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Definition of a measurement

  1. Jul 8, 2007 #1
    What is the definition of a "measurement" in quantum physics? The usual example used is bouncing a photon off an electron in order to measure the electron's position or momentum. However, do photons not bounce of electrons naturally without measurements being made by a conscious observer? Do these physical interactions cause the collapse of the wavefunction or does there have to be an observer present? Furthermore, measurements can be made in a more indirect manner, for example the tides allow us to infer the existence of the moon. Everything in existence is contingent upon everything else in existence, so in measuring one property of one particle are indirectly measuring many other things. Do the wavefunctions of all these things collapse?
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  3. Jul 8, 2007 #2


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    No. :rolleyes:
  4. Jul 8, 2007 #3
    "Measurement" always presumes existence of an observer who extracts some information/knowledge about physical system. If you think about it, science, in general, does not tell us what is "really" going on in the world. It only allows us collect, describe, and systematize our knowledge about the world. If somewhere a photon bounced off an electron and this event hasn't affected our measuring devices, then this effect hasn't changed our knowledge a bit. So, no wavefunction "collapse" has occurred.

    "Collapses" of wavefunctions or probability distributions occur around us all the time. There is nothing mysterious in them. Close your eyes and throw a die on the table. When the die has stopped you have a set of probabilities for which face is on top. The probability is 1/6 for each face. Open your eyes and this probability distribution "collapses". You get a definite result with 100% probability.

    The wavefunction collapse in quantum mechanics is really not much different. The only difference is that in the case of the classical die you can (in principle) predict (by analyzing all forces acting on the die) which face will be on top before opening your eyes and performing actual measurement. For quantum systems such a prediction is often impossible. So, there is an element of random behavior in quantum dice.

    There is a parallel thread "How observation leads to wavefunction collapse?" where you can also look for answers to your questions.
  5. Jul 9, 2007 #4
    I dont think the fact that the existence of one thing is contingent on the existence of everything else is generally disputed in philosophy. An electron only exists within space (ie relative to the spatial positions of other objects) and time (relative to time coordinates of other objects). It's a basic tenet of buddhist philosophy and of much of western philosophy (ie Aquinas' Cosmological Argument and Kant's arguments against it).
    The problem i have with measurement requiring an observer to collapse the wavefunction is that it seems to require a fundamental distinction between observer and object, wheras one might have assumed that an observer was made of the same matter as the object being observed and hence subject to the same laws. It seems to me to lead to the "consciousness causes collapse" theory.
  6. Jul 9, 2007 #5


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    Consciousness has nothing to do with it. Imagine a world exactly the same as ours but with no people in it. That world would still run along according to the laws of quantum mechanics exactly as our world with "conscious observers" runs along. The rods and cones in the eyes of a tiger measure photons just as well as the rod and cones in my eyes.

    Now you might start harping on eyes... the windows to the soul, and all that. But again, quantum mechanics has nothing to do with eyes... or tigers... the proof of this is left as a useful exercise for the reader.
  7. Jul 9, 2007 #6
    I'm thinking that "observation" is merely a recording of the correlation between two facts as opposed to probabilities being caused by observation. The only outside influence on the normal evolution of the wavefunction is our decision on which facts to correlate to one another. The normal wavefunctions of all things propagate as expected without "collapse". But when we chose where to start and end the propagation at certain possiblities, we get a probability of occurance.

    In other words, probabilities of occurance are not relevant when no choice is made between which two facts to correlate. And a probability results only when you know both which fact you started with and which fact you ended with. I suppose that a type of wavefunction does propagate for each particle in the universe. But I doubt that you could even write down a mathematical expression of propagation without at least knowing some initial conditions. For propagation exactly means how some known state changes with time.

    But things can only propagate from a more known state to a less certain state. There is more than one way things can develope, and one fact can lead to many other consequences, not just one result. So it is not enough to know that the first event may have possibly propagated to the second. You need to know that the second fact could also have lead to the first, in other words the end state must be able to propagate to the start, at least mathematically. The path integral formulation of QM includes paths from start to finish and also from finish to start. And this seems to mimick the logic that a conjunction of two facts (i.e. knowing that both exists) implies that the first state implies the second (propagation from start to finish) AND that the second implies the first (propagation from finish to start).

    Does any of this make an philosophical sense?

    P.S. have you checked your Private Messages, madness?
  8. Jul 9, 2007 #7
    I was writing in response to meopemuk's reply. He says
    ""Measurement" always presumes existence of an observer who extracts some information/knowledge about physical system. If you think about it, science, in general, does not tell us what is "really" going on in the world. It only allows us collect, describe, and systematize our knowledge about the world. If somewhere a photon bounced off an electron and this event hasn't affected our measuring devices, then this effect hasn't changed our knowledge a bit. So, no wavefunction "collapse" has occurred."
    This clearly states that when a physical interaction doesnt affect our knowledge, it doesnt collapse the wavefunction. Hence if no conscious observers existed to have their knowledge affected, no wavefunctions would collapse and the universe would indeed run along differently to our one with conscious observers.
  9. Jul 9, 2007 #8
    It is important to realize that in physics we are not trying to describe what "really" happens in the world while we are not looking. We are trying to describe results of measurements. It would be nice to know how physical systems behave when nobody is looking. However, quantum mechanics tells us that this knowledge would be incomplete. This knowledge is described by probabilities contained in the wave function. When a measurement is done a particular result gets chosen randomly among the set of possibilities presented in the wave function.

    Nobody knows why nature has this random behavior, but this is just the way it is. Quantum mechanics doesn't explain this randomness. It only allows us to calculate the probabilities of measurements. There are numerous "interpretations" of quantum mechanics, which attempt to describe what is going on while we are not looking, how the wave function "collapses", etc. In my opinion, the only role of these interpretations is to give us a "good feeling" that we understand about nature more than we can measure. In my opinion, all these interpretations are harmless, but not helpful.

    All objective and useful knowledge about nature is contained in results of measurements. These results are partly predictable and partly random. Quantum mechanics tells us everything about the predictable part. The random part is what it is - random. Consciousness has nothing to do with it.
  10. Jul 9, 2007 #9
    No. The measurement instrument must be the macroscopic object and obey the laws of classical physics (a photon is a quantum object, no collapse take place). The collapse is the consequence of the transition from Quantum World to Classical World.

    J. von Neumann Theory of Measurements is based on two independent assumptions:

    1) The experimental evidence of collapse is temporarily postulated (it was pointed out by J. von Neumann that it can’t be postulate since it is derivable feature of QM formalism-Spectral Decomposition Theorem);

    2) The state of the measured object and the observer is described by the direct product of the individual states (you completely correct that it means that the object being observed and the measurement instrument obey the same laws).

    It was demonstrated by J. von Neumann that these two assumptions lead to solipsism. Standard math method of Reductio ad absurdum .

    This is exactly what one should do in order to correct J. von Neumann Theory of Measurements. It automatically leads to the solution of the Measurement Problem (explanation of the collapse phenomenon).

    Regards, Dany.
  11. Jul 9, 2007 #10
    We have no business of asking how a universe without observers would run. Science is about objective facts, objective facts can be obtained only by measurements, measurements cannot be done without observers. Perhaps it would be nice to know how things are working when nobody is looking. However, even if we somehow obtained such a knowledge, it would be impossible to verify experimentally. Because in order to verify we need to look. Statements that cannot be confirmed by experiment are not a part of science.

    So, in some sense you are right, that consciousness is required for the wavefunction collapse. I just hope that you don't interpret this statement in a primitive way: "wavefunction wanders around space until it meets an intelligent human being, and then it happily collapses."
  12. Jul 9, 2007 #11
    This corresponds to state that in physics, things don't exist, when we are not measuring them. You quite state this, when you say:
  13. Jul 9, 2007 #12
    No, I am not saying this. Things do exist even when we are not measuring them. However, it is impossible to say anything useful and objective about things without performing a measurement. One can say all kinds of things about unmeasured properties (and different QM interpretations are quite good at that), but these statements are not scientific, because they cannot be verified experimentally.

    Laplace could say: An intelligence that would know at a certain moment all the forces existing in nature and the situations of the bodies that
    compose nature, and if it would be powerful enough to analyze all
    these data, would be able to grasp in one formula the movements of
    the biggest bodies of the Universe as well as of the lightest atom.
    And he could ignore the necessity of experimental verification of his predictions, because in the classical world his predictions were guaranteed to coincide with measurements (of course, if no mistakes were made in calculations).

    In the quantum world things are different. There is an inherent randomness in nature's behavior. So, our calculations are not telling us exactly what we will see in experiment. They tell us only probabilities. To see what truly happened one must make a measurement.

    One may say that measurement "collapses" these probabilities, and one may wonder about the mechanism of this collapse. In my opinion one should simply accept a certain degree of unexplainable randomness in nature. Then there is no need to explain the mechanism of the wavefunction collapse.
  14. Jul 9, 2007 #13
    First of all, the results of observations obtained using the measurement apparatus must be the same in every inertial system (up to irrelevant statistical nature of the macroscopic objects).

    If it happens that in some of them also the conscious object is present, it will lead in practice to misunderstanding and misinterpretation (the appropriate conclusion that the sun rising each morning still popular even today).

    The wavefunction is unobservable, Psi(x) x Psi(x)* is pictorially real (and mathematically). Only real quantities are observable.

    Regards, Dany.
  15. Jul 9, 2007 #14
    The only thing consciousness contributes is to choose which two events to correlate to each other. It doesn't seem likely that one could even write the mathematical expression for a wave function without some consciousness having already chosen which events to correlate with each other. So the wavefunction itself is an artifact of consciousness, and collapse requires no more consciousness than writing the wavefunction from which it collapses. Collapse is inherent in probability distributions not in measurement. You gauranteed a "collapse" when consciousness chose to write the wavefunction for the particular events it chose to correlate.
  16. Jul 9, 2007 #15

    A. Einstein dialog with W. Heisenberg (spring 1926) as quoted by W.Heisenberg:

    W.Heisenberg “It is reasonable to include in the theory only observable quantities…”

    A. Einstein:”Are you seriously assume that the physical theory includes only observable quantities?”

    A. Einstein:” Speaking more carefully, the remembering of what we are really observes and what we do not has probably some heuristic value. However, from the principal point of view, the attempt to formulate the theory based only on observable quantities is completely nonsense. Because in the reality everything that happens are just an opposite. Only the theory itself can decide what is and is not observable. You see, the observation, generally speaking, is very complicated notion…”

    A. Einstein to M.Born (12.10.1953):”For the planned in your honor collection of papers I wrote “physical” childish song, which slightly confused Bohm and de Broglie. It purposes is to demonstrate that your statistical interpretation of quantum mechanics is not necessary…

    In my last paper the mechanism of the wavefunction collapse is explained without probabilities but through the wave mechanical generalization of Newtonian mechanics (in compliance with the ideas of W.R. Hamilton and E.Schrödinger). Indeed, if you not need that you are not required to know.

    Regards, Dany.
  17. Jul 9, 2007 #16
    I dare to disagree with the great master on this point.

    What do you mean by the word "explain"? Can you use your theory to *predict* where each individual electron will hit the screen in the one-slit or double-slit experiment?
  18. Jul 10, 2007 #17
    Ok. How do you prove this statement? What does, in physics, mean the word "exist"?
    But in experiments like that performed by Alain Aspect and the more recent by Anton Zeilinger, they just verify experimentally the various QM interpretations!
    I see things in a different way. There is randomness only if we want to describe the phenomena using classical concepts, but this is as trying to read a book written in english using german language: you would conclude that there are "errors" and "randomness" in the printing.
    No, the problem is not to accept "randomness", but to understand that terms like "position" "momentum" ecc. can only be referred to classical, macroscopic objects. What is, for example, the "position" of a wave?
    Last edited: Jul 10, 2007
  19. Jul 10, 2007 #18
    I appreciate your courage. But your problem is much deeper. Notice that the above statement a posteriori may be considered as a definite prediction of quarks. It does not express the particular subjective POV of A. Einstein. It expresses the cumulative understanding and knowledge of E.Schrödinger, W. Heisenberg, P.A.M. Dirac, E.P. Wigner, C.N. Yang, Y. Aharonov, D. Bohm, R.P. Feynman, M. Gel-Mann, etc. You call “these statements are not scientific”!!! Suppose your reader (I) is ready to accept your POV. May you explain with whom and with what you left me? Using the mathematical notions I would say that your manifold is empty.

    W.Heisenberg, “The Physical Content of Quantum Kinematics and Mechanics”, Zeitschrift fur Physik, 43, 172 (1927):
    “We turn now to the concept of “path of the electron” By path we understand a series of points in space (in a given reference system) which the electron takes as “positions” one after the other. As we already know what is to be understood by “position at a definite time”, no new difficulties occur here. Nevertheless, it is easy to recognize that, for example, the often used expression, the “1s orbit of the electron in the hydrogen atom”, from our point of view has no sense.

    The QM is the field theory, theory of extended objects. Consider double-slit experiment. In that case in particular and for the measurement procedures in general the single isolated hit has no sense. You should be patient, wait 5 min (according to A. Tonomura) and you will obtain the complete knowledge. You use the standard “vanilla” requirement of repeatability and later the standard “vanilla” techniques of image and/or signal processing used in the classical physics. If you are ready to accept that P.A.M. Dirac understand something in physics (“Principles of QM”, Ch.1), you will comprehend that you see the amplified picture of the single electron.

    So, what happens here in the Theory of Measurements? The Measurement Problem is clearly the problem of the classical physics and not of QT.

    Let me present the “explanation” which is pure speculation since I have superficial knowledge in history of QM and no knowledge at all in psychology.

    First of all, the background:
    1)it is trivial point that the physical theories must use the matched mathematical languages;
    2)it is trivial point that the QM is more general description of nature than the classical physics;
    3)it is trivial point that the less general theory must fit the more general and not vice versa;
    4)it was clearly stated by J. von Neumann that the classical physics is dispersion free physical theory;
    5)it was clearly stated by N. Bohr that the measurement apparatus belong to the classical physics and therefore should behave according to the laws of the classical physics;
    6)it was initiated by E.P. Wigner et al the study of interconnection between the real/complex/quaternion/octonion Hilbert module structures;
    7)it was clearly demonstrated by E.C.G. Stueckelberg that the Real Hilbert space is dispersion free physical theory;
    8)E.P. Wigner, “Some Strangeness in the proportion”, p.457:”The real Hilbert spaces even I would know”;
    9)E.P. Wigner worked at IAS just around the corner near A. Einstein;
    10)the Theory of Measurements and the Measurement Problem were a “baby” of A. Einstein.

    The answer was just in front of his eyes: The Newtonian Mechanics should be reformulated in order to provide the natural explanation of the collapse of the wave packet.

    We consider the Analytical Mechanics the “mother” of the theoretical physics. I personally consider her the most beautiful and fundamental theory ever produced by the collective effort of human minds. At the beginning of the 20th century A. Einstein “raped” Analytical Mechanics. It was pure psychological “tabu” to accept that he should do it once again.

    Regards, Dany.

    P.S. I do not consider it “my” theory. I have no doubt that the presented results are invariant under the time translations
  20. Jul 10, 2007 #19
    I am a materialist. I believe that material world exists objectively. However, I also believe that the only way to get knowledge about material world is through observation. Of course, one can logically believe that everything is just our perception. This is too philosophical, and I don't want to go there.

    Which interpretation of QM was confirmed by Aspect and Zeilinger? I thought that they established that QM description is more preferable than classical "hidden variable" description.

    I think that phenomena should be described in terms of measurable quantities, like positions, momenta, spins, etc. This description does not depend on the type of the system: quantum (microscopic) or classical (macroscopic). The "randomness" is definitely a characteristic of microscopic ssystems.

    I know only waves which are complex systems consisting of many particles. For example, ocean waves are made of water molecules. Electromagnetic waves are bunches of photons moving in one direction. What other waves are there?
  21. Jul 10, 2007 #20
    I am afraid you interpreted my words in a too broad context. I was talking specifically about evolution of a physical system between its preparation and measurement and about "wavefunction collapse". My point was that it is not useful to speculate about what happens to the system while we are not watching and how exactly the collapse occurs. These are things that even in principle cannot be confirmed by experiment, because we already established the condition: "we are not watching".

    Having said that, I agree with you that it would be foolish to limit science only to things that can be directly observed. In physics it is useful to formulate models whose ingredients may not be observable at this particular moment, but we are hoping to observe them when more powerful techniques will be available in the future. This transition from a hypothesis to direct experimental observation is what science is about. Many physical concepts have passed (or still are passing) through this transition: atoms, nuclei, quarks,... I hope we are in agreement now.
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