What is Qm: Definition and 1000 Discussions

The hybrid QM/MM (quantum mechanics/molecular mechanics) approach is a molecular simulation method that combines the strengths of ab initio QM calculations (accuracy) and MM (speed) approaches, thus allowing for the study of chemical processes in solution and in proteins. The QM/MM approach was introduced in the 1976 paper of Warshel and Levitt. They, along with Martin Karplus, won the 2013 Nobel Prize in Chemistry for "the development of multiscale models for complex chemical systems".

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  1. B

    Why didn't radioactive decay probabilities cause the same uproar as QM

    It is equally puzzling why we are confined to probability amplitudes for RD as in QM measurements. Newtonian determinism is undermined in both, so why were there still Newtonian determinists around when QM hit the scene? We still have deterministic equations for both ofc but they are limited to...
  2. T

    B Tunneling through possible worlds

    Forgive my poor lingo. As my bio states, I only have a complete degree in HS. Is it possible to quantum tunnel across possible worlds? If so, is there a hyper-dimensional plane where this would be made possible? And if so, how would you conceptualize or describe it? Information breakage...
  3. R

    A Can I get electronic copies of the original QM papers?

    Hello, Is it possible to read the original, ground-breaking, pioneering papers of Quantum Mechanics - the papers by Max Planck, Niels Bohr, Albert Einstein, Louis De Brogile et al? In the form of electronic copy or a pdf format? I suppose the original 'copyrights' have expired and they should...
  4. entropy1

    I Is QM Deterministic in MWI?

    Is QM causal? Specifically, is it time reversable? Does that equate determinism? Specifically, in the case of MWI? Edit: Is QM deterministic? Specifically, in the case of MWI? Is it time reversable?
  5. F

    I It seems QFT and QM contradict with each other?

    In QFT a photon with arbitrary energy can interact with an electron.But in QM and Solid State Physics, if energy of photon smaller than energy gap of electron then photon can not interact the electron.So it seems to me there is a contradiction?
  6. nuclearfireball_42

    Why light reflected off of my calculator screen turns bluish?

    So, I was in class listening to my lecturer when I notice something intriguing. I was looking at the reflection of a lamp on the screen of my calculator. I paid close attention to the colour of the light reflected of my calculator and realized that when I rotate my calculator by about 90 deg...
  7. soothsayer

    I Questions about ensemble interpretation of QM

    I've been reading up on the ensemble interpretation (aka statistical interpretation) of QM and it's making a bit more sense to me that it did on the onset, but I still have some questions about how it is consistent with experimental observations of various QM experiments, especially...
  8. W

    Quantum Advanced-undegrad/junior post-grad books on QM mathematics

    Hi all, I was wondering if I could get recommendations for texts/notes that delve deeper into the mathematics of QM as well as QM itself than what one finds in standard "Intro to QM" texts. Ideally it'd be presented in the style of Nielsen and Chuang's Quantum Computation book, but in greater...
  9. fresh_42

    I Necessity of Complex Numbers in Quantum Mechanics

    Summary: Which properties of ##\mathbb{C}## are actually necessary? The following is speculative as well as a honestly meant question about the way QM is modeled. I don't want to create a new theory, just understand the necessities of the old one. Physicists use complex numbers for QM. But...
  10. DrChinese

    A How do entanglement experiments benefit from QFT (over QM)?

    I should first acknowledge 2 important points. I don't read papers on QFT, and therefore barely know how to spell it. And second, although I read many papers on entanglement (theory and experiment) I don't know if I have ever seen much reference to anything I might label QFT (that being...
  11. W

    I Is Quantum Mechanics Truly Time-Reversal Symmetric?

    I've bumped into a few interesting papers talking about time-reversal symmetry in QM (eg: https://arxiv.org/abs/1507.07745) but I can't seem to wrap my head around the concept. 1) What does it mean for one to say that standard QM isn't time-reversal symmetric? Does this have to do with the...
  12. H

    GR & QM: When Do They Disagree?

    The exact formula for gravitational time dilation is T_d = exp(m\phi / mc^2). (We usually cancel out the ms, but I want to show this as a ratio of energies.) Since m\phi is the potential energy, this says that the rates or frequencies of things in GR depend exponentially on their energy. On the...
  13. Auto-Didact

    A Collapse in QM: in conflict with SR?

    It is often claimed in these forums and beyond that collapse in QM is completely unnecessary for one of several reasons. To be explicitly clear, collapse seen purely as a probability theoretical concept of conditional updating - i.e. treating the wavefunction as an epistemic object - isn't...
  14. jdou86

    How do I understand spin in quantum mechanics for Physics GRE question 2?

    Summary: please help me understand the following questions from Physics GRE test Thank you very much! To be honest i really hate this formalism. Memorizing such things is pain. but like everything it is what it is
  15. W

    I Hermitian operators in QM and QFT

    I have always learned that a Hermitian operator in non-relativistic QM can be treated as an "experimental apparatus" ie unitary transformation, measurement, etc. However this makes less sense to me in QFT. A second-quantised EM field for instance, has field operators associated with each...
  16. Jamister

    I What is really that density matrix in QM?

    Summary: What are the basic assumptions of QM about the density matrix? The subject of density matrix in quantum mechanics is very unclear to me. In the books I read (for example Sakurai),they don't tell what are the basic assumptions and how you derive from them the results of the density...
  17. I

    Finding the Probability for a Particle in an Infinite Potential Well

    Homework Statement: Hi, I am self-studying QM and was wondering if you could check to see if my answers are right to an online exam I took. There are no solutions availabel Homework Equations: See below A particle with mass m is in an infinite potential well of length x=0 to x=a Q1a. Show...
  18. H

    I Why we need completeness of eigenfuntions for QM to be internally consistent?

    For quantum mechanics to be internally consistent, the following must be true: In other words, it is not alright for the solution space (to the Schrodinger equation) to span only a subspace of the Hilbert space. It has to span the entire Hilbert space. But why? What's inconsistent about it? I...
  19. Lynch101

    B Questions on QFT & QM: Is QM or QFT Absolute Time?

    It was recommended that I start separate threads, as I have quite a number of questions on QM & QFT. I'm including all the relevant information/quotes in this thread just for the sake of reference, but there are fewer questions. It might seem like an excessive amount of information but it's all...
  20. DarMM

    Quantum Interpretations of QM in different countries

    More a sociological question. I was talking to a former Japanese colleague by email recently and he mentioned that the Copenhagen interpretation is almost universally accepted there. I already knew Huzihiro Araki and many other researchers from Japan in mathematical field theory take a fairly...
  21. Lynch101

    B QFT & QM Questions (about time, locality, gravity, etc.)

    Apologies if there are already an abundance of threads on related questions. I have tried searching for threads on here and have read quite a few, as well as reading other sources. I've kind of reached a point where I need help to parse some of the information that I have been reading and to get...
  22. MichPod

    I Why observables are represented as operators in QM?

    Can somebody provide an explanation why the dynamical variables/observables are represented in QM as linear operators with the measured values being eigenvalues of these operators? For energy this is probably trivially and directly follows from the stationary Shrodinger equation which solutions...
  23. B

    Fourier transformation of the Wavefunction in QM

    Hello Physics Forum, I am not sure what to to in this task, because the wavefunction is only given as A_0. Maybe someone can explain it to me. Thanks in Advance, B4ckflip
  24. OldBeginnings

    I Isn't the local no-hidden-variable theory inconsistent with standard QM?

    Consider a pair of entangled particles described by a local theory without hidden variables. Because there are no hidden variables, the results of an experiment on one particle of the entangled pair must be perfectly random. Due to locality, the particles also have no way of coordinating the...
  25. QuasarBoy543298

    I Problem with the idea of identical particles in QM

    assume i have a gass made from N identical particles in a box and i want to calculate the probability for k out of N particles to be in the left side of the box. the problem is ,that if we treat the N particles as identical , each state in which exacly k of the N particles are in the left side...
  26. M

    I Can ambiguity in the word "measurement" allow contradictions in QM?

    Take 2 people P1 and P2. P1 claims that a Stern-Gerlach device collapsed an electron’s spin to + or - (mixed state if P1 doesn’t know which) while P2 may say it did not collapse, but instead remains in a pure, entangled state. If we continue this sort of thinking (2 people applying different...
  27. D

    A Soft photons and the ETH interpretation of QM

    As the thread on Jürg Fröhlich's paper on the "ETH interpretation" of QM has been closed, I want to consider here a specific point. Fröhlich points out that this interpretation requires the inclusion of massless particles and I think this is the key to getting a physical understanding...
  28. Jamister

    A How can we derive the results that we know about QM from QED?

    Summary: QED describes all physics under certain energy level. So how we derive results that we know from QM from QED? I want to know how to get quantum mechanics from QED (Quantum Electrodynamics). In QED we showed that the potential between two electrons goes like 1/r as in quantum...
  29. A

    I Let's talk about the classical limit of QM

    The classical limit of QM that have always puzzled me. There are common statement saying that you can recover classical mechanics by taking the limit of h->0 or by taking large quantum numbers. Other times times the Erhenfest theorem or the Madelung/hydrodynamics version of the Schroringer...
  30. F

    I Why are there so few experiments in the foundation of physics? Or am I wrong?

    Sabine Hossenfelder has a recent thread on her blog about experiments in physics, talking about how much money is spent on dark matter and what not. I actually wanted to open a thread even before that. My observation is that the field of experimental physics in foundation seem to be so thin and...
  31. SJay16

    B What’s more “important": GR or QM?

    Let’s say to the average Astronomer conducting research; generalizing the research to “astronomy” ; which would be more useful for the “average” Astronomer on a day to day basis: Quantum Mechanics or General Relativity? Obviously most are, but which so more? Which would be referred to more?
  32. F

    I Is this a new theory, an enhanced theory or just philosophy?

    I get new notification from new scientist usually about some virus or some weird anthropology theory. Once in a while a physics subject and this time I got this "https://www.newscientist.com/article/mg24132220-100-schrodingers-kittens-new-thought-experiment-breaks-quantum-theory/" Can somebody...
  33. F

    I Interpretation of QM in the Heisenberg Picture

    This is in reference to a question, never fully resolved, posed here: https://www.physicsforums.com/threads/interpretation-of-the-heisenberg-picture-in-qm.816449/ The von Neumann postulates for Quantum Theory - Evolution (Schrödinger's equation) and Projection (Born's rule) are always framed...
  34. Wrichik Basu

    I D'Alembertian in Sakurai's Advanced QM book

    In Sakurai's book "Advanced QM", he writes the Dirac equation (equation 3.31 to be exact) as: $$\left(\gamma _\mu \ \dfrac{\partial}{\partial\ x_\mu} + \frac{m\ c}{\hbar}\right) \ \psi= 0$$ which we can write as $$\left(\gamma _\mu \ \partial ^\mu \ + \frac{m\ c}{\hbar}\right) \ \psi= 0$$...
  35. C

    A QM Interpretations as applied to QFT

    [Moderator's Note: Thread spun off from previous discussion.] MWI and the TSVF are both easily generalized to QFT. One only needs to define the decoherence basis for the fields (see https://arxiv.org/abs/quant-ph/9510021) and then one simply interprets the decoherent histories as ontological...
  36. A

    A Are there conditions for the vanishing of geometrical phases in QM?

    Are there theorems for sufficient and necessary conditions for the vanishing of Berry and/or Wilzeck-Zee phases for a given quantum mechanical system?
  37. Q

    Book recommendations for learning QM

    Dear All, I am interested in learning quantum mechanics for curiosity. I know the basics from different sources and I've read the Quantum Mechanics (the theoretical minimum) book for Leonard Susskind and Art Friedman. What would you recommend as a book to further understand and learn quantum...
  38. Auto-Didact

    A Smolin: Realistic and anti-realistic interpretations of QM

    SPOILER WARNING: If you don't want to be spoiled about the book, stop reading now! I just finished the new book. First I would recommend the book to anyone who reads or takes part in discussions on QM foundations. Briefly put, Smolin offers a simple classification of almost all...
  39. I

    I am Imran, a retired Electrical Engineer with a QM question

    Hi , Could somebody explain how to DERIVE the canonical postulate [Xcap,Pcap]=ihbar ? The idea of Pcap operator's origin as -ihbar d/dx also is found puzzling .
  40. J

    I Hamiltonian in QM for QFT forces/fields effects

    In QM. Can you use the Hamiltonian (or kinetic plus potential energy) to describe the forces of nature that should supposedly use QFT or Lagrangian? I mean the fields have kinetic and potential energy components and what are the limitations in description and others if you only use them to...
  41. J

    A What is the relationship between QFT and QM?

    I know this question has been asked, in several ways, many times before. I have read many of the posts. And still I do not fully understand the situation: is QM in any way a subset of QFT? Apparently no: QM uses position variables, while QFT does not. QM has the Born rule and a wave function...
  42. P

    A No hidden variables in QM - a paradigm shift?

    If we would learn (somehow) that the pilot wave theory is false, that there are not even "non local" hidden variables, would this lead to a paradigm shift in physics? I ask because it would mean that a Laplace demon wouldn't even in theory able to predict the exact future motions of particles...
  43. E

    Quantum Textbook recommendations for self-studying QM

    I have a Physics degree, 1970, Durham University, England. I am semi- retired and have been re-studying using a modern text: Sears and Zemansky's University Physics updated by Young & Freedman. This has been brilliant. Plenty examples, questions and answers , ideal for self-study. I want to move...
  44. Demystifier

    A Proof that the thermal interpretation of QM is wrong

    Preface After a lengthy discussion of the thermal interpretation of quantum physics in https://www.physicsforums.com/threads/the-thermal-interpretation-of-quantum-physics.967116/ , now I think I can prove that it is wrong, i.e. that it doesn't solve the measurement problem in a way it claims it...
  45. J

    Good Textbooks for QM: Get Advice Here!

    I'm trying to do this by myself. I went through Dr. Susskind's 10 lecture series (the older one, not as much aligned with his book "Quantum Mechanics", which I own) taking notes, and am almost half way through with MIT Open Courseware's lectures in 8.04 (QM I) from 2013, again taking rigorous...
  46. Haorong Wu

    Courses Is Griffiths' QM sufficient before studying quantum computation?

    Hello, again. My current interest is quantum computation. I've finished Griffiths' QM for the first time. Because it only takes me a month studying the book, I may have study it superficial, so I plan to study it again and complete all the problems after each chapter. Then is this book...
  47. tomekrx

    Why do we take 4,5 and 5,13 instead of 4,6 and 13,15?

    Hello, can anyone tell me why do we take 4,5 and 5,13 instead of 4,6 and 13,15?Thats QM method
  48. LarryS

    I Do linear AND dispersive waves exist outside of QM?

    Outside of QM, do perfectly linear waves really exist in nature? I am referring to just those waves that also have dispersion - in which the wave components have differing phase velocities. ... or are we just using the mathematics of linear systems to approximate nonlinear systems? Thanks in...
  49. Demystifier

    A Complex Numbers Not Necessary in QM: Explained

    [Note from mentor: This was split off from another thread, which you can go to by clicking the arrow in the quote below] Actually they are not. See https://www.amazon.com/dp/3319658662/?tag=pfamazon01-20 Sec. 5.1.
  50. benorin

    B Would there be an upper limit to density in QM?

    Admitted I know very little about QM, but I've been thinking about black holes and I wondered if there would be an upper limit to density of an object of the smallest size allowable if the particles are not being observed by anyone (since black holes are black)? I ignorantly wondered that...
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