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Thread: Double Slit Questions

  1. #1 Double Slit Questions 
    Forum Senior bill alsept's Avatar
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    I had some questions about the double slit experiment and was hoping I could get some clarification. When it comes to firing electrons one at a time through a single slit, what creates the pattern on the other side (electrons or photons)?
    When it comes to entanglement experiments and they say the pattern collapses as soon as we observe or measure one of the slits what is actually involved (physically) with observing or measuring.
    Thanks


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    Quote Originally Posted by bill alsept View Post
    I had some questions about the double slit experiment and was hoping I could get some clarification. When it comes to firing electrons one at a time through a single slit, what creates the pattern on the other side (electrons or photons)?
    Electrons in an electron double-slit experiment, photons in a photon double-slit experiment.


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    In any case when we don't observe the particles before they go through the slit,they will behave as both wave and particle(superposition) but once we observe the superposition collapses and they behave just ℓιкє particles..

    Wheeler's delayed thought experiment showed that the future can affect the past(for a considerable milli-microsec) I.e when we delay before observing the particle(allowing it to go through both slits) and then observing,the particle we tend to show that it went through just one slit!
    "I am sorry for making this letter longer than usual.I actually lacked the time to make it shorter."###
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    When it comes to firing electrons one at a time through a single slit, what creates the pattern on the other side (electrons or photons)?
    You get single electrons hitting the detector, but their hits are distributed in a wave interference pattern.
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by Markus Hanke View Post
    You get single electrons hitting the detector, but their hits are distributed in a wave interference pattern.
    That is a good point. You cannot determine any pattern from a single electron (or photon). It is only after you have a large number of hits that you can detect a pattern.

    It is not really any different from a sheet of glass where you can see through it but also see a reflection. Any individual photon will either go through the glass or be reflected, and you can never know which. But light as a whole is split in a fixed proportion.
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    Quote Originally Posted by merumario View Post
    In any case when we don't observe the particles before they go through the slit,they will behave as both wave and particle(superposition) but once we observe the superposition collapses and they behave just ℓιкє particles..
    What does observe mean exactly? How is it done?
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    Quote Originally Posted by bill alsept View Post
    What does observe mean exactly? How is it done?
    Any measurement / detection of the electron (or photon).
    ei incumbit probatio qui dicit, non qui negat
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    Quote Originally Posted by Markus Hanke View Post
    When it comes to firing electrons one at a time through a single slit, what creates the pattern on the other side (electrons or photons)?
    You get single electrons hitting the detector, but their hits are distributed in a wave interference pattern.
    The part I don't understand is that if you fire an electron straight into the slit what stops it from continuing in a straight line?
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    Quote Originally Posted by bill alsept View Post
    The part I don't understand is that if you fire an electron straight into the slit what stops it from continuing in a straight line?
    It gets diffracted, like a wave. (Note: like a wave.)
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    Forum Senior bill alsept's Avatar
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by bill alsept View Post
    The part I don't understand is that if you fire an electron straight into the slit what stops it from continuing in a straight line?
    It gets diffracted, like a wave. (Note: like a wave.)
    Would't a single electron (or single anything) diffract like a billard ball?
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    Quote Originally Posted by PhDemon View Post
    You need to read about wave/particle duality (Wave). Every particle can also be thought to be a "matter wave" (the wavelength of which is inversely proportional to momentum) for particles such as photons and electrons where the momentum is small the wavelength is appreciable and the particle under certain circumstances shows wavelike behaviour (such as diffraction).
    I have and have been reading about it and that is why I am asking the questions. When you say " Every particle can also be thought to be a "matter wave" that doesn't mean much to me until I can piece it together with some kind of sequence of events. For example from an electrons point of view as it travels toward the slit it can see through the slit all the way to the wall on the other side. Why doesn't the electron continue on that straight path to the destination it can clearly see straight in front of it? And if after it goes through the slit it does change directions what caused it to accelerate or deflect?
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    Quote Originally Posted by bill alsept View Post
    Why doesn't the electron continue on that straight path to the destination it can clearly see straight in front of it?
    That is because it is not a classical particle, but a quantum mechanical object. Newtonian mechanics would dictate ( as you rightly say ) that it simply keeps going straight through the slit - experiment on the other hand tells us that in real life it does not behave that way. This was one of the motivations for the development of quantum mechanics. Electrons are not little billiard balls, after all.

    bill alsept, you have just discovered the quantum world

    And if after it goes through the slit it does change directions what caused it to accelerate or deflect?
    The electron ( like all quantum objects ) has a particle aspect and a wave aspect. The deflection is a result of its wave nature, the hit at the detector is a result of its particle nature. Note that there is no acceleration involved in deflecting a wave.
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    Quote Originally Posted by bill alsept View Post
    Would't a single electron (or single anything) diffract like a billard ball?
    No, because fundamental particles are not little solid balls. They have some aspects which are similar to everyday objects. Some of those are similar to particles. Some of them are like waves.

    Again, note the emphasis on "similar" or "like": they are not waves or particles. For example, as far as we can tell, an electron has no physical size ("diameter" of the "particle"); it is modelled as a point. And because of that its "spin" is not actually rotation, it is purely some intrinsic property it has that is similar to angular momentum. And so it goes on.

    So what is an electron, then? It is an electron. That's it. It is purely defined by its own properties. Not by billiard balls or ocean waves, not even by analogies to them.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by bill alsept View Post
    Would't a single electron (or single anything) diffract like a billard ball?
    No, because fundamental particles are not little solid balls. They have some aspects which are similar to everyday objects. Some of those are similar to particles. Some of them are like waves.
    When you say sometimes a particles aspects are like waves does that mean the particle itself moves outward like a wave or does that mean the universe or the particles surroundings interact with the particle from multiple directions to cause the interference?



    Quote Originally Posted by Strange View Post
    So what is an electron, then? It is an electron. That's it. It is purely defined by its own properties. Not by billiard balls or ocean waves, not even by analogies to them.
    When you fire an electron at a wall can the exact spot be determined and if so what does the impact look like compared to the the exact impact of a photon hitting that same wall?

    I am not disputing the existence of interference patterns obviously they can be seen. I am interested in their source.
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    Quote Originally Posted by bill alsept View Post
    When you say sometimes a particles aspects are like waves does that mean the particle itself moves outward like a wave or does that mean the universe or the particles surroundings interact with the particle from multiple directions to cause the interference?
    I'm not sure that question can be answered. We can only talk about the measured properties of something like an electron. You can think of it spreading out like a wave. Except when it is detected, it is detected at a single point. Like a particle.

    We can't actually say anything about how the electron gets from a to b. In order calculate the probabilities of an electron arriving at a particular point, you have to take into account every possible path it could take (weighted by the corresponding probability). To understand more, I recommend Feynman's QED lectures, available on videos or as a book.
    http://en.wikipedia.org/wiki/Path_in...interpretation

    When you fire an electron at a wall can the exact spot be determined and if so what does the impact look like compared to the the exact impact of a photon hitting that same wall?
    The exact spot can be determined to pretty much whatever accuracy you want, as far as I know. The same is true of electrons and photons. In both cases they will typically be detected by their interaction with an atom in the target, but the exact mechanism may be different for an electron and a photon (simply because of their different properties: charged vs uncharged, energy levels, etc.).
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    Quote Originally Posted by bill alsept View Post
    When you say sometimes a particles aspects are like waves does that mean the particle itself moves outward like a wave or does that mean the universe or the particles surroundings interact with the particle from multiple directions to cause the interference?
    What it means is simply that quantum objects are not like macroscopic, everyday objects - they have both a particle and a wave nature. Think about them as a new class of entity for which there is no real analog in the macroscopic world. Whether we see the particle aspect or the wave aspect depends on the experimental setup, but both are present in the object itself. This is what distinguishes quantum mechanics from classical mechanics.

    When you fire an electron at a wall can the exact spot be determined
    Yes it can.

    what does the impact look like compared to the the exact impact of a photon
    It would look the same. Both look like hits of a point-like particle.
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    Quote Originally Posted by bill alsept View Post
    When it comes to entanglement experiments and they say the pattern collapses as soon as we observe or measure one of the slits what is actually involved (physically) with observing or measuring.Thanks
    I'm not sure if this question was answered or not. How is a photon detected (at one of the slits) as it travels to the wall on the other side? How can you detect it without touching it or seeing it which are the same thing? If you have to touch it to detect it then of course the interference would stop because the photon wouldn't travel much farther than the slit where the detection is.Could someone help explain this part?Thanks
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    With electrons, you can measure the change in a magnetic field caused by the moving electrical charge. It may be small, but it's still measurable.

    With photons, you can take a circularly polarized photon and put a vertical polarizer on one slit and a horizontal one on the other. You can then measure the polarization of the photon well after the two slits.

    (Someone correct me if I got any of that wrong.)
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    Just a note...many experiments in Quantum theory have been replicated and thus validated over and over. They explain reality such as the dual property of a photon.

    If none of it makes sense....don't think you are necessarily missing something. It doesn't make sense to anyone in the physical way that our senses can detect. We see 'results'....the results are real. We can explain the results using theory but not because we can 'see' a process. Reality is very, very, very weird.YouTube has some good lectures and panel discussions on quantum happenings.

    What's ironic is that once you give into the idea of 'I'm just never going to understand this' that the pieces start to fit. We are logical beings and the logic is on a level outside of our perceptions.
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    Quote Originally Posted by MagiMaster View Post
    With photons, you can take a circularly polarized photon and put a vertical polarizer on one slit and a horizontal one on the other. You can then measure the polarization of the photon well after the two slits.
    It's worth noting that if the polarisation of the photon from each slit is at right-angles to each other, then there is no interference pattern even if one does not measure the polarisation to determine which slit the photon passed through.

    Measuring which slit the photon passed through, irrespective of how it is done, orthogonalises the states corresponding to the photon passing through each slit, and orthogonal states do not interfere. Making the polarisations at right-angles also makes these two states orthogonal, and therefore no interference pattern.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    So would you use a 90 degree polarizer and a 45 degree polarizer, or just use one polarizer and leave the other slit circularly polarized?
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    Quote Originally Posted by MagiMaster View Post
    So would you use a 90 degree polarizer and a 45 degree polarizer, or just use one polarizer and leave the other slit circularly polarized?
    I was referring to plane-polarised photons, not circularly-polarised photons. For photons, the two polarisations would differ by 90. If we were talking about electrons, then the difference would be 180 due to the spin- nature.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by bill alsept View Post
    I'm not sure if this question was answered or not. How is a photon detected (at one of the slits) as it travels to the wall on the other side? How can you detect it without touching it or seeing it which are the same thing? If you have to touch it to detect it then of course the interference would stop because the photon wouldn't travel much farther than the slit where the detection is.Could someone help explain this part?Thanks
    You can't detect a photon ( or anything else ) without interfering with it in some way. If you place a detection mechanism into the slits themselves, you will register exactly one hit in exactly one slit, without anything strange going on. The interference pattern only manifests if you allow a number of photons to cross the double slit undisturbed - you will still register single hits on the detection screen behind the double slit, but you will find that those hits are distributed in an interference pattern, as if the two slits where sources of wave fronts.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by bill alsept View Post
    I'm not sure if this question was answered or not. How is a photon detected (at one of the slits) as it travels to the wall on the other side? How can you detect it without touching it or seeing it which are the same thing? If you have to touch it to detect it then of course the interference would stop because the photon wouldn't travel much farther than the slit where the detection is.Could someone help explain this part?Thanks
    You can't detect a photon ( or anything else ) without interfering with it in some way. If you place a detection mechanism into the slits themselves, you will register exactly one hit in exactly one slit, without anything strange going on. The interference pattern only manifests if you allow a number of photons to cross the double slit undisturbed - you will still register single hits on the detection screen behind the double slit, but you will find that those hits are distributed in an interference pattern, as if the two slits where sources of wave fronts.
    It should be noted that by placing a barium borate crystal in front of the slits, a single photon passing through the slits is converted to an entangled pair of photons, and measuring the which-slit information from one of the photons prevents the other photon from being part of an interference pattern.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by Markus Hanke View Post
    You can't detect a photon ( or anything else ) without interfering with it in some way.
    That's what I was thinking. I have read about double slit experiments where they claim to use polarized photons and filters to determine the path of the photon etc. How can they do this without touching the photon which would block it and in so doing cancel the interference pattern?
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    Quote Originally Posted by bill alsept View Post
    Quote Originally Posted by Markus Hanke View Post
    You can't detect a photon ( or anything else ) without interfering with it in some way.
    That's what I was thinking. I have read about double slit experiments where they claim to use polarized photons and filters to determine the path of the photon etc. How can they do this without touching the photon which would block it and in so doing cancel the interference pattern?
    By measuring the photon's entangled partner, one measures the photon without interacting with it.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    By measuring the photon's entangled partner, one measures the photon without interacting with it.
    Maybe I am missing something. You are talking about observing another photon ( the entangled partner) and not the actual photon going through the slit. What I have read about the polarized double slit experiment is that just the act of observing the photon causes the interference pattern to stop. That would mean observing the photon going through the slit and not the other entangled partner.
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    Observing either of a pair of entangled photons is equivalent to observing both of them. That is pretty much what entangled means.
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    Quote Originally Posted by bill alsept View Post
    Quote Originally Posted by KJW View Post
    By measuring the photon's entangled partner, one measures the photon without interacting with it.
    Maybe I am missing something. You are talking about observing another photon ( the entangled partner) and not the actual photon going through the slit.
    Yes, I'm talking about measuring a different photon. Check out the quantum eraser experiment for more details.


    Quote Originally Posted by bill alsept View Post
    What I have read about the polarized double slit experiment is that just the act of observing the photon causes the interference pattern to stop. That would mean observing the photon going through the slit and not the other entangled partner.
    The point is that it is sufficient to place the photon in a state for which "the photon passing though one slit" and "the photon passing through the other slit" states are distinguishable for the interference pattern to be destroyed. It does not require that the photon forming part of the interference pattern be interacted with. That is, it is not the interaction that destroys the interference pattern.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by bill alsept View Post
    Quote Originally Posted by KJW View Post
    By measuring the photon's entangled partner, one measures the photon without interacting with it.
    Maybe I am missing something. You are talking about observing another photon ( the entangled partner) and not the actual photon going through the slit.
    Yes, I'm talking about measuring a different photon. Check out the quantum eraser experiment for more details.


    Quote Originally Posted by bill alsept View Post
    What I have read about the polarized double slit experiment is that just the act of observing the photon causes the interference pattern to stop. That would mean observing the photon going through the slit and not the other entangled partner.
    The point is that it is sufficient to place the photon in a state for which "the photon passing though one slit" and "the photon passing through the other slit" states are distinguishable for the interference pattern to be destroyed. It does not require that the photon forming part of the interference pattern be interacted with. That is, it is not the interaction that destroys the interference pattern.
    I understand what you guys are saying about entanglement. What I'm asking is if a photon in a double slit experiment is detected (observed) at either of the slits then wouldn't that photon be absorbed and have no possible way to create or be part of an interference pattern?
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    Quote Originally Posted by bill alsept View Post
    I understand what you guys are saying about entanglement. What I'm asking is if a photon in a double slit experiment is detected (observed) at either of the slits then wouldn't that photon be absorbed and have no possible way to create or be part of an interference pattern?
    You are correct, there will be no interference pattern if you place the detector into the slit itself.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by bill alsept View Post
    I understand what you guys are saying about entanglement. What I'm asking is if a photon in a double slit experiment is detected (observed) at either of the slits then wouldn't that photon be absorbed and have no possible way to create or be part of an interference pattern?
    You are correct, there will be no interference pattern if you place the detector into the slit itself...
    ... but that is not the point. The point is that one doesn't need to disturb the photon to destroy the interference pattern.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    ... but that is not the point. The point is that one doesn't need to disturb the photon to destroy the interference pattern.
    Well, that's true, but I don't think that's what bill alsept was asking - his question was specifically about whether or not there will be an interference pattern if you place a detector into the slits themselves. The answer is clearly no.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by KJW View Post
    ... but that is not the point. The point is that one doesn't need to disturb the photon to destroy the interference pattern.
    Well, that's true, but I don't think that's what bill alsept was asking - his question was specifically about whether or not there will be an interference pattern if you place a detector into the slits themselves. The answer is clearly no.
    I don't know if it needs to be "into" the slit or at the slit. All I mean is whatever it would take to detect a photon to determine which slit its going through. Other than a detector Absorbing the photon what else stops the interference pattern?
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    Quote Originally Posted by bill alsept View Post
    I don't know if it needs to be "into" the slit or at the slit. All I mean is whatever it would take to detect a photon to determine which slit its going through.
    If you detect a photon at the slit, there will be no interference pattern.

    Other than a detector Absorbing the photon what else stops the interference pattern
    Close one of the slits, and the pattern disappears too.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by bill alsept View Post
    I don't know if it needs to be "into" the slit or at the slit. All I mean is whatever it would take to detect a photon to determine which slit its going through.
    If you detect a photon at the slit, there will be no interference pattern.

    Other than a detector Absorbing the photon what else stops the interference pattern
    Close one of the slits, and the pattern disappears too.
    Of course, I guess my questions have more to do with single photon experiment where photons are sent one at a time through a slit and eventually an interference pattern builds up. If the photon goes through one of the slits where do you think the interference comes from? and how?
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    Quote Originally Posted by bill alsept View Post
    Of course, I guess my questions have more to do with single photon experiment where photons are sent one at a time through a slit and eventually an interference pattern builds up. If the photon goes through one of the slits where do you think the interference comes from? and how?
    It comes from the fact that photons are inherently quantum objects. This means they possess both a particle aspect and a wave aspect; that is the reason why you register individual hits on the screen ( particle ) which are distributed in an interference pattern ( wave ). There is no classical analogy for what happens at the double slit, since in our classical ( macroscopic, everyday ) world the wave aspect is so small as to be entirely negligible. We see only material objects, never waves. It is tempting to try and find a classical explanation/picture/visualisation for exactly what happens at the double slit, but such an endeavour is doomed to fail from the outset, because photons are not classical objects. All we can say is that one wave hits the double slit, two waves eminate from the other side of those slits, resulting in an interference pattern of particle hits.

    This is probably not very satisfying for you, but it is all that can be said. The quantum world just isn't classical or intuitive.
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    Quote Originally Posted by bill alsept View Post
    I don't know if it needs to be "into" the slit or at the slit. All I mean is whatever it would take to detect a photon to determine which slit its going through. Other than a detector Absorbing the photon what else stops the interference pattern?
    The important point is that, as KJW said, however you gain the information about which slit a photon went through (even if by detecting an entangled pair and not affecting the photon that goes through the slit) it will destroy the interference pattern.

    Read up on the "quantum eraser" or "delayed quantum eraser" experiments to understand how an entangled photon can be used. (These are both described on Wikipedia, but I'm not sure the descriptions there are terribly clear.)

    Quote Originally Posted by bill alsept View Post
    Of course, I guess my questions have more to do with single photon experiment where photons are sent one at a time through a slit and eventually an interference pattern builds up. If the photon goes through one of the slits where do you think the interference comes from? and how?
    The usual "explanation" (analogy/interpretation) is that the photon (somehow) goes through both slits and interferes with itself. I don't find that very satisfactory. After all, photons are supposed to be indivisible. And it implies that things like large molecules (the same experiment can be done with C60 "buckyballs") go through both slits.

    It may be better to think of it in terms of the fact that quantum effects are non-local (*) and so the paths the photon could take, and the probability of it landing at a given point (which is what creates the pattern) depends on the total environment of the photon. (This seems to fit better with Feynman's "sum over histories" approach - see his QED lectures, if you haven't already).

    (*) in time as well as space - which is why the delayed quantum eraser experiment makes sense.
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    Quote Originally Posted by Strange View Post
    It may be better to think of it in terms of the fact that quantum effects are non-local (*) and so the paths the photon could take, and the probability of it landing at a given point (which is what creates the pattern) depends on the total environment of the photon. (This seems to fit better with Feynman's "sum over histories" approach - see his QED lectures, if you haven't already).
    I don't agree. I think that Feynman's path-integral approach better fits the many-worlds interpretation.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by Strange View Post
    It may be better to think of it in terms of the fact that quantum effects are non-local (*) and so the paths the photon could take, and the probability of it landing at a given point (which is what creates the pattern) depends on the total environment of the photon. (This seems to fit better with Feynman's "sum over histories" approach - see his QED lectures, if you haven't already).
    I don't agree. I think that Feynman's path-integral approach better fits the many-worlds interpretation.
    There is an interpretation for everyone.
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    Quote Originally Posted by Strange View Post
    After all, photons are supposed to be indivisible. And it implies that things like large molecules (the same experiment can be done with C60 "buckyballs") go through both slits.
    Actually, the buckyballs example raises an interesting point: Organic chemists know that particular substituted biphenyls are chiral due to the inability of moving bulky substituents pass one another as the two benzene rings rotate with respect to each other. In other words, atoms, molecules, chemical bonds, etc behave as physical obstacles to other atoms, molecules, chemical bonds, etc. The parts of a molecules don't break apart to allow other parts of the molecule to move past it. Therefore, it would be incongruous for the buckyball to split to go through both slits unless the splitting is in some sense between different realities.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    [Therefore, it would be incongruous for the buckyball to split to go through both slits unless the splitting is in some sense between different realities.
    I am not too familiar with the many world's interpretation (not at all, in fact). I guess the bit that doesn't currently make sense to me is that if you say the splitting occurs in different realities, how can that affect the result we see in this reality? Is the idea that in our reality, we just happen to see the dots corresponding to particles that took a particular path that results in the interference pattern? But wouldn't that imply that there is another reality where the pattern doesn't form (just by chance)? But we never see that so ...
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    Quote Originally Posted by Strange View Post
    I guess the bit that doesn't currently make sense to me is that if you say the splitting occurs in different realities, how can that affect the result we see in this reality?
    I suppose part of the confusion lies in what exactly is a "reality", and I confess that I do add to the confusion by using the term in different ways without explanation. Part of the reason is the difficulty in translating the maths of quantum mechanics into something that regular folk can understand. The way I think of it, even if it's not completely accurate, is to consider that there are an infinite number of versions of me doing this exact same thing. If I were to perform some quantum experiment, then a proportion of those versions of me will see one of each different outcome. However, before the experiment, the question arises as to whether all the identical versions of me belong to different realities or whether it is just one reality. Since we are all existing in exactly the same way, is there a difference? I think they are all different because that would be consistent with the viewpoint that the realities are entire spacetimes (thus making it more amenable to relativity).

    This doesn't really help with the interference pattern because that involves subtleties that are difficult to translate from the purely mathematical viewpoint to the simplified picture that I presented above.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    This doesn't really help with the interference pattern because that involves subtleties that are difficult to translate from the purely mathematical viewpoint to the simplified picture that I presented above.
    Thus, when I do provide explanations of the interference pattern and why it is destroyed when the one-slit states become distinguishable, I tend not to place this within the bigger picture of the many-worlds interpretation, but simply indicate what the maths says.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    After chatting with Markus in the Non-Physical thread I'm starting to think that the field of observation is a real thing. Every particle moves through it, the field is omnipresent, if that's the right way of putting it. Because of this field we see the waves as an object. But I'm probably talking through my ass here.
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    Perhaps I should remark that to me, the mathematics of standard quantum mechanics IS the many-worlds interpretation... nothing more, nothing less. It differs from the Copenhagen interpretation only in that instead of selecting only one eigenvector statistically, all the eigenvectors are selected deterministically, though the observer only observes one eigenvector selected statistically, due to the orthogonality of the eigenvectors and the linearity of quantum mechanics.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by Markus Hanke View Post
    Quote Originally Posted by bill alsept View Post
    Of course, I guess my questions have more to do with single photon experiment where photons are sent one at a time through a slit and eventually an interference pattern builds up. If the photon goes through one of the slits where do you think the interference comes from? and how?
    It comes from the fact that photons are inherently quantum objects. This means they possess both a particle aspect and a wave aspect; that is the reason why you register individual hits on the screen ( particle ) which are distributed in an interference pattern ( wave ). There is no classical analogy for what happens at the double slit, since in our classical ( macroscopic, everyday ) world the wave aspect is so small as to be entirely negligible. We see only material objects, never waves. It is tempting to try and find a classical explanation/picture/visualisation for exactly what happens at the double slit, but such an endeavour is doomed to fail from the outset, because photons are not classical objects. All we can say is that one wave hits the double slit, two waves eminate from the other side of those slits, resulting in an interference pattern of particle hits. This is probably not very satisfying for you, but it is all that can be said. The quantum world just isn't classical or intuitive.
    It is the mystery that interest me and I hope it continues to interest others. Although the quantum world may not be classical I believe there is a better answer or explanation for the interference. Just because there is an interference pattern I don't see that as proof that the single photon goes through both slits or interferes with itself. It could be something else
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    Quote Originally Posted by KJW View Post
    Perhaps I should remark that to me, the mathematics of standard quantum mechanics IS the many-worlds interpretation...
    Surely that must be true of any interpretation? As they are all just interpretations/descriptions of the same underlying theory; none of them really say anything new about the theory or reality.
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    @bill, Like all of currently accepted science people have tried to prove the idea wrong, and failed.
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    Quote Originally Posted by bill alsept View Post
    It is the mystery that interest me and I hope it continues to interest others. Although the quantum world may not be classical I believe there is a better answer or explanation for the interference. Just because there is an interference pattern I don't see that as proof that the single photon goes through both slits or interferes with itself. It could be something else
    I'm not sure what you want as a "better" explanation. You seem to want a simpler or more classical explanation; that doesn't seem to be possible.

    Feynman, in his QED lectures gives the example of the way the number of photons reflected from the surface of a piece of glass depends on the thickness of the glass. It is almost as if the photon "know" how far away the far side of the glass is in order to determine the probability of being reflected. This gets even more complex if you add multiple layers of glass. The probability of being reflected is (apparently) determined by the paths they could take (even when they don't - which is perhaps where the "many worlds" view comes in).

    The double slit case is very similar. The photons form a pattern (which is identical to the classical interference pattern) based on their probability of arriving at certain places. It's not as if each photons forms a very faint interference pattern which gradually gets stronger with more photons. Each photons is detected at a single point. Initially, these points appear to be randomly scattered. It is only when enough are detected that a pattern starts to be seen.

    The places where a photon can be detected appears to be determined by all the paths they could take (i.e. the presence of slits or not) even if they don't take them. (Or maybe they do....)
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    Quote Originally Posted by PhDemon View Post
    So you don't see a definitive experiment as "proof" - no wonder no one takes you seriously. "It could be something else", really? like what? Come up with an experiment that shows it isn't going through both slits or stop ignorantly speculating. This is just laughable, in effect you're saying "I don't like this part of QM so I'll say it could be something else".
    Proof of what? That there is a pattern?? Of course there is. Can you prove its the photon causing the interference? NO. Just because you can except no definitive answer means that everyone else has to. I'm sorry you seem to have trouble discussing things without having childish tantrums but there are always going to be people who want to ask questions about things that don't have clear answers. You can't rule out that the interference could be caused by something else other than the single photon and teaching that as fact is wrong. For example in the game of PLINKO Plinko Probability 2.02 if you drop single balls and a pattern forms at the bottom (which it will) in your way of reasoning the pattern is caused by the ball and not the pegs. To make that analogy similar to the double slit experiment set up two plinko boards overlapping side by side with the slits being at the top of each board. You will get a pattern similar to the interference pattern and it was obviously NOT caused by the balls interfering with themselves. Your the one claiming that your answer is the only right answer so prove it.
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    Quote Originally Posted by PhDemon View Post
    Science doesn't do "proof",
    Then why did you say "So you don't see a definitive experiment as "proof"

    No one is disputting that there is an interference pattern just the cause of it. You claim the photon interferse with itself because everyone else told you so. Can you explain how it does this? NO. You except this strange behavior as just being inherent of quantum objects. I can't except a non answer as a final answer and that shouldn't bug you. Your answer just brings more questions. How does the photon go to both holes etc.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by KJW View Post
    Perhaps I should remark that to me, the mathematics of standard quantum mechanics IS the many-worlds interpretation...
    Surely that must be true of any interpretation? As they are all just interpretations/descriptions of the same underlying theory; none of them really say anything new about the theory or reality.
    Many interpretations (I'm not sure if they all are) are equivalent in the sense that they give the same answers. But I'm not sure if the mathematics itself is entirely neutral towards the interpretation. For example, where in the mathematics is it indicated that only one eigenvector is selected? The mathematics produces a weighting for each eigenvector, but is there anywhere in the mathematics that says that this weighting is a probability? What I'm saying is that Copenhagen (for example) is placing an interpretive spin on the mathematics that's not present in the mathematics itself. By contrast, the mathematics naturally points to the many-worlds interpretation because the notion of superposition is there in the mathematics to begin with and it requires nothing to carry that through to the macroscopic realm.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by bill alsept View Post
    No one is disputting that there is an interference pattern just the cause of it. You claim the photon interferse with itself because everyone else told you so. Can you explain how it does this? NO. You except this strange behavior as just being inherent of quantum objects. I can't except a non answer as a final answer and that shouldn't bug you. Your answer just brings more questions. How does the photon go to both holes etc.
    I think you are looking at this in the wrong way; the experiment doesn't say anything about a photon going through one or both slots. What the experiment tells us is only that photons have both a particle and wave aspect; remember that interference patterns are the result of waves, whereas each point in the pattern on the screen is a hit by a particle. Why would you argue which of the two slits the wave goes through ? It is a meaningless question. Likewise, if you detect a photon particle in one of the two slots, you can't keep wondering why there is no interference pattern; it is equally meaningless, since you have just detected a particle, and particles don't make interference patterns. All you are doing here is looking at two aspects of the same thing; there is really very little mystery to this, so long as you are willing to accept that there is more to the world than human perception allows in our macroscopic world.

    What this means is that the photon behaves as a wave at the double slit ( the wave behaviour itself is perfectly classical, btw ), but as a particle at the screen. It is this dual nature of quantum particles that lies at heart of this experiment and all quantum mechanics. The photon is neither wave nor particle, but an entirely different class of object which incorporates both aspects. A quantum object.

    There is nothing else that can cause an interference pattern but wave fronts.

    I can't except a non answer as a final answer
    I am sorry Bill, but this statement is just silly. What you can and can't accept has no physical relevance whatsoever; you are free to believe what you wish, but the fact remains that the wave-particle duality is the only consistent explanation for all observed quantum mechanical effects related to this, not just this isolated experiment. There is a vast array of experimental setups which demonstrate interference and diffraction of quantum objects ( not just photons ) under all manner of different circumstances; and there is one overarching model which applies to all such objects and explains perfectly how they behave, and why - that is wave-particle duality. You can choose not to "believe" in it, but you can't choose your own physical facts. You gain nothing by rejecting the obvious, and trying to substitute it with something else for which there is no evidence whatsoever. That is not how physics works.
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    Correct me if I'm wrong but doesn't the wave particle theory claim that the interference pattern (in the single photon version) is caused by each photon interfering with itself by somehow traveling through both slits or every possible path? Maybe that's not exactly right but I believe the theory involves both slits somehow or the pattern wouldn't develop. I don't think this theory will even work if the photon doesn't go through both slits so I would think the experiment would "say a lot about a photons going through one or both slits." Everyone believes that each photon goes through both slits only because they see an interference pattern form. Thats not proving the photon goes through both slits thats just claiming that it most be because there is an interference pattern. In the analogy I used with the plinko boards a pattern that looks like an interference pattern forms at the bottom but we know it was not caused by the balls interfering with themselves. Its not even caused by a wave. The point is there could be other possibilities for the interference pattern and it could involve an eather or particles filling the space between all the slits etc.. Why is that any harder to imagine than a photon being in two or more places at the same time? Also when I said I can't except a non answer as a final answer I just meant my personal curiosity would never be "willing to accept that there is more to the world than human perception allows in our macroscopic world." Even if thats true I would never stop wondering.
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    Quote Originally Posted by bill alsept View Post
    [You can't rule out that the interference could be caused by something else other than the single photon and teaching that as fact is wrong.
    The interference IS caused by something else other than the single photon: the presence of the two slits.

    For example in the game of PLINKO Plinko Probability 2.02 if you drop single balls and a pattern forms at the bottom (which it will) in your way of reasoning the pattern is caused by the ball and not the pegs.
    The pattern is (obviously) caused by the arrangement of all the pegs. But imagine you trace the path of a single ball and remove all the pegs except the ones it hit. Will you still get the same distribution? No, of course not. The pattern is caused by all the pegs, even though each ball only interacts with a tiny subset.
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    Quote Originally Posted by bill alsept View Post
    Maybe that's not exactly right but I believe the theory involves both slits somehow or the pattern wouldn't develop.
    Yes, precisely ! The point is that in this instance you are seeing the wave aspect of the photon; the particle aspect is entirely hidden. The detector, on the other hand, sees only the particle aspect, not the wave aspect at all; hence, when you place it into the slits, you get a well defined hit in a well defined slit. The photon itself is both particle and wave; which aspect you see depends on the observer, but at a given instant in a given experimental setup, you see only one of the two.

    I don't think this theory will even work if the photon doesn't go through both slits so I would think the experiment would "say a lot about a photons going through one or both slits."
    The experiment always works; it's just that if you change the setup ( such as placing the detector into a slit instead of behind it ), you might see a different aspect of the quantum object.

    It's much like a brezel - if you look at it edge-on, all you see is a flat, stringy piece of dough. But rotate the brezel, and you see a rather complex topological object of genus 3. Yet in both instances you are looking at the same thing. Same with the photon - depending on how you are trying to detect it you might see either a wave front ( with all its classical behaviour ), or a point particle; but in both instances it is the same object, a photon.

    I just meant my personal curiosity would never be "willing to accept that there is more to the world than human perception allows in our macroscopic world."
    To me that only means that whatever cannot be modelled by classical mechanics can't exist, in your opinion. A very naive and limiting point of view, if you ask me.
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    Quote Originally Posted by bill alsept View Post
    Correct me if I'm wrong but doesn't the wave particle theory claim that the interference pattern (in the single photon version) is caused by each photon interfering with itself by somehow traveling through both slits or every possible path?
    Those are two possible interpretations. If you like one of them then choose it and you will be happy. If not, there are about half-a-dozen other explanations you can choose from (many worlds, pilot wave, etc.) All are equivalent. All use the same mathematics to predict the result.

    Maybe that's not exactly right but I believe the theory involves both slits somehow or the pattern wouldn't develop.
    Yes, it must involve both slits.

    I don't think this theory will even work if the photon doesn't go through both slits so I would think the experiment would "say a lot about a photons going through one or both slits."
    But that is the tricky thing. We don't know (and we can't know, even in principle) what the photon does between the source and the screen.

    If we try and test which slit it goes through, or whether it goes through both, or ... Then we change the experimental setup and no longer get the interference pattern.

    Everyone believes that each photon goes through both slits only because they see an interference pattern form.
    I don't belive it goes through both slits (that is to say, that is not my favourite interpretation).

    I just meant my personal curiosity would never be "willing to accept that there is more to the world than human perception allows in our macroscopic world."
    Well, you seem to be with Einstein on that one. It appears you are both wrong.

    Why on Earth should nature at all scales behave the same way?

    You seem to be falling into the Forrest Noble fallacy of "the universe must be simple enough to be understood by a semi-evolved primate living on a small planet in the unfashionable corner of the galaxy." Why?

    Maybe it can only be understood by hyper-intelligent, pan-dimensional meta-beings who have ascended to a higher level of awareness.

    But there is no reason nature should even be fully comprehensible to anyone.
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    Quote Originally Posted by Strange View Post
    But that is the tricky thing. We don't know (and we can't know, even in principle) what the photon does between the source and the screen.
    We know that a photon traveled from the source through at least one of the slits to the final detection wall where an interference pattern builds up one photon at a time.
    What I have read about the single photon experiment is that somehow the individual photons interfere with themselves by going through both slits. Is this right? Above you suggest "there are about half-a-dozen other explanations you can choose from (many worlds, pilot wave, etc.) All are equivalent." Do your or Markus or anyone choose one of these explanations that you like? Please explain it. Nobody wants to speculate as to how the pattern is formed but they sure get upset if I want to wonder about these things. Below you seem to disagree with everyone else when you say you "don't belive it goes through both slits" but then you suggest sarcastically at Forest's and my expense that its ridicules to try or want to understand everything when really I'm only asking about a few things. I agree that the photons wouldn't go through both slits. Can you explain why you think so?

    In a double slit/single photon experiment what do you think causes the interference pattern?:
    (A) The photon somehow goes back and forth through both slits and interferes with itself. (or correct explaination if this is not described right)
    (B) Your own theory?....

    (C) There is no answer.

    (D) We can't understand everything so we should not ask, speculate, reason, or wonder about things like that.

    If you answer A please explain how this theory works if not then you are only speculating. At least my speculations come with a guess.
    If you answer (B) then great! I would like to hear it.
    I hope you wouldn't answer (C)
    If (D) is your answer then I guess I understand why you get upset when someone questions things.
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    Quote Originally Posted by bill alsept View Post
    What I have read about the single photon experiment is that somehow the individual photons interfere with themselves by going through both slits. Is this right?
    That is one popular explanation.

    Above you suggest "there are about half-a-dozen other explanations you can choose from (many worlds, pilot wave, etc.) All are equivalent." Do your or Markus or anyone choose one of these explanations that you like?
    I am in the awkward position of not fully understanding the math and not really like any of the interpretations I have come across.

    As far as I understand (which is obviously limited and may be wrong) it is just a consequence of the non-locality (in time and space) of quantum effects. An interpretation like "going through both slits" doesn't really help when it comes to larger objects. And doesn't help in the case of the delayed quantum eraser experiment. Non-locality explains these. And, presumably, so does the many worlds interpretation.

    but then you suggest sarcastically at Forest's and my expense that its ridicules to try or want to understand everything
    It took me a while to work out what you were referring to there as it is so far from what I intended to say. I wasn't ridiculing anyone. And I wasn't saying we shouldn't try to understand. I'm sorry if it came across that way.

    The point is: there is no reason to expect that the universe should be understandable beyond some level. Forrest's fallacy is to insist that the universe must be simple enough for him to understand (and therefore current theories are wrong because he doesn't understand them).

    That seems like an unreasonable argument.

    I certainly don't expect the universe to be simple enough for me to understand it (it is already well past that stage). It would be nice if it is possible for someone to understand it. But there is no guarantee of that. But that is no reason not to try.

    I agree that the photons wouldn't go through both slits. Can you explain why you think so?
    Non-locality. The photons are affected, to some extent, by everything around them (in the entire universe and over all time). But the size of that effect depends on the probability of the interaction - hence the pattern based on the probabilities of interacting with the two slits.

    This is my layman's interpretation of Feynman's path integral approach. It may be mistaken and it may turn out that Feyman's work is just a hacky approximation to something better.

    I will be very excited to see what new ideas science brings us in this area. But there is no point in someone like me trying to make up better theories, if Feynman couldn't do it.

    I guess this is somewhere between your A and C. I think B would be a waste of everyone's time.

    If (D) is your answer then I guess I understand why you get upset when someone questions things.
    I don't get upset when people question things. I think that is great; it is a good way of understanding things.

    I do get very slightly frustrated when people are completely irrational (e.g. Forrest's belief that his unreasonable assumptions, flawed logic and cherry picked evidence are somehow better than real science).
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    Quote Originally Posted by bill alsept View Post
    We know that a photon traveled from the source through at least one of the slits to the final detection wall where an interference pattern builds up one photon at a time.
    No, not really. All we know for certain is that a photon left the emitter, and a photon arrived at the detector; everything else is a matter of probabilities only.

    What I have read about the single photon experiment is that somehow the individual photons interfere with themselves by going through both slits. Is this right?
    If you sent a single photon through the setup, you get a single hit at the detector screen; there will be no pattern present ( you need many photons for that ). However, the important point to note is that you cannot predict precisely where the photon hits; every time you send a single photon, it will hit in a different spot ( statistically ). If you look at very many such hits, they will be distributed in a spread-out interference pattern. If the photon was a classical object, you could in principle predict precisely where on the screen it will hit if you are given its initial momentum vector; all photons with the same initial momentum vector would hit at the same spot. However, in reality this is not what happens; even though we get particle hits, the final trajectory behaves like a wave ( interference, and also deflection in both directions on the screen ), so photons do not behave like tiny classical objects. They are do not behave deterministically ( classical ), but probabilistically ( QM ).

    Nobody wants to speculate as to how the pattern is formed
    But we know how the pattern is formed, and is has been explained; what else do you need ?

    I agree that the photons wouldn't go through both slits.
    Of course it wouldn't, because the double slit acts on the wave aspect of the photon, not on a particle - one series of wave fronts comes in, and two series of wave fronts go out the other side, interfering with on another. The detector screen on the other hand acts only on the particle aspect, which is why you see only one hit.

    The photon somehow goes back and forth through both slits and interferes with itself.
    Definitely not, since photons cannot be accelerated, so you can't reverse their direction of propagation in transit.

    Your own theory?....
    Is not needed.

    There is no answer.
    There is - see above.

    We can't understand everything so we should not ask, speculate, reason, or wonder about things like that.
    Silly, because then we wouldn't need science, and don't need to discuss this in the first place.

    please explain how this theory works if not then you are only speculating
    The theory has been explained, the problem now is that you are refusing to accept the explanation, because you want everything to work deterministically and in terms of classical mechanics. So long as you insist on that you will never get anywhere over and above elementary high-school level physics, I'm afraid. You are trying to force an explanation that conforms to your ideas about what is rational and acceptable; you are not even willing to consider anything that falls outside those self-imposed parameters, it seems. Unfortunately the quantum world just does not work that way; classical mechanics is just a special case of how the world works, it is a subset of quantum mechanics for the limiting case of very small deBroglie wavelengths, and it explains both worlds, unlike classical mechanics ( correspondence principle ). To put it quite simply - you are stuck at the wrong end of the stick.

    So again - photons are not classical objects; the have both a wave and a particle aspect, yet they are neither wave nor particle. Depending on how you observe them you see either one or the other aspect, but never both at the same time; furthermore, they behave probabilistically rather than deterministically, so you can never predict the outcome of an experiment with 100% certainty - you can only give probabilities. There is no classical explanation for this; trying to find one is like chasing your own tail - it will never get you anywhere.
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