Proponents of the string theory approach to quantum gravity hold that the fundamental constituents of nature are not point particles but one-dimensional oscillating strings, and that this framework is the most mathematically consistent path toward reconciling general relativity with quantum mechanics. Advocates include a large portion of the theoretical physics community, particularly those working in high-energy and mathematical physics at research universities and institutes worldwide.

String theorists argue that the central obstacle to quantum gravity is the behavior of point particles under quantization. When physicists apply standard quantum field theory techniques to general relativity, they encounter non-renormalizable ultraviolet divergences - infinities that cannot be absorbed into a finite number of physical parameters. Advocates hold that replacing point particles with extended one-dimensional objects (strings) naturally regulates these divergences, because the string's finite length smears out interactions that would otherwise occur at a single point.

A central claim is that string theory predicts gravity rather than assuming it. The spectrum of a closed string's vibrational modes includes a massless spin-2 particle, which string theorists identify with the graviton - the quantum carrier of gravitational interaction. Advocates regard this as a significant theoretical achievement: gravity falls out of the mathematics rather than being inserted by hand. Edward Witten has remarked in lectures and interviews that the fact that string theory contains gravity is “the most important thing we know about it.”¹⁾

String theory in its modern form requires supersymmetry - a symmetry relating bosons and fermions - to be internally consistent. Advocates argue that supersymmetric string theories (superstring theories) eliminate tachyonic instabilities present in the original bosonic string and dramatically constrain the landscape of consistent theories. The requirement of ten spacetime dimensions (or eleven in M-theory) is not seen as a liability but as a theoretical prediction that physicists must account for through compactification of extra dimensions.

String theorists regard the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence, proposed by Juan Maldacena in 1997, as among the most important theoretical results in decades.²⁾ The duality relates a gravitational theory in a higher-dimensional space to a non-gravitational quantum field theory on its boundary, providing a concrete realization of the holographic principle and a well-defined non-perturbative framework for studying quantum gravity in certain backgrounds. Advocates argue that AdS/CFT has yielded results with real physical applications - including insights into strongly coupled quark-gluon plasmas and condensed matter systems - that demonstrate the framework's fertility beyond formal theory.

String theorists hold that the framework offers something no other approach to quantum gravity currently provides: a unified description of all fundamental forces and matter. The same strings whose oscillations produce gravitons also produce, in different vibrational modes, the particles mediating the electromagnetic, weak, and strong interactions. Advocates regard unification not as an aesthetic preference but as a theoretical requirement - the forces mix under renormalization group flow at high energies, suggesting a common origin.

String theory originated in the late 1960s as an attempt to describe the strong nuclear force. Gabriele Veneziano's 1968 dual resonance model, later understood to describe the scattering of one-dimensional strings, provided the seed.³⁾ After quantum chromodynamics succeeded as a theory of the strong force in the early 1970s, string theory was largely abandoned for that purpose, but a small group of theorists - including John Schwarz and Joel Scherk - recognized that the spin-2 mode in the string spectrum pointed toward gravity.

The first superstring revolution (1984-1985) followed the Green-Schwarz anomaly cancellation result, which demonstrated that certain superstring theories in ten dimensions were free of quantum inconsistencies.⁴⁾ This attracted widespread attention and a generation of physicists into the field.

The second superstring revolution (mid-1990s) was driven by Witten's identification of M-theory - an eleven-dimensional framework unifying the five distinct ten-dimensional superstring theories as different limits of a single theory - and by the discovery of D-branes by Joseph Polchinski, which opened new sectors of the theory and eventually enabled AdS/CFT.

Edward Witten (Institute for Advanced Study) is widely regarded as the most influential figure in modern string theory. His contributions span anomaly cancellation, topological field theory, M-theory, and the mathematical physics of supersymmetric gauge theories. He received the Fields Medal in 1990, the only physicist ever awarded the prize.

Juan Maldacena (Institute for Advanced Study) proposed the AdS/CFT correspondence, which is among the most-cited results in the history of theoretical physics.

Leonard Susskind (Stanford University) is a co-discoverer of string theory and a leading developer of the string theory landscape and the anthropic principle as applied to the cosmological constant. He is also a prominent popularizer of the field.⁵⁾

Joseph Polchinski (University of California, Santa Barbara; d. 2018) discovered D-branes and made fundamental contributions to string dualities and the black hole information paradox.

Brian Greene (Columbia University) is a string theorist and prominent public communicator whose books The Elegant Universe and The Fabric of the Cosmos have introduced the framework to general audiences.⁶⁾

Michael Green (University of Cambridge) and John Schwarz (Caltech) are the architects of the first superstring revolution through their anomaly cancellation work.

String theory does not predict a unique vacuum but instead permits an enormous number of possible compactifications of the extra dimensions - estimates range as high as 10^500 distinct solutions - each corresponding to a universe with different low-energy physics. Some advocates, including Susskind, embrace this “landscape” together with an anthropic or multiverse interpretation: we observe our particular vacuum because it permits observers. Others within the string community regard this as a serious theoretical problem or even a reductio of the approach, arguing that a theory that accommodates virtually any low-energy physics loses predictive power. This is an active and unresolved debate among string theorists themselves.

Critics within quantum gravity research (particularly proponents of loop quantum gravity) argue that string theory is formulated against a fixed background spacetime rather than treating spacetime geometry dynamically. Some string theorists acknowledge this as a limitation to be addressed; others argue that AdS/CFT, by defining the theory via a boundary conformal field theory with no gravitational background, represents genuine progress toward background independence.

A significant internal debate concerns string theory's relationship to experiment. The energy scales at which stringy effects are expected to appear (near the Planck scale, ~10^19 GeV) are far beyond any conceivable collider. Some advocates argue that indirect confirmation through mathematical consistency, AdS/CFT applications, and eventual predictions about low-energy supersymmetry is sufficient. Others argue that the field must find sharper contact with observable physics to remain scientifically productive.

Witten proposed M-theory as the overarching framework unifying the five superstring theories, but its complete formulation remains unknown. Some string theorists regard this incompleteness as the central open problem; others hold that the web of dualities and correspondences already provides sufficient structural understanding.

• Quantum Gravity - Main Topic
• Loop Quantum Gravity Viewpoint
• Causal Dynamical Triangulations Viewpoint
• Asymptotic Safety Viewpoint
• Quantum Gravity Debate
• Quantum Gravity - Physics Consensus
• General Relativity
• Supersymmetry
• Holographic Principle
• String Theory - History