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Paradoxes resolved by QM

  1. Mar 7, 2005 #1
    Puzzling paradoxes that classical explanations fail to resolve, but are resolved by QM include:
    1 Electrons don’t crash into protons.
    2 Two slit pattern for individual particles
    3 Entangled polarization issue as used by Bell
    4 Larger particle spins also used by Bell
    5 light stopped by H & V polar filters, by pass through H then Diag then V polar filters
    . . . (Or should 5 be considered part of 4)

    Are their any additional important paradoxes that it took QM to resolve??
     
  2. jcsd
  3. Mar 7, 2005 #2
    Any behaviour of a system where the quantity of action involved is comparable to Planck's constant cannot be described correctly without using quantum mechanics.
     
  4. Mar 7, 2005 #3

    SpaceTiger

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    Let's not forget the famous ultraviolet catastrophe.
     
  5. Mar 7, 2005 #4
    Essentially, we have

    1890 (sometime in that decade) -- Randomness of radioactive phenomena
    1900 -- Blackbody radiation
    1905 -- Photoelectric effect
    1915 (near that date) -- Stability of atoms
    1923 -- Matter displaying wave effects

    There maybe more, but these were the major ones before QM was discovered (as far as I know). After QM was discovered, a whole host of phenomena have been discovered. For example, anti-particle phenomena, entanglement etc.
     
  6. Mar 7, 2005 #5

    Cat

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    But this is easily explained using classical wave theory ...

    Cat
     
  7. Mar 9, 2005 #6
    But not of course when only individual photons are allowed to pass. I'll update wording.
    RB
     
  8. Mar 9, 2005 #7
    Paradoxes break down into 5 categories:

    1 - Light stopped by H & V polar filters, but passes H, then Diag, then V polar filters - even when only individual photons are passing.

    2 - Entangled issue as used by Bell: polarization for light, spins on larger particals

    3 - Two slit pattern for individual particles, (Matter displaying wave effects)

    4 - Electrons do not crash into protons. Including Paradoxes based on the quantized energy levels of electron orbits ie. Stucture of atom.
    . . . . .(ultraviolet catastrophe; Blackbody radiation; Photoelectric effect; Stability of atoms)

    5 - Randomness of radioactive phenomena

    Any others?
    Or 'category' corrections?
     
  9. Mar 9, 2005 #8

    Cat

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    But when has this been tried using "individual photons", and, if it has, how did they prove these were what they used? I don't know of any experiment that can truly distinguish between short pulses of light and "photons". When an instrument is interpreted as detecting a photon, the effect might (as some physicists recognise) be merely the result of the total intensity of signal plus noise exceeding some threshold. In other words the interaction at the detector (the photoelectric effect) can equally well be explained without photons. [*]

    Under wave theory there is no paradox. Even the shortest and weakest pulse of light is expected to obey Malus' law. Whatever its initial polarisation, it emerges from the first polariser as a pulse polarised in the direction of that polariser's axis. If it then encounters a polariser at 90 deg it is blocked, but if it encounters instead one at 45 deg a pulse of half the input intensity, polarised at 45 deg, emerges. This pulse suffers another halving of intensity at the final polariser. What emerges is a pulse at 1/4 that of the pulse from the first polariser. I don't know much QM but would assume that the predictions, when translated from intensity to probability language, agree.

    Cat

    [*] See Clauser, J E, “Experimental limitations to the validity of semiclassical radiation theories”, Physical Review A 6, 49 (1972)
     
  10. Mar 9, 2005 #9

    anti_crank

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    ZapperZ's journal contains a number of worthwhile reads on the photon picture and experimental status thereof:

    Part 1

    Part 2

    It seems the browser won't jump to the right entry; the entry I refer to as Part 2 is located at the very bottom of the page.
     
    Last edited: Mar 9, 2005
  11. Mar 9, 2005 #10
    explanation of why conductivity decreases as temperature increases for a semi-conductor.
     
  12. Mar 10, 2005 #11
    Lets see...there are a number of unresolvables that require QM besides those not yet mentioned:

    Well it probably not that important...but..er...

    *The exact ground state of the hydrogen atom; :bugeye:
    along with each electronic transition level (i.e., the exact reason for each absorption & emission spectra).
    Well its probably not that important... :tongue:

    Of course also...
    *The Compton effect

    *The allowable energies for a particle in a potential well and...
    the probability of tunneling through a potential barrier, as in alpha decay from radioactive nuclei.

    For that matter...
    *tunnelling in a Josephson junction in a superconductor...
    *The AC and DC Josephson Effect in a superconductor.
    *Flux quantization in superconductors

    * Allowable energy states for atomic harmonic oscillators...
    * Rotational energy levels in diatomic molecules
    *Vibrational energy levels in diatomic molecules,
    And ...
    *Zero point energy

    Don't forget...
    *Electron spin angular momentum and magnetic moments,
    *and electronic Orbital moments and
    *spin-orbit couplings and fine structure & hyperfine transitions
    *the exact value of the Bohr magneton
    *Exclusion principle

    *Nuclear spin & nuclear angular momentum
    *The nuclear magneton
    *Lamor precessional frequency and NMR

    *Stimulated absorption and stimulated emission (lasers and masers)
    *Fluorescence & phosphorescence

    *Flux quantization in superconductors
    *Quantum rotation in helium superfluids
    *Superfluidity in condensed systems
    *Superconductivity in superconductors
    *Specific heats of quantum solids (solid Helium)
    *Quantum Hall effect
    *ad infinitum
    *
    *
    Well, its probably not too important; but just thought a few more would prime the pump. :biggrin:

    Creator :biggrin:
     
    Last edited: Mar 10, 2005
  13. Mar 10, 2005 #12

    ZapperZ

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    There need to be some elaborations here.

    First of all, it is true that as far as the photoelectric effect is concerned, while the result makes a strong and compelling argument for the photon model, it cannot completely rule out the wave model of light. This, however, doesn't mean that the photon model is wrong.

    Secondly, there's more to the photoelectric effect that meets the eye, so to speak. The "standard" photoelectric effect is what we now call the single-photon photoemission process. This is where the only way a photoelectron can be produced is via the absorption of a single photon only (all or nothing). Under this scheme, if a photon doesn't have enough energy to liberate an electron from the cathode (hf is below the work function), then no matter how high the intensity of the light is, no electrons will come out. This is what we all learn in standard intro physics as being the photoelectric effect.

    However, we now know that this is not true. Ironically enough, this violation provides an even STRONGER evidence of the photon picture. If the intensity is large enough, meaning the photon density per unit area is large enough, then it is possible that even if the photon energy is below the work function, there is a strong possibility that 2 or more photons could cause an excitation that will still produce photoelectrons! This is where it gets interesting. If let's say I have a situation where (W is the work function)

    hf < W < 2hf

    then a 2-photon absorption is possible for high intensity light source. Similarly, for

    2hf < W < 3hf

    then a 3-photon photoemission is possible.

    We can TELL which is occuring by looking at either the spectrum of energy[1,2] of the emitted photoelectrons, or by looking at the photoelectron current versus light intensity.[3] These experimental observations CLEARLY shows the discrete quanta of absorption of light energy by the photocathode that occurs only in multiple of hf. Note that this discrete aborption cannot be attributed to the material since the conduction band and the vacuum level in the metallic cathode, especially, is CONTINUOUS.

    So yes, while the ordinary photoelectric effect experiment certainly cannot rule out the wave picture of light, higher-order multiphoton photoemission observations present a damn convincing case for the photon picture. As far as I know, there have been ZERO attempt at trying to explain this using the wave model - and the multiphoton photoemission phenomenon has been known for at least 15 years.

    Zz.

    [1] W.S. Fann et al., PRB v.44, p.10980 (1991).
    [2] U. Hofer et al., Science v.277, p.1480 (1997).
    [3] A. Damascelli et al., PRB v.54, p.6031 (1996).
     
  14. Mar 11, 2005 #13
    Don't put yourself down - I think there important,
    Most you mention I think fall into the general category of #3 from post 7:
    You also have some that look to belong to the with # 5,
    But you also have several I’m not familiar with,
    some that maybe - - - well do you think some are unique enough to separate from the others in their own category(s)?
     
  15. Mar 11, 2005 #14

    Cat

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    Hmmm ... that's all very interesting, but I'm not sure it is relevant to our particular situation, which concerns the transmission of light through an orthogonal polariser when one at 45 deg is inserted. I should still be interested to know if this experiment has actually been done with entities that were claimed to be "single photons". We can safely assume that we would use detectors sensitive to the frequency involved, so would not need to have high intensity to produce a detection. It would be just the ordinary photoelectric effect.

    Cat
     
  16. Mar 11, 2005 #15

    ZapperZ

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    I don't understand your question.

    I wasn't addressing the issue that is the topic of this thread, just your side comment on the photoelectric effect.

    I don't know what "detectors" you were refering to. Detector for the photocurrent? I can name a few - CCD dectector as in the Scienta SES100 spectrum analyzer and Integrated Charge Transformer that is used to measure current passing through a linear accelerator beamline. I have used both.

    The single-photon emission are verified the very same way as the multiphoton photoemission. The ordinary version of the photoelectric experiment is a STANDARD experiment in undergraduate intro physics labs (have you done one?) using Hg discharge lamps. Even the explanation using the "wave" picture is consistent with the dependence of the material's work function as some sort of a cut-off on the production of photoelectron. So if you question this, you are also questioning even the wave picture explanation.

    Just so you know, I am not just talking this based on what I read, or just what I have heard. I have experimentally done both single-photon photoemission experiments (angle-resolved photoemission to be exact) and multiphoton photoemission. If someone has a wave model to explain all this, I've yet to see one.

    Zz.
     
  17. Mar 12, 2005 #16

    Cat

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    Fair enough. Do I take it we are agreed that wave theory is just as good as QM in explaining what happens when light passes through several polarisers?

    I was referring to any detector designed for the wavelength in question.

    No, I've never done any experimental work but I've studied a considerable number of experimental reports and come to a few tentative conclusions. The whole picture is pretty complicated, since certain sources clearly do produce light in discrete units. Are these units "photons", though, or are they merely short bursts of light?

    You can try and distinguish the two by passing them through beamsplitters, but I've been wondering about these. In the past, it seems, they really did use "half-silvered mirrors", but these days they use, for instance, "polarising cubes". These are made of pairs of prisms with their diagonal faces separated by layers of dielectric and/or metal. The thickness of the layers is carefully engineered to be as near as possible exactly half or exactly 1/4 of the wavelength of interest. It is observed that when you have coherent light it has a strong tendency to exit such a cube almost entirely through one or other of the output ports, even if the cube is "non-polarising", and this is interpreted as showing photon behaviour. My idea is that maybe what it is really showing is the result of wave interference. The choice between one output port or the other might depend, for instance, on the exact frequency of the particular pulse, the exact interference effects it suffers as it is partially reflected at all the boundaries varying with the wavelength.

    Several of the light sources used in modern experiments almost certainly produce a series of pulses, each of slightly different frequency ... But this is a long story! It goes back for me to some interesting experiments on "induced coherence". The experiment that clinched the idea was one by Kwiat and Chiao in 1991. I can look up the ref if you're interested.

    Anyway, the long and the short of it is that I would expect the nature of the light source to be very important in determining the observed coincidences from a beamsplitter. An attenuated coherent laser beam might well give a different result from a PDC source.

    Tell me the experimental details and I might have a try!

    Cat
     
  18. Mar 12, 2005 #17

    ZapperZ

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    Nope, not when we are dealing with single-photon emitter. anti-crank cited a couple of experimental papers that dealt with this that I listed in my Journal. You may want to check those out first before settling to that conclusion.

    I'm not sure why this would affect anything. And I am not sure why you would bring this up, since I made no mention of any light/photon detectors. Photoemission deals with electron detectors and analyzers.

    No, a light "pulse" as in the normal sense is not a "photon". A light pulse may consist of a large number of photons. You need to be more explicit in describing these things rather than just "certain sources". If not, we would be talking about different things.

    I did! I mentioned angled-resolved photoemission spectroscopy (ARPES). If you have access to a Scienta SES200 electron analyzer, then try it. I have described at length the photoemission process (which is now one of the standard method at verifying a material's band structure) in another of my Journal entry, including several ARPES links done on materials such as High-Tc superconductors. In fact, my avatar is an actual raw experimental ARPES data on an overdoped Bi2212 high-Tc superconductor in the nodal region that I personally measured (and published).

    Zz.
     
  19. Mar 13, 2005 #18

    Cat

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    Am I correct in deducing from your journal that you are relying on reports such as Thorn et al's Am J Phys one for evidence that the photon does not split? I must re-read this.

    I've encountered many different "single photon" sources in my reading. They include Aspect's calcium radiative cascade, PDC sources, Santori et al's quantum dots and a few others. They are all low-energy sources. I think the rules for high energies are slightly different, in that for these a single "photon" is strong enough to trigger a detector response, whilst for the low energies I'm interested in it needs help, either from added noise or from a positive voltage bias.

    As I said, I am no experimenter! I have no access to a lab.

    How did we come to be talking about photoemission? What I was talking about was the detection of photons.

    Cat
     
  20. Mar 13, 2005 #19

    ZapperZ

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    You asked "Tell me the experimental details and I might have a try!" So I did, and I WAS talking about photoemission. Furthermore, how do you think most photon detectors work? A photomultiplier requires the first emission of an electron via a photoemission process, and then using subsequent secondary emission to cause a cascade. Or you could have a photodetector that essentially is a semiconductor that causes a current within a circuit.

    You also seem to have some confusion between "low photon" with low energy and high intensity with high energy.

    I would also like to say something else here. In all of the so-called arguments I've seen with people who are disputing the "photon" picture, here's the ONE thing that I have never seen - the prediction on where the photon picture fail but the wave picture prevails. All I have seen are arguments that say to the effect that "well, wave picture can explain that too".

    Now, you have to admit that, if the standard physics (as in QM description that has been so successful with light and elsewhere) indicates the validity of the photon picture, then if all the wave picture can do is agree with that, then what's the point? Classical physics CLEARLY indicates that particles and waves are NOT compatible. At some point, they HAVE to diverge. The which-way-type experiments have clearly started to show this divergence. Now people who are trying to push the wave picture needs to show and design an experiment that can support their arguments. They can no longer coattail on the same experiments that continue to confirm the photon model. And as I've said, no one has even bothered to attempt to explain these multiphoton photoemission results using wave models (because they can't!).

    Zz.
     
  21. Mar 14, 2005 #20

    Cat

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    As far as I can tell, the photon picture has never satisfactorily explained the most fundamental properties of light, in particular, interference effects and the association of a frequency with a particle-like photon. As Bohr said at the 1921 Solvay meeting:

    “[The hypothesis of light quanta] presents insuperable difficulties when applied to the explanation of the phenomena of interference ... [it] excludes in principle the possibility of a rational definition of the conception of a frequency ...” Hendry, John, “The Creation of Quantum Mechanics and the Bohr-Pauli Dialogue”, D Reidel Publishing Company 1984, page 28

    Some more recent facts that QM cannot explain satisfacorily are the results of the Bell test experiments. QM attempts of explain them simply by assuming a formula that fits selected observations, but it has never given any causal explanation for that formula and it seems to me no better than a myth. The photon model is causing people to ignore alternative explanations for the same results. The most realistic alternatives assume a wave model of light that allows the intensity of each "photon" (assumed here to be simply a pulse of classical light) to be split at a polariser. The raw data of some of the experiments can be fitted directly by such a model, simply assuming the classical version of Malus' law and then assuming "perfect" detectors that produce counts in exact proportion to input intensities. For other experiments we need to assume slight deviations from these assumptions, but still within the general picture of a wave model and an intensity reduction of every pulse at a polariser, as opposed to the QM idea of reduction in probability of passage.

    Clauser and Shimony in their 1978 report described the basic idea, though, as far as I can remember, they did not take the next step of considering the possibility that real apparatus might not convert input intensities exactly into probabilities. It seems to me that the conversion will never be exact, due to the existence of "dark counts" at the low end of the scale and of saturation at the high end. I have not been able to find any discussion of this point, all the papers assuming the ideal case.

    Yes, but I think too many of the papers on the subject have been by theorists who have not stopped to think about the actual mechanisms involved in each bit of apparatus! The idea I had yesterday about the choice between two outputs of a beamsplitter perhaps being due to tiny variations in frequency was probably, on second thoughts, wrong, but there are other possibilities. The experiments do seem to show that the intensity is not split 50-50 but they do not show convincingly that it is the all-or-nothing effect given by the photon picture. I suggest that we simply have not tried hard enough to update the wave model to allow for recent observations.

    Suggested experiments:

    1. Repeat a few Bell test experiments but this time investigating a range of different beam intensities, varying the beam intensities by at least two different methods. I predict that a wave model will fit the results better than the QM one, in which beam intensity plays no part.

    2. Try splitting an unpolarised beam then splitting again a few times and see if the resulting detections really do follow the expected pattern. If we start with N photons per sec, QM would predict N/2 after one split, or slightly less by a factor e, say, so call it eN/2. The prediction after k splits would presumably be (eN/2)^k. I predict that the counts will fall off faster than this, since the intensity per pulse will reduce at each stage and will reach a point where it is not distinguishable from the dark count. [I must admit that I wish I had access to a lab so that I could find out the pitfalls in the above before committing myself!]

    Cat
     
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