Your essay is a reasonable overview of the questions related to quantum gravity. My only disagreement might be with the issue of weirdness. Quantum mechanics, and I mean plain vanilla QM, is in many ways very weird. Quantum gravity is likely to have a lot of very strange features.

I suspect we may never come up with a completely fundamental quantum gravity that is not on some level an EFT. The Planck length is the shortest length that a quantum bit may be identified, at least in principle. We may be able to arrive at a reasonable quantum gravity close to the Planck scale. The reason for this is that quantum gravity may have close identification with the quantum measurement problem.

Quantum measurement ultimately involves a set of quantum states that encode the quantum states of a system. The occurrence of a classical stable state in the outcome of decoherence is something quantum mechanics is not able to compute. It may be that this process is a form of Godel loop or self-referential system of states encoding states. This then leads to the problem in mathematics of propositions that are true but unprovable. For quantum mechanics it might similarly mean there exist states, such as classically stable states and observed measurements, that are true but not provable by quantum mechanical "computers."

The issues with quantum information and black holes may ultimately reflect something similar. I suspect it could be that quantum gravity as a fundamental theory is not derivable or computable in any formal way.

I offer in my essay what I suspect is an effective theory, and in fact make various approximations, that might result in measurable outcomes in gravitational wave experiments. What is fundamental in the end is just what your feet stand on at the lowest level at the time.

Cheers LC

    Hi Dr Karen Crowther

    Nice observation on present day Physics.... "Ultimately, at any stage we may not be able to tell whether we've reached rock bottom, or even if there is a base level" ... and a checklist is a nice idea... dearDr Karen Crowther

    ............. very nice idea.... I highly appreciate your essay and hope you may please spend some of the valuable time on Dynamic Universe Model also and give your some of the valuable & esteemed guidance

    Some of the Main foundational points of Dynamic Universe Model :

    -No Isotropy

    -No Homogeneity

    -No Space-time continuum

    -Non-uniform density of matter, universe is lumpy

    -No singularities

    -No collisions between bodies

    -No blackholes

    -No warm holes

    -No Bigbang

    -No repulsion between distant Galaxies

    -Non-empty Universe

    -No imaginary or negative time axis

    -No imaginary X, Y, Z axes

    -No differential and Integral Equations mathematically

    -No General Relativity and Model does not reduce to GR on any condition

    -No Creation of matter like Bigbang or steady-state models

    -No many mini Bigbangs

    -No Missing Mass / Dark matter

    -No Dark energy

    -No Bigbang generated CMB detected

    -No Multi-verses

    Here:

    -Accelerating Expanding universe with 33% Blue shifted Galaxies

    -Newton's Gravitation law works everywhere in the same way

    -All bodies dynamically moving

    -All bodies move in dynamic Equilibrium

    -Closed universe model no light or bodies will go away from universe

    -Single Universe no baby universes

    -Time is linear as observed on earth, moving forward only

    -Independent x,y,z coordinate axes and Time axis no interdependencies between axes..

    -UGF (Universal Gravitational Force) calculated on every point-mass

    -Tensors (Linear) used for giving UNIQUE solutions for each time step

    -Uses everyday physics as achievable by engineering

    -21000 linear equations are used in an Excel sheet

    -Computerized calculations uses 16 decimal digit accuracy

    -Data mining and data warehousing techniques are used for data extraction from large amounts of data.

    - Many predictions of Dynamic Universe Model came true....Have a look at

    http://vaksdynamicuniversemodel.blogspot.in/p/blog-page_15.html

    I request you to please have a look at my essay also, and give some of your esteemed criticism for your information........

    Dynamic Universe Model says that the energy in the form of electromagnetic radiation passing grazingly near any gravitating mass changes its in frequency and finally will convert into neutrinos (mass). We all know that there is no experiment or quest in this direction. Energy conversion happens from mass to energy with the famous E=mC2, the other side of this conversion was not thought off. This is a new fundamental prediction by Dynamic Universe Model, a foundational quest in the area of Astrophysics and Cosmology.

    In accordance with Dynamic Universe Model frequency shift happens on both the sides of spectrum when any electromagnetic radiation passes grazingly near gravitating mass. With this new verification, we will open a new frontier that will unlock a way for formation of the basis for continual Nucleosynthesis (continuous formation of elements) in our Universe. Amount of frequency shift will depend on relative velocity difference. All the papers of author can be downloaded from "http://vaksdynamicuniversemodel.blogspot.in/ "

    I request you to please post your reply in my essay also, so that I can get an intimation that you replied

    Best

    =snp

    Hello Karen,

    Downloaded your essay, browsing it now, commenting on the passing scenery.

    Like your application of concept of inversion to non-fundamentals of mainstream physics. Important attribute of Clifford algebra is that it is invertible. If one is a fan of the geometric interpretation of the Hestenes community, then one would expect the invertibility to be easily visualized. Given that the geometric product of two vectors gives a scalar and a bivector, why is it that the product of a scalar and a bivector gives only a bivector? Can you help me understand that? True that the bivector is comprised of our two original vectors, however two individual vectors are topologically distinct from the bivector.

    The second section, where you "...outline some of the different ideas of fundamentality associated with modern physics", gets right to the question of a fundamental length. One can have but one fundamental length in a QFT. Effective field theories have two or more. I think all are agreed that Compton wavelength is best choice. Problem enters with renormalization. One needs a physical model that naturally contains the renormalization coefficients of QED. Requirements seem to be gauge invariance, finiteness, and confinement. Your question at the end of that section "Why are

    we currently digging for a more fundamental theory?" appears in large part answerable with a single word - renormalization. Tho of course there is more to it than that.

    You open the third section with the assertions that

    first, "...the framework is mathematically ill-defined". One might suggest that the geometric interpretation of Clifford algebra simply cannot be ill-defined, as in the present context it is simply the eight component Pauli algebra of interactions of the fundamental geometric objects our physical Euclidian space - one scalar, three vectors, three bivectors, and one trivector. It is not ill-defined. Hestenes' wonderful original text is imo the best reference if you're not into this. 50th anniversary 2nd edition was published a few years ago. Amazing how slowly good ideas propagate sometimes.

    secondly, you mention renormalization. Agreed on this. One needs a model that naturally contains the renormalization coefficients. Without that both singularity and boundary seem intractible.

    third is the problem of gravity. Again the Hestenes camp has this sorted out to some great degree, and particularly the Cambridge group. Equivalence of their gauge theory gravity and GR was demonstrated in a series of papers back in the 90s. So the possibility exists of a geometric algebra model in flat Minkowski spacetime, using interactions of the Pauli wavefunctions at the Compton wavelength say for instance of the electron and the event horizon of the Planck particle to make a model.

    darn. i'm new to posting here, hope i'm not hijacking your thread. Going point by point thru your essay, trying to be relevant. And know someone will kick me in the shins if this is over the top.

    point is geometric algebra is a great tool, need to start there if one wants fundamental understanding of how physical objects interact in physical space. Get the geometric structure right. Then throw in the fields and see what happens.

    still a ways to go to get thru your essay, but feel like i've gone on too long already. Hope to come back to it.

      Dear Karen

      Very nice essay and direct to the point, no heavy philosophizing and repeating arguments that has been heard a million times.

      I said this in my essay which essentially what you have said

      " This structure had to be simple, basic but showed all the present physics in a clear and COHERENT way. That is, what are space, time, mass, charge, spin, interaction and most of all why the electron, the proton and "photons" exist. They should be interrelated aspects of a fundamental system."

      My idea takes into account all the requirements that you mention

      https://fqxi.org/community/forum/topic/3127

      Thank you.

        Dear Karen,

        All of your 9 conditions are very real.

        Among all the essays without mathematics, you are among the best.

        My essay is tied to Plank's units. I would like you to tell me what conditions do not satisfy my views in the essay.

        With best wishes,

        Branko

          Dear Karen,

          Your contribution to our motley collection of essays here is much needed. You give a clearer picture than most physicists could give of what they're looking for in a fundamental theory, and I think it's important to understand how little clarity there is about this. To me what's most striking about physics is how much our current theories can explain about the world, while seeming to leave us almost clueless as to why a world should be based on such strange foundations.

          I want to mention that your excellent paper on "Decoupling emergence and reduction in physics" is directly relevant to this contest, since these two notions come up in many of these essays, and are usually taken to be directly opposed. Also, I was very glad to find your book/thesis on "Effective Spacetime" - it's rare to find such in-depth discussion of recent physics that a non-specialist can follow.

          I was interested in your comment, "The requirement of unification is hard to justify. Given that our manifest experience of the world is of diversity rather than a sameness of phenomena, seeking an explanation of heterogeneity seems counter-intuitive--surely a unified description would be more striking than a disunified one, and cry out for explanation?" Later you answer this by suggesting it's the "business of physics" to "explain diverse phenomena by appeal to simple, universal laws." I can't argue with that - and certainly the quest for unification has led to many an outstanding discovery, most lately in the Standard Model. But in my current essay I've tried to show that neither unification nor naturalness are reliable guides to a more fundamental theory. The essential argument is that diverse interaction-structures are necessary to make any kind of physical information measurable, or even meaningfully definable.

          I really appreciate the kind of work you're doing, digging out the real conceptual issues within the technical struggles of current research.

          Conrad

            Dear Karen Crowther

            Just letting you know that I am making a start on reading of your essay, and hope that you might also take a glance over mine please? I look forward to the sharing of thoughtful opinion. Congratulations on your essay rating as it stands, and best of luck for the contest conclusion.

            My essay is titled

            "Darwinian Universal Fundamental Origin". It stands as a novel test for whether a natural organisational principle can serve a rationale, for emergence of complex systems of physics and cosmology. I will be interested to have my effort judged on both the basis of prospect and of novelty.

            Thank you & kind regards

            Steven Andresen

            Dear Peter,

            Thanks very much for reading and commenting on my essay. I usually tend to think of the standard model as a collection of theories, but since it can be written as a single Lagrangian, it seems more usual to call it a theory (even if it is non-unified and rather inelegant one!) I also explored general relativity and quantum field theory instead of special relativity and quantum mechanics, because the latter are held to be less-fundamental than the former (SR being a special case of GR, and QFT being a combination of QM and SR).

            If we are talking about a TOE in the sense of a complete theory valid at the most fundamental level, as I do in the essay, then yes I think it is potentially within our power to formulate and understand such a theory. However, if you mean actually using such a theory to get results about everything (i.e., physics at all scales), then this is most certainly not possible. As you say, it would require some incredible level of computational power. In the essay, I used the assumption that it is possible "in principle" but, in honestly, I don't believe that -- in fact, I can't even make sense of such a statement.

            Thanks again,

            Karen

            Dear Diogenes,

            Thanks very much for your comments. Yes, in the essay I assume that it is possible "in principle" to derive results valid at larger length-scales from theories formulated at shorter length-scales. This is one sense of reductionism, but it is not one that necessarily conflicts with the possibly of emergent phenomena, particularly the examples you mention.

            Yes, I agree that there are many ill-defined concepts in mainstream physics, and I believe that part of my job is to help clarify these where needed (though being ill-defined is not always a bad thing, nor something that can in all cases be fixed). In this essay, though, I decided to work with the mainstream perspectives (this is also why I used the reductionist assumption, in spite of finding it problematic myself) --- this was partly for reasons of simplicity and accessibility for a short essay, but also because I wanted a better understanding of them. My aim next (i.e., my current project) is to critique these conditions (particularly their motivations, and consequences for other principles) from a philosophical standpoint.

            Best,

            Karen

            Sorry for the formatting in that post. The small "n"s that appear out of place at the start of some sentences were supposed to be new lines. I don't know why that happened, or how to fix it now!

            Dear Luca,

            Thanks very much for your comments! That's an interesting idea, that a fundamental theory cannot describe interactions. I will have to think about that, but there are simple systems with interactions that can be described without requiring perturbation theory.

            In regards to empirical adequacy, I did not think this to be a condition of fundamentality but a requirement for a theory in order that it be considered scientific at all. And, as I mention at the top of page 8, I take it for granted in the essay that we are considering only scientific theories. This saves me from having to explore the problem of demarcation in science, which is too complicated to discuss in a short essay! As you say, too, the issue of connecting theory with observation is certainly not a straightforward one.

            Best,

            Karen

            Sorry for the formatting in that post. The small "n"s that appear out of place at the start of some sentences were supposed to be new lines. I don't know why that happened, or how to fix it now!

            Sorry for the formatting in that post. The small "n"s that appear out of place at the start of some sentences were supposed to be new lines. I don't know why that happened, or how to fix it now!

            What do you mean "create a universe"? And how would that give us a fundamental theory?

            Dear Paul, Thanks very much! Yes, I agree the idea of naturalness is an interesting one for the reason you mention -- it may be desirable to have a theory that doesn't require fine-tuning, but we can't rule out the possibility that, at the fundamental level, we have a theory that is "unnatural". This is true of several of the requirements, including -- most obviously -- those of unification, and "no weirdness". It's very possible that, fundamentally, the world is described by an unsettling, non-unified theory. And yet, if we arrive at a theory that doesn't fulfil these conditions, then we will keep digging for a more satisfactory one. So, actually, it's possible that we have a final theory and yet continue to search in vain for something more. That's an interesting consequence of the epistemic worry that I hadn't considered, so thanks for that. Best, Karen

            Dear Lawrence B. Crowell, Thanks very much. Your comments touch on several interesting issues. Yes, I wonder about the no weirdness requirement, too, and am still not certain of its inclusion or interpretation. But, the idea generally expressed is that QM is not fundamental, precisely because of its weirdness. Many people (most prominently Penrose) argue that a more fundamental theory is necessary in order to solve these issues, particularly the measurement problem. Quantum gravity, although the moniker suggests otherwise, need not -- and probably, in fact, can not -- be a quantum theory in the usual sense. One reason is because quantum theories utilise space and time, and these are to be modified in quantum gravity. So, another reason why quantum mechanics is supposed to be non-fundamental is because of the expectation of the necessity of QG. If QG contains similar weird features, then this will push people to seek a more-fundamental theory, in turn. But, that said, you are right that QG is likely to be weird! The ideas of a shortest length scale and a shortest time scale are extremely difficult ones -- as is the possibility of formulating a theory that describes a non-spatiotemporal regime. Your suggestion that QG be an EFT valid "close to" this regime, is interesting, too. We would have to think more about the issue of UV completeness in this case. Best, Karen

            Thanks for your reply - I completely agree with what you've said here! Best, Paul

            Dear Karen

            Its nice to read an essay about physics that explores the issues without dragging in mathematics - not that I have anything against mathematics - I trained as one. But I do feel the essay ought to be about words and a pleasure to read and I very much enjoyed reading your essay. I'm glad that you pointed out that QFT = QM relativity as that particular point is not made often enough and it does show that progress has been made in integrating our two most fundamental physical theories. Congratulations on an informative essay!

            Best Wishes

            Mozibur Ullah

              It seems that the site slightly mangles up the formatting of posts by removing linebreaks for some reason.

              Hi Karen,

              I do not think, that a fundamental theory cannot describe interactions. On the contrary. But fundamental concept like mass, spin, momentum etc. are only defined in the free theory. Only if the meaning/definitions of these concepts are given, one can define, what interaction is. For instance force is something, that changes the momentum. I think that was Poincaré's view. Then whether given specific initial conditions, there exist a non perdurbative solution of the equations depends on the symmetry of that configuration. But I do not think that whether such a solution exists or not can be a criteria for a theory to be fundamental. But that might not be, what you intended to say.

              By the way I would be glad, if you could find the time to read and comment on my essay: The quantum sheep - in defense of a positivist view on physics

              Best regards,

              Luca