Thursday, June 06, 2013

Quantum gravity phenomenology \neq detecting gravitons

First direct evidence for gravitons.
I’ve never met Freeman Dyson, but I’ve argued with him many times.

Almost every time I give a seminar about my research field, the phenomenology of quantum gravity, I find myself in the bizarre situation of first having to convince the audience that it is a research field. And that even though hundreds of people work on it. I have been organizing and co-organizing a series of conferences on Experimental Search for Quantum Gravity, and in each installment we had to turn away applicants due to space limitations. The arXiv is full with papers on the topic, more than I can keep up with on this blog, and it’s in the popular press more often than I’d like*. Why are my fellow physicists so slow to notice? I make Freeman Dyson responsible for this.

Dyson has popularized the idea that quantum gravity is inaccessible to experiment and thereby discouraged studies of phenomenological consequences of quantum gravity. In a 2004 review of Brian Greene’s book “The Fabric of the Cosmos” he wrote:
“According to my hypothesis [...] the two theories [general relativity and quantum theory] are mathematically different and cannot be applied simultaneously. But no inconsistency can arise from using both theories, because any differences between their predictions are physically undetectable.”
And in a 2012 essay for the Edge Annual Question, he still pushed the idea of quantum gravitational effects being unobservable:
“I propose as a hypothesis... that single gravitons may be unobservable by any conceivable apparatus. If this hypothesis were true, it would imply that theories of quantum gravity are untestable and scientifically meaningless. The classical universe and the quantum universe could then live together in peaceful coexistence. No incompatibility between the two pictures could ever be demonstrated. Both pictures of the universe could be true, and the search for a unified theory could turn out to be an illusion.”
The problem with this argument is that he equates the observation of a single graviton with evidence for a quantization of gravity. But the two are not the same. If single gravitons were unobservable, it would not imply that “theories of quantum gravity are untestable and scientifically meaningless.”

It might indeed be that we will never be able to detect gravitons. One can estimate the probability of detecting gravitons and even with extremely futuristic detectors the size of Jupiter put in orbit around a Newton star, chances would be slim. (See this paper for estimates.) Clearly not an experiment you want to write a grant proposal for.

But we don’t need to detect single gravitons to find experimental evidence for quantum gravity.

Look around. The fact that atoms are stable is evidence for the quantization of the electromagnetic interaction. You don’t need to detect single photons for that. You also don’t need to resolve atomic structures to find evidence for the atomic theory. Brownian motion famously provided this evidence, visible by eye. And Planck introduced what is now known as “Planck’s constant” before Einstein’s Nobel-prize winning explanation for the photoelectric effect.

If we pay attention to the history of physics, it is thus plausible that we can find evidence for quantum gravity without directly detecting gravitons. The quantum theory of gravity might have consequences that we can access in regimes where gravity is weak, as long as we ask the right questions.

Some people have a linguistic problem with calling something a “quantum gravitational effect” if it isn’t actually an effect that directly involves quanta of the gravitational field. This is why I instead often use the expression “Planck scale effects” to refer to effects beyond the standard model that might be signatures of quantum gravity.

Interestingly, Christine recently pointed me to a writeup of a 2012 talk by Freeman Dyson, in which he discusses the possibility of detecting gravitons without jumping to the conclusion that an inability to detect gravitons means that quantum gravity is a subject for philosophers. Instead, Dyson is very careful with stating:
“One hypothesis is that gravity is a quantum field and gravitons exist. A second hypothesis is that the gravitational field is a statistical concept like entropy or temperature, only defined for gravitational effects of matter in bulk and not for effects of individual elementary particles… If a graviton detector is in principle impossible, then both hypotheses remain open.”
A hooray for Dyson!

Unfortunately, there are still other people barking up the same tree, for example by pulling the accelerator argument. For example John Horgan writes:
“String theory, loop-space theory and other popular candidates for a unified theory postulate phenomena far too minuscule to be detected by any existing or even conceivable (except in a sci-fi way) experiment. Obtaining the kind of evidence of a string or loop that we have for, say, the top quark would require building an accelerator as big as the Milky Way.”
Horgan is well known for proclaiming The End of Science, and it seems indeed he’s run out of science when he wrote the above. To begin with, string theory doesn’t “postulate... phenomena,” what would be the point of doing this? It postulates, drums please, strings. And I’m not at all sure what “loop-space theory” is supposed to be. But leaving aside this demonstration of Hogan’s somewhat fuzzy understanding of the subject, if we could build a detector the size of the Milky Way, we’d be able to test very high energies, all right. But that doesn’t mean we can conclude this is the only way to find evidence for quantum gravity.

Luckily Horgan has colleagues who think before they write, like George Musser who put it this way:
“[Q]uantum gravity” and “experiment” are… like peanut butter and chocolate. They actually go together quite tastily.
(I had meant to write a summary of which possible experiments for quantum gravity pheno are presently being discussed and how plausible I think they are to deliver results, but I got distracted by Dyson’s above mentioned paper on graviton detection. The summary will follow some other time. Update: The summary is here.)

*Almost everything I read in the popular press about evidence for quantum gravity is wrong or misleading or both. But then you already knew I would complain about this :p


  1. Looking forward to the post which you have announced in the brackets. That was exactly the (logical) question I wanted to ask ;).

  2. The application of our current theories of gravity on observation on large scales leads us to phenomenological models of dark matter, dark energy and something that fuels inflation. Is it too difficult to link explanations for this trio to quantum gravity? (Are these three what Brownian motion was for atomic theory?)

  3. Brown described Brownian motion in 1827 while Einstein explained it only in 1905. The Brownian motion analogy suggests that the phenomenon is observed long before it is connected to the underlying structure. That is the motivator for the previous question.

  4. There are several attempts to explain dark matter and/or dark energy as a quantum gravitational effects. So far I don't find any of these particularly convincing, but the idea is around. There are many other models that try to find qg imprints on other cosmological observables. Presently particularly fashionably are tensor-modes and non-gaussianities, though the latter is rapidly going out of fashion after the Planck results...

  5. If we have a "plausible" theory of quantum gravity, it must provide fully finite calculations (i.e., it must provide "computable" observables) and hint what is truly observable (and what it is NOT) in the context of "quantum spacetime". String theory or loop quantum gravity are candidate theories of quantum gravity, but, until current time, any "prediction" or "physical effect" coming from those theories have been found. Even with Lorentz violating theories, or modified dispersion relationships we have found "null experiments", i.e., nothing "new". If quantum gravity does exist, it seems to obey the Freeman Dyson "undetectability principle". However, it could be possible that the true quantum gravity theory is something different, nonperturbative and nonlinear itself, so our current models are all flawed (or almost, since LQG or strings provide tools to make "quantum gravity calculations" computable...). However, I am convinced that it is not only an issue on phenomenology what we need. There are some "contradictions" between quantum theories and relativity. E.g.: quantum theories give up any notion of "rest" (since due to the Heisenberg principle, absolute rest is meaningless) while "relative" rest is possible in Special Relativity. And there are many others. QFT is a nice theory but it can not be the final story, like SR or GR, BUT, the nocion of "vacuum" is important, very important...It is showed by the Unruh effect, the Hawking effect, or the Schwinger effect. Interestingly, these 3 effects (non-perturbative like) are yet to be "measured" as well. I follow the searches for the Schwinger effect in pulsed lasers, graphene and other systems. I believe that the technology to measure these 3 effects (if possible) in "strong fields" could be also interesting for quantum gravity searches.

  6. Theory fails when matter is fit with vacuum photon symmetries, hence symmetry breakings. "Violations" are diagnostics. Parity violations are failed theory - quantum gravitation, dark matter, and SUSY. Solve problems by acting orthogonal to their presented forms. Do it the other way.

    Theory can be a trollop. Superluminal neutrinos were a defective fiberoptic connection; and arxiv:1303.3271, 1304.6264, 1301.0708. Fermionic matter does not see gauge boson photons' rigorously isotropic vacuum. Physics denies emergent property geometric chirality. Dark matter is Noetherian leakage as Milgrom acceleration, parity violations are diastereotopic divergences. Gravitation assuming the Equivalence Principle is geometric chirality-falsifiable, on a bench top, within 90 days.

    The Emperor cannot be clothed until somebody looks.

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  8. I wish physicists would move out of the strait-jacket of insisting the either the quantum converge on GR or that GR converges on the quantum. There are analogues in our physical environment right here on Earth for what is going on.

    Gravity is weaker by 42 orders of magnitude than the strong force. Lets see, is there anything in our physical environment that might approximate that? I'm no mathematical physicist so I can only guess. But my guess is that the ratio would be best approximated by the force exerted on a stationary body by an atmospheric high or low pressure system and the force exerted by a tornado on a stationary object.

    It's important to differentiate wind from the force exerted by high and low pressure systems. You experience the force of wind when there is flow between high and low pressure systems. You also experience wind in a tornado or hurricane.

    You do not experience wind just because you are in either a high or low pressure system. It is simply an energy density gradient and if you could quantify it I'm sure the force might be dozens of orders of magnitude smaller than either of the other two kinds of atmospheric disturbances. Maybe those more able in mathematical physics could quantify this?

    That force of gravity may just be completely different in origin from what is measured in qft. The wind would be similar conceptually to what is measured in qft.
    But the underlying energy density in which the wind expresses itself would be gravity.

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  10. "It's important to differentiate wind from the force exerted by high and low pressure systems"

    I meant to say it's important to differentiate wind generated by being between high and low pressure systems and the tiny tiny force generated by the density gradient when being within either a high or low density system.


  11. Bee says: "The fact that atoms are stable is evidence for the quantization of the electromagnetic interaction."

    However, if nature's geometry is conformal and involves discrete self-similar scaling, then the correct statement would be as follows.

    The fact that atoms are stable is evidence primarily for the quantization of the gravitational interaction, and secondarily for the quantization of the electromagnetic interaction.

    This is of course a new idea and it is received with the expected amount of resistance. But if one really wants to make progress in quantum gravity, resolve the vacuum energy density crisis, and solve the hierarchy problem of particle physics, not to mention explain the physical meaning of the fine structure constant, then one must seriously consider this new way of modeling nature. It makes definitive predictions and they are verified.

  12. @RLO
    "This is of course a new idea."

    Not to any of us here. You keep repeating the same old ideas you are so attracted to, even though I doubt any of us agree with your theory. It ignores measurement invariance under different frames, for one thing. You can't solve it simply by believing the constants in the fine structure constant change over time.

    People have thought about that since Dirac. You keep rehashing an old idea that people have thought about for 70 or 80 years. And which did not work out. Still, you have the pomposity to exclaim how original you are. We all yawn.

  13. Or, I should add, changing the gravitational constant. It all adds up to cheating observational invariance, which you can't do.

  14. Well, one part of your problem is that nobody famous has endorsed your field. If Nima Arkani-Hamed gave a talk saying that quantum gravity phenomenology is the way to go, and that only stooopid people work on anything else, your problem would be solved overnight. Are you sure you would like that? :-) Another problem though is the pervasive despair in the field. Every recent claim to have found something exciting has turned out to be wrong, and if someone in your field found something interesting, many people [especially, sadly, young people] would mutter, "superluminal neutrinos" and ignore it. This is the current reality. Sure you want tenure?

  15. I have read this blog for a couple of years and enjoy it. So, I regret that my first comment is to nitpick. However, I don't see how the stability of atoms is direct evidence for quantization of the electromagnetic field. Historically, this was deduced from the ultraviolet catastrophe in blackbody radiation, not from the stability of atoms. One can predict the stability of atoms just from Schrodinger's equation with the hamiltonian modeled after Coulomb's law (as we all do as undergrads). One misses the Lamb shift this way, of course.

  16. Hi Sabine,

    Are there any models of quantum gravity that doesn't rely on gravitons?

    Cheers, Paul.


  17. Eric opines: "People have thought about that since Dirac. You keep rehashing an old idea that people have thought about for 70 or 80 years. And which did not work out."

    Maybe that's because nobody ever did it right, i.e., recognized that discrete dilation invariance would work where continuous dilation invariance would not.

    You go right ahead and "yawn" all you want, but do you really want to ignore an elegant, natural and highly unifying idea before you even have a crude understanding of it?

  18. Robert, Eric,

    Could you please live out your disagreements over your own research elsewhere? Best,


  19. Captain,

    Well, the graviton is a quantized weak-field perturbation. You'll get it very generically in the weak-field limit of any quantized theory of gravity. I'm not sure what you mean with your question. Most existing approaches to quantum gravity do not "rely on gravitons" in the sense that they're not starting with the perturbative quantization. They all should get something like a graviton though in the respective limit. The only way I see this might not be the case is if gravity is classical indeed and it's somewhat of a stretch to call this a "model for quantum gravity". Best,


  20. Stephen,

    You're misquoting me. I did not write that the stability of atoms "is direct evidence for quantization of the electromagnetic field". Sorry to nitpick myself, but my wording was more careful than yours. Best,


  21. Rastus,

    "Another problem though is the pervasive despair in the field. Every recent claim to have found something exciting has turned out to be wrong, "

    This is why I am so dismayed at what the pop sci press "reports" about qg pheno - because it leaves people like you with a very wrong impression about the status of the field. Very, very, few people have made claims that observable tests would deliver evidence by now, and almost nobody has claimed that something has been found already. Most people see the situation much more realistically. Arguably, we're not there yet. But, look, sooner or later we either have to find observational evidence for quantum gravity, or we lay the whole idea to rest and leave it to philosophers.

    The bottomline is this: if you're an institute that pretends to do physics and hires people working on quantum gravity, you should also hire people working on the phenomenology of quantum gravity or stop pretending it's physics and call it what it is: mathematics or philosophy.

    And, yeah, I want tenure. Best,


  22. What an absurd approach.

    It reminds me of a motto we engineers use to say: "If it works don't fix it"

    But I guess physicists shouldn't think this way. I don't know what Dyson wants to say here but I guess these are the implications when you take into extreme the usual mantra that a new theory has to make new predictions that needs to be verified experimentally.

    And let me give an example:

    Let's say that we have a TOE that derives theoretically the experimental values of all the unknown parameters of the SM with a prefect match, but it makes no new predictions that could be verified experimentally in the foreseeable future.

    Will you accept this theory like you accept GR for example or you will reject it since the theory doesn't make new predictions that could be verified experimentally?

  23. Hi Giotis,

    I suspect he's trying to be provocative. On that level, I can understand it, because I'm saying the same, really: If your theory isn't experimentally testable, it's not science, so better look for observational consequences. I just don't understand why he's so obsessed with graviton detection in particular.

    Be that as it may, regarding your question, if you had such a theory, I think it would be highly convincing and most people would believe it captures some deep truth about nature. Beyond that, it would be intellectually satisfactory but practically somewhat useless. It seems extremely unlikely to me however that such a theory would not also make some other prediction, so it's not a scenario I'm worried about. Best,


  24. "But, look, sooner or later we either have to find observational evidence for quantum gravity, or we lay the whole idea to rest and leave it to philosophers."

    Sabine I completely disagree with this statement. If you don't find experimental evidence of QG in our low energy effective world, it means that physicists should stop trying to find a consistent theory of QG?

    Please re-think this for the love of God:-) (see also the example I gave above with a TOE including gravity)

  25. Hi Giotis,

    "If you don't find experimental evidence of QG in our low energy effective world, it means that physicists should stop trying to find a consistent theory of QG?"

    I wouldn't put it this way because a) our world has not always been "low energy" and it's not everwhere "low energy". And b) It's hard, if not impossible, to know that a theory can't deliver experimental evidence if you don't have that theory... What I'm saying is not that we should stop trying. What I am saying is that if we don't care about experimental evidence we can as well stop trying, because then picking the right theory is just opinion, not science. Best,


  26. How did the author manage to have such a long discussion on the testability of quantum gravity without proposing a single test

  27. Nice article,

    I have immediately to send this to a colleague with which I had a recently a discussion about this very issue in the context of a popular not well enough informed article.


  28. Bee: fair enough. Let me try to better express myself. (I basically had a question in mind. I'm not sure why yesterday I was in the mode of attempting to provoke an answer via criticism rather than directly asking the question.)

    What do you think is the most direct way to deduce quantization of the electromagnetic field from the stability of atoms? I find it a fun game to consider (with benefit of hindsight) how theorists might conceivably have deduced various facts about physics without using as many hints from experiment as they actually did.

    My best answer is this: by treating the position/momentum degrees of freedom of electrons orbiting the nucleus in a quantum-mechanical way, one predicts that atoms have ground states, thus resolving the problem that classical electrons should radiate electromagnetically and spiral in to the nucleus. Then, knowing that the position/momentum degrees of freedom of electrons are quantum-mechanical, one is forced by consistency to believe that electromagnetic field should be quantum-mechanical; a superposition of electron positions should induce a corresponding superposition of electromagnetic fields. Thus one would be led to try to quantize the electromagnetic field.

    I would be curious whether you have thought of different lines of reasoning, especially ones that diverge further from history than mine. (Of course, this is somewhat tangential to the original topic of the post, so I understand if you do not wish to get into detail and derail the comment thread.)



  29. /* I propose as a hypothesis... that single gravitons may be unobservable by any conceivable apparatus */

    Dense aether model proposes instead, that the gravitons are stuff, which everyone can detect with his TV set, because they represent the 2-spin component of notoriously known CMBR noise. This controversy just illustrates, here is a huge causal gap between formal description of phenomena at the rigorous level and its qualitative understanding.

    In another words, being clever to able to predict/describe some phenomena/artifact doesn't imply, you're clever enough to know, where to look at it - and vice-versa.

  30. BTW In this model the notoriously soughed gravitational waves would correspond the gravitons and therefore the CMBR noise in the same way, like the photons correspond the light waves at phenomenological level.

  31. As they say 'Never say never". There is an experiment which if there is ever an observable effect implies that an energy of ~10^15 GeV is involved which is not so far from the Planck energy. That would be looking for 'proton decay' whereby the decay is mediated by the X-boson (or something involving 10^15 GeV) where the proton spontaneously decays to end products of 2 gamma-ray photons. Not related to graviton searches but the Georgi-Glashow GUT model. Interesting that the experiment relies on time and the internal natural machinery of the particle involving the mediating 10^15 GeV energy which we would not see but we would know it is there.

  32. /* that an inability to detect gravitons means that quantum gravity is a subject for philosophers */

    The purpose of quantum gravity is not to describe some esoteric phenomena at the whole boundary of the observable Universe - but all these common life phenomena, which do exists between mass/energy density and distance scales of quantum mechanics and general relativity. It's actually very materialistic theory free of esoteric - the only problem is, even the quantum theorists itself don't realize it.

  33. Hi Stephen,

    I don't think we ever 'deduce' a theory from data. We just find consistent explanations. Or we conclude that an explanation we have is not consistent with the data. This is what I was trying to say: the stability of atoms tells you that classical electrodynamics cannot be the end of the story.

    Regarding your question, it's difficult if not impossible to answer since we all know of quantum mechanics, so we have a hindsight bias... I think you are right, concluding that there must be a ground state that doesn't radiate seems straight forward. I don't think that this alone gives you a quantization condition for position and momenta. You could implement this by brute force by some other requirement. Eg, you could require wavelengths to be quantized 'just because', not saying anything about wave-functions. Either way, you'd probably strap along your way with some phenomenological models before you find a consistent theoretical approach. Best,


  34. /* I don't think we ever 'deduce' a theory from data. We just find consistent explanations...*/

    The development of theory isn't just a random guessing - it's logical process - at least for me. But your remark points clearly, how the contemporary physicists ignore the logical reasoning for their theories - they tend to consider it as a mechanical game with equations. It's immanent portion of their formal philosophy: math, math is everything, which moves us forward in understanding of nature.

  35. Hello Philip,
    because black holes are very "non-newtonian" I guess Newton stars are all non-black-holes.
    (Maybe a Freudian typo?)

  36. Bee, I have a feeling the research you and your colleagues are doing on the phenomenology of quantum gravity is worthwhile, and will in all likelihood yield meaningful and possibly very important results, both physically and mathematically.

    However, I must say I'm impressed by Dyson's position, not simply his argument re the two mirrors, but his realization that quantum mechanics and general relativity may well be incommensurate. After all, as the Pythagoreans discovered, to their horror, even mathematics itself contains incommensurable elements. Instead of leading to a dead end, that realization led to extremely important developments in math.

    The effort to reconcile qm and gr looks very much to me like the incommensurables we see in math and geometry, especially the most famous example being the impossibility of squaring the circle.

    I think Bohr had the right word for how to think about this "impossible" situation: complementarity.

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  39. Freeman Dyson

    I thought it important that the quote be given to Freeman Dyson that sets up the question with regard to the Graviton and Dyson's influence to your blog post?

    More specifically, here

  40. Sorry for the deleted posts...

    Is it true, that a picture can paint a thousand words?

    Diagram of the Lagrange Point gravitational forces associated with the Sun-Earth system.

    In a certain sense a perfect fluid is a generalization of a point particle. This leads to the question as to what is the corresponding generalization for extended objects. Here the lagrangian formulation of a perfect fluid is much generalized by replacing the product of the co-moving vector which is a first fundamental form by higher dimensional first fundamental forms; this has as a particular example a fluid which is a classical generalization of a membrane; however there is as yet no indication of any relationship between their quantum theories.A Fluid Generalization of Membranes.

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  42. Paul Steinhardt:It is amazing that an abstract study of tiles and building blocks can lead to a keen insight about the fundamental constituents of matter and a classification of all possible arrangements of them. It is a classic example of physicist Eugene Wigner referred to as the "unreasonable effectiveness of mathematics in the natural sciences." Quasi-elegance

  43. Hi Markus,

    That's an interesting reference. What is Schwinger in that quote referring to? Is he really referring to a graviton or to a graviational wave?



  44. Hi Doc,

    I don't see how saying "complementarity" answers the question of what is the gravitational field of an electron in a double-slit experiment. Best,


  45. Hi Bee,
    I think your response to Doc is a bit of a straw man. You are putting up a statement unrelated to what Doc said and then refuting it. Easy enough to do.

    I generally don't take too much with complimentariness as Bohr described it when referring to the quantum world. But in the case of gravity vis-a-vis qft I find it a very reasonable position to take. YMMV.

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  47. Hi Eric,

    I'm just saying that deferring to some "complementarity" doesn't solve the puzzle for me, in particular the one that I referred to, and I also doubt it is a convincing solution for many of my colleagues. Best,


  48. "I don't see how saying "complementarity" answers the question of what is the gravitational field of an electron in a double-slit experiment."

    Since there is no way of determining both the position and the momentum of an electron at any given time, then I don't see any possible way to determine the gravitational field. So the answer to your question would be that the gravitational field of an electron in a double-slit experiment is indeterminate.

    "The retrospective eventing of the event can never constrain the projective momentum of the moment."

  49. "Dyson has popularized the idea that quantum gravity is inaccessible to experiment ..." I claim that Milgrom is the Kepler of contemporary cosmology. STUDY THE EVIDENCE!
    Kroupa, P., Pawlowski, M., Milgrom, M.: The failures of the standard model of cosmology require a new paradigm. Int. J. Mod. Phys. D. 21(14), 1–13 (2013).
    If you are a quantum gravity researcher, THEN YOU SHOULD STUDY MILGROM'S WORK! Find a theory that explains Milgrom's acceleration law — Milgrom's law is the main target for quantum gravity theory — BELIEVE IT OR NOT!!!

  50. Even if gravity is not fundamental, it would still be subject to quantum mechanics. Gravitons would still effectively exist, they would be quasi-particles like phonons.

  51. "Horgan is well known for proclaiming The End of Science, and it seems indeed he’s run out of science when he wrote the above."

    That is a great piece of English writing!

    That is all.

  52. Count,

    "Even if gravity is not fundamental, it would still be subject to quantum mechanics."

    I don't even know what this is supposed to mean. Best,



  53. John Horgan is just one of those people who are dismayed by the lack of definitive predictions and testability in certain "sectors" of theoretical physics.

    He is far from alone.

    Science was founded on the ideas of definitive predictions and empirical testing of those predictions.

    Abstract analysis and pure Aristotelean "reasoning" don't cut it in science unless their plastic products are empirically tested for structural soundness and scientific integrity.

    Robert L. Oldershaw
    Discrete Scale Relativity/Fractal Cosmology


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