Tuesday, September 23, 2014

String Theory

In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. In string theory, the different types of observed elementary particles arise from the different quantum states of these strings. In addition to the types of particles postulated by the standard model of particle physics, string theory naturally incorporates gravity, and is therefore a candidate for a theory of everything, a self-contained mathematical model that describes all fundamental forces and forms of matter. Aside from this hypothesized role in particle physics, string theory is now widely used as a theoretical tool in physics, and it has shed light on many aspects of quantum field theory and quantum gravity.

Many theoretical physicists believe that string theory is a step towards the correct fundamental description of nature. This is because string theory allows for the consistent combination of quantum field theory and general relativity, agrees with general insights in quantum gravity such as the holographic principle and black hole thermodynamics, and because it has passed many non-trivial checks of its internal consistency.

We live in a wonderfully complex universe, and we are curious about it by nature. Time and again we have wondered--- why are we here? Where did we and the world come from? What is the world made of?It is our privilege to live in a time when enormous progress has been made towards finding some of the answers. String theory is our most recent attempt to answer the last (and part of the second) question.

So, what is the world made of? Ordinary matter is made of atoms, which are in turn made of just three basic components: electrons whirling around a nucleus composed of neutrons and protons. The electron is a truly fundamental particle (it is one of a family of particles known as leptons), but neutrons and protons are made of smaller particles, known as quarks. Quarks are, as far as we know, truly elementary.
Our current knowledge about the subatomic composition of the universe is summarized in what is known as the Standard Model of particle physics. It describes both the fundamental building blocks out of which the world is made, and the forces through which these blocks interact. There are twelve basic building blocks. Six of these arequarks--- they go by the interesting names of updowncharm,strangebottom and top. (A proton, for instance, is made of two up quarks and one down quark.) The other six are leptons--- these include the electron and its two heavier siblings, the muon and thetauon, as well as three neutrinos.
There are four fundamental forces in the universe: gravity,electromagnetism, and the weak and strong nuclear forces. Each of these is produced by fundamental particles that act as carriers of the force. The most familiar of these is the photon, a particle of light, which is the mediator of electromagnetic forces. (This means that, for instance, a magnet attracts a nail because both objects exchange photons.) The graviton is the particle associated with gravity. The strong force is carried by eight particles known as gluons. Finally, the weak force is transmitted by three particles, the W+, the W- , and theZ.
The behavior of all of these particles and forces is described with impeccable precision by the Standard Model, with one notable exception: gravity. For technical reasons, the gravitational force, the most familiar in our every day lives, has proven very difficult to describe microscopically. This has been for many years one of the most important problems in theoretical physics-- to formulate aquantum theory of gravity.
In the last few decades, string theory has emerged as the most promising candidate for a microscopic theory of gravity. And it is infinitely more ambitious than that: it attempts to provide a complete, unified, and consistent description of the fundamental structure of our universe. (For this reason it is sometimes, quite arrogantly, called a 'Theory of Everything').
The essential idea behind string theory is this: all of the different 'fundamental ' particles of the Standard Model are really just different manifestations of one basic object: a string. How can that be? Well, we would ordinarily picture an electron, for instance, as a point with no internal structure. A point cannot do anything but move. But, if string theory is correct, then under an extremely powerful 'microscope' we would realize that the electron is not really a point, but a tiny loop of string. A string can do something aside from moving--- it can oscillate in different ways. If it oscillates a certain way, then from a distance, unable to tell it is really a string, we see an electron. But if it oscillates some other way, well, then we call it a photon, or a quark, or a ... you get the idea. So, if string theory is correct, the entire world is made of strings!
Perhaps the most remarkable thing about string theory is that such a simple idea works--- it is possible to derive (an extension of) the Standard Model (which has been verified experimentally with incredible precision) from a theory of strings. But it should also be said that, to date, there is no direct experimental evidence that string theory itself is the correct description of Nature. This is mostly due to the fact that string theory is still under development. We know bits and pieces of it, but we do not yet see the whole picture, and we are therefore unable to make definite predictions. In recent years many exciting developments have taken place, radically improving our understanding of what the theory is.
If you want to learn more, visit the sites listed below. I also highly recommend the popular science book "The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for The Ultimate Theory" (W. W. Norton & Company, 1999), written by Prof. Brian Greene, a well-established string theorist.
• The Official String Theory Website: An excellent introductory site for the nonspecialist, including RealAudio interviews with leading string theorists, and a tour of the Big Bang.
• The Elegant Universe: The companion site for the 3 hour PBS series based on Brian Greene's book. You can even watch the series online. (Unfortunately, it is nowhere near as good as the book! It is visually stunning, but the physics content is extremely low and the approach is excessively propagandistic. You can read a critical NY Times review of the series here.)
• Particle Adventure: A very nice tour through the main ideas of the Standard Model of particle physics.(Also available in Spanishand other languages).
• The Science of Matter, Space and Time: Another nice presentation of the concepts of particle physics.
• Einstein's Unfinished Symphony: An article on string theory that appeared recently in TIME magazine, on occasion of TIME's designation of Einstein as the person of the 20th century.
• Strings '00 Pictures: Photos of some of the world's leading string theorists, at the most important yearly conference on the subject (see also the ITP M Theory Program and Strings 96 pictures).
• Black Holes, Strings and Quantum Gravity: A public lecture by Prof. Juan Maldacena, undoubtedly the most influential string theorist in the last few years (also available in Spanish).
• Superstrings! Home Page: A nice site where many aspects of string theory are explained.
• Duality, Spacetime and Quantum Mechanics: A public lecture by Prof. Edward Witten (IAS Princeton), arguably the world's most influential theoretical physicist, and a leading contributor to string theory
• The Theory of Strings--- A Detailed Introduction: An extensive description of the basic ideas of the theory, by Prof. Sunil Mukhi, a leading string theorist.
• String Reviews: List of downloadable review articles on various topics in string theory. Most of them are highly technical, but some of the articles in the "Colloquia/Semipopular" category are intended for a wider audience.
• String Theory and the Unification of Forces: An explanation of the ambitious goal of string theory, by Prof. S. Mukhi.
• The Second Superstring Revolution: A (somewhat technical) report on the latest developments in string theory, due to Prof. John Schwarz, one of the fathers of the theory.
• Black Holes, Quantum Mechanics and String Theory: A series of 10 lectures on string theory, intended for the general public, by Prof. Finn Larsen.
• ITP Teachers' Educational Forum on String Theory: Is it the Theory of Everything?: A set of lectures on string theory, intended for highschool teachers.
• M-theory, the theory formerly known as Strings: A brief introduction to some ideas of the theory, presented by the Relativity & Gravitation group at Cambridge University.
• String Theory in a Nutshell: A brief account of string theory, including some words on the latest developments, by Prof. J. M. Figueroa-O'Farrill.
• What is String Theory?: A longer introductory description of the basic ideas of the theory, due to K. LLoyd.
• Superstring Theory: An explanation of some of the ingredients of the theory, written some time ago by Prof. Brian Greene, for the graduate program at Cornell University.
• String Theory: A short introduction to string theory by Prof. Robert Dijkgraaf, written for physicists.
• M-Theory: strings, duality and branes: a non-technical article by Prof. J.P. Gauntlett, which appeared in Contemporary Physics (available in Postscript only).