Quantum Mechanics: The End of the War of Interpretations

    Quantum mechanics is surrounded by a halo of mystery. Often, this halo arises due to the fact that popular sources present the material without adhering to any specific interpretation, and sometimes they try to squeeze modern facts into the Procrustean bed of the old Copenhagen interpretation.

    A list of more than 17 interpretations can be found here . This whole long list arose from the fact that the formulas of quantum mechanics themselves guessed pretty quickly, but for a very long time did not understand what they mean .

    Ingredients of the Copenhagen Interpretation

    The Copenhagen interpretation occupies a special place: it is one of the first, if we take the chronology of occurrence. But, most importantly, it is the standard for the de facto popular explanations of quantum mechanics. This is where all the problems go.

    What parts does modern QM consist of? (Relativistic) quantum mechanics is a framework of formulas filled with specifics about what elementary particles are (the so-called menagerie ) plus the parameters of the Standard Model . These are 19 magic dimensionless numbers, which theory does not explain in any way. But this is a topic for another discussion. All that is described above, is a part with pure formulas that is dear to theorists. Now two additional, magical ingredients. We touch one of them for now.

    Wave function collapse

    It is a consequence of measurement. Measurement is a process when almost weightless quantum-mechanical systems come into contact with classical measuring instruments, approximately the same as during the time of Niels Bohr:

    Despite the fact that for a huge number of experiments the interpretation worked like a clock, none of its supporters could clearly explain what is measurement . As a rule, recursion occurs sepule sepule sepulcari measurement / observation / observer ... which measures.

    The presence of an observer, coupled with the fact that the collapse is partial (in the presence of a reasonable observer who can make conclusions) even gave rise to the interpretation of Von Neumann-Wiener , which is calledthe dream of the mind creates monsters The collapse created by consciousness . I have nothing against Neumann and Wiener - this is just an illustration of how bad everything was. Even the conversation about alternative interpretations became, if not taboo, then considered useless chatter:

    Current situation

    Recently, especially thanks to research in the field of quantum computing, measuring systems from huge instruments with arrows have shrunk to tiny systems - sometimes measuring on the order of several atoms. And the border between the quantum-mechanical system and the classical systems, which had been blurred before, had completely erased.

    Why is this system of a pair of atoms a measuring device, but this one is not? Where do atom groups have the “I'm measuring device” flags attached? The Copenhagen interpretation has reached a philosophical impasse (which does not diminish its historical and pedagogical role).
    However, physicists working in the field of quantum computing have not suffered from these problems for a long time, and that's why

    New Hope

    I will not talk about Many Worlds Interpretation here . There is a lot of information about this theory (and often incorrect). Of interesting facts, it should be noted that the first to come upon her was not Everett but Schrödinger .

    However, we will be interested, and why did the new theory not win the minds right away (in addition to rejection due to the fact that it is “stoned”)? Mainly because it contradicted the observed facts. Looking at the experiment with the Schrödinger cat, the theory predicted that we should have seen this:

    But you will never see real objects in superposition. Therefore, for the time being, MWI was just a crazy hypothesis, until it was discovered

    Quantum decoherence

    Decoherence was detected at the tip of the pen in the early 70s. In the 80s there was an active study of this area. Now without it, modern quantum mechanics and, especially, quantum computers cannot be imagined. In half of the articles on quantum computing, the word decoherence is in the title, and in almost every - in the body of the article.

    Decoherence shows that in the interaction of a complex system (observer) with a quantum (or other other system) diagonalization occurs. That is, instead of two blurry silhouettes of a living and a dead cat, the system turns into two almost independent ones: a sad observer who sees a dead cat, and a joyful one who sees a living cat.

    Unlike the collapse of the wave function, decoherence does not need the magic of “measurement” - this happens with any system that has many degrees of freedom (which is true for the brain and macroscopic observation devices). The decoherence process itself is physical, that is, due to the exchange of an observer and an object by photons (for example), it does not occur instantly, and the phenomenon itself propagates no faster than the speed of light.

    But what about the collapse of the wave function?

    The position of the supporters of the Copenhagen interpretation is very weak. After all, now they have not one, but two whole agents, providing what collapse was previously responsible for. Obviously, this cannot be so. Clutching a straw, they try to declare decoherence an explanation of the collapse and even put it as a victory (they say, the collapse used to be magic, but now it is explained) The

    fatal problem of this approach is that decoherence is weaker than the collapse - it explains why we do not see foggy a mixture of different states of cats, but does not say that there is only one state of the cat! That is, the adherent of this approach must either de facto adhere to the MWI, or still drag the saw into the theory, which will cut off the “unnecessary” branches.

    The Bourne Rule

    The Bourne Rule is the last missing ingredient for MWI. Roughly speaking, this is a crutch explaining how the thickness of branches affects reality. You are going to work. In one of the branches of the multiverse, your brain twisted, you stopped and rushed to rob a bank, and instead of work you ended up in a jail. Fortunately, the intensity of this branch is very low. The rule explains that we hardly observe such thin branches. There is a rule in other interpretations, in particular, in Copenhagen. And in February of this year, the crutch was removed .

    As decoherence derived from the formalism of quantum mechanics supplanted collapse, the Bourne rule was (finally !!!) deduced in a purely mathematical way. The last nail in the lid of the coffin of the Copenhagen interpretation was hammered a month ago. The story is over. Time to take stock.


    Let us compare the current state of MWI and the Copenhagen interpretation and, as always, dispel myths. So,

    Copenhagen = Formulas + Collapse of the Wave Function + Bourne Rule
    MWI = Formulas + ... and nothing else is needed.

    Thus, MWI is an absolutely minimalist theory (it is sometimes even called NULL interpretation ). It does not imply any additional principles other than the formalism that has been known for a long time. Many believe that Occam’s razor cuts out MWI because MWI “ postulates the existence of other, unobserved branches of reality.”" MWI just does not postulate anything. The existence of these branches inevitably follows from the formulas of quantum mechanics, as the structure of space inside a black hole follows from Einstein's formulas. On the contrary, so that there are no additional branches, you just have to postulate the presence of a “saw”, which continuously saws off these branches. Well, or langolerias:

    Suddenly, in MWI, quantum mechanics is deterministic, objective and local - well, just a warm tube theory. Of course, it is determined only globally (if you look at the development of all branches) - this is called the birds view, and from the point of view of the internal observer, the “frogs view” - events in the universe are random (but statistically obey the Bourne rule).

    (again from an article by Max Tegmark)

    Some believe that MWI is a desperate attempt to bring determinism back into science.

    Spoiler: the old man was right!

    No and no again. At physicsforums, I somehow wondered about the motivation of die-hard realists who fought a desperate war with Bell's theorem (at the time, not all loopholes in the experiments were closed). Bell's theorem prohibits the existence of local realistic theories in the framework of quantum mechanics - however, this theorem does not apply to MWI. Accordingly, for lovers of realism, switching to the MWI flags would solve all problems (as it seemed to me). But no - thunder and lightning struck me - the price of recognizing the multiverse for realists was absolutely unacceptablemore unacceptable than even breaking up with their beloved realism. This leads to the conclusion:

    MWI - you have to pay for everything just once. Accepting madness in one thing –– and everything else will become easier. Or all life revolve in chimeras such as collapse , confusion, instantly changing the state of entangled particles, no matter how far they may be , or particle-wave dualism . By the way, the well-known Hawking was for MWI ( proof by Martin Gardner reports Hawking saying that MWI is "trivially true").


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