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Laymans question about observational effect

  1. Feb 19, 2010 #1


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    Could someone please explain in laymans terms if possible how the process of effecting things by observing them actually works.
    Perhaps rather pretentiously me and a friend are having a dispute. He is saying its simply a matter of the measuring device bouncing photons or whatever off whatever you are trying to measure (or observe).
    However I have rather ambitiously considering how ill informed I am , said that this is not the case but the actual act of observation seems to effect the observed without there necessarily being any physical copntact - until you observe it it actually exists in more than one state.
    So how do you observe these tiny things exactly ?
  2. jcsd
  3. Feb 19, 2010 #2
    You must absorb some energy in order to measures anything. So by measuring you always interfere with the particle. Absorbing a photon is tricky as you need to absorb enough energy to make a measurement and you would likely have no photon left. It might be possible to have a high energy photon and take a bit of it's kinetic energy away, however in order for a photon to give up any kinetic energy it must be localized at that moment which would collapse the wave function.

    This act of a wave localizing to a particle when measured is in large what QM is all about. Every time we measure a particle it is only ever in one location, though QM proposes that before we measures it, it is a wave covering all space. And only with this explanation do our calculations meet experimental results because all particles have wave like properties in many experiments (double slit, Q tunneling, etc)
  4. Feb 19, 2010 #3


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    Theoretically speaking, you are correct. The act of "observing" (whatever that means in this case) will collapse the wavefunction from a mixture of eigenstates of the observable into one eigenstate of that observable. This is a fundamental postulate of standard QM, and requires no interpretation into the method of observing. Whatever can theoretically constitute an observation, can collapse the wave-function.

    However, physically speaking, you MUST interact with the particle in some way in order to observe it. So, the physical mechanism that will make the collapse occur is interaction.
  5. Feb 19, 2010 #4


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    I think my friend thinks observing the particle is a bit like trying to assess the position of a snooker ball on a snooker table by bouncing another snooker ball off it.
    However in the case of Schrodingers cat the observation is merely observing which of two holes the particle has passed through - no interatcion is actuially postulated other than observing which route the particle took in the past.
    Is my friend correct with his snooker ball analogy - is that the real reason that w effect the result by observing it - a simple matter of causality - or is it more complex ?
    To really understand quantum physics properly what areas of mathematics and physics would I need to study (I dont have the opportunity to study a whole physics degree at the moment - I was wonderin if I could just read the relevant parts) ? The last thing i remember are quadratic equations - however I am prepared to learn the maths and physics necessary to gain a better understanding of the theory - I find it intriguing.
  6. Feb 22, 2010 #5
    For a solid foundation of wave mechanics


    For an introduction to LVS and QM Core, less mathematical


    For in-depth mathematical QM Core

    If you have not taken Calc, you will need:
    You need to know LA
    You need to understand DEs

    If that is just too much, then check out this non-mathematical book which goes in-depth on the double slit experiments and others:

    Last edited by a moderator: May 4, 2017
  7. Feb 22, 2010 #6


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    No thats not too much at all, I realise this willl take time but I am determined -so I will start with calculus - you could never get me to do this at school and now look - volunteering ! - thankyou very much for your help !
    Last edited by a moderator: May 4, 2017
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