Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

String theory and foundations of quantum mechanics

  1. Nov 9, 2006 #1

    Demystifier

    User Avatar
    Science Advisor

    String theory is supposed to be a theory of "everything".
    In your opinion, should string theory, or some other "theory of everything", solve the problem of measurement in quantum mechanics and other related foundational problems of quantum mechanics?

    Anyway, for my contributions to that issue see
    http://arxiv.org/abs/hep-th/0512186
    http://arxiv.org/abs/hep-th/0605250
     
  2. jcsd
  3. Nov 9, 2006 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

  4. Nov 9, 2006 #3

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Hrvoje (or "Harvey"), here is another recent paper about foundations of QM:

    http://arxiv.org/abs/quant-ph/0609109
    Could quantum mechanics be an approximation to another theory?
    Lee Smolin
    10 pages, no figures

    "We consider the hypothesis that quantum mechanics is an approximation to another, cosmological theory, accurate only for the description of subsystems of the universe. Quantum theory is then to be derived from the cosmological theory by averaging over variables which are not internal to the subsystem, which may be considered non-local hidden variables. We find conditions for arriving at quantum mechanics through such a procedure..."

    ===sample quote===
    ...evidence that quantum mechanics is an approximation to
    a deeper theory. Among the reasons for this belief are;

    • The unresolved difficulties in extending quantum
    theory to cosmology. If this cannot be done then
    one possible explanation is that quantum mechanics
    does not in fact extend to the whole universe.
    It must then be an approximation to a more fundamental
    cosmological theory, which applies only
    for small subsystems of the universe.

    • The difficulties in solving the measurement problem
    in the context of a theory with a realistic ontology.

    • The success of quantum information theory,which
    reinforces the viewpoint that the quantum state
    represents the information that observers have of
    a system.

    • The experimental evidence against the Bell inequalities
    tells us that any theory quantum mechanics
    is derived from must be non-local. It is
    then natural to hypothesize that this non-local theory
    is a cosmological theory, which is more adequate
    than quantum mechanics for the investigation
    of cosmological problems.

    ===endquote===
     
    Last edited: Nov 9, 2006
  5. Nov 9, 2006 #4

    CarlB

    User Avatar
    Science Advisor
    Homework Helper

    I would like to see an expanded version of what one gets when one looks at creation and annihilation operators from a Bohmian perspective.
     
  6. Nov 9, 2006 #5
    I'm wondering if all these quantum effects might be due to information inside a sphere being limited to the surface entropy of a black hole of that size. Some have suggested that the surface entropy of black holes can be generalized to any spherical region, the entropy can't be any more than if the region were a black hole, right?
     
  7. Nov 10, 2006 #6

    Demystifier

    User Avatar
    Science Advisor

    Marcus, my name is not pronounced as "Harwey".
    But you can call me Hrvoje if you like.

    I know about the paper of Smolin you mentioned. In fact, I had a brief discussion on that paper with him. I particulary like his solution of the problem of single-valuedness of wave functions.
     
  8. Nov 10, 2006 #7

    Demystifier

    User Avatar
    Science Advisor

  9. Nov 10, 2006 #8

    CarlB

    User Avatar
    Science Advisor
    Homework Helper

    Hrvoje,

    Regarding the fermions in the Dirac equation in:
    http://xxx.lanl.gov/abs/quant-ph/0302152

    Interesting that you split the particles from the antiparticles. This is the way I think it should be done as well. And the operator:

    [tex]\Omega^{(P)}(x,x') = \sum_k u_k(x)u_k^\dag (x')[/tex]

    reminds me of the density operator way of rewriting the Dirac equation. I'm slowly typing up a density operator formalism for the standard model, the results so far are here:
    http://www.brannenworks.com/dmaa.pdf

    I have to do the same thing, but it is by a sleight of hand that the reader is not intended to notice. In my way of looking at it, the Dirac equation works because it is the simplest equation you can write in a Clifford algebra. But when using it for the electron, you get the positron, an embarassment. The massless Dirac equation works better as a start for a preon description of the elementary fermions.

    Yes! Yes! Here, take a look at the presentation I gave at the DPJ2006 meeting a few days ago. After the details of the arithmetic in the first few slides, the above points, that the vacuum is not needed and that the creation and annihilation operators do not make sense when examined alone, are the subject:
    http://www.phys.hawaii.edu/indico/c...?contribId=114&sessionId=116&confId=3

    It gives me great pleasure to see these connections between Bohmian mechanics and density operators. What I am doing in quantum spaces of finite dimension (i.e. internal states), you are doing with Bohmian mechanics for position and momentum.

    Carl

    Also, I love your elegant and correct use of English. It could not have been easy.
     
  10. Nov 16, 2006 #9
    Hi Demystifier and everybody,

    I'm no physicist but I share the same basic question as your original post:
    If we will ever have a TOE, one of the things we surely expect from it is that it will resolve (interpreet) all the weird Quantum puzzles.

    So far for what I have read about Superstring theory or LQG, they seem to be rather quests at finding mathematically valid relationships of the known phenomena, without caring so much about if they actually reflect any realistic physical facts or not.
    Surely the concept of "cosmic music" in Superstrings is appealing as physical principle, but there are still so many gaps that many wonder if it's just a beautiful mathematical construction without any physical relevance at all.
    After all, in many cases it's possible to find different mathematical constructions that lead to the same final result. Which is the right one?

    Wouldn't it be possible to find some (or more than one) mathematical representation of ANY phenomena ? (except for the truly random ones).
    Many believe that the mathematical realm is far more extensive than the physical realm, that "reality" is a sub-set of all the possible mathematically consistent scenarios.

    I read your papers but I don't have enough technical background to grasp the basic final conclusions (e.g. you advance in the introduction your proposal for interpreting how does string theory resolve the measurement problem, but I don't see such explanation in plain words).

    Would you please (and/or anybody else) give in layman terms your words on what do Superstring theory / LQG have to say about the typical quantum puzzles:

    - measurement problem (role of consciousness / observer)
    - non-locality
    - the double-slit experiments
    - the "retarded choice" experiments
    - the quantum zeno effect
    - multiverse yes or no?
    - decoherence
    ....

    Do we agree that anybody who is enthusiastic about a TOE must address these issues?
    Even if not resolved at the present moment, do Superstring / LQG enthusiasts believe that their theories will eventually provide the answers?

    I saw that similar fragmented questions appear in scattered posts, but I like to take this one to summarize these issues.

    Thanks !
     
    Last edited: Nov 16, 2006
  11. Nov 16, 2006 #10

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Nada, as I think you intended to bring out. However some of the alternative QG models, like CDT and Group Field theory, do address the roots of quantization, and presumably have some input on those questions. Please note that some of your questions, like delayed choice, have reasonable explanations within the existing family of interpretation of standard (non-Bohm) QM.
     
  12. Nov 17, 2006 #11
    Huh ?? :uhh: Where did you get that idea ?
     
  13. Nov 17, 2006 #12

    Kea

    User Avatar

    I have to agree with Careful here. CDT and GFT are not fundamental theories.
     
  14. Nov 17, 2006 #13

    Demystifier

    User Avatar
    Science Advisor

    I fully agree with you selfAdjoint. What the Bohmian interpretation solves is the problem of measurement, other problems on the list are not really problems in the conventional interpretation of quantum mechanics.

    To answer to Gerinski, my point is the following. I derive (not simply postulate) the Bohmian equation of motion from the assumption that particles are not really pointlike, but extended as in string theory. How do I do that? In the Schrodinger picture the string coordinate depends on the parameter along the string (sigma) but not on time (tau). But this is not world-sheet covariant, in a covariant approach sigma and tau should have equal roles. To restore covariance in the Schrodinger picture, it turns out that the string coordinate has to have a dependence on tau exactly as in the Bohmian interpretation. This, of course, is a nontechnical explanation of my result, presented in a way that even non-physicists can understand it. The technical presentation is in the first paper above. In the second paper I explain how the Bohmian interpretation reinterprets T-duality of string theory.
     
  15. Nov 17, 2006 #14

    selfAdjoint

    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Demystifier, that is actually a cogent explanation! (Translation: on my wavelength:redface: ). I will take a look at the paper, no guarantees.

    I have been reading your Honorable Mention Essay. Toward the end of it you say

    Well, no. It is derived from your desire to have a covariant particle description which is not the same thing.

    But what if the particles we see are not a covariant phenomenon? Consider the Unruh effect; scalar field plus acceleration equals particle spectrum, and from underlying particle spectrum plus Nelson, we get quantum mechanics of particles. What if all the particles the experimentalists see in their experiments are just contingent effects of the enormous accelerations involved when the beam hits the target? Can we rule this out?
     
    Last edited: Nov 17, 2006
  16. Nov 21, 2006 #15

    Demystifier

    User Avatar
    Science Advisor

    SelfAdjoint, you are right that we cannot rule this out. But I hope that you will agree that my desire to have a covariant particle description is not unreasonable.

    In the case of strings, I similarly require to have a covariant string description. Such a requirement seems even more reasonable because there are indications that string field theory may NOT be the correct way to treat strings. (See e.g. Sec. 4.2 in http://arxiv.org/abs/hep-th?papernum=9411028 )
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?