What is Measurement problem: Definition and 78 Discussions

In quantum mechanics, the measurement problem considers how, or whether, wave function collapse occurs. The inability to observe such a collapse directly has given rise to different interpretations of quantum mechanics and poses a key set of questions that each interpretation must answer.
The wave function in quantum mechanics evolves deterministically according to the Schrödinger equation as a linear superposition of different states. However, actual measurements always find the physical system in a definite state. Any future evolution of the wave function is based on the state the system was discovered to be in when the measurement was made, meaning that the measurement "did something" to the system that is not obviously a consequence of Schrödinger evolution. The measurement problem is describing what that "something" is, how a superposition of many possible values becomes a single measured value.
To express matters differently (paraphrasing Steven Weinberg), the Schrödinger wave equation determines the wave function at any later time. If observers and their measuring apparatus are themselves described by a deterministic wave function, why can we not predict precise results for measurements, but only probabilities? As a general question: How can one establish a correspondence between quantum reality and classical reality?

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  1. Marco211298

    I Doubts regarding my interpretation of Wigner’s Friend Scenario

    I have some doubts regarding my personal interpretation that i was contemplating about in the context of Wigner's friend experiment (also tested in the laboratory recently). Could it be that a system is always in a superposition, and when we perform a measurement, we obtain a definite value due...
  2. D

    I What can we scientifically measure directly?

    I am interested in finding out what we can objectively measure. How to define what we can measure directly, without (human) models getting involved, as in model-dependent realism as described by Hawking and Mlodinow? It seems as if everything we measure is either mass or energy (or spin?). Time...
  3. L

    I Copenhagen Interpretation vs Pilot Waves

    I was unsatisfied with the measurement problem so I was looking at pilot wave (PW) Sabine says one downside of pilot wave is there is no QFT version for pilot wave yet. And a significant problem in replacing the QFT is that PW is non-local, and that copenhagen very much depends on being local...
  4. Peter Morgan

    I The collapse of a quantum state as a joint probability construction

    The titular paper can be found here, https://doi.org/10.1088/1751-8121/ac6f2f, and on arXiv as https://arxiv.org/abs/2101.10931 (which is paginated differently, but the text and equation and section numbers are the same). Please see the abstract, but in part this 24 page paper argues that we...
  5. Ashish Somwanshi

    Measurement problem quantum mechanics

    I was not able to attempt since I don't know which formula or method can be used to solve the problem
  6. M

    I What does it take to solve the measurement problem? (new paper published)

    Yesterday Jonte Hance and Sabine Hossenfelder published this preprint on the arXiv: https://arxiv.org/abs/2206.10445 What does it take to solve the measurement problem?
  7. U

    B Special Relativity and the Measurement Problem of QM

    Given what we know about special relativity and its implication for time and the observer, could this in any way be linked to why the isolated processes of QM are exhibiting everything happening at once and then collapsing to classical physics when bigger objects interact - the measurement...
  8. M

    I Measurement problem in simple experiment

    I’m trying to understand the measurement problem using the simplest experiment I can think of--passing a particle P through a 50/50 beam splitter S, sending it down “path A” or “path B” with equal probability. Each path has a detector that can tell us if P was in that path. The detectors...
  9. Killtech

    I Weak measurement and the measurement problem

    When I think about it my biggest issue with the interpretation of QT can be boiled down to this question. There are the axioms of measurement in QT, which have their limiting nature and claim to be the only way to extract information from a quantum system - but that is just a claim with enough...
  10. M

    I Questions about Adler paper "Why Decoherence has not Solved ...."

    On page 8, I don't understand the last paragraph. It says if ##|A \rangle = U|0\rangle ## and ##|B \rangle = U|0\rangle ## then ##\langle A | B \rangle = 1##. Of course ##U |0\rangle## is a unique state (##A## and ##B## are the same by this definition). So I assumed what he meant was more...
  11. M

    I Decoherence references and a question about the measurement problem

    Couple questions here. 1. What do you think is the best source to read about decoherence, both from an intuitive/high level pov, but also with mathematical details. I'd partially read a recommended paper by Schlosshauer a while back, is that still the bible for this? 2. If probabilities in...
  12. bland

    I Is the measurement problem equally as vexing as entanglement?

    Maybe I've got this wrong, because from what I've been able to glean, how a particle measurement affects another particle which can be on the other side of the Universe in principle is agreed to be 'spooky' and it seems that everyone is happy to simply say, yes this is very mysterious and...
  13. allisrelative

    Doesn't Wigner's Friend Experiment solve the measurement problem?

    If you look at the recent Wigner's Friend experiment, it seems to support Carlo Rovelli's Relational Interpretation which says there's no real measurement. Wiger's Friend carries out a polarization measurement. Before he does, the quantum system is in a superposition of horizontal/vertical...
  14. MichPod

    I What did Heisenberg know about the measurement problem?

    While reading a book "Physics and Philosophy: The Revolution in Modern Science." by Werner Heisenberg, 1958, I was much surprised by the following passage near the end of Chapter 3: "The measuring device deserves this name only if it is in close contact with the rest of the world, if there is...
  15. entropy1

    I What does it mean to influence a quantum measurement's outcome?

    This just occurred to me and I don't expect to be the first one to address it: It is said that in a specific measurement basis, the outcome of a measurement in this basis is determined by chance. But in how far is this the case, since if the eigenvectors are for example ##\overrightarrow{A}##...
  16. E

    I Macroscopicity and the measurement problem

    Assuming the validity of the Heisenberg-von Nuemann cut, does the measurement problem influence the macroscopic nature of the macro world? Quantum effects, roughly speaking, 'lose' their quantum nature over the many degrees of freedom of many-particles systems(cars, chairs...) via averaging out...
  17. entropy1

    I Conservation of energy and measurement problem

    If we have a two dimensional measurementbasis, then we have two possible outcomes of the measurement. Now I figured: considering the law of conservation of energy, if one particle goes in, one and only one can come out. So outcome "both results simultaneously" cannot happen, for that would...
  18. A. Neumaier

    A Weinberg on the measurement problem

    Summary: A link to excerpts from a paper behind paywalls I just found this link featuring excerpts from a 2017 paper by Steven Weinberg on the measurement problem, which I couldn't read before, it being behind paywalls...
  19. C

    I "Spatial Width," "Coherence Length," and the Measurement Problem

    A new (short 3 pg) paper, making quite a grandiose claim, caught my eye last night: https://arxiv.org/abs/1908.07719 Abstract: We propose an experiment of two-path interference in which the optical path difference between the two interferometer arms is much larger than the spatial spread of...
  20. Peter Morgan

    I Against Collapse: A Mathematical Argument

    One sees "Collapse" language all the time, and yet it seems there's a very simple argument that shows that it's not necessary. Suppose we measure ##\hat A## twice, at an earlier and at a later time, then the joint probability density for the measurement results being ##u## and ##v##...
  21. Auto-Didact

    A Ballentine: Decoherence doesn't resolve the measurement problem

    This thread is a direct shoot-off from this post in the Insights thread Against "interpretation" - Comments. I am usually not a big fan of Ballentine, but I tend to fully agree with him on the following issue (taken from this paper, credits to @bhobba) Decoherence theory is a pragmatic...
  22. Auto-Didact

    I Human eyes can detect single photons?

    I'm not sure where this thread belongs; it is essentially a topic at the intersection between quantum optics, human physiology and even psychology, but I will focus more or less on the implications for quantum physics, so I just opted for this subforum. There is a recent article over Scientific...
  23. Robert Friz

    B Are Quantum Entities in Macro Sized Objects Entangled?

    Hi. This is my first posting on the Physics Forum so please forgive any issues as a result. I am a (reasonably educated) lay person with a strong physics interest with extensive readings -- so please be patient with my questions. :-> My questions and interest in these issues are sincere. I...
  24. haushofer

    I Some (unrelated) questions about the measurement problem

    Dear all, every now and then I get this itchy feeling and start to think about quantum mechanics. Which raises, of course, some questions. These concern the measurement problem. I decided to post them in 1 single topic, so I enumerate them. If someone has some insights clarifying my confusions...
  25. A. Neumaier

    Insights Interview with Mathematician and Physicist Arnold Neumaier - Comments

    Greg Bernhardt submitted a new PF Insights post Interview with Mathematician and Physicist Arnold Neumaier Continue reading the Original PF Insights Post. Arnold will welcome science questions and comments only.
  26. J

    A Discrete measurement operator definition

    Consider the Gaussian position measurement operators $$\hat{A}_y = \int_{-\infty}^{\infty}ae^{\frac{-(x-y)^2}{2c^2}}|x \rangle \langle x|dx$$ where ##|x \rangle## are position eigenstates. I can show that this satisfies the required property of measurement operators...
  27. N

    SR830 lock-in amp measurement problem

    Hello I am currently have difficulties trying to measure dc-modulated voltages while applying a dc current, via Sr830 Lock-in Amplifier (LIA). Basically, I apply an 2 GHz ac current (amplitude modulated@10kHz and referenced to the LIA for lock-in measurement) and a dc current to my DUT...
  28. A. Neumaier

    I On the Heisenberg uncertainty relation

    Are there fundamental limits on the accuracy for measuring both position ##q## at time ##t## and momentum ##p## at time ##t+\Delta t##, with tiny ##\Delta t##? If yes, why? If no, why can't one then measure (in principle) both ##q## and ##p## arbitrarily well at the same time ##p## (which is...
  29. A. Neumaier

    A Key problems in classical and quantum measurement

    Quantum mechanical foundations are usually phrased in terms of measurement. I believe this is the main cause why these foundations remain shaky after almost 100 years of a good mathematical basis. Classical mechanics never had any reference to measurement in its foundations, and hence it was...
  30. pittsburghjoe

    I Explanation for the Quantum Measurement Problem?

    Do we yet have an explanation for the Measurement Problem that Einstein would have been satisfied with?
  31. E

    Calculating steel round bar's max capacity to support?

    First off, I am new to this forum.. However I wanted To find out if anyone would be able to help me calculate How much a steel bar (2 in. In diameter, 5 ft. Lengthwise) Would be able to hold up without bending?
  32. ShayanJ

    A Measurement problem in the Ensemble interpretation

    The ensemble interpretation asserts that QM is only applicable to an ensemble of similarly prepared systems and has nothing to say about an individual system and in this way, it seems, it can prevent the need for introducing the concept of wave-function collapse and so it may seem that there is...
  33. psycho_physicist

    Errors/ Uncertainty in measurement problem

    Homework Statement [/B] The surface tension (T) is measured by capillary rise formula T = \frac {rh ρg}{2 cos\Theta} . The quantities of ρ, g and θ are taken from the table of constants while the height and diameter are measured as h = (3.00 + 0.01)cm and D = (0.250 ± 0.001)cm Find the...
  34. L

    Meter Bridge: Finding Error in Resistance

    Homework Statement Homework Equations How to find error in resistance ? The Attempt at a Solution I found out the resistance without error. But What is the way to figure out an error of an instrument ?
  35. R

    Measurement problem and many worlds

    Hello, sorry if I created new thread that is already open, but I did not find answer. I would like to ask you about measurement problem (double slit experiment) and many worlds. When interference pattern is created, the dot on screen just show us in which branch or world we are. But if we...
  36. jk22

    Measurement problem and computer-like functions

    Suppose we define the measurement of an observable A by v(A) v being an 'algorithm giving out one of the eigenvalues each time it is called' (we accept the axiom of choice) In this context we have in particular v(A)≠v(A) since when we call the left hand side and then the right handside the...
  37. atyy

    Measurement problem in classical probability

    This question is inspired by ephen wilb's https://www.physicsforums.com/threads/quantum-theory-of-others.816818/#post-5130620. In Bohmian Mechanics, everything is exactly as in classical probability theory. How, from the point of view of BM, does the measurement problem arise? Since the...
  38. M

    Quantum Measurement Problem: Nature of Matter or Nature of Probability?

    The objective of this post is to: (i) generate a discussion of whether results of the double-slit experiment address solely the nature of matter, or do the results also address the nature of probability; and (ii) to determine if there have been any comparably structured experiments outside of...
  39. K

    Measurement Problem & Objectivity

    I have never been to the pyramids in Egypt. I'd love to go one day. I am sure their configuration will be just as others have seen, and as the visitor map describes. Why is it that my experience will correspond exactly with others' experience if I am collapsing quantum potential with respect to...
  40. D

    Pump drive torque measurement problem

    Hi all! Basically I will soon be required to measure the torque required to drive a plunger pump which operates by an eccentric cam on the pump's driveshaft pushing a plunger up and down. So obviously the torque will vary through the driveshaft's revolution. The setup is as follows: Motor...
  41. Feeble Wonk

    Decoherence relationship to the measurement problem

    Decoherence relationship to the "measurement problem" I have heard the argument that "decoherence" in quantum states causes the quantum collapse to occur, and that this solves the "measurement problem". But I'm still left with a nagging question... Does decoherence only produce quantum collapse...
  42. Quarlep

    What is the meaning of a measurement expressed as 2.0 cm -/+ 0.05 cm?

    Hi I saw something in Wikipedia and I am confused. I saw something like this 2.0 cm -/+ 0.05 cm Whats that mean? My first opinion Between 1,5 and 2,5 My second opinion 1,5 cm or 2,5 cm
  43. bhobba

    Interesting View Of Quantum Mechanics and The Measurement Problem

    I have been going through the following interesting paper on the foundations of Quantum Mechanics: http://arxiv.org/pdf/0911.0695v1.pdf 'We define the state of a system as that mathematical object from which one can determine the probability for any conceivable measurement. Physical theories can...
  44. S

    Does Conscious Observation Influence Quantum State Collapse?

    Do you believe that "conscious" observation has anything to do with "collapsing" superimposed quantum states, or do you simply believe that interaction with a macroscopic, or sufficiently large system, is what causes superimposed quantum states to collapse? Why?
  45. S

    Vapor Pressure Measurement Problem

    Hi all, Just have a quick question regarding the measurement of vapor pressure. Firstly, let me explain my scenario. I am carrying-out some measurements of steam condensation in a vacuum. The steam is being condensed in a air-cooled heat exchanger. Initially, I let in a quantity of steam...
  46. P

    Does Decoherence Solve the Measurement Problem Completely

    As the Title describes, Is the measuremet problem completely solved by the decoherence Program? In specific I would like the following question addressed. Is there is clear explanation as to what it means to Record Infromation? Can it explain the behaviour of a photographic plate? What happens...
  47. O

    General/special relativity-a measurement problem

    [b]1. The problem statement: all variables and given/known Describe a Non-Local Expirement that will lead to the conclusion that a body is in fact in a revolving system and not a system of constant acceleration. Non-Local meaning that the expirement uses 2 measurement instruments at 2...
  48. D

    Measurement problem query

    In the measurement ansatz below, must the state |O,ready> be orthogonal to |O,reads up> and |O,reads down>]? |+x> |O,ready> =1/(sqrt2)[|+z> + |-z>] |O,ready> (1) ---------→ 1/(sqrt2)[|+z> + |-z>] |O,ready> (2) --------→ 1/(sqrt2)[|+z>|O,reads up> + |-z>|O,reads...