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Quantum Physics: Double Slit Experiment

  1. Jul 23, 2014 #1
    I have a few problems understanding the double slit experience and the behavior of photos due to simple observation.

    I am not really looking for in depth explanation here (great if you have it) but more for links or references to read more about it.

    Here are some questions:

    1. What does "observation" mean? Is it mere act of looking passively or is something interfering with the photon such as shining light on it during observation?

    2. Observation is made at the slit, and at the board after the slit. When observed it acts as a particle and when not observed it acts as a wave. Does this imply something about "time" or am I missing something else?
     
  2. jcsd
  3. Jul 23, 2014 #2

    bhobba

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    Its an interaction with the system that usually leaves somre kind of mark here in the common-sence macro world.

    The so called wave particle duality is a crock of the proverbial - see the FAQ:
    https://www.physicsforums.com/showthread.php?t=511178

    Here is a correct analysis of the double slit experiment:
    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    Its got nothing to do with acting like waves or particles.

    Thanks
    Bill
     
  4. Jul 23, 2014 #3
    Thanks, this makes me feel a little better. But can I ask you, are you unique in this view, or are you an outlier, or is this the consensus view? How prevalent is your view in the real Physics community of scientists?
     
  5. Jul 23, 2014 #4

    bhobba

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    It's the view of those that have studied the details in the more advanced textbooks like Ballentine:
    https://www.amazon.com/Quantum-Mechanics-A-Modern-Development/dp/9810241054

    Its basically what the FORMALISM of QM says - devoid of interpretative views. Interpretations are a mixed bag with a wide variety of views about amongst those interested in such things.

    I would say most physicists don't really care about the fundamental, and even philosophical issues, lay people seem fascinated with - possibly from reading/seeing sensationalist popularisations of dubious value such as What The Bleep Do We Know Anyway (a total load of rubbish BTW) or watching Kaku and others on TV. Most physicists fall into the shut up and calculate brigade - which, basically (I add a couple of bells and whistles to do with decoherence - but no need to go into that here) is my view.

    The FAQ I referred to was written by Zapper who is a professional physicist that does this stuff for a living. You can trust his view as a correct reflection of scientific orthodoxy. I am a guy formally trained in applied math self taught in physics - so while my information comes from highly regarded standard textbooks, and is trustworthy in that sense, its not quite the same as others steeped in this day to day.

    I understand their confusion - I shake my head when I see Kaku and others on TV saying an electron is in two places at once and other sensationalist 'half truths'. Its not in two places at once - what it is when not observed is very interpretation dependant, but the formalism is silent about such questions - yet they state it like its an accepted fact - it isn't.

    Thanks
    Bill
     
    Last edited by a moderator: May 6, 2017
  6. Jul 23, 2014 #5
    It makes sense to me that the actual physicists who know what the math is about spend all their time doing the math rather than getting into philosophy or showmanship or vudu.
     
  7. Jul 23, 2014 #6
    So what about Schrödinger's cat? Is this merely a simplified example for the layperson?
     
  8. Jul 23, 2014 #7

    bhobba

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    Most are like that - but a few are into foundations and in that community all sorts of views abound eg
    http://users.ox.ac.uk/~mert0130/ [Broken]

    I am a bit interested in foundations, but my main interest is the most elegant way to view QM so it seems reasonable, even natural. A tall order - but progress has been made:
    http://arxiv.org/pdf/quantph/0101012.pdf

    The modern view along those lines, and personally what I believe, is that in modelling physical systems it turns out, from a few reasonable assumptions, that there are only two candidates. Ordinary probability theory and QM. But QM is singled out if you want either to be able to have continuous transformations between states or entanglement. You really do want continuous transformations because if a system can change from one state to another in say one second, in doing that it would change to something in half a second.

    Thanks
    Bill
     
    Last edited by a moderator: May 6, 2017
  9. Jul 23, 2014 #8

    atyy

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    bhobba is giving the textbook answer. Two other excellent textbooks that give this answer are Landau and Lifshitz https://www.amazon.com/Quantum-Mechanics-Third-Edition-Non-Relativistic/dp/0750635398, and Weinberg https://www.amazon.com/Lectures-Quantum-Mechanics-Steven-Weinberg/dp/1107028728.
     
    Last edited by a moderator: May 6, 2017
  10. Jul 23, 2014 #9

    bhobba

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    No - it was actually proposed by Schroedinger, but its import is not what the populist press make out.

    First you need to understand the standard Copenhagen interpretation (it not the one I hold to but my view is similar):
    http://motls.blogspot.com.au/2011/05/copenhagen-interpretation-of-quantum.html

    Under that view the solution to Schroedinger's cat is utterly trivial. In Copenhagen QM is a theory about observations that appear here in an assumed common-sense classical macro world. That occurs at the particle detector in Schroedinger cat - everything is common sense classical from that point on. The cat is never dead and alive, in some weird superposition, it is alive or dead - period.

    The issue Schroedinger's cat is about, is how can a theory that assumes the existence of a classical world explain that word if it assumes it existence in the first place. If you don't assume it you run into the issues the thought experiment was meant to highlight. What is needed is a purely quantum theory of measurement not involving this classical quantum distinction.

    In recent times a lot of progress has been made in resolving the issue with investigations into decoherence:
    http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

    But some issues, of a technical nature, do remain.

    In that connection you may find the following interesting:
    http://www.fisica.ufmg.br/~dsoares/cosmos/10/weinberg-einsteinsmistakes.pdf

    Einstein and Bohr would argue about QM in their famous debates. But as Weinberg points out - the jokes on both of them - they were both wrong. The moon is there when we are not looking because its observed by its environment all the time.

    This is the modern view decoherence has engendered.

    Thanks
    Bill
     
    Last edited by a moderator: May 6, 2017
  11. Jul 23, 2014 #10
    The moon is there when we are not looking because its observed by its environment all the time.

    This was my problem with the double slit experience's observation. It is always observed by its environment.
     
  12. Jul 23, 2014 #11

    bhobba

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    Yes - but photons interact weakly with the environment - which is why you can get the experiment to work easily.

    It can be done with electrons, but that is much harder because they strongly interact - eg you need a hard vacuum.

    Thanks
    Bill
     
  13. Jul 23, 2014 #12

    atyy

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    As an example of what bhobba is saying, here is a double slit experiment with buckballs in which the need for a high vacuum is clearly stated: http://arxiv.org/abs/quant-ph/0303093. This paper studies how increasing the gas pressure in the environment leads to decoherence: http://arxiv.org/abs/physics/0508197.
     
  14. Jul 23, 2014 #13
    I missed this point regarding the vacuum environment for a second there, thanks.


    One thing I still am a little confused about. When you say that when the experiment is not observed, it acts as a wave, who is observing it acting like a wave? And when the observer sees it act like a particle through the two slits, are we talking about two observers here now? Or is there only one observer?

    I think "Observer" has a specific meaning in this context, no? "Observer" is a tool, right?

    I mean, you can have 100 scientists observing the experience, but it wouldn't be the "Observer" spoken of in the experiment, right?
     
    Last edited: Jul 23, 2014
  15. Jul 23, 2014 #14

    bhobba

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    I don't think anyone on this forum has said that.

    Most people here, understanding the difficulties with the wave particle duality eschew it - exactly as the FAQ says.

    Its got nothing to do with observers. Can you have a read of the paper I linked to and use that to discuss what's really going on:
    http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

    Indeed that is one of the issues here - observation in QM is simply an interaction with a classical system - an observer is simply a classical system capable of registering some kind of 'mark' - its not what the semantics of normal use suggest.

    The observer in the double slit experiment is whatever registers the photon hitting the screen at the other end - eg a photographic plate. Its got nothing to do with scientists, human beings, etc etc.

    Thanks
    Bill
     
  16. Jul 23, 2014 #15
    There is so much misinformation in the media, TV, youtube, it is unreal.
     
  17. Jul 23, 2014 #16

    bhobba

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    Without doubt.

    But regarding the observer thing that is a semantic trap even some beginning textbooks fall into.

    The more advanced ones like Ballentine however are much more careful.

    Thanks
    Bill
     
    Last edited: Jul 23, 2014
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