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M-theory for the layperson

  1. Nov 15, 2003 #1
    Anyone have another/easier way to describe M-theory to someone, not technically/scientifically inclined, as something other than that illusive connection, or web, between types I, IIB, IIA, Heterotic-0 and E and 11D supergravity .... and then have to go on and explain each of those parts individually, concluding with the explanation of strings, themselves - this being the easiest part of the whole dilema?

    I haven't taught physics since 2000 and am looking for some updates in simpler language...

    Any leads on the net? I ask this here because I've been surfing for a few hours with no real luck and I'm sure there are articles out there that speak to other than the scientifically-trained.

    metaphors acceptable...

    Thanks. (maybe this should be in the TOE forum?)
  2. jcsd
  3. Nov 16, 2003 #2
    madcat11, you will not find an easy introduction to M-theory, because id handles many abstract an difficult principles, so its description must necessarily be technical. If you like M-theory, my advice is that you try to learn Matrix theory, that is a formulation of M-theory introduced by Susskind and Banks in 1996 (though Lubos Mottl claims that he was who introduced the theory, but this is not true). Matrix theory is a nonperturbative theory based on a form of light-cone quantization called "discrete light cone quantization". Matrix theory is described in terms of D0-branes (that are pointlike branes)which live in an eleven dimensional spacetime. Their spatial positions
    are determined by the eigenvalues of nine matrices.
    Try this introduction to Matrix theory:
    I've started to read it and I like it
  4. Nov 16, 2003 #3
    Thanks, Meteor.
    The abstract is indeed interesting - and approaches M-theory from a new light (no pun intended...). And it's a keeper, bedtime reading - absolutely - and I'm printing it out for just that use.

    However, I'm still in the dark in my attempts at explaining all this (M-theory) to someone who knows nothing about 'degrees of freedom' or matrices. I want to use terms that are non-technical and I find myself at a loss...

    But thanks again for the great link!
  5. Nov 16, 2003 #4
    When did Motl ever claim that??
  6. Nov 16, 2003 #5
    Ooops, Ambitwistor, I'm just starting to learn this stuff, so I saw this link a couple of days ago:
    where there's a paragraph that says something like Motl being the originator of Matrix string theory, so I thought that he had claimed that, but it's possible that I was misleading. My apologies to Motl if that is not true
  7. Nov 16, 2003 #6
    Motl is the originator of matrix string theory, but that is not the same as originating "the matrix formulation of M-theory". In fact, the first citation of Motl's first matrix string theory paper is to the original BFSS matrix model paper, "M Theory as a Matrix Model: A Conjecture". The original BFSS model was designed to describe an 11-dimensional sector of M-theory; matrix string theory attempts to extend these ideas to describe ordinary 10-dimensional perturbative string theories.
  8. Nov 16, 2003 #7


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    It is an understandable error on the part of whoever made that website since Motl was a grad student at Rutgers working under Tom Banks and wrote some papers with Banks---maybe about M theory. So there is a close association at some point, and then also Lubos is flamboyant and highly vocal. So one could get that idea. And who knows since Lubos admits to being a space alien maybe he did teach Matrix theory to Leonard Susskind and Tom Banks (his human contacts) and so it might be true after all
  9. Nov 16, 2003 #8

    do you have a reasonable explanation why "the closed strings" (gravitons) would leave the brane? What mechanism provokes the separation? A cutter knive? And how can gravitons affects (gravitationally) other strings - open-end- strings on the brane when they are floating somewhere at a distance?
  10. Nov 16, 2003 #9

    Well, picture the geometry: you have an open string, both of its ends confined to (indeed, defining) the brane. If it emits a closed string, then it pinches off a loop of itself, and you have an open string with a closed string sitting right next to it. In general, the closed string won't have to be on the brane itself, because the open string doesn't have to be (except for its endpoints).

    Closed strings can travel anywhere: off the brane or in the brane.
  11. Nov 16, 2003 #10
    Matrix string theory

    Hi physics fans,

    I was told that someone wrote that I wrote that I discovered Matrix theory. Well, it is not the case. A similar statement that would mostly be correct would have to talk about "Matrix string theory", not "Matrix theory". :wink: These are two different things.

    "Matrix theory" was the first known way to define M-theory in 11 dimensions using a quantum mechanical model that involves a couple of matrices - the quantum-mechanical operators x,p (transforming as nine-vectors) have two extra indices m,n that go from 1 to N - therefore the operators x,p behave as matrices. Well, there are also some fermionic matrices that transform as spinors.

    The Hamiltonian of this Matrix theory is the dimensional reduction of the 9+1-dimensional supersymmetric Yang-Mills theory down to 0+1 dimensions (0+1 dimensions means that the time - the number 1 - is the only continuous variable on which the operators depend; therefore we deal with a quantum mechanical model, not a higher-dimensional field theory).

    The authors of that paper (Banks, Fischler, Shenker, Susskind i.e. BFSS)


    have explained that this simple matrix model in fact contained the whole physics of 11-dimensional supergravity including the membranes (this was mostly known before BFSS - a paper by de Wit, Hoppe, and Nicolai) as well as the correct high-energy completion of this theory - that is called M-theory. Matrix theory provided us with the first known definition of the 11-dimensional magical M-theory; the word "Matrix" became another justification of the letter "M" beyond Membrane, Mother, Magic, Mystery (or upside-down "W" for "Witten", as Glashow said).

    Matrix string theory was started by my paper (L.Motl)


    Well, originally the matrix strings were called "screwing strings" and I had no idea about the extra meaning of that phrase. A more complete work on the same subject was - sort of independently - written 2 months later by more renowned scientists, namely Dijkgraaf, Verlinde, Verlinde (DVV).


    They were able to use conformal perturbation theory to describe the details of the interaction process. See our recent paper with Dijkgraaf


    to see some new progress in the field, as well as the fact that we talk to one another :-) although, of course, I must confess that I find it a bit annoying if I see a paper that talks about matrix string theory and cites DVV only. But this is not DVV's fault, I would say. ;-)

    Another early paper on Matrix string theory was written (before DVV) by Banks and Seiberg (an extension to type IIB string theory and so on),


    Matrix string theory (MST) is, unlike the original BFSS Matrix theory, a 1+1-dimensional super Yang-Mills theory on a cylinder: the spatial dimension is compactified. An essential feature of this theory is that there are long/matrix/DVV strings that can be wound around the cylinder. MST was the first known nonperturbative definition of string theory - something that string theorists had dreamed about for years - that could have been showed to reduce to the well-known perturbative calculations in the limit where the coupling constant is small, and it convinced many that the Matrix theory is correct, indeed.

    I apologize for the graphical imperfections of this text; it is my first posting here.

    All the best,
  12. Nov 16, 2003 #11
    If it emits a closed string by next mechanism: ...... ?

    What is the mechanism that holds the interactive binding between the floating gravition (closed string) and ... a (gravity related) open-end string? Since there are only strings at this level ... by other - also floating - strings?
  13. Nov 16, 2003 #12
    Re: Matrix string theory

    Thanks Lubos,

    Great to have you here. I am sure everyone appreciates your presence. :smile:. For sure a lot of people will have questions for you!

  14. Nov 16, 2003 #13
    Quantum strings have a finite probability amplitude to spontaneously break; a closed string is emitted in string theory when part of a string pinches off and breaks.

    I don't understand your question.
  15. Nov 16, 2003 #14
  16. Nov 16, 2003 #15
    Re: Forum

    Thanks Lubos,

    I am already downloading your first link. ;-) I am sure others are doing/going too. That's the beauty but also the power of Internet. I believe that this type of interaction - you being an 'expert' and we being 'motivated but knowledge-hungry people' communucating - has a great value. So your feed-back and even short visits will always be highly appreciate :wink:

    Thanks again!

  17. Nov 16, 2003 #16
    But the breaking can also be a decay. There is another question. Since a string is the smallest piece ... how can it break?

    Sorry I will rephrase. I assume gravity means that there is some kind of link between string X (an open-end string on the brane) and a somewhere floating graviton (closed string). If there is not such link the open-end string has no 'gravity effect'. It will receive a gravity effect from a somewhere floating graviton. Correct? So how is that 'particle' open-end string (which is on the brane) "communicating" with that somewhere in the blue floating graviton? The only way this can be done is by other floating strings, so we need to make extra glue-strings(!), thus adding complexity ... meaning less unification. (Unification mean IMO the less parameters)

    Or the assumption that the graviton is separated from the open-end string (particle type) is wrong. (as shown by Greene on the Nova program)
    In that case the gravition is always connected with the open-end string. In this last case the gravitons are just part of the strings on the brane. That would fit Occam's razor (reduce complexity, go for the simplest solution).
  18. Nov 16, 2003 #17
    Strings can (in principle) be any size. So there is no problem with a string breaking into smaller strings.

    An open string on the brane is influenced by gravity a closed string merges with it. (This is the reverse process of a closed string breaking off from an open string.) Two strings interact gravitationally when a closed string breaks off of one, flies off, and merges with the other.
  19. Nov 16, 2003 #18
    So all the strings making the bottle of beer on my desk must always wait to 'express' gravity till a graviton is 'available' to merge with a 'particle' string of the bottle? If some would not be available then the bottle would 'express' less gravity? That really makes me . Ambitwistor, that's a very nice try but I am not eating that! :wink:. There is something very wrong conceptually. Discussions were made if gravity was even faster than light! In the concept you try to sell me gravity maybe 'uncertain'!
  20. Nov 16, 2003 #19
    This isn't any different than, say, how electrons interact electromagnetically via virtual photons in quantum electrodynamics. In QED, you sum over all possible ways that two electrons can exchange a photon, and in string theory, you sum over all possible ways that two strings can exchange a string.

    These aren't real strings, just like in QED you're not talking about real photons. They're virtual. They don't "run out"; there are infinitely many of them being exchanged at any time between any two particles (or strings) in the universe.

    I think you need to understand something about how ordinary quantum field theory works before you try to understand strings. Feynman's book QED: The Strange Theory of Light and Matter is good.
  21. Nov 16, 2003 #20
    ;-). Thanks Ambitwistor for your answers. Really appreciate. The QED book of Feynman lays in fact 20 cm from the beer can on my desk. I know what he wrote ;-). I think the last paragraph is very interesting ... explaining how we use calculations but don't know what these calculations repressent, neither we know they come from. Which is honnest, but too.
    I'll take another drink.
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