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Discussion in 'Physics & Math' started by arfa brane, Jan 14, 2022.

  1. arfa brane call me arf Valued Senior Member

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    Well that's just wrong, and I say you need to support your claim.

    Maybe you could start with an electron in a superposition of spin-up and spin-down, with equal probability; make this electron one of a pair of entangled electrons. Which one is in a superposition of spin?
     
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  3. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Eh? I never said it couldn't. I'm talking about you trying to link entanglement with superposition... and you ask me why I don't think an electron can be in superposition?? Maybe if you explain how that has anything to do with entanglement you might start to make a coherent rebuttal to the point I'm making.
    There is no relation to superposition, other than being possible properties of a quantum system. They are different phenomena. My dog may have 4 legs and be black, but the two are unrelated other than being about my dog. So, please, actually show why you think the two things are related, how they are related, rather than just spout forth quantum phenomena as though they are related.
    You'd get the classical interference pattern. Why do you think you'd get anything different?[/quote]
     
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  5. arfa brane call me arf Valued Senior Member

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    Of course you would, because the diffraction of electrons isn't the right kind of experiment for detecting if any electrons are pairwise entangled. Recall, you have to choose what to look for in any quantum experiment, you need to fix something and once you do, it makes other information unavailable.

    I think that's exactly the kind of mistake you're making with your claim of no relation between superposition and entanglement. These are distinct when you do experiments that "distinguish" one from the other, because choices in QM have different consequences than in classical information-sharing.
     
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  7. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    No, you need to show that they are linked. You're claiming they're related / linked. You show it. Provide the argument.
    They both are. Until one is observed.
     
  8. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Then show them to be linked. Show them to be related. Provide the argument. Don't just wave your hands around and assume it.
     
  9. arfa brane call me arf Valued Senior Member

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    Observed how? What would the observer need to know?

    Ahem: Frank Wilczek says that entanglement requires that an observer has partial information. The requirement is a lack of information . . .
    But there is something about your dog you didn't mention: your dog can't be in a superposition of not black and black, for example.
     
    Last edited: Jan 23, 2022
  10. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Let's just stick with the simple concept of measurement, shall we. If the observer measurers the state, that collapses the wavefunction.
    Not a lack of, but partial. I.e. you know something about it. And measuring partial information is one way of creating entangled states.

    So, again I ask: what does this have to do with superposition? Make your case. Coherently, if you can. Don't just throw out quotes, links, allusions to things being related. Actually put an argument to show why/how they are related.
    And before you do, maybe you want to do a quick Google about it, see what others have to say? E.g. https://physics.stackexchange.com/q...m-superposition-be-considered-the-same-phenom - note, I'm not saying this is your position, but the responses given to it may help inform any argument you choose to make for your own position.
     
  11. exchemist Valued Senior Member

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    Some of this is confused. It is nonsense to speak of superposition of a wave and a particle. Superposition applies to quantum states. There is no quantum wave or particle state. That is not what quantum states are.
     
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  12. arfa brane call me arf Valued Senior Member

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    And yet, an electron behaves like a wave in a 2-slit experiment, then it behaves like a particle. Which is it?

    It's already known that an electron is actually a superposition of three quasiparticles, this is new research so a new surprise. The trio consists of a particle with electric charge, a particle with mass, and a "ghost" spin particle. I'm not making that up.
     
    Last edited: Jan 23, 2022
  13. arfa brane call me arf Valued Senior Member

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    Ok, I'll try to be at least as coherent as you.

    It's simple really, can you answer the following question: Are Bell states entangled, or are they in superposition, or both? Or is that an abuse of terminology, perhaps?
    Think about how you appear to accept that I can place an electron in a superposition of spin, then entangle it with another electron. I say that means the two electron states are in superposition.

    If that isn't true, it should be easy to determine which one is in spin superposition and which one isn't--you should see separable states, right? Unless nobody tells you anything and hands you two electrons (in an electron container).
    . . . from PhysicsWorld:

    |Ψ+〉 = (1/√2)(|0〉1|1〉2 + |1〉1|0〉2)

    |Ψ–〉 = (1/√2)(|0〉1|1〉2 – |1〉1|0〉2)

    |φ+〉 = (1/√2)(|0〉1|1〉2 + |1〉1|0〉2)

    |φ–〉 = (1/√2)(|0〉1|1〉2 – |1〉1|0〉2)

    Each Bell state represents a coherent superposition of two possibilities. In the top two states, |Ψ+〉 and |Ψ–〉, the two qubits are different and in the bottom two states, |φ+〉 and |φ–〉 they are the same. The key point is that the single-particle states are “entangled” in these superpositions. This means that the two-particle Bell states cannot be written as linear combinations of single-particle states.
     
    Last edited: Jan 23, 2022
  14. arfa brane call me arf Valued Senior Member

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    Sorry, I just noticed the formulas above for Bell states didn't paste properly; the 1s and 2s to the right of each ket are subscripts for qubits (1 and 2).

    I'll just comment on wave/particle duality. This is gradually falling out of use. A particle is just a small-ish wave packet; there are no waves or particles in the Hilbert space, after all. Those words refer to observations of particle behaviour, but neither has the precise meaning the logic seems to require--for everything.

    It's still ok I suppose, to mention deBroglie wavelengths for wavepackets.
     
  15. exchemist Valued Senior Member

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    11,150
    The maths of QM provides for QM entities to have some of the character of "particles" and some of the character of "waves", without being either as such. There is no requirement for nature to be force-fitted to our human notions of how matter behaves at the macro scale. Wave-particle duality is intrinsic to the model. An entity in any single QM state has both particle-like and wave-like behaviour. It is nothing to do with superposition of QM states.

    On your second (unrelated) assertion, the electron is, so far as a I am aware, considered to be elementary. Modelling it by means of superpositions of quasiparticles may well be something that is mathematically possible, but as quasi-particles are not real, it does not mean the electron is really made of a combination of sub-entities. But I have not come across the research you refer to: if you can provide a link I'd be interested to read it.
     
    Last edited: Jan 24, 2022
  16. arfa brane call me arf Valued Senior Member

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    Do you know where, specifically, the math provides for this? I haven't seen it yet.
    So you're saying the observation (in the macro world where humans are) of waves or particles doesn't require us to force-fit that onto QM? Like I sort of said?
    Ok I used the word "particle" as if there really is a thing like a little blob of matter; I used it in the modern sense--a field excitation.
    Is it? where does the theory say that?
    Have a look at this, it's not a scientific paper but a bit of science journalism, if you want to know more about spinons, magnons and holons, I found quite a lot of stuff with google. The article has a one-paragraph byline on the research; it includes images of the substrate and of the wavelike pattern that appears, the imaging is STM/SEM with I presume Lorentz microscopy, but I haven't looked into it.

    p.s. Quasiparticles are real (since the transistor was invented, scientists have had to deal with them); I think you need to seriously update what you think you know about this stuff. See if you can explain what a spin liquid is without resorting to the existence of real, physical quasiparticles.

    But don't come at me, trying to tell me how it is. Ok? I've actually studied some of this stuff, so nyah nyah.
     
    Last edited: Jan 24, 2022
  17. exchemist Valued Senior Member

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    You think I haven't? I took the Quantum Chemistry supplementary subject as part of my degree, attending lectures given by: https://en.wikipedia.org/wiki/Charles_Coulson and: https://en.wikipedia.org/wiki/Peter_Atkins, both of whom are pretty well-known, and I passed an an exam in it. I admit it was 40 years ago, but I assure you I have not forgotten it all

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    . But never mind all that.

    What you said, which I am pointing out is incorrect, is that QM entities are a "superposition" of a particle and a wave. Superposition applies to quantum states. There is no quantum state corresponding to a "particle" and none corresponding to a "wave". So there are no such states to be superposed. A quantum state is a wave function: it is these wave functions that can be combined in a superposition of states.

    Born's interpretation of the wave function is that its square modulus represents a probability density: the probability of the QM entity being detected - as a particle - in a given volume of space. That is what I mean by wave-particle duality being intrinsic to the maths of the model. The square modulus of the "wave" is the probability density for detecting the "particle". Superposition of states does not come into it.
     
    Last edited: Jan 24, 2022
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  18. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    They are both, but this has nothing to do with wave particle duality, or how "single electrons become entangled in a double slit" - i.e. you're continuing to not address the point I actually made.
    I'm sure you can point to where I disagree with this? Or are you ever going to address the criticism I actually raised? If you won't, just tell me, and I'll stop wasting my time with you.
     
  19. arfa brane call me arf Valued Senior Member

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    But, there isn't any particle. This idea is from humans, particles are not included in the mathematics, they appear in the language humans use to describe what they see. That's the reality. Particles are just heuristic devices, and the meaning of the word has changed a lot in the context of physics.
    Superposition is the defining feature of quantum behaviour. I seriously doubt it "does not come into it"; into what?
    If you claim there are things that behave like waves, or like particles (even if that isn't really accurate, maybe it's still ok, I mean, seeing how it's lasted this long), and that these things can be in superposition, why would you then claim a superposition of wave and particle isn't a possibility?
     
  20. arfa brane call me arf Valued Senior Member

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    A Bell state has nothing to do with wave-particle duality, or electrons being entangled with a pair of slits (or with paths from the slits), you say?
    Superposition and entanglement are distinct, you say?

    Both those statements are just wrong. And, again, you seem to accept that a Bell state is a superposition of states. So which answer is right? Are superposition and entanglement related, or aren't they?

    I can find plenty of online material that refutes what you say. Electrons entangle with both slits, and you can represent this with the notation, there are online courses that use notation to explain interference. The other way to explain it is to say each electron is in a superposition of two states, one for each slit it "passes through".

    So let's see if you can now address this statement:
    Once we agree on who it was that said it.
     
    Last edited: Jan 24, 2022
  21. arfa brane call me arf Valued Senior Member

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    Ok so, let's go over the lesson here.

    We have two people telling us (actually, telling me), that superposition and entanglement are not related. They seem to be unable to provide anything definitive to back this up, and I already know it's not true--they are related. I thought I had provided the clincher with that post about Bell states from PhysicsWorld. Maybe they just can't see it.

    The wave or particle thing. Like I say, those are heuristics, that's all they are. In solid state physics it's ok to use single complex numbers to represent transmitted and reflected waves at a boundary--the particle in a box thing also comes to mind. Is an electron or a photon a complex number?
     
  22. arfa brane call me arf Valued Senior Member

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    Let's be clear about what I said. In the context of the heuristics, a particle can be said (by us humans who know better) to be in a superposition of both. Is it possible to do an experiment in which we see wave-like and particle-like behaviour simultaneously?

    So the words particle and wave, have shifted in meaning somewhat; in QM they really only apply to observations, and we suspect that neither entity actually ever exists. So today, an electron can be a wave or a particle, or neither.

    p.s. I have to ask, do you still think quasiparticles aren't real?
     
    Last edited: Jan 24, 2022
  23. DaveC426913 Valued Senior Member

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    "...superposition is necessary for entanglement, but they are not the same."
    "For entanglement you need superposition AND the possibility of having more than one system (formally called a tensor product)."
    https://www.researchgate.net/post/A...uperposition,formally called a tensor product).

    "All entangled states are special cases of superposed states. But not all superpositions are entangled states."
    https://physics.stackexchange.com/q...m-superposition-be-considered-the-same-phenom

    (Caveat lector: I do not vouch for the veracity of these sites.)
     
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