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Lamb and The Photoelectric Effect Without Photons

  1. Jan 25, 2010 #1

    bcrowell

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    Lamb and "The Photoelectric Effect Without Photons"

    I've recently been pointed by two different people to this paper by Lamb (yes, that Lamb) and Scully: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680009569_1968009569.pdf . The title is "The Photoelectric Effect Without Photons." Despite its famous first author, I think the paper is clearly bunk (and therefore it's not surprising it was never published in a refereed journal). Below is my analysis, cut and pasted from a FAQ I maintain at http://www.lightandmatter.com/cgi-bin/meki?physics/faq . I may, of course, be wrong. If you think so, let me know :-)

    Didn't Lamb and Scully show that you don't need photons to explain the photoelectric effect?

    No.

    There is a 1968 paper by Willis Lamb, Jr. and M.O. Scully, "The Photoelectric Effect Without Photons," that seems to have developed a life of its own on the internet. It was never published in a refereed journal. What you can find on the web is a pdf file made from scans of an internal publication of the Center for Theoretical Studies at the University of Miami.

    It may help to understand something about the historical context. Einstein hypothesized the photon in 1905, but his paper was ahead of its time and was not widely accepted. For decades afterward, even once the quantum-mechanical nature of the *atom* was assumed by all physicists, the quantum-mechanical nature of *light* was considered suspect. Bohr was influential in pushing a theory in which atoms were quantized, but the light they absorbed and emitted was classical. Lamb began his career during this era.

    If you read the Lamb-Scully paper, the first thing you notice is that they explicitly state that photons are absolutely necessary in order to explain phenomena such as blackbody radiation, Compton scattering, spontaneous emission, and the Lamb shift. Any internet kooks who are trying to quote Lamb as an authority against quantization of light are way off base.

    As in Bohr's old-fashioned dead-end approach, they then treat the atom as a quantum-mechanical system and the electromagnetic field as a classical one. They are able to reproduce the Einstein relation E=hf-W, where E is the maximum energy of the electron once it leaves the cathode, h is the quantum-mechanical Planck's constant, f is the frequency of the light, and W is the energy required for the electron to escape through the surface of the cathode. This is not particularly surprising or impressive in a bastardized quantum/classical calculation like this one; essentially it just says that the light wave has to have the energy taken out of it at a resonant frequency that matches its own frequency.

    They also show that the transition rate is nonzero even when the light is first turned on, saying that their transition rate "certainly does not imply the 'time delay' which some people used to expect for the photoelectrons produced by a classical e.m. field." This result is not as impressive as they make it sound, since the classical prediction is what one expects for a classical light wave impinging on *classical* atoms. In fact, the transition rate they derive shows the real problem with their calculation. Their calculation treats every atom as *independent* of all the other atoms. Therefore if a single photon illuminates the cathode, it may ionize more than one atom, violating conservation of energy. This unphysical result shows the opposite of what they claim; it shows that their mixed quantum-classical Frankenstein fails to provide a physically acceptable explanation of the photoelectric effect. What they really need is a quantum-mechanical entanglement between the different parts of the photon's wavefunction, so that if the photon is observed at atom A, it is guaranteed not to be observed at atom B. Without this quantum-mechanical "spooky action at a distance," their theory violates conservation of energy.
     
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  3. Jan 27, 2010 #2
    Re: Lamb and "The Photoelectric Effect Without Photons"

    First of all, thanks to bcrowell for posting the link to the paper which was referred to elsewhere in the discussion minus the actual link. I found the paper to be good and bad in ways that are different from the criticism posted here. What I found missing from the paper was a good physical description of how the process goes. This is disappointing because the great strength of the semi-classical approach is that it does indeed give real-time description of the process from start to end, unlike the SQM idea of the quantum leap where you're not allowed to ask how the system gets from A to B. To some extent I have to forgive the authors for this lapse because what they present instead is also something very necessary: a careful mathematical calculation using standard methods of SQM for what should be very straightforward problem: evaluate the transition rate from the ground state to the unbound states for an atom in the presence of a oscillating electric field.

    Is this not the kind of calculation which a qualified practitioner of quantum mechanics should be able to carry out? The author's look for the well-known identifying characteristics so often talked about in the debate over waves vs particles: the frequency dependence, the cutoff frequency and prompt emission. They find that these phenomena all appear in their model just as they do in the SQM version involving photons.

    BCrowell criticizes this as being the same as "Boh'rs old-fashioned dead-end approach". I don't see his point at all. In quantum mechanics you have an atom with its bound and unbound states, and you can also have an external potential; if you choose, surely you can allow this potential to oscillate sinusoidally. Then you take what is widely known as the "most accurate theory known to man" and apply it to this problem. Isn't that what the authors have done?
     
  4. Jan 27, 2010 #3

    ZapperZ

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    I still find this approach of treating the photoelectric effect as being the interaction of a photon with an atom to be rather naive. The photoelectric effect is done on metals, and the collective effect of the metal to produce the conduction band seems to have been ignored in here. The photoelectrons in a standard photoelectric effect comes from this conduction band, not from a particular atom of the metal.

    Zz.
     
  5. Jan 27, 2010 #4
    Re: Lamb and "The Photoelectric Effect Without Photons"

    That's one of the things I didn't like about the paper. This statement is made without any backup and doesn't have anything to do with the body of the article.
     
  6. Jan 27, 2010 #5

    sylas

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    I disagree. I think this is a very important point, lest the paper be misunderstood as arguing that light is not quantized. The statement here doesn't need backup; it is elementary background information, properly cited if anyone actually needs it, given as a useful and relevant context to what is in the paper.

    Having such comments in a paper like this is crucial, because otherwise the paper becomes far too easy for the cranks to abuse in support of something that the paper is not actually saying.

    The actual claim of the paper appears to be that the photoelectric effect itself is insufficient on its own to establish the quantum nature of light. This claim can be evaluated on its own merits, and it's good to have that done here.

    The title of the paper is unfortunate, as it lends itself to misunderstanding by those who want to deny the quantum nature of light altogether. So it's a very good thing that the paper explicitly shows this to be invalid, and that the author recognizes the well established quantum nature of light.

    By the way, although the paper was not published in a journal, it was published, I understand, as part of a volume of papers. The reference is:
    • Lamb W E and Scully M O (1969) "The photoelectric effect without photons", in "Polarization, Matière et Rayonnement", Volume in Honour of A. Kastler (Presses Universitaires de France, Paris, 1969)

    Felicitations -- sylas
     
  7. Jan 27, 2010 #6

    atyy

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    Does Einstein's first treatment address this objection?
     
  8. Jan 27, 2010 #7

    ZapperZ

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    Einstein first treatment, i.e. the photoelectric effect equation, isn't the issue that I was addressing.

    Zz.
     
  9. Jan 27, 2010 #8
    Re: Lamb and "The Photoelectric Effect Without Photons"

    The paper has its own merits and I agree with Conway.
    It is abbusive the statement that others effects can not have a nonquantum explanation.
    Because of the strong momentum of QM it was simpler to made those remarks.

    " they explicitly state that photons are absolutely necessary in order to explain phenomena such as blackbody radiation, Compton scattering, spontaneous emission, and the Lamb shift."

    one can find :
    http://www.springerlink.com/content/t7n71u8161824842/"
    abstract "We prove that a correct nonquantum treatment of electromagnetic radiation leads to Planck''s distribution (including zero-point energy). The only principles that we invoke are general invariance principles or, alternatively, the impossibility of violating the second law at a macroscopic level."
     
    Last edited by a moderator: Apr 24, 2017
  10. Jan 27, 2010 #9
    Re: Lamb and "The Photoelectric Effect Without Photons"

    Semiclassical Compton scattering:

    http://www.iop.org/EJ/article/0049-1748/7/7/A10/QEL_7_7_A10.pdf" [Broken]
    V A Dubrovskiĭ and B G Tsikin 1977 Sov. J. Quantum Electron. 7 832-836

    http://www.springerlink.com/content/m80x2818r44850h3/"
    Summary It is shown that the classical interaction between a charged particle and a radiation field (Thomson scattering) involves a hierarchy of processes analogous to that for photon-electron scattering (Compton scattering) in quantum electrodynamics. Statements that the double and multiple Compton effects are intrinsically quantum-mechanical processes are incorrect; it is shown that double Thomson scattering is the classical counterpart of the double Compton effect. However, it is only the inverse of double Compton scattering (two photons scattered into one photon) which has a classical counterpart; the direct process does not. Familiar examples of the interaction of three waves in nonlinear plasma theory are shown to be particular cases of double Thomson scattering.

    http://www.springerlink.com/content/r10am90am8v1a18p/"
    Abstract

    Scattering of a plane electromagnetic wave by free and bound wave packets is analyzed by semiclassical radiation theory. It is shown that the theory gives the correct answer to the question of radiation intensity in the photoelectric effect and to the correlation problem in Compton scattering. The expression for the intensity of the scattered radiation differs from the cross section which is derived from the model based on the particle nature of the electromagnetic field. The meaning of this difference is discussed. Low frequency spectrum of the scattered radiation on a bound charge is obtained.
     
    Last edited by a moderator: May 4, 2017
  11. Jan 27, 2010 #10
    Re: Lamb and "The Photoelectric Effect Without Photons"

    "[URL [Broken] shift semiclassical
    http://www.opticsinfobase.org/abstract.cfm?URI=josab-16-1-173"
    Abstract
    It is often remarked that an explanation of spontaneous emission and the Lamb shift requires quantization of the electromagnetic field. Here these two quantities are derived in a semiclassical formalism by use of second-order perturbation theory. The purpose of the present paper is not to argue the validity of QED but rather to develop a semiclassical approximation to QED that may nonetheless have certain computational advantages over QED. To this end, the vacuum of QED is simulated with a classical zero-point field (ZPF), and as a consequence the resulting theory is entitled semiclassical random electrodynamics (SRED). In the theory, the atom is coupled to the ZPF and to its own radiation-reaction field through an electric dipole interaction. These two interactions add to produce exponential decay of excited states while they cancel each other to prevent spontaneous excitation of the ground state; the Lamb shift appears in the theory as an ac Stark shift induced by the ZPF. The spontaneous decay rate of an excited-state derived in SRED is equal to the Einstein A coefficient for that state, and the Lamb shift agrees with that of nonrelativistic QED. Moreover, SRED is shown to be useful for the numerical simulation of spontaneous decay.

    Spontaneous Emission in Semiclassical Radiation Theory[/URL]
    It has recently been shown by Jaynes and collaborators that semiclassical radiation theory contains a description of spontaneous emission of radiation and of radiative level shifts. The present paper gives a gauge-invariant derivation of the radiant energy production rate, using only Maxwell's equations and the usual definitions of electric current and charge density for a many-particle material system described by Schrödinger's equation. A complementary derivation, using the time-dependent Schrödinger equation, verifies the instantaneous conservation of energy and probability during the radiation process. The semiclassical rate of spontaneous emission differs from the usual formula (Einstein's A coefficient) because it depends on the occupancy of both initial and final states of the material system. The implications of this with regard to thermal equilibrium and Planck's law are examined. If a new hypothesis is introduced, postulating the decomposition of the equilibrium radiation into incoherent components, each interacting with a specific pair of energy levels of the material system, then Planck's law is shown to hold for the total intensity of radiation. If this hypothesis is not introduced, the equilibrium conditions for different transitions are incompatible, and the semiclassical radiation theory is incapable of describing thermal equilibrium.
     
    Last edited by a moderator: May 4, 2017
  12. Jan 27, 2010 #11

    ZapperZ

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    But I can describe charge transport in metal via semiclassical description as well. It doesn't mean that it works ALL THE TIME!

    Again, the problem here is that many of these are tackling the NAIVE phenomenon known as the "photoelectric effect". It is ignoring the fact that this phenomenon has gone on to greater heights (photoemission phenomenon) in which there are no classical or semi-classical formalism to describe things such as angle-resolved photoemission spectroscopy (did anyone come up with a semiclassical anything to replace the Spicer 3-step model?), multiphoton photoemission, resonant photoemission, etc. In other words, there is MORE information that resides in the energy and momentum of the emitted photoelectrons that tells you of the mechanism of the emission. These seems to have been overlooked in all of the so-called classical and semiclassical description.

    Zz.
     
  13. Jan 27, 2010 #12

    jtbell

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    Just to single out that sentence from the abstract [I added the boldface emphasis].
     
    Last edited by a moderator: Apr 24, 2017
  14. Jan 27, 2010 #13

    sylas

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    Even more explicit...

    heldervelez's comments actually demonstrate just how important it is for an author like Lamb to include appropriate statements of their own real position. This prevents helderverlez and such ilk from distorting the author's own real position; he is reduced to merely denigrating Lamb for refusing to share his own curious views. Most readers, I expect, will recognize that Lamb actually means what he says. He actually disagrees with helderverlez on the nature of electromagnetic radiation. Attempts to paint this as a lack of integrity in the face of the "momentum" of QM are transparently self-serving.

    Sylas
     
    Last edited by a moderator: Apr 24, 2017
  15. Jan 27, 2010 #14

    Physics Monkey

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    Suppose we ignore questions of history and educational merit.

    I think we might agree on the following statements?

    1. The notion that the photo-electric effect (in its most primitive form) simply proves the existence of photons is scientifically a bit too naive. Again, I'm ignoring the history (maybe it was taken historically as really solid evidence, I don't know) and the educational question.

    2. Lamb and Scully's model does appear to account for some aspects of the photo-electric effect. There is also a cartoon in terms of photons which predicts many of the same features. Given that we accept that photons are the ultimately more useful description, it is still an interesting question ask to just how far the classical model can take you. Perhaps the classical model is much more practical for many calculations.

    3. The most relevant physics criticism may be ZapperZ's since he points out that the simplest experimental photo-electric effect is observed in metals, etc. An interesting question is then does Lamb's method work here as well, with some simple model of the metal? For example, if we treat the electrons in the metal as classical a la Drude (which is adequate for some transport phenomena) can we still recover the photo-electric affect without assuming photons?
     
  16. Jan 27, 2010 #15

    atyy

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    Agree with 1 and 2 fully, and 3 partially. For 3, I would say relevant wrt to what? If it's with respect to 1, then we should also ask the criticism would apply to Einstein's analysis; but with respect to 2, then some of the other stuff that Lamb (spontaneous emission, lamb shift etc) mentioned would be equally relevant. Also, Drude is entirely classical isn't it? I think Lamb's point is that while the (naive) photoelectric effect is quantum, the quantumness could be in either the material or light. What's one step up from the Drude model?
     
  17. Jan 27, 2010 #16
    Re: Lamb and "The Photoelectric Effect Without Photons"

    It was not my intention. Just to say that the papper has a positive assertion about one issue, and it is simpler to verify the steps and assumptions.
    The negative assertions he made about others issues, not in analyse, are just an oppinion not neccessary to the article.
    Just to prove my point I did a google search and came up with those papers that apparently claim differently.

    Not all statements are equally strong (verifiable).
    As an example: I, living in a world of black tables, can say 'this table is black' but can not verify 'there are no non black tables' (without checking every single one).

    We are humans and the context always matters, even in physics. It is not about integrity.
     
  18. Jan 27, 2010 #17

    sylas

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    We disagree there. The value of those statements is particularly clear from this thread.

    Well, you got at least two of them wrong, even quoting what is in the extracts you provided here.

    If you think it is not about integrity, then you should not have said "Because of the strong momentum of QM it was simpler to made those remarks." That's a pretty clear imputation of a lack of integrity. How about assuming he made the statements because he felt it was true and useful context?

    That's what I certainly think!

    A more reasonable assumption is that Lamb made those statements precisely to clarify that light certainly DOES have a quantum nature, even if in his estimation this cannot be proved just from the photoelectric effect. This is important to help clarify that he is not giving reasons to doubt the quantum nature of light.

    Cheers -- Sylas
     
  19. Jan 27, 2010 #18

    ZapperZ

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    I don't think it does. Einstein's photoelectric effect model makes no assumption about the mechanism at the cathode other than the requirement of some minimum energy for emission (the work function). There's no attempt to model "atoms".

    Then that makes the Lamb's model even MORE severely wrong, because the quantumness of the material is definitely very unlike an atom, which has no conduction band.

    Zz.
     
  20. Jan 27, 2010 #19

    bcrowell

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    I would actually turn your argument around a little bit.

    Historically, it is certainly much too naive to see the photoelectric effect as proof of the quantum nature of light.This is clear because people like Bohr were arguing for a classical theory of light several decades after Einstein's 1905 paper. Einstein's paper didn't really propose a complete photon model; it was (as others have pointed out) a cartoon model.

    Scientifically, I don't think the statement is too naive at all. Lamb and Scully produced a model that violated conservation of energy. If a classical flash of light comes along with an energy equal to 1.5W, where W is the work-function of the metal, then conservation of energy dictates that the number of electrons ejected through the surface is either 1 or 0. Lamb and Scully's model says that with quite high probability, the number of electrons ejected will be 2 or more. Since nobody has been able to produce a model that simultaneously (1) has a classical electromagnetic field, and (2) satisfies conservation of energy, I think it is absolutely fair to say that the photoelectric effect suffices to prove the quantized nature of light.

    Educationally, what I really abhor is the textbooks that introduce the photon via blackbody radiation. It's a horrible mess to try to get that across to students. And what are the advantages? Scientifically, it's not any better than the photoelectric effect, because either observation suffices to prove that light is quantized. Historically, it's also no better than the photoelectric effect, because Planck didn't interpret his calculation in terms of quantization, and the success of his calculations was no more successful than the photoelectric effect in convincing people like Bohr.
     
    Last edited: Jan 27, 2010
  21. Jan 27, 2010 #20

    bcrowell

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    Re: Lamb and "The Photoelectric Effect Without Photons"

    Maybe I'm missing your point, but I'm not sure that their model even really assumes atoms. They do use the *word* "atoms," but all they assume is the following: (1) each electron is independent of the others; (2) each electron has available to it a ground state plus a quasicontinuum (the latter representing the vacuum, not the metal). I don't know if it matters whether you consider the ground state to be a state localized to an atom or a state in the conduction band.

    I'm actually confused by your statement that the PE effect comes from the conduction band. The PE effect can't occur for an electron in a vacuum, because it would violate conservation of mass-energy. (In the center of mass frame, the final mass-energy would be less than the initial mass-energy.) I've always understood this as the essential reason for the sharp Z-dependence of the photoelectric effect. Z=0 would be a vacuum. The fact that it's proprtional to a power of Z, and not of a screened charge, would suggest to me that it's a phenomenon that preferentially affects the innermost electron shell.

    In the specific case of metals, we observe that the ability to absorb gammas via the PE effect is highly correlated with Z, so, e.g., we use lead as shielding, not aluminum.

    Is there perhaps some aspect of this situation that is different for visible light photons than for gammas? My experience is with gammas.
     
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