What is Wavefunction collapse: Definition and 48 Discussions

In quantum mechanics, wave function collapse occurs when a wave function—initially in a superposition of several eigenstates—reduces to a single eigenstate due to interaction with the external world. This interaction is called an "observation". It is the essence of a measurement in quantum mechanics which connects the wave function with classical observables like position and momentum. Collapse is one of two processes by which quantum systems evolve in time; the other is the continuous evolution via the Schrödinger equation. Collapse is a black box for a thermodynamically irreversible interaction with a classical environment. Calculations of quantum decoherence show that when a quantum system interacts with the environment, the superpositions apparently reduce to mixtures of classical alternatives. Significantly, the combined wave function of the system and environment continue to obey the Schrödinger equation. More importantly, this is not enough to explain wave function collapse, as decoherence does not reduce it to a single eigenstate.Historically Werner Heisenberg was the first to use the idea of wave function reduction to explain quantum measurement.

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  1. K

    B Can we say a photon doesn't exist until the wavefunction collapses?

    I am sure this sounds silly but.. Between being emitted and interacting, a photon gives us no information as to where it is in space. If we know both where a photon was emitted from and where it then interacted, we can say that it travelled in a straight line. But we actually dont know. So in...
  2. R

    B Double slit experiment, expensive gear?

    I have an idéa how to run the double slit experiment that could give new insight to whats hapening. As I understand, when the photons are observed by someone the wawefunction colapses and the photons become particles, this can be seen as the interference pattern dissapears in the experiment...
  3. bhobba

    I Interestingly Bohr Did Not Believe in Wavefunction Collapse

    I was reading an article 'What Einstein Really Thought about Quantum Mechanics' in Scientific American. There they mentioned something I didn't know - Bohr did not believe in Wave Function Collapse as an issue (I don't either - but that's just my opinion, so means nothing). I found this...
  4. S

    I Nima Arkani-Hamed's opinion on Many Worlds?

    I am writing a blog about physics and one of the sections is about the different interpretations of quantum mechanics and some of its supporters. I was wondering what was the opinion of the physicist Nima Arkani-Hamed towards the Many Worlds interpretation... I ask this because I found...
  5. S

    Nima Arkani-Hamed's opinion on Many Worlds?

    I was wondering what was the opinion of the physicist Nima Arkani-Hamed towards the Many Worlds interpretation. Is he open to the possibility of it being true? Does he support it?
  6. A

    I Master` Equation for the Penrose-Diosi wavefunction collapse

    I'm trying to understand equation 209 on page 81 here: https://arxiv.org/pdf/1204.4325.pdf Here's what I understand so far: - We are to imagine a system in a superposition of |X> and |X'>, where these describe distinct particle configurations. - On the RHS of the eqn, the first term is...
  7. Denselight93

    B The Virtue of Individual Perception RE: Wavefunction Collapse

    Hey guys! New to the forums so unsure if this is exactly the right place to present this idea I have. I hope that you will find it interesting. I am all too familiar with claims of quantum consciousness woo. I am also aware that the role of 'wave function collapse', if such a thing even...
  8. Erik Ayer

    I Does downconversion cause pump photon wavefunction collapse?

    I saw a paper on an experiment where a pump beam first went through a double-slit, then was downconverted with BBO. Recently a friend with a PhD in quantum physics said the downconversion will cause the pump's wavefunction to collapse and the implication for this experiment is that the pump...
  9. F

    I How to Determine a Photon's Wavefunction After it Collapses?

    Suppose one measures the position of a photon without destroying it. From my understanding, the wavefunction of the photon should collapse, and will return to a more spread out state over time. How would one calculate this, specifically the rate at which the wavefunction spreads out from the center?
  10. MichPod

    I Is information lost in wavefunction collapse?

    Having in mind the idea that "information is not lost" (referring to the black hole information paradox), is not the same rule violated in the wave function collapse? I.e. during the decoherence process information is not lost as this process of entanglement of some object with its environment...
  11. P

    I What's the significance of HUP if Ψ collapses?

    If I understand it correctly, a particle doesn't have a definite momentum and a definite position, but is in a superposition of multiple positions and momenta. And when we measure either of the two quantities, say, position, the wavefunction collapses to tell us where the particle is. Now when...
  12. fluidistic

    I How to pick an operator onto which the wavefunction collapses

    Imagine a system of 1 particle in a superposition of eigenstates of some operator(s). If one were to make a measurement of a property of that particle, how is the operator (or observable) "picked" so that the wavefunction collapses into an eigenstate of said operator? In other words, how do one...
  13. D

    I Superposition state and wavefunction collapse

    Hi. My question is a general one but I will use an infinite well as an example. Without knowing details of the exact wavefunction I presume it can exist as a linear superposition of an infinite number of energy eigenstates ? Without knowing the exact wavefunction ; does that mean that when the...
  14. T

    I Regarding consciousness causing wavefunction collapse

    What are the experiments that disprove the idea that consciousness causes wavefunction collapse?
  15. E

    Does the Wavefunction Collapse Violate Information Conservation?

    I have read that "Information is conserved", or at least that it should be. I read this in some discussion about whether or not black holes destroy information. (ref: Leonard Susskind, Stephen Hawking, The Black Hole War, etc..) It is believed systems in distinct states evolve into distinct...
  16. J

    Hydrogen Atom: Wavefunction collapse after measurement of Lz

    Homework Statement Suppose we have a wavefunction with n=4. If we measure the orbital angular momentum along the z-direction(no spin in this problem) and get 2*hbar then what are the possible values of the total angular momentum and what is the most general wavefunction after the measurement...
  17. G

    Why does not the graviton cause wavefunction collapse?

    The modern view of the measurement problem is that any interaction of a particle (say an electron) will cause its wavefunction to 'collapse' in the process called decoherence. No need for conscious observers, interaction with any other particle will cause decoherence hence collapse of the...
  18. T

    Wavefunction Collapse: Ideas & Scenario Analysis

    Hi Everyone I am not sure of what exactly happens when a wavefunction collapses but I have some ideas about which I would be grateful for your thoughts. Firstly, it is often said that wavefunctions collapse when they are "observed" or "measured". To be honest, I do not think that...
  19. M

    Measurements and wavefunction collapse.

    Hey everyone.I have a couple of question regarding wave function collapse. I can accept that one cannot make a measurement with absolute precision and have a usable wave function afterwards due to the uncertainty principle... Consider a particle moving along the x-axis with a wave function...
  20. Superposed_Cat

    Mathematically what causes wavefunction collapse?

    Hi all, I was wondering mathematically ,what causes wave function collapse? and why does it exist in all it's Eigen states before measurement? Thanks for any help and please correct my question if I have anything wrong.
  21. andrewkirk

    Is destruction of dbl-slit interference pattern wavefunction collapse?

    I have read in several places (such as this StackExchange reply) that wavefunction collapse is 'non-physical'. The explanations I have read seem clear and understandable to me and have helped me to understand EPR and the implications of Bell's Theorem. I am trying to reconcile this in my mind...
  22. B

    A thought experiment of wavefunction collapse

    I propose a simple thpught experiment; A scientist A measure the spin of an electron and finds it "up". After that he removes all evidence of his experiment and goes away, only to die in a road accident. Another scientist, obviously unaware of the experiment done by A come and measures the...
  23. M

    Wavefunction collapse - experimental proof?

    Have you recently came across and remember titles of papers about real experiments concerning the famous double slit electron behaviour? I have found some about electron diffraction, but I'm still looking for those that showed how interference pattern is broken when we watch where the...
  24. A

    Wavefunction collapse and antisymmetry

    Hey guys, I was reading a book about the philosophy of science, and in the chapter about QM the author uses a well known example in order to explain quantum entaglement and illustrate the non-separability of individual system in QM. He describes a system composed of two spin-1/2 particles...
  25. V

    Degenerate states and wavefunction collapse

    So, in QM making a measurement collapses the state into an eigenstate of that observable. Thus, if the system is properly isolated, then the same measurement should return the same value. But the eigenvalue for that state is degenerate, then does that mean the state might actually collapse to a...
  26. X

    Wavefunction collapse with a single photon?

    Hello, I was wondering what exactly happens when you observe part of the wavefunction of a particle, does this always cause collapse? Or only when the probability distribution decides that the particle is indeed there? What I mean is, is an observation in the form of photons interacting with...
  27. J

    Wavefunction Collapse: Timeline & Effects

    Simple question. So the energy of a particle is observed to be E_1 (for example) at time t=0. At time t=0 the wavefunction psi(x) collapses to phi(x)exp(-i(E_1)t/h). At time t>0 the wavefunction is also in this state (right?). Is it in this state until it interacts with another particle or...
  28. R

    Wavefunction collapse and dirac delta functions

    What is the experimental evidence that a wavefunction will collapse to a dirac delta function, and not something more 'smeared' out?
  29. D

    Wavefunction Collapse: Measuring Electron Spin

    If you were to measure an electron's spin, for example, will the wavefunction associated with its position also collapse?
  30. S

    What causes wavefunction collapse?

    I've always been confused about something -- I'd love for someone to clear up my ignorance. I understand that the position of a particle can be modeled as a wavefunction (a probability distribution, to my understanding) where we can describe the position as fundamentally random, but it takes on...
  31. O

    Wavefunction collapse on degenerate states

    Hello, I am a beginner on the sbject so please correct if I'm using some sloppy terminology. I'll try to be clear. Consider a Hamiltonian with degenerate energy eigenstates (say the degeneracy is on angular momentum as in hydrogen atom). Which of the degenerate eigenstates would the wave...
  32. 3

    Wavefunction Collapse: Exploring Particle & Wave Behavior

    Would it be fair to say that before an observation, a wave-particle is in a superposition of many possible states but that after the observation, the wave-particle is found only in one state? Would that be analogous to saying that it goes from behaving in a very wave-like manner to behaving...
  33. M

    Help a Quantum Noob with the Wavefunction Collapse?

    If matter goes from a superposed state to a collapsed state when measured, how did scientists see the interference pattern in the double slit experiment, doesn't the surface that the photons hit after passing through the slits count as a measuring device? Also, is there any updated...
  34. S

    When does the wavefunction collapse?

    Hi guys, Quantum mechanics gives well-defined probabilistic predictions for the value we get when we measure position or momentum; one simply takes the absolute square of the wavefunction in either the x-basis or the p-basis. However, I am not so clear on how we would predict at what time we...
  35. K

    Observational data and wavefunction collapse

    Okay, let me begin by saying that I do NOT have a good foundation in quantum mechanics, but I have been completely captivated by the wavefunction collapse. I have looked over many explanations of the double-slit experiment over the course of a few months, and I cannot find that one single...
  36. W

    Wavefunction collapse and measurement

    So, rookie question, I know, but I'm having a little trouble with the idea of wavefunction collapse as it pertains to stationary states: If a measurement of energy collapses a wavefunction into an energy eigenstate, it stays there forever (unless perturbed). But my impression is that although...
  37. H

    Observer types to make the wavefunction collapse.

    Hi there, I'm not extremely adept at understanding what I like to think of as the "Philosophical" side of QM but I find I have a problem with some aspects of an interpretation of the wave function collapse. I also could be misunderstanding what people are saying. When people talk about...
  38. F

    Schrodinger Equation and wavefunction collapse

    If we solve the Schrodinger Equation for hydrogen atom, we get discrete energy levels that agree with experiment. But no where we need the wave function collapse. So my question is where the wave function come from and why do we need it?
  39. A

    The effect of wavefunction collapse on spacetime?

    Hi folks, I'm not sure if it's best to ask this question here, or in the Special & General Relativity section - it's probably more appropriate for this forum. I've been wondering about the following question: what effect does wavefunction collapse have on space-time? For example, if we...
  40. 2

    Wavefunction collapse => increase in entropy?

    Wavefunction collapse ==> increase in entropy?? I just read an article in Scientific American by Sean Carroll, called something like Does Time Run Backward in Other Universes. In it, he says that the reason wavefunctions only collapse and never un-collapse is because collapsing represents an...
  41. S

    Clarification on wavefunction collapse

    I am just beginning to understand this concept. Some help would be appreciated. Let me know if I am wrong in saying the following: "The wave function (say \Psi] collapses to an eigen vector of the operator corresponding to the physical quantity(say \lambda) being measured. This is because the...
  42. L

    Wavefunction collapse: is that really an axiom

    Can the wavefunction collapse not be derived or is it really an axiom? How can the answer to this question (yes or no) be proven? If it is an axiom, is it the best formulation, is it not a dangerous wording? Let's enjoy this endless discussion !
  43. L

    I won't debate on the wavefunction collapse

    I won't debate on the "wavefunction collapse" ... ... since this is just a lazy debate started from a misunderstanding. Clearly when a small system interacts with a measuring device, the wave function of the small system just loses any meaning. There is only one "larger" wavefunction for...
  44. M

    How observation leads to wavefunction collapse?

    Hi all I know I raised a similar question in the thread "Wave particle duality", but it is already so full of many other questions, that I'd not be able to discuss this topic fully there. So, in the double slit experiment, if a photon observes an electron, the interference pattern...
  45. N

    String Theory & Wavefunction collapse

    Does string theory provide an explanation for the collapse of the wave function in QM? Thank you in advance for your comments.
  46. Loren Booda

    Is wavefunction collapse limited by the speed of light?

    Doesn't complete information about a probability distribution presuppose a physically determined wavefunction collapse? How can we have knowledge about statistics of all existent quanta for the wavefunction except by light signals in the first place, whose correspondent reversed process should...
  47. M

    Double Slit Experiment: Explaining Wavefunction Collapse

    there's something that is annoying me, because I can't find a explanation It's about the classical double slit experiment, where you have two screens, and one of them has 2 narrow slits then you launch a photon against the screens, and if there are no detectors in the slits, you observe an...
  48. Loren Booda

    Reversing wavefunction collapse

    Does the observational process quantum-->classical ever reverse?