Quantum mechanics Definition and 994 Threads

  1. Demystifier

    I Ontology is to quantum theory what hardware is to computation theory

    In interpretations of quantum mechanics there are two types of physicists: those who care about ontology and those who don't. The ontologists, or realists, want to know what is the world made of. The non-realists, on the other hand, think that this question is not relevant to physics. Usually...
  2. P

    I Quantum Mechanics as a Probabilistic forecast of reality

    Is Quantum Mechanics a Probabilistic Forecast of nature?Someone I know told me their interpretation of QM is that QM only a probabilistic forecast of systems like electrons around atoms. I would like someone to analyse this interpretation and say if its valid or not. According to this person we...
  3. lektroon

    I Are Nicolas Gisin's Intuitionist Mathematics Theories Compatible with SR and GR?

    Hello, As a layman in physics, I wonder the ideas of people who have more knowledge in physics than I do about the theories of Swiss Physicist Nicolas Gisin and his arguments about the intuitionist mathematics. Is there a way to reconcile these ideas with more fundamental theories like SR and...
  4. graviton_10

    I Showing that operators follow SU(2) algebra

    For two quantum oscillators, I have raising and lowering operators and , and the number operator . I need to check if operators below follow commutation relations. Now as far as I know, SU(2) algebra commutation relation is [T_1, T_2] = i ε^ijk T_3. So, should I just get T_1 and T_2 in...
  5. L

    B Bell inequality test without polarisation?

    Was trying to understand the inequality test. The only article ever that I've found that explains it simply is the 1981 article, Bringing home the atomic world: Quantum Mysteries For Anybody. All other explanations require trust and understanding of polarisation, which is a huge deal. So i now...
  6. hachi44

    I Principles of Quantum Mechanics - Hans C. Ohanian - 0137127952

    Hi. I looked everywhere for a specific book but I cannot find any pdf copy of it. The book specifics are below: Publication Name: Principles of Quantum Mechanics Author: Hans C. Ohanian Publisher: Benjamin Cummings Publishing Company ISBN-10: 0137127952 ISBN-13L 9780137127955 I would...
  7. HadronPhysics

    A Numerical method to Lippman-Schwinger equation

    There is a question that puzzle me when I apply numerical method to principal value integral. Let me descibe it. We make use of the fact that the integral ##\int_0^\infty \frac{dk}{k^2-k_0^2}## vanishes, namely, $$ \int_0^\infty \frac{dk}{k^2-k_0^2} = 0 . $$ We use this formula to express a...
  8. H

    Why a particle with spin=0 can't posses a magnetic dipole moment?

    Hi, I would like to know why a particle with spin=0 can't posses a magnetic dipole moment? Using Wigner-Eckart theorem for ##\langle j,1,m,0|j,m \rangle## I get ##\langle j'|| \vec{J}|| j \rangle = \hbar \sqrt{j(j+1)} \delta_{jj'}## It seems like the right hand side is the magnetic dipole...
  9. H

    Probabilities of measuring ##\pm \hbar/2## along ##\hat{n}##?

    Hi, Given a spin in the state ##|z + \rangle##, i.e., pointing up along the z-axis what are the probabilities of measuring ##\pm \hbar/2## along ##\hat{n}##? My problem is that I'm not sure to understand the statement. It seems like I have to find the probabilities of measuring an eigenvalue...
  10. V

    A How to derive Quantum Mechanics in curved physical space?

    I am following [this YouTube lecture by Schuller][1] where he finds the appropriate formalism for the quantum mechanics in the physical curved space. Everything makes sense to me but at the very end I see that we find the pull backed connection one-form on the base manifold. He says to the end...
  11. O

    I Limit of quantum mechanics as h -> 0

    Starting from the Heisenberg equation of motion, we have $$ih \frac{\partial p}{\partial t} = [p, H]$$ which simplifies to $$ih \frac{\partial p}{\partial t} = -ih\frac{\partial V}{\partial x}$$ but this just results in ## \frac{\partial p}{\partial t} = -ih\frac{\partial V}{\partial x}## and...
  12. K

    B Searching for Quantum Mechanics Lecture on Delayed Choice Experiment

    Hi everybody. Some years ago I came across a video on youtube where they talked about an experiment with small and large envelops, when the small ones were placed into the large ones and then it resulted in something interesting. It might have been an instance of delayed choice, but I am not...
  13. J

    I Why is quantum mechanics not valid for large objects?

    It this correct explanation why quantum is not valid for large object? Why would small object want to hide their information?
  14. C

    A Multiparticle Relativistic Quantum Mechanics in an external potential

    It is often argued that Dirac Equation is not valid as relativistic quantum mechanics requires the creation of antiparticles. But, there are also some arguments that suggest otherwise. For example, I saw Arnold Neumaier's website on this that there are multiparticle relativistic quantum...
  15. CuriousLearner8

    A Eigenvalue Problem of Quantum Mechanics

    Hello, I hope you are doing well. I had a question about the eigenvalue problem of quantum mechanics. In a past class, I remember it was strongly emphasized that the eigenvalues of an eigenvalue problem is what we measure in the laboratory. ##A\psi = a\psi## where A would be the operator...
  16. H

    Unitary vector commuting with Hamiltonian and effect on system

    Hi, I'm not sure to understand what ##| \phi_n \rangle = \sum_i \alpha_i |\psi_n^i## means exactly or how we get it. From the statement, I understand that ##[U,H] = 0## and ##H|\psi_n \rangle = E_n|\psi_n \rangle## Also, a linear combination of all states is also an solution. If U commutes...
  17. yucheng

    I Fermi's golden rule: why delta function instead of density states?

    Sakurai, in ##\S## 5.7.3 Constant Perturbation mentions that the transition rate can be written in both ways: $$w_{i \to [n]} = \frac{2 \pi}{\hbar} |V_{ni}|^2 \rho(E_n)$$ and $$w_{i \to n} = \frac{2 \pi}{\hbar} |V_{ni}|^2 \delta(E_n - E_i)$$ where it must be understood that this expression is...
  18. N

    Average value of components of angular momentum for a wave packet

    I have typed up the main problem in latex (see photo below) It seems all such integrals evaluates to 0, but that is apparantly unreasonable for in classical mechanics such a free particle is with nonzero angular momentum with respect to y axis.
  19. H

    I Spinors and eigenspinors confusion

    Hi, While studying the spin 1/2, I'm facing some confusions about the spinors and the eigenspinors. I understand that ##\chi = \begin{bmatrix}a \\ b \end{bmatrix}## is the spinor with ##\chi_+ = \begin{bmatrix}1 \\ 0 \end{bmatrix}## and ##\chi_-= \begin{bmatrix}0 \\ 1 \end{bmatrix}## the...
  20. 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...
  21. Paul Colby

    A Recent papers refuting real-valued quantum mechanics

    So, in a rare instance I actually read APS News, I came across “New Experiment Suggests Imaginary Numbers Must be Part of Real Quantum Physics.” In November 2022, Volume 31, Number 10. Since complex numbers are isomorphic to a real 2x2 matrix algebra, I was confused how such a claim can be...
  22. H

    Allowed energy for a potential in quantum mechanics

    Hi, I'm working on a problem where I need to find the different energies allowed for a potential, and I found this link https://quantummechanics.ucsd.edu/ph130a/130_notes/node151.html, which is similar of what I'm doing. I'm using mathematica to find the values of E. However, I'm not sure how...
  23. Ashish Somwanshi

    Measurement problem quantum mechanics

    Is the answer 0.748?
  24. Q

    Quantum Advanced Quantum Mechanics Textbooks: Derivations of Equations

    Hi I’m looking for a textbook that shows the derivations of equations such as the different forms of the schrodinger equation fully and step by step.
  25. I

    Quantum Mechanics problem: Determine the value of the constant

    I have no idea where to start with this problem. I am interested in any hints, or ways to proof this. But i would especially like to know how the commutator is connected to the identity.
  26. 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
  27. S

    More Nobel prizes in quantum information?

    Alain Aspect, John Clauser & Anton Zeilinger have rightfully received the Nobel prize for their contributions to quantum information, as they were three of the main pioneers of quantum information. However, is it now impossible or very unlikely that other physicists working on this field (e.g...
  28. B

    Checking assumptions in boundary conditions of double well system

    The idea here (as I'm told) is to use the boundary conditions to get a transcendental equation, and then that transcendental equation can be solved numerically. So I'm making a few assumptions in this problem: 1. The potential ##V(x)## is even, so the wavefunction ##\psi(x)## is either even or...
  29. Graham87

    Condensed Matter Physics - Fermi velocity, etc.

    I have made solutions a-d, but my fermi velocity seems too big.
  30. H

    I Quantum mechanics stationary state

    Hi, I have hard time to really understand what's a stationary state for a wave function. I know in a stationary state all observables are independent of time, but is the energy fix? Is the particle has some momentum? If a wave function oscillates between multiple energies does it means that the...
  31. S

    I Energy from quantum systems in an expanding universe?

    I found a paper (https://arxiv.org/pdf/astro-ph/0411299.pdf) which talks about quantum systems emitting energy due to spacetime expansion. Is this true or only a hypothesis?
  32. sol47739

    I Postive rays in cathode ray tube experiments?

    I read in the following book A history of the sciences by Stephen F. Mason. About the discovery of the electron the write what I attached in the picture. I wonder what do these positive rays traveling in the opposite direction they talk about consist of? Some ions or what? I understand that the...
  33. pokespriter

    I About the position of electrons (uncertainty)

    Hello guys, I don't know if this is the right place to ask, so please be kind :/ I have a question regarding the location of an electron that belongs to an atom. A teacher told me that the probability of an electron to be found within its orbital is around 99%. When I asked about the remaining...
  34. sol47739

    I How Do Radio Antennas Function in Quantum Mechanics?

    In classical electromagnetism I think I have understood the following(please correct me if something is wrong): A charge produces an electric field, a charge moving with constant velocity produces a magnetic field, an accelerating charge emits electromagnetic radiation. In radio antennas this is...
  35. sol47739

    I Polarization of photons quantum mechanically

    What is it of the photon that gets polarized from a quantum mechanical perspective? In the classical perspective it is often thought that it is the oscillating electric field that gets polarized. But in the quantum case: Is it the de Broglie wave function? Or is it the spin and in case it is the...
  36. sol47739

    A Cause of spontaneous emission?

    I am reading this chapter 3 from the book called The Quantum Vacuum by P.Milonni.(Attached in the pdf, look at chapter 3.2 Spontaneous emission)There they say that spontaneous emission is due to both quantum fluctuations and radiation reaction. They say the transitions induced by the quantum...
  37. E

    I Understanding No Energy Degeneracy in Sakurai's Quantum Mechanics

    Hello, I'm hoping someone can help me understand a statement in Sakurai Modern Quantum Mechanics (3rd edition). In particular, in the section that describes free particle in infinite spherical well (page 198, section 3.7.2), after the text has shown that for a given ##l## value, the energy...
  38. Graham87

    Quantum Mechanics - Matrix representations

    I have found J^2 and Jz, but I am not sure how to find Jx and Jy. I’m thinking maybe use J+-=Jx+-iJy ? But I get unclear results. Thanks!
  39. R

    On the origin and the evolution of information in the Universe

    [Mentor Note -- thread moved from the schoolwork forums to the technical forums] Homework Statement:: Tentative Note and summary on the origin and the evolution of information in the universe. Relevant Equations:: none As a teacher of physics I got many questions asked by my students when...
  40. Graham87

    Intro to quantum mechanics - Spin and linear algebra

    So this expression is apparently in Sz basis? How can you see that? How would it look in Sy basis for example? The solution is following. They are putting Sz as a basis, bur how do you know that Sz is the basis here? Thanks
  41. Graham87

    Intro to Quantum Mechanics - Formalism normalisation

    I can't figure out how they get i/sqrt(2) for normalisation of c1. Why is it a complex number? If I normalise c1 I just get 1/sqrt(2) because i disappears in the absolute value squared. Thanks
  42. G

    I At which point is gravity inconsistent with quantum mechanics?

    I'd like to understand how gravity does not combine with quantum mechanics. At least there is no accepted theory of quantum gravity, so I assume it is not solved? I'm only starting to learn QFT and eventually GR. Maybe, someone can already outline where those theories fail to combine and comment...
  43. Graham87

    Plot the Expectation Value of Spin - Intro to Quantum Mechanics Homework

    I have found an answer to all of them (a-e) but I don’t know how to plot the function. Thanks!
  44. Graham87

    Intro Quantum Mechanics - Dirac notations

    I am learning Dirac notations in intro to quantum mechanics. I don’t understand why the up arrow changes to down arrow inside the equation in c). My own calculation looks like this:
  45. Graham87

    Quantum mechanics - Find S_x and S_y

    I have a lecture slide that shows how to find S_x and S_y. I get all the steps except the last row. Where did 1/2 come from? I think my linear algebra needs polishing. Thanks!
  46. J

    I Black hole singularity vs. quantum mechanics

    I'm wondering about some aspects about black holes (BH) and singularities, but since all my questions have to do mostly with quantum mechanics, I placed this thread in here. OK, let's assume there IS a singularity in the middle of a BH. A) Pauli exclusion principle (PEP) says no two fermions...
  47. D

    I "No objective reality" in quantum mechanics?

    As per title and the TL;DR, I'm curious if there could be some truth in these statements of the headlines I had read recently or are they just sensationalist fluff. Personally, I find these statements very hard to believe. In fact, impossible to believe. But I'm not a QM expert, not even an...
  48. Delta2

    I Earth's orbit not perfect ellipse

    Listen to the following arguments: Earth's orbit isn't perfect ellipse because classically there is the gravitational field of moon and possibly of Mars and Venus which affect it According to general relativity isn't perfect ellipse because there is the curvature of space time which doesn't...
  49. J

    I Dimensions of quantum cell automata's state space

    In the paper C. S. Lent and P. D. Tougaw, "A device architecture for computing with quantum dots," in Proceedings of the IEEE, vol. 85, no. 4, pp. 541-557, April 1997, doi: 10.1109/5.573 about quantum dots, it is stated that the basis vectors in the space of quantum states for a single cell...
  50. A. Neumaier

    A Is Quantum Mechanics Truly Underdetermined by Evidence?

    David Wallace, The sky is blue, and other reasons quantum mechanics is not underdetermined by evidence, Manuscript (2022). arXiv:2205.00568. From the Abstract: ''I argue that there as yet no empirically successful generalization of'' [Bohmian Mechanics and dynamical-collapse theories like the...
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