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Heisenberg Uncertainty due to interaction of the inherent reality?

  1. Jul 10, 2014 #1
    Coming from a Chemistry background, the Uncertainty Principle always seemed to be described as an inability of precision due to strictly physical and real reasons (i.e. you have to interact with an object to measure something about it, and this certainly will alter the object). It seems as if this was historically the original naive explanation of it as well before QM came along in vogue and made us wonder if reality itself was uncertain (in the sense of there not being defined states before measurement).

    Without a fundamental understanding of what underpinning reality exists (or doesn't), how do we have confidence in the validity of our measurements? How do we know that our measurements are not, via interaction, deceiving us from their true states?

    I wager I'm missing something from EPR and Bell's here...but I'll leave that gambit open for now. If it helps anyone in explanation or context, I believe thinking of the typical 3-polarizer experiment (two 90 degrees, with one 45 degrees inbetween) was what got me to pondering these concerns.
  2. jcsd
  3. Jul 10, 2014 #2


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    Staff: Mentor

    The uncertainty principle is not about measurements. It is an intrinsic property of the particle itself.
    Naturally, measurements will show this uncertainty, but they are not its cause. This can be verified by clever measurements that do not suffer from the "measure the position and you will change its momentum" problem.

    Bell's theorem is indeed the answer: the world cannot be classical. And quantum mechanics is the best theory we have to describe the derivations.
  4. Jul 10, 2014 #3
    Weak measurements sounds pretty to the spirit of the quandary in my mind, but geeze, I find it difficult to conceive of how that could be possible. That wiki article unfortunately read a bit like gibberish to me, but I'll do some googling to see what context I can dig up.
  5. Jul 10, 2014 #4
    Alrighty, I've done a bit of reading on that. Correct me if I'm wrong, but the sentiment I'm getting is that weak measurements are NOT truly measurements but rather inferences taken from indirect measurements that provide effectively the utility of an average measurement for the system. Nonetheless, these weak measurements stick with QM predicted probabilities.

    If this is so, it doesn't seem like the uncertainty principle itself directly suggests that the uncertainty is inherent, and it seems that uncertainty is at least inevitable with actual measurement. Rather, any inherent uncertainty comes from the interpretation you get out of the math of QM in general. Am I on the right track or way off hah?
    Last edited: Jul 10, 2014
  6. Jul 10, 2014 #5


    Staff: Mentor

    Mate I think you might be interested in an approach to QM based on what are called POVM's, which are a generalisation of the usual quantum measurements which are known as Von Neumann Measurements. They contain weak measurement MFB is talking about. In fact a POVM is always derivable from a Von Neumann measurement by interacting a measurement apparatus whose observation is a Von Neumann measurement with the thing to be measured. When that interaction is weak - guess what - you get a weak measurement.

    To see its power check out, by starting with POVM's you can actually derive the Born rule fairly readily:

    Neat hey.

    BTW since your background is Chemistry a specific QM book would likely help:

    Last edited by a moderator: May 6, 2017
  7. Jul 10, 2014 #6


    Staff: Mentor

    They are an example of what are called generalised measurements.

    As I mention in the previous post they result from interacting a system with a measuring apparatus.

    Check out:

    Last edited: Jul 10, 2014
  8. Jul 11, 2014 #7
    I definitely saved that pdf. That had some good information on some of the formalism and mathematical background I was missing. I have a taste of how weak measurement could be done from it now, though that's going to take considerable reading for me to discuss it further in any appreciable way. If I presume no major objections then that leads to another question. If these weak measurements do indeed demonstrate inherent uncertainty (again, not simply due to the measurement itself), doesn't this empirically rule out a good handful of realist QM formulations? It would seem to be recognizable evidence of a lack of defined states outside of observation, and I would have to wonder how theorists such as the Bohmians would respond.

    Also, that book sounds pretty good. I'll add it to my summer reading list.
  9. Jul 11, 2014 #8


    Staff: Mentor

    Of course not.

    It like aether theories in relativity - they are deliberately concocted so its 'reality' is not directly observable, nor can any experiment be devised to determine its existence one way or the other.

  10. Jul 12, 2014 #9
    Bah, I keep forgetting that about the Bohmian setup. Maybe that's just personal bias pushing that thought to the back since it makes the idea seem a bit ad hoc when recognized.

    Anyways, I feel pretty sated on all this. I'll have a bit of reading to do to truly be so, but that's going to happen over a much slower course. Thanks.
  11. Jul 12, 2014 #10


    Staff: Mentor

    You hit it in one.

    It's purely a personal opinion of course but like the aether I find BM far too ad-hoc. Don't like the world view engendered by some theory - simple - create experimentally undetectable ad-hoc hypothesis to give the world view you want.

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