What is String theory: Definition and 829 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.

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  1. marce

    A String theory and locality of small dimensions

    - I have in essence never understood the concept of curled up dimensions in string theory ; in the sense that if it occurs in this universe isn't that included in our 3 dimensional space ? My question is more fundamental in the sense from a topology viewpoint : was it correct to involve curled...
  2. K

    I String theory vs Quantum field theory

    So QFT treats particles as excitations of their underlying quantum field, meaning that fields are more fundamental than particles. Then String theory comes in and says that actually strings are the fundamental building block of the universe and that the different particles are vibrating strings...
  3. T

    Recommendations on research topic for string theory (M-theory)

    TL;DR Summary: Currently applying for a scholarship for a PhD, and I feel like I need some help finding interesting research propositions/questions when it comes to M-theory. I would be thankful for all the help I could get! I'm currently attending a masters program in theoretical physics and...
  4. M

    A Basic Scheme and Fundamental Equations of String Theory

    In Classical Mechanics you solve for a particle’s position through F=ma or the Lagrangian and Hamiltonian Methods. Quantum Mechanics you solve for the wavefunction or density matrix either the Schrodinger or Von Neumann Equations respectively or use Operator and Path Integral Methods...
  5. P

    B String theory suggests that dark matter is hiding in extra dimension

    Here is the article titled "In a ‘Dark Dimension,’ Physicists Search for the Universe’s Missing Matter" by Quanta Magazine https://www.quantamagazine.org/in-a-dark-dimension-physicists-search-for-missing-matter-20240201/
  6. bhobba

    A The Emergence of Space-time in String Theory

    A few yesrs ago now I read a First Course in String Theory. In that book strings were part of normal space-time plus for consitency some extra dimensions. Spin 2 partices natually emerged and so did GR. I didnt think anything of it at the time (pun intended), but I recently saw a...
  7. R

    Quantum What books/materials/subjects are needed to understand String Theory?

    Hi, I have heard all this hype about a Physicist named Ed Witten and how instrumental he is to something called “String Theory”. I tried listening to one of his lectures on YouTube and reading one of his papers and I couldn’t follow anything. I was hoping to figure out the prerequisite...
  8. K

    I Is loop quantum gravity dual to CFT?

    this paper arXiv:2310.02958 [pdf, other] What if Quantum Gravity is "just'' Quantum Information Theory? Aron C. Wall Comments: 6 pages, 2 figures. Additional references added to arxiv version Journal-ref: Proc. 28th Solvay Conf. Phys., ed. D. Gross, A. Sevrin, P. Zoller, World...
  9. R

    I What if unification does not work?

    I will try to word this in a way that does not violate forum rules. As I look at the struggles of string theory and loop quantum gravity and the struggles of trying to unify quantum mechanics and general realtivity in general such as the need for a framework for quantum physics which does not...
  10. P

    B Looking for help understanding string theory

    String theory is a vast topic, and no grade 9 book will have content on it. However, no site or video gives it the attention this topic deserves, please input your understanding of string theory here. here are a few general questions to think about: 1)What is string theory? 2)Why was it made...
  11. apostolosdt

    A NANOGrav waves support string theory?

    NANOGrav waves are real observational data, and now this: https://arxiv.org/abs/2307.08601. I don't know much in this area of research, except for the basics on LIGO and the like. Any comment from the knowledgeable members here?
  12. jaketodd

    B Balancing theories and empirical data, especially concerning string theory

    The following discussion is from reading this short paper: The dangers of non-empirical confirmation https://arxiv.org/abs/1609.01966 The following are quotes from the paper, and my observations and questions. "excessive reliance on non-empirical evidence compromises the reliability of...
  13. GeniVasc

    I What maps are considered in the Polyakov path integral?

    Recently I've came to some references on mathematical aspects on string theory that deal with the Polyakov euclidean path integral. An example is the book "Quantum Fields and Strings: A Course for Mathematicians. Volume 2", where it is stated roughly that the path integral is $$A =...
  14. Conn_coord

    B Long String Theory: Unveiling the Universe's Secrets

    In theories of particle physics based on string theory, the characteristic length scale of strings is assumed to be on the order of the Planck length, or 10−35 meters. I am sure few people have heard about the long strings theory. If there is information on the topic big request to share it...
  15. 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?
  16. 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...
  17. 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...
  18. 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...
  19. MathematicalPhysicist

    Suggestions for future reading — Supergravity by Freedman or A First Course in String Theory by Zweibach?

    What do you recommend to read first, Supergravity by Freedman or A First Course in String Theory by Zweibach? For future consideration.
  20. alan123hk

    Roughly how many people in this world actually know about string theory?

    Roughly how many people in this world actually know about string theory? I'm guessing no more than a few hundred because the math of string theory is so esoteric.
  21. K

    I String theory calculation of Extremal black hole entropy problem

    one of the claimed successes of string theory is its ability to derive the correct Hawking-Bekenstein equations to calculate the quantum entropy of a black hole without any free paramenters, specifically Extremal black hole entropy using supersymmetry and maximal charge. I was wondering if...
  22. 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?
  23. 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...
  24. MathematicalPhysicist

    A What possible experiments will falsify String Theory?

    Or will confirm its predictions? As far as I can tell, you can only raise the bar on the energies required from the accelerator, but you cannot give an upper bound, where beyond it the theory is doomed... This isn't science... we might as well say we need infinite energies. 🙃
  25. StenEdeback

    Why must the second term on the right-hand side vanish in string theory?

    I am doing private studies in string theory and am reading "A first course in string theory" by Barton Zwiebach. Below equation 6.52 the author says "Since the second term on the right-hand side must vanish...". I do not understand why this term must vanish, and I would be grateful for an...
  26. 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 (https://arxiv.org/abs/hep-th/9907209) which is closely related to the concept of branes (https://en.wikipedia.org/wiki/Brane) 1. The authors of the paper mention that the branes in the model could...
  27. topsquark

    Quantum Looking for a good String Theory text

    (Sorry about writing a book! But I felt more information was the better mistake.) Okay, here's the deal. I have been teaching myself Physics and Math beyond my MS in QFT. I have been doing this for the last 20 or so years and I've gotten reasonably decent at it but String Theory is a bit...
  28. 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?
  29. 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!
  30. 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!
  31. 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: https://arxiv.org/abs/hep-ph/9905221 With k as large as the Planck scale, the exponential will be so small and...
  32. 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...
  33. 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 (https://www.snowboundbooks.com/book/9780385542746): “Kaku also explains the intense controversy swirling around this theory, with...
  34. 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?
  35. 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 '}...
  36. 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 =...
  37. D

    A BRST operator Q in string theory and string field theory

    In string theory, physical states satisfy QBΨ = 0, where QB is the BRST operator. This equation of motion can be obtained from an action S = ∫ QBΨ*Ψ + Ψ*Ψ*Ψ There is a gauge invariance under δΨ = QBΛ. what is the framework in which the role of the BRST operator QB is understood in open string...
  38. 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...
  39. snypehype46

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

    Hi, I was given the task to read this paper https://arxiv.org/abs/hep-th/0204051 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...
  40. D

    I Does string theory falsify the theory of special relativity?

    Hello, I am spending time learning more about the theory of special relativity and string theory. One of the things that I have read about string theory is that it includes other dimensions in relation to space (space has 9 dimensions in string theory, supposedly). However, from what I...
  41. S

    I Brane cosmology without string theory?

    There are several models of brane cosmology (https://en.wikipedia.org/wiki/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...
  42. 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...
  43. 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...
  44. D

    High Energy String Theory Books: Intro & Advanced Level | GSW, Polchinski, McMahon, Becker

    Some good introductory string theory books that I know are GSW Polchinski McMahon Becker What are the good books at a more advanced level? Are there any such books, or do I have to dig it out of the research papers? There is a set of books called "mirror symmetry" and "dirichlet branes and...
  45. 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...
  46. 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...
  47. Bancrates

    A What is the Best Resource for Learning String Theory as a Side Project?

    I've completed my PhD and am leaving the field to take up a career elsewhere, however I'm interested in developing my knowledge of string theory as a (potentially lifelong) side project. I have a solid understanding of GR and some extensions (my PhD was in relativistic effects in cosmology, and...
  48. S

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

    Physicist Joseph Polchinski wrote an article (https://arxiv.org/pdf/1412.5704.pdf) 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...
  49. Demystifier

    A Can String Theory Finally Win a Nobel Prize?

    Given that Penrose now got the Nobel prize for a theory that is almost impossible to verify experimentally in a near future (that is, for theorems that predict singularities inside black holes), does it mean that now string theory can also get a Nobel prize? (If so, Witten and Schwarz would be...
  50. Iliody

    A Sum over backgrounds in String Theory

    Usually, I saw that string theory (perturbative, or matrix models) are made in a fixed background. Even if you consider that the metric is quantized and etc. there is an apparent physically motivated need for making a sum over topologies (manifolds, conifolds, orbifolds, and etc), for example...