Quantum and general physics Definition and 151 Threads

Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.
Classical physics, the description of physics that existed before the theory of relativity and quantum mechanics, describes many aspects of nature at an ordinary (macroscopic) scale, while quantum mechanics explains the aspects of nature at small (atomic and subatomic) scales, for which classical mechanics is insufficient. Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large (macroscopic) scale.Quantum mechanics differs from classical physics in that energy, momentum, angular momentum, and other quantities of a bound system are restricted to discrete values (quantization), objects have characteristics of both particles and waves (wave-particle duality), and there are limits to how accurately the value of a physical quantity can be predicted prior to its measurement, given a complete set of initial conditions (the uncertainty principle).
Quantum mechanics arose gradually from theories to explain observations which could not be reconciled with classical physics, such as Max Planck's solution in 1900 to the black-body radiation problem, and the correspondence between energy and frequency in Albert Einstein's 1905 paper which explained the photoelectric effect. These early attempts to understand microscopic phenomena, now known as the "old quantum theory", led to the full development of quantum mechanics in the mid-1920s by Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Max Born and others. The modern theory is formulated in various specially developed mathematical formalisms. In one of them, a mathematical entity called the wave function provides information, in the form of probability amplitudes, about what measurements of a particle's energy, momentum, and other physical properties may yield.

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  1. SunRay-dvsh

    A Why light beams attract or repel each other even when they don't have charge

    Hi, Can someone please explain as to why light beams attract or repel each other even when they don't have charge. Seems like it behaves like two current carrying parallel wires. There is very little material about this which goes completely above the head. Thanks
  2. T

    Expectation of energy for a wave function

    Homework Statement At ##t = 0##, a particle of mass m in the harmonic oscillator potential, ##V(x) = \frac1 2 mw^2x^2## has the wave function:$$\psi(x,0)=A(1-2\sqrt\frac{mw} {\hbar} x)^2e^{\frac{-mw}{2\hbar}x^2}$$ where A is a constant If we make a measurement of the energy, what possible...
  3. T

    Studying Quantum Engineering: Embracing the Future with Cutting-Edge Technology

    Hello. I am a new member of this forum. My name's Patrick and I'm 1st year Wrocław University of Science and Technology student. I decided to study, as in title, quantuum engineering, because I think, saying succinctly, everything in the future will be based on quantuum technology - AIs, nanobot...
  4. Aaron M

    Thinking about getting an MSc in CS

    Hello all, I have recently graduated with a BSc in physics and have been accepted into the MSc program for Applied Physics. However, due to the job outlook for the position I actually want to do I am deeply interested in pursuing my second interest, which is, AI. Question: My main question is...
  5. P

    Calculation of no. of spectral lines for group of similar atoms

    Homework Statement The maximum no of spectral lines for a single atom during it's electron's transition is given by [∆n(∆n+1)]/2 . But I don't seem to arrive at the answer when a group of atoms are present . The question was - What is the maximum number of spectral lines possible for Balmer...
  6. I

    Quantum QFT: groups, effective action, fiber bundles, anomalies, EFT

    Hi, I am looking for textbooks in QFT. I studied QFT using Peskin And Schroeder + two year master's degree QFT programme. I want to know about the next items: 1) Lorentz group and Lie group (precise adjectives, group representation and connection between fields and spins from the standpoint of...
  7. M

    A How can quantum mechanics contribute in a Nuclear reactor

    Hi, does treating neutrons as waves make any difference when it comes to micro cross sections ? , does quantum mechanics help more than classical mechanics in nuclear reactor physics ? Noticing that I am a nuclear engineering student
  8. S

    I Are quantum harmonic oscillators and radiation modes equal?

    Are quantum harmonic oscillators, vacuum fluctuations and radiation modes the same? Is vacuum energy caused by vacuum quantum fluctuations?
  9. D

    Exploring the Wonders of Physics: A High School Student's Journey

    Hi guys! I'm a high school student and I love Physics. I hope this site will help me learn and understand a lot of (new) things! :)
  10. T

    Deriving the ionization energy of a hydrogen atom

    Hi, I've just gotten started with basic quantum physics in physics class and we've just talked about ionization energy. It is stated that the energy of a hydrogen atom is -13.60eV (or -2.179aJ). I assume this is the potential energy (and that this is the reason the atom has a lower mass than the...
  11. Boosh

    How to Derive the General Solution for the Hydrogen Radial Equation?

    I am going through my Quantum textbook, just reviewing the material, i.e. this isn't a homework question. We are solving the radial equation for the Hydrogen Atom, first looking at the asymptotic behavior. My issue is I am completely blanking on how to solve the differential equation...
  12. J

    A Is the pole in this integrand integrable?

    I am trying to numerically integrate the following complicated expression: $$\frac{-2\exp{\frac{-4m(u^2+v^2+vw+w^2+u(v+w))}{\hbar^2\beta}-\frac{\hbar\beta(16\epsilon^2-8m\epsilon(-uv+uw+vw+w^2-4(u+w)\xi...
  13. J

    Operator in three level system -- Eigenvalues/Eigenvectors

    There is an operator in a three-state system given by: 2 0 0 A_hat = 0 0 i 0 -i 0 a) Find the eigenvalues and Eigenvectors of the operator b) Find the Matrix elements of A_hat in the basis of the eigenvectors of B_hat c) Find the Matrix Elements of A_hat...
  14. T

    Rapidly changing Hamiltonian and an observable

    Homework Statement Consider an experiment on a system that can be described using two basis functions. We begin in the ground state of a Hamiltonian H0 at a time t1, then rapidly change the hamiltonian to H1 at the time t1. At a later time tD>t1 you preform a measurement of an observable D...
  15. N

    I Do quantum linked rings (quantum knots) have energy?

    How can we harness that energy? Is there any engine, heater, battery or other thing that uses it?
  16. ThunderLight

    B What is the Quantum of Sound in Gases and Air

    It is usually referred to as Phonons for sound waves in solid. But, where it gets confusing, is in gases and air. Some still call it Phonons, others say, Phonons can only be used in solid states. So what is the Quantum of Sound in Gases/Air? And if possible, refer to any text which speaks of...
  17. S

    I Which ψ do I use for the Expectation Value ?

    I have to calculate the Expectation Value of an Energy Eigenstate : < En > The integral is ∫ ψ* En ψ dx I have : A ) ψ = √L/2 sin nπx/L , a single standing wave of the wave function B ) ψ = BsinBcosD , the wave function of the particle C ) ψ = ΣCn ψn = C , sum of all the...
  18. B

    How to calculate relative effective mass?

    Homework Statement A simplified E versus k curve for an electron in the conduction band is given. the value of a is 10 Å. Determine the relative effective mass m* / m0. Homework Equations [/B] m* = ħ2 / (d2E /dk2) m0 = 9.11 E-31kg 3. The Attempt at a Solution The second derivative must...
  19. N

    A Hologram theory and quantum physics = existential crisis

    Hello, here there are 2 things (one about hologramic universe and the other about a quantum physics test similar to the slit lamp. The first one is about the Bohm and Pibram as well as one about Alain Aspect 1982 study: articles about their theories...
  20. shihab-kol

    Speed,mass,energy and a question.

    Suppose instead of moving at the speed of light , someone moves at a speed of 2*10^8 m/s .Then will his mas and energy exist simultaneously ? I mean mostly energy and some mass ?
  21. Z

    A Atom-Light Interaction: Understanding d.E vs p.A Hamiltonian

    I am reading Cohen-Tannoudji's Atom photon interactions (2004 version), in the Appendix he explains that for atom-light interaction, the electric dipole Hamiltonian (d.E form) is got from the original, "physical" (in line with his language) p.A form Hamiltonian by a time-independent unitary...
  22. J

    A How Do Time Correlation Functions Reveal Particle Dynamics in Quantum Mechanics?

    What exactly does class of time correlation functions measure of particle in quantum mechanics? In particular, I would love to know the answer in the context of Kubo-transform time correlation function.
  23. G

    I What is the physical interpretation of eigenstates in quantum mechanics?

    Hey everyone, I've been doing some quantum mechanics but I think I have yet to fully grasp the meaning of eigenstate. What I mean is, I understand that an eigenstate ##x## is such that, if we have an operator ##\hat{A}##, it satisfies ##\hat{A} x=\lambda x## and so ##\hat{A}## represents a...
  24. Q

    Introduction to Quantangular Moe

    Greetings everyone! I have been trained as a mechanical engineer, but I retain a strong interest in quantum theory and read what I can. In case you didn't know, my handle is a reference to Richard Feynman's comment on angular momentum from The Feynman Lectures on Physics. I look forward to...
  25. munirah

    How to differentiate GHZ state and W state?

    Good day, I try to differentiate GHZ-state and W-state using three tangle. Suppose The value three tangle for GHZ-state equal to 1 while W-state equal to 0. I used three tangle formula, $$\tau_{ABC}=\tau_{A(BC)}-\tau_{AB}-\tau_{AC}=2(\lambda^{AB}.\lambda^{AB}+\lambda^{AC}.\lambda^{AC})$$...
  26. munirah

    What is the Relationship Between Concurrence and Three Tangle in Quantum States?

    Good day, May anyone help me to confirm the formula use for three tangle using concurrence. From my reading, three tangle, $$\tau=\tau_{A(BC)}-\tau_{AB}-\tau_{AC}$$ and it can be related to concurrence $$\tau=C^2_{A(BC)}-C^2_{AB}-C^2_{AC}$$ and I used formula for C is...
  27. E

    A Is Gravity Directly Related to Groupings of Atoms?

    According to observations and known models mass is equal to the amount of atoms in a given space, and gravity is proportional to the amount of mass in a given space (ie. Sun > Earth > Moon) If those observations are undeniable then my question remains: is gravity directly related to groupings of...
  28. munirah

    A How to Multiply SU(4)XSU(2) Matrices to Form a 8x8 Matrix?

    From my reading, the X between SU(4)XSU(2) mean Cartesian product. But How the way to mutiply two matrix A in SU(4) and B in SU(2). Example the matrix A=\begin{pmatrix} a & b & c & d \\ e& f & g & h \\ i & j & k & l \\ m & n& o & p \end{pmatrix} and B=\begin{pmatrix} 1 &2 \\ 3 &4...
  29. munirah

    How can I make singular matrix become nonsingular matrix?

    << Mentor Note -- thread moved from Homework Help forums to General Math >> Good day, I run coding in Mathematica. But, I get singular matrix A at certain loop. In theory, how can I make matrix A become orthogonal A=\begin{pmatrix} 0& 0 & 0 & 0 & 0 & 0 & 0 & 0\\ 0& 0 & 0 & 0 & 0 & 0 & 0 &...
  30. M

    Temporary magnetization of Casimir effect plates?

    Have any experiments on the Casimir effect tried what happens if the plates are temporarily magnetized when they are pulled together by the effect? E.g. measurements of what happens to passing laser beams and the like?
  31. B

    I Schrödinger's Equation Infinite Potential Well

    Given the equation ##\frac{d^2 \psi (x)}{{dt}^2}+\frac{2m}{{\hbar}^2}(E-V(x))=0## the general solution is: $$\psi (x)=A_1 e^{ix \sqrt{\frac{2m}{{\hbar}^2}(E-V(x))}} +A_2 e^{-ix \sqrt{\frac{2m}{{\hbar}^2}(E-V(x))}}$$ If we have an infinite potential well: ## V(x)=\begin{cases} \infty \quad x\ge...
  32. munirah

    Term of Tripartite: Explaining Three Qubit System & Entanglement

    Good day, I get confuse about the term used. Can anyone help me to explain: Tripartite system. Three qubit system. Tripartite entanglement. I really need help for me clarify these term. Thank you very much for helping me.
  33. O

    I Harmonic Oscillator in 3D, different values on x, y and z

    Hi, For a harmonic oscillator in 3D the energy level becomes En = hw(n+3/2) (Note: h = h_bar and n = nx+ny+nz) If I then want the 1st excited state it could be (1,0,0), (0,1,0) and (0,0,1) for x, y and z. But what happens if for example y has a different value from the beginning? Like this...
  34. Nipuna Weerasekara

    B How is the Wave Function Derived in Quantum Mechanics?

    Everybody knows what is the Wave Function is. $$\Psi=\space e^{i(kx-\omega t)}$$ or $$\Psi=\space cos{(kx-\omega t)} \space - \space isin{(kx-\omega t)}$$ But can anyone tell me how it is derived. Since Schrodinger Equation is derived so easily using this Wave Function. I think it is necessary...
  35. munirah

    How to calculate negativity measurement of quantum state?

    Good day, From my reading according to negativity for tripartite state, it is given as below; $$N_{ABC}(\rho)=(N_{A-BC}N_{B-AC}N_{C-AB})^{1/3}$$ with $$N_{I-JK}=-2\Sigma_i\sigma_i(\rho^{TI})$$ where $$\sigma_i(\rho^{TI})$$ being the negative eigenvalues of $$\rho^{TI}$$, the partial...
  36. L

    I What oscillates in matter waves?

    like in sound waves pressure varies(oscillates) and in light electric and magnetic field oscillates
  37. A

    B How do I calculate electron acceleration by gravitational waves

    If the amplitude of gravitational waves, frequency of gravitational waves and the vector potential of magnetic field in surrounding of such waves are known then what would be the easiest way to calculate resultant acceleration of electrons? My above question is based on the various researches...
  38. munirah

    How can I do when I make a Log2 towards zero?

    Good day, Homework Statement I want to make a measurement on qubit by using formula von Neumann entropy using Mathematica given as below; Homework Equations (ρ)=−Tr(ρlog2ρ) The Attempt at a Solution The ρ={{0.5,0},{0,0.5}}My problem is, when I make the log2{{0.5,0},{0,0.5}} I get the...
  39. EngPhy

    A Effect of visible light on conductivity

    Are metals better conductors in the presence of visible light? Considering photons in visible spectrum are not energetic enough to induce emission from a metal surface but they do cause the ejection of 'conduction electrons' which are bound to the metal by a few electron volts.
  40. T

    I Two Identical non-entangled Particle System

    $$|\psi(x_1,x_2)|^2=|\psi(x_2,x_1)|^2$$ $$\psi(x_1,x_2)=+/-\psi(x_2,x_1)$$ How do they convert they former into the latter one? Is it due to the modulus? I know the latter can also be written as $$\psi(x_1,x_2)=e^{i\alpha}\psi(x_2,x_1)$$ where the exponential is the phase used to replace +/-...
  41. P

    I How are different potentials implemented experimentally?

    Hi. I'm wondering how different potentials, such as the Dirac-Delta potential, linear potential, quandratic potenial, etc., are implemented experimentally. I only understand how the Schrodinger equation is solved if these are the potentials and I'd like to have a better understanding of quantum...
  42. Q

    Name of Online Journal with Immediate Online Review

    Hello everyone. A Philosophy of Science friend showed a site where you can upload your paper and then you will be fairly quickly reviewed by your peers and this will be published alongside your paper. It seems to me this is a very good way to proceed; you may get assessment from the very top...
  43. 1

    B Where did the energy of red shifted CMB go?

    i know it sounds stupid, but i can't seem to find an answer to it where did the energy went? you can't just just destroy energy, but when you red shift a gamma CMB into a microwave CMB, the energy has go to go somewhere...? it's not like one gamma photo spitted into many microwave, cause that...
  44. Swapnil Das

    Reasons for Electrostatic interactions

    Hello. Well, each and everyone of us know that Like Charges repel, while unlike charges attract. That's a normal high school physics fact. But what's the reason for that? Is it just an experimental fact, or even does have theoretical significance? Please do help. Thanks a lot Swapnil Das.
  45. Abhishek Sethi

    I Time evolution of a wave function

    Hi, I just completed my second year of my physics undergraduate degree. And recently did a course on Quantum Mechanics. I have a few questions regarding the basic theory and postulates, probably, because due to lack of full clarity. So, Consider a wave function ψ(x,o), which is well behaved and...
  46. B

    Electron shell further away from nucleus higher Energy lvl?

    Using electrical potential energy =1/4πεo Q1Q2/r , a particle further away from nucleus has lower magnitude of energy Using coulomb's law, a particle further away from nucleus experiences weaker attraction, hence less energy is needed to maintain orbit* around that e-shell compared to a...
  47. B

    Why does an electron shell further away from nucleus has higher energy?

    Using electrical potential energy =1/4πεo Q1Q2/r , a particle further away from nucleus has lower magnitude of energy Using coulomb's law, a particle further away from nucleus experiences weaker attraction, hence less energy is needed to maintain orbit* around that e-shell compared to a...
  48. erbilsilik

    A What's meant by D dimensional gas?

    As Avagadro states: "One mole of an ideal gas at STP occupies 22.4 liters." This doesn't seems counterintuitive since we know that we need a volume to compress the gas. But how can we explain quantum ideal gases. Especially in my case, Bose gases. Since well-known example is Helium 4, we might...
  49. J

    How do I find eigenstates and eigenvalues from a spin operator?

    Homework Statement I have a spin operator and have to find the eigenstates from it and then calculate the eigenvalues. I think I managed to get the eigenvalues but am not sure how to get the eigenstates.Homework Equations The Attempt at a Solution I think I managed to get the eigenvalues out...
  50. I

    Showing f is a solution to quantum oscillator SWE

    Homework Statement For a 1-dimensional simple harmonic oscillator, the Hamiltonian operator is of the form: H = -ħ2/2m ∂xx + 1/2 mω2x2 and Hψn = Enψn = (n+1/2)ħωψn where ψn is the wave function of the nth state. defining a new function fn to be: fn = xψn + ħ/mω ∂xψn show that fn is a...
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