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Thread: Diffraction of a massive particle in a slit experiment

  1. #1 Diffraction of a massive particle in a slit experiment 
    Forum Sophomore Le Repteux's Avatar
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    Hi Everybody,

    If it is easy for me to understand how a wave on water will be diffracted by an obstacle, it is less easy to understand how a massive particle will be diffracted while passing through a slit. By friction and gravity, momentum of water molecules is affected by the obstacle, and transfer part of this momentum to the molecules around, whereas a massive particle is not affected by friction from the slit if it is sent in the middle. What force changes its direction then? How does it know it has to change direction? Does it interact with the border of the slit in such a way that, if it is a bit closer to the right, it will turn right, and if it is a bit closer to the left, it will turn left? Maybe the theory says that it is its wave form that will be affected and that this wave will behave as a water wave, but a wave contains information, and this information cannot travel instantly from the middle of the slit to its borders and vice-versa. Is there an answer to that question in the actual theory of diffraction?


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  3. #2  
    KJW
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    Huygens-Fresnel principle


    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Forum Sophomore Le Repteux's Avatar
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    Hi KJW,

    This is the answer for waves, but massive particles are not only waves, they contain a massive center that has to interact with the borders of the slit in order to change direction, no?
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  5. #4  
    exchemist
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    Quote Originally Posted by Le Repteux View Post
    Hi KJW,

    This is the answer for waves, but massive particles are not only waves, they contain a massive center that has to interact with the borders of the slit in order to change direction, no?
    To get anywhere with this I suspect we need to check what you want to talk about.

    What do you have in mind when you speak of a "massive particle"? An electron? A proton? Or a truly macroscopic object such as a golf ball?
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  6. #5  
    Forum Sophomore Le Repteux's Avatar
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    Hi Exchemist,

    I am talking about any massive particle or molecule that has been diffracted in a slit experiment yet.
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  7. #6  
    exchemist
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    Quote Originally Posted by Le Repteux View Post
    Hi Exchemist,

    I am talking about any massive particle or molecule that has been diffracted in a slit experiment yet.
    It's been done with electrons. I suspect for anything more massive, the momentum is too high for the wavelength to be long enough to produce much of an observable effect.

    The reason the phenomenon occurs is because matter, at the atomic or subatomic scale, has a wavelike aspect. You can read about it here: Matter wave - Wikipedia, the free encyclopedia

    So it works for the electron just as KJW's link says - because the electron is a wave as well as a particle. This is the basis of the structure of the atom. The wavelike nature, I mean.
    Last edited by exchemist; October 9th, 2014 at 04:45 PM.
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    Forum Sophomore Karsus's Avatar
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    It's been done with C60 Fullerenes. They are 60-atom carbon spheres measuring about a nanometre across with about 1.3 million times the mass of an electron.

    Fullerene Diffraction

    The problem with molecules that size and larger is that the van der waals force between the edge of the slit and the molecule start becoming disruptive. Also, the de Broglie wavelength is tiny, making first order diffraction angle so small that you need a very long apparatus in order to see any separation at your detector. The one pictured in that link looks about 3-4 metres long.
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  9. #8  
    KJW
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    Quote Originally Posted by Le Repteux View Post
    Hi KJW,

    This is the answer for waves, but massive particles are not only waves, they contain a massive center that has to interact with the borders of the slit in order to change direction, no?
    The point is that the Huygens-Fresnel principle explains the phenomenon that is called "diffraction" which causes waves to turn corners. The particular point I was making was that diffraction is not caused by interaction with the edges of the slit. Indeed, as indicated by Karsus, interaction with the edges of the slit is detrimental to the effect.

    Although a non-zero mass does change the precise wave-equation, and therefore the precise behaviour of the wave solution, I believe that the Huygens-Fresnel principle still applies as a general principle satisfied by solutions of wave-equations in general.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  10. #9  
    exchemist
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    Quote Originally Posted by Karsus View Post
    It's been done with C60 Fullerenes. They are 60-atom carbon spheres measuring about a nanometre across with about 1.3 million times the mass of an electron.

    Fullerene Diffraction

    The problem with molecules that size and larger is that the van der waals force between the edge of the slit and the molecule start becoming disruptive. Also, the de Broglie wavelength is tiny, making first order diffraction angle so small that you need a very long apparatus in order to see any separation at your detector. The one pictured in that link looks about 3-4 metres long.
    Thanks for this, Karsus. In fact, now that you mention it, I think this may have come up previously in this forum. I just forgot about it. The techniques are getting better and better, whereas my brain is getting older and older…….
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  11. #10  
    Forum Sophomore Le Repteux's Avatar
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    Quote Originally Posted by KJW View Post
    The point is that the Huygens-Fresnel principle explains the phenomenon that is called "diffraction" which causes waves to turn corners. The particular point I was making was that diffraction is not caused by interaction with the edges of the slit. Indeed, as indicated by Karsus, interaction with the edges of the slit is detrimental to the effect.
    It is probably too much interaction that is detrimental. There has to be interaction, no? Otherwise, the slit would not change anything. As I said, for water waves, the edges of an obstacle changes the momentum of the molecules, and since massive particles also carry a momentum, something has to force them to change direction. For instance, if the wave associated with a particle hits the edges, it might diffract that wave as in the H-Fresnel principle, but that wave has to influence the mass of the particle in some way, and the mass has to respond to that influence too. How a massive particle can change direction to follow information from a wave? How does that work? Is there any theoretical study of that question?
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  12. #11  
    exchemist
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    Quote Originally Posted by Le Repteux View Post
    How a massive particle can change direction to follow information from a wave? How does that work? Is there any theoretical study of that question?
    But surely, is that not exactly what quantum mechanics is all about? Particles with mass behave according to the information contained in their wavefunction. That could be said to be QM in a nutshell. So yes, thousands of theoretical physicists and chemists, since the 1920s, have devoted their careers to study of aspects of this insight into the nature of matter.

    If you would like to know more, you too can study QM, as many contributors to this forum have done.

    Actually, on your specific question about how it is that a particle can "change direction" at the slit, I think this may have a bearing on the question Nic321 asked in another thread about the "width" of a photon. I am very rusty on all this but I suspect the reason is the uncertainty principle, i.e. after the slit, although the particle might classically be thought to travel onward in a straight line, there is non zero probability that it may be found to have deviated from that line. The uncertainty principle is a wave-related phenomenon, of course.
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  13. #12  
    KJW
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    Quote Originally Posted by Le Repteux View Post
    There has to be interaction, no? Otherwise, the slit would not change anything.
    No. I suggest you re-read the Huygens-Fresnel principle and how it causes diffraction.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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