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Why the photon is disturbed by a hole?

  1. Oct 10, 2013 #1
    Please help me to understand why the path of the photon changes when travels close to a material (for example the edge of a hole)?
    I'm aware that the path of the photon can be calculated by the QM principles as a probability wave and by this the change of the wavefront follows the Huygens–Fresnel principle. Also I know that because of the uncertainity the smaller size of the hole results bigger possible change in the path.

    But what is exactly the physical effect that causes the photon to change its path? Maybe the electrostatic force of the protons and electrons that builds up the edge of the hole? Or simply the spacetime curvature that is stronger because of the mass of the close atoms? If there would be a hole inside a plate made of neutrons (like the neutron stars), the photon path would be disturbed also in it?

    Thank you for your help.
  2. jcsd
  3. Oct 10, 2013 #2


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    Are you talking, perhaps, about a photon passing close by a 'black' hole? Or just any old hole?
  4. Oct 10, 2013 #3


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    Sounds like you are asking about the physics of diffraction?


  5. Oct 10, 2013 #4


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    I don't think the photon ever changed paths. I'm not sure you can say where the photon is in the time between emission and absorption.
  6. Oct 11, 2013 #5


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    This is a problem that needs to be approached from the direction of Waves and not Photons. You never have any idea of where a Photon actually went between being emitted and detected (that's one of the basics of QM). Applying the principles of Diffraction will give you the probability that the photon arrived (or will arrive) at a particular spot but, if you were actually to intercept and detect a photon on the way, it would not arrive at the target so you would have destroyed the experiment.
    There is nothing 'wrong' with using the appropriate analysis for a particular situation and it makes sense to treat this sort of problem without trying to introduce photons. They just confuse the issue and get you no further, here.
  7. Oct 11, 2013 #6
    Thanks to all of you for the replies.

    I'm aware that we have information only about (and at) the emission and the detection. I also know that (only) the probability of the place of the detection can be calculated.
    But I'm talking about the case when the photon is already arrived. By applying some filters, we can collect some informations (for example in a double-slit experiment we can find out in which slit (hole) the photon was going through, and by doing this we lose the interference pattern).
    And for example also we usually say that the path of the photon is changed by the curvature of spacetime. So you can find lots of places when we are talking about the path or trajectory of the photon (even if it is only a presumption)

    So if we have already collected the data and know the staring-ending point of the path and we also know that the photon had the only opportunity to go through the hole, we assume its path. Also clear that normally (without any disturbance) the path of the light is a straight line.
    But in case of diffraction caused by a hole the path was obviously not a straight line, I assume that we need to have an explanation what effect caused the break in the path.

    Spacetime curvature? Electric force? Other?

    (Let's consider a gravity lens caused interference. In this case I know for sure that the path of the photon was disturbed by a spacetime curvature. Is this the same when the diffraction is caused by a slit in a lab? Is there any other effect that can disturb the path of the photon?)

    Thank you!
  8. Oct 11, 2013 #7


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    Why are you distinguishing between the effect of two slits and a hole? Each slit has a finite width and will introduce its own contribution to the overall diffraction pattern. (Wave approach says it all.)
    I also don[t see how you can say the photon has already arrived, yet go back and find which way it went. This is contradictory. I see no point or meaning in any discussion about the 'path taken' by a photon, in any case. There is no model, afaik, for describing the extent or location of a photon except when it is interacting with something. If the interaction is very subtle (as in refraction or relativistic bending) then the extent of the photon is over the whole refracting region and, again, is much better described in terms of waves.
    I think it is very risky to look for a "what's really happening" kind of answer at this level of Physics.
  9. Oct 11, 2013 #8
    Your assumption that the photon ought to follow a straight line just isn't valid. Not in Quantum Mechanics. The Path integral formulation of quantum mechanics, for instance, is founded in the postulate that all possible paths contribute equally to the propagation of the photon. Let me say that again: ALL possible paths must be taken into account. The straight paths have no (a priory) privilege. The propagation of light following a straight line (sort of), that we observe in our daily lives is obtained as a consequence of the calculation (for free space), not as a postulate, and wont be valid if obstacles are present.
  10. Oct 12, 2013 #9
    I think that your state is about how to calculate the (probable) behavior (path) of the photon when it is going to be released. However if it is already arrived, we have certain (more or less) sure points of the path. The straight line comes to our calculation if we count with the all possible paths and most of that are destructed by the effect of the (self) interference. So finally the staight line will be the privileged shape.
    If there are (disturbing) obstacles, only then we can calculate with other kinds of path shapes than (more or less) straight line.

    However the question is not this problem, I'd like to know exactly why the obstacles changing the situation? What kind of the property of the obstacles count in this situation?

    For example when a photon hits an atom, we can say that the atom is a "disturbing" obstacle because of its proper electron which will receive the energy packet (and the atom gets excited). And what kind of force that we are talking about in this case? The electric force, because surely the photon is not disturbed by the spacetime curve in this case, but it is interacted by the other property of the electron: its electric field. This is a concrete explanation why an atom disturbes the path of the photon.

    But what happens in case of a hole?

    Let's imagine a two-slit experiment with electrons. There is a wall with two slits. In this case I'm sure that we have to count with the static electricity of the wall, because there will be some kind of electrostatic lens inside the holes (slits). But if the wall is neutral, still there are atoms at the edge of the slits that may have effect of the path of the electron by their non-consistent electric shape. For example if the wall is made of ice, then there are H2O molecules inside and surely we know that they are polar. I don't know if this counts or not. If the electric field of the atoms (molecules) are too weak to count with then what force disturbes the electron inside the slit? (Obvious that if the slit is too big, then there will be noting to disturb the straight line path of the electron). And to top it all, the experiment can be done with photons also, so we must say something what force or effect counts as "disturbing effect" in case of photon.
  11. Oct 12, 2013 #10
    I agree that it is difficult to say something about the photon if it is not released yet. Also the "path" of the photon can be only an imaginary property.
    But we can try to imagine a path if we already know that the photon was released and arrived. And we can easily exclude places where we can be sure that the photon was not there. For example if you (laser) light through a tube (pipe) and the arrived measured energy fits to the released energy of the laser machine, then we can know for sure that the path of the photons was through of the tube.
    Also if we create a wall from the same material (as the tube) and we found that the photon (energy) can't across it, then we'd be stupid to not assume that the material of the wall is not blocking the path of the photon in any other case. This two simple experiment ensures that the path of the photon was inside the tube.
    Also known the refreaction and the refractive index of materials. These effects are well known from their ability to change the path of the light.
    Take a look at this picture: light path
    Why we can see a path made of straight lines? Because there were lots of single photons, trying to behave similarly, following a path, and some of them were "disturbed" by the atoms of the material and by this they arrived to the camera.
    I don't want to explain more why we can talk about "path" in case of photons and why it is important to know what effect is changing it. Please agree if you can accept this.
    Last edited: Oct 12, 2013
  12. Oct 12, 2013 #11
    How obstacles mangle a wave function: (what is a wave function, that I don't necessarily know)

    Photon and a small hole in a glass pane: Momentary slowing of propagation of wave function and partial reflection of wave function.

    Photon and a small hole in a mirror: Partial reflection of a wave function.

    Photon and a small hole in a black cardboard: Partial collapse of a wave function by many measurements, where the result of each measurement is: "no photon here".
  13. Oct 12, 2013 #12


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    I think you are bound for confusion if you want to maintain the Wave / particle ideas at the same time (that's a general principle). How can you think that there is any fundamental difference between the two slit and the single hole situation in principle (except in a 'nonsense' ideal case for infinitely thin slits)? If the slits are of finite width then you are just dealing with a diffraction pattern in both cases. The nuts and bolts of what happens to the photon (or electron) as you consider its path through the screen are not relevant (just adding details of complication but nothing in principle). Introducing electrons doesn't help, actually. It just involves a different set of parameter values. Of course the electric fields of the atoms around the edge are relevant. Without those fields, the electron would sail through the screen like a neutron and we would have a totally different situation. (Likewise for gamma photons, actually)
    It is important to avoid red herrings and to keep to the essence of the situation. An ideal screen with an ideal hole or finite slots is quite acceptable to get the principle - which is much too philosophical to bother with the odd practical detail like screen material.
  14. Oct 12, 2013 #13
    I mentioned the (double) slit experiment(s) because they are well known. Also known that it is an important question (in this case) that we know the "which slit" information or not. But I didn't wanted to discuss this topic, I only used this experiment to show that the path of the photon is important (and the term "path" is important). In this topic I don't want to make difference between "two slits" or "one hole" or any kind of close material that makes the photon to participate to produce diffraction (interference) pattern. The important thing is now that without any disturbance (big hole or no barrier (obstacle) at all), we can count with a straight line as path (and on the detector screen there will be a point-like-shape). But if there is a wall with a hole (or two slits, doesn't matter) we will definitely get a different shape on the detector screen (interference pattern or probability distribution), but in fact we can count every single photons released and arrived. By this if we'd like to imagine a path for the photons, we must consider that at least some of them was not straight.

    I think I'm not confused by the duality. The wave function describes how to count with the path, and the obstacles between the source and the destination point cause that the path is not straight.

    1: We can not talk about "path" of the photons. I agree that maybe there is no "movement". Then what happens? Maybe the photon teleports from the source to the destination. But this "teleportation" have strict rules (depending on the obstacles, spacetime curve, etc)... but these rules define a path. I think that there is no other way than considering a path. Let's close this issue.

    2: Wave behavior. Doesn't matter, the photons can be counted one by one even if the wave property is visible (the arrive points of the single photons form an interference pattern). Let's close this issue too.

    3: The path is obviously can be changed by the effect of material. No more words needed.

    4: The path is also changed even if we know that the path is close to a material, but not "crossed" it. Why? The near obstacles disturb the path. But what kind of force counts in this case? A little help: we know that the spacetime curvature can provide a phenomenon like this, also the electric field can bend the path of the electron, both of these cases are similar to ours.

    (It can be that the answer is not a direct effect, but probabilistic, but if this is the case, I need an explanation how this have connection to the wave function. For example I've heard that someone tried to explain gravity with the entropy change, maybe my problem can be explained this way, but I think that there is a simple answer how the physicists are thinking about when they are try to explain why the path is changed by a close atom)
  15. Oct 12, 2013 #14


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    1. The question about "movement". There is a delay, due to c and the actual time taken for the resulting detection will have a statistical spread, just as the point of arrival has. (You still seem to demand that the photon has to be in existence whilst the energy is travelling from A to B; why?; it doesn't have to)
    2. You may be able to tell when a photon arrives but you can't tell when it left without disturbing it or destroying it.
    3. No more words - I agree.
    4. The concept of the need for an actual 'path' is flawed', I feel. The energy flows through an aperture, which may vary in size on the way, from an infinitely wide space to number of restricted apertures. From the wave approach, it is the surface integral over the whole of space that determines the way the wave develops along its journey (even going back to Huygens) so why should a particular 'ray' be relevant? The actual nature of the 'disturbance' or obscuring of the light between source and detector doesn't really affect any of this. Why can't you allow a photon to be just a quantity of energy and leave it at that? We surely acknowledge that we can't tell where it is when it goes through two slits so why should we ever know?
    Yes, it is of interest to know how the beam / wave / photon is affected by the presence of matter and this implies that mass and charge will have an effect. It is usual to split the problem into two and to look at the propagation separately to the effects introduced by obstacles or apertures or even when there is scattering (another separate issue).
  16. Oct 12, 2013 #15
    The photon can be detected and the light can be emitted only in "packages", quantums, photons (not as waves). The wave function can not be detected. The only reason of the wave function is to calculate the probability the place where the photon will arrive. But we can detect a photon, (no more) in a specific and almost exact place (compared to the spread size of an interference pattern for example). If we build up an experiment that consequently tries to find the path of the photon, we can see the path of it (as I mentioned the picture above, it is an easy experiment for it). One arrived photon represents one particular ray. This is why I think that it is relevant. (And also relevant because of the fact that by knowing the part of the path may cause the change the result of the double-slit experiments, etc, so it is important because it have some kind of connection with the behavior of the light)

    Despite this I'd like to try to accept your explanation. Waves. Then first of all we need to know what disturb the wave function, what kind of property have the material that counts? You can calculate the wave function if you know the followings: the places of the obstacles, the behavior of the obstacles (reflective or absorber, etc)... so how the photon would behave if the material of the obstacle has been hit by the photon. If we assume that this is based on the electric force of the electon (of an atom) then we can say: the path of the photon through the hole is disturbed by some kind of a "remainig" electrostatic effect of the material that surrounds the hole. (and the spacetime curvature is negligeable) Is this close to your opinion?

    I think that we can tell that on which slit it went through, so we know some point(s) of the path, the only problem with it that in this case the interference pattern will disappear. But this doesn't matter in our topic because in this case the path is still not a straight line in every case.
  17. Oct 12, 2013 #16
    Thanks jarsta, this helped me a little.
    What is your opinion, why the obstacle slows down/reflects/or collapses the wave function?
  18. Oct 12, 2013 #17


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    I can't think why you are drawing a distinction between the diffraction phenomenon due to two slits and for a single hole or slit. In neither case is there a 'direct path' through the aperture(s).

    I have a feeling that you are still hanging on to the 'little bullet' model of a photon, in your head. You need to ditch this completely. If you still feel inclined to hang onto it then I suggest you try to apply this view to the photons of a long wave radio transmission, with a wavelength of 1.5km. How big would your photons be in that case? Any model you have for light and gamma must also fit in with these and even longer wavelengths. The concept of the 'Extent of a photon' is basically a non-starter. Whether you call the interference pattern a wave phenomenon or a probability distribution, neither description involves a photon of defined size and they are, of course, mathematically equivalent.
    People like to use the term 'wave packet' as if that implies some small entity but it's just playing with words and really has no implication about size.
  19. Oct 12, 2013 #18

    The black cardboard is sitting there and observing photons. The wave function of a photon collapses into that point on the cardboard were a photon is observed.

    When no point of the cardboard observes a photon, then the photon wave function collapses into the hole on the cardboard. Before the collapse the wave might have been 10 cm wide, after the collapse the wave is as wide as the hole.

    A wave function of a black cat and a wave function of a photon collide. The result is a more energetic black cat wave function. The possible energy levels of the cat were such that aforementiond result is possible. Actually pigment molecules on the surface of the cat have those energy levels. Pigment and photon interact somehow.

    Or should I say pigment wave function and photon wave function interact somehow. Do you know how black paint and light interact?
  20. Oct 13, 2013 #19
    I don't hanging on the "bullet" model of the photon, this is why I mentioned the energy transfer as "teleportation", it doesn't matter. Even if it is a (delayed by c) teleportation, in correct experiments you can (and logical to) consider a path for it. I hope you can understand this, please let me know if this is not acceptable for you. The "bullet form" is an easy way to imagine that if the freq/energy is high, the effect of the photon can be limited to a small area (point-like spot on the detector screen) which is much more exact than what the wave function can describe -- exactly because the reason of the wave function is not to define exactly the annihilation place, but gives only its probability, so the "result" of the calculation of the wave fuction is obviously a wider range than when what we can determine as "path" or "ray".
    If we are counting with long wavelengths (low freq/low energy) then this is the same: you can define a path for it, which will be a (more-or less) straight line, especially if we compare it to some kind of an interference pattern of it (which will be a very wide range in case of long waves, I think). We can say the same: if we "disturb" the log waves, we will get a different path. (however I have heard about several models where the phenomenon of the "bullet-like" photon is fit also for the long waves, but doesn't matter now, this is different topic)

    "I can't think why you are drawing a distinction between the diffraction phenomenon due to two slits and for a single hole or slit. In neither case is there a 'direct path' through the aperture(s)."

    I dont wanted to draw a distinction between these, I wanted to show that in both of the cases we can imagine the effect as the path was disturbed.
    Now I'll try to explain once again why I think that it is OK, if we want to describe a 'direct path', for any cases, sorry for my bad english. I'll try to make a numbered list (steps), please let me know in which point you don't agree.

    1. We can define 2 point-like areas/spots for the photon: the source and the destination. We can detect the photon in an (more or less) exact place when it is arrived to the detector screen. By the wave function we can't determine this place so exactly, because the reason of the wave function is not to give a place, but a wider range and probability levels in this range.
    2. These 2 points can be connected. First of all physically: we can make it sure that the source energy arrived to the destination. Secondly: we can imagine a path, for example we can draw a straight line between this 2 points. (in the begining this is up to our imagination)
    3. We can make lots of similar experiments, and we can conclude that the energy transfer (lets say photon) behave similarly every time.
    4. By putting obstacles between or near to this imagined straight line, we can aware the followings: there are obstacles/material that reflects/blocks the energy transfer (the photon). By several experiments with obstacles, we can draw a conclusion: sometimes the path can be a straight line, sometimes not. For example in case of a pipe, or a mirror...
    5. If there is no disturbing obstacle, we can measure the size/width of the area, where we can detect the photons destination places. By this we can assume a width of a straight path (maybe we are wrong, but we can assume), by this we get a "more or less" straight path area.
    6. Now lets take an obstacle made of a special material. We experienced earlier that if we block fully the more-or-less straight path with this obstacle, then it will fully blocks the transfer and absorbs the energy. But now we will just put this obstacle not exactly between the 2 points, but close to this more-or-less straight path. The obstacle will not even touch this imagined path.
    7. Now if we measure the energy transfer we will find 2 things: 1: a very little energy portion will be absorbed by this obstace. (or: if we do the experiment photon by photon then sometimes it will block the photon before it arrives to the screen) 2: the destination place area is bigger compred to the earlier mesurement (step 5). We will call this phenomenon as "diffraction"
    8. On the new destination place we will find such arriving points of individual photons, which can not be connected by a straight line from the emission point.
    9. We can now conclude the followings for sure: A): If there is no disturbing obstacle, we can imagine a straight line as path for every individual photon every time. B): If there is a disturbing obstace, then there are cases when there is no straight line available (the material disturbs the path, this is ok) C): if the obstace is close enough to the imagined path that we described in A) (but not blocks it at all) it may result that some start-end points can't be connected by a straight line.
    10. There are other scientifically accepted cases, when we can talk about the modified path of the photon (without "direct touch" of an obstacle/material), for example one of the proof of the relativity when the curved path of the photon was detected near to the Sun, and the modification was caused by the spacetime curvature. In this case it was exactly given why the path was not straight. Also it was important to know what was exactly the force/field/effect that caused the "disturbance".
    11. It is not a stupid state if someone assumes that in case of 9.C the photon path was disturbed not by a "direct touch" of the material of the obstacle, but there is some effect near to it (in the hole) that caused the disturbance of the path.
    12. The case 9.C and 10 is similar, for the first view it differs only by the size of the experiment (ok, the similarity can be only an illusion)
    13. I found two cases for 9.C: 1: at the surround of the obstacle we can imagine some effect that causes the path disturbance 2: the obstacles(material) have an additional feature -- it can not only reflect/absorb the energy but also can "draw it closer" (ok, this sounds stupid :-) )
    14. It is clear that 2 kinds of "force" can cause any "disturbance" for the photon itself (electric and spacetime curvature). By this if we want to imagine a path change of it and this seems to be not a "direct touch" then the cause of the change is also some kind of an (indirect) effect of these forces.

    Thank you for your patience.
  21. Oct 13, 2013 #20


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    The above is fairly reassuring (that the little bullets are not part of your model :smile:)
    I don't think we are disagreeing much about this, in fact.

    If there is no obstruction between source and detector then, once the photon has been detected, you can draw a metaphorical line between them. Does that really mean that a photon has actually travelled along this line? I don't think so. It's just a way of looking at things, imo. It is true that, with no significant restriction to the wave front in an optical system, you can draw a straight geometrical line between emitter and detector, with a ruler and that 'ray' is a fair description of how the optics have worked. That is the most trivial case and it is because all other possible points of arrival have zero probability. Any 'selection' by blocking or reflection will spread the probability function out and the simple 'ray' will not give a satisfactory answer. With enough computation, you can calculate the diffraction pattern (or whatever else you care to call it). I don't agree that this method could be any less accurate than any other method as they all boil down to the same thing - involving calculations across the whole of the width of the optical system and the detail of interaction every 'obstacle'. Whether you discuss what goes on in terms of collapsing wave functions or diffraction, there is no basic difference afaics.

    This thread seems to be about "what's really happening" more than anything else and you seem to be bringing in various practical situations in order to resolve the question. I guess it's always true to say that the specific problem may call for a particular 'best' approach. But I don't think it's any deeper than that. There are many cases when a very conventional Ray Tracing method will yield excellent predictions (that's going to the extreme classical approach, if you like). Just because it doesn't consider waves or QM doesn't invalidate it, when it gives good results. Likewise, one would choose between different modern methods to suit the problem. I say this because I believe that there is no absolute truth in this (or any Science ftm) and we just use the best available model for the purpose.
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