String Theory

What is string theory?

String theory is at this moment the most promising candidate theory for a unified description of the fundamental particles and forces in nature including gravity. As a theory of quantum gravity string theory is at present our best hope to give concretely computable answers to fundamental questions such as the underlying symmetries of nature, the quantum behaviour of black holes, the existence and breaking of supersymmetry, and the quantum treatment of singularities. It might also shed light upon larger issues such as the nature of quantum mechanics and space and time. In string theory all the forces and particles emerge in an elegant geometrical way, realizing Einstein's dream of building everything from the geometry of space-time.

String theory is based on the (deceptively simple) premise that at Planckian scales, where the quantum effects of gravity are strong, particles are actually one-dimensional extended objects. Just as a particle that moves through spacetime sweeps out a curve (the worldline)

string will sweep out a surface (the world-sheet)

In contrast with particle theories, string theory is highly constrained in the choice of interactions, supersymmetries and gauge groups. In fact, all the usual particles emerge as excitations of the string and the interactions are simply given by the geometric splitting and joining of these strings:

In this way the usual Feynman diagrams of quantum field theory are generalized by arbitrary Riemann surfaces

Much recent interest has been focused on D-branes. A D-brane is a submanifold of space-time with the property that strings can end or begin on it.

More information on the web

Een Nieuwe Revolutie in Stringtheorie, a popular article in Dutch about recent developments, Afleiding 1 (1996) 7-11 (in Dutch, ps-file)

Nonperturbative String Theory, a somewhat more technical review in English of recent developments.

Superstring Theory, by Brian Greene.

String Theory and the Unification of All Forces, by Sunil Mukhi.

The Second Superstring Revolution, by John Schwarz

Some recent popular literature on string theory

G. Taubes, A Theory of Everything Takes Shape, Science 269 (1995).
P. Townsend, Unity from Duality, Physics World, Sept 1995.
E. Witten, Reflections on the Fate of Spacetime, Physics Today, April 1996.
M. Mukerjee, Explaining everything, Scientific American, January 1996.

Good starting points in the scientific literature

M.B. Green, J.H. Schwarz, E. Witten, Superstring Theory, two volumes (Cambridge University Press, 1987).

Still the most complete treatment of the pre-1987 material.

J. Polchinski, What is String Theory? in Les Houches 1994, `Fluctuating Geometries in Statistical Mechanics and Field Theory,' hep-th/9409168.

A good review of the material that ends just before the `1994 revolution.' It also includes the matrix models and random surface ideas of 1989/90.

J. Polchinski, Tasi Lectures on D-branes, hep-th/9611050.

All you want to know about D-branes, the single most important ingredient to understand non-perturbative string theory.

B. Greene, String Theory on Calabi-Yau Manifolds, hep-th/9702155.

A excellent review of (super)conformal field theory, supersymmetric sigma models, mirror symmetry etc.

H. Ooguri, Z. Yin, TASI Lectures on Perturbative String Theory, hep-th/9612254.

A compact lecture series that treats the essentials of perturbative string theory in an elegant way.

R. Dijkgraaf, Les Houches Lectures on Fields, Strings and Duality, to be published in Les Houches 1995, `Quantum Symmetries.' hep-th/9703136

My lecture notes on the more mathematical aspects of modern quantum field theory and strings.

T. Banks, Matrix Theory, hep-th/9710231.

A review of matrix theory, at this moment the hot topic in string theory.

Superstring Theory

Superstring theory resolves the most enigmatic problem of twentieth century theoretical physics: the mathematical incompatibility of the foundational pillars of quantum mechanics and the General Theory of Relativity. In doing so, string theory modifies our understanding of spacetime and the gravitational force. One recently discovered consequence of this modification is that spacetime can undergo remarkable rearrangements of its basic structure requiring the fabric of spacetime to tear apart and subsequently reconnect. Such processes are at best unlikely and probably impossible in pre-string theories as they would be accompanied by violent physical effects. In string theory, on the contrary, these processes are physically sensible and thoroughly common.

Superstring Theory.

The usual domains of general relativity and quantum mechanics are quite different. General relativity describes the force of gravity and hence is usually applied to the largest and most massive structures including stars, galaxies, black holes and even, in cosmology, the universe itself. Quantum mechanics is most relevant in describing the smallest structures in the universe such as electrons and quarks. In most ordinary physical situations, therefore, either general relativity or quantum mechanics is required for a theoretical understanding, but not both. There are, however, extreme physical circumstances which require both of these fundamental theories for a proper theoretical treatment.

Prime examples of such situations are spacetime singularities such as the central point of a black hole or the state of the universe just before the big bang. These exotic physical structures involve enormous mass scales (thus requiring general relativity) and extremely small distance scales (thus requiring quantum mechanics). Unfortunately, general relativity and quantum mechanics are mutually incompatible: any calculation which simultaneously uses both of these tools yields nonsensical answers. The origin of this problem can be traced to equations which become badly behaved when particles interact with each other across minute distance scales on the order of 10 ^-33 cm ( 10 ^-34 in)--- the Planck length.

String theory solves the deep problem of the incompatibility of these two fundamental theories by modifying the properties of general relativity when it is applied to scales on the order of the Planck length. String theory is based on the premise that the elementary constituents of matter are not described correctly when we model them as point-like objects. Rather, according to this theory, the elementary ``particles'' are actually tiny closed loops of string with radii approximately given by the Planck length. Modern accelerators can only probe down to distance scales around 10 ^-16 cm ( 10 ^-17 in) and hence these loops of string appear to be point objects. However, the string theoretic hypothesis that they are actually tiny loops, changes drastically the way in which these objects interact on the shortest of distance scales. This modification is what allows gravity and quantum mechanics to form a harmonious union.

There is a price to be paid for this solution, however. It turns out that the equations of string theory are self consistent only if the universe contains, in addition to time, nine spatial dimensions. As this is in gross conflict with the perception of three spatial dimensions, it might seem that string theory must be discarded. This is not true.

Kaluza-Klein Theory.

The idea that our universe might have more than the three familiar spatial dimensions is one which was introduced more than half a century before the advent of string theory by T. Kaluza and by O. Klein. The basic premise of such Kaluza-Klein theories is that a dimension can be either large and directly observable or small and essentially invisible. An analogy with a garden hose can be helpful. From a distance, a garden hose looks like a long one dimensional object. From a closer vantage point (or from a long distance using a visual aid) an additional dimension --- the circular dimension winding around the hose --- becomes evident. Thus, depending on the scale of sensitivity of the observer, the hose will either appear as one or two dimensional. Kaluza-Klein theories state that the same thing can be true of the universe. No experiment rules out the possible existence of additional spatial dimensions curled up (like the circular dimension of the hose) on scales smaller than 10 ^-16

cm ( 10

in), the limit of present day accessibility. Although originally introduced in the context of point particle theories, this notion can be applied to strings. String theory, therefore, is physically sensible if the six extra dimensions which it requires curl up in this fashion.

A universe with both extended dimensions (two shown) and curled up dimensions (two shown).

A remarkable property of these theories is that the precise size, shape, number of holes, etc. of these extra dimensions determines properties such as the masses and electric charges of the elementary `particles'.

Gravitational Fluctuations and the Topology of Spacetime.

A number of issues, unresolved at present, prevent the application of string theory to the analysis of the kind of spacetime singularities described above. The theory can be successfully applied, though, to another class of singularities which control the topology of the universe.

Topology is a mathematical concept that embodies those properties of a geometrical space which do not change if the space is stretched, twisted or bent but not torn. A doughnut and a sphere are distinct from the topological viewpoint because there is no way to deform one into the other smoothly, that is, without tearing either object. A doughnut and a teacup, both of which have one hole, can be continuously deformed into each other and hence have the same topology.

General relativity predicts that the fabric of spacetime will smoothly deform its size and shape in response to the presence of matter and energy. A familiar manifestation of this spacetime stretching is the expansion of the universe. The topology of the universe, however, remains fixed. A long standing question is whether there might be physical processes which, unlike those familiar from general relativity, cause the topology of the universe to change. There is a heuristic reason for suspecting this possibility based on a naive application of quantum mechanics. Namely, a universal feature of quantum mechanics is that on the smallest distance scales even the most quiescent systems undergo `quantum jitter': the value of quantities characterizing the system fluctuate, sometimes violently, averaging out to their measured values on larger distance scales. This notion, applied the fabric of spacetime, yields the image of a frothing, undulating structure on small distance scales which averages out on larger scales to the smooth geometrical description of general relativity. It is conceivable that, behind the veil of quantum jitter, the fabric of spacetime could momentarily tear and subsequently reconnect in a manner resulting in a change of the topology of the universe. Prior to the advent of string theory, the incompatibility of general relativity and quantum mechanics made it impossible to address this possibility in a quantitative manner.

Topology Change in String Theory.

Due to the above reasoning, the possibility of spacetime topology change was suggested as a novel characteristic of the union of gravity and quantum mechanics. String theory, which achieves this union, has recently been shown to permit physical processes which do result in a particular kind topology change, at least in the extra six dimensional component of spacetime.

There is a well studied mathematical operation called a flop which is a systematic procedure for changing the topology of a geometrical space in a ``minimal'' manner. It involves singling out a sphere in the space, continuously shrinking its volume down to zero (leaving the rest of the space fully intact) and then blowing its volume back up, but in an orthogonal direction. The point at which the volume is zero is the singularity which may be considered as a minimal tear. The result of this operation is a new geometrical space whose topology is different from the original. The change in topology is not as drastic as that between a doughnut and a sphere, but nonetheless it is different.

Mathematically, this is a rigorously defined and well studied operation. It can, for instance, be applied to the curled up six dimensional part of spacetime in a theory based on strings. The crucial question is whether this operation is physically realizable. The criterion for determining this is simple: can this operation be achieved in a manner which does not result in any catastrophic physical consequences? In general relativity the answer to this question is no as the physical model ceases to make sense at the singular point --- the point at which the chosen sphere has zero volume. Since string theory differs from general relativity on short distance scales, it is conceivable that a different answer might emerge. At first sight, however, even the equations of string theory appear difficult to analyze in this context. Only with the tool of mirror manifolds can this question be addressed.

Mirror Manifolds.

Four years ago it was shown that the interpretation of string theory using the Kaluza-Klein idea of curled up dimensions comes with a remarkable twist. Two completely different possibilities for the curled up space (different sizes, shapes and number of holes) can, if properly chosen, give rise to identical observable physics. This is completely unexpected from a point particle viewpoint. The reason for this is that in point particle theories, the physical and mathematical descriptions of a geometric space are both based on considering it to be a collection of an infinite number of points grouped together in a particular manner. In string theory, the physical model is based on tiny loops and hence differs markedly from the mathematical description. This, in turn, allows two mathematically distinct curled up spaces to yield physically identical string models. This is a purely string theoretic phenomenon which relies profoundly on the extended nature of a string.

Although either member of a mirror pair gives rise to the same physical theory, the technical description of a given physical process very often differs drastically between the two constructions. In fact, certain processes which have an extremely complicated, and difficult to analyze, description when one curled up space is used, have a transparent, and easy to analyze, description when the mirror is used.

Recently, the mirror description of the topology changing flop operation discussed above has been analyzed. This results in a remarkable simplification of the string equations governing this process. An analysis of these simplified equations has revealed that there are no catastrophic physical consequences of this topology changing process. In fact, the mirror description makes it clear that such topology changing events are not only physically realizable, but commonplace as well. Thus, using the tool of mirror manifolds, it has been shown that the long suspected possibility of topology changing processes can be explicitly realized in string theory.

Bibliography.

Superstrings: A Theory of Everything?, ed. P.C.W. Davies and J. Brown, Cambridge University Press, 1988;
Mirror Manifolds, ed. S-T. Yau, International Press, (1992);
B. R. Greene, P. S. Aspinwall and D. Morrison, Multiple Mirror Manifolds and Topology Change in String Theory, Physics Letters B303, (1993) 249;
E. Witten, Phases of N = 2 Theories in Two Dimensions, Institute for Advanced Studies Preprint IAS-HEP-93-3.
B. Greene, "The Elegant Universe"; (Articles) 2)
E. Witten, "Duality, Spacetime and Quantum Mechanics", Physics Today, May 1997; 3)
M. Mukerjee,"Explaining Everything", Scientific American, Jan. 1996; 4)
G. Taubes, "String Theorists Find a Rosetta Stone", Science, July 23, 1999.

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