Essay Abstract

I argue that it is possible for a theory to be neither quantized nor classical. We should therefore give up the assumption that the fundamental theory which describes gravity at shortest distances must either be quantized, or quantization must emerge from a fundamentally classical theory. To illustrate my point I will discuss an example for a theory that is neither classical nor quantized, and argue that it has the potential to resolve the tensions between the quantum field theories of the standard model and general relativity.

Author Bio

Sabine is an assistant professor at Nordita in Stockholm. Her work is mostly focused on the phenomenology of quantum gravity. In her free time she blogs at backreaction.blogspot.com

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Hello Sabine

Thanks for this essay. It summarises what appears to be a real problem and outlines what might be a real solution. It does not give many details of either the problem or the possible solution, but they may be in the references. This is all good. The questions which come to mind are:

1. Does it make sense to say that Planck's constant varies? Might it be a quantity, like the speed of light, which is taken as an absolute unit? Are there any other natural absolute units which might be used instead? The equation you give on page 3 for G relates G and c and Planck's constant and the Planck mass. Is there a good reason for regarding any one of these as less fundamental than the others, and so as dependent on them?

2. If Planck's constant varies, what might be the equation (or whatever) describing how it varies? What might be the "suitable potential" which could be added to the Lagrangian? Are there any physical or geometric criteria for choosing any particular potential?

If these points can be given satisfactory answers, this all looks like promising research.

Regards, Alan H.

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    Sabine,

    A rose by any other name would still be a rose, I think? Or would it require another theory?

    I like your well written essay and to the point. And you are not afraid of taking some risk. Cool!

    I was hoping you were going to enter this contest, so I could thank you for making my entry possible. One of your blogs on "backreaction" gave me enough confidence to submit "An Elephant in the Room" and have some fun.

    Thank you,

    Don L.

      Sabine,

      You wisely set a modest and an achievable goal, though well-presented.

      "I hope to have convinced the reader that giving up the assumption that

      a theory is either classical or quantized can be fruitful and offers a new possibility to address the problems with quantum gravity."

      My ruminations about gravity tended toward observations that could be questioned and details that could point elsewhere.

      Jim

      Hi Sabine,

      Your essay is very nicely structured, although there's little I can reasonably comprehend. Therefore, I'm certain:) what's really needed here are some insights from an uninformed pedestrian passerby...

      - There seem to be two alternative, mutually exclusive characteristic properties of matter: kinetic self-propagating momentum and potential inertial mass. By that I mean that there may be some constant particle emission energy that is configured by external conditions of emission, vacuum energy density, for example, to allow propagation or to prevent it, perhaps as a fluctuating configuration state. Perhaps matter is either physically configured as a linearly directed waveform or as a self-contained sphere of inwardly directed energy. In its inwardly directed spherical configuration, applied external energy might be absorbed, increasing effective inertial mass.

      - The effects of gravity simply do not seem to appear in the interactions of quantum particles. Gravity seems to exist as a property of matter when material mass is aggregated, localized within a region of space. Since the principal effect of gravity seems to be the acceleration of discrete objects of mass from space towards objects of superior mass, perhaps gravitation is produced by an interaction between potential mass-energy and the kinetic vacuum energy contained within space. The presence of both aggregated masses and unoccupied space would be required to produce the interaction identified as the the gravitational effect. Essentially, the localizing potential energy of mass would have a contracting effect on vacuum energy, increasing its local density and its accelerating effect on other objects of mass (which produce their own external gravitational effects in proportion to their potential mass-energy).

      I think this type of physical interaction could directly produce the effects described by general relativity using its system of abstract spacetime dimensional coordinates.

      It seems there might be no quantum gravitational effect, only quantum potential mass-energy; the gravitational effect requires its interaction with substantial amounts of external vacuum energy.

      That's the way things appear to this outside observer, anyway.

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        Dear Sabine,

        Good luck in the competition. Your essay is about an interesting and important subject which is very relevant to the essay question. With so many possible subjects to consider, you probably made a wise choice applying your expertise to just this one.

        PS. I do read your thought provoking and educational blog from time to time- and watched your music video.

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          Just pointing out, in case anybody missed it, that the essay "Gravity can be neither classical nor quantized" has a companion article at http://arxiv.org/abs/1208.5874 called

          "A possibiliity to solve the problems of quantizing gravity"

          It helps to read both. I expect the two articles together will be a landmark and will noticeably change the way quantum relativists look at the problem of a combined theory of geometry and matter. The result can be something more interesting, and better, than either a classical or a quantum theory.

          I have to say--the Hossenfelder idea is such a bold stroke, and seems to have enough possibility of succeeding, that it is actually entertaining to think about. It is *enjoyable* to mull over such a creative yet fundamentally simple proposal.

          Sabine,

          A very interesting essay. A few questions come to mind.

          1. You mostly describe quantum theory in the language of operators. Do these ideas still make sense, and what do they look like, using Feynman's sum-over-histories approach?

          2. It seems natural at first glance to try to apply these ideas to inflation and dark energy. This may be hopelessly naive, but have you thought about letting your "Planck field" change sign in some regime to produce a negative G?

          3. Are you envisioning sharp phase transitions or gradual ones? The reason I ask is because if you went to an even more radical paradigm and incorporated discreteness, you might possibly make use of the phase transitions of random graph dynamics.

          To get a better idea of what is motivating these questions, you might take a look at my essay

          [link:fqxi.org/community/forum/topic/1386]On the Foundational Assumptions of Modern Physics[link]

          Take care,

          Ben Dribus

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            Hi Don,

            It makes me happy to hear that my blogging has encouraged you to write down your thoughts. I'll have a look at your essay. Best,

            B.

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            Dear Georgina,

            I'm glad to hear you find it interesting. The video... was fun. I guess I'm overcompensating for my conservative colleagues ;o) Best,

            B.

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            Hi Patrick,

            Planck's constant is dimensionful, so if you want to speak of it varying, you should strictly speaking normalize it to make a dimensionless constant first. I did mention this in the arxiv paper - the normalization that suggests itself is the measured value of the Planck constant (at low temperatures). I'm not entirely sure what you mean with "variation". Do you mean a spatial variation or a temporal one? In the case I discussed there shouldn't be a spatial variation of Planck's constant unless you are in strong curvature regimes, ie towards black holes or towards the Big Bang singularity. (Think of particle masses, ie the higgs vev, it doesn't vary either.)

            What might be a suitable potential, well, it has to be one that leads to a symmetry breaking at high temperatures and at the same time have quantum corrections that allow the convergence of the perturbative expansion, as I explained towards the end of the text. I don't know if such a potential exists. To begin with it would depend on the particle content of the theory. I don't think there's an easy answer to that, but all I wanted to say is that it does not seem impossible, and I believe it's a possibility worth investigating.

            Best,

            B.

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            Hi Jim,

            I don't know what you mean when you say "The effects of gravity simply do not seem to appear in the interactions of quantum particles." They appear if you put in a gravitational interaction, and you can do that in a perturbative quantization coupled to the rest of the standard model. The problem is just that this theory only makes sense as an effective theory, not as a fundamental one. Best,

            B.

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            Hi Ben,

            These are all very good questions that I don't have quick answers to. As to 1: I can only say I certainly hope it does, it would be more elegant. As to 2. I think this might be similar to a signature change. It might be very interesting to think about further. And regarding 3. I haven't been envisioning this one way or the other. I'd think this is a question that would have to be addressed by experimental constraints, for example, as you say, inflationary imprints in the CMB because it affects what it means to have a quantum fluctuation to begin with. In fact, this seems to me the most fruitful direction to make contact with observation. Best,

            B.

            Dear Sabine Hossenfelder,

            If the universe is with matters in continuum, gravitation is considered as the tensor product on deformation of matters, in that, matters are eigen-rotational strings. To describe this gravity of the universe with an infinite sum of string-lengths, quantization is inevitable in that a different framework of quantization to be adapted that is non-perturbative and conformal.

            With best wishes,

            Jayakar

            Hi Bee,

            A quick hypothetical question.

            In a toy cosmos wherein all masses were quantized, would gravitational interactions (even if intrinsically classical) be quantized by default?

            I have asked this question in several discussion forums, but I don't think I have ever gotten a straight answer. Is the question flawed in some way?

            Rob O

              I am a big fan of arguing discreteness is an emergent property, however isn't discreteness effectively a two state proposition, either its there or it isn't. Since planks constant has arbitrary units, it seems unnatural to me to talk about it evolving in time. I am thinking you mean that some fundamental ratio is evolving with time, but I am not sure if that is a correct characterization.

                Hi Sabine,

                You write: "Classical general relativity predicts the formation of singularities under quite general circumstances. Such singularities are unphysical and should not occur in a fundamentally meaningful theory." That is a powerful statement.

                As I understand it, the theoretical physics community proposes to get past the problem of gravitational singularities by a clever definition in quantum gravity; instead of investigating the possibility that gravity is wrongly defined.

                An alternate ansatz to describe gravity, which will not lead to singularities, you will find in my essay "Rethinking Geometry and Experiece", I really would appreciate your time and a feedback.

                Regards

                Anton

                Hi B.,

                Of course I prefaced my comments by indicating that I'm not a physicist, much less a quantum theorist, but, that being said, I was referring to observational evidence of gravitational interactions among quantum particles. While models can be constructed describing various forms of gravitational interactions, is their observational evidence supporting a quantitative perturbative interaction among particles?

                Of course, that was not the main point of my little comment, but you're certainly free to dismiss or ignore it in its entirely...

                B.,

                Actually, shouldn't a quantum gravitational effect be manifested more specifically as an effective attraction interaction among particles - proportional to their mass?

                Has any such interaction among particles been observed?

                Jim