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B Photon dimensions

  1. Sep 14, 2016 #1
    I know photon is considered a point particle, so it means it has no physical dimensions, if I'm not wrong. Plus, it has a probability wave function associated to it.
    It also interacts as a wave which consists of an electrical and magnetic field perpendicular to each other and delayed by a phase.For long wavelength it is easier to imagine them interacting as waves.

    Personally I don't understand the particle existence.
    As a particle, clearly, has no size, but what about the wave? If a photon is emitted in discrete packs of energy then is it a way to calculate the length of a pack in radians for a given frequency ? Can we at least say they have the same length, like for instance 2PI radians?
  2. jcsd
  3. Sep 15, 2016 #2
    Can Quantum Physics attribute it a size, or only Classical Physics can ?
  4. Sep 15, 2016 #3
    Classical physics cannot describe photons, at least not most of the interactions it does.
    Quantum Physics describe photon as the electromagnetic field itself.
    You cannot think of it as point particle and not as a simple wave. it has the particle-wave duality and it has quantum amount of energy but it is no a particle and also not a wave.
    Are you familiar with the uncertainty principle? If not you can check it on wikipedia, it is very important for the questions you asked.
    Are you familiar with the double slit experiments? if not please take a look at wikipedia as well.
  5. Sep 15, 2016 #4
    Actually, recently, I've been focused especially on the double slit experiment and also did a bit of work studying and interpreting the uncertainty principle. However, apparently, it is not clear for me, what descriptions belong to classical physics and which belongs to QP, because my approach was to find correlations between different experiments and statements from various theories regardless of what they belong to.

    I understand sometimes it behaves like a wave other times like a particle but why can't we say anything about it's dimensions? This has to do with the uncertainty principle. For example, cross section of a photon can be less than a certain value say 1 mm radius for a particularly wave length, say 350nm , if you fire a beam of say 0.5mm photons through a 1mm hole and you find no interactions.
    On the longitudinal axis it should be a similar limit, varying with wavelength.
    PS. You are right, I just realized I can't do the thought experiment in the above example due to the uncertainty principle.
    Last edited: Sep 15, 2016
  6. Sep 15, 2016 #5


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    You should not start to study quantum theory by choosing the most complicated case. Photons are far from any classical-particle description. The do not even have a clearly defined position observable. They are the Fock states of the quantized electromagnetic field, which is a pretty abstract construction and mathematically challenging.

    Further note that the socalled "wave-particle dualism" is an outdated point of view from the times before the full modern quantum theory has been discovered in 1925. Before that, QT was a pretty confusing subject with no clear concepts, and that's why (after an astonishing short time of only 25 years from the discovery of the breakdown of classical physics by Planck's work on the black-body spectrum to Heisenberg's famous Helgoland paper).

    Better start with non-relativistic quantum mechanics to understand the fundamental concepts underlying quantum theory.
  7. Sep 15, 2016 #6
    The reason for uncertainty of position times momentum seems to me clearly explained by the fact that every interaction in the universe is an interaction between waves. You can't measure waves using other waves hoping to find precise information about particles that probably don't even exist the way we want.
    For me the most convincing theory is the de Broglie - Bohm Theory which says that waves are real not just mathematical functions but we simply cannot measure them correctly. Therefore they must have precise dimensions in space.
  8. Sep 15, 2016 #7


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    Well, that's not a very accurate view and again, please note that photons do not even have anything like a position. It just doesn't even make sense to say that a photon has a position. All you can say, given a photon state, is the detection probability at the location of the photon detector!
  9. Sep 15, 2016 #8
    You are right. According to all variation of the double slit experiment, including quantum entanglement, what you're saying is correct. In other words we assume by definition that these experiments are correct and draw conclusions. But, I am actually trying to figure out if something is wrong with them, that is why I need the answer for my question. So, obviously, this kind of question I've asked can't help me in this respect.
    Last edited: Sep 15, 2016
  10. Sep 15, 2016 #9


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    We most certainly do not "assume that these experiments are correct" - that notion is absurd on the face of it. The experimental observations are what they are, and the question is which theories most accurately predict these observations. So far quantum mechanics is the best (and most convincing) theory on offer. That doesn't necessarily mean that it is correct, but you aren't going to get very far looking for improvements on the quantum mechanical explanations if you don't even know what they are. @vanhees71 is giving good advice when says
    Unfortunately, there is a tremendous amount of misinformation out there - we could easily have a thread about why QM generates more misleading popularizations than other branches of physics. The odds are very high that:
    - Anything you think you understand about photons that you did not learn from a graduate-level textbook on quantum electrodynamics is wrong (or oversimplified to where it is useless as a starting point).
    - Anything that you have heard about wave-particle duality or how something will sometimes behave like a wave and sometimes like a particle is wrong (or oversimplified to where it is useless as a starting point).
    - Anything that you have heard about the uncertainty principle being related to measurement difficulties is wrong (or oversimplified to where it is useless as a starting point).
    There really is no substitute for a proper college-level first-year textbook as a starting point.
    Last edited: Sep 15, 2016
  11. Sep 15, 2016 #10
    Thank you. I've heard this before and it definitely makes sense.
  12. Sep 15, 2016 #11
    I know science doesn't consider any theory or experiment as 100% certain.
    By "correct" I was actually meaning something more like well trusted and logical with no better alternative.
    I also consider QM is based on empirical evidence. It offers a mathematical model of the real world that is capable of making extremely accurate predictions with certain probabilities.
  13. Sep 20, 2016 #12
    Probably I'm messing everything up but after thinking about how EM waves interact it started to make sense to me, what you said. If you position perpendicular to the direction of travel the intensity of the EM field decreases with distance by a certain factor but it is never 0. Just like gravity. So the size of any photon as a wave is always increasing.
  14. Sep 20, 2016 #13


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    Not quite true these days.

    We understand the foundations of QM much much better than the early pioneers:

    As Wienberg says:
    “It is striking that it has so far not been possible to find a logically consistent theory that is close to quantum mechanics, other than quantum mechanics itself.”

    It would be better to say the experiments verify what our current understanding tells us rather than are the starting point, even though beginning textbooks don't tell the story that way.

    For the double slit here is a much better explanation based on that current understanding:

    Yes physics is an experimental science and as Feynman says if it disagrees with experiment its wrong - end of story:

    But QM when you get into the details is a little different. As Weinberg says it seems contsained somehow - if its wrong ie if it disagrees with experiment we really are in very very deep do do because it looks pretty much the only way to go:

    Last edited: Sep 20, 2016
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