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Don´t we need quantum gravity to study wormholes ?

  1. Sep 30, 2007 #1
    Hi there,

    I am still a lay person in this subject, so I have to ask.
    Don´t we need quantum gravity to study wormholes ?


  2. jcsd
  3. Sep 30, 2007 #2
    The idea of a wormhole is derived from valid mathematical solutions to equations in General Relativity. So while a quantum gravity theory might contribute in that study, for now, we can use GR.
  4. Sep 30, 2007 #3
    can quantum gravity help?

    I know wormholes are solutions to Einstein´s equations. What I tried to say (ask) is whether a quantum theory of gravity can help in the study and problems of wormholes.
  5. Oct 1, 2007 #4
    Actually, wormholes are solutions of equations of GR, but they could be created only when matter of negative energy is introduced. However, so far nobody saw or created matter with negative energy. Perhaps Dark Energy could help, so here we need to understand particle and high energy physics very well. And whar could be more suitable that complete theory of interactions?
  6. Oct 1, 2007 #5
    Whormholes have attracted recently a lot of interest because of diferent reasons.

    I´ll speak first about Lorentzian, i.e. transversable, wormholes. As it has been told they are solutions which connect separated asymptotic regions of the universe. One postulates a (family of) metric that holds a certain reasonable, and necessary for the purpose,conditions. Later one asks for the kind of matter (represented by it´s energy momentum tensor or a lagrangian from which that energy moentum tensor can be derived) and anlizes it´s characterisitics.

    As also has been told that matter must violate the null energy condition (NEC for brevity). Matter whith that characterisitcs is anlikely to exist in ordinary scenarios. One posibilty is to get that matter as some kind of hall effect of ordinary matter.

    Even thought the mos interesting things have been raised by the discovery that the universe is acclerating. One way to get that accelerating universe is dark energy. Some forms of dark energy, for example the so called phantom energy, can be described by special types of free fluid like energy momentum tensors and can be shown to allow the existence of stable transversable wormholes. The big point with these is that these dark energy is not introduced to allow wormholes but to allow an observed phenomena, the acelerating universe. It´s "utility" to permit wormhles is subsidiary.

    There is another way to support an acelearting universe, the existence of a cosmological constant. Well, one way, inspired by string theory, to get a forud dimensional cosmological constant is in the arena of braneworld universes, as the very famous Randall-sundrum warped universes. IN these the observed universe would be a brane with possitive tension (corresponding to a positive cosmological constant). In addition ther would be a bulk, which would be a portion of an anti de Siter universe (AdS) and a second braned, with neggative tension. In these kind of scenaries one can get an energy-momentum tensor which doesn´t violate the NEC condition in the brane (but it does so in the bulk). Once again the big point here is that the existence of these models is not designed ex-profeso to get wormholes and the allowance of them is a secondary characteristic.

    As womholes appear to be allowed by similar mechanisms that the ones that allow an acelerating universe it could be reasonable that they would exists afther all. Their existence implies the existance of some observable phenomena (because of the fact that a magnetic field wich would tranvserse one should look as some kind of big macroscopic magnetic monopole). Beeing observables som epeople is asking for specific experiments whichoch would show their existence. Obviously at to now there is nothing observed.

    Another way in wchich transversable wormholes can exist is in modified theories of gravity, such as Brans-Dyckes theories of gravity. In such theories wormholes could exist without the need of NEC violating matter.

    There is a different way to get a source for the necessary matter for the wormhole. These requires string theory. Under certain conditions it can be shown that the dilaton, and the antysymnmetric tensor field common to the NS-NS spectrum of any string theory could behave as matter which allows an stable Lorentzian wormhle. Here the big point is that these fields wouldn´t need to violate the NEC.

    These is about Lorentzian womholes. There is another kind of worholes which are interesting in the theory of gravity. hey are the euclidean wormholes. These represent saddle points in the (euclidean) path integral approach to quantum gravity. By it´s analogy with similar situations in Yang-Mills theories they are called also gravity instantons. Their main role in a theory of quantum gravity is that they would give the mayor contribution to topolgy changes in the space-time. One interesting and puzzling feature of these euclidean woromholes, due to Coleman (if I am not wrong) is that they could be a source of violation of coherence in quantum theory. A laymen explanation of these is that they could create baby universes and send information into them which would not be observable in our universe, resulting so in a source of decoherence. One of the formers ways to study these phenomena was throeught the use of the wheller-de witt equation whcich is some kind of semiclasical approach to quantum gravity (these equation appears in the pionners works triying to cannonically quantize gravity, as a constraint equation, but also can be shown to appear as an WKB aproximation to the euclidean path integral formulation of quantum gravity). there is a recent paper by Polchinsky et all which claims to have solved these apparent problem within an string theory approach.

    There are many articles in arxiv about wormhles and a bibligraphy would be too extensive. I greatly recomend the book by M. Visser, "Lorentzian wormholes...". It is fromm the 98 or so and it doesn´t covvr many recent advances, but still is very interesting well writen and easy to read.

    Well, these is the "official" scince, but in strict confidence I can assure that wormholes really exist, what else did you think that a palantir actually was :wink:.
    Last edited: Oct 1, 2007
  7. Oct 5, 2007 #6


    thanks for your response, it was very useful. I will get the Visser´s book when I have the opportunity.


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