Is our world made of strings?

This article is the preface written by Academician Cai Ronggen for the 2021.4 issue of Science World magazine

With the discovery of the Higgs particle in 2012 at the European Large Hadron Collider (LHC), the Standard Model of particle physics describing the fundamental components of matter and their interactions was a great success. The Standard Model of Particle Physics successfully describes three fundamental interactions (strong, weak, and electromagnetic) in addition to gravity, but it still has a range of problems with canonical hierarchy, cosmological constants, dark matter, and more. These questions have inspired physicists to look for new physics beyond the Standard Model of particle physics, and one of the important ideas is the supersymmetry of the ferron degrees of freedom and boson degrees of freedom.

In the study of hadron physics in the 20th century, in order to describe Reggie (T. Regge) sees the relationship between the spin and mass of hadrons, Yoichiro Minami, L. Saskander ( L. Regge ) . Susskind et al. first proposed in the 1970s the idea of "using the motion of 1-dimensional strings instead of the motion of point particles". A 1-dimensional string can be a closed loop, i.e., a closed string, or an open string, or an open string. Elementary particles come from different vibration modes of the strings. However, with M. Gell-Mann The success of the quantum chromodynamics (QCD) theory proposed by Gellman and others gradually obscured the light of string theory, but its ideas have not been interrupted. Because closed strings naturally contain a massless particle with a spin of 2 (gravitons), it has been speculated that string theory may not exist as a theory of strong interactions, but as a theory of quantum gravity.

The original string theory was Bose string theory, in which there were only bosons and no fermions, and there were problems such as containing unstable tachyons and having to exist in 26-dimensional space-time. The combination of supersymmetric thought and string theory, adding fermions of freedom to the world to make it supersymmetric, gave birth to superstring theory. The introduction of supersymmetry modifies the perverse calculation so that the space-time dimension requires only 10 dimensions. In addition to the 4-dimensional space-time we live in, the other 6 dimensions are curled up on the Calabi-Yau manifold. This idea of extra dimensions dates back to the 1920s when T. Kaluca Kaluza) and O. Klein Klein) proposed the idea of high-dimensional space-time, and Einstein also tried to unify the theory of gravity and electromagnetic interactions. In this sense, superstring is a way to achieve great unity in physics through the idea of extra dimensions.

The development of superstring theory was accompanied by two revolutions. First, in 1984, M. Green Green) and J. Schwarz discovered the anomalous elimination mechanism, the first time physicists have discovered a self-consistent string theory. Subsequently, 5 different string theories were gradually established, which was the "first superstring theory revolution".

Then in 1995, E. Witten Witten discovered that these 5 seemingly different string theories were actually different approximations of some higher-dimensional M theory, unifying them in principle. Therefore, M-theory is also known as the "ultimate theory that describes the world". J. Potzinski Polchinski) found that in addition to the 1-dimensional string, there should be an extended object in theory, that is, a D film. In 1997, J. Mardasina Maldacena) discovered the adS/CFT duality based on the D-membrane theory, i.e. the equivalence of the superstring (M) theory on anti-Desitter space-time (AdS) and its conformal field theory (CFT) on its boundary. The AdS/CFT duality links gravitational theory and quantum field theory and, for the first time, gives a quantitative connection to the properties of gravitational holographics. Because of its extensive connections and intrinsic properties, the AdS/CFT duality has received great attention and attention from the theoretical physics community since it was proposed. At the same time, string theory has also greatly promoted the development of black hole physics. Based on D-membrane physics, Strominger (A. Strominger) and C. Wafa (C. Vafa) is the first to give an explanation of the microscopic degrees of freedom for the entropy of black holes of a class of supersymmetric black holes. In view of the emergence of these important achievements, this golden age of string theory research has been called the "second superstring theory revolution".

Because of its excessive energy scale and the requirement of additional dimensions, superstring theory is extremely difficult to confirm or falsify in traditional collider experiments, which has caused some controversy about superstring theory. One possible opportunity comes from cosmological observations. Recently, more and more people are studying the limitations that string theory can impose on the universe in which we live at low energy. The idea of string swamps gives many conjectures in cosmology that can be observed and tested, which may lead to evidence that confirms or falsifies string theory.

Although it remains to be proven whether string theory is a unified theory describing our world, superstrings as a theoretical framework have become an important method for studying physical problems, bringing many unexpected surprises to the mathematical and physics communities. Expect more people to understand it, explore it and develop it.

Source: Institute of Theoretical Physics, Chinese Academy of Sciences

Edited by: Dogcraft, yrLewis