical level but who have some idea about quantum field theory and the concept of extra dimensions. I will review the basic vacua of string/M-theory in 10-11 spacetime dimensions and their basic relationships.

It turns out that almost each of them may give rise to a particular, idiosyncratic class of realistic universes with 3+1 large dimensions that we may inhabit.

**So what kinds of string theory are there?**

First, I must say that this very question is obsolete if it is phrased in this way. In the 1980s, people would be talking about “different string theories” (and non-experts are doing so even today). But in the mid 1990s, string theorists have understood that all the “different string theories” are actually just environments in a single theory.

You should imagine that string/M-theory is a single theory with many “fields” and similar objects and if you tune these fields (think about scalar fields) to various values, you will obtain a universe with properties that are described by what used to be called “a particular string theory”. And string theory dictates how these points in the configuration space or “landscape” are connected, too. For example, the number and types of low-energy fields depend on the point in the configuration space, too.

Since the 1990s, we know that there is just one string theory and not many.

**Higher-dimensional vacua**

Fine. But we may still use the vocabulary of the 1980s for a little while. What string theories do we have if we don’t allow any compactification? There are six of them: all of them live in spacetime whose dimension is either 10 (string theory) or 11 (M-theory).

String theory was originally born in the late 1960s and within a few years, people understood that the right spacetime had 26 dimensions. But this was a different, older, not quite healthy string theory, the so-called “bosonic string theory”. This theory predicted that there were no fermions which is a bad starting point to describe our reality with lots of fermions. Even more seriously, bosonic string theory did include a (bosonic) tachyon, a particle that naively moves faster than light (but it’s surely not a neutrino) and that makes the spacetime of bosonic theory unstable (much like the “h=0” point of the Higgs field is unstable).

So I will not treat bosonic string theory as a part of the genuine, fully consistent string theory (although there are interesting papers that describe hypothetical dynamical processes that may change a 26-dimensional spacetime to a 10-dimensional one or vice versa). We will only focus on the string theories in 10-11 dimensions and assume that the 26-dimensional “theory” is just a toy model, not a fully consistent one, to learn the actual theory that matters and works, namely superstring/M-theory.

**The six theories in the maximum dimension**

The list of uncompactified string theories is short: it only contains 6 entries:

- type I string theory with spin(32)/Z_2 gauge group
- type IIA string theory
- type IIB string theory
- heterotic E_8 x E_8 string theory
- heterotic spin(32)/Z_2 string theory
- M-theory

The first five entries should be called “string theory” because vibrating 1-dimensional strings are the most important objects they contain. All of the string theories contain closed strings (e.g. the graviton is always a closed string); type I string theory is the only one on the list whose strings are unorientable and that also contains open strings. The last entry in my list is eleven-dimensional M-theory and contains no strings; instead, it has other extended objects, namely M2-branes and M5-branes. The numerals in the brane nomenclature count the number of spatial dimensions; so strings in string theories are also known as F1-branes (“F” stands for “fundamental”); they may also be obtained as M-theory’s M2-branes with one dimension wrapped around the compact dimension of the M-theory spacetime…… Continue reading Where are we in extra dimensions?