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OT: Oh oh... Possible error in QED with very strong experimental evidence.

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  • OT: Oh oh... Possible error in QED with very strong experimental evidence.

    While not a subject of much attention for most the Theory of Quantum ElectroDynamics (QED) is one of the best established and solid (they thought) theories in physics. It deals with how electromagnetism and light interact with matter. It has everything to do with atomic level electronic interactions and optical systems, reflection, lasers, lenses and all related phenomena. Without QED, Quantum Mechanics cannot deal with light. QED is an essential part of Quantum Field Theory. Everything we think we know about how light works is based on QED.


    It has just been reported that an experiment performed at NIST has produced a result that does not agree with QED. The discrepancy is very large and the certainty is 5 sigma. The experiment is relatively simple and easy to replicate. The observation is simple to make: Under certain fairly uncommon circumstances highly ionized titanium emits a different colour of light than QED predicts. This goes directly to the heart of QED. It remains to be cross checked and replicated but it doesn't appear to be an experimental mistake. The paper has been published by Physical Review Letters, one of the most respected journals in science.

    http://prl.aps.org/abstract/PRL/v109/i15/e153001

    Popular story:

    http://www.nanowerk.com/news2/newsid=27724.php
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  • #2
    Perhaps it is incomplete, and not incorrect.

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    • #3
      Im curious as to what Fasttrack thinks.

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      • #4
        Perhaps it is incomplete, and not incorrect.
        Not in this case. This is an effect that it predicts but appears to predict incorrectly.
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        • #5
          Originally posted by Evan View Post
          The Popular story states

          If 20 of titanium's 22 electrons are removed, it becomes a highly charged ion that looks in many ways like a helium atom that has been shrunk to a tenth its original size ..


          I don't understand how the titanium ion can look smaller than a helium atom, since the Ti nucleus is larger than the He nucleus. Are they talking about the relative sizes of the electron clouds?
          Allan Ostling

          Phoenix, Arizona

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          • #6
            The actual size of the nucleus of an atom plays no part in the electromagnetic properties. It is the overall charge that matters. The "size" of an atom depends on its ionic radius. The more negative it is the larger the ionic radius and the more positive it is the smaller. An non-ionized helium atom has a particular ionic radius. A titanium atom missing 20 electrons is very strongly positive and so has a very small ionic radius. The positive ion has a stronger attraction on the remaining electrons so it pulls the electron shells closer. The negative ion has less unit attraction per electron (from the protons) so the shells expand, keeping in mind that the electrons repel each other and aren't subject to the strong force.
            Last edited by Evan; 11-29-2012, 12:41 AM.
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            • #7
              I am betting on QED. To even do the QED calculation you have to ignore many things. Your justification for doing so is the assumption that the ignored terms are small and do not contribute to the answer. These things are called perturbations by those who do the calculations. You can check to see that a given term is small enough to ignore, but it is at time hard to tell if you left anthing out. You have to account for all of the important perturbations to get the correct answer. So they may not have all of them present and accounted for. Another possibility is an interaction that QED does not account for at all. For example, QED does not take nuclear forces or gravity into account. If, for example, nuclear forces did make a difference in this case, QED is not wrong, it simply does not describe the details of this experiment. Time will tell, but QED has been called into question in the past, but it won out in the end.

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              • #8
                I caught this thread early! Usually I miss the interesting threads until they hit 4 pages or longer...

                The "popular story" and the press headlines are extremely misleading. In fact, the title of the PRL paper is the key to this conundrum:

                Testing Three-Body Quantum Electrodynamics with Trapped Ti20+ Ions: Evidence for a Z-dependent Divergence Between Experiment and Calculation
                (Emphasis added)

                Three body QED is not an analytic theory. In fact, as an undergraduate, I worked on an experiment very similar to this one studying the three body problem in QED. About two years ago we found a divergent discrepancy between a popular theory and experiment for helium at very low energies that was published in PRL.

                I think the quote from Jonathon Sapirstein (professor of physics at Notre Dame) is the best summary of these new results (snipped from the popular story):

                "What the NIST experiment found is interesting enough that it merits attention. Independent calculations should be done to confirm the theory, and other experiments should also confirm the findings. However, if no errors are found in the theory and the NIST experiment is correct, some physics outside of QED must be present."
                As Tony Ennis suggests, this may very well be a case of incompleteness and not incorrectness. What the popular article doesn't reveal is that this wasn't a test of QED but a test of our understanding of the three body problem in QED. At this level, it's not a matter of simply having a formula and plugging parameters in to get a result. Scattering in QED requires fairly sophisticated mathematics; mathematics beyond our normal algebra ... meaning that the objects in our formulas don't obey basic relations like AB = BA but may still be single objects (e.g. not matrices or tensors). For instance, one approach to the quantum field theory describing fermions (e.g. the electrons in this experiment!) requires the use of things called "Grassmann numbers", which are anti-commuting. If A and B are Grassmann numbers and X is a "normal" real number, then we have the following algebra:

                AB = - BA
                AX = XA
                A^2 = 0

                These are true for any Grassmann number and any real number.

                Well that was a little diversion, but the point is ... the popular story over simplifies the problem with the result that it makes a somewhat extraordinary claim. Certainly, this is an exciting result, but it is not evidence that fundamental QED is incorrect.

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                • #9
                  Originally posted by Bruce Griffing View Post
                  I am betting on QED. To even do the QED calculation you have to ignore many things. Your justification for doing so is the assumption that the ignored terms are small and do not contribute to the answer. These things are called perturbations by those who do the calculations. You can check to see that a given term is small enough to ignore, but it is at time hard to tell if you left anthing out. You have to account for all of the important perturbations to get the correct answer. So they may not have all of them present and accounted for. Another possibility is an interaction that QED does not account for at all. For example, QED does not take nuclear forces or gravity into account. If, for example, nuclear forces did make a difference in this case, QED is not wrong, it simply does not describe the details of this experiment. Time will tell, but QED has been called into question in the past, but it won out in the end.

                  Very well said! Currently, I am working on a process known as VBF: "Vector Boson Fusion" which could provide us with a sensitive experimental test of the Standard Model by examining collisions in the ATLAS detector at the LHC. To make a long story short, there are many subtleties involved in these types of studies. Everything from gauge choice/invariance to interference to "perturbations". Typically, the precision is set by the number of perturbations you include but understanding the interference is where the real difficulty lies.

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                  • #10
                    Originally posted by Evan View Post
                    A titanium atom missing 20 electrons is very strongly positive and so has a very small ionic radius.
                    Your explanation of atomic "size" makes the situation perfectly clear.

                    Now we await independent verification of the anomalous results. I wonder which would be a bigger scientific discovery -- the upcoming announcement of the Mars discovery, or this potential flaw in QED. These are heady times!
                    Allan Ostling

                    Phoenix, Arizona

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                    • #11
                      Originally posted by aostling View Post
                      Now we await independent verification of the anomalous results. I wonder which would be a bigger scientific discovery -- the upcoming announcement of the Mars discovery, or this potential flaw in QED. These are heady times!
                      Anything discovered on Mars will likely be a bigger discovery than the results reported here. As I explained above, the popular story is extremely misleading. All you have to do is read the title of the paper. If you make it a little farther, read the last few lines of the abstract [Phys. Rev. Lett. 109, 153001 (2012)]:

                      Our result for titanium alone, 4749.85(7) eV for the w line, deviates from the most recent ab initio prediction by 3 times our experimental uncertainty and by more than 10 times the currently estimated uncertainty in the theoretical prediction.
                      An ab initio prediction is a particular "brand" of numerical or computational methods used to solve multi-body QED. The fact that the theory and experiment are in disagreement does not mean that QED is wrong but that the numeric method used to solve the equations generated by QED are not useful for this type of interaction. "Ab initio" refers to the fact that these methods can be tailored to converge to exact solutions (when we can calculate exact solutions!) when some very specific criteria are met. In short, it's just as Bruce Griffing said. It's not at all trivial to do these calculations. It is very likely that the method of calculation is not suitable for this experiment and it is very unlikely that the fundamental QED governing the interaction is incorrect.

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                      • #12
                        I have difficulty seeing how there can be any equivalence between the 3 body problem at the macro scale and the quantum scale. It seems a bit misleading to even call them by a similar name. The fact that 3 body QED is not analytic and macro 3 body is also not analytic seems more a matter of coincidence (in actual fact there is a useless analytical solution to the regular 3 body problem, but not the general case).

                        Regardless, the result of the experiment is as I described and it remains to be seen if it can be verified with other elements. The authors state they have taken care to account for confounding factors and obviously there were no glaring errors seen upon peer review. We shall see. This experiment seems to be on much better ground than the faster than light neutrinos.
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                        • #13
                          NASA has completely backed off from the earlier statement by the project leader. They are now saying the results will be "interesting" to those that follow the program.
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                          • #14
                            An ab initio prediction is a particular "brand" of numerical or computational methods used to solve multi-body QED.
                            My understanding is that simply refers to a calculated set of beginning conditions as per QED (In this instance. It could be anything). The final conditions are then calculated based on the ab initio values and further inputs to determine the QED predictions. The experimental values are then compared.
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                            • #15
                              Core electrons at low levels are responsible for cold fusion, you know?

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