What is Quantum and general physics: Definition and 149 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. Vamsi9955

    B How can string theory be proved

    The fundamental building blocks of the universe is thought of super strings, if proved can solve the mysteries of the universe but if proved than how? And how can it solve the mysteries of dark energy &dark matter and black holes?
  2. Vamsi9955

    I The energy released from antimatter annihilation -- New uses?

    The pure energy released from antimatter annihilation can there be any use of this clean energy for research purposes like the use of light in LIGWO for gravitational waves study can we use it efficiently like light in LIGWO I think we can and it could lead us to reveal mysteries of universe
  3. H

    A What is the functional representation of D(E) for a given energy interval?

    Data = np.array([-1.61032636, -1.23577245, -0.50587484, -0.28348457, -0.18748945, 0.4537447, 1.2338455, 2.13535718]) print("Data is: ", Data) print(Data.shape) n,bins,patches = plt.hist(Data,bins=4) print("n: ",n) print("bins: ",bins) plt.savefig("./DOS")
  4. P

    A Relativistic derivation of E=1/2MV^2 from QFT or Diriac or other

    It is easy to derive E=1/2mv^2 from the Schroedinger equation for the nonrelativistic one dimensional case where e^ipx-iEt/\hbar is the free traveling wave function: i\hbar x -iE/\hbar x e^ipx-iEt/\hbar = - - \hbar^2/2m x p^2/2m x e^ipx-iEt/\hbar which reduces to E=1/2mv^2 Where should I start...
  5. P

    Finding the unitary matrix for a beam splitter

    Hello, I have some trouble understanding how to construct the matrix for the beam splitter (in a Mach-Zehnder interferometer). I started with deciding my input and output states for the photon. I then use Borns rule, which I have attached below: To get the following for the state space...
  6. C

    Courses Course advice for an enthusiast

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  7. H

    B Taking Quantum Computers into Space

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  8. GANTI_RAVITEJA

    I One-Dimensional System: Boundary Condition Applicability

    In one dimensional system the boundary condition that the derivative of the wave function Ψ(x) should be continuous at every point is applicable whenever?
  9. Zeynel

    I Transistors and Quantum Physics

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  10. L

    Help with finding the expectation value of x^2

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  11. M

    What is quantum physics and how can I learn more about it?

    So I love Quantum but unfortunately I don't have much idea of what its about. I would love to grasp the concept better. Can anyone discuss it with me?
  12. C

    I Why limited discussion of only looking at history

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  13. Hawkingo

    I Want some suggestions for research papers on quantum entanglement

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  14. Sophrosyne

    B How to think of molecular orbitals quantum mechanically

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  15. Danny Boy

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  16. C

    B Treating a galaxy as a quantum system

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  17. kenneththo85431

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  18. Danny Boy

    A What is quantum synchronization?

    Can someone provide a clear basic description or reference to how the phenomena of synchronization manifests at the quantum scale? Thanks.
  19. Jaden159

    A Is energy contained in matter wave equals hv like EM waves?

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  20. Dennis Plews

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  21. twist.1995

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  22. twist.1995

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  23. Bobs

    Quantum version of Larmor precession

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  24. S

    I What exactly is Weizsäcker's ur-alternatives theory?

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  25. meyol99

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    Can someone explain the meaning of the k vector and the general meaning of this equation?
  26. HastiM

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  27. patric44

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  28. S

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  29. Safder Aree

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  31. B

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  32. Richard Feynman - Character of Physical Law lecture series: Lecture 1 - The Law of Gravitation

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  33. Richard Feynman - Character of Physical Law lecture series: Lecture 2 - The Relation of Mathematics to Physics

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  34. Richard Feynman - Character of Physical Law lecture series: Lecture 3 - The Great Conservation Principles

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    Lecture Series: Character of Physical Law
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  35. Richard Feynman - Character of Physical Law lecture series: Lecture 4 - Symmetry in Physical Law

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  36. Richard Feynman - Character of Physical Law lecture series: Lecture 5 -The Distinction of Past and Future

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  37. Richard Feynman - Character of Physical Law lecture series: Lecture 6 - Probability and Uncertainty

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  38. Richard Feynman - Character of Physical Law lecture series: Lecture 7 - Seeking New Laws

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  39. Danny Boy

    A Probability of obtaining general quantum measurement outcome

    The Fundamental Theorem of Quantum Measurement is stated as follows: Every set of operators ##\{ A_n \}## ##n =1,...,N## that satisfies ##\sum_n A_n^{\dagger}A_n = I## describes a possible measurement on a quantum system, where the measurement has ##n## possible outcomes labeled by ##n##. If...
  40. I

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  41. Danny Boy

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  42. Danny Boy

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  43. Danny Boy

    A How Do Quantum Measurements Impact System Coherence?

    The following, regarding quantum measurement, is stated in the paper "Limitation on the amount of accessible information in a quantum channel" : "Our discussion of measurement will be based on a specific physical model of measurement, to which we now turn. Suppose we have a quantum system ##Q##...
  44. D fatima

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  45. SemM

    A What separates Hilbert space from other spaces?

    Hi, I have the impression that the special thing about Hilbert space for Quantum Mechanics is that it is simply an infinite space, which allows for infinitively integration and derivation of its elements, f(x), g(x), their linear combination, or any other complex function, given that the main...
  46. M

    Quantum Jump that produces an infrared line?

    1. A certain type of atom has only four energy levels, as shown in the diagram. The "spectral lines" produces by this element are all visible, except for one infrared line. The quantum jump that produces the infrared line is: (see attached file) A) state 4 to 3.B) state 4 to 1.C) state 2 to 1...
  47. Jianphys17

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    Hi at all. I'm wondered is necessary learn well analytical mechanic ( eg learning The Goldstein's book), for understand and study QM ?
  48. A

    Diatomic Hydrogen Molecule

    Homework Statement Consider an ##H_2## molecule where the protons are separated by a wide distance R and both are located on the z-axis. Ignoring the spin degrees of freedom and treating the dipole-dipole interaction as a perturbation, use perturbation theory to estimate an upper limit for the...
  49. SunRay-dvsh

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

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  50. T

    Expectation of energy for a wave function

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