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How is the presence of a *single* electron determined?

  1. Dec 17, 2014 #1
    The 2 slits experiment can be carried out by firing a single electron at a time and then over time observing the gradual build up of what appear to be interference patterns characteristic of a wave passing through the 2 slits on the screen behind the slits. This is despite the fact that the point of detection of each electron as it arrives appears to be random. It appears then that the path of each single electron passing through one or both of the slits has an effect on the path taken by subsequent electrons.

    If the assumptions above are correct there is no obvious classical explanation for what is observed.

    Q. What characteristics are measured to confirm the presence of a SINGLE electron being fired?

    Q. What characteristics are measured to confirm the presence of a SINGLE electron arriving on the screen behind the slits?

    I presume the determination of the presence of a SINGLE electron is achieved by measuring the quantity of specific properties such as charge and spin. How though were the determinative quantities being utilised originally bench marked? If for example an electron were only able to exist as a pair of particles then the smallest reading of such properties would equate to a pair of particles.

    The method of propelling each electron towards the slits would be expected to be inherently random in determining the initial trajectory of the particle. Thus the presence of interference patterns indicates some effect that alters the initially random trajectory of a majority of the electrons such that the majority of the electrons are directed to specific regions of the screen.

    Such an effect could be envisaged as the presence of a continuous ever present field that is propagating as waves through the slits and generating interference patterns beyond the slits. As each electron enters the field its random trajectory will more often than not be affected by a region of peaks and troughs which will then channel the electron toward one of the specific "interference" regions of the screen.
     
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  3. Dec 17, 2014 #2

    mfb

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    If you never get a splitting of them, then there is no difference to a single particle (but that would be in strong disagreement with its spin measurements). You can do the same experiment with atoms, for example, where it is known that they consist of many smaller particles, and it does not change the result. Whatever appears as single particle to us is called "electron", its internal structure is not relevant here.
    There are tons of measurements of the electron charge, so "the charge of a single electron" is well-measured and can be used in the double-slit experiment.

    Actually, the majority hits the material with the slits (but outside the slits). For those passing through: yes.

    What is a "continuous ever present field" and how does it depend on the slits and the electron? How is that consistent with electrons coming from different angles (producing a different interference pattern), for example?
     
  4. Dec 17, 2014 #3

    DrChinese

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    Welcome to PhysicsForums, Jonathan!

    These 2 references (same experiment) may help:

    Very basic overview:
    http://www.hitachi.com/rd/portal/research/em/doubleslit.html

    Download Tonomura.pdf from this one:
    https://www.u-cursos.cl/ingenieria/2007/2/FI34A/1/material_docente/?id=139739#o139739
     
  5. Dec 17, 2014 #4

    Nugatory

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    Why should you draw that conclusion? All that is necessary to produce the pattern is that each individual electron have a higher probability of landing in some areas than in others.
     
  6. Dec 17, 2014 #5

    DrChinese

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    If that were the case, how does any of this explain the key effect observed? Namely, that interference only occurs when the path is not known. Your explanation would produce an interference pattern all the time (even if on just one side).
     
  7. Dec 17, 2014 #6
    By "continuous ever present field" I am postulating that the trajectory of the electrons are influenced by the environment through which they travel. No environment in which quantum particles (and their associated waves exist) can be entirely inert. Assuming the presence of electromagnetic waves, gravity, anti-matter and virtual particles there is the potential for interference with electron particle/wave pairs.My proposition is that the trajectory of an electron particle/wave may be altered by the presence of waves and their resulting interference patterns. The analogy would be boats launched at randomly differing angles into a river containing a number of discrete channels with very strong currents each of which flows to one of the interference pattern points; most boats would quickly get swept up into the first strong channel they reach , though some boats would escape into quieter eddy currents and avoid the interference pattern points. As far as I am aware neither the composition of an electromagnetic wave nor its method of propagation through a vacuum are understood; in the absence of such understanding it seems premature to accept the current explanation for interference in the 2 slit experiment.


    Actually, the majority hits the material with the slits (but outside the slits). For those passing through: yes.

    What is a "continuous ever present field" and how does it depend on the slits and the electron? How is that consistent with electrons coming from different angles (producing a different interference pattern), for example?[/QUOTE]
     
  8. Dec 17, 2014 #7

    mfb

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    At least by the double slit for sure.
    I don't think that makes sense.
    That is not your proposition, that is standard quantum mechanics. In terms of interpretations, you get everything from "the wave is just a mathematical tool" to "the wave exists and it is everything that is there".
    "Flows towards points" do not work, as those points depend on the direction of the incoming electron (before it hits the slit), for example. You can also shift the pattern with electric or magnetic fields (even if the electron never passes through the field!).

    And please keep in mind that we do not allow personal speculations that go beyond existing theories here (forum rules).
     
  9. Dec 17, 2014 #8

    bhobba

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    VERY good point.

    To the OP a different approach may help your understanding.

    QM is actually an approximation to a deeper theory called Quantum Field Theory.

    Strangely, at the beginner level looking at it from that viewpoint avoids a number of beginner issues. Here is a good cheap book that takes that approach:
    https://www.amazon.com/Fields-Color.../ref=tmm_kin_swatch_0?_encoding=UTF8&sr=&qid=

    Caveat - that book is generally good - but it does have a few minor issues with what Feynman thought and some other things - but overall it gives a correct account.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  10. Dec 18, 2014 #9
    From what I have understood about the experiment interference ceases to occur in the presence of an ACTIVE detector that detects a particle travelling through one of the two slits. Under these circumstances the path is known by the human observer and from what is observed one could infer that the particles change their behaviour purely based on this knowledge. Alternatively one could look for an explanation that does not require the particles to act intelligently but rather from the physical effects of any known changed variable in the environment which in this case is the presence of an ACTIVE particle detector (perhaps something along the lines of the Aharonov–Bohm effect?). Assuming the presence of actual waves (not probability waves) within the environment of the experiment prior to the release of any particles and in the absence of an ACTIVE particle detector there could be interference at the slits which could affect the trajectory of any particle/wave that is released through the slits. My assumption would be that ACTIVATION of a particle detector may introduce a secondary field that would then broadly negate the effect of the primary field.
     
  11. Dec 18, 2014 #10
    All that's needed to spoil the interference pattern is for the "which way" information of the particle to become entangled with something else. That's certainly the case for a human observer becoming aware of the which way information—in which case, the path information has become entangled with a huge amount of environmental variables leading to full blown decoherence—but that's overkill. The "something else" could be another quantum degree of freedom which can be disentangled again if one wishes. That's the quantum eraser experiment, in a nutshell. There's no "intelligently acting particles" or "second fields" involved in any of this. It's just the observation that mixed states aren't capable of interference in general.
     
  12. Dec 18, 2014 #11
    From what I have read about QM I was under the impression that the scientific community treated the Quantum Wave Function strictly as a mathematical formula to calculate the probability of the location and velocity of a quantum particle whereby the precise underlying processes contributing to the calculated results are as yet unknown. Accepting that the formula works in practise my interest is in trying to uncover the underlying processes. My proposition is based firmly on the notion of ACTUAL waves as opposed to probability waves.

    I take it from your comments that you envisage the trajectory of an electron to be fixed at the point of exit from the emitter such that its destination on the screen is pre-destined without any possibility of changing course due to environmental factors in the interim? If for example the electron behaves as a wave when travelling through the slits it could be affected by its own interference and /or the interference of any other waves propagating through the slits.
     
  13. Dec 18, 2014 #12

    mfb

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    Most physicists don't care for their work - "shut up and calculate" works well. If you ask for favorite interpretations, then many different answers come up.
    Then you have to find an experiment that would give a measurable difference between different interpretations.
    The word "actual" is meaningless. Several interpretations treat the wave functions as physical objects, some of them are deterministic (no probabilities involved).
    No one said that, and I have no idea how you got that impression. Obviously interactions on the way change the behavior of the electron.
    That's exactly what we see in the double-slit experiment (in nearly all interpretations).
     
  14. Dec 18, 2014 #13
    Do you think my proposition is a plausible alternative explanation to that of entanglement?
     
  15. Dec 18, 2014 #14
    I disagree that the word "actual" is meaningless in this context. An actual wave is for example a detectable electromagnetic wave whereas a probability wave is a wave in an abstract mathematical "space".

    In your previous response you said "I don't think that makes sense" and "Flows towards points" do not work, as those points depend on the direction of the incoming electron (before it hits the slit), for example. I took it from this that you didn't accept the viability of my proposition that the destination of an electron could be influenced by waves and interference around the slits to arrive predominantly at the sites of the interference patterns on the screen. I take it you now accept this as plausible but with the obvious challenge of designing a suitable experiment to verify. Do you have any ideas for such an experiment?
     
  16. Dec 18, 2014 #15

    Nugatory

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    Not until it's been through the peer review process that is an essential part of all contributions to the physical sciences. Until then, we're on the wrong side of the PhysicsForums rules about personal theories and acceptable sources, so this thread is closed.
     
  17. Dec 18, 2014 #16

    mfb

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    I do not see a proposition that could be used to predict anything (where are the calculations to show that this proposition gives the correct results in all setups?) - and that would be against the forum rules anyway.
    Playing with words is not physics. Words can be used to describe formulas, but they cannot replace them.

    Edit: Sorry Nugatory, I was writing that post already and did not see your post. But I see we posted the same thing.
     
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