What is Quantum physics: Definition and 701 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.

View More On Wikipedia.org
  1. T

    I Generic Form of Hermitian Matrix

    I am reading the following thesis: https://www.kip.uni-heidelberg.de/Veroeffentlichungen/download/6387/pdf-6387.pdf Specifically, I am confused about equation (2.5), where they give the generic form of the matrix ##\mathcal{M}## due to the Hermiticity of ##\mathcal{H}## and the commutation...
  2. T

    I Exploring the Ambiguity of Wick Rotation in Time-Ordered Correlators

    Hi all, I am reading the following post: https://physics.stackexchange.com/questions/336336/from-euclidean-correlator-in-cft-to-time-ordered-correlator-in-real-time-how-do and was confused regarding the portion in the accepted answer where they made a distinction for the ##t>0## and ##t<0##...
  3. murtazashab

    I In the delayed-choice quantum eraser, why is a stepper motor used?

    It appears to misunderstand the notion of the detectors D3, D4 providing path information and affecting the results D0. My question is: When there is no path information being provided, why does the experiment still call for moving D0? I understand that by moving D0 we can modify the arrival...
  4. D

    I Klein-Gordon equation and continuity equation

    Hi I am using the textbook "Modern Particle Physics" by Thomson. Working from the K-G equation and comparing with the continuity equation he states that the probability density is given by ρ = i ( ψ*(∂ψ/∂t) - ψ(∂ψ*/∂t) ) He then states that the factor of i is included to ensure that the...
  5. genxium

    I Need help understanding the first single attosecond pulse measurement

    In the 2001 paper Attosecond Metrology, the authors presented the first attosecond pulse setup and explained the measurements. This paper cites another paper quite often to mention similarities in the setup, but emphasized that they chose different observables: and only the choice made by...
  6. T

    I Density Matrix of Multiple Qubits

    Hey all, I am having trouble relating probabilities with the density matrix of multiple qubits. Consider we have a system of 3 qubits: the first qubit is in the state ##\ket{\psi} = \frac{1}{\sqrt{2}}(\ket{0}+\ket{1})## and the remaining 2 qubits are prepared in the state described by the...
  7. Hak

    B Frequency of an EM wave in Classical and Quantum Physics

    In Classical Physics, when a charged particle oscillates, it emits an electromagnetic wave, and the frequency of the wave depends on the frequency with which the particle oscillates. But in Quantum Physics, when an excited atom emits a photon, the energy of the photon depends on the amplitude of...
  8. cemtu

    I Thomson Scattering when low-intensity light meets an orbital electron

    Can you explain to me the reason why Thomson Scattering can not explain what happens when light meets an electron at low intensity, and what does that have to do with light being a wave or particle or relativistic/QM effects? https://en.wikipedia.org/wiki/Compton_scattering
  9. A

    Nuclear Quadrupole moment

  10. Quarinteen

    B Double Slit Experiment: Dumb question that needs to be asked

    So the double slit experiment. If I understand correctly when electrons are shot through 2 slits and no one is monitoring, measuring or watching they create an interference pattern if they are being measured they create a 1 to 1 pattern. I keep seeing it be said that the mere act of monitoring...
  11. R

    I Multiverse theory, I dont like it...

    In think the popular science discussion about multiverse theory has problems. For example, they suggest that in another universe, coffe is green. Problem, coffe in our universe is brown for a reason, it reflects brown light. If it would reflect green light, it must contain other chemical...
  12. Islam Hassan

    B Particle Movement in Quantum Mechanics

    Niels Bohr famously said --and I paraphrase-- that QM is an abstract description of nature and that it can only prescribe what we can say about nature rather than what nature is. What does QM say about the movement of a particle? Is this movement positively ascertained to be smooth and...
  13. vanhees71

    I The notion of locality in (Quantum) Physics should be clearly defined

    Unfortunately one of the threads about entanglement and Bell tests has again been closed prematurely. It has not been clarified what "locality" means. In the physics community, not involved in philosophical arguments about foundations of QT, it's clearly defined as the property of a...
  14. D

    I Solutions of the Bloch equations for MRT

    Hello all, I have a question about the relationship between resp. and the Bloch equations . Are these upper equations special solutions of the Bloch equations? If yes, under what condition(s) do the solutions hold? Thanks in advance for helpful support!
  15. C

    A Is the Fractional Quantum Hall Effect an emergent property?

    I do not think that true emergent properties -- as defined by behavior of matter that cannot be reduced to fundamental physical law -- exist. Yet I have been told that the Fractional Quantum Hall Effect is an example of an emergent property. What is the consensus?
  16. J

    A QFT S-matrix explanations are incomprehensible

    The first look at a scattering process is something like this: We define an initial state |\textrm{in}\rangle = \int dp_1dp_2 f_{\textrm{in,1}}(p_1) f_{\textrm{in,2}}(p_2) a_{p_1}^{\dagger} a_{p_2}^{\dagger} |0\rangle Here f_{\textrm{in,1}} and f_{\textrm{in,2}} are wavefunctions that define...
  17. R

    B Double slit experiment, expensive gear?

    I have an idéa how to run the double slit experiment that could give new insight to whats hapening. As I understand, when the photons are observed by someone the wawefunction colapses and the photons become particles, this can be seen as the interference pattern dissapears in the experiment...
  18. dom_quixote

    I Double slit as a function of time

    It is known that in the double slit experiment, when successive photons are fired, the photons are statistically distributed on the target screen, as if it were a wave. What is the variable that changes between one shot of and another? Probably the slits change places as a function of time.
  19. ARoyC

    Questions from Quantum Measurements

    [Mentor Note: Two similar thread starts merged] The questions are from MIT OCW. First of all, I cannot understand what is the meaning of the measurement outcome being 0. How can an eigenvalue be 0? I tried doing the problems guessing that by 0 they mean the posterior state will be |0>. The only...
  20. ShayanJ

    A Probabilistic game of life as a toy model for Everett's interpretation

    A while back I was watching an interview with Sean Carroll and for some reason his explanation of the Everett's interpretation at that particular interview clicked for me and I became a proponent of it. But I always thought it made more sense to look at it at as the universe being a giant...
  21. ARoyC

    I Confusion Regarding a Spectral Decomposition

    Hi. I am not being able to understand how we are getting the following spectral decomposition. It would be great if someone can explain it to me. Thank you in advance.
  22. C

    I Two-photon cascade question

    Hello. I have a question about the two photons emitted from a radiative atomic cascade (such as the calcium radiative cascade used by Aspect et al. in their tests of Bell's theorem). The short version of my question is this: Do the two photons have any directional correlation (any correlation...
  23. T

    I Antisymmetrizing a Factorized Polynomial Vanishes?

    Hi all, I am having trouble understanding the argument below equation (3.5) in https://arxiv.org/pdf/cond-mat/9605145.pdf where they claim that "Upon antisymmetrization, however, a term with ##k## factors of ##(z_{i}-z_{j})## would have to antisymmetrize ##2k## variables with a polynomial that...
  24. M

    I Help with a derivation from a paper (diatomic molecular potential)

    Hello! I am confused about the derivation in the screenshot below. This is in the context of a diatomic molecular potential, but the question is quite general. Say that the potential describing the interaction between 2 masses, as a function of the radius between them is given by the anharmonic...
  25. jaketodd

    I Hawking's latest contribution, is this an implication?

    Does this mean that everything, including us, will turn into photons/electromagnetic waves some day? Sounds fun! A universe of matter converted entirely to light! I couldn't find any papers yet on this on scholar.google.com Here are some good articles though...
  26. J

    B Relationship between a material wave and the uncertainty of position and momentum

    I'm just an ordinary person who's very interested in physics. I'm posting a question because I'm curious about quantum mechanics. The wavelength of the material wave that can be obtained when a baseball with a mass of 150 g is thrown at 40 m/s is 1.1×10^-34m by the h/mv formula. As you can see...
  27. Slimy0233

    Should I have studied E&M before learning Optics, Waves & Oscillations

    I was trying to learn physics from a coaching institute and they started optics before they started Electricity and Magnetism and the lecturer went on saying somethings which I didn't completely understand. Is the coaching institute doing it wrong teaching me Optics before they taught me...
  28. H

    I When an operator can be written in different ways, it seems to affect its partial derivative, resulting in contradicting expectation values

    Suppose Q=2x+t and x=t2, then ∂Q/∂t=1. But Q can also be written as Q=x+t2+t, then ∂Q/∂t=2t+1. We now have 2 different answers. But I think there can only be one correct answer. In reference to the equation in the image, no matter we write Q=2x+t or Q=x+t2+t, <Q> should be the same, so the LHS...
  29. H

    A Exploring the Cabibbo & CKM Matrices

    Hi Pfs, I read a paper about the Cabibbo matrix and the CKM matrix. The first one is a 2*2 real matrix and the other a 3*3 matrix with complex entries. In this article i read that a n*n matrix has 2 n*n real degrees or freedom. The unitarity (orthonormal basis) devides this number by 2. I read...
  30. ergospherical

    <x|P|x'> in Dirac formalism

    What is ##<x|P|x'>##? (for particle in 1d, and ##\hbar = 1##)?\begin{align*} <x|P|x'> &= \int dp' <x|P|p'><p'|x'> \\ &= \int dp' \ p' <x|p'> <p'|x'> \\ &= \int dp' \ p' \frac{1}{\sqrt{2\pi}} e^{ip'x} \frac{1}{\sqrt{2\pi}} e^{-ip'x'} \\ &= \frac{1}{2\pi} \int dp' \ p' e^{ip'(x-x')} \end{align*}
  31. ergospherical

    I Average magnetic moment of atom in magnetic field ##B##

    from the partition function - am trying to show that ##\langle \mu \rangle = \beta^{-1} (\partial \log Z / \partial B)## where ##Z## is the canonical partition function for one atom, i.e. ##Z = \sum_{m=-j}^{j} \mathrm{exp}(\mu_0 \beta B m)##, and ##\mu = \mu_0 m##. The average...
  32. P

    I Colorless Compounds and electromagnetic radiation

    I was trying to understand why some compounds appear colorless (transparent) and tried to give an explanation. I take benzene as an example: it is a chromophore group in which there is π-conjugation, so a certain energy gap is generated between HOMO and LUMO. This energy gap is such that in...
  33. shunyadragonvv

    I Quantum decoherence and the emergence of space/time and gravity

    Quantum decoherence. and the emergence of continuous space/time and gravity In another forum I have experienced a lot of combative dialogue asserting that continuous time/space is a property of the smallest Quantum scale. My present knowledge indicates this not true, and that the goal of the...
  34. J

    Nobody understands quantum physics?

    Listen just 1 minute, what does it mean when he said nobody understand quantum mechanics? This sound like comedy
  35. Vectronix

    B Quantum Physics and Atomic Physics

    I think Atomic physics gets into specific examples and experiments but I think I’m probably wrong. So, I have to ask, what is atomic physics and how is it different than quantum physics?
  36. Slimy0233

    I Heating a tungsten filament to find out the maximum brightness emitted

    I was taking notes from a lecture on Quantum Physics and during the introduction, they gave an example of what led to the discovery of Quantum Physics: The electric bulb example where the brightness and colour of light depended on the temperature of the filament(see...
  37. S

    I Intuition for why linear algebra is needed in quantum physics

    I'm watching a nice video that tries to explain how linear algebra enters the picture in quantum physics. A quick summary: Classical physics requires that physical quantities are single-valued and vary smoothly as they evolve in time. So a natural way to model classical physical quantities is...
  38. ThiagoMNobrega

    A What is the biggest problem to be solved in Quantum Physics in 2023?

    Hey everyone, A quick question that I hope all can participate: In your educated opinion, what is the biggest question or the biggest problem to be solved in quantum mechanics for 2023? Warm regards, Thiago Munhoz da Nóbrega
  39. jim mcnamara

    Uh-oh The movies and "quantum physics" as they see it....

    This trend is likely to drive a lot of misguided threads on science forums - IMO. Trend: https://www.cnn.com/style/article/multiverse-movies-newfound-popularity/index.html
  40. J

    I Two hydrogen atoms and Pauli's exclusion principle

    Hello, I recently came across the following (apparent, I hope) paradox: suppose we have two H atoms. Now, a hydrogen atom is made up of one proton and one electron (fermions), so it is a boson. Then one could have two hydrogen atoms which are in the exact same state (including position). This...
  41. S

    B I have question regarding quantum physics (localizing an electron)

    As QP says it is uncertain that where you found electron. But its not the same phenomena that one fan blade running so fast that you cant see where the blade is. You can only see the blade on a particular position if you use high resulation camera and its position depend on when you take the...
  42. S

    Nima Arkani-Hamed's opinion on Many Worlds?

    I was wondering what was the opinion of the physicist Nima Arkani-Hamed towards the Many Worlds interpretation. Is he open to the possibility of it being true? Does he support it?
  43. Lotto

    What is the length of an infinite potential well for an electron?

    I have a nanoparticle of cadmium selenide with a diameter d. When it emits a photon with a wavelenght lambda, it happens because an electron jumps from the conduction band to the occupied band across a forbidden band. I can suppose that jump as a jump from a higher energy level (the conduction...
  44. Ashish Somwanshi

    Measurement problem quantum mechanics

    I was not able to attempt since I don't know which formula or method can be used to solve the problem
  45. Ashish Somwanshi

    Matrix representation in QM Assignment -- Need some help please

    This screenshot contains the original assignment statement and I need help to solve it. I have also attached my attempt below. I need to know if my matrices were correct and my method and algebra to solve the problem was correct...
  46. Ashish Somwanshi

    Finite potential well problem penetration depth

    I don't understand where I went wrong, the formula and calculations which I have attached are correct...please do help if anyone can spot the mistake.
  47. Ashish Somwanshi

    Infinite potential well problem normalization

    I have attached my attempt and proof that my attempts were incorrect.
  48. pokespriter

    I About the position of electrons (uncertainty)

    Hello guys, I don't know if this is the right place to ask, so please be kind :/ I have a question regarding the location of an electron that belongs to an atom. A teacher told me that the probability of an electron to be found within its orbital is around 99%. When I asked about the remaining...
  49. fresh_42

    B Positive Definite Cartan Matrices in Quantum Physics

    As I was looking for an example for a metric tensor that isn't among the usual suspects, I observed that the Cartan matrix I wanted to use is positive definite (I assume all are), but not symmetric. Are the symmetry breaks in quantum physics related to this fact?
  50. V

    B How does exchange of elementary particles result in a force?

    Homework Statement:: I came across the following in an online article. I am unable to understand how these elementary particles cause a force to exist. "Each of the four forces results from the exchange of force-carrier particles.". Above statement is taken from...