What is Quantum physics: Definition and 723 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. Demystifier

    A How ‘spooky’ is quantum physics? The answer could be incalculable

    A new mathematical result seems to have very important implications for quantum mechanics. https://www.nature.com/articles/d41586-020-00120-6?fbclid=IwAR0DxBdyEW10Te3C1Vfyn17TZshyCRrO8nX6D5Iq4CGahZ_DXgfeQ_Ii0Z8 The original paper is a very hard read: https://arxiv.org/abs/2001.04383
  2. CrosisBH

    Courses Taking both Classical Physics and Quantum Physics at the Same Time.

    Hello! I'll be entering my second semester of my junior year of my Physics Degree. My uni doesn't have a bad physics program, but it's really small so I have to take my upper level classes as soon as they're offered or I won't take them for another 2 years or so. They are on a cycle so I will be...
  3. C

    The Combination Of Philosophy And Quantum Physics

    Is there any evidence that Panpsychism may hold its own in the quantum world? What quantum physics principals violate the ideology of Panpsychism? This is meant to be fun. I am curious what other people think.
  4. T

    Finite quantum well, multiple choice question

    Homework Statement:: Consider an electron trapped in a one-dimensional finite well of width L. What is the minimum possible kinetic energy of the electron? A) 0 B) Between 0 and h^2/8mL^2 C) ≈h^2/8mL^2, but it is not possible to find the exact value because of the uncertainty principle D)...
  5. Killtech

    I Quantum physics vs Probability theory

    Because I do have a background in the latter it was originally very difficult for me to understand some aspects of QP (quantum physics) when I initially learned it. More specifically whenever probabilities were involved I couldn’t really make full sense of it while I never had any problems...
  6. E

    I Can a simulation hypothesis simulate quantum physics?

    From the point of view of science and physics, is the simulation hypothesis true? What is the scientific refutation of this hypothesis? What do physicists say about this?
  7. D

    B Linking Quantum Physics and Relativity: Theory of Everything?

    I am going to get laughed out of this forum but at least it will put my mind to rest when this happens. I've been thinking about this for a few years now and what better place for an answer than a physics forum? Amongst many other things I have read A Brief History of Time seven or eight times...
  8. warhammer

    Question on Quantum Physics- Probability of finding a particle

    I calculated the complex conjugate of both the given wavefunctions. For ψ1: ∫re^((-2)mod(r)x)dx=1 with upper limit ∞ & lower limit -∞. I replaced the upper and lower limit after breaking down the function inside integration as follows- r*∫e^(2rx)dx from -1/r to 0 and r*e∫e^(-2rx)dx from 0 to...
  9. Greg Bernhardt

    B Guidelines for Quantum Physics Forum

    Brief Description of New Quantum Forum Structure A common topic of discussion regarding quantum mechanics is the foundations of QM as a theory structure and different interpretations of QM. These discussions are of interest to many PF members, but they present some unique challenges since they...
  10. N

    B Classical physics vs quantum physics

    Is classical physics independent from quantum physics? Or is classical physics an approximation derived from quantum physics? Is it dependent on interpretations? What quantum interpretations support the latter above?
  11. A

    Studying Self-studying plan for modern science

    Hey guys, I want to build a strong and straight plan for my next years of studying and once finish I am able to do something on my own and come up with crazy ideas and actually test them, build some awesome algorithms, all that cool stuff, but I'm kinda stumble so it would be nice if someone...
  12. K

    Quantum The Philosophy of Quantum Physics by Cord Friebe

    Summary: CordFriebe et al. Has anybody read this book? I am wading through this tome for the second time. Was wondering what others whom might have read it thought. https://www.amazon.com/dp/3319783548/?tag=pfamazon01-20
  13. schwarzg

    A Simultanious eigenstate of Hubbard Hamiltonian and Spin operator in tw

    Please see this page and give me an advice. https://physics.stackexchange.com/questions/499269/simultanious-eigenstate-of-hubbard-hamiltonian-and-spin-operator-in-two-site-mod Known fact 1. If two operators ##A## and ##B## commute, ##[A,B]=0##, they have simultaneous eigenstates. That means...
  14. HaZe-

    Quantum physics (blackbody radiation)

  15. Sasho Andonov

    A Uncertainty vs Randomness in Quantum Physics

    In Quanta Magazine there is article on this link...
  16. Futurestar33

    Shelf in a box, treating the shelf as a weak perturbation

    In this problem I am supposed to treat the shelf as a weak perturbation. However it doesn't give us what the perturbed state H' is. At the step V(x) = Vo, but that is all that is given and isn't needed to determine H'. This isn't in a weak magnetic field so I wouldn't you use H'=qEx and then...
  17. P

    I According to quantum physics, can our universe be a simulation?

    What does quantum physics say about the simulation hypothesis?
  18. P

    Constant of proportionality in probability of superposition of states

    Using the fact that Pa ∝ |α|^2 and Pb ∝ |β|^2, we get: Pa = k|α|^2 and Pb = k|β|^2 Since the probability of measuring the two states must add up to 1, we have Pa + Pb = 1 => k = 1/(|α|^2 + |β|^2). Substituting this in Pa and Pb, we get: Pa = |α|^2/(|α|^2 + |β|^2) and Pb = |β|^2/(|α|^2 + |β|^2)...
  19. lastItem

    I Calculating Momentum Operator Matrix Elements from <φ|dH/dkx|ψ>

    Is there a relationship between the momentum operator matrix elements and the following: <φ|dH/dkx|ψ> where kx is the Bloch wave number such that if I have the latter calculated for the x direction as a matrix, I can get the momentum operator matrix elements from it?
  20. J

    How to find von-Klitzing constant based on graph?

    Hi all, Given that the question: From what i know , I am not sure how this equation can help me estimate the von-klitzing constant? Or is there another way? Thanks!
  21. B

    A LS vs jj couplings and their selection rules

    Two questions, where the 1st is related to previous discussion regarding thes couplings: The selection rules for LS coupling is quite clear - it's based on calculating the compatible electric dipole matrix element. However, in the case of jj coupling we end up with different selection rules...
  22. C

    I Confusion on binding energy and ionization energy.

    1) I know that the binding energy is the energy that holds a nucleus together ( which equals to the mass defect E = mc2 ). But what does it mean when we are talking about binding energy of an electron ( eg. binding energy = -Z2R/n2 ? ). Some website saying that " binding energy = - ionization...
  23. J

    I David Deutsch (1985) attempt to solve the incoherence problem

    Can anyone elaborate on Deutsch's attempt to solve the incoherence problem? He postulates a continuously infinite set of universes, together with a preferred measure on that set. And so when a measurement occurs, the proportion of universes in the original branch that end up on a given branch...
  24. A. Neumaier

    Insights A Classical View of the Qubit - Comments

    Greg Bernhardt submitted a new blog post A Classical View of the Qubit Continue reading the Original Blog Post.
  25. lekh2003

    How Does Energy Transformation Occur in Quantum Orbital Mechanics?

    Homework Statement Homework Equations I've used: ##mv^2/r## = Centripetal ##q^2/r^2## = Force pulling the electron in A bunch of other ones which I really can't be bothered listing. The Attempt at a Solution I managed to get Part A of the question using pretty simple methods. I set ##mv^2/r...
  26. R

    Does Evolution Occur in Quantum Jumps? Exploring a New Hypothesis

    Is it possible that evolution happens in quantum jumps as no intermediate lifeforms were ever found? Analogous to an electron jumping from lower energy level to higher energy level without intermediary states.
  27. A. Neumaier

    I The thermal interpretation of quantum physics

    A complete description of the thermal interpretation of quantum physics can be found in my just finished papers (for the bare bones, see Section 2.5 of Part II) Foundations of quantum physics I. A critique of the tradition, Foundations of quantum physics II. The thermal interpretation...
  28. F

    B Quantum entanglement phenomenon

    Hi there, Question from a biologist with very poor background in physics, but willing to understand quantum physics. I think quantum entanglement shocks everyone, even if it has been proven right. I would love to know if there is any hypothesis or crazy theory out there to explain why or how...
  29. Demystifier

    Against "interpretation" - Comments

    Greg Bernhardt submitted a new blog post Against "interpretation" Continue reading the Original Blog Post.
  30. P

    I Van der Waals force in quantum physics

    According to QFT, are there hydrogen bonds or Van der Waals force? Or this an outdated concept of classical physics?
  31. Zeynel

    I Transistors and Quantum Physics

    My question is: What is the contribution of Quantum Physics to the discovery, of the transistor? In Adam Becker's book What is real? I read that, "the discovery of quantum physics in the early twentieth century led directly to the [discovery] of silicon transistors..." He implies that, the...
  32. RUTA

    Insights The Quantum Mystery of Wigner's Friend - Comments

    Greg Bernhardt submitted a new blog post Wigner's Friend Continue reading the Original Blog Post.
  33. thariya

    A Quantization of the electric field inside a box

    Hello all, The second quantization of a general electromagnetic field assumes the energy density integration to be performed inside a box in 3D space. Someone mentioned to me recently that the physical significance of the actual volume used is that it should be chosen based on the detector used...
  34. RUTA

    Insights The Unreasonable Effectiveness of the Popescu-Rohrlich Correlations - Comments

    Greg Bernhardt submitted a new blog post The Unreasonable Effectiveness of the Popescu-Rohrlich Correlations Continue reading the Original Blog Post.
  35. G

    B Computer-simulated quantum physics?

    There is so much discussion about what does "really happen" at the quantum level, because we can not directly observe the most minute details of quantum systems, and much is left to inference and to the "interpretations". Does the system really collapse by transferring state information to its...
  36. J

    I Causality and quantum physics

    Let me present what I think is the understanding of a particular situation in quantum mechanics, and ask people to tell me whether I am right or wrong. To say that everything happens randomly in QM would be misleading at best. We get at least statistical prediction. But discussions such as the...
  37. Warda Anis

    Expectation value <p> of the ground state of hydrogen

    Homework Statement How should I calculate the expectation value of momentum of an electron in the ground state in hydrogen atom. Homework Equations The Attempt at a Solution I am trying to apply the p operator i.e. ##-ihd/dx## over ##\psi##. and integrating it from 0 to infinity. The answer I...
  38. A

    I Dressed electrons are not defined as point masses....

    In @A. Neumaier 's excellent Physics FAQ, he notes under "Are electrons pointlike/structureless?" that "Physical, measurable particles are not points but have extension. By definition, an electron without extension would be described exactly by the 1-particle Dirac equation, which has a...
  39. Pouyan

    Finding state vectors for pure states

    Homework Statement Is the following matrix a state operator ? and if it is a state operator is it a pure state ? and if it is so then find the state vectors for the pure state. If you don't see image here is the matrix which is 2X2 in MATLAB code: [9/25 12/25; 12/25 16/25] Homework...
  40. Suppaman

    B Theories of Quantum Physics, do they exist?

    Every time I watch another YouTube video about the "spooky" stuff or multiple slit experiments they never have a proposed explanation of how they work. It would seem to me that if you had a proposed idea then people could think of experiments to prove or disprove the idea. But with no ideas it...
  41. D

    I Finding expansion coefficient of a 3-d Gaussian wave packet

    I'm having trouble with trying to find the expansion coefficients of a superposition of a Gaussian wave packet. First I'm decomposing a Gaussian wave packet $$\psi(\textbf{r},0) = \frac{1}{(2\pi)^{3/4}\sigma^{3/2}}\text{exp}\left[ -\frac{(\textbf{r} - \textbf{r}_0)^2}{4\sigma^2} + i\textbf{k}_0...
  42. S

    Schools What are the best universities for Quantum Physics in Canada

    I would love to know about not only the rankings of the universities (Which I see a lot) but more what makes each one special. Which means I appreciate personal experience related to the courses as well as for example if there is any research done (or a way to get some hands-on experience in the...
  43. P

    I Why do you need infinite size matrix which commute....

    ...to give a number? https://ocw.mit.edu/courses/physics/8-04-quantum-physics-i-spring-2016/lecture-notes/MIT8_04S16_LecNotes5.pdf On page 6, it says, " Matrix mechanics, was worked out in 1925 by Werner Heisenberg and clarified by Max Born and Pascual Jordan. Note that, if we were to write xˆ...
  44. Vectronix

    Quantum Is this a good book to learn quantum physics?

    Quantum Mechanics of Particles and Wave Fields by Arthur March... Does anyone know of the author? I think I found an error in this book. :(
  45. Safder Aree

    Wave packet width given a wave function

    Homework Statement Find the wave packet Ψ(x, t) if φ(k) = A for k0 − ∆k ≤ k ≤ k0 + ∆k and φ(k) = 0 for all other k. The system’s dispersion relation is ω = vk, where v is a constant. What is the wave packet’s width? Homework Equations [/B] I solved for Ψ(x, t): $$\Psi(x,t) =...
  46. Safder Aree

    Harmonic Oscillator violating Heisenberg's Uncertainity

    Homework Statement Does the n = 2 state of a quantum harmonic oscillator violate the Heisenberg Uncertainty Principle? Homework Equations $$\sigma_x\sigma_p = \frac{\hbar}{2}$$ The Attempt at a Solution [/B] I worked out the solution for the second state of the harmonic oscillator...
  47. Safder Aree

    Quantum Quantum Physics textbooks with worked examples

    I'm currently in my first QM course and we have griffiths as our textbook. Although Griffiths is a great read I'm having trouble really conceptualizing QM ideas. We've only covered infinite potential well and wave packets as of now. Are there any additional resources or textbooks in which they...
  48. J

    I Can something be caused and be ontologically random?

    Or does ontological probability exist? I was reading an article that came up in my google searches ( https://breakingthefreewillillusion.com/ontic-probability-doesnt-exist/ ) ignore the free will philosophy stuff. But the author makes the claim that ontological probability simply does not...
  49. bland

    I Does quantum physics predict all isotope decays

    I am particularly wondering about zircon, element 40. I notice that its isotopes decay by either electron emission positron emission or electron capture, and I was curious to know whether this is established experimentally or if quantum physics is able to predict which isotopes will decay via...
  50. Giulio Prisco

    I Einstein-Bohr "photon box" debate and general relativity

    I see this has been already discussed but the old threads are closed. EPR before EPR: a 1930 Einstein-Bohr thought experiment revisited "In this example, Einstein presents a paradox in QM suggesting that QM is inconsistent, while Bohr attempts to save consistency of QM by combining QM with the...
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