What is Quantum mechanics: Definition and 995 Discussions

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. MugiwaraNoSanji

    Quantum Quantum Mechanics and Quantum Chemistry books

    Hi, I've been self studying physics for a few years and I'm looking for some new books, specifically on Quantum Mechanics or Quantum Chemistry. It doesn't matter if they are PDF files or actual books, and price doesn't really matter. I've read Feynmans Lectures books and I'm looking for the next...
  2. N

    Why quantum mechanics looks so different from classical one?

    In many textbooks, the non-commutativity for the canonical pair is considered to lead to the major variaty from classical mechanism(CM) to quantum mechanism(QM), and change the Possion bracket into quantum commutator is a standard procedure called as canonical quantization. But in fact the...
  3. M

    How Does Light-Matter Interaction Challenge Classical and Quantum Views?

    I have just started reading about a classical electromagnetic treatment of light-matter interaction (beginning with dispersion relations, and then moving on to the standard phenomena - reflection, refraction, etc.). The discussion begins with a forewarning that light is not 'continuous' as the...
  4. R

    Quantum Mechanics: Wave Equation Probability

    Homework Statement Normalize the wave function $$ \langle x|\psi\rangle = \left\{ \begin{array}{l l} Ne^{-kx} & \quad x>0\\ Ne^{kx} & \quad x<0 \end{array} \right..$$ Determine the probability that a measurement of the momentum p finds the momentum between ##p## and ##p + dp## for this wave...
  5. T

    Elementary quantum spin in Sakurai

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  6. Z

    Relativity Books for relativity and quantum mechanics

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  7. R

    Quantum Mechanics: Translation and Wave Function

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  8. A

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  9. C

    Quantum Mechanics is a set of Laws

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  10. R

    Quantum Mechanics: Wave Mechanics in One Dimension

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  11. P

    Role of quantum mechanics in cosmology

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  12. Hamza Abbasi

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  13. J

    Requirements prior to Quantum Mechanics

    Hello! What are the requirements in physics and mathematics that somebody must have in order to start learning Quantum Mechanics by himself?
  14. B

    Exploring Quantum Mechanics & Time's Direction

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  15. T

    Stern Gerlach Spin 3/2, Quantum Mechanics

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  16. B

    Quantum Mechanics, time independant solution in Dirac notati

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  17. L

    Motion on a ring. Quantum mechanics.

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  18. Rimmonin

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  19. M

    Operators in quantum mechanics

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  20. T

    Reading list for learning Quantum Mechanics please

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  21. S

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  22. R

    Matrix representation in x and y basis for spin operators

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  23. R

    How can we compute expectation values for spin states using Pauli matrices?

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  24. R

    Quantum Mechanics: Angular Momentum Operators

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  25. W

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  26. R

    Quantum Mechanics: Probablities

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  27. Robsta

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  28. R

    Quantum Mechanics: Eigenstates of ##\hat{\mathbb{S}}_x##

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  29. R

    Quantum Mechanics: Three Spin Observables

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  30. R

    Uncertainty Relation for Eigenstate of Spin-1/2 Particle

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  31. G

    Formulation of quantum mechanics

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  32. kini.Amith

    The original postulates of quantum mechanics

    I am trying to find the basic list of postulates that lay the foundation for QM, but i see a different list of postulates in different textbooks and different places. In MIT lectures, Prof Allan Adams gives 3 basic postulates: 1. State of a system given by Wave function. 2. Mod squared psi gives...
  33. Greg Bernhardt

    Quantum Mechanics Video Lectures

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  34. mr_sparxx

    Deriving commutator for angular momentum components

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  35. O

    Pigments, EM radiation and quantum mechanics.

    An article in Wikipedia tries to explain pigments. One particular section has the following: "A wide variety of wavelengths (colors) encounter a pigment. This pigment absorbs red and green light, but reflects blue, creating the color blue." Questions arise... They may see stupid, but please...
  36. B

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  37. Ahmad Kishki

    Quantum Discovering an Unknown Quantum Mechanics Book: An Unconventional QM Journey

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  38. gfd43tg

    Quantum Mechanics learning resource videos

    Hello, I was wondering if there exists some videos that explain quantum mechanics. Something similar to Khan Academy, but that website only has introductory material. I am not looking for online lectures, but more like Khan Academy or some sort of step by step problem solving skills that is...
  39. R

    Quantum Mechanics: Linear and Circular polarization states

    Homework Statement Evaluate the matrix elements ## {\mathbb S}=\left( \begin{array}{cc} \langle x|\mathbb{\hat J}_z|x\rangle& \langle x|\mathbb{\hat J}_z|y\rangle\\ \langle y|\mathbb{\hat J}_z|x\rangle &\langle y|\mathbb{\hat J}_z|y\rangle\end{array}\right)## by expressing the linear...
  40. R

    Quantum Mechanics: Matrix Representation

    Homework Statement What is the matrix representation of ##\mathbb{\hat J}_z## using the states ##|+y\rangle## and ##|-y\rangle## as a basis? Homework Equations ##|\pm y\rangle =\frac{1}{\sqrt{2}}|+z\rangle \pm \frac{i}{\sqrt{2}}|-z\rangle##The Attempt at a Solution A solution was given...
  41. L

    Homogenous and linear differential equations

    Homework Statement I am currently in quantum chemistry, and in class one day my professor spent some time talking about Maxwell's equations. I am looking at my notes, trying to piece together Maxwell's equations, differential equations, and the principle of superposition, since this is not in...
  42. R

    Quick derivation question Quantum Mechanics

    Homework Statement Having one of those days where nothing makes sense, here is the derivation: how does he get the two circled terms from what he's derived above? Homework Equations N/A The Attempt at a Solution I tried substitution but get nowhere I just can't see how the early...
  43. R

    Quantum Mechanics: Transformation Matrix

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  44. LachyP

    Calculate Complex Conjugate of Ψ(x,t) for x=4, t=9

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  45. Ahmad Kishki

    Recommended References for Mathematical Methods in Quantum Mechanics

    I am self studying quantum mechanics from griffiths at the moment, and the formalism chapter (to me) seems to be an overview, and since most of the linear algebra i did was in real numbers, most of the stuff are new to me. So i want you to recommend a reference for math to go along with...
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