string theory

History

The idea arose in the late 1960s when physicists were looking for a way to describe the strong interaction—the force that holds atomic nuclei together. They noticed that a formula proposed by Gabriele Veneziano described interactions similar to the vibration of strings. In the 1980s–1990s, the theory turned into a candidate for a 'theory of everything,' promising to unite all fundamental forces.

How it works

Under normal conditions, strings are invisible: their size is about 10⁻³⁵ meters, unimaginably small. The theory predicts that at extreme energies, comparable to the conditions of the Big Bang, strings stretch and their effects become noticeable. Analogy: if you strongly heat a guitar string, it will sound at a new, previously inaudible frequency.

History

The idea arose in the late 1960s when physicists were looking for a way to describe the strong interaction—the force that holds atomic nuclei together. They noticed that a formula proposed by Gabriele Veneziano described interactions similar to the vibration of strings. In the 1980s–1990s, the theory turned into a candidate for a 'theory of everything,' promising to unite all fundamental forces.

How it works

Under normal conditions, strings are invisible: their size is about 10⁻³⁵ meters, unimaginably small. The theory predicts that at extreme energies, comparable to the conditions of the Big Bang, strings stretch and their effects become noticeable. Analogy: if you strongly heat a guitar string, it will sound at a new, previously inaudible frequency.

History

In 1968, Gabriele Veneziano discovered that Euler's beta function describes the scattering amplitude of mesons. This led to the dual resonance model, which Yoichiro Nambu, Holger Nielsen, and Leonard Susskind interpreted as a quantum theory of relativistic strings. In the 1970s, the bosonic string suffered from tachyons, but the addition of supersymmetry and fermions led to superstring theory (John Schwarz, Michael Green). In 1984–86, the 'first superstring revolution' showed that 10-dimensional superstrings are free of anomalies and can incorporate the Standard Model. The mid-1990s—the 'second superstring revolution'—revealed dualities and a unifying M-theory (Edward Witten).

How it works

The string action generalizes the action of a point relativistic particle: instead of a worldline—a worldsheet (two-dimensional surface) in spacetime. The quantum behavior of the string leads to the requirement of a conformal field theory on the worldsheet. The mass spectrum is discrete, and for a closed string, the massless spin-2 excitation corresponds to the graviton, automatically incorporating gravity. Interactions are described by Feynman diagrams, but with the topology of worldsheets—without ultraviolet divergences, yielding a finite quantum theory of gravity. Upon compactification, the extra dimensions determine the set of massless fields and symmetry groups of the effective theory. The superposition principle applies to string states.

Open problems & fun facts

Lack of experimental confirmation: the Planck scale requires energies unattainable in accelerators.; The landscape problem: a huge number of possible vacuum states (~10^500) makes it difficult to single out our Universe.; Understanding non-supersymmetric solutions and the cosmological constant.; Background independence: a complete formulation of the theory without relying on a fixed spacetime is not finished.

💡 In some models of string theory, there exist 'cosmic strings'—macroscopic topological defects that could be detected through gravitational lensing; this is one of the few potentially testable signals.