Saturday, November 09, 2019

How can we test a Theory of Everything?

How can we test a Theory of Everything? That’s a question I get a lot in my public lectures. In the past decade, physicists have put forward some speculations that cannot be experimentally ruled out, ever, because you can always move predictions to energies higher than what we have tested so far. Supersymmetry is an example of a theory that is untestable in this particular way. After I explain this, I am frequently asked if it is possible to test a theory of everything, or whether such theories are just entirely unscientific.


It’s a good question. But before we get to the answer, I have tell you exactly what physicists mean by “theory of everything”, so we’re on the same page. For all we currently know the world is held together by four fundamental forces. That’s the electromagnetic force, the strong and the weak nuclear force, and gravity. All other forces, like for example Van-der-Waals forces that hold together molecules or muscle forces derive from those four fundamental forces.

The electromagnetic force and the strong and the weak nuclear force are combined in the standard model of particle physics. These forces have in common that they have quantum properties. But the gravitational force stands apart from the three other forces because it does not have quantum properties. That’s a problem, as I have explained in an earlier video. A theory that solves the problem of the missing quantum behavior of gravity is called “quantum gravity”. That’s not the same as a theory of everything.

If you combine the three forces in the standard model to only one force from which you can derive the standard model, that is called a “Grand Unified Theory” or GUT for short. That’s not a theory of everything either.

If you have a theory from which you can derive gravity and the three forces of the standard model, that’s called a “Theory of Everything” or TOE for short. So, a theory of everything is both a theory of quantum gravity and a grand unified theory.

The name is somewhat misleading. Such a theory of everything would of course *not explain everything. That’s because for most purposes it would be entirely impractical to use it. It would be impractical for the same reason it’s impractical to use the standard model to explain chemical reactions, not to mention human behavior. The description of large objects in terms of their fundamental constituents does not actually give us much insight into what the large objects do. A theory of everything, therefore, may explain everything in principle, but still not do so in practice.

The other problem with the name “theory of everything” is that we will never know that not at some point in the future we will discover something that the theory does not explain. Maybe there is indeed a fifth fundamental force? Who knows.

So, what physicists call a theory of everything should really be called “a theory of everything we know so far, at least in principle.”

The best known example of a theory of everything is string theory. There are a few other approaches. Alain Connes, for example, has an approach based on non-commutative geometry. Asymptotically safe gravity may include a grand unification and therefore counts as a theory of everything. Though, for reasons I don’t quite understand, physicists do not normally discuss asymptotically safe gravity as a candidate for a theory of everything. If you know why, please leave a comment.

These are the large programs. Then there are a few small programs, like Garrett Lisi’s E8 theory, or Xiao-Gang Wen’s idea that the world is really made of qbits, or Felix Finster’s causal fermion systems.

So, are these theories testable?

Yes, they are testable. The reason is that any theory which solves the problem with quantum gravity must make predictions that deviate from general relativity. And those predictions, this is really important, cannot be arbitrarily moved to higher and higher energies. We know that because combining general relativity with the standard model, without quantizing gravity, just stops working near an energy known as the Planck energy.

These approaches to a theory of everything normally also make other predictions. For example they often come with a story about what happened in the early universe, which can have consequences that are still observable today. In some cases they result in subtle symmetry violations that can be measurable in particle physics experiments. The details about this differ from one theory to the next.

But what you really wanted to know, I guess, is whether these tests are practically possible any time soon? I do think it is realistically possible that we will be able to see these deviations from general relativity in the next 50 years or so. About the other tests that rely on models for the early universe or symmetry violations, I’m not so sure, because for these it is again possible to move the predictions and then claim that we need bigger and better experiments to see them.

Is there any good reason to think that such a theory of everything is correct in the first place? No. There is good reason to think that we need a theory of quantum gravity, because without that the current theories are just inconsistent. But there is no reason to think that the forces of the standard model have to be unified, or that all the forces ultimately derive from one common explanation. It would be nice, but maybe that’s just not how the universe works.

121 comments:

  1. "The reason is that any theory which solves the problem with quantum gravity must make predictions that deviate from general relativity. And those predictions, this is really important, cannot be arbitrarily moved to higher and higher energies." and "I do think it is realistically possible that we will be able to see these deviations from general relativity in the next 50 years or so."

    If this is the case, wouldn't such tests be a top priority for experimental funding? Do these involve other topics that have tangible results besides testing predictions of TOEs? It seems to me that tests with such advantages would be more valuable than ever-larger particle accelerators, but I am incredibly naive about these things so would be happy to be shown I am wrong.

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    1. Why does a correct theory have to deviate from general relativity? Would an agreement be more desirable? Of course, we would then have to test some other aspect.

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    2. At the moment we are presented with commercial electric ZPE- and/or anti gravity propulsion units, we will know the truth.

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  2. We all are gonna die without knowing nothing about the possible GUT or TOE even if existing. It's sad to think in this way but all the odds point in that direction. The stagnation in theoretical physics is going to be here for a longggg time.

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    1. Nima Arkani-Hamed speaks directly to this as quoted in Bee's book Lost in Math. Speaking implicitly to physicists, he says: "Nobody promised you a rose garden. This is a risky business. You want certainty, you do something else with your life. People spent centuries barking up the wrong tree. That's life".

      It's a great quote. Right or wrong, love him or not, agree with him or not...there's a reason why he's a thought leader among physicists.

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  3. I just tested Future Strings, a project of Catrin Finch and Sekhou Keita. Not only beauty-proof but also reality-proof.

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  4. "...predictions that deviate from general relativity."

    The limit of General Relativity has long been known.
    https://ui.adsabs.harvard.edu/abs/1985AN....306...77S/abstract
    We need a theory that provides the additional term 0.3"/r^2 for the light deflection. Schmeidler has specified as limit 5 sun radii. At this distance the force of gravity of the Sun corresponds approximately to the force of attraction on the surface of the Earth. Therefore, this limit may be detected in the earth laboratory.

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    1. The extent of the sun corona outwards in the space is given with five sun radii. So there must be refraction by the media of the corona.

      First we have calculate the refraction effect - then we can calculate something other, e.g. something hypothetical.

      Anything other way is senseless, not science.

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    2. @weristdas: Do you presume you are the first person to think of this? So nobody in the last 100 years has thought of this and examined this possibility?

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    3. @weristdas: I'll go one further than Lawrence Crowell: why not get the relevant data and do your own calculations?

      The relevant physics is pretty straight-forward (taught in many senior high schools I'd guess), and the mathematics likewise. Yes, a path through a sphere may be a bit of a challenge, but you could make it planar to simplify things. You'd get an estimate that would surely be good enough, at an order of magnitude level, which is all you need here.

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    4. weristdas: "Anything other way is senseless, not science."

      Or this is the long-awaited gate to New Physics.

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    5. JeanTate11:07 AM, November 11, 2019

      "why not get the relevant data and do your own calculations?"

      I'm not a well financial supported academic and no rich man - that's all why I don't do that and has not to do that.

      But I pay taxes for people who HAVE to do that because it's their job. And I ask modest, why they don't do that.

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    6. Lawrence Crowell7:17 AM, November 11, 2019
      "So nobody in the last 100 years has thought of this and examined this possibility?"

      So I beg you to show the calculations. A link, bibliographical data of a paper, a book, whatsoever.

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  5. The TOE = GUT + QG, QG = quantum gravity, can be a whole lot of things. Currently with the standard model we have the group structure of the 3 gauge interactions, nuclear QCD, weak QFD, and electromagnetic QED, as SU(3)×SU(2)×U(1). This is a twisted bundle construction and some saw this as less than satisfactory. The idea was to embed this into SU(5) with the 3×3 of the 5×5-bar matrix irrep defining QCD, the 2×2 QFD and the trace as QED. This was the Glashow-Salaam GUT. It predicts a proton decay that has not been found. It has empirically been largely ruled out. The double cover is SO(10), which has a little longer proton decay time, but the super-Kamiokande has largely ruled that out. What next? We can then appeal to the Cartan decomposition with embedding or groups

    E8 ⊃ E6 ⊃ SO(10) ⊃ SU(3)×SU(2)×U(1).

    E6 is interesting for a number of reasons, such as the Yaguchi-Hansen SU(3) metric in decomposition to SO(10) and that the proton decay time can be set to “infinity..” We can also collapse this nesting so the intermediate groups E6 and SO(10) play a minor role and the theory is standard model SU(3)×SU(2)×U(1) all the way to the GUT or TOE scale.

    It might turn out that what we call unification happens in entirely different ways. It might be there is some n-partite entanglement of states which makes these gauge groups indistinguishable. The idea of embedding groups into each other an climbing to higher symmetries seems appealing. Yet this appeals to more degrees of freedom and structure, and honestly it might just be that what nature does involves a reduction in degrees of freedom.

    I think quantum gravitation might reveal itself in gravitational waves. In the last post here we bounced around the issue of LIGO detection of gravitational radiation. The peak of the time reversed chirp signature is not well known. I think it is possible there are quantum gravitational signatures there. I have been looking at quantum hair, just a generic scalar field for now, or BPS charges on horizons right at the point of collision. It is a sort of quantization of Mirzakhani's entropy dynamics on hyperbolic spaces. I make no preference for what ever structure this quantum hair has, where with follow on work this should work with any possible GUT/TOE.

    Experimental tests of quantum gravity and GUT/TOE obviously can't be done with colliders. Such a collider would be a loop larger than the circumference of the Milky Way galaxy. We are going to have to rely upon the far more abundant energy nature can provide in processes such as black hole coalescence.

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  6. Nit-pick time. Second para, "on the same table" should be "on the same page".

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  7. Sabine wrote:

    “The other problem with the name “theory of everything” is that we will never know that not at some point in the future we will discover something that the theory does not explain. Maybe there is indeed a fifth fundamental force? Who knows.”

    I think that the common use of the term TOE is that indeed it would be the final word, there would be no room for any new forces.

    “Though, for reasons I don’t quite understand, physicists do not normally discuss asymptotically safe gravity as a candidate for a theory of everything.”

    The reason ASG is not considered TOE is that ASG usually refers to looking for a UV fixed point in pure gravity. Someone could in principle start an ASG program that tries to incorporate all existing forces and find a UV fixed point. They might discover that to get to such a fixed point they need to add additional particles and forces. If they ended up finding such a combination that miraculously flows to a UV fixed point, it would be a TOE.

    This explains my first point. In such a TOE there would be no room for any new forces, because anything that you would add would drive the theory away from the fixed point.

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    1. Udi,

      "I think that the common use of the term TOE is that indeed it would be the final word, there would be no room for any new forces."

      The point is, Udi, that we would never know that this is indeed the case.

      "The reason ASG is not considered TOE is that ASG usually refers to looking for a UV fixed point in pure gravity."

      Not so. The current ASG program does include all existing forces and, as I said in my video, can include a GUT.

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    2. Asymptotic safety seem to be a method rather than an independent theory. For instance, it depends on fixed point type what kind of theory is applicable or what you will get. Several functions or functionals can define finite divergence by perturbations or not. Perhaps we are still wondering what would be the basis of foundation theory in which asymptotic safety would be emerged...

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  8. Sabine, you (and most physicists?) believe that GR will fail to fit reality at the smallest scales rather than SM-predicted behaviors sort of "fading away" as we look more and more closely. Why is the latter hard to conceive?

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  9. I find the term TOE for explaining gravitiy and the other three forces of QM misleading anyway. Because a 'Theory of Everything' should have a serious Answer For
    Everything. But unfortunately current TOEs don't cover 'dark energy' or (the gravitational-effect of) DM.
    The term TOE that science is using is so far not really a TOE for me.

    A critical question from my side:
    What makes scientist believe that 'dark energy', DM and the phenomenon of consciousness is not coming from another (so far undiscovered) fundamental force ?

    kind regards,

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    1. The cosmological constant is "dark energy". There is nothing here to explain. As to dark matter, all GUTs predict new particles and there are usually candidates for dark matter among them. So it is plausible (though of course we don't know for sure) that a TOE will also contain an explanation for dark matter.

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    2. I think there is a lot more. The cosmological constant Λ = Λ(φ, φ-dot) is very small currently Λ ≈ 10^{-52}m^{-2}, but during inflation (suing that paradigm) it was Λ ≈ 10^{60}m^{-2}. This is a horrendous value. To call this dark energy is just a label. We have from the standard commutator [a, a^†] = 1 that for a quantum field this commutator occurs for every value of frequency ω_n and we then sum over this sum_n ħω_n = E that is the Planck energy, or near to that. This assumes a box normalization. This then captures some aspect of the inflationary manifold; the sum over vacuum modes produces a huge zero point energy. This is then captured in the scalar-tensor gravitational Lagrangian √(g)φR, a construction first proposed by Dicke.

      It is not too difficult to see that this theory for φ → ⟨φ⟩_vev on a physical vacuum. The much smaller vev means the inflationary expansion is reduced, and in the case of the false vacuum to physical vacuum transition this was over 110 orders of magnitude. This is not hard to understand with the Lagrangian √(g)φR + ½ (φ-dot)^2 - ½mφ ^2 with this inflaton field φ a quadratic field. However, how does this really adjust the vacuum? The vacuum has modes of a wide range of fields, fermions, gauge fields etc, and by some means this inflaton field on the physical vacuum sets up a near zero vacuum so the cosmological constant is very small. This is really weird.

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    3. Lawrence Crowell 10:25 AM, November 10, 2019

      But inflation is just speculation. It has not been confirmed by observation. You don't know that the Cosm. Const is "very small" or the situation is "really weird" as there is no current explanation for the value of the Cosm. Const. You are heading into Fortunate Cosmos: How Baby Jesus' Papa Made the Universe territory.

      ***** Speculative theories are not confirmed physics. *****

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    4. Inflation is more than speculation. The piece of evidence still waiting is polarization anisotropy modes in the CMB. However, on a number of fronts it is supported by data.

      The cosmological constant, or Hubble constant, is pretty well known. There is a bit of an issue over whether H = 67km/sec-Mpc or 74km/sec-Mpc based on galactic z red shift data vs CMB data. Yet this is a very small cosmological constant.

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    5. If inflation is more than speculation why do you write that we are still waiting for a piece of evidence?
      Inflation is like String Theory, motivated by a good idea but now a huge collection of theories that are unfalsifiable and can fit pretty much any data. There are question marks against and serious critics of inflation.

      Contrast with a true empirical fact: time dilation. Easily falsifiable, tested quintillions of times, everybody accepts it's true.

      I've explained what an empirical fact is to you several times now. Have you any intention of mending your ways or will you continue to pretend phenomena that have never been observed are real?

      The value of the cosmological constant may be precisely known, but the reason for its value, if there is one, is not known. Therefore, to describe it as "very small" doesn't make sense. Very small compared to what? Compared to cosmological constants of other universes of which there are, despite what crazy Luke claims, zero examples. There may be a perfectly good physical reason for its value. I don't know what you might mean to write that it's "very small".

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    6. Lawrence Crowell 8:38 PM, November 12, 2019

      "Inflation is more than speculation."

      So what exactly is inflation and how has it been falsifiably tested?

      E.g. The inertia of matter was unexplained. Theorists speculated about the existence of the Higgs Field - a well-understood physical object. The existence of the field was confirmed by the observation of the Higgs Boson.

      No such precise theory or observational confirmation exists for inflation. Because it's speculation.

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    7. I suppose Lawrence means the same as Nima Arkani-Hamed when he says “If you believe there’s only one underlying vacuum, then there’s some formula out there for what the vacuum energy is, given in terms of some dimensionless numbers: the ratio of the vacuum energy to electron mass; some dimensionless quantity given in terms of 2s, πs; something like that. The fact that our estimates give us something that’s 60 to 120 orders of magnitude off [(“really weird” ~Lawrence)] suggests there must be some magical mechanism that’s solving the problem. If there is a unique vacuum, you have to look for something, which is making the answers small.”

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    8. Emmette Davidson 6:54 PM, November 14, 2019

      Right, so it's the estimates that are badly out rather than the measured value of the Cosm. Const. that is "very small". The reason it's important to make this distinction is because calling the measured value "very small" is a portal into empirically unverified claims, some nonsensical, about universal fine-tuning, anthropic principles, multiverses, inflation and whatnot. And sure enough Lawrence claims inflation is not speculation.
      Also, the fact that the estimates are out by 60 orders is not necessarily a measure of how badly wrong the theory is. It might just be a tweak to the theory that is needed, rather than a "magical mechanism". More unjustified speculation.

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    9. Inflation has more empirical support than string theory. Inflation is just where the cosmological constant, or maybe we should say vacuum energy parameter, is very large. The Einstein field equation

      R_{μν} - ½ Rg_{μν} + Λg_{μν} = (8πG/c^4)T_{μν}.

      has this additional term Λg_{μν} for the expansion of space. This has the effect of generating a flow of space, or dynamics of spatial surfaces. This meets a number of criteria, such as observed cosmological flatness, and the main test is the existence of B-modes in the CMB. This is where inflation induced gravity waves imprinted their polarizations on later acoustic waves.

      The physical vacuum of the observable universe is very small and that of the inflationary or false vacuum is > 110 orders of magnitude larger. Various string theoretic ideas of brane wrapping with different gauge fluxes have been calculated and so forth. However, nothing along those lines seems to capture this.

      Supersymmetry was one way it was thought to get rid of this large zero point energy. The vacuum energy for bosons is positive and fermions it is negative. So with boson and fermion pairs the vacuum energy would sum to zero, exactly in fact. This may still be so, or nearly so with some small supersymmetry breaking, but for curious reasons the supersymmetric partners are absent. Supersymmetry is broken when the vacuum energy is nonzero, and for inflation the vacuum energy was enormous. Yet how SUSY figures into this is not well understood. It could be analogous to how water can remain in a liquid state at temperatures far below freezing if there are no particles or sites of nucleation for the phase change.

      Things to think about.

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    10. Nima's words; I merely point to them for their worth. It is what Luke conveys: you may judge that.

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    11. Emmette Davidson7:40 AM, November 15, 2019

      "Nima's words; I merely point to them for their worth."

      Ah, I was just supposed to behold his words in awe? I responded by pointing out that if the estimate is 120 orders larger than the measured value, then the measured value is not "small", but the estimate is not very good. The measured value is in nature; it is not big or small, but literally natural.
      Do you agree?

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    12. Lawrence Crowell 6:02 AM, November 15, 2019

      There is unexplained data e.g. the flatness and homogeneity of the universe. But there is no direct evidence of the cosmological constant being much larger in earlier times than now. And the predicted B-modes in the CMB have not been detected.

      So there is some unexplained actual data, a vague theory which may or may not be physical, and an unfulfilled prediction.

      So it's speculation, isn't it.

      Then there is the usual list of speculative ideas:

      "inflation induced gravity waves imprinted their polarizations on later acoustic waves." Speculation
      "The physical vacuum of ..inflationary or false vacuum is > 110 orders of magnitude larger." Speculation
      "Various string theoretic ideas of brane wrapping with different gauge fluxes have been calculated" Speculation
      "Supersymmetry" Speculation
      "small supersymmetry breaking" Speculation
      "but for curious reasons the supersymmetric partners are absent." Not curious, it's nature.
      " Supersymmetry is broken when the vacuum energy is nonzero" Speculation
      "for inflation the vacuum energy was enormous." Speculation
      "SUSY" Speculation

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    13. You pander the fisher, and ask me for a fish.

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    14. Emmette Davidson 7:38 PM, November 15, 2019

      "You pander the fisher, and ask me for a fish. "
      You quoted vague speculation in support of Lawrence's vague speculation. If an estimate is out by 60 orders then the estimate is bad rather than the measurement small. Do you agree?

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    15. Lawrence Crowell 6:02 AM, November 15, 2019

      "Inflation has more empirical support than string theory. "

      Observations:
      Flatness and homogeneity of universe measured to high degree of precision.
      Observations since the 90s suggest the current expansion of the universe is accelerating, but there remain question marks against these measurements.

      I am failing to see how these observations show that inflation is "more than speculation". Are you referring to other empirical evidence? (The B modes haven't been observed.)

      Do you agree therefore that inflation is speculation?

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    16. I disagree. It solves a number of previously open questions. I think this raises it above not just a speculation but puts it somewhere between a hypothesis and a phenomenology or theory.

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    17. Well, I'll split the difference and agree it's a vague hypothesis. I'm relieved you're not claiming it's an empirical fact.

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  10. There is a nice article in the European Physical Journal H (2015), Fischbach recounts the story of "fifth force." We read: This story is of interest in that it provides yet another example of how the scientific community gives birth to an idea, tests it, and then accepts or rejects it based on the results of experiment."

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  11. "But there is no reason to think that the forces of the standard model have to be unified, or that all the forces ultimately derive from one common explanation"
    Are there no compelling reasons to suggest that there may be a TOE other than an appeal to beauty?

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  12. The Xiao-Gang Wen reference above looks interesting. More recently:

    Four revolutions in physics and the second quantum revolution
    – a unification of force and matter by quantum information
    Xiao-Gang Wen
    arXiv:1709.03824

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  13. Hi Sabine. I'm afraid that one of your slides refers to "asymptotically save gravity" rather than "asymptotically safe gravity". Don't you just hate it when that happens during a presentation :-) Apologies if this has already been spotted.

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    1. Gosh, no I hadn't noticed. Well, too late to fix that now. At least I did remove the textbox which said "New text here"...

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  14. Excerpt from an AIP 2009 interview with Joe Polchinski: "I think it’s an important question to as whether quantum gravity has an ultraviolet fixed point, and if it does, it’s important to know. Being a skeptic, I suspect it doesn’t, but that’s just because I tend to be skeptical of anything new."

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    1. If there is a duality between the UV and IR then asymptotic safe gravity with a fixed IR fixed point would imply a fixed UV point. Such an idea might be expressed as

      UV quantum gravity = IR quantum fields

      which in a way is the Einstein field equation.

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  15. Come on now, Sabine...

    Any attempt at an all-inclusive description of reality that does not provide an explanation of what consciousness (life) is, can in no way, shape, or form be called a “Theory of Everything.”

    At best, what you are talking about would be a theory of everything we presume to know about physical matter.

    Indeed, the existence of the physical properties of the universe would be utterly meaningless if life and consciousness did not exist.

    So how in the world could a “Theory of Everything” ignore such a profoundly important aspect of reality?

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    1. We have zero reason to think that consciousness is not physical. The brain is made of particles and a theory that describes what the particles do will explain what the brain does.

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    2. There was this idea of the ultimate bootstrap theory, back in the 1960s when S-matrix held attention, which had the only universe that can exist is one that brings forth intelligent observers. Today there are aspects of this with anthropic principle ideas and string theory has some roots in the old S-matrix theory.

      We might imagine that observers looking into the further past of the universe, say observing gravitational waves and neutrinos back to the earliest factions of a second, up into inflation etc, of the universe make measurements of such physics. This might then be compared to a Wheeler Delayed Choice Experiment (WDCE) that in effect sets the structure of the universe. In this setting the observed universe may be the only one that exists, or one of some very select few that can exist.

      These other cosmologies in the so called multiverse would then be quantum virtual off-shell configurations. We might then ponder whether there is some quantum cosmological form of the Lamb shift where these then have some physical implications for quantum gravitational states.

      I am not proposing this as something I think can be taken very seriously. I am not sure how this can be empirically supported.

      When it comes to consciousness, we have a problem with first, second and third person narratives. Science requires that results be communicated from a first person narrative to second, "Here are my results that you can verify." Where upon that person does so, communicates this one to others and it becomes a third person narrative. The experience of consciousness is not of this nature, and generally is a purely subjective first person narrative. This is a problem with making the so called hard problem of consciousness into a scientific proposition.

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    3. @Lawrence Crowell: “When it comes to consciousness, we have a problem with first, second and third person narratives.”

      I suggest that the only true problem we have with respect to consciousness is how it is possible that a wholly unique and inexplicable feature of reality...

      (consciousness/mind)

      ...could magically emerge from a specific arrangement of non-conscious particles of matter.

      Sabine seems to believe that some “future theory” of how particles behave will reveal the answer.

      However,...

      (and I say this with nothing but the greatest of respect for Sabine and her work)

      ...to me, that’s just a situation of someone kicking the proverbial can down the road in order to avoid the issue.

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    4. “We have zero reason to think that consciousness is not physical. The brain is made of particles and a theory that describes what the particles do will explain what the brain does.”

      Here’s a reason I wonder whether consciousness is physical: Prof. Sabine actually experiences “qualia.” She has a first-person conscious experience. She does not consciously experience Thomas’s qualia. Both Prof Sabine and Thomas’s brains are made of particles yet why does Sabine only experience her own “qualia.”

      It is incompatible with reductionism to treat a system as a non-quantum observer performing measurements on a different quantum system. Is it not also incompatible with reductionism for Prof. Sabine to experience only her own unique set of qualia? I don’t see how reductionism can account for conscious observers?

      I don’t understand how the standard model could give rise to “qualia” (personal, conscious experience). A logically consistent position is that qualia do not actually exist, but I think that this ignores the most basic observation one could make, that one actually does have personal, conscious experience.

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    5. No, it is not incompatible with reductionism. Any sufficiently large set of particles is "unique" in this universe simply because the chances that there is another set of particles with identical arrangements and identical history is basically zero. What you call "qualia" is some processes in the brain and that's that.

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    6. "It’s not the physics picture of matter that’s the problem; it’s the ordinary everyday picture of matter."

      Consciousness Isn’t a Mystery. It’s Matter.
      By Galen Strawson

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    7. There are reasons to think that consciousness, or the process that gives rise to consciousness, is physical. We also know the Cartan theater idea is just begging the question. In that if there is a deux ex machina then this ghost observer then needs explaining, which requires another nested layer and you have an infinite nested series. If there were such a ghost then it must also act on the physical body. In doing so it ultimately changes the energy and momentum state of particles. This would then lead to a violation of basic physical principles. So you might then say this ghost is “smart” and just transfers energy and momentum from one particle to another. However, if you conserve momentum that way then by Noether's theorem this means there is a symmetry involved, which means this ghost is just a stand in for some physical process.

      Daniel Dennett wrote a book Consciousness Explained that gives a good set of arguments that may or may not be right, but they are good ways of thinking about this. He basically advances the idea that consciousness emerges from a heterophenomenology of different neural drafts of how the exterior world is perceived or how the person will act on it. This is sort of Darwinian in a sense, and the winning draft is what appears as this “illusion”:we call consciousness. In this setting consciousness is an epiphenomenon that has no actual function on the world; it is an illusion. This might turn out to be the most accurate concept of consciousness.

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    8. @Sabine Hossenfelder: “We have zero reason to think that consciousness is not physical. The brain is made of particles and a theory that describes what the particles do will explain what the brain does.”

      Sabine, you are far too quick to dismiss the philosophical complexities associated with consciousness and the mind.

      For example, when we fall asleep, our central consciousness (the eye of our mind) will oftentimes experience an extremely vivid dream – a dream whose phenomenal features seem to be almost as objectively real as the phenomena we experience outwardly in the universe.

      In considering the ontological status of the internal workings of a dream, we are talking about an epiphenomenal feature of the brain that, for all practical purposes, represents a separate dimension of reality unto itself...

      (like a subjectively-based “parallel universe”)

      ...whose inner features would be every bit as inaccessible (un-measureable) as the inner features of one of Everett’s branched universes.

      Now with the above in mind, do you really believe that the three-dimensional phenomenal structures appearing within our dreams...

      (a house, or a tree, for example)

      ...are made of quantifiable (measureable) particles that would be accessible to our measuring devices in the same way that the particles that form the fabric of our brains are accessible?

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    9. Gill,Crowell,Payne

      Qualia are subjective information.
      QM is objective information.
      So Sabine is correct.
      Hints;Unitarity, Lie algebra, Wittgenstein

      Delete
    10. You should start your own blog where such inchoate fluff can be bandied about. Sabine is trying to discuss physics.

      -drl

      Delete
    11. We don't have a good theory of emergence, but we do have some general principles from both physics and computing where we regularly build one abstraction on top of another:

      1. The emergent system can have quite surprising properties, and we don't need to find any essential part of them in the base abstraction. For example we can have life, consciousness, and videogames without needing living atoms, conscious molecules, or Super Mario circuits. It's easy to build an abstraction that runs very general software, but very hard (mathematically impossible) to inspect the software and put bounds on what it can do.

      2. Causality always goes up the stack, from particles to things, from brains to minds, from hardware to software. This is totally compatible with minds or software feeling they have autonomy in the space of their thoughts. But for causality to go down we need to build hooks in the underlying hardware. If my thoughts move my fingers on the keyboard and then the software makes letters appear on the screen, it's because physically there are neurons and circuits for that.

      3. The main requirement for emergence seems to be a mathematically robust abstraction that has enough base complexity to form structure and enough stability to let it do so. A long-lived universe that's bound by mathematics. A planet that doesn't melt every couple of million years. A genetic code that's not destroyed by radiation or heat. An operating system that doesn't keep crashing.

      The point here is that materialism leaves a lot of room for minds, consciousness, free will, spiritual experiences and all that. Really there's a surprising amount of room for emergence and that's a scientific discovery that's still unfolding. We don't like dualism or essentialism manly because they're unnecessary assumptions. You don't need anything special in the underlying physics for life and consciousness to emerge. Isn't that something to be celebrated?

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    12. Most physicists are reluctant to talk about consciousness, and less willing to make consciousness a part of physics. We really do not know with any clarity what consciousness is. It was at one time said that physics would become merged with psychology, but if that is to be that day is quite some time in the future.

      Wittgenstein is so interesting because he is so enigmatic. My father, a professor of linguistics, gave me his Tracticus when I was in high school and I couldn't make bugger all out of it. Even today it is somewhat bizarre, and he seemed to be skirting around issues more clearly laid out by Turing and Gödel. He did say something to effect, "That which we cannot speak we must pass over in silence." The situation of consciousness in physics is about like that.

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    13. Goldman (and anyone else)

      "Qualia are subjective information. QM is objective information. So Sabine is correct."

      My point is that the existence of subjective information (conscious experience) does not seem to be able to be explained purely in objective/physical terms. For example, if a person trapped their entire life in a blue room possessed all knowledge capable of being communicated about the color red (perhaps they had a computer so powerful they could simulate entirely from the standard model a human body interacting with photons of the appropriate wavelength), would that person not learn something new if they actually stepped out of that blue room and saw something red for the first time? They would now know what it's like to consciously experience seeing the color red, information that could not be communicated to them without the subjective experience of seeing red.

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    14. Thomas,

      The logic of the story about Mary the color-blind scientists is so obviously broken that even the guy who invented it later admitted it's not an argument for anything. It confuses having knowledge about the experience X with actually having the experience X. Would Mary have a new experience if she one day actually had red light fall on her retina? Yes. Does this prove that one cannot describe this experience in terms of brain states? No. Because her knowledge of what these brain states look like is not the same as her having those brain states.

      The existence of subjective information is explainable in objective terms. I do not know what makes you think it is not. If you think of a number and don't tell me what number, you have subjective information and nothing about that is incompatible with science.

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    15. Prof. Sabine,
      I suppose I was under the impression that for a purely reductionist theory, there would be no difference between "having all knowledge about experience X" and actually "having the experience X" My understanding was that, if I am thinking of a number which I don't tell you and you have both all information about me and the ability to parse it, you could retrieve the number I was thinking of from my brain state. You couldn't, though, retrieve the actual subjective experience of me thinking of that number. I think you're saying both that: of course you couldn't retrieve that subjective experience, and this doesn't matter because by definition we are two different subjects.

      Also, thanks so much for taking the time to engage with commenters on your blog! I had, until now, considered "Mary the color-blind scientist" such a convincing thought experiment.

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    16. @drl: “You should start your own blog where such inchoate fluff can be bandied about. Sabine is trying to discuss physics.”

      Yes, and Sabine is very good at it.

      However, in this instance, she is discussing the proposed aggregation of constituents that make up the formulation of a “Theory of Everything.”

      To which I pointed out that you simply cannot have a “Theory of Everything” that treats consciousness...

      (which, arguably, is the singular most important phenomenon in all of reality)

      ...as an insignificant by-product of the behavior of particles of matter.

      Now you can refer to this as being “inchoate fluff” if you wish.

      However, if you have any doubt as to the importance of consciousness, then I defy you (or Sabine, or anyone else) to come up with one single reason for the existence of anything whatsoever, if life and consciousness did not exist.

      I mean, what possible purpose could there be for the existence of the multifarious features of the universe (suns, planets, heat, light, gravity, quantum particles, etc., etc.) if there existed nothing to consciously experience such phenomena?

      Again, the point is that you simply cannot have a “Theory of Everything” that does not include - as its most important and critical component - the one and only aspect of reality (consciousness) to which a “Theory of Everything” would actually have meaning.

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    17. It could simply be that computations have feelings and that's all we ever get to know. We may easily rule out any special physics or strange forces or funky math as having anything to do with consciousness, and yet never be able to explain how or why some computations have feelings. We might get clues of what it takes for a computation to have feelings, or what kind, but the mechanism of how subjective experiences arise may remain forever resistant to explanation.

      Another way of saying this is that strict materialism is compatible with panpsychism (although there's nothing to motivate the religious anthropomorphism some ascribe to the latter) and that may a brute fact.

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  16. How much do you value the explanatory power of theories? For example Einstein provided some more accurate formulae for calculating paths in spacetime, but also showed there's no absolute motion or time. On quantum measurement you can take the practical approach (CI) or try to explain what's happening (MWI and others). Recently people are trying to derive spacetime from qbits or abstract parameters in Hilbert space, and if they succeed another explanatory leap may be round the corner.

    What do you think of these efforts? From reading your blog one could take the message that only predictions count and maybe explanations are fluffy philosophy? In this case we only care about emergent spacetime if it provides a more accurate theory of gravity. Or is it that predictions are objective and whatever one thinks of the explanatory value is subjective? For the non-physicist certainly the explanatory leaps (knowing how the world functions, how big it is, where it came from, etc) are very interesting.

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  17. Great presentation! Listening to your explanation and reading the text is very helpful. Perhaps a discussion of the differences in the geometric conception of space by the theories would further clarify the situation.

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  18. "The description of large objects in terms of their fundamental constituents does not actually give us much insight into what the large objects do."

    Does this mean that reductionism breaks down at some scale?

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    1. No, it just means that you don't learn a lot from writing down the standard model Lagrangian for every quark and gluon and electron in the human body.

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    2. Ok, but why?
      Is it just a practical limit of the computations needed to derive any macro property from a large number of micro constituents, or is there a more fundamental problem, where even in theory there's no way to derive such macro property from only the laws governing the micro costituents?

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    3. "even in theory there's no way to derive such macro property from only the laws governing the micro constituents" (Claudio)

      That seems to be the case. Our theories are just different kinds of tools in a big toolbox that we use to build models for (as we parse the world) quantum, chemical, biological, psychological, sociological (and whatever else) domains. We are floundering about, just getting by the best we can.

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  19. I am more and more convinced that the search for a theory of quantum gravity is a mistake. The mistake that many physicists do is to misunderstand the EPR argument and Bell's theorem. They think that they have to choose between locality in realism, they choose locality (so they trash realism) and as a result they believe that the classical framework must be wrong, General Relativity must be wrong so it has to by "quantized".

    Let's take a look at EPR's reality criterion:

    "If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of reality corresponding to that quantity."

    Let's formulate the argument in a context of an EPR-Bohm experiment with spin 1/2 particles where the measurements are performed in such a way that a light signal cannot get from A to B:

    1. It is possible to predict with certainty the spin of particle B by measuring particle A (QM prediction).

    2. The measurement of particle A does not disturb particle B (locality).

    3. From 1 and 2 it follows that the state of particle B after A is measured is the same as the one before A is measured (definition of the word "disturb")

    4. After A is measured B is in a state of defined spin (QM prediction)

    5. From 3 and 4 it follows that B was in state of defined spin all along.

    6. The spin of A is always found to be opposite from the spin of B (QM prediction)

    7. From 5 and 6 it follows that A was in a state of defined spin all along.

    Conclusion: QM + locality implies that the the true state of A and B was a state of defined spin. The superposed, entangled state is a consequence of our lack of knowledge in regard to the true state. So, QM is either an incomplete (statistical) description of a local deterministic hidden variable theory or it is non-local.

    Let's take a look at Bell's theorem now. What is less known is that Bell was not a replacement of EPR, but a refinement of it. The purpose of Bell's theorem is to choose one of the two remaining options after EPR (local realism and non-locality). If one agrees that Bell's theorem successfully ruled out local-realism the conclusion would be that physics is non-local. But most physicists forgot EPR and they put back on the table non-realism, so they arrive to the wrong conclusion that realism has to go.

    The correct view however is to observe that the independence assumption in Bell's theorem is false in theories with long range forces (like classical EM and GR) so one can simply ignore Bell and fall back to the most reasonable position after EPR, that QM is incomplete.

    In the light of the above argument, the first step to unification should be to find the local hidden variable theory that gives QM as a statistical limit, not to introduce superpositions in gravity. The fundamental description provided by this hypothetical hidden variable theory (which has to be a field theory to avoid Bell) should be much easier to combine with GR.

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    1. "3. From 1 and 2 it follows that the state of particle B after A is measured is the same as the one before A is measured (definition of the word "disturb")"

      No, before the measurement on A, B was in a superposition of states, and this superposition was entangled with the similar superposition of A.
      Hence, the rest of your logic chain is false.

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    2. Claudio, after A is measured, according to QM, both A and B are left in a spin eigenstate - a state of defined spin. If we assume locality (the measurement of A did not disturb B) it follows that the state of B, before A was measured was the same (state of defined spin). True, QM describes the pre-measurement state as a superposition, but, if we hold to locality, we must conclude that the superposition does not represent the true state of the particle but our incomplete knowledge about it.

      Please pay attention to the argument, there is nothing wrong with it. More, if you deny 3 you can either deny 1 or 2 or show that 3 does not follow from 1 and 2. Disagreeing with the conclusion of the argument while being unable to point out an error in the argument is not logically valid.

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    3. We already know that intuitive physics doesn't apply in the quantum world. What's less appreciated is intuitive logic doesn't apply either. When reasoning that B is in some state, or A doesn't affect B, these may be questionable assumptions. Quantum theories come with both a kinematics and a logic for how to talk about these things.

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    4. Pavlos Papageorgiou,

      "We already know that intuitive physics doesn't apply in the quantum world."

      We do not know that. Such a conclusion is based on false premises and faulty arguments of the type I presented above.

      "What's less appreciated is intuitive logic doesn't apply either."

      What do you mean by "intuitive logic"? I use logic as it is used in all science, physics included. Denouncing logic in order to save a bad argument is a desperate and useless choice.

      "When reasoning that B is in some state, or A doesn't affect B, these may be questionable assumptions."

      I do not need to make any assumption about B being in some state before A is measured. What we know is that after A is measured B is in a state of defined spin. This is a confirmed prediction of QM.

      The assumption that A doesn't affect B is called "locality". Of course you can deny it if you want, but given the evidence we have it does not seem a reasonable choice.

      So, IF you want to preserve locality it logically follows that B must have been in a state of defined spin even before A is measured. This is a conclusion, not an assumption.

      You may believe whatever you like about B before A is measured. Maybe it didn't exist at all, or it didn't have any spin, whatever. If any of the above were true it means that the measurement of A did instantly changed B so you have a non-local phenomenon.

      "Quantum theories come with both a kinematics and a logic for how to talk about these things."

      QM presents us with a piece of mathematics. In itself it tells you nothing about any kinematics or logic, it's just a formula. The next step is to decide what relationship this formula has with the real world and EPR criterion of reality is a crucial tool to that end. This simple argument tells us that the world described by quantum mechanics is a classical world (objects are always in well defined states) and that the quantum formalism is an incomplete, statistical description of this world. The alternative is a non-local world that looks like voodoo.

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  20. Is the "Theory of Everything" not only another kind of search after symmetry and beauty in nature?

    Why should there be only one idea underlying the whole world? Is such a thinking more than "kindergarten"? Why?

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  21. Reminds me of the story about the student that studied only about the rain worm for his biology exam. On the exam he got a question about the elephant. His answer was 'The elephant is a mammal with a trunk that resembles a rain worm. The rain worm is...'

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    1. Sorry, this was meant as reply to Andrei.

      Delete
  22. It would have been nice to have a TOE after merely 150 years since we got serious about this physics business. The only question left then would have been how to keep our minds busy in the eons ahead.

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  23. The concept of "a theory of everything" is unsound, both scientifically and philosophically. It's not just that we wouldn't know if we had found it (because there could always be something else discovered later), but that (1) "everything" is an impossibility ontologically and epistemologically, (2) it would be the end of science and the end of a humanity that would have lost its curiosity.

    Altogether a crazy idea, that is useful only for grant money,

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  24. The best synopsis of reasoning to Quantum Gravity is the essay by Bryce DeWitt, Quantum Gravity:The New Synthesis (pages 680-745, Einstein Centenary Survey). DeWitt wrote "the results obtained are too beautiful not to be believed." and his concluding paragraph argues for "the answer proposed by Everett."

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    1. A friend of mine has a t-shirt that says: Science Doesn't Care What You Believe. Someone should apprise Mr DeWitt of that sentiment.

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    2. Advances in scientific understanding have often been in the form of elegant simplifications.

      There is no blasphemy in the belief that advancing beyond the Standard Model may lead to something that is more elegant and simpler.

      The fallacy is the perversion, indeed inversion, of that idea into the principle that says "my idea must be true BECAUSE it is simple and elegant".

      There is an informal principle in aerodynamics which says that if an aeroplane looks right it will fly right. This idea, based on long experience, is that the best aircraft designs often look beautiful too. It was never intended as a principle that one could throw the aerodynamic rule book in the bin, design an aesthetically pleasing aeroplane, and the job was done.

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  25. Regards "Dreams of a Final Theory" Steven Weinberg: "...known as asymptotic safety, according to which, although there are an infinite number of interaction types their interaction strengths are not arbitrary but fixed by the condition that at very high energy these interaction strengths (suitably defined) all approach finite constants. So far this has passed some mathematical tests, but has not led to any specific theory." (May 2015, Sci Amer Blog).

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  26. Interesting if it could come out of quantum computing ...

    Bulk entanglement gravity without a boundary: Towards finding Einstein’s equation in Hilbert space
    https://journals.aps.org/prd/pdf/10.1103/PhysRevD.97.086003
    ChunJun Cao* and Sean M. Carroll

    "We will also argue that more general emergent geometries can be assigned to quantum error correction codes, where the code subspace
    naturally separates the geometric and matter contributions to entanglement entropy."

    * Chales Cao - https://quics.umd.edu/people/charles-cao

    "His current research focuses on an approach to quantum gravity where space-time and Einstein gravity can emerge from quantum manybody systems."

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    1. Of particular interest I think is the many-body work of Perelman and the Toda lattice. This has exceptional E8 symmetry, with the 8 weights as the golden field quaternions. When this symmetry is broken one can get the Zamalodchikov mesons as a split form of the golden field quaternions, or removal of state degeneracy, and these I think play a role with high temperature super-conduction. Think of phonons interacting with electrons, where when symmetry is removed, such as U(1) breaking in standard BCS superconductivity, there is a mass state for the polaron or Cooper pair. In a general setting this may be with the AdS, where the unbroken symmetry is on the boundary or think of the UV end of RG flow on the stretched horizon of a black hole, and off this boundary or horizon one has broken symmetry.

      With quantum gravitation the AdS_5 = O(4,2)/O(4,1) exploits the SO(4,2) ~ SU(2,2) as the center of the QECC. The E8 constructs an (3, 5, 8) error correction or Hamming distance correcting system. This may be the symmetry that transforms quantum gravitation states in a way that conserves qubits. Just with the high-TC case above, the broken symmetry phase would just mean that lost qubits are simply carried off in other degrees of freedom, such as transverse modes of fermions with mass.

      Entanglements such as Bell states are just individual quantum states of spin that are lost in the entangled state. For 4-qubits we have the states on CP^3, and the CP^1×CP^1 \subset CP^3 are separable states and the remainder are entangled. In general such quantum states are mixtures of these. We can then have entanglement phase diffused out into a reservoir with partial entanglements. As an hypthesis we might suppose that the symmetries of entanglements are dual to those of gauge fields or gravitation, and this would then mean entanglement spreading has some relationship to the breaking of symmetry. This might then carry to the breaking of E8 into Zamalodchikov mesons.

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  28. IMO a real theory will always be testable. It will suggest methods of being tested that are not even clear at the moment. They emerge from the new structure. There is no clear answer to your question yet because there is no clear path to harmony between SM and GR. One is ontologically rooted in the Yang-Mills potential and the other in Riemannian geometry. There cannot be a unification unless those ontologies can be melded. Recasting GR as a tetrad gauge theory to make it more like YM doesn't work. That leaves very few alternatives. What would represent progress is to find a role for the YM potential within the GR ontology. That points to a reassessment of Weyl's original idea. (It can be done.)

    -drl

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    1. drl,

      “... Weyl's original idea. (It can be done.)” – what do you mean by this?
      And yes, had Weyl instead used the imaginary i as in e^(i...) Einstein probably would not have scolded him for his gauge theory – see here (enlarge “+” to see more pages).

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    2. Yes I know that work. I mean, in the context of a conformal extension to Riemannian geometry. It cannot work in a simple form in 4D (thanks Albert), but it can in 6D (3 timelike dimensions). My at the moment not completely formed idea is that there are amplitudes (original Weyl) and phases (YM gauge) involved.

      -drl

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  29. If gravity is Asymptotically safe then does this mean it gets weaker at higher energies like QCD?

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    1. This comment has been removed by the author.

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    2. It seems that this would change the dynamics of Planck-scale and slightly-above-Planck scale black holes in such a way that quantum gravity wouldn't behave like general relativity any more in that regime. Have people thought about this at all?

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    3. Since the gravitational coupling = GMm that this should really scale with the square of energy. So quantum gravitation should become stronger at high energy. If one uses the Planck scale as a blunt tool this should cut-off at the Planck energy.

      Renormalization theory implements counterterms that cancel divergent loop integrals. With quantum gravity number of the counterterms required to eliminate all divergences proliferate infinity. Asymptotic safe gravity imposes a UV fixed point for RF flow on the UV critical surface embedded in the theory space. In the theory space there is a divergence at UV, but the observable aspects of q-gravity are on the UV critical surface. This is really a veiled form of holography.

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  30. I don't think we necessarily need testing since we already have the basic information. What is needed is a consistent theory that produces all the phenomena of reality (particles masses and strength of forces) from a basic principle that shows all the relations.

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  31. There was a scientist who promised her Everything, but all she got was a Theory.

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  32. Hi Sabine,
    the Asymptotically Safe Gravity is what looks to me more of a bottom's up approach to the problem of quantum gravity that does not require supersymmetry. I have read up elements of the theory (Friederich 2017) and in the article it is mentioned that it attributes fractal like properties to space time. There has been some recent work disproving the Penrose cosmic censorship conjecture that seems to lead to the conclusion that a fractal space time exists beyond the Cauchy horizon in a black hole (Quanta magazine May 17 2018 Kevin Harnett). Has anyone looked at a possible link between the two and using the Cauchy horizon as a boundary for the AS theory somewhat in the same spirit Einstein used Mach’s principle.

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  33. @Lawrence Crowell: Why do we call the cosmological constant, the cosmological constant. Obviously we do this because Einstein introduced it as a term in his gravitational field equations.

    However, we don't call gravity by the gravitational constant G that measures its strength. Likewise we *ought* not to call the cosmological constant, a cosmological constant but by a name that properly names what it is.

    Could we think of it as a force, so calling it the cosmological force? If so, then Einstein discovered a new force, which adds to his already mythical status...

    I can't say I think too much about Wittgensteins Tractatus, after all, he himself repudiated it in his later philosophy. It's a logicists view of the world from the idealist point of view. 'The world is my world', yes, but it is also mine, yours and Gods. Spinoza did it better and he himself was a footnote to Platonism. I suppose Wittgenstein cut God out of that particular equation hence his appeal to generations of rationalists. They ought to make that more explicit, like calling it athiestic philosophy, rather than naming it as philosophy as such, as it gives the mistaken impression, that it is *the* philosophy. Far from, in my humble opinion.

    -----

    I like the videos. I recall a time that physicists, as a joke, were saying that the equation for the theory of everything was near at hand, and it ought to be able to print it on a t-shirt. But of course, that would mean nothing, after all, we can print Newtons Laws of motion on a t-shirt, but that would not mean very much unless one knows how to apply them - that is actually get the physics out of them. Thats takes several years of a physics education, many lectures and several fat books, and which is the compressed essence of the thinking of several generations of physicists.

    It just occurred to me, just now, whether quantising gravity will turn out to be akin to the fifth axiom in Euclidean geometry, that is the whole paradigm shift was required. We can only wait and see. Personally, I happen to like LQG where we have area and volume operators whose eigenvalues give the quanta of area and volume. I really ought to spend more time learning about it.

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    1. The cosmological constant is called a constant because of the legacy with Einstein. Einstein realized that matter in the universe would all implode, which lead to uncomfortable conclusions. Einstein did not like the idea of the Schwarzschild solution because it appeared to violate the principle of least action with conservation of phase space volume. So Einstein thought it a nice idea to add this curvature term Λg_{μν} to the Einstein field equation

      R_{μν} - ½ Rg_{μν} + Λg_{μν} = (8πG/c^4)T_{μν}.

      This has the effect of generating a flow of space, or dynamics of spatial surfaces. For Λ ≥ 0 this generates a repulsive flow, which corrected the problem Einstein saw. However, Hubble demonstrated the universe was expanding, and this seemed to remove the need for this constant. It was Einstein's self-admitted greatest blunder. However, it turns out this term is real and defines the Hubble parameter as H^2 = √(Λ/3), and is large enough to repel the universe into accelerated expansion. For Λ < 0 this gives a strange type of spacetime related to the Poincare ½-plane or disk that we call the anti-de Sitter spacetime.

      We generally have for this constant a situation where Λ = Λ(φ, dφ/ds) for φ a scalar field. Now Λ is no longer strictly a constant. On the high energy vacuum, where the vev of this scalar field is large, this cosmological constant is over 100 times larger in magnitude, eg 10^{110} or so, than it is now. This is a ferocious accelerated expansion. However, the small value now is where this scalar field has some small expectation and is no longer varying so Λ = Λ(⟨φ⟩) and we have this as a constant FAPP. There are some ideas about this transitioning into a Λ = 0 configuration or at least smaller than it is now. This would then be some bubble of vacuum energy transition that would be sweeping through at the speed of light. I am not sure about those idea. There is also the quintessence hypothesis, which has been to some degree ruled out, that this scalar still evolves and there are regions of the universe with different vacuum energy.

      I can't think of who it was who said that with Alan Turing we got the computer and with Wittgenstein we are left with just Wittgenstein. His ideas are odd, and he back away from the Tracticus thesis later on.

      If quantum gravity is like the Euclid 6th axiom it would just mean various theories so far are wrong.

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    2. "The cosmological constant is called a constant because of the legacy with Einstein."

      Well to my understanding, the CC is constant because it can't change. It represents the arbitrary length scale in Riemannian geometry. The covariant derivative of the metric is identically 0 for all metrics, so you can put a constant in front of it and add it to Rmn and get something that will result in the same Rmn;p = 0. Because parallel transport only changes the phase, not the amplitude, of a vector, there is a global, arbitrary length scale. Einstein was right (of course) that the CC was a critical defect of his theory and he worked his entire life to be rid of it.

      I know of a field theory based on Weyl geometry in 6D that does not have this degeneracy, because parallel transport changes both amplitude and phase. If you want Einstein GR in 4D to be a limit, then the CC has to be zero. Thus the appearance of the arbitrary non-zero CC represents the decoupling of the metric with the Weyl gauge field.

      -drl

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    3. Oops - in the above I meant Gmn (Einstein tensor Rmn - 1/2 R gmn), not Rmn.

      -drl

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    4. drl,

      “... arbitrary non-zero CC represents the decoupling ...” - sounds interesting - a bit in reverse like the central extension of SR [K,P]=(1/c²)H when 1/c² is not zero anymore and therefore couples time and space.

      Maybe your 2 extra “timelike dimensions” are related to the circumstance that QM needs complex numbers?
      I know you only like the SM “as a classification scheme”, but it is precise up to 8 digits, so it cannot be too wrong.

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    5. Typo: [K,P]=i(1/c²)H
      Forgot an imaginary i - those tiny little buggers are terrible important.

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    6. The cosmological constant is a measure of so called dark energy, which for good reason is just the vacuum energy of the observable universe. This is determined by the vacuum expectation value (VEV) of a scalar field that in turn determines this cosmological parameter. If this VEV is absolutely stable then the CC is as you say constant. On the other hand the observable universe is FLRW or de Sitter (like) and these spacetime vacua are not perfectly stable. They may be stable for 10^{10^{10^{70}}} years, which is a terribly long time, but it is not eternity.

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    7. @LC I understand the creative re-interpretation of the CC in the modern world. What I am pointing out, is what it actually is, in the context in which it was born. It is not a quantum field, it is not dark energy, it is the ghost of a structural element of Riemannian geometry as used for the basis for a theory of gravitation. What I stated above about decoupling is not a conjecture, it is an accomplished calculation.

      -drl

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    8. @reimond, This isn't the place to discuss all that (contact me offline and we could do that), I just wanted to point out that the modern interpretation of the CC is very much opposed to its actual role in the theory of gravitation. The folklore about "Einstein's blunder" is part of all that. How this came about is not entirely clear to me, but I would guess that the prejudice against geometry as a foundational element in gravitation, and the repeated attempts to reclassify GR (e.g. as a spin-2 quantum field) to be rid of Riemannian geometry in the effort to quantize gravitation, are the ultimate source. The most interesting explicit view of this schism is found in Weinberg's book "Gravitation and Cosmology", in which he refers to "the geometric analogy" (! - ch. 6 sect. 9) in the last paragraphs before introducing the EFE, and then develops the theory along the very geometric lines he discredits in that preamble. In its actual context in which GR is useful, and within the ontology GR actually defines, the CC is a very simple thing, and is indeed an arbitrary constant.

      -drl

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    9. The cosmological constant can simply be seen as an integration constant. From the Bianchi identity as ∇_aT^{ab} = 0, the Einstein field equation is the integral of this and the cosmological constant is just a constant of integration. This means is "flaps in the breeze" so to speak.

      This means this is set by some exterior information or data. This can be either observational data on the Hubble constant or maybe some sort of theory that supersedes general relativity.

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    10. @LC It is not a constant of integration. No integration is involved. It is a degeneracy of the possible field equations consistent with the Bianchi identities Gmn;n = 0. The degeneracy arises in the arbitrary length scale in Riemannian geometry.

      -drl

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    11. PS for some insight, start here.

      Veblen, O. - Projective and Affine Geometry of Paths, PNAS December 1, 1922 8 (12) 347-350

      See followup work with T. Y. Thomas and L. P. Eisenhart. This is not even physics.

      -drl

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  34. Latest (Nov 2019) journal entry (Classical and Quantum Gravity) into the asymptotic approach to gravity: "Over the last years the asymptotic safety program has matured into a serious candidate for a quantum theory of gravity compatible with observations..." (Knorr, Ripken, Saueressig).

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  35. Eugene Wigner: "I believe that this is true: we have no right to expect that our intellect can formulate perfect concepts for the full understanding of inanimate nature's phenomena." (1950, Limits of Science).

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    1. That's what people who think they are really smart say when they can't figure something out.

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  36. My impression is that the ultimate workings of the universe aren't "testable," because the behavior is algorithmic, not formulaic ... you would need to use half of the universe to predict the behavior of the other half!
    Formulaic descriptions can be arbitrarily close to fully accurate, but there will always be edge cases not accounted for using mathematical shortcuts.

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  37. @Lawrence Crowell: I realise that its due to Einstein that we still call it the cosmological term. I was suggesting that we call it something that signifies what it means.

    You write 'it turns out that this parameter is real .and defines the Hubble parameter ... and is large enough to repel the universe into an accelerated expansion.'

    Thus my suggestion above of calling it a cosmological force, which in fact is a form of 'anti-gravity'. Perhaps it's a suggestion that might catch on, but I won't hold my breath.

    You also write:

    'On the high energy vacuum where the vev of this scalar field is large, this cosmological constant is over 100 times greater in magnitude ...'

    Presumably this is the high energy vacuum that obtains in the very early universe. From what you write it seems that in QFT the cosmological constant of gravity is identified with the vacuum energy and which is understood as a scalar field. (Which, given what I've written above, is just further evidence we should not be calling it a constant and a better name should really be chosen).

    Personally, I think Turing gets too much credit for inventing the computer. Its like Newton getting all the credit for thinking up universal gravitation whereas Copernicus already had that idea:

    "I am at least of opinion that gravity is nothing more than a natural tendency implanted in particles … by virtue of which, they, collecting together in the shape of a sphere, do form their own proper unity and integrity. And it is to be assumed that this propensity is inherent also in the sun, the moon, and the other planets - Copernicus on the idea of universal gravitation, 1543 (quoted by Mach)"

    Newton 'just' quantified it.

    Dark mass is one solution to the problem that Einsteins gravity does not correctly predict gravity on a cosmological scale. Another possibility is simply that Einsteins theory itself has to be modified at that scale. Perhaps this is why quantised GR turns out to be not renormalisable - its the wrong theory of gravity to quantise!

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    1. I have a somewhat different idea about asymptotic gravity. Weinberg formulated it so it is similar to QCD. This is in some ways a brilliant idea. However, it is clear that gravity is "free" at low energy and self-binding at extreme energy where quantum black holes appear. So the RG flow is opposite QCD, but this is alright if we have a duality between IR and UV physics. As I keep reiterating

      UV-quantum gravity = IR QFTs

      is a way of writing the Einstein field equation.

      We could also think of gravity and QCD related in SU(4) with 15 roots, 8 of which are the RYB colored gluons and the other 7 correspond to what might be called OGP (orange, green, purple) gluons. These other gluons form entanglements or glue-balls so a bipartite entangled colorless gluon system is a graviton. These also have opposite RG flow. The trick of course is to make the flip SU(4) --> SU(2,2). This is an interesting toy model to consider, even if it is probably not entirely correct.

      The most important this is that if gravitation is a manifestation of entanglement, then we might think of gravitation as one that maximizes entanglement entropy. Entanglement entropy is energy E = S + δS, with S = -sum_np_n ln(p_n) the standard von Neumann-Shannon quantum entropy and δS a quantum correction. This quantum correction is the tough part, and with black holes it corresponds to a quantum extremal surface that is not necessarily coincident with the event horzon. Instead of gravitation being an extremal principle for paths in curved spacetime or the action principle for spatial surfaces, it has an underlying extremization of entanglement entropy.

      In this way the underlying quantum principle of gravitation is determined in this extremal quantum surface, which is a holographic surface, with it being mapped in what is called a majorization to the stretched horizon in LOCC maps. This is also the surface where the RG flow occurs, and this is where things should be "nice" or acceptable. In the bulk space is the emergent classical gravitation that has nonlinearities that are no amenable to quantum physics. Weinberg's UV critical surface is I think a quantum extremal surface that is well behaved, while the embedding configuration space permits classical properties that are not themselves quantizable.

      Copernicus did not have gravitation. Kepler was closer. The period^2 ~ radius^3 of Kepler's 3rd law allowed Newton with his derivation of centripetal force F = mω^2r to equate this "ma" part with the force F = kr^n to easily find the exponent as n = -2.

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    2. Lawrence

      A lot of what you write is over my head, especially about holography effects for gravitation. But I am just an amateur trying to keep afloat. I have written amateur papers online about using entanglement to make metrics (and those metrics seem to mimic gravitation effects).

      Also I have written online and in other nearby threads about the dependency of electric charge on QCD colour charge. That is a broken symmetry which is true for my preons but is not true for quarks after aggregation.

      IMO weak charge is a specialised form of electric charge and hence should also depend on QCD colours. The specialisation IMO being needed because of the weak fields inhabiting the vacuum eg higgs.

      So in my toy model the four forces become gravity and the colour-dependent forces. I have also suggested that gravity is also a colour-dependent force where the colours are very weak in intensity. And gravity could be an amalgam of QCD, electric and weak but all at much weaker strength, like a weaker generation of bosonic forces in the same way that the fermions lie in different generations. [In a post somewhere near here you cite fermion effects as maybe preventing a big crunch. Penroses' CCC circumvents that by having no fermions left at the crunch or end-of-cycle. With a preon model one can use the same bag of preons to make fermions-only, or bosons-only, or a mixture.]

      I have another amateur paper on negative masses, with a computer simulation which seems IMO to reproduce both dark matter and dark energy effects. That can be related to a weak-colour gravity using spin 1 gravitons. A problem you and others identified with my colour gravity effect is that GR shows a graviton should be spin 2. I have a spin 2 graviton in one model, but spin 1 sits better with a TOE having all forces basically looking alike, i.e. all colour forces at high energy. But I realise that this might be a beauty prejudice.

      QCD exhibits asymptotic freedom which has a weaker force at closer quarters but at larger distances has a stronger force which confines the quarks to remain close together. I take it that this needs to be reversed (you mention opposite RG flow, which may or may not be connected?) for gravity to be coloured. You mention 7 extra OGP bosons.

      I used a very weak form of RGB for gravi-colour where a QCD-red quark can be any gravi-colour. [Regarding a separate post ... I am very unclear how opposite flow relates to 'asymptotically safe', if at all.]

      QCD-colour forces must be the overriding determiner of how atoms in a say planet are QCD-coloured. Gravi-colours, if they are so weak, would be distributed fairly chaotically as a subservient effect. This may mean that an effect comparable in principle to quark confinement could not occur with gravitation as say a star and a planet would never be pure gravi-colours which attracted strongly.

      Anyway, a spin 1 graviton with associated positive and negative masses could show the dark matter and dark energy effects whatever the source of the force, coloured or otherwise.

      As the thread is about testabiity, I suggest that anomalies in the nature of dark matter may eventually be revealed to show whether or not negative mass is at work.
      No idea how to test for gravi-colour.

      Austin Fearnley

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    3. @Lawrence Crowell:

      Is that reply above directed to me? Because your reply makes no sense given what I was asking and referring to - For example I make no mention of asymptotic gravity - I only heard of it recently and this was due to this blog.

      Of course you could be writing in 'code', in which case your reference to Turing above would be apposite, because amongst his many accomplishments he was a key code-breaker of the WWII German Enigma code when he was at Bletchly Park as part of the war effort and which - and this makes me really pissed off - makes his eventual murder by the British security forces a complete scandal. He actually committed suicide, but given the allegations that he was hounded by the British security services I personally consider it as murder. The time is well overdue for a proper public investigation into this case.

      It is only when I got to the end of your rambling post that I realised that it must have been directed to me because it is only then you decide to mention Copernicus. The point I was making, and which you decided against even acknowledging, is that I considered that Newton getting all the credit for gravity was risible, given the contributions by others. What this really points is just how badly the history of sciences is taught to the point where it has become a tissue of lies and half-digested myths.

      As for your statement: 'UV-quantum gravity = IR QFTs is a way of writing Einsteins field equations' is just another way, it seems, of writing one of your key obsessions surely - I mean AdS/CFT duality. Why not refer to it simply by its traditional name?



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  38. Physicist Eugene Wigner does not have to "think he is really smart." Eugene Wigner had a brilliant mind. I am inclined--after merely 50 years studying physics-- to believe Eugene Wigner is correct in his "belief." The dilemma for physicists (smart or otherwise) would be for them to have achieved a "full understanding of inanimate nature's phenomena," for then they would be out-of-business. Physicists, indeed, have "no right" to expect that they can achieve a full understanding.

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  39. Thank you for the link to causal fermion systems. I was not aware of that. Looks really promising to me, for whatever that may be worth. I like it! It must be true! Just kidding about that last part. I do like it but it still might not be the best approach.

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  40. @Gary Alan: I doubt that Wigner himself would have put it in those terms given what he says in his autobiography. He felt that his own accomplishments was quite modest.

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