Wednesday, November 19, 2014

Frequently Asked Questions

[Image source: Stickypictures.]

My mom is a, now-retired, high school teacher. As teenager I thought this was a great job and wanted to become a teacher myself. To practice, I made money giving homework help but discovered quickly I hated it for a simple reason: I don’t like to repeat myself. I really don’t like to repeat myself.

But if I thought spending two years repeating how to take square roots - to the same boy - was getting me as close to spontaneous brain implosion I ever wanted to get, it still didn’t quite prepare me for the joys of parenthood. Only the twins would introduce me to the pleasure of hearing Jingle Bells for 5 hours in a row, and re-reading the story about Clara and her Binky until the book mysteriously vanished and will not be seen again unless somebody bothers to clean behind the shoe rack. “I told you twice not to wash the hair dryer,” clearly wasn’t my most didactic moment. But my daughter just laughed when the fuse blew and the lights went off. Thanks for asking, we got a new dryer.

And so I often feel like I write this blog as an exercise in patience. Nobody of course bothers to search the blog archives where I have explained everything. Sometimes twice! But today I will try to be inspired by Ethan who seems to have the patience of an angel, if a blue one, and basically answers the same questions all over and over and over again. So here are answers to the questions I get most often. Once and forever I hope...
  1. Is string theory testable?

    The all-time favorite. Yes, it is. There is really no doubt about it. The problem is that it is testable in principle, but at least so far nobody knows how to test it in practice. The energy (densities) necessary for this are just too high. Some models that are inspired by string theory, notably string cosmology, are testable with existing experiments. That it is testable in principle is a very important point because some variants of the multiverse aren’t even testable in principle and then it is indeed highly questionable whether it is still science. Not so though for string theory. And let me be clear that I mean here string theory as the candidate theory of everything including gravity. Testing string theory as means to explain certain strongly coupled condensed matter systems is an entirely different thing.

  2. Do black holes exist?

    Yes. We have ample evidence that supermassive black holes exist in the centers of many galaxies and that solar-sized black holes are found throughout galaxies. The existence of black holes is today generally accepted fact in the physics community. That black holes exist means concretely that we have observational evidence for objects dense enough to be a black hole and that do not have a hard surface, so they cannot be a very dim stars. One can exclude this possibility because matter hitting the surface of a star would emit radiation, whereas the same would not happen when the matter falls through the black hole horizon. This horizon does not have to be an eternal horizon. It is consistent with observation, and indeed generally believed, that the black hole horizon can eventually vanish, though this will not happen until hundreds of billions of years into the future. The defining property of the black hole is the horizon, not the singularity at its center, which is generally believed to not exist but for which we have no evidence one way or the other.

  3. Why quantize gravity?

    There is no known way to consistently couple the non-quantized theory of general relativity to the quantum field theories of the standard model. This only works in limiting cases. The most plausible way to resolve this tension is to quantize gravity too. It is in principle also possible that instead there is a way to couple quantum and classical theories that has so far been missed, or that the underlying theory is in some sense neither classical nor quantum, but this option is not favored by most researchers in the field today. Either way, the inconsistency in our existing theories is a very strong indication that the theories we have are incomplete. Research in quantum gravity basically searches for the completion of the existing theories. In the end this might or might not imply actually quantizing gravity, but Nature somehow knows how to combine general relativity with quantum field theory, and we don’t.

  4. Why is it so hard to quantize gravity?

    It isn’t. Gravity can be quantized pretty much the same way as the other interactions. It’s just that the theory one arrives at this way cannot be a fundamental theory because it breaks down at high energies. It is thus not the theory that we are looking for. Roughly speaking the reason this happens is that the gravitational equivalent of a particle’s charge is the particle’s energy. For the other known interactions the charge and the energy are distinct things. Not so for gravity.

  5. Is quantum gravity testable?

    Again, yes it is definitely testable in principle, it’s just that the energy density necessary for strong quantum gravitational effects is too high for us to produce. Personally I am convinced that quantum gravity is also testable in practice, because indirect evidence can prevail at much lower energy densities, but so far we do not have experimental evidence. There is a very active research area called quantum gravity phenomenology dedicated to finding the missing experimental evidence. You can check these two review papers to get an impression of what we are presently looking for.


  1. Oddly I dislike playing the same song twice, originally is as important to me as concentration on an observation.
    But some requests eventually had me memorize the lyrics and music

    Lately I see a higher level debate on what the Higgs data says ( Technicolor? ) more dramatic than the SUSY LQG wars. But it seems clear to me that both theories apply.

    I enjoy tutoring, seeing awakening minds. But to what expectations have I led them into? I mean child rearing is a most awesome responsibility and no one has written the definitive book.

    Good post, bares repeating. Good teaching, great mother whose kids will recall and understand the more important things. Then again I am just stating the underrated obvious :-)

  2. "Roughly speaking the reason gravity is so hard to quantize is that the gravitational equivalent of a particle’s charge is the particle’s energy."

    At every point in space a gravitational field is defined by a particular length contraction and a particular degree of time dilation. So, rather, than treating mass (energy) as the equivalent of charge, why not consider the existence, (at some energy not yet explored), of quantums of length of both polarities (contract or expand the local metric), and quantums of time of both polarities (retard or advance the rate of local time flow). With both variables being bi-polar, infinities would be cancelled.

    Hopefully I'm not violating forum rules.

  3. A possibly useful distraction when reading the same story to a child for the nth time: what exactly is happening in their brains while they listen to it for that nth time? My guess is that it's something interesting and complicated beyond mere entertainment.

    What additional thing are they learning the twentieth time they hear it that they didn't learn the tenth time?

  4. ""“I told you twice not to wash the hair dryer,” clearly wasn’t my most didactic moment. But my daughter just laughed when the fuse blew and the lights went off. ""

    Hallo Bee,
    there is no doubt, Your daughter will
    follow an experimental career :=)

  5. @CapitalistImperialistPig I have been told that one important thing the child is learning (and reinforcing) is the story will continue to be same each time you re-read the book.

  6. 1) Empirically falsify a string theory postulate. The Equivalence Principle defines vacuum symmetries. Resulting theory suffers parity violations, symmetry breakings, chiral anomalies, baryogenesis, Chern-Simons repair of Einstein-Hilbert action. Geometric Eötvös experiment: one vertical side is four space group P3(1)21 alpha-quartz single crystal test masses (right-handed), the other is space group P3(2)21 (left-handed). Test spacetime geometry with geometry.

    2) Black holes are divine revenge for eating a pomegranate in Eden. (Apples, native to Kazakhistan (The Botany of Desire), aren't quantized.)

    3) GR "achiral" spacetime curvature (Ashtekar, Barbero, Immirzi!) may be Einstein-Cartan-Kibble-Sciama gravitation chiral spacetime torsion. (1) is diagnostic.

    4) Gravitation fails quantization because Luboš won't do it the "other" way.

    5) Reject theories' excuses and curve fits. Are you economists?

  7. /* but at least so far nobody knows how to test it in practice */

    There were some experimental tests of string theory already. Regarding the postulates, the quantum gravity theory suffers with similar inconsistency and resulting fuzziness.

  8. Zephir: You have a real problem extracting information from what you are reading. I am not saying this to insult you but because you should take note and try to improve. It should be abundantly obvious that Peter's title is sarcastic, it should be even more obvious if you read the first paragraph (which I am not sure you did), but you entirely failed to grasp this. You also didn't understand my explanation. I'm not sure if you have a problem with reading English - your writing is pretty bad - or if you have a problem with the written word generally, but I hope you find some time to work on this. Best,


  9. Everybody agrees that strong quantum gravity effects should be seen at length scales of the
    order of Planck length. Hence this type of QG experiments are still beyond our reach. However,
    there is a quantum gravity effect which has been already measured, and that is the value of the
    cosmological constant. It is clear that the only way to explain the amazingly small value of the CC is to take into account QG, and the non-trivial question is which QG theory to use. My answer is to use Regge Quantum Gravity (see arxiv:1407.1394,1407.1124,1402.4672), because this is a QG theory where the value of CC can be calculated and one can obtain the observed value.

  10. Mikovic:

    "Everybody agrees that strong quantum gravity effects should be seen at length scales of the
    order of Planck length."

    I don't agree and I think that most of my colleagues wouldn't agree either.

  11. /* It should be abundantly obvious that Peter's title is sarcastic, it should be even more obvious if you read the first paragraph */

    So why he quotes the links like the "String Theory Fails First Major Experimental Test"? These links are sarcastic too? In addition, we can find a links to many other experimental tests of string theory, which don't look overly sarcastic for me (1, 2, 3, 2, 4, ...). Also, we don't need to test theory as such - the experimental tests of its postulates (existence of extradimensions, for example) is sufficient for falsification of theory by itself.

  12. "Everybody agrees that strong quantum gravity effects should be seen at length scales of the
    order of Planck length."...

    ...I don't agree and I think that most of my colleagues wouldn't agree either....

    Why not?

  13. The reason why I, and QG people I know, believe that the QG effects will be significant at the
    Planck scale is based on the fact that QG perturbation theory is an expansion in powers of
    (l_P^2)R ~ l_P^2 /l^2, where R is the spacetime curvature and l is the size of the spacetime
    region having the curvature R. Hence the QG corrections become large in regions of large R,
    which means small l. So, the smaller the l, the larger are the QG corrections.

    It is possible that there are non-perturbative QG effects which will be significant at extra-
    large (cosmological) length scales L. The reason for this is that at our length scales l_0,
    which are much bigger than l_P, we do not see such effects, so that the only possibility is that
    they appear at L >> l_0.

  14. i suppose that a QG largely visible effect, happens during fusion / fission of atomic nucleons. how otherwise justify dE=mc^2?

  15. /*i suppose that a QG largely visible effect, happens during fusion / fission of atomic nucleons */

    Everyone of us is QG effect: the quantum gravity is the matter of just CMBR/human observer scale.

  16. Mikowic:

    You know say that "QG effects will be significant at the Planck scale". I agree on this. Your previous statement was that QG effects "should be seen at length scales of the order of Planck length" which I, as I said, do not agree on, and I think most of my colleagues wouldn't agree on either.


    As usual, you failed to understand what I was saying. I don't know what the Springer paper you link to is supposed to document. Why does Peter quote the links? Because, if you follow his blog, it should be obvious to you that these pop sci headlines are nonsense. At the very least you must have noticed the recurrent word "hype" in his writing, did you? Maybe look it up.

  17. So you haven't been introduced to Frozen yet?

  18. Paul:

    No... The girls don't really follow stories that long. We still do the features with the short clips, Shaun the Sheep is presently en vogue. I'm also honestly not very interested in seeing Frozen. I think I'll opt for the Lion King if I can find a version where they didn't translate the songs, I always liked that.

  19. Bee - US President Bush the Lesser embraced The Pet Goat's political allophany.

    Avoid The Pet Goat. Try "Twinkle, twinkle little star, power equals I-squared R." Mozart is good.

  20. IMHO when it comes to quantum gravity, people are stuck because they have a blind spot, and can't see what they're missing. For example see the three points in your paper:

    1. A photon goes through both slits. It isn't some billiard ball, it has an E=hf wave nature. Check out weak measurement and think in terms of a seismic wave, then think of detection performing something like an optical Fourier transform.

    2. General relativity doesn't predict black hole point singularities. The MTW interpretation of GR does, but the "field interpretation" predicts a frozen-star black hole with no point singularity.

    3. The black hole information loss problem is an invented problem, and Hawking radiation is a fairy story that remains hypothetical. Forget it.

  21. @johnduffield
    A photon passing through a slit exists as a spherical wave that passes through said slit in both directions. You are not privileged to only consider the forward direction, add probabilities, and postulate they sum to 1.

    (Zero thickness slit along direction of travel re umbra and penumbra, absolutely straight sides re apodization, etc.)

  22. Mikowic,

    Do you think this makes sense to you.

    page 21

    The idea that the field concept has to be used with great care lies also
    close at hand when we remember that all field effects in the last resort can
    only be observed through their effects on matter. Thus, as Rosenfeld and
    I showed, it is quite impossible to decide whether the field fluctuations
    are already present in empty space or only created by the test bodies.52

  23. There could be a place where our interpretations of dark energy and energy are the same description on all scales especially mass locations.
    Is a point or zero width string abstractly the concept of a black hole singularity? e, as trsncendental modulus may not be as absolute as uncertain at some relative scales. That is asymptopic freedom breaks down.
    Yes, uncle AI, time like nature does not privilege direction but also it does.
    Let us say Preons and patrons are some dimensional issue. What unity of scale defines horizons?
    With zero and positive expression we could imagine a black hole object is without a singularity left over after the quarks are ejected as matter and its horizon itself may fade away.
    This speculation seems a plausible conclusion even if unobserable.
    I am just suggesting that we raise the theory bar a little rather than trying to conform nature into falsely complete seeming views and lesser confining arguements. The standard model stands as nature solves over all the spectrum of energy but not in our narrow views of symmetry, dimensions, groups, and mathematics as geometric or informational formalism. In discussions even the great philosophers missed each others points.

  24. Typo : dark energy and GRAVITY the same thing. quantized or not. Gravity also needs a much higher and dynamic definition.

  25. /* if you follow his blog, it should be obvious to you that these pop sci headlines are nonsense */

    Of course they aren't. The string theory was really tested at LHC - the existence of extradimensions and supersymmetry in particular (a two important prerequisites of string theory). These tests failed, or at least current wisdom is such (I don't think this wisdom is correct at all).

    At any case, if the prerequisites of some theory can be tested, it just means, the theory as such can be falsified with these tests (the supersymmetry is not crucial prerequisite of string theory, but the extradimensions are).

  26. Nemo: Fusion and Fission are nuclear processes. They happen at energy scales of the order MeV. That's 20 Orders of magnitude below the Planck energy. It has absolutely nothing to do with quantum gravity. Best,


  27. Zephir: Why don't you at least try to understand what I am saying and what Peter is saying? Tests of extra dimensions and supersymmetry are NOT tests of string theory. That's the whole point of Peter's writing. If you can't understand his posts, try to understand at least the name of his blog (or book for that matter). This should be even more obvious now because if that was so then string theory would be ruled out already -- Oh, I see, that's what you believe has happened... Best,


  28. /*Tests of extra dimensions and supersymmetry are NOT tests of string theory*/

    Of course they aren't - but at least without extradimensions the string theory cannot work. And the supersymmetry is required for to have it predictable at least a bit, so that the string theorists are pretty motivated into its confirmation too..

    So that if we prove, that the extradimensions don't exist, then the string theory can never work also - which means, it has been falsified in this way.

    It's trivial logic, my dear Watson/Bee. Please, refrain of notes, I don't understand something of your or whoever else comments. If nothing else, it doesn't add anything to arguments.

  29. Zephir: I have deleted several of your comments. The amount of misinformation in them is more than I am willing to tolerate. *Searches for* extra dimensions and supersymmetry cannot prove that they do not exist, and consequently negative results do not tell you anything about the viability of string theory - so much about your, uhm, logic. Now please stop clogging this comment section with nonsense. Best,


  30. So Zephir,
    You are raising points of philosophy which could be science some day so defined and understood, perhaps a day. Infinitely. Far away
    Consistency or inconsistency is thus a philosophic stance that an ordinary boring truth is not justified in the now by any narrow assertion or result of experiment felt as plain as the nose on our face. Have we falsified QM at its foundations by not finding evidence of extradimenitional supersymmetry said to be developed from it. Have we falsified looping QM phenomenology by not finding mini black holes likewise?
    Here, perhaps without enough or limits to observable data philosophy and science fiction meet in debate. If time travel is constrained physically do we in facts assert or must forever speculate,
    Where did the idea of tachyons vanish in the SUSY some ask in an alternative view of its purity?
    On the light cone at least philosophic time seems a higher balanced symmetry (over all geometry and equations with mirrors and assumed deeper connections of diverse math forms).
    But what of the potentially infinite past? Would our worry over eternal soul vanish into inherent asymmetry an that broken then? Half QM cats alive?
    The child in me enjoys stories repeated many times along with intuitive errors.
    Sabine explains such utility of determined facts of path defects if we read her right. But my philosophic frontier is a little shaky for I misread or got backwards what we mean by something more than or less than the. Planck level as a matter of scale.

  31. It is OK if in the interest of the continuity of the comment section you delete my reply to Zephir, Professor Hossenfelder. No problem. Others had some good comments relevant to which I would have rather taken time to address. Best

  32. By saying that the QG effects should be seen at Planck scale, I meant that they should be
    significant at the Planck scale. However, there could be non-perturbative QG effects at
    cosmological scales.

    The value of the cosmological constant is a QG effect, which can be determined perturbatively (see my papers), but since CC is a constant (independent of a length scale) we are able to measure its value.

  33. I found your little Q&A on quantum gravity very helpful in (better) understanding the tension between general relativity and quantum mechanics.


  34. I get annoyed if someone keeps asking me the same question, but on the other hand I have noticed that when people say something which they think is a good idea they may repeat it, even immediately after they first said it. I may even have done this myself.

    You should publish a book of these great essays.

  35. Maybe because people believe/ afraid not to be understood correctly

  36. maybe gravity doesn't couple to energy, energy is simply a proxy, gravity couples to something else that is effected by energy.

  37. Hmmm... after seeing the amplituhedron video something deeper is hinted at yet still vaguely that bypasses the question of QM and GR physics as unity and locality derived as a four way deal by simple drawing. After all the idea of gluons being unexplained as periodic has always seemed to me too simple and rather absurd. But the terminology we have inherited is way too complicated. Let that be a future question of speculations where we try to say something like gravity coupled to energy or still debate at what level of things philosophically are free will or determinism that does not carry over to the acid test as physics. We are like the Esquimo's who have small slits in their wooden glasses to ease the snow white glare, many world or one world,many dimensions or a few, through these quantum slits beyond duality we see deeply with our mind.

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  40. Nice article.

    You make large mistake when you say

    "most researchers in the field today" .

    If history is any guide (and surely it is) then we know that on the bigger questions, most researchers in the field at any given time are absolutely wrong.

    Also please look up 'this time its different' - a term from economics, which applies equally well to scientific endeavour.

  41. Tom:

    First you claim that it is wrong that "most researchers favor this option", then you say history says most researchers are wrong - what is it that you want to say?

    It is not true that history says most researchers are generally wrong, this is a ridiculous statement. It is true that in some historical cases this has been so. Further, I didn't take a survey but I am very sure if you'd do one you would find that it is correct that most researchers today favor this option. In summary, I don't know what you think it is wrong.

  42. Sabine,

    Sorry you misunderstood. I fully agree that most researchers think that we must quantize gravity. If history is any guide, the collective wisdom of science has always been wrong - at least on the big picture stuff.

    I was saying your mistake was to assume that since most researchers think that we need to quantize gravity, it gives more weight to the likelihood of that path bearing fruit. After a generation of little progress, it's likely that the quantize gravity program will not work.

  43. Tom,

    "I was saying your mistake was to assume that since most researchers think that we need to quantize gravity, it gives more weight to the likelihood of that path bearing fruit."

    I never said that, please stop putting things into my mouth.

  44. " It is in principle also possible that instead there is a way to couple quantum and classical theories that has so far been missed, or that the underlying theory is in some sense neither classical nor quantum, but this option is not favored by most researchers in the field today. "

    Half of your post is an advert for quantum gravity, you diss alternatives with the above quote. Its fine to think that quantum gravity is the way forward, but groupthink should carry 'anti weight' if the history of physics is to be respected.

  45. I am not "dissing" anything or anybody - I am simply informing the reader about what is going on. If you don't like what's going on that's an entirely different matter.

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  47. Nemo: I appreciate your interest, but please do not discuss your work here. This is not a good place to do it. Best,


  48. Sorry, that's right!

  49. 2. Black Holes exist.

    er, so if a big Goblin is eating all the photons at the centre of our galaxy that's also a "Black Hole" is it?

    Sorry Sabine, but the Event Horizon is not just the opposite of a hard surface, it is a mathematically precise prediction constructed using classical General Relativity

  50. In case my comment was misunderstood, by "Goblin" I mean "unknown physical mechanism".

    eg. Maybe all particles get converted to the lowest stable supersymmetric particles at the extreme Quantum Gravity regime within the "black hole" - so only dark matter gets produced - so nothing visible.

    or a zillion other possible weird QG things that we have no idea about.

    Faith in classical GR is a bit too much imho



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