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B Psi ontic question

  1. Jun 18, 2017 #1
    I'm reading a lot of references about this today.. but I'm kinda confused.. so I need to know some basic to aid in the understanding and able to scrutinize the material.

    Wave function needs more 3 coordinates to specify a quantum state.. like 6 dimensions for 2 properties (like position and momentum) of one particle.. so it is obviously a mathematical operation.

    But I keep reading about wave function being ontic or real. Its like saying the formulas in my iphone apps are real which doesn't make sense. Are they proposing physical space is infinite dimensions so all these wave function and HIlbert space is factual? or do they mean wave function is real in the sense the amplitude squared really corresponds to the location of the object probabilities without saying the physical space has infinite dimensions (but this is simply the Born rule and I don't think this is related to the ontic thing)? What else does it mean?

    When one reads about this psi ontic thing or wave function being real. What really and exactly do they mean?
  2. jcsd
  3. Jun 18, 2017 #2


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    This question is beyond physics and thus you can pick whatever point of view you like. The only thing physical is that you have a formalism called quantum theory, and the physical meaning of states is probabilistic, i.e., given a state (for a pure state of a single non-relativistic particle without spin you can take the wave function ##\psi(t,\vec{x})##) you know about any observable of the system with which probability you'll get one of its possible values (the probability distribution for position at time ##t## is ##|\psi(t,\vec{x})|^2##). From a physics point of view there's no other meaning than that from a physical point of view.

    Whether you consider it ontic or real is your personal decision. These notions are so unsharply defined by the philosophers using them that I'm unable to understand their meaning at all.
  4. Jun 18, 2017 #3
    I wasn't asking if they were physical or not which I know is beyond physics.. I was just asking what people meant by ontic or physical. In BM, the wave function still occurs in higher configuration space and only the local particle is real. In MWI, I wonder if only the entanglement is real or in what sense the wave function is real in MWI. I can't understand. In Objective Collapse, something collapses but does it mean physical space has thousands of dimensions corresponding to the dimensions of the quantum properties of Hilbert space or does Objective Collapse means only the collapse is ontic.. what do they say how nature computes the wave functions or are followers of Objective Collapse agnostic of what goes on before the particle collapses?

    Please share how you guys understood by this ontic and physical thing based on materials you encountered in the literature. Thank you!
  5. Jun 18, 2017 #4


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    As I said, I don't understand, what's meant by "realistic", because this notion is so unsharply defined that it means different things for different philosophers of science. For me realistic is, what's objectively observable and quantifiable by measurements.
  6. Jun 18, 2017 #5
    The message was meant for others like Demystifier who is expert on it and the so called PBR theorem that is supposed to be as famous as the Bell Theorem. But I forgot today is sunday. Demystifer only logs in during weekdays Monday to Friday maybe he doesn't have any internet at home. So meantime while we await his masterful insights. We can discuss some lower facts.

    I'm thinking about Hamilton phase space. If six dimensions are needed to describe one particle, then ten particles in a system would require sixty dimensions. This classical picture doesn't require spacetime actually to require sixty dimensions. Likewise in Hilbert Space. It doesn't mean spacetime needs thousands of spacetime dimensions to store the information. So maybe how spacetime or particles can conform to the say Dirac Equation is how spacetime/particles dynamics can represent Hilbert space without spacetime needing thousands of dimension? Can anyone elaborate on this.
  7. Jun 18, 2017 #6


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    Surely you didn't mean to use the Dirac equation as an example?

    In any case, you have it backwards. The universe doesn't "conform to" the mathematical models we use, it just does what it does. We choose to use mathematical models that can be used to describe that behavior, so the match between model and universe is altogether tautological.
  8. Jun 18, 2017 #7

    Simon Phoenix

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    A good place to start is Leifer's excellent review which can be found here :


    As I understand it, which isn't all that well, an 'ontic' state is (roughly speaking) something that is a direct representation of something 'real'. An epistemic state is a representation of our knowledge (or assumed knowledge) and is not a direct representation of something 'out there'.

    You can see I'm kind of struggling to put this in precise terms - as Vanhees says it's all rather vague. As vague as my Aunt Doris after a bottle of Auchentoshan, but a lot of fun.

    If you write out the generalized coordinates and momenta for a single particle it's an ontic description. We would say that the particle really is there and moving that-a-way at that speed, for example. So the numbers (the coordinate values) really correspond to something 'out there'. But if we had an Avogadro's number of particles we couldn't possibly hope to provide a complete ontic description so we resort to a probability distribution. A probability distribution isn't real is it? It's not representative of some 'thing' that's 'out there' - it's a modelling tool that connects with 'reality' in things like expectation values and variances and so on. So this is an epistemic description that in some sense represents our knowledge or information about the system we're interested in.

    The question is, is whether the wavefunction, or state, in QM has an ontic character (is it some kind of representation of something that's physically 'out there') or does it have more of an epistemic character (is it a modelling tool that encodes what can be known)?

    It's a damnably perplexing question and unless you really want to end your days mumbling to yourself on street corners it's probably best avoided.

    All joking aside, though, the fact remains that if you want to think of the wavefunction as something 'real' that gets 'projected' upon measurement to some new state, according to the 'standard' rules of QM then you're free to do so - you'll get the right answers. If you want to think of a measurement as providing information by which we update our encoding of knowledge giving us a new wavefunction (like probability distributions get altered when we receive new data) then you're also free to do so - and you'll get the right answers. Which is really just a long-winded way of saying what Vanhees said. There's no current way, as far as I know, that will allow us to experimentally distinguish between these viewpoints or any shade in between.

    On Mondays, Wednesdays and Fridays I'm hanging with the onts. On Tuesdays, Thursdays and Saturdays I'm an ep. Sunday is my day off and I head round to Aunt Doris for a wee tipple of Auchentoshan.
  9. Jun 18, 2017 #8
    In lecture.. where I want accuracy in words and semantics to avoid confusion. I need to know something about the phrase "is wave function real". Isn't it like the phrase "is Dirac Equation real?". I mean it's more semantically right to ask "Is the electron or particle real?" instead of "Is Dirac Equation real".. likewise.. maybe I should avoid the words "is wave function real" altogether and instead use the words "Is the quantum state real" for more clarity? Or do you think words such as "Is Dirac Equation real?" is still valid words to use.. and why? How do you understand "Is Dirac Equation real?"?

    In some authors. When they say "Is wave function real". They really meant it and not just describing whatever mathematical object we are using to describe the quantum state of the system. Or if the latter is what they meant. How could the dirac equation or mathematical object be real? Can you justify or give an example for it or elaborate what it meant or justify the words "is wave function real?" is still very valid phrase to use?
  10. Jun 18, 2017 #9

    Simon Phoenix

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    A very laudable desire, but sometimes, alas, words aren't enough. I'm not really sure I understand what you're asking above, but of course the equations, the mathematical objects aren't 'real' - except in the sense that they exist as thoughts in our heads or squiggles on a bit of paper.

    Suppose you set up an experiment with a ball that squirts ink - set up a device to project it at different angles of inclination and different initial speeds and set this device next to a blank wall. We fire the ball and we find that the ink traces a parabola on the wall. We can model that and we find our model works - if we keep the launch speed fixed and alter the angle of inclination, for example, we'll be able to predict the exact parabola that gets traced for a given angle and describe it using an equation. The equation isn't the line on the wall - but it faithfully describes the line on the wall.

    So in some sense here the mathematical representation is directly reflecting the reality we observe. If you like, the physical system is doing an analogue computation of our mathematics.

    Is Newton's first law of motion 'real'? I don't know how to answer that properly. I wouldn't know what is meant by 'real' here. So if someone writes "the Dirac equation is real" I wouldn't know what to make of that either. We could say that every body continues in a state of rest or uniform motion unless something acts to change that state of motion is, to a very good approximation, a very good description of the way objects behave. So the law represents the way 'real' things behave. Is the law 'real'? I can't answer that question except to say that it represents the kinematic behaviour of objects we think of as 'real'.
  11. Jun 18, 2017 #10
    When they asked "is wave function real?". Do they meant the physical system doing the analogue computation of the wave function is real or do they mean the wave function (or state vector) is really real or there floating in space and computing?

    Also another way to put it. Is E=mc^2 real? This seems nonsensical question. So why is the question "Is wave function real?" not non-sensical? Maybe in the case of E=mc^2, it's not pointing to an actual location in space. While the wave function being real or not thing is directly pointing to an actual location in space so we think the latter has more sense of being asked if it is real?

    Maybe we must not used the words "is wave function real?" at all. Maybe we should just say "is the object where the mathematical representation of the wave function reflect is real?". But someone may say of course the object (keyboard) is real. Maybe we should say "is the quantum stuff (whatever this is) where the mathematical representation of the wave function reflect is real?" What do you think?

  12. Jun 18, 2017 #11


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    Why are you so sure that it's any less nonsensical? Simon Phoenix's thoughtful (and admirably patient) answers above do a pretty good job of showing the limits of any discussion about "what is real", and I doubt that Physics Forums will be able to provide any better answer than that.

    This thread is closed, but as with all such closures, any member can PM me or one of the other mentors to ask that it be reopened for something new to say - we just don't want a thread that run endlessly and without resolution.
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