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Quantum theory of (others)

  1. May 31, 2015 #1
    QM or quantum mechanics is quantum theory of particles
    QFT or quantum field theory is quantum theory of fields (containing QED, QCD, and maybe QG)

    are there none beside particles or fields.. why is there no quantum theory of other stuff?
     
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  3. May 31, 2015 #2

    Vanadium 50

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    What "other stuff" are you talking about?
     
  4. May 31, 2015 #3
    No. I was just asking if there are just particles and fields... what category does particle and field fall under.. what else in the category and why only the two present?
     
  5. May 31, 2015 #4

    jtbell

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    Fields (described by QFT) and particles (described by QM) are not different kinds of "stuff." They are different descriptions of the same "stuff." The description that uses fields and QFT is more fundamental and applies under more circumstances than the description that uses particles and QM. Under some circumstances the fields behave in ways that we label as "particles" that follow the rules of QM, which are easier to work with.
     
  6. May 31, 2015 #5

    atyy

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    There are also quantum theories of strings and membranes, spins stuck to a lattice and lattice gauge theory.
     
    Last edited: May 31, 2015
  7. May 31, 2015 #6
    In jtbell context... particles, fields, strings, membranes are "different descriptions of the same "stuff.""? so fields are literally strings and vice versa?
     
  8. May 31, 2015 #7

    atyy

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    Yes and no. Heuristically, fields can be thought of as quantum theories of many particles. Also, fields and lattice gauge theory are heuristically the same things. It is also conjectured that fields in a lower dimension can be equivalent to strings in a higher dimension. Of the preceding heuristics, the most rigourous one is the notional equivalence of fields and lattice gauge theory. Thinking of quantum field theories as quantum theories of many particles is less rigourous, but most quantum field theory books do things this way. The relationship between fields and strings is an area of active research.
     
  9. Jun 4, 2015 #8
    Here's something I'm getting my head into. Do all fields have to be quantized? For example. If fields cause wave function collapse.. does it has to be quantum mechanized? I presume being quantized and quantum mechanized is the same.. But gravity as field doesn't necessarily mean it's quantized.. so how do you tell whether a field can remain classical or should be quantized (quantum mechanized)?
     
  10. Jun 4, 2015 #9

    atyy

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    In the traditional "Copenhagen" interpretation of quantum mechanics, there is a practical division of the universe into a classical part and a quantum part. It is the interaction of the classical part with the quantum part to produce a measurement result that causes collapse. The division of the universe into a classical part and a quantum part is subjective, so each observer can do it differently (or even the same observer can do it in several different ways). In this traditional interpretation, the "whole universe" is not necessarily quantum. However, it should be possible to formulate a quantum theory of any part of the universe that we can observe - so in that sense "everything" is quantum.

    The basic philosophy here is that we have an intuitive idea as to what a measurement result is, and quantum mechanics is not necessarily about a "story of reality that is continually unfolding", but rather a way to predict probabilities for what an observer observes.
     
    Last edited: Jun 4, 2015
  11. Jun 4, 2015 #10
    I understand what is the Wigner Friend all about. It's anti realism of QM due to conflicting accounts with Many Worlds as only alternative. Have you come across any daily life analogy of the Wigner Friend.. such as for example if all wear uniform or certain weather, there is some so and so conflict (just an example of daiy life). I'd like to know if the Wigner friend scenario is a common one or rare one and what's it correlates in other field or topic.
     
  12. Jun 4, 2015 #11

    atyy

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    Yes, it is common throughout QM. But the response need not be anti-realism or Many Worlds. The response could be that QM is a practical theory, and in practice we always know how to divide the universe into a classical part and a quantum part. Wigner's friend and associated problems may simply indicate that QM is incomplete. This was Dirac's own view. Since Bohm's work, we are able to write down explicitly for non-relativistic QM some possibilities for these more complete theories without the Wigner's friend problem. Whether and how we might do so for relativistic QM is still being researched.
     
  13. Jun 4, 2015 #12

    ShayanJ

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    I think paradoxes like Schrodinger's cat and Wigner's friend can be solved using decoherence. Also decoherence tells us that we don't need the Heisenberg cut any more.
     
  14. Jun 4, 2015 #13
    I was asking if you have analogy of Wigner Friend in other subjects for example, economics, bathrooms, lego objects, toys.. I can't give an example that is why I was asking.
     
  15. Jun 4, 2015 #14

    bhobba

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    Well Wigner's Friend represents a bit of a misunderstanding of QM anyway. It relates to the misunderstanding Von Neumann made in his conciousness causes collapse interpretation that he and Wigner held to. Later Wigner woke up to the issue and did a 180% about face. So no - the same misunderstanding doesn't crop up in other fields so its pretty useless outside QM - and these days within QM as well.

    Thanks
    Bill
     
    Last edited: Jun 4, 2015
  16. Jun 4, 2015 #15
    Going back to this. In special relativity, objects can time dilate and length contract with respect to one another.. this shows reality is malleable.. so what is wrong with the more literal "story of reality that is continually unfolding" in QM? If it can happen in relativity, why not in QM?
     
  17. Jun 4, 2015 #16

    bhobba

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    Its shows space-time is described by Minkowskian geometry - not reality is mailable - whatever that means.

    Thanks
    Bill
     
  18. Jun 4, 2015 #17
    Minkowski geometry is flexible. it bends, expands, contracts etc. So it is not difficult to make minkowski geometry and the fields in it appear and vanish in instances related to wave function collapse. Since relativity is relative.. then the answer to Wigner Friend is maybe QM has relativity of observations too akin to SR?
     
  19. Jun 4, 2015 #18

    bhobba

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    Really - then how come the metric is constant?

    You are thinking about the pseudo Riemannian geometry of GR - but flexible is not the appropriate analogy there.

    The key to GR is space-time geometry is dynamical - not 'flexible' which has connotations not appropriate to what's going on.

    Thanks
    Bill
     
  20. Jun 4, 2015 #19

    atyy

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    Last edited: Jun 4, 2015
  21. Jun 4, 2015 #20
    Do you know of any function or device in mathematics that can bind discrete and continuous (QM and GR) that can transform into each other and even make them emergent of this more primary aspect?
     
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