# Quantum and general physics Definition and 131 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. ### 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.
2. ### 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)...
3. ### 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...
4. ### 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...
5. ### I Irreducible state solution

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...
6. ### 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...
7. ### 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...
8. ### 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.
9. ### 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...
10. ### 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?
11. ### Kinds of motion

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?
12. ### 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...
13. ### I Would Black Holes Be Coherent Under Quantum Mechanics?

I just want to know if Black Holes would be quantumly coherent.
14. ### 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...

37. ### 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?
38. ### 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
39. ### A How to calculate density of states (DOS) from 8 energy eigenvalues of a Quantum model calculated by exact diagonalization?

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")
40. ### 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...
41. ### 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...
42. ### Courses Course advice for an enthusiast

Hey all, firstly let me give some background, i`m a Cloud Computing Engineer and have been in the I.T. industry for the past 20 years since I left university. I have always had an interest in science and astronomy (My dad was a keen amateur astronomer) but never really taken it very seriously...
43. ### B Taking Quantum Computers into Space

Can the low temperatures in Space be employed to cool a Quantum computer installed on an operational Space vessel doing away with the need of a super fridge or whatever they use here on Earth? Thank you.
44. ### I One dimensional system

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?
45. ### 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...
46. ### Help with finding the expectation value of x^2

The question is as follows: A particle of mass m has the wave function psi(x, t) = A * e^( -a ( ( m*x^2 / hbar) +i*t ) ) where A and a are positive real constants. i don't know how to format my stuff on this website, so it may be a bit harder to read. Generally when i write "int" i mean the...
47. ### I Why limited discussion of only looking at history

When you look at a bird in a tree with binoculars, that is not the bird as it exists at the moment but the bird that was there before the light traveled to the scope, which for practical purposes is the bird. You can shoot the bird of before, and it will fall to the ground. However, when you...
48. ### I Want some suggestions for research papers on quantum entanglement

I want to know more about quantum entanglement and I am a undergrad student in physics.Can someone suggest some research journals or articles to easily understand the concept deeply?
49. ### B How to think of molecular orbitals quantum mechanically

The electrons in a molecule are said to be in a quantum superposition state in terms of their position/spin/momentum. But when you look at a molecule like water at a chemical level, it has a very specific shape corresponding to the P orbitals of the outer shell of the Oxygen atom. The two...
50. ### A Quantum synchronization description used in a paper

In the paper "Steady-state spin synchronization through the collective motion of trapped ions" it states the following: "Steady-state synchronization of atomic dipoles forms the foundation for ultra-stable optical lasers utilizing narrow-linewidth atoms coupled to a lossy cavity mode. The...