What is M-theory: Definition and 93 Discussions

M-theory is a theory in physics that unifies all consistent versions of superstring theory. Edward Witten first conjectured the existence of such a theory at a string-theory conference at the University of Southern California in the spring of 1995. Witten's announcement initiated a flurry of research activity known as the second superstring revolution.
Prior to Witten's announcement, string theorists had identified five versions of superstring theory. Although these theories appeared, at first, to be very different, work by many physicists showed that the theories were related in intricate and nontrivial ways. Physicists found that apparently distinct theories could be unified by mathematical transformations called S-duality and T-duality. Witten's conjecture was based in part on the existence of these dualities and in part on the relationship of the string theories to a field theory called eleven-dimensional supergravity.
Although a complete formulation of M-theory is not known, such a formulation should describe two- and five-dimensional objects called branes and should be approximated by eleven-dimensional supergravity at low energies. Modern attempts to formulate M-theory are typically based on matrix theory or the AdS/CFT correspondence.
According to Witten, M should stand for "magic", "mystery" or "membrane" according to taste, and the true meaning of the title should be decided when a more fundamental formulation of the theory is known.Investigations of the mathematical structure of M-theory have spawned important theoretical results in physics and mathematics. More speculatively, M-theory may provide a framework for developing a unified theory of all of the fundamental forces of nature. Attempts to connect M-theory to experiment typically focus on compactifying its extra dimensions to construct candidate models of the four-dimensional world, although so far none has been verified to give rise to physics as observed in high-energy physics experiments.

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  1. 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...
  2. J

    A Can Membrane Field Theory Elucidate M-Theory?

    Hey guys, I just wanted to know if you think that a membrane field theory could ellucidate the non-perturbative framework of M-theory? Let me specify and explain what I mean by that: String field theory was intoduced to study the non-perturbative regime of string theory and some achievements in...
  3. Paige_Turner

    B What is the nature of dimensionality in 11 dimension M-theory?

    They're dimensions, so they DO have a metric equation, right? Does energy flow cyclically between pairs of dimensions? To me, that's what rotation is.
  4. ergospherical

    A M-Theory: Bosonic Fields - Need Help With Part III

    Need help with part iii) i) Under ##C \rightarrow C + d\Lambda##, and since ##dG = d^2C = 0 \implies d(\Lambda \wedge G \wedge G) = d\Lambda \wedge G \wedge G##, then neglecting the surface terms\begin{align*} \int_D d\Lambda \wedge G \wedge G = \int_D d(\Lambda \wedge G \wedge G) &=...
  5. D

    I M-theory and Calabi-Yau compactifications

    The bundles like U(1) and SU(3) and so on, how are they related to Calabi-Yau compactifications? For example, how does one relate Calabi-Yau compactifications to SU(2)? does the Calabi-Yau compactification give rise to SU(2)?
  6. 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...
  7. saadhusayn

    A Calculating the ghost field in the Becker and Becker paper

    This is the paper that I refer to. I'm trying to figure out the ghost action (Equation 2.16) in the background field gauge. I am attempting to use Srednicki's (chapter 78) expression for the ghost field in the background gauge. However, I am missing out on a √g coefficient in front of the term...
  8. 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...
  9. S

    A Is M-Theory more fundamental than String Theory?

    M-Theory is a theory of membranes which are the fundamental objects of the theory (M2 and M5 branes), however these objects are considered solitons, solutions of supergravity. How can membranes be "fundamental" if they are solitonic solutions of supergravity? Or am I missing something? And is...
  10. M

    A What is the role of homotopy super Lie-n algebras in M-theory?

    PF regular @Urs Schreiber has coauthored a new paper: http://arxiv.org/abs/1805.00233 Higher T-duality in M-theory via local supersymmetry Hisham Sati, Urs Schreiber (Submitted on 1 May 2018) By analyzing super-torsion and brane super-cocycles, we derive a new duality in M-theory, which takes...
  11. S

    A What does this article mean? (The holographic principle and M-cosmology)

    I was reading an article about holographic principle and M-cosmology and how could they be used to interpret QM mechanics. But there's a part where it talks about holographic principle in a strange way to me. What does this article mean with this?: "Since for the participant observer the...
  12. B

    B LHC Constrains on Compactified M-theory SUSY particles

    I just finished Gordon Kane superb book "String Theory and the Real World" in one sitting. It answered most of my current questions about the field. I need some hard data. He mentioned: "More generally, where did the predictions for superpartner masses come from? Until recently there were no...
  13. K

    I M-theory and loop quantization of higher dimensional SUGRA

    A new duality between Topological M-theory and Loop Quantum Gravity Andrea Addazi, Antonino Marciano (Submitted on 17 Jul 2017) Inspired by the long wave-length limit of topological M-theory, which re-constructs the theory of 3+1D gravity in the self-dual variables' formulation, we conjecture...
  14. Urs Schreiber

    Insights Super p-Brane Theory Emerging from Super Homotopy Theory - Comments

    Urs Schreiber submitted a new PF Insights post Super p-Brane Theory Emerging from Super Homotopy Theory Continue reading the Original PF Insights Post.
  15. A

    Need help to begin research on M-theory

    Hi, I am very interested in superspring and M-theory and hope to begin reserches on these fields. Could anone provide me some advice on how to enter into this field ? or could anyone suggests some foremost paper for me to read ? Thanks a lot ! by the way, I am a student studying physics...
  16. P

    A Does String Theory have any equations yet? I chose not to....

    Back when I was studying at Princeton University, J.A. Wheeler introduced me to Ed Witten while we were having lunch at the Institute for Advanced Study (IAS). I knew Feynman’s and Wheeler’s views on “String Theory,” and I had looked into it a bit as a field of research, but I couldn’t seem to...
  17. R

    Equation linking superstring vibration with mass?

    Is there an existing equation, or set of equations, that directly links the specific vibration of a superstring filament with the resultant particle mass? A brief explanation of any mathematics would be helpful. Thank you.
  18. N

    In M-theory, is a baryon a D4 brane with three strings?

    I've read in a few string theory documents that have 10d spacetime that a baryon is a D4 brane with three strings (each being a quark), but what about in M theory?
  19. Urs Schreiber

    Insights It Was 20 Years Ago Today -- the M-theory Conjecture - Comments

    Urs Schreiber submitted a new PF Insights post It Was 20 Years Ago Today -- the M-theory Conjecture Continue reading the Original PF Insights Post.
  20. nuclearhead

    In what way is M-Theory different from Loop Quantum Gravity?

    I have been reading about M-Theory and LQG, and from what I read they are actually very similar. It seems that just a a string-world sheet is the same as a sum of an infinite number of Feynman diagrams, a membrane would be the sum of an infinite number of graphs or networks. Or as Susskind...
  21. twistor

    Is M-Theory the only possible UV completion of Supergravity?

    Is M-Theory the only possible UV completion of Supergravity?
  22. S

    How does M-Theory deal with space time?

    How does M-Theory deal with space time? One of my friends said something like it verifies it. Is anything known about it?
  23. Milan Nikolic

    Cosmology as quantum, ie. M-theory

    I wonder, whether cosmology come to an end as quantum, ie. M-theory?
  24. P

    Question about SO(32) heterotic - Type I duality, M-theory perspective

    I'm having trouble understanding the arguments in http://arxiv.org/abs/hep-th/9510209 They propose that M-theory on the orbifold R^9 \times S^1 \times S^1 / Z_2(1) yields Type I' on R^9 \times S^1, wheras M-theory on R^9 \times S^1 / Z_2 \times S^1(2) is E_8 \times E_8 on R^9 \times S^1. What...
  25. W

    What characterizes each dimension in M-theory?

    according to M-theory, there should be 7 ekstra spatial dimensions curled up at the plank lenght, but why do they need to be curled up? if light and matter is bound to the 3rd dimension like waves of the ocean is bound to the 2D surface of the water that wouldn't that be sufficient for us not to...
  26. R

    Schiff conjecture and LQG or m-theory?

    Recently heard about the Schiff conjecture saying that any reasonable theory of gravitation should adhere to the ideas of EEP and UFF. I realize that this isn't a "strong" idea (only a conjecture after all), but to anyones understanding, does either loop quantum gravity or m-theory appear to...
  27. Demystifier

    What is quantum theory about? From quanta to M-theory

    https://www.physicsforums.com/blog.php?b=3873 What do you think? :-p Can you shed some light on that mess? Or even better, can you further confuse it?
  28. D

    Are 10 spatial dimensions necessary for M-Theory?

    Hi Everyone, I have not understood or even studied string theory. However I was wondering whether the extra dimensions demanded by M-theory (10 spatial dimensions in that case), other string theories or even any other field theories necessarily demand the dimensions higher than four...
  29. H

    Why does M-theory fit so well and why is it not just arbitary?

    why is reality described by 11 dimensions and strings? why is the most fundamental thing a string and not a very small gummybear-shaped thing? why does it fit so well? High-End math just appears to deduce more and more results from one set of axioms to the next set of statements. The...
  30. R

    How well accepted is M-Theory?

    Lately popular science 'rags' have been selling M-theory especially after Stephen Hawking and Brian Green's recent books and tv shows. Just how well accepted or widely rejected is M-theory? Are we seeing a paradigm change or is just the media tying to sell papers?
  31. H

    M-theory and Chaotic inflation

    Hi, I am not currently studying, have never studied Astrophysics, so will no doubt come across as rather inadequate in trying to clarify, what my question actually is. Please forgive me for not using appropriate terminology, and, if this forum is not for the curious laymen in the general...
  32. S

    In short, I don't know if there is a clear answer to your question at this time.

    Hi there PF My question here is: Is the Bagger-Lambert-Gustavsson action (http://en.wikipedia.org/wiki/Bagger%E2%80%93Lambert%E2%80%93Gustavsson_action) a way of proving the "M-theory as a matrix model: a conjecture" (http://arxiv.org/abs/hep-th/9610043) or is it a different approach? And how...
  33. D

    Understanding the M-Theory Equation: Wavefunction and Constant Explained

    In the following equation: Γ^i [X^i ,λ]ψ Does psi refer to the wavefunction? If not what does it refer to? Also, is i the square root of negative 1 or is it a constant? Thanks in advance.
  34. M

    Exploring ABJM-like Theories and M-theory on dS4 x X7

    The ABJM model is a super-Chern-Simons theory in 2+1 dimensions which is dual to M-theory on AdS4 x S7/Z_k. Other ABJM-like theories are being explored which are dual to M-theory on other AdS4 x X7 backgrounds. One difficulty of dS/CFT is that you can't match up supersymmetries as you do in...
  35. narrator

    Understanding M-theory and Its Implications for Dimensions and Scales

    Hi, Reading up on dimensions, like the m-theory's 11 dimension model, I'm unsure what is meant by this. Does it mean 7 more dimensions beyond space and time, or does it mean something akin to 11 universes in the multiverse (ours being 1 of the 11)? When relating this to string theory, does it...
  36. D

    Planck Mission and M-Theory Cosmology

    A ways back, I seem to remember reading on the Planck Mission page that it's observations of the CMB could help foster support for M-theory cosmology and weaken support for Inflation, or vice-versa. I thought it had something to do with detection of Non-Gaussianity in the CMB. I could not...
  37. S

    Uncovering the Solution to M-Theory: A Lagrangian Approach?

    Hi there PF. I am a bit curious of how we are going to solve M-theory. Do we just need to find a Lagrangian, describing all of the dynamics of the theory? or is more needed? And how do one find such a Lagrangian? From what principles do we derive it? I am no expert on string theory, I have...
  38. S

    Why is QM incompatible with GR and how does string theory solve this? Or M-Theory?

    I always hear that these two things are incompatible but I never really hear why. The most I know is that QM assumes a quantized spacetime whereas GR assumes a dynamic one, but I don't really understand if this is correct (nor do I get what that really means). I don't see how GR would fail...
  39. R

    Isn't M-theory and the multiverse idea a bit discouraging?

    I have been reading about the multiverse idea recently and when I came to think of it , as much as I find it extremely interesting, isn't it a bit discouraging as well? I mean, if there was "time" before the big bang and if there are infinite number of other "bubble" universes in the "11th...
  40. C

    The Mysteries of the Other Dimensions: Uncovering the Secrets of M-Theory

    From what I have gathered, M-theory states that there are 11 dimensions. We live in 3 of these dimensions(unless time is considered its own dimension), and I believe the common understanding is that these other 8 dimensions are incredibly small. At the time of the Big Bang, these 3 dimensions...
  41. Demystifier

    Evidence for UV finiteness of M-theory?

    Is there any evidence that M-theory (where M may stand for Matrix, Membrane, or whatever ...) could be UV finite? I am not asking for a proof (because I know there is none), but only for a piece of evidence or an argument which is more than a pure wish. If you know about such evidence, an...
  42. nomadreid

    Closed and open strings in M-theory

    Given my limited background in Physics, I am restricted to the popular literature on M-theory. After going through this literature (including articles from the very helpful "annotated list of useful literature for String Theory" posted in this rubric of the Forum), I am still puzzled by the...
  43. J

    Supergravity as the low energy effective action of Superstring- / M-theory

    Hello. I have a question for the specialists on Superstring Theory: I am looking for a reference to original research, where the following is proven / shown / evidence is given / made plausible / etc. - 10D supergravities in their various forms are the low energy limits of superstring...
  44. M

    M-theory phenomenology - homework exercise

    In the current long thread discussing string theory, https://www.physicsforums.com/showthread.php?p=2982921": I replied So here is the promised thread. As I said, I think it's unlikely that this number from the 11-dimensional theory would show up so directly in 4 dimensions. But it would be...
  45. MTd2

    Why do matrix models fail to be The M-Theory?

    In what aspect do they fail (IKKT and BFSS) to be?
  46. C

    Ed Witten & M-Theory: Physics I Project Ideas

    I must write an essay with a visual representation (like a model, etc.) for my high school physics class (I'm only in Physics I, forgive me for my ignorance). My essay is on Ed Witten and his M-Theory. However, I'm stuck on the visual part. Does anybody have any ideas? I'd rather not do a poster.
  47. S

    How does M-theory explain genesis of the 11th dimension?

    I read Stephen Hawking new book and did a little research on M-theory. Apparently in the 11th dimension there are different brains that crash into each other. 2 of these brains crashing supposedly rendered our big bang, and explains why stuff came out in blobs (not homogeneous). But how...
  48. J

    M-theory and gravitational waves

    I've read that the discovery of gravitational waves will further support the big bang theory at the expense of M-theory. What's the specific difference between the two models that relates to gravitational waves? Is it that a brane collision would not produce the waves at the beginning of the...
  49. T

    Experimental Evidence for M-Theory

    I am A Ph.D em Chemistry and not so good in High Mathematics than most of you. Therefore forgive me if, if my idea is a little bit naive. Superparticles as an evidence are dificult to create, because of hughe energy ammounts necessary for it. But I was told that our STRINGS are much, much...
  50. T

    M-Theory: is a String A digital Data Register ?

    Dear Friends: I am a PhD. in Chemistry, born and studied in Germany, whereas I am working in Brazil in industry. My High Mathematics knowledge might be inferior to most of you. Therefore I beg your pardon, if I explain my ideas somehow naively. The question is, if STRINGS may vibrate at...