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. Shourya Tripathi

    I What is the fabric of space-time made up of?

    Also why can't we apply laws of gravity to the quantum world? Can gravitational forces affect the subatomic particles such as protons and neutrons.
  2. 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...
  3. StanislavD

    A Pairing of electrons in superconductors

    Crystals may contain electronic real-space-eigenstates as ground states, which are spatially much larger than one unit cell, such as impurity states, standing waves at Brillouin zone edges, states of Anderson localization, etc. Every eigenstate is usually occupied by two conduction electrons...
  4. Rayan

    The time-dependence of the expectation values of spin operators

    So first I derived the expressions for the dynamics of the spin operators and got: $$ \frac{d\hat{S}_y}{dt} = w\hat{S}_x^H $$ $$ \frac{d\hat{S}_x}{dt} = w\hat{S}_y^H $$ $$ \frac{d\hat{S}_z}{dt} = 0 $$ Now I want to calculate the time-dependence of the expectation values of the spin operators...
  5. E

    Physics Opinions about a career path in quantum computing

    Hi, I'm 20 and I'm courrently persuing a barchelor's degree in Computer Science in Italy and now I'm in the middle of the second year. Thinking about a career path I've always been sure to specialize in Cyber Security but I recently came across Quantum Computing that I find fascinating. After...
  6. AshIsH_0001

    Probabilities out of non-normalizable functions?

    a and b were fairly easy to solve; but the c part which actually demands the probability! How are we suppose to fetch the value if the function can't even be normalized; I tried to make some assumptions like making the system bounded; but I don't think that it's the right way to do so... What...
  7. Rayan

    Sequential Stern Gerlech experiment

    So I thought that when the $m_l = 1$ beam passes through the second SG-magnet, it should split into 3 different beams with equal probability corresponding to $ m_l = -1 , 0 , 1 $ since the field here is aligned along z-axis and hence independent of the x-axis splitting. And I thought that the...
  8. Half Infinity

    I How did Sommerfeld arrive at his quantum condition?

    Wherever I read, they just simply state the Bohr-Sommerfeld or the Wilson-Sommerfeld quantum condition to be integral (p.dx) over closed path = nh But nowhere can I find the insight using which Sommerfeld arrived at this equation. I mean, he could'nt have just guessed that. There must have...
  9. P

    B Need help with understanding time travel

    Ridiculous! I know. But as a 9th grader, there are a few things that have been bugging me. I had a conceptualisation of how 'time travel' could work. However, it's based on the presumption ( a very large one at that) that the many-worlds interpretation of quantum mechanics is correct...
  10. P

    B Looking for help understanding string theory

    String theory is a vast topic, and no grade 9 book will have content on it. However, no site or video gives it the attention this topic deserves, please input your understanding of string theory here. here are a few general questions to think about: 1)What is string theory? 2)Why was it made...
  11. namelessuser

    I Quantum Mechanics vs Einstein: Light Btwn Black Holes, Double Slit Experiment

    Quantum mechanics vs Einsteins theory of relativity: How does light move between two black holes if we create a double slit experiment in front of the light? Do the light waves distribute themselves equally on the screen or does gravity distort light waves at the edges? So just imagine doing...
  12. T

    I Time dependence of operators in the Schrödinger picture

    I found in wikipedia following formula describing the derivative of operator ## A_H ## considered in Heisenberg picture, where ## A_S ## is it's representation in Schroedinger picture: ## \frac{d}{dt}A_\text{H}(t)=\frac{i}{\hbar}[H_\text{H},A_\text{H}(t)]+\left( \frac{\partial...
  13. Q

    A particle in an infinite square well

    What I am lost about is b, rather the rest of B. I am not sure what it means by probability density and a stationary state.
  14. gggnano

    I Negative energy when a laser goes through lithium niobate crystal?

    A screenshot from a book which describes it: So I am trying to picture this one: 1. A laser is "pumped" through a cylinder made from lithium niobate which is placed at 90 degrees, perpendicularly...so that the laser passes through the body of the cylinder (and not through the 2 round ends)...
  15. ThiagoMNobrega

    B Wave function won't collapse under a microscope?

    So what am I doing wrong here? I can clearly observe it, I'm nearly sure I can tell which particles are going throw each slit if I used another laser too. My suspicion is that the electrical current of the photon detector that uses germanium or silicon to detect the particles are influencing the...
  16. mohamed_a

    I Classical analogy approach to quantum mechanics

    I have read about several approcahes to bypass some classical restrictions to quantum facts such as the electron being in a torus-like shape to avoid ,the greater than speed of light, rotation paradox . Could you recommend websites , sources or books that give good classical analogy to quantum...
  17. G

    I Understanding the Irreducible Solution in Classical Harmonic Oscillators

    Hellow. I am doing an introductory to Quantum Mechanics course, and the irreducible solution appeared in the harmonic oscillator. When we talk about the irreducible solution, this is the solution as a linear combination of the eigenbasis of the system. This is understandable, however, if I have...
  18. schrodingers_cat

    Quantum Good resources for a beginner getting into Quantum Physics?

    Summary:: I’m a beginner trying to get into Quantum Physics, and want some good resources (e.g. books) to get me started. Hi everyone! I’m a beginner trying to get into Quantum Physics, and want some good resources (e.g. books) to get me started. I have a very basic knowledge of physics (10th...
  19. schrodingers_cat

    What are some resources for beginners in quantum physics research?

    Hi! I’m @schrodingers_cat, and I am new to this forum. While I currently have a limited knowledge of physics, I am excited to learn more about the field. I am interested in the idea of researching quantum physics, however I do not know where to get started. I hope this forum helps me in my...
  20. hello_world30

    A Hamiltonian in second quantization

    Hello ! I require some guidance on this prove :I normally derive the Hamiltonian for a SHO in Hilbert space with a term of 1/2 hbar omega included. However, I am unsure of how one derives this from Hilbert space to Fock space. I have attached my attempt at it as an image below. Any input will be...
  21. F

    I Determining Momentum from Wavefunction

    The goal I am trying to achieve is to determine the momentum (2D) in a quantum system from the wavefunction values and the eigenergies. How would I go about this in a general manner? Any pointers to resources would be helpfull.
  22. A

    Programs Should I become a physicist or a mathematician?

    I am a person who likes to think deep and can't keep himself from delving deeper into concepts. Even though math is the subject in which I outshine my counterparts, it is physics that I find more fascinating. Also I feel that physics gives math a sense of direction to work upon. But I am a...
  23. H

    I A statement in a superposition of being true and false?

    If a cat can be in a superposition of being dead and alive, why can't a statement be in a superposition of being true and false?
  24. J

    Exploring Motion in Physics: Beyond Translation, Rotation, and Oscillation

    In high school I learned about three kinds of motion in classical mechanics - translation, rotation, and oscillation. Are there any other kinds of motion in the physical world?
  25. D

    B 2D-cross-double-slit experiments for understanding quantum world

    How did you find PF?: gmail The regular double-slit experiment is the "the basic Mystery" in quantum (Feynman). now I have done several cross-double-slit experiments with different configurations, My question is: how photons “sense”: (1) which slit they pass through; (2) what photons they...
  26. iVenky

    I Microscopic and macroscopic physics

    Quantum Mechanics seems to explain phenomenon happening on a microscopic scale, where measurements aren't definite and probability plays a role. Classical physics explain macroscopic phenomenon where measurements are definitive without probability of that event happening. Is it happening just...
  27. Danny Boy

    A Quantum Ising model correlation function query

    In this paper, on quantum Ising model dynamics, they consider the Hamiltonian $$\mathcal{H} = \sum_{j < k} J_{jk} \hat{\sigma}_{j}^{z}\hat{\sigma}_{k}^{z}$$ and the correlation function $$\mathcal{G} = \langle \mathcal{T}_C(\hat{\sigma}^{a_n}_{j_n}(t_n^*)\cdot\cdot\cdot...
  28. allisrelative

    Doesn't Wigner's Friend Experiment solve the measurement problem?

    If you look at the recent Wigner's Friend experiment, it seems to support Carlo Rovelli's Relational Interpretation which says there's no real measurement. Wiger's Friend carries out a polarization measurement. Before he does, the quantum system is in a superposition of horizontal/vertical...
  29. A

    What are the Latest Discoveries in Quantum Mechanics?

    Hi! I'm a second year physics student from Bucharest, Romania. Physics is my passion and I love everything about it. I dream to become a theoretical physics researcher and teacher, and right now my main interest is quantum mechanics. I'd love to help other students if I can, and I'm excited to...
  30. kellys

    Help with this quantum circuit textbook problem about controlled gates

    Summary:: My skills are very very basic and I'm more a networking major but i had to take a quantum mechanics class, i have trouble with this xcercise from textbook quantum mechanics a general introduction [Mentor Note -- Thread moved from the Technical forums so no Homework Template is shown]...
  31. Unbreakabletoon

    I Exploring the Physical Properties of Human-Sized Worm Holes

    Hello Everyone in the forum: I have a theoretical question about Worm Holes. So for the sake of this question let's just assume we have the technology and the power source to fire up a small human size worm hole. My question would be regarding the edge of the worm hole. Would you be able to...
  32. U

    I Quantum Mechanics Particle in a Box

    I need help .I did not A) E < V0 for T =? (passing coefficient ) B) E = V0 for T = ? C ) E > V0 for T =? A
  33. G

    Calculating Φ - Wrong Answer Found

    I calculated <+|Φ> to be (1-√2i)/4√2 + (3-2i)/4√2. When I squared this I for 16/32 but this is no the right answer.
  34. I

    Quantum Mechanics Infinite Potential Well -- Check Answers please

    I'm self studying so I just want to ensure my answers are correct so I know I truly understand the material as it's easy to trick yourself in thinking you do! A particle of mass m is in a 1-D infinite potential well of width a given by the potential: V= 0 for 0##\leq## x ##\leq## a =...
  35. RealKiller69

    Why Does My Calculation Have an Extra Factor of 2?

    My solution is not the same, it has an additional *2 factor and i can't see any mistakes.
  36. S

    Three quantum mechanics questions (about Uncertainty p. and comp. conjugate)

    Is the last inequality correct? Should it not be ##|A|^2 \cdot 2(1+\cos{(ka)})##? How is the time calculated here? Given ##\Delta v > 10^{-34}##... How come ##mv \Delta v = \Delta (\frac{mv^2}{2})##? Where does the ##(1/2)## come from?
  37. J

    A Explanation of the Diagram of the Rashba Effect

    Hello, I find something about the Rashba Spin Orbit coupling but I am not sure how to understand it. Hope you can help me. Consider the following picture of the Rashba effect i found: Well i don't really know what's about the arrows here. They should showing the spin direction i guess but i...
  38. J

    A Rashba-Effect and Intrinsic Spin Hall Effect

    Hello everybody, Im looking for a good explanation for the Rashba effect or Rashba Spin Orbit coupling (when it is the same effect with 2 names?) . If somebody can help me here that would be awesome. Moreover i try to understand the intrinsic mechanism of the Spin Hall Effect with the Berry...
  39. 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...
  40. Nonametheone

    Mini magnetosphere within Earth's Magnetosphere

    Summary: Can such magnetosphere exist on Earth? The moon has mini magnetospheres produced from its crust shown below. Can Earth have similar magnetic properties in its crust like the surface on the Moon to create a mini-magnetosphere? Are ther places on Earth where a mini magnetosphere exist...
  41. A

    I Classical treatment of quantum eraser?

    The below experimental setup is of the sort one finds frequently discussed in the delayed choice quantum eraser (QE) experiments, such as that of Kim et al. (https://arxiv.org/abs/quant-ph/9903047) I extracted only the essential part I'm wondering about and did not find a satisfying answer in...
  42. Michael Price

    A Resolving power of a radio telescope array: Quantum or classical?

    My question is: is the resolving power of an array of radio telescopes a quantum or a classical effect? The increase in resolving power of a single telescope, as aperture size increases, is easy to explain in terms of Heisenberg's uncertainty principle. But when we go an array of telescopes are...
  43. patric44

    Quantum What is the most suitable Quantum Mechanics book for these questions?

    i want to know what is the most suitable quantum mechanics that will enables me after studying it to answer these specific questions ? i mean based on the hardness of these questions ( what book would you suggest to study these subjects ) . and by the way what is the level of these QM questions...
  44. N

    I Impact of atmospheric turbulence on secret key rate in QKD

    I have the calculation on secret key rate over the turbulence channel using Python code. The calculation is as below: import numpy as np import math import re import random import cmath #import itertools from math import pi,e,log import time #import xlwt import matplotlib.pyplot as plt from...
  45. S

    I "It from Bit" and the multiverse?

    Did physicist John Archibald Wheeler propose the existence of multiple universes through his "It from Bit" or "Participatory Universe Principle" or "Law without Law and Pregeometry"? Is the multiverse related to Wheeler's ideas? I've been told that It can produce multiple universes, obviously...
  46. S

    I Are all Cellular Automata models of universes?

    Both Stephen Wolfram and nobel laureate Gerard 't Hooft think that the universe is a Cellular Automata. As far as I know, 't Hooft developped a series of frameworks to build different models of Cellular Automata and Wolfram also proposed a framework where network nodes could produce different...
  47. S

    A Is the Wheeler-DeWitt equation related to Wheeler's It from Bit?

    In this article: https://arxiv.org/pdf/hep-th/0211048.pdf by physicist Andrei Linde, it mentions Wheeler-DeWitt equation and he relates it to Wheeler's Participatory Universe model. Also, I found other sources that seem to establish a connection between Wheeler-DeWitt equation and Wheeler's It...
  48. J

    I Value of beta in Boltzmann Statistics taking degeneracy into account

    Hello, The relationship between entropy ##S##, the total number of particles ##N##, the total energy ##U(β)##, the partition function ##Z(β## and a yet to be defined constant ##β## is: $$S(\beta)=k_BN \cdot \ln(Z(\beta)) - \beta k_B \cdot U(\beta)$$ Which leads to: $$\frac{dS}{d\beta} =...