THE QM DISCUSSION THREAD

  • Which has more serious flaws Quantum mechanics or Rossi?


    https://www.quantamagazine.org…eality-go-wrong-20181203/



    Rossi.


    RB - I'm so glad you raised this link. I was thinking of starting a thread on the (recently popularised in NS) Frauchiger Renner "Paradox". Like many foundational QM paradoxes - and many other paradoxes - its value lies in highlighting loose thinking on the part of those baffled by it, rather than inherent problems with the topic. A good example would be Russell's paradox (not a paradox if you understand axiomatic set theory - otherwise a real paradox).


    In the case of QM Wigner's Friend and related paradoxes do tell us something deep about what any physically accurate theory must be, but don't (in any way) show a flaw in quantum mechanics. Unlike what NS (seldom accurate on fundamental physics) headlines.


    I've found (prior to your posting this) a really good discussion of the FH paper written up as a Foundations of QM tutorial article with minimal maths (just some linear algebra). It is highly recommended for anyone who feels that the foundational aspects of QM hint at something wrong with the theory. They do perhaps hint at something incomplete about it - but then we knew that - we all want a decent GR QM unification!


    OT for this thread - I'll start a new thread if no-one else does.

  • I hear that QM does not deal with negative energy. Do you think that this assumption that negative energy cannot be real is a problem?


    Axil: negative energy is of course real in various simulated conditions: under which it can be defined and observed - and QM allows that.


    Otherwise I think your question is meaningless unless you can define precisely what you mean in some context relevant to physics. It would indeed be a problem if some phenomena observed and described using negative energy could not be modelled using QM (assuming it is not related to GR - since QM does not currently mesh well with GR).

  • Comments on the Frauchiger Renner "Paradox".


    This is a recent proposal updating the Wigner's Friend paradox that claims to expose an inconsistency in QM. In fact it exposes an inconsistency in any naive understanding of QM. Working though it does show the necessarily spooky nature of the experimentally validated physical world - and therefore knock out a wide range of non-QM theories.


    Recommended: detailed while also being not too technical:


    Tausk18


    Also very good (Tausk references) Lazarovici & Hubert 18



    As somone who has always primarily seen QM observations in terms of a simply evolving wave function - since to superimpose collapse on this makes little sense mathematically - I've always naturally tended towards interpretations that do not suffer these paradoxes, and have many worlds -like characteristics, with no wave function collapse. If however you want something else you need to be careful about when do wave functions become truly decoherent, since this (irreversible from 2nd law of thermodynamics) defines an "observation" of the type that collapses a wave function.

  • From Tausk (but you will want to read the whole thing for much more context) analysing the reasoning of Frauchiger Renner:


    W, F correspond to Wigner and Wigner's friend in the Wigner's friend paradox

    Wbar, Fbar correspond to the same in a copy of this, linked coherently to the the state of W,F, as specified in

    Frauchiger Renner.


    From Tausk18:

    Now let us review the incorrect reasoning that leads to the conclusion that
    it is impossible for both W and Wbar to get the outcome “ok”. This reasoning
    in based on the following statements:
    (a) analysing the experiment “from Fbar’s point of view”, we obtain that
    if Fbar gets the outcome “tails”, then W will get the outcome “fail”;
    (b) analysing the experiment “from F’s point of view”, we obtain that
    if F gets the outcome “up”, then Fbar got the outcome “tails”;
    (c) analysing the experiment “from Wbar’s point of view”, we obtain that
    if Wbar gets the outcome “ok”, then F got the outcome “up”.
    Combining (a), (b) and (c), we conclude that if Wbar gets the outcome “ok”,
    then W will get the outcome “fail”. Now let us discuss statements (a), (b)
    and (c) individually.


    [Lots of discussion: basically - (a) is false, and involves a misconception that part of a (necessarily in this case) quantum system can be isolated. (b), (c) are true, and can be shown true from both a many worlds and a Bohmian interpretation.]

  • Axil: negative energy is of course real in various simulated conditions: under which it can be defined and observed - and QM allows that.


    The term negative energy has been introduced by people with superficial knowledge of physical definitions. Energy once has been defined as a quantity that always must be positive. Otherwise all physics falls apart.


    But people were eager to get attention and introduced the fancy term "negative energy" only because one term in a formula had a "-" sign.


    As already said: If there would be negative energy there must also be negative energy photons and thermal energy must flow backwards against all laws.


    Thus this terminology is nonsense.


    What we see are energy holes. But these holes are always parts of a larger system that somewhere carries the surplus. An energy hole is a term in a formula with a "-" sign in form of an energy ... In a closed system and physics only works in closed systems, there never ever can be something like negative energy.