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

I just want to know if Black Holes would be quantumly coherent.
27. ### 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...