Time and information

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

  1. arfa brane call me arf Valued Senior Member

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    Waves and particles are necessary (at the human level) to explain observations, but waves are not the same as particles.
    Ha. So you're saying that's a relation between superposition and entanglement?
    Of course, not all superpositions are entanglements, entanglement means you have at least two (systems of) particles. Note please: I use the word particle in the modern, QM sense.

    Why are they different (even though there's a mathematical relation)? one difference is that superposition "exists" after a quantum measurement, verification of entanglement requires classical measurement, so is detected post-hoc, whereas superpositions are "destroyed" by detectors.
     
    Last edited: Jan 24, 2022
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  3. DaveC426913 Valued Senior Member

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    I'm simply attempting to support your assertion that the two are related.
     
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  5. arfa brane call me arf Valued Senior Member

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    Well, thanks. I'm fairly comfortable with the relation already; I wonder how comfortable other posters are? (As if I care).

    Oh wait, I do care. I care about understanding this stuff, not so much about whether anyone else can follow what I'm saying.
    What I can see already is people skipping over evidence that something they posted, some definitive statement like-- "quasiparticles aren't real", or "an electron doesn't entangle with a pair of slits"--is just wrong. They would fail a course at a university (as if that's some kind of standard around here).
     
    Last edited: Jan 24, 2022
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  7. arfa brane call me arf Valued Senior Member

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    Here's a slightly different argument. There is a relation between time and classical information (see if you can spot it, it has to do with a transfer of energy/momentum from one system to another).

    Entanglement/superposition posits a relation between time and "quantum information".
    These two quantum effects are related to each other, so how are they related to information itself, bearing in mind that reading quantum information seems to involve transforming it into classical information, so the momentum-transfer thing must be in play. After this, the quantum particle is unkown--it can't contribute anything more.

    Although it follows that an electron still exists after it's detected by a particle-counter, nothing can really be said about it except it's in a generalised state.
     
    Last edited: Jan 24, 2022
  8. arfa brane call me arf Valued Senior Member

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    More, ah, supporting evidence from a science journo:

    --https://theconversation.com/explainer-what-is-wave-particle-duality-7414
     
  9. exchemist Valued Senior Member

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    Certainly the terms wave and particle have to be used with care when dealing with QM entities, since they have elements of both and neither, but, as I have been explaining, that is not superposition in its QM sense. A single QM state is described by a wave function, the square modulus of which is the probability density for detecting the entity as a particle. So you have wave-particle duality already, without involving linear combinations of multiple wave functions, which is what you have with superposition.

    Re quasiparticles, it seems that you didn't provide a link to the article you were referring to - at any rate I couldn't see one. Certainly my current understanding of quasiparticles is from things like phonons, which are phenomena (in that case, lattice vibrations) of systems, which can to some extent be modelled as if they are QM entities, but have no independent existence in the way an electron or a proton does. So maybe I should have said no independent existence.

    But I'd be more than willing to read the article if you think I am wrong.
     
  10. exchemist Valued Senior Member

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    Sure. Wave/particle duality. Notice there is not a word about superposition in this explanation.

    The "missing information" Einstein had in mind was due to his reluctance to accept the uncertainty principle ("God does not play dice" and all that).

    As I'm sure you know, various people have tried to fill in the missing information by constructing "Hidden Variable" theories, to get rid of the uncertainty principle and recover the deterministic universe that Einstein believed in. All attempts to date have failed. Einstein seems to have been wrong, God does seem to play dice, and there really do seem to be limits to what can be defined in a quantum system. Heisenberg wins!
     
  11. exchemist Valued Senior Member

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    That sounds about right to me.

    Superposition comes up in a fair bit in chemistry, as it provides the basis for the QM of chemical bonding. But none of that involves entanglement, as one is dealing in principle with superpositions of the possible states a given single electron can occupy.

    Whereas entanglement obviously requires 2 or more entities, physically separated. So it's a special case of superposition, with intriguing properties.
     
    Last edited: Jan 24, 2022
  12. Write4U Valued Senior Member

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  13. arfa brane call me arf Valued Senior Member

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    Would it qualify as superposition in an "observational sense"? You know the argument about having two different objects (subject to classical observation), which are then hidden, perhaps in identical containers? which object is in which container? are the objects entangled, or is their information entangled?

    Entangled with what?

    My question, now I look at it more closely, appears to be about telling the difference between two classical objects I have partial information for, and two quantum objects (namely a wave and a particle), I also have partial information for.
     
  14. arfa brane call me arf Valued Senior Member

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    Is that an admission that entanglement is a superposition? Like some guy at PhysicsWorld thinks (and he's not the only one).
    So if entanglement in QM is a superposition of two (or more) particle states, how goes the theory that entanglement and superposition are . . . different physical things?
     
    Last edited: Jan 25, 2022
  15. arfa brane call me arf Valued Senior Member

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    Yes, that looks erm, ok. Quasiparticles are excitations that depend on the medium (a crystal lattice), the presence of fields, etc.

    In transistors, electrons leave holes behind when they propagate. The holes are called quasiparticles, and you're right about their independence, the hole quasiparticles cannot have an independent existence outside the transistor (likewise for any quasiparticle in some device).

    Nonetheless, the holes are 'moving' with the same speed as the electrons, there is a current of positive charge. This can't affect the electron current because the "hole-flow" is restricted to the device.

    If you want to see some published literature on quasiparticles, I dunno, maybe start with Wikipedia. I recommend a look at quantum spin liquids, mainly because it's brand new stuff.
    It's brand new because of developments in materials science, the substrates that they can play with now just didn't exist, even a few years ago.
    My guess is there will be more discoveries; to find new quasiparticles does involve some exotic physics, so if quasiparticles exist naturally it might not be for very long.
     
    Last edited: Jan 25, 2022
  16. arfa brane call me arf Valued Senior Member

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    This part probably isn't going to go down very well.

    The wavefunction of a quantum particle does not describe or define a physical wave that propagates through space.
    It's a function, not a physical thing. Probabilities are only real when measurements are made--you can't put probabilities in a box, or in a bottle. There are plenty of examples in physics where "something real" has measurement problems: global phase of a particle, time, superpositions of states . . . lots of things we take for granted that we have no way of showing the actual existence of.

    Or perhaps an alternative is that these non-measurable real things are destroyed by a measurement but otherwise exist somewhere or other. Wave-particle duality is in this category it seems; and again, there are matter-waves with deBroglie wavelengths that also explain interference effects.

    Once again, the theory does not describe waves or particles, it describes quantum states--humans are responsible for trying to shoehorn them in, with the result that wave-like, or particle-like behaviour is observed. It's just a handy word for a thing that isn't either unless we say so. Why should a particle care what we say about it?

    Perhaps someone can wring a description of a wave or a particle (or both!) from the following diagram (of a quantum state):

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    Oh hell, that's one of those "real but not measurable" things, right there. Dang.
     
    Last edited: Jan 25, 2022
  17. arfa brane call me arf Valued Senior Member

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    I know that wavefunctions are often described as "probability waves", a lot of the literature states that because of the wavelike properties of quantum probabilities (they can oscillate for instance), this is why quantum particles behave like waves--except you need to decide to look for wave-like behaviour in an experiment.

    But, for instance, an electron has mass, charge, and spin, a wavefunction does not. The squared modulus gives a probability, which is a number, not a mass or a spin or a charge . . .

    Numbers struggle to be real physical things, I think, possibly because they just aren't. That dot you see appear on a screen isn't mass, charge or spin either, and it certainly isn't a number.
     
  18. arfa brane call me arf Valued Senior Member

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    Here an image of a spin liquid, in which the electrons are split into chargons, each occupying a centre, and spinons arranged around the centres.

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    --https://newscenter.lbl.gov/2021/08/19/exotic-particle-out-of-body/
    The electrons were injected onto the surface during a routine STM scan, then the pattern was imaged using another technique, again the usual routine, but the spin waves were a surprise, mainly because the (explanation was) electrons were undergoing spin-charge separation.
    That effect isn't a new one, although it's still young.
     
  19. exchemist Valued Senior Member

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    Admission?

    Sure, entanglement is one of many situations in which a superposition of states is involved. Here is a 5 minute video by Sabine Hossenfelder explaining very clearly what a superposition of states is and what entanglement is:


    Superposition of states is very general in QM, because any linear combination of solutions to the wave equation is itself a solution also. That's just a feature of linear differential equations, I gather. (I do not pretend to be a mathematician, just a chemist

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    .)

    Chemists use this, for instance in developing "hybridisation" of atomic orbitals in chemical bonding. For example, a carbon atom with 4 σ-bonds can be represented as having 4 identical "hybrid" orbitals, constructed from linear combinations of the s and the 3 p atomic orbitals in the valence shell. Since the s and p orbitals correspond to solutions of the wave equation, these 4 hybrids do as well, so they are valid states for an electron to occupy and make more sense when the atom finds itself in an environment with tetrahedral symmetry, as it does in many molecules. This is the approach taken in the Valence Bond model of bonding. There is also the Molecular Orbital model, which again uses linear combinations of atomic orbitals: https://en.wikipedia.org/wiki/Linear_combination_of_atomic_orbitals

    Entanglement involves two entities prepared in such a way that a given property (often spin) is not determined for either but the total is known. That is represented by a superposition of states in which the left one is spin up and the right one spin down and vice versa, as each is equally probable - until one of them is determined. So that is one, very special, case of superposition of states, involving a pair of entities.
     
  20. exchemist Valued Senior Member

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    Yes it looks as if we are converging on the same understanding.

    I would say a "hole" is not a "real" entity, in that it has no independent existence. Whereas an electron does.

    But a hole is a real phenomenon, certainly, as is a Cooper pair, or a phonon. And these phenomena can be often modelled as if they are QM "particles", even though they are not. Hence the prefix term "quasi-"
     
  21. exchemist Valued Senior Member

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    Wave functions are more than "probability waves", though ( more properly, probability amplitudes, as it is the square modulus that corresponds to probability density - just as it is the square modulus of radiation amplitude that gives the radiation intensity).

    The wave function also contains within it the other properties of the entity, which can be extracted by use of the appropriate operator. The case of mass and charge may seem curious, as they are intrinsic to the entity, but there is an interesting discussion of exactly this here: https://quant-math.org/wp/2016/07/08/is-there-a-mass-quantum-operator/
     
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  22. arfa brane call me arf Valued Senior Member

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    Sure, you can say that. "Real" is a bit loaded, these days.

    Quasiparticles exist when electrons move through any medium, however. Electrons are quasiparticles when they propagate in less than 3 dimensions. Actually they are when propagating in three dimensions too, like along a copper wire, say.

    In a vacuum I guess they're "all electron", the point being the need to account for the interactions between a medium and an electron (or particle of choice).

    An electron tends to move in a cyloidal motion, along a 2d surface.
    What "really" is an electron, sorta depends on what it's doing.
     
  23. arfa brane call me arf Valued Senior Member

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    Ok, that is interesting, I admit I haven't seen a "mass operator" before, but a spin operator is familiar.

    Interesting too, because recent research shows that spin-charge separation means we can "operate" on the intrinsic charge of an electron so it splits off, leaving behind a quasiparticle with mass and spin. This electron-splitting occurs when you have a lot of electrons "packed together" on a surface, and introduce a nanowire close to it. An electron jumps, or tunnels into the nanowire and separates into a quasiparticle with only charge (and zero momentum), and a quasiparticle with the remainder--the names given to these, like chargon or holon, are overloaded. They're new, so it looks like researchers will settle on which name to use.
    Y'know, eventually.

    So the intrinsics, and operations on them, actually do something that might be useful in a quantum computer.
     

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