Friday, June 12, 2015

Where are we on the road to quantum gravity?

Damned if I know! But I got to ask some questions to Lee Smolin which he kindly replied to, and you can read his answers over at Starts with a Bang. If you’re a string theorist you don’t have to read it of course because we already know you’ll hate it.

But I would be acting out of character if not having an answer to the question posed in the title did prevent me from going on and distributing opinions, so here we go. On my postdoctoral path through institutions I’ve passed by string theory and loop quantum gravity, and after some closer inspection stayed at a distance from both because I wanted to do physics and not math. I wanted to describe something in the real world and not spend my days proving convergence theorems or doing stability analyses of imaginary things. I wanted to do something meaningful with my life, and I was – still am – deeply disturbed by how detached quantum gravity is from experiment. So detached in fact one has to wonder if it’s science at all.

That’s why I’ve worked for years on quantum gravity phenomenology. The recent developments in string theory to apply the AdS/CFT duality to the description of strongly coupled systems are another way to make this contact to reality, but then we were talking about quantum gravity.

For me the most interesting theoretical developments in quantum gravity are the ones Lee hasn’t mentioned. There are various emergent gravity scenarios and though I don’t find any of them too convincing, there might be something to the idea that gravity is a statistical effect. And then there is Achim Kempf’s spectral geometry that for all I can see would just fit together very nicely with causal sets. But yeah, there are like two people in the world working on this and they’re flying below the pop sci radar. So you’d probably never have heard of them if it wasn’t for my awesome blog, so listen: Have an eye on Achim Kempf and Raffael Sorkin, they’re both brilliant and their work is totally underappreciated.

Personally, I am not so secretly convinced that the actual reason we haven’t yet figured out which theory of quantum gravity describes our universe is that we haven’t understood quantization. The so-called “problem of time”, the past hypothesis, the measurement problem, the cosmological constant – all this signals to me the problem isn’t gravity, the problem is the quantization prescription itself. And what a strange procedure this is, to take a classical theory and then quantize and second quantize it to obtain something more fundamental. How do we know this procedure isn’t scale dependent? How do we know it works the same at the Planck scale as in our labs? We don’t. Unfortunately, this topic rests at the intersection of quantum gravity and quantum foundations and is dismissed by both sides, unless you count my own small contribution. It’s a research area with only one paper!

Having said that, I found Lee’s answers interesting because I understand better now the optimism behind the quote from his 2001 book, that predicted we’d know the theory of quantum gravity by 2015.

I originally studied mathematics, and it just so happened that the first journal club I ever attended, in '97 or '98, was held by a professor for mathematical physics on the topic of Ashtekar’s variables. I knew some General Relativity and was just taking a class on quantum field theory, and this fit in nicely. It was somewhat over my head but basically the same math and not too difficult to follow. And it all seemed to make much sense! I switched from math to physics and in fact for several years to come I lived under the impression that gravity had been quantized and it wouldn’t take long until somebody calculated exactly what is inside a black hole and how the big bang works. That, however, never happened. And here we are in 2015, still looking to answer the same questions.

I’ll restrain from making a prediction because predicting when we’ll know the theory for quantum gravity is more difficult than finding it in the first place ;o)


  1. It seems to be rooted in the Hamiltonian formulism.

  2. "how detached quantum gravity is from experiment." The only predictive gravitation is geometric, thus 90 days in a geometric Eotvos experiment. Everything exactly cancels except geometry where the most extreme composition and field contrasts are inert. A non-zero signal is definitive. "Experimental Search for Quantum Gravity: The Hard Facts" Green's function imposes mirror-symmetry. There is only contrary evidence that the vacuum is exactly mirror-symmetric toward matter. Baryogenesis eludes theory, Sakharov conditions or otherwise.

    Ashtekar (plus Immirzi) is GR chiral decomposition. The
    Coupe du Roi also shows how a perfectly symmetric ball has hidden structure.

  3. "...discourage people who follow long standing established research programs..."

    So, people like Lee Smolin? He's been doing LQG for how long and GR still hasn't been derived from it in any kind of limit? He says, "The emergence of general relativity from the semiclassical approximation of the path integral is understood." which is clearly a lie. What does he mean by "understood"? If it hasn't been explicitly shown, then it is not "understood".

    It doesn't surprise me that he says all these things. I remember a while back Lee Smolin hyping up a particular research program concerning braids in LQG and how it was going to show us that the Standard Model can be shown to emerge from the dynamics of LQG. Nothing has come from that.

    At this point, I take very little of what he says seriously.

    To be honest, I think all these research programs are more or less worthless. The program that comes out ahead is string theory since, at least, it allows for some kind of unification.

    Even quantum gravity phenomenology is semi-worthless since, by definition, quantum gravity cannot be observed with current methods. Quantum gravity can only be observed at extremely high energies, way beyond what we are capable of.

    Oh well....

  4. Call me an optimist.

    It is taken a few decades, but we are approaching an era in which voluminous and precise astronomy observations, coupled with computational power that would have been almost unimaginable when the Standard Model was formulated and Sting Theory started to take shape, can provide genuine empirical tests of various proposals for inflation; dark energy/cosmological constant; the behavior of particles in very strong field regime of white dwarfs, neutron stars, and black hole fringes; the possibility the dark matter phenomena are cause is part or predominantly by modifications to gravity.

    The litany of null results out of the LHC for dark matter candidates or other new physics also isn't nothing. A huge swath of parameter space for new physics has been definitively ruled out.

    A space telescope program with an LHC scale budget could take that to a whole new level. Something as simple as sending space telescopes to opposite ends of the solar system to allow more observations to be calibrated against parallax measurements could greatly reduce systemic error in gobs of data that we already have in hand.

    Progress in particle physics and engineering has also pushed our instrumentation that allows us to test every detail of gravitational phenomena at the solar system level to almost maximal theoretically possible precision.

    We also have a very deep bench of investigators worldwide who through mechanisms like arVix are sharing information with each other with near theoretically minimal friction. There are more new publicly available papers on GR and quantum gravity written by well trained PhDs each week than there would be for whole years for the first half century of GR.

    The biggest threat we face, I think, is group-think. Because so many thousands of investigators are so intimately in touch with what each other are thinking, the risk that conventional wisdom will discourage out of the box thinking and destroy the benefits of having a legion of skilled people doing the work is a very real one. There might be something to be said for figuratively locking a few hundred of the most innovative and divergent physicists in a box at some institution in the middle of nowhere to at least have two independent communities of investigators to pursue their own sequence of insights for a few decades, imitating for the theoretical community the notion of having dual independent experiments at Tevatron and the LHC.

  5. The class of solutions that Lee and Sabine (and 99.99% of the physics establishment) all seem to think have weight have one major problem - they all assume that QM is some sort of bedrock. The fact that 1000's of PhDs and postdocs have been spent on chasing the Quantization of Gravity should mean one thing.

    It can't be done.

    Lee does show some light when he says: "I believe that quantum theory requires a completion, in a deeper theory that allows a complete description of individual processes. I see no other way to resolve the measurement problem."

    There are not many non linear physical theories which we know work, other than General Relativity. Yet the world of physics is so stuck in the linear QM world that it is GR which is assumed to be some approximation, when it is more than likely that the exact opposite is the case. Recent experiments show that QM like behaviour can emerge from classical fields, so it follows that GR may have the strength and flexibility to build QM, rather than the other way around.

  6. I guess you're not posting my comment. It's not like I said anything inappropriate. I just offered a critical view. Last I checked, this wasn't The Reference Frame.

    Oh well...

  7. "... how detached quantum gravity is from experiment ..." If the space roar number is 6 ± .1 then MOND from string theory with the finite nature hypothesis yields 4.99 ± .03 for the number in the photon underproduction crisis. Is string theory with the finite nature hypothesis a revolution waiting to happen? "In the physics I have learned there were many examples of where the mathematics was giving infinite degenerate solutions to a certain problem (classical mechanical problems e.g.) There the problem was always a mistake in the physics assumption. Infinity is mathematical not physical, as far as I know." — Maria Spiropulu See Maria Spiropulu, THE LANDSCAPE, . Is MOND empirically valid because a complete infinity does not occur in nature?
    “The failures of the standard model of cosmology require a new paradigm”, Jan. 2013

  8. Vince,

    Sorry for the wait, but I can't sit at my computer 24/7 and approve comments. Please give me at least 24 hours, more when traveling. I know it's annoying, but I've gotten really tired of all the crackpottery in my comment sections. I generally don't check email between 7pm and 7am.

  9. Tom,

    You didn't actually read what I wrote, did you?

  10. Vince,

    Regarding your comment about qg pheno, you are talking about direct detection, and you're just demonstrating you don't know a lot about the research area if you think that's it. For starters, please read this.

  11. I think you are too impatient. What are 15 years of the life of a person in comparison with the entire life of the cosmos?

    And I think, that a physicist must truly excel in mathematics (and must always be angry of his/her ignorance) and must have a strong mathematical curiosity and a mathematical mind. If one underestimates mathematics compared to physics as you do, one is automatically led astray. Because mathematics is the language of nature and there exists and there shall never exist any other language. Even good experiments, when they are carefully planned, have some kind of mathematical/logical design.

    If one neglects sophisticated mathematics, one neglects seeking for the appropriate means of expression in which a thought in physics can be properly uttered. If one needs to devote time to problems on convergence and stability it is because the problems in physics that one is addressing require the consideration of those problems. A blackhole is a singularity and this has both, a mathematical and a physical meaning.

    Mathematics is also wonderful for the sake of itself and that is the reason why most physicists are attracted by her. And never fatally attracted: Mathematics truly satisfies all human needs of some people (even people who live below a bridge cannot fail to do mathematics if they happen to be mathematicians: I have seen this) and one can speak of a happiness that is so full of joy that one happily renounces to the world of riches, pomps and vanities (the 'physical' world there where it is lacking in humility) for the Platonic world of joy and order with which God delights the mind of those who are strong enough and which appear as weak, poor and masochistic in the world of pomps and riches. All good physicists that I know want always to learn more mathematics and to express things clearly and rigorously.

    A theoretical physicist is a poet of the real things and his/her word is the equation. An equation, where all terms are properly defined, belongs to the realm of mathematics and opens the realm of the infinite. One has to study an equation in itself in order to know where the equation cannot fail to be valid. The unified theory of physics should be valid in every instance. The complement of the set of things explained by theory is God, whose beauty and magnificence cannot be grasped with our words, concepts and equations.

  12. I find the interview with Smolin quite useful and informative, thanks for sharing. And I will also read your preprint.

  13. I have read your article and I have found it very interesting. I think your point is reflected in the quantization condition, where you introduce the field alpha, so that you can tune the quantization condition and decouple gravity as hbar tends to zero (the gravitational coupling constant G being proportional to hbar). I find the idea nice and simple.

    In fact, I had also a very similar approach that I did never try to publish (I am not an specialist in quantum gravity and did not know what to do with that idea, even when I considered it interesting because of linking quantum mechanics with things in which I was then involved). What I considered, instead of your alpha, was the mean field r of a system of globally coupled nonlinear oscillators described by a Kuramoto model. The coupling constant of the model was as in your case so that, when hbar ->0 one has an incoherent population of oscillators (and hence the average order parameter r ->0). This would represent your 'unquantized' state. When hbar is nonzero, however, one has a synchronized phase emerging out of incoherence, the order parameter r -> 1 as more and more oscillators are synchronized, and one approaches the traditional quantization condition.

    I did not know how to connect the Kuramoto model with gravity. Now, in reading your article, I have got another interesting idea and probably it is time to rescue those old crazy exercises with nonlinear oscillators. I share these ideas here freely for if you have any suggestion to make. I have to study your work in more detail. The monk Zacharias is also interested.

  14. Sabine,

    Thanks for your article, which I did read - I can see how I was perhaps too hard on you. You do seem to want a way of the stagnant morass that theoretical physics has become.

    Emergent QM along the lines of Bush, Couder, Brady and others is something that mainstream physics wrongly ignores. In fact all emergent phenomena are often looked at as something 'below physics'. Perhaps we don't need new fundamental equations to advance physics at all.

  15. Tom,

    I wrote about Couder's theory here. I haven't had time to read the more recent paper. Best,


  16. Hermannus,

    How interesting that you had a similar idea! Unfortunately I don't know anything about the Kuramoto model. I'm not sure what you mean with 'globally coupled', I'd hope they are locally coupled, otherwise you'll run into trouble combining your model with gravity. Best,


  17. Ok, thank you very much 'globally coupled' do not mean globally coupled in space but in their phases. I considered, in a rather crazy way, that each point in space time contains (locally) an infinite collection of oscillators. When they are decoupled then I reproduced the Poisson brackets of classical mechanics and when they are all coupled Heisenberg's commutation relationship. When there is a 'something in between situation' then I had something as your Eq. (3), with r, the order parameter of the collection of oscillators, replacing your alpha.

    A wonderful introduction to the Kuramoto model is provided by Strogatz (I have teach this article at the University)

    If you have problems in downloading the article let me know. And if you are interested I can send you an script with more details when I finish it. I cannot reveal you my identity, however, because this would go against the benedictine law to which I am subject and which I must carefully observe.

  18. Usually, the lectures on the Kuramoto model I gave where accompanied with an experiment that I did on synchronization of metronomes, which constitute an example of application of the Kuramoto model:

    What I did consider is that we do not have strings but a local collection of 'metronomes' that do something as above.

    One solves the equations of motion of a system of metronomes horizontally coupled by a common support. The equations of motion reduce to the Kuramoto model in the limit of weak coupling (introduced by the common support and by the conservation of momentum of the center of mass of the whole system).

    Best regards

  19. Dear Sabine!
    I am interested what do You think about approach like this one:

    J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)
    (Submitted on 17 May 2005 (v1), last revised 6 Jun 2005 (this version, v2))

    We provide detailed evidence for the claim that nonperturbative quantum gravity, defined through state sums of causal triangulated geometries, possesses a large-scale limit in which the dimension of spacetime is four and the dynamics of the volume of the universe behaves semiclassically. This is a first step in reconstructing the universe from a dynamical principle at the Planck scale, and at the same time provides a nontrivial consistency check of the method of causal dynamical triangulations. A closer look at the quantum geometry reveals a number of highly nonclassical aspects, including a dynamical reduction of spacetime to two dimensions on short scales and a fractal structure of slices of constant time.

  20. fiksacie,

    I wrote about CDT here and most recently here.

  21. "How do we know this procedure isn’t scale dependent? How do we know it works the same at the Planck scale as in our labs? "

    Nobody is claiming this, so I think you are banging on open doors here.

    Such approach should work in the perturbative regime though with weakly coupled Langrangians. At strong coupling if you a have a continuous limit you are ok.

    But don't forget that there are theories with no classical limit and without Langrangians.

    For QG it is quite obvious for me that you need new degrees of freedom, the new degrees of freedom are stringy ones.

  22. I read the interview with Lee and saw his remark that asymptotically safe gravity has a problem with stability. I wasn't quite sure what he was referring to at first. I'm thinking now that he must have meant that at any larger structure size than a proton the weak force comes into play and particles and energy can escape. (Remember the fission nuclear bomb. If that is what he is thinking then he is overlooking something big.

    ASG depends on a closed, finite universe. That isn't so strange. If you can't define the borders of what you are attempting to define then there is no hope for solving it. Just assume it and see how far you can get. You can get pretty darn far! If one assumes a closed universe then any coming apart of a bound structure, at any scale you can name, will release energy that will accelerate something else in the universe that will then become bound together with that same asymptotically safe force. The only difference between a proton, which seems to be unconditionally stable, and the universe as whole with an asymptotically safe structure, is that the stability hops from one structure to the next. This is all dependent on a closed universe. Why not?

  23. I guess I should add that the fission atomic bomb was used to deploy the fusion atomic bomb. That should lead to some basic intuition about the "global" stability of asymptotically safe gravity.

  24. Sabine,
    Thanks for the reference. I'll have a look. I do have an open mind. :-)

  25. Hi Bee:
    Does Ashtekar have his own theory of quantum gravity, different from Smolin's LQG? Can you summarize results in few lines?(!!)

  26. kashyap, It's both the same theory but with different variables, that's the short story.

  27. Eric,
    No, that's not what he meant. He probably meant that it's not known whether the fixed point has a Hamiltonian that is bounded from below, and last time I looked they still didn't know that.

  28. I wanted to describe something in the real world and not spend my days proving convergence theorems or doing stability analyses of imaginary things.

    Bravo, Bee, bravo!

    Now to read the rest of the article :)

  29. When we do not know what the majority of the gravitating mass in the universe consists of; we know little about it except its gravitational effects - why do we think we are in a position to produce a unified theory? Is it because we seem to know that dark matter interactions are so weak that there are no additional forces, beyond the electroweak, strong and gravity?

    Of course, not understanding the matter content of the universe has nothing to do with quantizing gravity, except in unification scenarios like string theory?

  30. Do we know the way to quantum gravity?

  31. @Arun: Gravitation ignores composition and field - black holes, neutron stars, white dwarfs, hydrogen stars, Nordtvedt effect, lab stuff. Don't describe it or challenge it with such.

    Newton does not parameterize to GPS. Quantum gravitation is not predictive and the standard model has no SUSY. Perfect derivation creates non-empirical models. A founding postulate is geometrically anomalous at the starting gate where physics "knows" it need not look. Nothing else matters, by observation. Listen to the dog that does not bark. First heretical experiment, then applicable theory.

  32. I haven't (yet) read any of Smolin's books. One problem with popular-science books (I'm not sure if his fit into that category) is that someone with a background in the science in general, but not in the topic of the book (e.g. physics but not quantum gravity) learns little, if anything, new. (Such books might be OK for "interested laymen" who are interested in a very broad-brush overview.) On the other hand, one hasn't the time to read the technical literature outside of one's own field. There is a real need for something in between. For example, in 1991 Narlikar and Padmanabhan wrote a review called "Inflation for astronomers". Another good example: John Barrow's The Book of Universes (the level is not as high as that of the review, but higher than the typical popular-science book).

    So, could I learn anything from Smolin's books?

  33. Phillip,

    Yes, I know what you mean. I read popular science books in physics primarily because I have an interest in writing. Did I learn anything from that book? It's been a decade ago that I read it and honestly I can't recall very much about it. I think I didn't previously know anything about spin networks, and that was the first time I heard about it. I vaguely recall having to look up "node" in a dictionary :p

    I sometimes find lecture notes quite useful to get an introduction to a field I'm not so familiar with, but then you're not always lucky and find something suitable.

  34. Arun,

    I think it's because most particle physicists expect that whatever the unified theory is it will contain a suitable dark matter candidate. To them it's kind of exactly the opposite: instead of dark matter standing in the way of unification, dark matter is a motivation for unification.

  35. I read a lot of divulgative book on the same arguments (gravity) because I wanted and I want to understand it. I stopped to buy divulgative books when I discover to main authors: Einstein and J.A. Wheeler. I haven't stop yet to read and rad again. Every time I understand a bit more.
    Some authors are really.... Fantastic!
    For sure I'll never miss a Sabine's book!

  36. Hi Bee,

    As with three Roads to Quantum Gravity we see where Lee evolves his perspective over time. Subject to change, of course, his perception may evolve too.

    Anyway it has been sort of enlightening when one sees what we are doing in context of Quantum Cognition utilizing quantum theory as a foundational base when exploring our potentials. Why not, when it comes to Quantum Gravity? :)

    Que dos to MarkusM.


  37. Perhaps we should try a road less traveled?

  38. Something noticeable about 'stringy' culture, is that almost exclusively people from that culture use extreme put-downs of basically anything that looks another way. Look above at the attack on Lee Smolin. It's not that he's wrong, or been wrong in the past about this matter or that matter. Stringy people don't look at things that way, I think because, phrase like that, it's self-evident there is no case to answer: nothing wrong with trying things and being wrong.

    So what they do, the stringy people, is take things to a personal and couch their 'criticism' in terms of dishonesty, lies, deliberate omission, theft, and so on.

    Now, sometimes in life it's true that there is gross dishonesty and deliberate omission and all the rest. At times like that, it's right to call a spade a spade. Problem is, dishonest, lying, strategies are just as likely to take the offensive, if not more likely.

    Therefore the old adage, that when someone is being attacked by someone else on personal grounds, involving dishonest, lying, etc, etc, then someone is always guilty of exactly that. But which one?

    One way to resolve this in detective logic, is to observe that it's not actually easy to fallaciously attack people the way Smolin is attacked. Severe moral and ethical compromises need to be made. And it's just one of those things, that we can't do that, and then stay the same in the other areas of our lives. We can't switch it on and off. If we give up our standards, we go to the new standard that we effectively choose. The whole of us.

    So from that we can identify which side have sold themselves out for less along the way. They will be targeting indiscriminately. Their views will display cynicism across the board except for their 'own'.

    How about something more local like that can be demonstrated here in the comment. Well yeah, that's doable, because the one thing that always HAS to go, when an intellectual makes that compromise so that he can project onto his victim, is the normal high-standards practice, of seeking always to see past communication shortcomings, and secondary items, flawed instantiations of examples in what the other person is saying, so as to to 'see' as far as possible, what the other person is 'seeing' for the fundamentally critical purpose of putting their position to its strongest form, and answering it, only there.

    And for why? Because if you don't do that, you are answering a wholly different matter, that not only is not what that person is saying, but not what anyone is saying or has ever said. You don't even know what your answering. And that's a corrosive harmful infliction on yourself. That's the price. Of selling out for less.

    Look here at the answer to Sabine's very good, highly plausible, and original idea.


    ""How do we know this procedure isn’t scale dependent? How do we know it works the same at the Planck scale as in our labs? "

    Nobody is claiming this, so I think you are banging on open doors here."


    That's coming from a stringy friend. Does he address a reasonable proxy for her position here? I am seeing the mirror opposite of that.

    Maybe I'm the dishonest one in putting that example down? Or it representative of a lot of what's coming out of the string place these days? If it's me, then I'm sorry, because that isn't something I'd want to do.

    And that means I don't think it is me. But if it is me it's settled immediately by the fact no one else recognizes anything of the sort from their observations and experiences of the stringy friends.

  39. Giotis,

    If I'm banging on open doors, then all the rooms seem to be empty ;) If you apply a different quantization prescription to strings you get a different theory (I believe Thiemann wrote a paper about this 10 years ago or so), so it's an assumption that matters. I don't know if you can include the prescription in the effective action and if that makes hbar (and possibly other constants) run.

  40. This was shot down by Helling, Policastro

  41. Giotis,
    Thanks for the reference. I'm not sure what you mean though. I'm not saying that this is a good quantization method or one that one should use, I was just using this as an example that the assumption of the quantization method makes a difference for the outcome.

  42. "Have an eye on Achim Kempf and Raffael Sorkin..."

    By the way, his name is Rafael. Great guy, too!

  43. Lee Smolin was correct...there is a theory of quantum gravity in 2015. In fact, there are any number of theories of quantum gravity in 2015. He then further stipulates that experiment must validate that theory, but which experiment he does not stipulate. Any experiment? I doubt that just any old experiment would satisfy Smolin.

    My own sense is from the outside looking in and qg seems to be caught in a recursion of space and motion and continuous time. Science builds its theories with space and motion and continuous time, but there are other conjugate axioms besides space and motion. The Schrödinger equation works well for other conjugates like discrete matter and time. Why Smolin and others do not build their theories on discrete instead of continuous time is a mystery to me.

    The answer seems so obvious...

  44. I thing the Freidel-Leigh-Minic preprint that Smolin mentions [] and also the same authors' previous paper [] are fascinating, and may be the most important pair of QG papers I've read in a long time. And as you suggest, they are explicitly doing something other than the standard form of quantization -- in fact they have a rather plausible-sounding argument that to quantize gravity, or as they put it equivalently to gravitize quantum mechanics, you have to do an extrapolation of Born's suggestion: both the space-time and the quantum momentum space need to have curvature metrics, and these both need to be dynamical. Which normally would cause horrible failures of locality and unitarity, but they show that for string theory, it doesn't. Seriously, go read these two papers.


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