String theory Definition and 136 Discussions

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. String theory describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Thus string theory is a theory of quantum gravity.
String theory is a broad and varied subject that attempts to address a number of deep questions of fundamental physics. String theory has contributed a number of advances to mathematical physics, which have been applied to a variety of problems in black hole physics, early universe cosmology, nuclear physics, and condensed matter physics, and it has stimulated a number of major developments in pure mathematics. Because string theory potentially provides a unified description of gravity and particle physics, it is a candidate for a theory of everything, a self-contained mathematical model that describes all fundamental forces and forms of matter. Despite much work on these problems, it is not known to what extent string theory describes the real world or how much freedom the theory allows in the choice of its details.
String theory was first studied in the late 1960s as a theory of the strong nuclear force, before being abandoned in favor of quantum chromodynamics. Subsequently, it was realized that the very properties that made string theory unsuitable as a theory of nuclear physics made it a promising candidate for a quantum theory of gravity. The earliest version of string theory, bosonic string theory, incorporated only the class of particles known as bosons. It later developed into superstring theory, which posits a connection called supersymmetry between bosons and the class of particles called fermions. Five consistent versions of superstring theory were developed before it was conjectured in the mid-1990s that they were all different limiting cases of a single theory in 11 dimensions known as M-theory. In late 1997, theorists discovered an important relationship called the AdS/CFT correspondence, which relates string theory to another type of physical theory called a quantum field theory.
One of the challenges of string theory is that the full theory does not have a satisfactory definition in all circumstances. Another issue is that the theory is thought to describe an enormous landscape of possible universes, which has complicated efforts to develop theories of particle physics based on string theory. These issues have led some in the community to criticize these approaches to physics, and to question the value of continued research on string theory unification.

View More On
  1. codebpr

    A Does the Maximum Lyapunov exponent depend on the eigenvalues?

    I am currently reading this paper where on page 8, the authors say that: This correlates with Figure 8 on page 12. Does it mean that there is a real correlation between eigenvalues and Lyapunov exponents?
  2. codebpr

    A Can a black hole horizon act as a source of Chaos?

    I was going through this paper where on page 5 they argue that in the given Poincare section: I am a bit confused by this statement. How does the given saddle point correspond to the black hole horizon and is it necessary that it acts as a source of chaos? Any explanation would be truly...
  3. codebpr

    A The kinetic term of the Hamiltonian is not positive definite

    I am trying to reproduce the results from this paper. On page 10 of the paper, they have an equation: $$ \frac{S}{T}=\int dt\sum _{n=0,1} (\dot{c_n}{}^2-c_n^2 \omega _n^2)+11.3 c_0^3+21.5 c_0 c_1^2+10.7 c_0 \dot{c_0}{}^2+3.32 c_0 \dot{c_1}{}^2+6.64 \dot{c_0} c_1 \dot{c_1} \tag{B12} $$ where they...
  4. ohwilleke

    I Trying To Conceptualize Two Dimensions Of Time

    Some BSM theories, often but not exclusively string theory inspired, have an extra time dimension, as well as extra spatial dimensions. I'm trying to make sense of what it even means to have more than one time dimension, ideally, with a concrete example that illustrates how the second time...
  5. M

    I Is quantum gravity still an immensely popular field?

    I was just wondering how much work is being done in the field of quantum gravity nowdays. Is there still a huge volume of research published on the topic? Are we closer to a "solution" nowdays than we were a few years ago? And also, what exactly would constitute a solution to such problem?
  6. A

    A Loop Quantum gravity or String theory?

    I understand that string theory has almost no testable predictions, however loop quantum gravity is an enticing candidate for only quantum gravity and it doesn't explain much of symmetry, constants, mixing angles etc in Standard model. There is obviously not enough evidence to create a full...
  7. S

    A Branes with any number "n" of dimensions and laws of physics?

    I had a few questions about this paper by Nima Arkani-Hamed, Georgi Dvali and Savas Dimopoulos ( which is closely related to the concept of branes ( 1. The authors of the paper mention that the branes in the model could...
  8. TheHeraclitus

    I String theory in 4 dimensions?

    Non-critical string theory, on Wikipedia it says: Has it been experimentally disproven or are other reasons for its apparent obscurity?
  9. TheHeraclitus

    I Is Supersymmetry required by String theory?

    From what I understood Supersymmetry means there are more particles than we currently know about and they are predicted by (some/all, I do not know) versions of String theory. Is it so important to String theory or can it work without SUSY? Thank you!
  10. TheHeraclitus

    I Bohmian mechanics and String theory

    Is it easy to combine them or impossible? Is there a tension due to the non-locality found in Bohmian mechanics? Thank you!
  11. Safinaz

    A How Randall-Sundrum model has solved the hierarchy problem?

    I'm trying to understand how the RS model solved the hierarchy problem from this mass relation $$ M^2_p = \frac{M^3}{k} \Large[1- e^{-2k\pi r} \Large],$$ Equ. 16 in their paper: With k as large as the Planck scale, the exponential will be so small and...
  12. Safinaz

    A Deriving an action from a metric

    Hello! The paper I study is related to string theory and modified gravity theories topics. As they say in page 5 “The four-dimensional effective theory now follows by substituting Eq. (13) into the original action, Eq. (4)” I wonder how did they drive a 4- dimensional effective metric...
  13. S

    Nobel laureates for and against string theory?

    According to Michio Kaku, Nobel laureates have taken both sides towards string theory since some of them accept it and some of them reject it, as he says here ( “Kaku also explains the intense controversy swirling around this theory, with...
  14. S

    Nobel laureates supporting string theory?

    Are there any living Nobel laureates in physics (apart from David Gross, François Englert and perhaps Gerard 't Hooft and Steven Weinberg) who have made research in string theory or at least find it attractive?
  15. snypehype46

    Computing OPEs of linear dilaton CFT

    I'm trying to do the following question from David Tong's problem sheets on string theory: > A theory of a free scalar field has OPE $$\partial X(z)\partial X(w) = \frac{\alpha'}{2}\frac{1}{(z-w)^2}+...$$. Consider the putative candidate for the stress energy tensor $$T(z) = \frac{1}{\alpha '}...
  16. snypehype46

    I Mass spectrum of open bosonic strings

    I'm learning string theory from the book by Zwiebach and others. I'm trying to understand the quantisation of the open string and its mass spectrum. In light-cone gauge the mass-shell condition of an open string is given by: $$M^2 = 2(N - 1)/l_s^2$$ where ##N =...
  17. E

    Other Should I give up string theory due to its job prospects?

    I'm currently a fresh grad student in theoretical physics, and I'm still deciding to choose which research group to join. My current understanding (maybe I'm wrong) is the PhD theme pretty much determines the topic for future post-doc research so I kinda need to choose very carefully. I'm...
  18. snypehype46

    I Help with a paper on semiclassical strings in anti-desitter space

    Hi, I was given the task to read this paper entitled "A semi-classical limit of the gauge/string correspondence" by Polyakov. On page 7 of this paper it is mentioned that the maximal radial coordinate of the string is p0 and that the string is constrained by...
  19. S

    I Brane cosmology without string theory?

    There are several models of brane cosmology ( and several physicists working in this field (e.g Lisa Randall and Raman Sundrum), but as you will notice, apparently they are all directly related to string theory. This has several consequences, for...
  20. S

    A String Theory in N dimensions?

    String Theory and related theories like M Theory have strong constraints in the number of dimensions where they can be formulated (for example, in the case of M theory, it is only allowed in 11D or in the case of bosonic string theory is only allowed in 26D. Since string theory and related...
  21. J

    I Movement of a guitar string at relativistic speeds

    Hey! I'm and undergrad in the third year of my applied physics program. I'm taking a course in Special Relativity, and due to Corona the exam has been replaced by a pretty free project where we delve deeply into a topic related to the course. I'm interested in music, so my professor suggested I...
  22. S

    A Different symmetries or no symmetries in string theory?

    I was reading the book "A Fortunate Universe" by Geraint Lewis and Luke Barnes and something caught my attention: At page 195 the authors say that universes with different symmetries could be modeled and they would have dramatic results like having different conservation laws. I asked Mr...
  23. J

    I What is the Kalb-Ramond field?

    All String theories include the massless bosonic fields ##G_{\mu\nu}##, ##B_{\mu\nu}## and ##\Phi##. I understand that ##G_{\mu\nu}## is the spin-##2## field of the spacetime metric and ##\Phi## is the spin-##0## dilaton field. The ##B_{\mu\nu}## is called the Kalb-Ramond field and is said to...
  24. S

    A What would it mean if symmetries in physics would not be fundamental?

    Physicist Joseph Polchinski wrote an article ( where he considered the possibility that all symmetries in nature may not be fundamental. He says at page 36: "From more theoretical points of view, string theory appears to allow no exact global symmetries, and...
  25. AhmadKhaqan

    A Constant scalings of 4-vectors

    If anybody has studied the book: A First course in String Theory - Barton Zweibach - 2nd edition This statement is present in 6th chapter of book on pg 110
  26. P

    A First order formalism of Polyakov action

    In the notes of Arutyunov, he writes down the equation of Polyakov action in what he calls a first-order formalism(equation 3.19). But here I did not understand how he got this equation. Can someone help? Moreover, can someone explain how he got the constraints in equation 3.25? And why they...
  27. nomadreid

    I What use is the AdS/CFT correspondence in a flat universe?

    All the accounts which I have read (and which are accessible to my limited knowledge of General Relativity and its mathematics) on the holographic principle says vaguely that the AdS/CFT correspondence is very enlightening, but with the caveat that, well, we don't happen to live in an AdS space...
  28. S

    A Can Cosmic Inflation and String Theory change fundamental laws?

    Is there any version of string theory or cosmological inflation that allows the most fundamental laws and constants change between universes? String Theory and Cosmological Inflation are two theories or models that allow multiple universes to exist. Laws and constants of physics could change...
  29. C

    B How can a dimension be "curled up" and have a finite extent?

    As I understand it, dimension is a way of describing direction, with the first three spatial dimensions being straight lines which extend infinitely in one direction, perpendicular to each other. In string theories, several additional dimensions are required, sometimes up to nine or 10, I...
  30. nomadreid

    I Is M-theory (a) self-consistent (b) externally consistent?

    I do not pretend to understand M-theory or even any of the string theories which make it up, so the answers to these questions do not need to get too much into detail. The two (related) questions: (a) Forget for a moment that this is supposed to be describing our known universe. Is there a...
  31. J

    I Why don't strings have a Planck mass?

    I understand that strings have a size of roughly the Planck length ##l_P## of ##10^{-35}## m. If that is the case then one would expect that their mass would be roughly the Planck mass which is an enormous ##10^{19}## GeV. (Strings that have small spins, like standard model particles, are...
  32. S

    A Is the Randall-Sundrum model background independent?

    Randall-Sundrum model ( is related to string theory. String theory can be background independent ( "String theory is usually formulated with perturbation theory around a fixed...
  33. I

    Programs Topics at the junction of AdS/CFT and quantum information

    I have been asked to draft a (informal) research proposal for a PhD thesis. I have some background in quantum information and my interests are leaning towards the AdS/CFT correspondence. I was wondering if you could suggest a few (preferably recent) theory papers at the junction of quantum...
  34. Vamsi9955

    B How can string theory be proved

    The fundamental building blocks of the universe is thought of super strings, if proved can solve the mysteries of the universe but if proved than how? And how can it solve the mysteries of dark energy &dark matter and black holes?
  35. I

    Admissions Role of AdS/CFT correspondence in the context of integrability

    I have a masters degree. I studied general relativity and quantum field theory. I was interested in applying to PhD programs for AdS/CFT. I was wondering how integrability fits in the context of AdS/CFT. As I understand, the AdS/CFT correspondence postulates a duality between gravity theories...
  36. Ryu

    Theoretical Higher spatial dimensions question

    So I had a topic which I would like to fact check from an informed scientific source. Basically there is an argument about whether or not an object that naturally exists in a fourth dimensional space, would by default have more than countably infinite times the energy of a 3 dimensional Object...
  37. P

    I Electrostatic in string theory or LQG

    How the repulsion between electrons occurs in String theory and in the loop quantum gravity? The electrons will also create electrostatic fields, or will it be the another mechanism?
  38. R

    PhD Inquiry, Modified Gravity or String Theory

    Hello everyone, I am currently finishing my Masters in Theoretical Physics and looking forward to continue in a PhD. I always thought I would love String Theory, and now that i am actually doing it I see that my feeling was correct. I do not know why but trying to understand its concepts...
  39. A

    B Why are we trying to unify all the physical forces?

    I've been reading a book called hyperspace by D.R Michio Kaku. In this book he talks about unifying all the fundamental forces of nature (gravity , electromagnetic force , weak and strong nuclear forces.) My question is, what if they are not unified and we are just trying in vain. I don't...
  40. V

    I Has anyone worked on higher dimensional strings?

    Possibly naive question. Wikipedia describes string theory as follows: The obvious next step (since string theory hasn't succeeded in describing our universe) would be to define elementary particles as 2D surfaces or 3D volumes or 4D space-time volumes, which may have vibrational modes similar...
  41. W

    Studying Mathematics to Understand String Theory/SuperString

    I just recently graduated with a mathematics degree. Lately, I've been very fascinated with quantum mechains and string theory, but when I try to do research I am a little overwhelmed by all the varying topics of advanced mathematics I have to know. Can anyone suggest mathematical topics to...
  42. J

    A Does it make sense to break T duality at fundamental level? Here is an attempt to make bohmian mechanics compatible with string theory. It posits that T duality breaks at the fundamental level, and that the is no minimal length in the theory. Does this proposal make sense?
  43. J

    A Is Bohmian Mechanics incompatible with String Theory

    Bohmian mechanics requires a preferred reference frame. But String theory doesn't have any preferred frames, so isn't BM incompatible with ST?? Maybe it would be compatible with other theories of quantum gravity that permit a preferred frame (LQG, CDT)...
  44. P

    Courses Is particle physics that important?

    Hey! I will start my third year on the theoretical physics program. I have taken an introduction course in particle physics, just the basics, not much math. (quark and Feynman diagrams the forces and interaction , CRM matrix and cabibbo angle etc. ) Now I'm choosing between relativistic...
  45. A

    B Is string theory a deterministic hidden variable theory?

    There are several interpretations of QM which differ from being deterministic or non deterministic, with or without hidden variables and local or non-local. As I understand it ST poses the existence of vibrating strings, that is, physical objects with definite properties, moving along a world...
  46. tatkhj

    How do the Mobius and Klein models illuminate string theory?

    I'm a speculative fiction writer and playwright, a retired archirect with a master of architecture degree in theory, and a theater producer, director, and acting improvisor. I'm currently working on a TV series of 169 episodes exploring life in a contemporary parallel universe very much like...
  47. Extremal Black Holes in String Theory (1) by Ashoke Sen

    Extremal Black Holes in String Theory (1) by Ashoke Sen

    Speaker : Ashoke Sen ( Harish- Chandra Research Institute, Allahabad, India ) Date and Time : 10 Aug 2009, 04:00 PM Venue : AG 66, TIFR, Mumbai In these lect...
  48. Extremal Black Holes in String Theory (2) by Ashoke Sen

    Extremal Black Holes in String Theory (2) by Ashoke Sen

    Speaker : Ashoke Sen ( Harish- Chandra Research Institute, Allahabad, India ) Date and Time : 11 Aug 2009, 04:00 PM Venue : AG 66, TIFR, Mumbai In these lect...
  49. Extremal Black Holes in String Theory (3) by Ashoke Sen

    Extremal Black Holes in String Theory (3) by Ashoke Sen

    Speaker : Ashoke Sen ( Harish- Chandra Research Institute, Allahabad, India ) Date and Time : 12 Aug 2009, 04:00 PM Venue : AG 66, TIFR, Mumbai In these lect...
  50. ohwilleke

    I BSM conference '17 notes

    This 54 page review article from a September 2017 conference in Portugal is a good starting point for people wanting an overall summary of the state of BSM physics theories that is reasonably up to date, and may suggest questions about specific theories that deserve more attention in this forum...