# What is Qm: Definition and 999 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. ### Other Which Springer books to buy? (QM, GR and statistical mechanics)

Hello, Springer books are on sale this week so I wanted to buy some textbooks to support my studies and (eventual) future career. I'm an undergrad (in europe) and my courses next year will be QM, GR and statistical mechanics, so I was looking for books about these topics, but any suggestion on...
2. ### I A Heuristic View of QM: Axioms & Gleason's Theorem

Strictly speaking, this isn't about interpretations; it's about whether there is a way to justify QM formalism more intuitively. I put it here because it may help when discussing interpretations. I will let others decide on its implications or even if it is a valid way of viewing it...
3. ### QM interpretations in fiction

I recently finished reading a current novel whose main plot device depended on the Many Worlds interpretation as well as Observer-Created Reality. I wasn't too put off because every science fiction - type story is allowed a certain degree of temporary suspension of disbelief. As I plowed on...
4. ### QM Eigenstates and the Notion of Motion

I'll risk a quick off-topic answer here, since I think it's straightforward QM, not vague "interpretation" stuff. :oldbiggrin: In QM (e.g., Ballentine p81), for a free particle, ##H = \frac12 \, M\, V\cdot V + E_0##. So in the ground state ##|E_0\rangle## we have ##H|E_0\rangle = E_0...
5. ### I Very basic questions about operators in QM

Could you tell me if I have understood following about operators in QM correctly? Wavefunction takes all generalized coordinates of the system as arguments. for example if we have a system of proton and electron (in 3-dimensional space) then the wavefunction of this system has 7 arguments...
6. ### A Can the double slit experiment distinguish between QM interpretations?

This paper claims that the double slit experiment can be used to distinguish between QM interpretations: https://arxiv.org/abs/2301.02641 IMO, the paper goes astray right at the start, when it points out that time is a parameter in the Schrodinger equation, not an operator, so that equation...
7. ### Understanding Complex Conjugates in QM (Griffiths pg. 13)

Am looking at page 13 of QM by Griffiths - have become stuck on minor point. He is proving that a normalised solution of Schrodingers eqn stays normalised. The bit I don't get is how can you just take the complex conjugate of Schrodingers eqn and assume its true. (ie how does he get from Eqn...
8. ### Matrix representation in QM Assignment -- Need some help please

This screenshot contains the original assignment statement and I need help to solve it. I have also attached my attempt below. I need to know if my matrices were correct and my method and algebra to solve the problem was correct...
9. ### A Peter Morgan (QM ~ random field, non-commutative lossy records?)

"One way to ground everything in reality is to think purely about the records of experiments that are stored in computer memory. Very often, that's a list of times at which events happened." -- Peter Morgan, old thread meaning-of-wave-function-collapse "If we are to understand the relationship...
10. ### I Einstein summation convention in QM

Hi For an operator A we have Aψn = anψn ; the matrix elements of the operator A are given by Amn= anδmn My question is : is this an abuse of Einstein summation convention or is that convention not used in QM ? Thanks
11. ### I Where does the QM creation operator get its energy?

We are all familiar with ladder operators, such as QM harmonic oscillators or in QFT to produce energy states which are interpreted as particles. But when a creation operator raises the energy level of a system, where does that energy come from?
12. ### B QM concept of photon.... still a bit of a mystery today?

Hi PF. I desire deeper understanding of fundamental physics, and quantum mechanics can be a challenge for me (compared to other topics in physics). I read in the intro of the physics textbook chapter that QM explains many phenomena, e.g. why copper conducts electricity and glass does not. In...
13. ### A How does Bohmian Mechanics actually replicate QM?

I was recently trying to understand how Bohmian Mechanics could model quantum theory. In an old lecture of Sidney Coleman's called "Quantum Theory with the Gloves off" available here: https://www.damtp.cam.ac.uk/user/ho/Coleman.pdf He shows with a "physicist's proof" that QM predicts truly...
14. ### B Is All Motion Discrete or Continuous in QM?

We were discussing how much weight it would take to stop the mechanism from rotating in this thread: https://www.physicsforums.com/threads/weight-required-to-hang-straight-down-with-known-torque.1016470/#post-6646777 I wondered if there were actually a range of weights that would stop it...
15. ### A QM and free will

I do not like superdeterminism because it makes reality Newtonian. However, the objection that superdeterminism means that there is no free will and free will is required for science is silly. Computers lack free will, but they still figure out what's going on. from the introduction to the...
16. ### Requesting guidance on Commutators & Intro QM

I have approached this question step by step as shown in the image attached. I request someone to please guide if I have approached the (incomplete) solution correctly and also guide towards the complete solution, by helping me to rectify any mistakes I may have made. I'm still unsure how to...
17. ### Question on Intro QM pertaining to Harmonic Oscillator

Hi. I have attached a neatly done solution to the above question. I request someone to please check my solution and help me rectify any possible mistakes that I may have made.
18. ### The correct interpretation of QM.... according to a "language model" AI

In the last few years, many of us have heard of GPT-3, a "language model" trained on terabytes of Internet prose, with a remarkable ability to generate essays, dialogues, and other original works. There is a similar but less powerful model called GPT-J which can be accessed freely at the...
19. ### I Realistic interpretation of QM

If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck. Why is there not a standard quantum mechanics interpretation that represents the most straight forward interpretation of what we see in experiments? By this, I mean: 1. Particles are real and...
20. ### Cracking Exams: Overcoming Knowledge Gaps for Intro QM

Summary:: I understand the consensus on PF about studying for knowledge and not merely for "cracking Semester Exams" but I urge you all to go through below thread before attaching to that feeling in my case. Hi. So I have my Exams on Intro QM approaching very soon, which will be a combination...
21. ### [Intro QM] Verification requested on possible solution for a question

Below I have attached an image of my possible solution. I have replaced all the relevant limits. For some reason, I am getting the final value for (i) part as ψ(x)= with an additional √2pi in the denominator. Have I made any errors or is it fine if I take it within the constant A.. In...
22. ### Normalizing Wavefunction: Hard QM Question)

So I have come up with my solution(attempt) which is: where ( $$\psi_ 1 \triangleq Asin(kx),0<x<L$$ $$\psi_ 2 \triangleq Be^{-sx},x>L$$ $$k \triangleq \sqrt{\frac{2mE}{\hbar^2}}$$ $$s \triangleq \sqrt{\frac{2m(V-E)}{\hbar^2}}$$) But this has a serious problem about boundary: I think...
23. ### Why does a free particle in an infinite well have uncertainty bigger than h/2 ?

So I think I use the right approach and I get uncertainty like this: And it's interval irrelevant(ofc), So what kind of wave function gives us \h_bar / 2 ? I guess a normal curve? if so, why is normal curve could be? if not then what's kind of wave function can reach the lower bound
24. ### B What sort of an experiment can refute QM or QFTs?

I assume most people when they refer to Quantum Theory, they sort of referring to QFT. So my question boils down to, what sort of an experiment could potentially refute QFTs (its plural because there are QF theories)?
25. ### 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...
26. ### I Odds of getting pairs -- QM version

As a spin out of How to calculate these odds? https://www.physicsforums.com/threads/how-to-calculate-these-odds.1012014/ I have a question for quantum objects. Question : An atom system has 16 different energy levels filled with 16 electron pairs of opposite spin direction. We pick up or...
27. ### I Does it make sense to speak about the Grandfather paradox in QM?

Since QM is not deterministic, the future state B is not determined by the previous state A (at time A, B was only a possibility, not a certainty). Then, when we are at time B, and assuming we could move back in time (of course, we cannot do that, but let us make a Gedankenexperiment), it just...
28. ### I What is the role of Relativistic QM?

In non-relativity then there is QM, but in relativistic regime then there is QFT. Then what is useful role of Relativistics QM nowaday, or it is only has a historical meaning?Does non-existance wave function in relativistic regime make RQM meaningless?
29. ### A Interpreting QM without Schrödinger's equation

Rovelli, in his recent paper, writes: https://arxiv.org/pdf/2109.09170.pdf Do you think Rovelli is correct in implying the wave function is redundant and misleading? If we disregard the wave function's significance, beyond convenience, say we forgot it even existed, how would that change (or...
30. ### I QM and mental states,sure this isn’t philosophical discussion

Various people like VN,Wigner thinks QM links to mental states,we know that. People introduced decoherence to kick consciousness out of the realm,as we widely accept.But even Zurek himself admits that there is a fundamental problem of why we only perceive one state and that solving the problem...
31. ### A About the rigged Hilbert space in QM

In Quantum Mechanics, how can you justify the use of distributions like the delta functional without introducing a rigged Hilbert space? I see that some texts do not make any reference to this subject.
32. ### Quantum Modern Quantum Mechanics 3rd Ed: J. J. Sakurai & Jim Napolitano Review

Is Modern Quantum Mechanics, 3rd Edition, by J. J. Sakurai and Jim Napolitano a good book to learn quantum mechanics from?
33. ### Quantum QM Book for non-physisists

Hi all, I have a simple question (apologies of this has been asked elsewhere, I am new here): I am looking for a source on Quantum Mechanics for non-physicists. A little more detail on what I am looking for: I am a PhD in Discrete Maths/Optimization and I come from a pure Computer Science...
34. ### I How does QM explain that we see electrons circulating in a magnetic field?

Hi Pfs. I think that QM can explain the classical things explained by classical physics. Using mean values and so on. We know that in a constant magnetic field an electron will rotate on a circle (at the macroscopic scale approximation) I have the answer for the Larmor precession but how to...
35. ### I Is formal QM correct in its entirety?

Can we tell, given the ample accuracy of formal quantum mechanics, that the (formal) theory is correct in its entirety?
36. ### I Problems with Paper on QM Foundations

I was on you-tube and saw a video from Oxford on QM foundations. I didn't agree with it, but that is not an issue - I disagree with a lot of interpretational stuff. The video mentioned a paper they thought essential reading...
37. ### What is the process of QM studies? (Not a simple question)

New physics guy.(This means totally unknowledged and ignorant.) I've been surfing google scholar,researchgate etc. Seeing many exotic articles,claming things like 1.unitary quantum mechanics fails to describe dynamics,so is ruled out. 2.quantum randomness is illusional! 3.we can recover...
38. ### I New papers on randomness and QM, need insight please

Hi,new college guy here,previewing next year's work. Been seeing these articles,written by reputed physicists. 1. https://www.mdpi.com/1099-4300/23/5/519 this one claims that quantum randomness is false.does this argument be strong enough to rule out QM's so-called randomness?I understand that...
39. ### I Scientific realism and QM

I read in an article that "Quantum physics is a highly mathematical theory that describes the nature of reality at the atomic and subatomic level". I also read another article that says quantum physics does not tell anything about reality. Can you give me some context about it in a way that is...
40. S

### I Does QM offer a solution to Zeno's paradoxes?

I understand that QM identifies the nature of the universe as being granular (discrete?) rather than continuous. In Xeno's race, it seems Achilles problem is he must divide distance an infinite number of times before chasing down that speeding tortoise. But I understand that 'Plank Length' is...
41. ### B Is an experiment planned to discern determinism and randomness in QM

I can remember reading something about a future experiment which alledgely could decide if there is an underlying deterministic layer governing quantum phenomena or if pure, empty chance rules suppreme (which I can't imagine). It had something to do with arrival times but I can't imagine how...
42. ### Variational approximation and QM

Hello. I should find the energy aproximatelly using the variational approximation for this physical hamiltonian: ##bx^4 + p²/2m## Imediatally, i thought that the better trial wave function would be the one correspondent to the ground state of the harmonic quantum oscilator. THe problem is, in...
43. ### Angular momentum and QM

I was thinking a little about how the absorption of angular momentum occurs from the point of view of QM. For example, suppose we have an atom A and an electron $e^-$. The electron $e^-$ is ejected from a source radially in direction of the center of the atom. Suppose that the atom has net...
44. ### I QM: Interesting View - Get the Inside Scoop

I recently came across the following lecture on what QM is: I was shocked by how close it is to my view. What do others think? Thanks Bill
45. ### I Relational QM Example, Contradiction?

I started reading about the “relational” interpretation of QM (RQM). I’m stuck on what appears to be a contradiction. The simplest way I can think to explain it is with a slight complication added to the Wigner’s friend experiment, adding an extra operation after the friend F measures the...