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I QM has never made a prediction contradicted by experiment

  1. Sep 16, 2016 #1
    I have had many people claim that QM has never made a prediction that has been contradicted by experiment.
    Yet as i understand it Qm predicts the vacuum energy density is 10^122 erg per cubic cm whereas the measured energy density of 10^-8 erg per cubic cm. So how is that not an example of an experiment contradicting QM?

    of course Im aware that most people think there will be some modification to QM to correct this but who does one say theres no experimental evidence to contradict QM? It seems to me there clearly is, where have I gone wrong?
     
  2. jcsd
  3. Sep 16, 2016 #2

    Vanadium 50

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    Please show us this calculation.
     
  4. Sep 16, 2016 #3
  5. Sep 16, 2016 #4

    atyy

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    You have gone wrong because that is not a prediction of QM. It is only a prediction of a naive way of doing QM. A more accurate way of thinking will show that there is no problem of prediction.
     
  6. Sep 16, 2016 #5
    Thank you for you quick reply, is there not a problem here though in that when QM gets something so obviously wrong we call it naive ?After all from your answer it seems to suggest a hope that one day Qm will get it correct. I would bet this problem will be solved, but dont we have to admit that right now it isn't solved? and that right now the predictions of the theory as it currently understood do not only not match observations but they are the worst predictions in the history of science?
     
  7. Sep 16, 2016 #6

    mathman

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    This issue is not a question of quantum mechanics or general relativity, but the fact that it has been observed the expansion of the universe is accelerating. To explain it, the concept of dark energy has been proposed, but the entire subject is an area of physics where almost nothing is known. Vacuum energy is an attempt at an explanation, but as noted, quantitatively it is way off.
     
  8. Sep 16, 2016 #7

    atyy

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    The problem right now is solved.

    It is related to Einstein's "greatest mistake". The equations of general relativity contain a "cosmological constant". Einstein went back and forth over whether that term should be included. First he excluded it because it wasn't pretty. Then he included it for some wrong reasons. Then he excluded it after learning the reasons were wrong. However, the "cosmological constant" is consistent with all the reasoning that Einstein used to reach general relativity, and so can be included. Whether its value is zero or not is simply a matter for experiment. Experiment simply shows that the cosmological constant is not zero, thus resolving the problem.

    There is a deeper lesson involving speculative theories like string theory, but you should learn the basic one first: there is no problem, and quantum mechanics is consistent with all known observations.
     
    Last edited: Sep 16, 2016
  9. Sep 16, 2016 #8
    Actually there are other anomalies relating to QM, for instance the anomalous muon magnetic moment and the 17 MeV anomaly in Beryllium nuclear decays. "Anomaly" means, theory doesn't match experiment. In every case we guess there must be some new force or particle - or something - to explain it. If so there's nothing wrong with QM theory per se.

    It's important to note there have been dozens of anomalies over the years (EPR is, sort of, one example). Every time it turned out the theory was right; some other misunderstanding, mistake, or incomplete knowledge was responsible. So it's reasonable to suppose these currently-unexplained anomalies will, also, not invalidate the theory.

    In the specific case of so-called vacuum catastrophe, it involves gravity. The intersection of QM and gravity is problematic; there are definitely unanswered issues. This is just one of the most visible.

    In my opinion this optimism is, in fact, justified. Most likely all these anomalies are due to something other than QM. Nevertheless at the moment we don't know that. So I agree with your implied point: we really shouldn't make the blanket statement that QM never made a wrong prediction. There should be a caveat: "except in a few questionable cases, where QM will probably turn out to be right when the anomalies are completely understood".
     
  10. Sep 17, 2016 #9

    vanhees71

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    Well, I'd say the value of the "cosmological constant" and its physical meaning is an enigma (as are the many parameters of the Standard Model with the tendency to become even more for models beyond the Standard Model). We can just take them as parameters and determine them from experiment. There's no theoretical concept that can explain them. I think concerning the "cosmological constant" or "dark energy" or however you want to label the enigma, the status is not much advanced from the status summarized in Weinberg's famous paper on the subject:

    S. Weinberg, The cosmological constant problem, Rev. Mod. Phys., 61 (1989), p. 1.
    http://www.itp.kit.edu/~schreck/general_relativity_seminar/The_cosmological_constant_problem.pdf [Broken]

    10 years later he wrote another summary

    http://arxiv.org/abs/astro-ph/0005265
     
    Last edited by a moderator: May 8, 2017
  11. Sep 17, 2016 #10
    I take it you are taking the approach to the vacuum catastrophe that Carlo Rovelli implied in this paper: http://arxiv.org/abs/1002.3966 . If I've understood it correctly , he is denying that the cosmological constant is the vacuum energy , is that right? In which case I agree this is a potential solution but I think we have to accept its not widely accepted as "the" solution by the community.
     
  12. Sep 17, 2016 #11

    A. Neumaier

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    No. It means ''not normal'', a difference to naive expectations. Most things called anomalies are in fact today well-understood; they just need more complex models.
    You should understand that QM is not a unique theory making predictions of everything. It is a framework in which to model everything. As a framework, it has been undefeated.

    However, depending on the model, predictions may be good or bad. Thus quantum mechanical models can be wrong or inaccurate. This doesn't affect quantum mechanics as a theory - which is the collection of facts and techniques that can be applied to particular models to yield the predictions.

    To say that QM has never been contradicted means in more detail that whenever a quantum phenomenon was better understood it could be correctly represented by a quantum model. A few things are still open - but essentially everyone expects that they can be accommodated within standard quantum mechanics once the models are sufficiently improved. The problems with dark energy, the value of the cosmological constant, etc., are due to specific models, not to the theory as such.
     
  13. Sep 17, 2016 #12

    bhobba

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    Its actually zero - see normal ordering:
    https://en.wikipedia.org/wiki/Normal_order

    But it is the the first sign of a sickness in QFT that renormalization is needed to fix.

    Thanks
    Bill
     
  14. Sep 17, 2016 #13

    Vanadium 50

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    This thread is going in circles, because this false statement has gone unchallenged. Or rather, the OP hasn't answered the challenge.

    QM does not make this prediction. A prediction involves the expression you are calculating, an equals sign, and a right hand side where every element is known. We don't have anything like this. At best we have guesses. But "when we guess, we get nonsense, therefore QM is wrong" is not a very good argument.
     
  15. Sep 17, 2016 #14

    atyy

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    Rovelli has his own idiosyncratic view on it, but the solution is accepted by the community (way before Rovelli's papers). You can see Nima Arkani-Hamed state it in his Messenger lectures http://www.cornell.edu/video/playlist/nima-arkani-hamed-on-future-of-fundamental-physics.

    The challenge of the cosmological constant to theoretical physics does not come from any current observations and currently accepted quantum theory, but from theoretical considerations relating the still unknown quantum theory that is needed at the Planck scale.

    Current quantum theory can accommodate all current observations.

    If you know about the hierarchy problem and low-energy supersymmetry, the situation is the same. Both the hierarchy problem and the cosmological constant problem are fine-tuning problems. Fine-tuning is not a challenge from observation to current theory. Rather fine-tuning has to do with how we think about the relationship between still unknown theories and current theory.
     
    Last edited: Sep 17, 2016
  16. Sep 17, 2016 #15
    OP's basic concern - is QM really 100% consistent with experiments? - is better addressed by the anomalies mentioned below.

    What about the anomalous muon magnetic moment and the 17 MeV anomaly in Beryllium nuclear decays? They can be explained by postulating currently unknown (possibly virtual) particles, but - according to physicists working in the field, as you can see via Google - there's a tiny chance that QM doesn't "accommodate" these observations. Meaning, the theory might need modification. Very unlikely, but the anomalies remain unresolved.
     
  17. Sep 18, 2016 #16

    Vanadium 50

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    Neither of those, if confirmed, means QM is wrong.
     
  18. Sep 19, 2016 #17
    It seems there are also discrepancies for the Stark effect:
    https://www.physicsforums.com/threads/stark-effect-theory-vs-experiment.885330/

    Let's try to gather more of them.
    I got the Stark effect from Gryzinski's book where he lists also other situations he claims QM gives nonsatisfactory agreement with experiment (especially for various scattering situations) - I would like to test his claims.
    He had 25+ articles 1957-2000 in top journals (Phys. Rev. class), in which he uses purely classical calculations, getting surprisingly good agreement with experiment, especially for various scattering situations - these papers have currently ~3000 total citations: https://scholar.google.pl/scholar?hl=en&q=gryzinski

    It is not about being wrong, but inaccurate in some situations.
    For my physics PhD I was working on Maximal Entropy Random Walk (https://dl.dropboxusercontent.com/u/12405967/MERWsem_AGH.pdf [Broken]) which shows that standard choice of diffusion models is approximation of the way statistical models should be chosen (principle of maximal entropy) and it turns out that doing it right we get exactly quantum probability densities - in contrast to popular view, properly made diffusion is not in disagreement with thermodynamical predictions of QM.
    A long story short, I see QM as effective description of dynamical equilibrium - it is great for static situations, but might have issues with trying to predict e.g. scattering - it would be very valuable to understand well these weaknesses.
     
    Last edited by a moderator: May 8, 2017
  19. Sep 20, 2016 #18
    There is lots of strange stuff about vacuum energy. For example, why doesn't it have mass? The Casimir effect makes it pretty clear that something is going on but if E=MC^2 holds then quantum foam should be observable by its gravitational properties. One theory is that the particles appear and disappear in conjugate pairs. But pairs of what? According to QM and experimental data matter and antimatter both have positive mass. If QF had even a minuscule mass it would create an observable cosmological constant. If the QF particle pairs were of positive and negative mass then the mass might cancel out. But, as far as I know there are no negative mass particles predicted by QP. Maybe it's a deSitter and anti deSitter space kind of thing. The standard model doesn't have much to say about gravitation so in that sense it can be considered incomplete.
     
  20. Sep 20, 2016 #19
    You can assign an arbitrary 0 point to the energy scale. So, you can make the vacuum energy whatever you want, and there's no way to measure the energy of the vacuum, except relative to something.
    The cosmological constant depends on the choice of the 0 point of the energy scale. Now, if we fix the value of the cosmological constant to 0, then this assigns a particular value to the vacuum energy that we can measure via the expansion of the universe. We don't know how to calculate this value of vacuum energy.
     
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