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What are the kinds of mesurements that affect the wave function?

  1. Jan 22, 2013 #1
    First I should point out that I don't have any scientific knowlege in Quantum Mechanics. I am just enthusiast physicist not professional. I am interested in physics and the only thing that I know for now is Classical Mechanics (Newtonian, Lagrangian and Hamiltonian reformulations) and some Special Relativity. All that I learned form Leonard Susskind's Lectures that are uploaded on youtube. The things that I will ask about QM I all got from popular books so the questions and statements i make can be very inaccurate or even stupid. I am sorry If the questions that I ask have been asken or things like that but I am new to the forum.

    As far as I know the Wave Functions represents the probability of the particle to be in certain state and it is experimental fact that mesurements cause some change in the WF which make the particle to establish well defined state. The thing that I realy wonder is what kind of action can be described as a mesurement.
    For example in the double slit experiment with a single particle. When the particle is emited its wave function splits through the slits then interfere with itself and then when it is detected ( a mesurement is performed) it establishes well defined state which we can see as a dot on a screen (if it can detect single particles). If detectors are put to the slits the wave function collapses there and after a long periond of time there is no interference pattern on the screen. But how the slits themselves don't affect the wave functions? They also are made of some kind of particles that have kinetic energy and are interacting with the wave.
    Also this experiments have also been made with big atoms or molecules. In this case how the molecule itself can hold its wavefunction without collapsing? I mean the particles in the atoms are interacting electromagnethicaly ( they are exchanging photons) and there are quarks and gluons in the protons and neutrons. They all have energy and they all interact.
    What is the difference between me emmiting some photon to a particle then after the particle emits it back to me and I detect it and a molecule or atom where the particles exchange photons?

    I am not sure about this but I've heard that quantum effects where particles behave like waves have even been observed in living cells ( for example during photosyntesys) when electrons can move as a waves. Living cells have very high entropy and they are made of many particles interacting which should be counted as a mesurement.

    So how at some point a mesurement can affect the system and other times it does not affect it? What can be described as mesurement in QM?
  2. jcsd
  3. Jan 22, 2013 #2


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    Welcome to PhysicsForums, Sayajin!

    These are great questions, and I would say some of these have no clear answers at this time. I can tell you what is a measurement or not from the "context" of a particular setup, true enough. Generally, if you can directly observe or otherwise deduce information about an observable, its wave function will experience collapse.

    But the fact that there are things called "quantum erasers" throws this into some confusion. If something is measured but later the results are properly made ambiguous, then the wave function may not collapse as would otherwise be expected. (This is an example of a reversible process.)

    In addition, there are things called "weak" measurements. Weak measurements do not cause complete collapse, only partial collapse.
  4. Jan 22, 2013 #3


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    I agree with what DrChinese said. Also, 'measurement' (as a useful definition for QM), is really about how the eigenstates associated with a particular 'measurement' compare to the system being measured.

    For example, in the double-slit experiment, when we say that we place detectors so that we can 'measure' which slit the electron went through, we are defining our 'measurement' such that its eigenstates are the states "through left slit" and "through right slit" So in this example, by our definition, our 'measurement' is making the electron go through only one slit or the other. (not some superposition of both).

    So I guess I'm saying that you need to look at what the definition of 'measurement' is, for each specific example. Then use that definition in the framework of QM to find the answer to the problem.
  5. Jan 22, 2013 #4
    Thanks for the responses.
    I think that my confusion is now cleared. As far as I understood it not the process of mesurement itself ( the particle doing something physically to the other particle) but the information that the "detector" can get for the state of the system that causes the collapse.

    I have heard about this quantum erasers for the delayed choice experiment ( where using entangled particles scientist can cause collapse due to mesurement which is performed in the future) but I thought that this property had more to do with the entanglement itself.

    This realy makes me think that information is actualy something very physical and real not some property we only use to describe objects but that don't have physical meaning like was thought for space and time before Relativity which shows how time and space are very real and have some strange properties.

    I am sorry that I ask questions which I am obviously not ready for because I don't even know tha basics of QM but studying this subjects takes a lot of time. I barely have the spare time to watch lectures but at the same time Physics (especialy modern stuff) is very interesting thing to me.
  6. Jan 22, 2013 #5


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    Keep up the good learning, I am sure you will gain a lot from it!
  7. Jan 22, 2013 #6
    A property of a system is "measured" if it is present in the observer's mind. This is a consequence of the MWI interpretation and all interpretations yield the same results, so this is basically what QM says.

    It's not said however that the observer's mind causes the collapse. As well there may be something happening in the way from the quantum event to our sensory organs that causes it. But we didn't discover anything like that yet. We know that when we get to know some observable, then the quantum state is collapsed. If we don't, then it is not. We don't know what happens in between.

    The candidates for the definition of a "measurement" are at least (but not limited to): getting something to know by a concious mind, magnifying a quantum state to the macroscopic realm, leaking the state enthalpy into environment and others. It also may be that the state was never in a superposition and the world is classical from the very beginning (hidden variable theories). In this case, the measurement would be the same thing as in classical physics. There are even some "mystical" interpretations of QM, including the idea that we are not able to perform just any measurement but only those that would give the proper results or the idea that it all has something to do with soul :).

    As said, we have no way to differentiate between any of QM interpretations yet. You can assume anything and you will get the same results.
  8. Jan 22, 2013 #7
    That sounds like you think that "information" is "out" there independent of us, kinda like "objective" or mind-independent information? There are some interpetations that favour that view (Bohm and Hiley, for example). I'm not sure if you've looked at this experiment but you might find it interesting:

    Realization of quantum Wheeler's delayed-choice experiment

    Full article arxiv preprint:
    Revisiting Bohr’s principle of complementarity using a quantum device

    Another university news article discussing the paper:
    [Nature Photonics]Quantum optics: Wave–particle superposition
    Last edited by a moderator: May 6, 2017
  9. Jan 22, 2013 #8


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    Cant agree with that - or rather it is only measured if that occurs - it certainly is measured when that happens. Say for example the result of a measurement is recorded in computer memory and that result is only viewed by a sentient being decades or even centuries later. Is it only at that time the system is measured? As a result of the measurement the system is in an eigenstate of the measurement operator but that occurs when the measurement is done - not when it registers on a mind.

    I think the only reasonable assertion is that measurement apparatus measure a property interdependent of this thing called 'mind'. Decoherence sheds a lot of light on what going on - but doesn't resolve all the issues to everyone's satisfaction - to me it does (you need some minimal extra interpretative assumptions though such as the consistency condition of Decoherent Histories) - but opinions vary.

    Of course all interpretations, MWI or even consciousness causes collapse, leads to exactly the same predictions but some carry 'baggage' with them that many would reject. IMHO its sort of like solipsism - it leads to exactly the same experience of the world as if you accept or reject it - but most reject it because its an unnecessarily weird view.

    Last edited: Jan 22, 2013
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