Qm Definition and 1000 Threads

just for personal enrichment I going to do a second reading of QM. my first was 20+ years ago and found the topic interesting. my education was exclusive to position space and solving problem after problem using SWE with a bazillion different boundary conditions and Hamiltonians, my calculus is...

Hi. First off, I'm not sure if this is the right sub to talk about QFT. Apologies if t isn't. I'm halfway through an introductory QFT course and I still don't get what it is. What is different in its formalism that makes it able to tackle problems that quantum mehanics can't deal with? From...

A particle in a 1-D Hilbert space would have position basis states ## |x \rangle ## where ## \langle x' | x \rangle = \delta(x'-x) ## A 3-D Hilbert space for one particle might have a basis ## | x,y,z \rangle ## where ##\langle x', y', z' | x,y,z \rangle = \delta(x'-x) \delta (y-y') \delta(z-z')...

Hi everyone, Weinberg uses spatial translation invariance to derive the momentum operator. But the way he does it puzzles me. Here is an excerpt of the book. Equation 3.5.1 is the definition of the unitary operator ##U(x)## for translation invariance: $$U^{-1}(x)XU(x) = X+x,$$ with...

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >[/color] Problem is: If the behavior of ψ( r,t ) as r->inf is dominated by r-n, what values can n assume if the integral ∫A(ψ*∇ψ-ψ∇ψ*)⋅nda taken over the surface at infinity is to vanish. I...

Homework Statement Consider the double well potential with two wells separated by delta function in middle. V(x) = V0 > 0 for x<-a and x>a 0 for -a<x<0 and 0<x<a αδ for x =0 1. Find Bound state energies 2. Find odd solns and their eigenvalue equation. Give solns in...

... except one weak exception of the entanglement? Assuming that unitary QM (aka MWI) is a-local (c) Demystifier, and the ultimate reality is the Hamiltonian, somehow mapped into our 3D space, it is more surprising that the nature we observe is local. If would be more logical to expect multiple...

In the Hamilonian for an H2+, the kinetic energy of the electron (KE of nucleus ignored due to born-oppenheimer approximation) has a negative sign in front of it. I understand the signs for the potential energy operators but not for the KE apart from the strictly mathematical point of view. Can...

Hello everyone, I don't get how the second-order derivative ##\partial^2 S/\partial x_i \partial x_j## of the phase S arrives here. If one performs a power series expansion of the Hamiltonian around ##\nabla S##, then I do get where the first term ##A## comes from, but then adding higher-order...

In the thought experiment known as Schrodinger's cat a cat is placed in a sealed box, and its life or death is tied to an uncertain quantum event such as radioactive decay. If the radioactive particle decays, the cat dies. If not, nothing happens. According to probability collapse QM, as...

Hi everyone, I'm a bit puzzled by the derivation of this formula, in particular since the definition of the "overbar" notation is a bit fuzzy (see Formula 6.4.1). Does anyone have a more formal definition of the correlation function in this setting (I know what a CF is, in general)? In this...

Wondering what educators / researchers in the field think is the most important and logical flow of topics. The way I was instructed was solving Schrodinger's wave equation, SWE, for every possible problem that could be solved with a pen and paper in say 2 or 3 pages max. The problems were...

Suppose we have an observable with a certain number of eigenstates. We would normalize all these possibilities to 1 in order to give each eigenstate an appropriate probability of being measured. Can we then only consider the data of many measurements for only a subset of those eigenstates and...

I believe this question will be a no-brainer for many members. I want to conceptually grasp how do atoms (and molecules) behave using QM in contrast to the classical physics model. Now there's uncertainty principle and atoms don't have a fixed position and momentum, and that's fine, I know it's...

I'm from a country in Africa where I didn't have the privilege of learning the Standard Model, so I'll try and rephrase my question here: https://www.physicsforums.com/threads/why-is-the-electromagnetic-field-not-a-charge-field.857927/#post-5384284 In a way where my ignorance doesn't outrage...

<< Moderator note: Split from https://www.physicsforums.com/threads/why-do-we-need-quantum-mechanics-so-much.859210/ >> What about particle physics? It bases on QFT therefore on QM.Is that right?

Hi, I don't get how one goes from 3.6.17 to 3.6.18 on Page 80 (Galilean invariance) regarding the zeroing of boost generator commutators. I do get that this is a special case of the Lorentz invariance (which I understand), but this particular step eludes me. Thanks for your help.

Hi All, Was there any use of the concept of amplitudes of probability before their use in quantum mechanics? In connection to this question, who invented or was the first to use this resource? Best wishes, DaTario

Homework Statement Homework EquationsThe Attempt at a Solution I'm fine with parts a) and b) However I don't understand what part c) is asking me to do. How do I 'measure' an operator? There are only two things I can think to do: 1. Find the expectation values of A for <Φ1|A|Φ1> and...

Nielsen & Chuang list three axioms for QM. I paraphrase them as follows: 1. States are unit vectors. 2. The evolution of a state is unitary and given by the Schrodinger equation. 3. The measurement of a state yields a value from a probability distribution. The state just before the...

Is there already consesus in the (mainstream) scientific world that the various interpretations of QM can't and won't be reconsiled? (unless, perhaps, they find a completely different mathematical framework?) And what is the cause of that? :smile:

If I show you three face down cards (normal playing cards, so can be either a red or black suit), and tell you that no matter which two you pick, they would be different colors, I have no doubt you'll say I'm being weird (to put it mildly). Well, nature does pretty much the same thing. This is...

How do CRTs work well and electrons can be sent to exact location on screen in CRT monitors if electrons can behave like wave? Is there something in old TVs (for example measurement device) along the road that electron travels to avoid behave like wave?

Hi guys, I'm a newbie in quantum physics and it has blown my mind so far. I feel a bit confused because it seems very unintuitive, but I'm ready to learn more and I need some help on this forum. I have few questions. 1) I red that the wavefunction of an electron is spread all over the universe...

It is said the axiom of QM is observation.. but if observation is secondary effect of more primary dynamics that don't involve observations.. could it still be called QM, and what's it supposed to be called? In other words. Say QM is emergent from a deeper reality.. and we are to study the...

And back again with a strange/odd layman question: Actually, this remark of Jilang is a perfect illustration of what I am wondering about: It seems as if realism and locality are behaving like two eigenstates in a space of interpretations of QM. Most people seem to suggest that you must have...

Homework Statement In Zettili's QM textbook, we are asked to find the trace of an operator |\psi><\chi| . Where the kets |\psi> and |\chi> are equal to some (irrelevant, for the purposes of this question) linear combinations of two orthonormal basis kets. Homework Equations...

It’s easy enough to find the interpretations that break counterfactual definiteness in order to maintain locality. But I’m not sure how to find the ones with something (perhaps information) moving backwards through time. The reason I ask is I’ve always had trouble with the idea that a...

I will be taking a first course on Quantum Mechanics and just wanted to know what kind of ordinary differential equations must i know before going into the course. Thank you!

I am new to this forum and relatively new to learning physics itself. I have a very basic question. What is the motivation for looking for a unified theory of quantum gravity? Other than the aesthetics of having a unified understanding of reality, do we have any physical or mathematical reason...

Can we describe the intensity of spectral lines using Markov theory? No matter what is the initial state vector of the system, the final state will be reduced to a stationary vector whose elements represent the intensity of the spectral lines.

Homework Statement I am struggling with one of the end of chapter questions in my QM textbook (see attachment as I don't know how to show calculus on PF). It has thrown me because the chapter introduces some of the key principles in QM by talking about probability but then it randomly chucks in...

Reading through David Tong lecture notes on QFT.On pages 43-44, he recovers QM from QFT. See below link: [QFT notes by Tong][1] [1]: http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdfFirst the momentum and position operators are defined in terms of "integrals" and after considering states that...

Hi. I hope this is the rightplace to ask this. Sorry if it isn't. I'm looking for a resource (book, web page, etc) and I don't know if it exists so I'll describe it. I want something that lists actual experiments that complement or back up the topics listed in a typical QM textbook, so I can...

There is a group of researchers that say that there is a problem in QM (the spectral gap problem) that is undecidable in generalhttp://www.nature.com/news/paradox-at-the-heart-of-mathematics-makes-physics-problem-unanswerable-1.18983?WT.mc_id=TWT_NatureNews...

My layman understanding of the many-worlds interpretation of QM is that if a person had to make a decision between A and B and he chose A, then the universe splits into 2 branches, one where A was chosen and another where B was chosen. First of all, is this correct? Secondly, regarding the...

Whole my life I have been interested in Quantum Physics. I have a bachelor degree in IT. I did not finish my studies. I have always been quite sloppy in studying in school. As a result, my mathematics skills are terrible. I often get pointed out on this forum that I better start with the...

I just finished a first course in quantum mechanics based on Griffiths' textbook. I have to take an advanced qm course next sem which starts in Feb. I was planning to do some reading by then mostly on some specific topics like mathematical formalism of QM, spin and angular momenta, and identical...

I'm an undergraduate in Physics and just cracked open some QM books, some seem to start with polarization and a strange way of introducing operators, while others start with the wavefunction and then go into harmonic oscilators, then the hydrogen atom etc.. What does polarization have to do...

I am reading an article by Tachikawa on the Nekrasov partition function ("A review on instanton counting and W-algebras"). The article is meant to be pedagogical but I have some trouble with what is supposed to be "baby" examples. The first one involves susy QM on \mathbb{C}^2 . He says the...

I am wondering about the meaning of 'preservation of causality' in quantum mechanics. Is there causality in QM? And does it act back in time? I have some ideas of myself, but want to learn about the general accepted view first.

So far I got: Griffiths => easy, ok it's not so easy,not really what I ment, but it covers the basics without going much into the real deeper algebraïc stuff, emphasis on calculus, that's what I mean with easy, I mean a decent, solid book for undergraduate level. But suppose I tackle Griffiths...

Depending on interpretation of QM, can hilbert space be considered just as real as space time? In MWI the wave function is real, but still lies in hilbert space, so would hilbert space be considered a real space according to this interpretation?

Homework Statement I was reading this textbook: https://books.google.com/books?id=sHJRFHz1rYsC&lpg=PA317&ots=RpEYQhecTX&dq=orbit%20center%20operators&pg=PA310#v=onepage&q=orbit%20center%20operators&f=false Homework Equations In the equation of the page (unlabeled), we have $$...

Homework Statement Given a wave function psi, \psi (x) = A \sqrt{|x|} e^{- \beta x^2} where \beta is a constant (take the positive square root) . Normalise the wave function and hence find A. Homework Equations ? The Attempt at a Solution This is my first attempt at a problem like this, and...

Homework Statement a particle is in a linear superposition of two states with energy E_0 \ and\ E_1 |\phi> = A|E_0> + \frac{A}{(3-\epsilon)^{1/2}}|E_1> where: A \ > \ 0, \ 0\ <\ \epsilon \ <\ 3 What is the value of A expressed as a function of epsilon Homework Equations P(E_0) \ +\...

Hello folks! New to this forum, so hoping I'm not retreading old ground. The Pauli matrices are spin angular momentum operators in quantum mechanics and thus are axial vectors. But in Clifford algebra in three dimensions they are odd basis elements and thus polar vectors. Hestenes, Baylis, other...

I thought of posting this on astrophysics or general relativity forums since it seems completely classical, but given the extraordinary claim, I am posting here. Note that one of the guys is a notable string theorist. http://arxiv.org/abs/1510.03396v1 Journey Beyond the Schwarzschild Black...

SO(3) is subgroup of Poicare group.Does Relativistic Quantum Mechanics obey rotational symmetry.If it is,why we do not still keep the non-relativistic concept of angular momentum(orbit angular momentum plus spin) for relativistic concept of angular momentum,but we instead replace the concept by...

Is there a relationship between QM probability and normal distribution ? I'm thinking about drawing probability densities as functions of phase. Thanks