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B Where is the field?

  1. Dec 14, 2017 #1
    A charged particle is passed through a beam splitter and progresses down two divergent paths.

    How does the charge field document this?

    Does the field recognize both components? Only one? And once the field is detected, defining which path was taken, does the field alter it's nature through all of space to reflect the fact that the particle took a specific path?

    Discussion of this would be helpful.
  2. jcsd
  3. Dec 14, 2017 #2


    Staff: Mentor

    What do you mean by "the charge field"?

    Also, you marked this thread as "A", which indicates a graduate level knowledge of the subject matter. What background do you have in this area?
  4. Dec 14, 2017 #3
    You need to get familiar with the concept of a superposition. If you aren't yet familiar with that, I have found this video from MIT to be good:

    The first few minutes deals with admin stuff about assignments, tests etc so you can skip to around 09:30
  5. Dec 15, 2017 #4
    I am referring to the electric field as interpreted as a charge polarization of space.

    Does the field record an electron moving on both paths? The average of half an electron moving on both paths? Does it record nothing until there is some kind of interaction with an observer?

    It seems to me that the field must document one of the possibilities that the particle will follow, path A, path B or both A and B.

    If the particle is not observed then it is claimed that it travels along both A and B. Observation can be delayed as long as possible, so the field can grow arbitrarily large. and if it is found that the particle took path A for example, then the field must re-organize itself through it's entire extent so that this observation is consistent with future observations. This has implications for how fields change over time.
  6. Dec 15, 2017 #5
    "you can skip to around 09:30"

    Actually I've seen that lecture and the ones that follow, and they don't speak to the topic I am addressing, which is how a distributed field documents an electron's wave function. In this example, one in which the electron (without observation), can be interpreted as being multiple places at the same time.

    The field should record this.... What does it record, and how does that record change when the particle is localized?
  7. Dec 15, 2017 #6
    In a sense, you are right. The fact that we have a superposition, by itself, doesn't "document" what would happen once a particle is actually found somewhere. To complete the picture, you need the ideas of collapse and Born rule. That tells us that (a) an actual measurement will yield only one of the results that compose the superposition and (b) squaring the amplitude for a given measurement tells us the probability of seeing that measurement.

    But I think Dr Allen Davies gets to these points somewhere in that lecture.

    There are interpretations of QM that try to 'document' an underlying cause that decides when a particle will be found here and when it will be found there, but AFAIK, there is no unanimous acceptance for these views.
  8. Dec 15, 2017 #7
    Along with the collapse of the particles wave function there is going to have to be a collapse of the wave function of the entire electric field that is associated with the particle and which is distributed though and recorded into space.

    This immediately raises the question, how does space document the electron moving down both paths in a way that is consistent.
  9. Dec 15, 2017 #8


    Staff: Mentor

    Where do you get that from? - its bollocks.

    You may be thinking of the intuitive idea behind charge screening where the polarization of the field on virtual particles screens the charge. That's equally bollocks as well - but needs it own thread. Basically though that is the result of the so called re-normalization group:

    At about the most elementary level I know see the following:

    Since this is an advanced level thread the above should be understandable by the OP.

  10. Dec 15, 2017 #9


    Staff: Mentor

    Well I suppose you could come up with an actual interpetation along those lines - I have never seen one though.

    It's usually a misconception from popularization's. Don't worry though - I fell for it and I had read quite advanced books on QM like Ballentine, Dirac etc. Its an easy trap to fall into. It wasn't until posting here I realized it was an interpretation, and not an official one at that - at least none I could locate.

    What is an electron really - well that depends on what you mean by 'really', reality - you know all those words that get bandied about in trying to understand modern physics, until you realize it cant be done that way - but I will let you figure that one out for yourself - if I were to discuss it, and I have many many times, I know it will lead precisely nowhere. QED describes it to amazing accuracy - that we can say for sure - beyond that - it's really philosophy not physics.

  11. Dec 16, 2017 #10


    Staff: Mentor

    Where are you getting this from? Do you have a reference?

    It doesn't. The electron is separate from the electromagnetic field; they each have their own wave function.
  12. Dec 16, 2017 #11


    Staff: Mentor

    Based on the OP's further posts, I have changed the level of this thread to "B".
  13. Dec 16, 2017 #12
    It’s only a “B” if he observes it...
  14. Dec 16, 2017 #13
    Journal of Physics B: Atomic, Molecular and Optical Physics

    Vacuum polarization by a Coulomb field. Analytical approximation of the polarization potential
    A G Fainshtein, N L Manakov and A A Nekipelov

    Journal of Physics B: Atomic, Molecular and Optical Physics, Volume 24, Number 3

    Back when I was in university, vacuum polarization was the principle means of obtaining Coulomb's law from an infinite electron point charge.

    With that out of the way, perhaps you can now answer the question that was posited regarding how the wave function of an electron moving down two independent paths is recorded by the electromagnetic field that is conducted through space.
  15. Dec 16, 2017 #14
    "The electron is separate from the electromagnetic field; they each have their own wave function."

    Excellent. How does the separate wave function of the electromagnetic wave in space record the wave function of an electron that is passed through a beam splitter and is constrained to move along two separate paths?

    How do the vector components of the electromagnetic field record the path taken by the electron?
  16. Dec 16, 2017 #15
    "Based on the OP's further posts, I have changed the level of this thread to "B"."

    Excellent. You should find the question easy to answer then.

    So far I haven't seen any attempt to do so.

    When do you think someone might try?
  17. Dec 16, 2017 #16
    "Since this is an advanced level thread the above should be understandable by the OP."

    Thank you for your references. Unfortunately they do not apply to the question being asked.

    Perhaps you could try again.
  18. Dec 16, 2017 #17


    User Avatar
    Staff Emeritus
    Science Advisor

    Probably never since you got yourself banned. Anyone else with questions on this topic should make a new thread.

    Thread locked.
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