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Physics today rely on an observer

  1. Jul 15, 2003 #1
    Considering that certain areas in physics today rely on an observer, such as when considering the collaspse of a wavefunction, I think that we need a good physical definition of an observation.
    So what is an observation? I like to think of it as a change in configuration or state. Think about it. When we make observations we look around us and adapt to the situation at hand. We change. And chimps, cats, or viruses are just as good at being obsrevers as we are. But what of inanimate objects? Surely a particle behaves in the same way, abeit not conciously. it changes to it's environment, for instance if hit by a photon it goes to a higher energy level. so particles and everything else is an observer.
    I like this explanation, it makes things simpler. Consider Schroedinger paradox. the paticle dector observe the atoms decay and the wave function collaspes for it. the cat observes the poison gas and knows it dies, it doesn't need a human to tell it so. But we have not ob served it so the wavefunction does not colaspse for us.
    I was just wondering what you thuoght of this.
     
  2. jcsd
  3. Jul 17, 2003 #2
    The wavefunction of a system collapses when the system is measured, not when the system observes its surroundings. The difficult thing is to define what is a measure. There are several scientists, Wigner one of the most known, that says that is necessary conscience to perform a measure, but this is a very much anthropocentric idea, and the majority of the investigators agree that a measure don't need to involve humans
     
  4. Jul 17, 2003 #3

    jcsd

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    There is no mathematical or well-defined description of a quantum measurement which is probably the largest flaw of the conventional theory of quantum measurement, the Copenhagen intpretation.

    On top of this you have the Quantum Mechanical measurement problem which begs the question, should the detector be considered part of the system or should it be treated classically? Of course if you do treat the detector as part of the system then you'll need another detector to detect that system, which could again be argued to be part of the system ad infinitum.
     
  5. Jul 23, 2003 #4
    might an object in the area of plants, and what thy do with light, fit the bill as a good definition of an observation without wave function.
     
  6. Jul 23, 2003 #5

    selfAdjoint

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    That ain't a bug, that's a feature!

    Bohr stated that quantum mechanics isn't a theory about the micro world, it's a description of our observation of the micro world. You use it to design and interpret macroscopic experiments, and to do that you don't need a theory of collapse.

    To use QM to interpet the universe, in Bohr-thinking, is like using a ladder to pound a nail.
     
  7. Jul 24, 2003 #6

    Ivan Seeking

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    Measurement in Quantum Theory

    "From the inception of Quantum Mechanics (QM) the concept of measurement has proved a source of difficulty. The Einstein-Bohr debates, out of which both the Einstein Podolski Rosen paradox and Schrödinger's cat paradox developed, centered upon this difficulty. The problem of measurement in quantum mechanics arises out of the fact that several principles of the theory appear to be in conflict. In particular, the dynamic principles of quantum mechanics seem to be in conflict with the postulate of collapse."

    http://plato.stanford.edu/entries/qt-measurement/

    Please see also the links at the bottom of the linked page.
     
  8. Jul 24, 2003 #7

    jcsd

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    No, it is widely recognised as a flaw of quantum measurement (particularly the Copenhagen intepretation) as quantum mechanics is meant to be a universal theory. If QM is only useful in describing the microscopic world it begs the question "where does the microscopic end and the macroscopic begin?".

    Oddly enough a group led by Stephen Hawkings have used quantum mechanics to interpret the universe at large by constructing the 'wavefunction of the universe' which has yielded results that seem to explain the large scale structure of the universe.
     
  9. Jul 26, 2003 #8

    selfAdjoint

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    Widely recognized by people who insist on reifying the wave function and, yes, interpreting QM as a universal theory. What I was decribing is caricatured as "shut up and calculate" and it is a perfectly reasonable take on the facts of QM. You don't get a "universal theory" but you don't get the insoluble collapse problem either.

    As for the wave function of the universe, talk to Marcus. I believe LQG has supervened.
     
  10. Jul 26, 2003 #9

    jcsd

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    The wavefunction of the universe is still a very-much ongoing project which has recently yielded results which help to explain the large scale structure of the universe.

    Quantum mechanics is accepted theoritically at least as being a universal theory and if you do not accept this the quantum mechanical measurement problem becomes even accuter (as it would seemingly forbid explanations like decoherence). You cannot define exactly what is a quantum measurement or on what scale classical measurement theory takes over. The many worlds theory does seem to get around this problem, however it creates other problems that has stopped it from becoming the mainstream interpretation.
     
  11. Jul 26, 2003 #10
    The only problem I see is trying to attach a "real" meaning to the wave function and its collapse. If you just look at it as a prediction tool, no problem. But when you try to make the wavefunction out to be an event in itself, there is going to be problems.

    I think the "problem" really goes back to "superposition", trying to attach too much meaning to it, then that in turn leads to problems where you [[want/have]] to attach an independent meaning to the wavefunction, which leads to Schrodinger's Cat, etc.

    It can be stopped right at superposition. In the strict Copenhagen Interpretation it does, too. They don't really talk about superposition of states, but note that you can't talk about states of something that isn't measured .. and then cop out on the definition of a measurement. And the collapse of the wave function and Schrodinger's Cat fall out as something that doesn't need to be considered.
     
  12. Jul 26, 2003 #11

    jcsd

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    The quantum mechanical measurement problem is deeply tied in with the Copenhagen Interpreation and it's inabilty to satisfactorily define a quantum measurement (i.e. a measurement that collapses the wavefunction), should you not also treat the measuremnet appartus as part of the quantum system rather than treating them classically? This is basically what the QM measurement problem is about.
     
  13. Jul 26, 2003 #12
    I have to disagree. I put no independent meaning on the wave function. Its use is only a predictive tool, and also, only after you have found/decided on an initial state. I think that is overlooked .. try formulating the Schrodenger's Cat problem without knowing there is a particle in the enclosure to begin with, or something about that particle .. one that will trip the lever to release the poison. The wave function will be of no use. How come? if it leads an independent existance?

    I left something out in my post .. superposition is not the only thing that can lead to viewing the wave function independently .. intanglement also tends to give the wave function a life of its own. Its much harder to deny that a problem as it is superposition.

    I think Bohr did it the same way as denying superposition though, that there is no such thing as a "state" until a measurement is made .. even if after the measurement was made it corresponds to predictions before the measurement.

    Granted Bohr/Copenhagen wouldn't define what constituted a measurement, but I don't see as the wave function posing problems in it .. only if you suppose the wave function goes beyond predictions.
     
  14. Jul 26, 2003 #13

    jcsd

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    The wavefunction (which you correctly state is viewed by the Copenhagen interpretation as a mathematical construct only with no physical analogue) isn't the problem here, it's the collapse of the wavefunction. The fact that a measurement cannot be described properly is a deep problem. Why shouldn't the appartus be treated as if it where in a supposition of states bewteen a postive and negative results rather than treated classically, why shouldn't WE be in a suppositon of states between having read a postive and negative result? These are problems which decoherence (a theory on why quantum behaviour does not seem to amplify to a macroscopic level in problems like Schroedinger's cat) tries to explain.
     
  15. Jul 26, 2003 #14
    I touched on that in my first post, that that is what I thought the problem actually was, superposition, rather than "collapse" of the wave function. If you take superposition out of the picture, there is no need for trying to give any meaning to the "collapse" (neglecting entanglement, which I also touched on). Just don't try to think of the collapse as a real thing that has to happen, think of it only as that the predictive power of the wave function ceases.

    Another thing has happened here, down the line somewhere. People have given a proper name to this, "collapse of the wave function", just like we have given a name to an electron, or a photon. Well, those names themselves tend to conjure up properties that aren't necessarily there. Like an electron, it tends to conjure and make you (me) think of an electron as a little round ball. The name has done that; not any properties of the electron that we know about. Somewhere along the line "collapse of the wave function" has grown to add its own properties/connotations to it, and made it out to be something physical rather than an idea thought up in somebody's brain and the idea/equations written down on paper.

    Take superposition out of the picture, then you don't have to worry about a combined quantum state of the entity being measured and the measuring device. You only have to worry about the quantum state of the measuring device. That's tough enough.

    IMO, superpostion (in particle aspects) is denied by strictly adhering to the Copenhagen Interpretation. If you're looking only at the wave aspects of a system, then that's another matter.
     
  16. Jul 27, 2003 #15

    jcsd

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    superpoistion, is part of QM and so is the collapse of the wavefunction. The collapse of the wavefunction cannot be described in mathematical or a purely abstract context. Superpoistion is also part of the Copenhagen interpretation and is a clear mathematical property of Hermitian matrices

    The QM measurement problem is still there, what I think you're missing is that it is more of a practical than philosophical problem and it will probably take a theory of quantum gravity to solve.
     
  17. Jul 27, 2003 #16
    Superposition is part of Copenhagen QM as it applies to wave properties, not particle properties. And part of the problem is trying to overextend the wave function to particle aspects, such as particle detection. In short, the wave function ceases to give any predictive value once a particle is detected, and IMO, the collapse of the wave function is nothing but a metaphor (or simile, or whatever that correct term is). There is no transition between wave/particle .. it's either/or. When you talk of one, the other has to be totally left behind.
     
  18. Jul 27, 2003 #17

    jcsd

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    Forget the distinction between wave and particle as that can be rather arbitary as particles can display both properties at the same time in experiments like the delayed choice experiment, anyway it's not important to the QM measuremnet problem.

    The problem is how can you say when a quantum system's evolution goes from being decided by the time-dependent Schroedinger equation to a collapse into one of the eigenstates? I mean you can say from the behaviour of the system but defining this collapse mathematically cannot be done.
     
  19. Jul 27, 2003 #18
    I don't believe that to be true, and IMO it is important. In that instance the wave and particle properties are not taken at the exact same time in the exact same measurement, and simply, you don't have to take a measurement for wave aspects to appear. When the particle measurement is taken, those wave properties disappear, even in a delayed choice experiment. These two things are totally distinct, and cannot be taken in combination, or viewed/measured at the same time.

    That's simply one of the "funny" things about QM, and is not limited to the Copenhagen Interpretation.

    Let me say it this way, because there is general agreement, at least I believe so. When an electron jumps from one orbital to another because of emitting or obsorbing a photon, it's generally agreed there is no transition state. The electron is in one orbital 1 [[moment]], and another orbital the next [[moment]. I don't know of persons that believe in the collapse of the wave function as being a physical happening in measurements as talking about this instance in respect to a transition.

    But the word "collapse", and how it is used in "collapse of the wavefunction" has connotations of a transition. If that is the case, it ought to be possible to detect both wave and particle aspect at the same time. That might be the whole basis of what you are saying, in trying to determine what constitutes a collapse of the wave function, trying to find/define a moment of transition.

    IMO, that's like trying to kill a dead skunk! ;). It seems to me it is a need to try to retain some aspects of Classical physics that there is transition, when there isn't transition.

    I've enjoyed the discussion!
     
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