# What is Quantum: Definition and 992 Discussions

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. ### I Virtuals particles, vacuum and infinite energy

Hi guys I have a question for you. Virtual particles can appear anywhere and when they have enough energy they turn into real. And if it happens long enough in a vacuum, will it remain a vacuum? If not, then is matter infinite?
2. ### I Where Do You Stop? (Do electrons or photons move faster than the speed of light?)

The title is from a great book by Eric Kraft, who plays around with one's physical-being in elemental terms in an excellent novel. He is very funny. To get down to my question: Do electrons or photons on anything move faster than the speed of light?
3. ### I Question about Stern-Gerlach experiment

I just learned about the Stern-Gerlach experiment and have some questions: 1: clearly there's no objective "up" or "down"--the directions are measured relative to the magnetic field, correct? And well always find just 2 spots of equal and opposite distance on the detector, implying the magnetic...
4. ### Band gap and lattice spacing

How can we link the band gap to lattice spacing? For (a), if we purely do dimension analysis, then I would guess $$a=\frac{\hbar c}{E_g}$$. But what's the reason behind this answer, and will the true lattice spacing be larger or smaller? For (b), I guess $$\lambda=\frac{\hbar c}{E_g}$$ due to...
5. ### I Multiverse theory -- Why don't strange things happen here sometimes?

If I have a brittle piece of rock and hit it with a hammer, can a round ball split of in some universe, verses in our universe a piece with rugged ends always form? If so, why do we always, in our universe seem to get "expected" results? Why dont strange things happen here sometimes? Why is our...
6. ### I Can you increase the mass of a closed system with information?

Let's assume that there is a closed box, with mass M. There are some random quantum processes inside it, say radioactive decay. Let's assume that we can manipulate the decay from the outside somehow, thus 'putting information' into the box. Can that affect its mass?
7. ### B QM randomness vs chaotic determinism?

Can you swap out the RNG that is the wave function collapse with a suitable deterministic chaotic process that matches the wave function (squared)? I can picture a multi leg pendulum swinging around drawing out the wave function. The point where you measure is the point the pendulum was at. Is...
8. ### I Delayed choice quantum eraser setup with no beam splitter

Hello Dear Physicists, I know this question probably discussed many times before. But I need a clear answer about this setup in case there is no beam splitter. What is gonna happen in this situation? My classical intuitions say I will see a correlated interference pattern on both screens(or...
9. ### Sequences of measurements in quantum mechanics

ATTEMPT AT SOLUTION: I understand if looking for positive this will be +hwo/2 (hbar) for Sz so must find |a|^2. and if looking for negative this will be -hwo/2 (hbar) so must find |b|^2. If asked to find say Sx and original question in Sz, we must find new eigenstates associated with this state...
10. ### A K-point mesh and Monkhorst-Pack

Are the K-point mesh and monkhorst pack same. I was reading research papers using VASP and in some papers as the number of cells increases the Monkhorst Pack increases but in others the K-Point mesh decreases.
11. ### I Phenomena: Recent + Documented

This might seem like a rather peculiar observation and question. However, a rather strange physical anomaly was noted about a decade ago. While gazing thorough a living-room window. A flying-insect was seen with a portion of its torso fixed within the pane of glass. It seemed, as if, it...
12. ### I Can we now explain the Fine Structure Constant?

In this video Dirac talks about the Fine Structure Constant 1/137. Can we now explain why this is?
13. ### B Interferometry: Photons Needed for Interference Pattern

If you have, say, a two-mirror interferometer that is producing an interference pattern, how many photons are interfering? Two, or just one?
14. ### I Are non-perturbative methods in physics limited to quantum physics?

Non-perturbative methods are critical in parts of quantum field theory, such as QCD, and have at least some applications in quantum electrodynamics. You can also have mathematical problems that don't have perturbative solutions. But, it isn't clear to me if classical physics can ever have...
15. ### B Is it possible to ionise a solid, so that it has no electrons left?

Hi, is it possible to ionise a solid, so that it has no electrons left? Josiah
16. ### Making light of CsPbBr3 - precisely enough to be "unsplit"

A perovskite, normally used for solar energy collection, may find an application in quantum computing. The article is article is published in Nature (with a paywall), It is also covered in SciTech Daily. This is not the first material that has been able to generate photons so precisely...
17. ### I Electromagnetic Absorption in the Ocean

The recent rescue operation of a sub in the ocean made me wonder how difficult it is to communicate with underwater vessels. Is it that all parts of the EM spectrum gets absorbed at extreme depths that things like GPS is not possible. I wondered what wavelengths of the spectrum were absorbed...
18. ### I Where do the vibrational modes of molecules come from?

Hello everyone. First, sorry for my english. Second, I have got question where vibration mode of H2+ molecule (I think it is the most simple molecule for this topic explanation) comes from. If I should get basics before asking this tell me :). By my count the most important factor behind "being"...
19. ### I Interpreting Unitary Time Evolution

Hi all, This should be a simple question but it has been bothering me for a bit: Consider 2 Hamiltonian terms ##H_{1},H_{2}## that satisfy ##[H_{1},H_{2}] = 0##. Suppose we are working in the Heisenberg picture and we time evolve some operator ##A## according to ##A(t) =...
20. ### I A hurdle in quantum computing I've never been able to figure out

So the whole idea with quantum entangled computing, is that particles in superposition can compute more than one thing at the same time, right? But how does a system know which computed result is which? Maybe like a hashtag that separates one from another? But wouldn't that get jumbled, and...
21. ### A How can a 'Principle' produce a 'Force'?

In a collapsing star, the expression for what goes on is "degeneracy pressure". The way it's put is that the Pauli Exclusion Principle just doesn't allow more than one fermion to exist in one place (state). So the star reaches a certain volume and, on the way, produces a lot of Energy. I can...
22. ### How many branches are possible after measurement/splitting?

Say you have a simplified 1d Gaussian wave function describing location of a particle. Many worlds says that every outcome is a separate branch. Copenhagen says you will get one of those branches. So how many distinct positions, imaginary or real, can you generate from a fixed segment of a...
23. ### I Spin operator and spin quantum number give different values, why?

Assume spin 1/2 particle So the spin operator gives +/- hbar/2 eg. S |n+> = +/- hbar/2 |n+> But S= s(s+1) hbar = sqrt(3)/2 hbar So I'm off by a factor of sqrt(3). I suspect I am missing something fundamental about my understanding of spin. My apologies and thanks in advance.
24. ### I Questions about Grover's algorithm for quantum searches

Hi! I have studied Grover's algorithm for quantum search and I just want to make sure that I understood it correctly: to make a number k of calls to the oracle one needs to have k physical copies of the gate producing the oracle. In quantum circuits there are no loops, hence a physical gate...
25. ### I Ontology is to quantum theory what hardware is to computation theory

In interpretations of quantum mechanics there are two types of physicists: those who care about ontology and those who don't. The ontologists, or realists, want to know what is the world made of. The non-realists, on the other hand, think that this question is not relevant to physics. Usually...
26. ### I Quantum Mechanics as a Probabilistic forecast of reality

Is Quantum Mechanics a Probabilistic Forecast of nature? Someone I know told me their interpretation of QM is that QM only a probabilistic forecast of systems like electrons around atoms. I would like someone to analyse this interpretation and say if its valid or not. According to this person...
27. ### 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...
28. ### Classical Looking for book about relativistic classical field theory

Hi, I am trying to learn relativistic classical field theory as a preparation for studying quantum field theory. I am currently reading chapter 13 i Herbert Goldstein's Classical Mechanics edition 3, but I think that this book is a bit too brief and does not fully derive and explain the...
29. ### Studying Comparing Candidate Theories of Quantum Gravity

Hi, you all, I have been for a couple of semesters interested in quantum gravity as a problem, but truth is I never have been properly introduced to any of the candidate theories. Actually, there are multiple candidates and I would like to compare them. The question then is the following: do...
30. ### I Do black holes destroy quantum states?

This pop article popped up (isn't that what they do, by definition?) on my google news page. https://www.sciencenews.org/article/black-hole-paradoxes-quantum-states It claims that a thought experiment shows that doing a double-slit experiment near a black hole event horizon can reveal...
31. ### I Quantum Entanglement and Parallel Worlds/Existence

Hey! I'm new to the forums so its nice to be here. I don't have a deep deep background in physics (I plan to self study after I finish my math studies). However, I recently learned about the notion of quantum entanglement. My basic understanding of it is that quantum entanglement (will use QE...
32. ### I Principles of Quantum Mechanics - Hans C. Ohanian - 0137127952

Hi. I looked everywhere for a specific book but I cannot find any pdf copy of it. The book specifics are below: Publication Name: Principles of Quantum Mechanics Author: Hans C. Ohanian Publisher: Benjamin Cummings Publishing Company ISBN-10: 0137127952 ISBN-13L 9780137127955 I would...
33. ### Quantum Workforce Elements, Toolbox for QIS Programs at Universities

TL;DR Summary: Imagine you have infinite funds and cooperation to build a very ideal curriculum and student organization, both dedicated to quantum information science, at a university. What does this look like pedagogically? How would this be structured? What resources do students need access...
34. ### Quantum spin superposition interpretation

Here is my workings out:  If a particle's spin of magnitude ##\frac {\hbar}{2}## is prepared along direction ##\vec r_1## and subsequently its spin is measured along direction ##\vec r_2 ## at an angle ##\vec \theta ## to ##\vec r_1##, the probability of its being found "spin up" along is...
35. ### B Quantum Computers and their special properties?

I believe at the current time quantum computers can't get as much done as fast as normal computers, but do quantum computers have access to information by its own nature that allows it to run special calculations that normal computers can't? In particular physics or biology simulations using...
36. ### Quantum Trying to remember an unusual quantum textbook

Not sure whether it was a quantum physics, quantum information, or quantum computing textbook It spent the first few chapters/sections developing some sort of model theory in a very elementary context, in which (multi)functions were represented as boxes and outputs/inputs were represented as...
37. ### B Quantum Entanglement & Hybridization

I'm conflicted about how hybridization and quantum entanglement can simultaneously co-exist. I'm first confused about how quantum entanglement was proven. I tried to read to proves (I'm in grade 11 and planning on writing an ee on this) and it flew relatively over my head. Hybridization states...
38. ### B Quantum entanglement and simultaneity

I know very little about quantum physics. I was looking up the definition of quantum entanglement and asked ChatGPT to explain it. Here is an interesting phrase in its answer: "Once the particles are entangled, measurements made on one of the particles will instantaneously affect the state of...
39. ### Quantum Theory, particle in a ring

hello i would to get some help with my homework. 1. true 2. i dont know 3. true 4. i dont know 5, false 6. i dont know about 2,4,6 i really have know idea what to think I really appreciate help
40. ### Basic Quantum Circuit: States of Individual Qubits

I have done part A so far below, but I'm a bit behind on my reading, so I don't quite understand the action of the controlled-NOT gate on a single qubit. What I have so written so far for part B is: Let ##\mathcal{H}=(\mathbb{C}^2)^{\otimes 3}##. Let ##|\psi _{q_i}\rangle_k## , ##(i\in\left...
41. ### B Does anything happen in the quantum world?

Was Neils Bohr justified to claim that the quantum world does not exist? Are we stuck in a loop when having to use classical-world tools to probe and understand the micro world? Does anything happen in the quantum world?
42. ### I Bra-Ket Notation Manipulations: Quantum State Expansion

I suspect it will help if you know about my background: I did some linear algebra in university but never used it and am now in my mid 60s. I am interested in understanding the mathematics of quantum physics. I have read a number of layman's texts on quantum mechanics, but they all gloss over...
43. ### B Quantum entanglement and hidden variables

Hi, I have a basic understanding of quantum physics. I was reading a Wikipedia article on hidden variables, https://en.wikipedia.org/wiki/Hidden-variable_theory . The article says the following. I was confused about the words "local" and "nonlocal" in the quote above so I checked out another...
44. ### A Quantum analog of Boltzmann entropy?

In classical statistical physics, entropy can be defined either as Boltzmann entropy or Gibbs entropy. In quantum statistical physics we have von Neumann entropy, which is a quantum analog of Gibbs entropy. Is there a quantum analog of Boltzmann entropy?
45. ### I Does an electron have a quantum phase frequency?

Stationary solutions to the Schrödinger equation factor into a spatial part, e.g. atomic and molecular orbitals, and a temporal part that gives the phase rotation frequency. It is often assumed that adding a constant to the potential leaves the physics unchanged. And clearly, any "spectroscopic"...
46. ### I Repeated measurements on a quantum system interacting with other quantum systems

In quantum mechanics if I repeat a measurement of the same observable in succession I get the same quantum state if it is not a degenerate state. If I make the system under consideration interact with another quantum system and meanwhile keep measuring it what happens? Does the system not...
47. ### I Quantum effects near 0 Kelvin

In https://phys.org/news/2016-09-cold-black-holes.html it is stated that a supermassive black hole interior could be 10^-14 degrees Kelvin. Is there a limit, perhaps due to quantum effects, below which a temperature (in a black hole or elsewhere) can go? Or do the possibilities approach 0...
48. ### A How to derive Quantum Mechanics in curved physical space?

I am following [this YouTube lecture by Schuller] where he finds the appropriate formalism for the quantum mechanics in the physical curved space. Everything makes sense to me but at the very end I see that we find the pull backed connection one-form on the base manifold. He says to the end...
49. ### I Limit of quantum mechanics as h -> 0

Starting from the Heisenberg equation of motion, we have $$ih \frac{\partial p}{\partial t} = [p, H]$$ which simplifies to $$ih \frac{\partial p}{\partial t} = -ih\frac{\partial V}{\partial x}$$ but this just results in ## \frac{\partial p}{\partial t} = -ih\frac{\partial V}{\partial x}## and...
50. ### I Ways of measuring open quantum systems

At the heart of the theory of open quantum systems is the idea that the measurement statistics of many-body systems can be expressed in terms of a reduced density matrix, obtained by tracing over degrees of freedom that are irrelevant to the system of interest. In general, given a pure state...