Quantum Definition and 999 Threads

In physics, a quantum (plural quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantization". This means that the magnitude of the physical property can take on only discrete values consisting of integer multiples of one quantum.
For example, a photon is a single quantum of light (or of any other form of electromagnetic radiation). Similarly, the energy of an electron bound within an atom is quantized and can exist only in certain discrete values. (Atoms and matter in general are stable because electrons can exist only at discrete energy levels within an atom.) Quantization is one of the foundations of the much broader physics of quantum mechanics. Quantization of energy and its influence on how energy and matter interact (quantum electrodynamics) is part of the fundamental framework for understanding and describing nature.

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  1. A. Neumaier

    A Tumulka on Bohmian QED

    Roderich Tumulka (a long-time Bohmian) demonstrates in his newest preprint, R. Tumulka, A Vision for a Bohm-Style Theory of Quantum Electrodynamics, arXiv:2409.07784. that for the interpretation QED, the Bohmian interpretation has nothing to offer (apart from speculation called vision), and...
  2. S

    Funny QM Video

    TL;DR Summary: Funny (but very well informed) video about QM. Hope this is not considered off-topic, but anyone so inclined and with 4+ mins to spare might enjoy this (intentionally funny) QM video by eigenchris:
  3. A. Neumaier

    I New book on algebraic quantum physics

    I am happy to announce my book A. Neumaier and D. Westra, Algebraic Quantum Physics, Vol. 1: Quantum mechanics via Lie algebras, de Gruyter, Berlin 2024. It features a mathematically rigorous but still physically lucid account of quantum mechanics from the point of view of symmetries...
  4. B

    I How we are in a matter dominated universe (without antimatter annihilation)

    Assuming the situation where there is an infinite inflation stream, from which bubble universes are appearing in locally slowed regions. There could be many many such bubble universes and so very rare occurrences in some bubbles can sometimes happen. Also assumed, as is often postulated, that...
  5. damarkk

    Electric sinusoidal field on a hydrogen atom - Quantum Mechanics

    Hello to everyone. I have some doubts about one problem of quantum mechanics. My attempt. I need to calculate the coefficient ##W_{ij}=<\psi_i | H' |\psi_j>## where ##H' = -eE(t)z## is a perturbation term in the hamiltonian and ##|\psi_i> = |\psi_{nlm}>##. We have four states and sixteen...
  6. T

    I About the results of trying to build quantum gravity from gravito-EM

    So one of the major topics in physics is trying to reconcile quantum mechanics and general relativity. And the classical expression of electromagnetism is Maxwell's equations, and its linked to quantum models of electromagnetism. And then there's also a gravitational analogue of Maxwell's...
  7. Q

    A How does QFT treat Young’s Double-Slit Experiment?

    How does QFT treat the Young’s DSE? Is there a wave function (wave packet) attached (and created at the moment of launching of the photon) or the modes of the EM quantum field are pre-existing due to experimental configuration (including the screen) and do they play the role the wave function is...
  8. E

    How to explain Bell's theorem to non-scientists

    How did you find PF?: Wandering on the internet looking for answers about Bell's theorem I teach adult education at 2 universities near Denver and I'm writing a book for non-scientists interested in quantum mechanics and its foundations. The challenge is to present the concepts digestibly...
  9. W

    I How Does the Heisenberg Uncertainty Principle Relate to Electron Confinement?

    https://physics.stackexchange.com/questions/824561/two-element-theory
  10. M

    Reflection probability in Quantum Mechanics

    For this, The solution is I have doubt where they got the reflection probability formula from. Someone may know how to find it. I think that ##R + T = 1##. But I'm not sure where the transmission probability formula comes from either. Kind wishes
  11. M

    Quantum Mechanics Boundary conditions

    For this problem, The solution is, I have a doubt about Step number 3 about boundary conditions. Someone maybe be able to solve that doubt? Kind wishes
  12. E

    I FTL signals connected to a previous non-FTL discussion are possible?

    So basically assume both the star and earth have high-tech machines and a mapping of natural changes in orbit such that the detectors will always be the same settings on star and earth, and the ship in between earth and star will move in such a way that the alignment will be exactly halfway in...
  13. J

    Master in Physics from Buenos Aires University

    Suppose that Bob and Alice decides to do a quantum experiment, more precisely, The Shroedinger Cat Experiment. Alice goes to the Andromeda Galaxy, and wait for Bob’s results. According with the MWI, the wave function never collapses, suppose that in one world Bob found the cat alive, in the...
  14. K

    I The standard model from loop quantum gravity via spinors octonions

    in an interview Michio Kaku was asked about Loop Quantum gravity and he replied that it is only a theory of pure gravity, but that the universe also contains particles, the particles of the standard model, and only superstring theory unifies both quantum gravity and the standard model. the...
  15. PotatoMan

    What Are the Latest Insights in Quantum Physics?

    Recently I’ve been learning more about quantum physics, and Physics Forums has had a lot of good answers to questions I had. I’m looking forward to learning even more, and asking questions that I’ve beenconfused about.
  16. RUTA

    Insights PBS Video Comment: “What If Physics IS NOT Describing Reality”

    Continue reading...
  17. B

    Is My Approach to the Gaussian Wave Packet Problem Professional?

    hi, I'm solving this statement, We split into parts Can some expert in Quantum say that my working is professional? Kind wishes to you
  18. B

    Particle in one dimension wave function from Quantum Mechanics

    Hi, I try and solve this problem I have solved the problem in different parts But me not sure how to plot the graph. Maybe someone knows? Merci
  19. D

    Need resources to help me understand quantum computing please

    How did you find PF?: Good Need documents to help me understand quantum computing!
  20. Q

    I Could Quantum Dots Between Optic Fibers Create Photon Interference?

    A quantum dot is placed in a line between two optic fibers The dot can emit a photon in every direction which is unknown. If the fibers are combined at a beamsplitter would there be interference if the photon is not observed outside the fibers? Do you know if somebody made such experiment...
  21. PeterDonis

    A A possible quantum field theory of tachyons?

    I have come across the following paper (arxiv preprint link, it looks like it is published in Phys Rev D) that claims to have found a consistent quantum field theory of tachyons, overcoming three issues which have been said in previous literature to make such a theory impossible...
  22. CHANDANKUMAR S

    What is Quantum Information Science?

    I am currently a research scholar specializing in open quantum systems.
  23. P

    A Hard-Core Boson Model in K space

    Hello, I am interested in the following model: $$ H = \sum_{<i,j>} -t (c_i c_j^{\dagger} + \text{H.C.}) + U (n_i n_j) + \sum_{<<i,j>>} -t' (c_i c_j^{\dagger} + \text{H.C.}) + U' (n_i n_j) $$ where \( <i,j> \) indicates nearest neighbors, and \( <<i,j>> \) indicates next-nearest neighbors...
  24. gentzen

    I Can I take the transactional interpretation seriously?

    My first "active" encounter with the transactional interpretation happened when I tried to react to Ruth Kastner's criticism of consistent histories. My next encounter happened when I was fighting with reciprocity occuring for crystal effects in electron scattering, and hoped for help from time...
  25. Darmstadtium

    Admissions Direct Entry to PhD Program -- Advice Please

    I am a second-year combined honour in physics and math with prerequisites for 4th-year math courses. I am hoping for a direct entry (if possible) into a top US PhD physics program in quantum with a current unofficial cGPA of 3.9/4 (88%). Since I am in Canada, we only have percentage grades that...
  26. Grinkle

    B What does it mean for a glass bead to be in a quantum state?

    https://www.guardianmag.us/2023/02/scientists-levitate-glass-nanosphere.html "the nanosphere was suspended in its lowest quantum mechanical state, one of extremely limited motion where quantum behavior can start to happen." To my reading, the glass bead was cooled down and then suspended with...
  27. M

    QM time independent perturbation theory

    The lowest two energy level corrections (l=0, s_{z}=-1/2 and l=0, s_{z}=1/2) are easy to work out since the eigenvalues are not degenerate and the unperturbed energy levels also happen to be eigenstates of H'. However I have three degenerate energy levels for the third eigenvalue of the form...
  28. keyzan

    Probability of the outcomes of ##J^2## and ##J_{z}##?

    Hi guys, I have a problem with point 2 of this exercise: The electron of a hydrogen atom is initially found in the state: having considered the quantum numbers n,l,m and epsilon related to the operators H, L^2, Lz and Sz. I am asked: determine the possible outcomes of a measurement of J^2...
  29. L

    Quantum Mechanics - Time dependent solution - x's and t's not mixed up

    Next, we assume a solution in this form: Which simplifies (according to my notes) to this: In the middle equation, we have factorised out the F(t). My question is why is it wavefunction(x,t) rather than wavefunction(x). I first thought it was a mistake in the notes, but it uses the same...
  30. Jim Kata

    A Why is matter-free gravity not ultraviolet finite?

    In reading Weinberg volume 1 I learned gravity is not renormalizable by Dyson power counting. This means that it has an infinite number of free parameters, and such theories lose their predictive power at energies of the common mass scale. This being said, T Hooft and Veltman showed miraculously...
  31. PhysicsEnjoyer31415

    B What Are Gravitational Waves?

    Many more questions....please enlighten me on this topic. thank you!
  32. B

    I Quantum Eraser Experiment at home

    I was tryingt to find away to build a setup for conducting the quantum erase experiment, demonstrating the observer effect. It seemed impossible without expensive equipment for sending single photont and detecting single photons, then however I bumbed into this guide from scientific american...
  33. D

    B Dark Energy: Are We All Missing The Obvious?

    Well, I am not making any exciting claims or anything, just asking a question that I cannot stop asking myself. WE all know the Quantum theory on spontaneous generation of particles in a vacuum etc and many have asked is this linked to dark energy somehow. I like to try Einstein type Thought...
  34. L

    Normalising the wavefunction - two answers

    Hi, when normalising the wavefunction I get two answers. Is this correct? My notes only has 1/sqrt(L) = A.
  35. adf89812

    Shielding or principal quantum number predicts Covalent or Ionic?

    TL;DR Summary: Why is HgCl2 more covalent than CaCl2 via shielding? Hg^2+ in HgCl_2 has more shells of p-orbitals and s-orbitals than Ca^2+ in CaCl2 - Do those extra p and s shells of Hg^2 in HgCl2 shield the two chlorines in HgCl2 from the effect of Hg^2+'s nucleus attraction, or leave the...
  36. SWIRF

    I Breaking the Light Speed Barrier?

    If we have a pair of super-asymmetrical entwined particles, and move them a light year away so that they retain their quantum entanglement, and we set a clockwise spin as 0 and a counter-clockwise spin as 1. Would it be possible to transmit binary data faster than the speed of light? If we hold...
  37. Elaren

    Quantum Better advanced Quantum Mechanics book than Sakurai?

    Sakurai's Modern Quantum Mechanics is 1/2 of a very good graduate textbook. Unfortunately Sakurai passed away midway through writing it, and it is very obvious exactly where it swapped from their writing to just using their notes. The back half of the book, while by no means bad, is notably less...
  38. mortimertz

    I am a physics enthusiast with some tendency in Quantum Gravity

    Student of master of physics.
  39. A. Neumaier

    A Teaching ideal quantum measurement, from dynamics to interpretation

    A.E. Allahverdyan, R. Balian, and T.M. Nieuwenhuizen, Teaching ideal quantum measurement, from dynamics to interpretation, Comptes Rendus Physique 25 (2024), 251--287. https://www.researchgate.net/publication/380862732
  40. K

    I Are gravitons necessary?

    If gravity is not a force, but rather curvature of spacetime, are gravitons necessary to couple with gravitational field?
  41. S

    Can quantum mechanics be combined with game theory?

    I suppose, anybody here knows about the Elitzur–Vaidman bomb tester and the counterfactual definiteness: https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester https://en.wikipedia.org/wiki/Counterfactual_definiteness I have a question: can this experiment be performed at the level of...
  42. S

    I Color charge is not scalar -- do their components have dimensions?

    Color charge is not scalar. Still, do their components have dimensions (in metrological terms)?
  43. K

    Quantum Well Centred at the Origin Doubled in Size at time t=0. Symmetry seems to render the question incorrect

    Firstly I have found the eigenstates for both the original well and the new well as the following $$\psi_{n,\frac{L}{2}} = \begin{cases} \sqrt{\frac{2}{L}} \cos{\frac{n \pi x}{L}} \; \; \; \; \; n \text{ odd} \\ \sqrt{\frac{2}{L}} \sin{\frac{n \pi x}{L}} \; \; \; \; \; n \text{ even}...
  44. N

    A X measurement modeled in non-separable Hilbert space

    Am reading a book (Ballentine, "Quantum Mechanics: A modern development) which I have found very helpful. Am now puzzled by section 3.4, where the position operator satisfies Q|x> = x |x> (I have simplified from 3 dims to 1 dim). Here, x is any real number. There are, thus, uncountably many...
  45. H

    I Is quantum interference an instantaneous consequence of superposition ?

    A physicist (I'm not a physicist by profession, as you'll have gathered) told me, without being more specific, that interference is not an 'interaction' in the strict sense of the word, in other words in the physical sense of the term. I can only guess at what is meant by this (but perhaps...
  46. billtodd

    A Quantum Groups as a Generalization of String Theory

    Every once in a while I use my ancient trick of searching something in google with keywords, and found the above article. I don't think there's a free copy of it, because it's from 1989. I guess I need to read the pink book on foundations of Q-Groups by Majid. You know who also has written a...
  47. Justina

    Enthusiast of Quantum mechanics

    Hi everyone I'm Justina. I'm excited in joining physics forums and I'm high school student who is interested in Quantum mechanics a lot .I used to spend lot of time on knowing Quantum mechanics and i learnt few little things. And this is my first time, joining an online community hope I will...
  48. Q

    "Retired" but still active

    I am a long-retired physics lecturer, with the bulk of my lecturing focused on quantum and relativity. I am still active in research. I have completely lost contact with the challenge of explaining this stuff to students, and was curious to see how these challenges are met on this forum, not...
  49. Penny57

    Show that energy E always exists such that junction conditions have a solution

    I know this wavefunction should behave as a symmetric cosine function (possibly as Cos( (k∗x)/(hbar) ?). I also know for a bound state, the wavefunction must decay exponentially outside the well. Additionally, r = (-β+ik)/(β−ik) . However, aside from that, I do not know how to get this question...
  50. S

    I Does entanglement always conserve something?

    IIUC, entanglement sometimes plays a role in conserving come quantity like momentum or spin: the quantities measured for two particles must be correlated in order to get a certain total value. But is this always the case? For example, what, if anything, is conserved in the Hong-Ou-Mandel...
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