Quantum physics Definition and 717 Threads

(Disclaimer: this is not a HW question. I am self-studying, and this felt like the type of question I've seen in this forum. If there is somewhere better for me to share this doubt, please let me know and I'll transfer it right away.) I am currently reviewing Chapter 7 of Introduction to QM by...

Hello and welcome. In my free time I like to think about science. Currently, AI helps me with this. I build a hypothesis as a foundation, and AI checks whether it can describe it mathematically and what conclusions can be drawn. When it turns out that it is nonsense, I write AI not to worry...

Disclaimer: I’m new to QM, so things I say might be wrong, please forgive me for possible inaccurate info. So as for what I know, quantum gravity is the study of merging GR and QM on gravity. Here’s a few questions I’m interested about on the quantisation of gravity: String theory 1. What is a...

I’m new to quantum physics, and I probably don’t understand a lot of things. But there is one question I wanted to ask: What would happen if there were no observers? Nothing that could measure? (Edit) After doing a little bit of digging, apparently a thing doesn’t need to be observed to exist...

In Sakurai, section 4.4, says that if ##\psi(x,t)## is a solution of Schrodinger equation then ##\psi^*(x,-t)## is always another solution. However, in the corrected version of "Many-body Quantum Theory in Condensed Matter Physics, Bruus and Flensberg, 2016", section 7.1.4, restricts this...

I've calculated density matrix $$ rho_{ABC} = \frac{1}{2} \left( \left| 100 \right\rangle \left\langle 100 \right| + \left| 100 \right\rangle \left\langle 001 \right| + \left| 001 \right\rangle \left\langle 100 \right| + \left| 001 \right\rangle \left\langle 001 \right| \right)$$ and ##...

Video on Brownian matter that got me thinking Brownian motion is a fundamental concept in physics, describing the random movement of particles suspended in a fluid. However, the apparent randomness of this motion is largely due to our limited understanding and computational power. As...

I'm 64. My background prior to 2001 - Aviation, experimental aircraft and aerodynamics, robotics, mechanical and software engineering, disaster recovery. Since 2001 - Human sustainability and global recovery, researching every related sector in depth in order to understand what can now be proven...

I am interested in the topic of vacuum phase transitions in models of the universe. One popular instance of this is a vacuum decay from a metastable vacuum energy level to a "true" one (in which the vacuum would sit at the lowest possible energy level depending on the model) I have 4...

Below is the derivation of E1so, the first-order correction to the Hamiltonian due to spin-orbit coupling of the election in hydrogen atom. My question is whether it's valid to use [6.64] (see below). ##<\frac{1}{r^3}>## I believe is ##<\psi_{nlm}|\frac{1}{r^3}|\psi_{nlm}>##, but ##\psi_{nlm}##...

First, a warning: I do not know much about hidden-variable theories, so my apologies in advance when some of my questions seem rather obvious to people better versed in them than me. Without the Born rule, much of quantum physics falls apart, and the Born rule does wonders at predicting data...

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So I've seen that radar can be used in space which is interesting and brings questions to mind as well as theories. Unrelated Sonar which uses sound waves don't work as sound isn't transmittable through space. Questions: 1) What techniques are used in radar to prevent "wondering" or the radio...

Hi, as discussed in this recent thread, for a particle without spin the quantum state of the particle is described by a "point" in the Hilbert space of the (equivalence classes) of ##L^2## square-integrable functions ##|{\psi} \rangle## defined on ##\mathbb R^3##. The square-integrable...

so i was reading about Objective collapse theory and i found out that apparently one of the main problems/criticisms these theories have is they violate conservation of energy. From wikipedia "According to collapse theories, energy is not conserved, also for isolated particles. More precisely...

Hello, I have been interested in physics and math since I was a student. Now I am just retired and have time to dedicate myself to deepening the study of these subjects. Above all, I would like to know better the theoretical foundations of quantum physics and relativity. I have been a teacher...

I think 10 more IQ points would help me understand quantum physics at the level I'd like. That said, I'd like to know more about Qbits. I've watched several videos about them, and they say that they can be 1 and 0 at the same time. Is that just an analogy to the polarization of the old...

Hello! Im a freshman in college, taking pretty basic chem classes and Ive found myself in a deep dive regarding quantum physics. Im sure this is pretty simple and easy compared to everyone else on here but I feel like I keep getting oversimplified answers that just leave me with more...

Question: Is it meaningful to think of the repulsion of mutual color charge and the attraction of three different color charge in QCD as being indicative of the classical concept of work taking place? Exactly, how is this explained in the context of three charges needed to elicit the...

TL;DR Summary: A particle of mass m, placed in an infinite rectangular one-dimensional potential well that confines it in the segment between x=-a/2 and x=a/2 Hi guys, I need help with this exercise which reads: a particle of mass m, placed in an infinite rectangular one-dimensional potential...

The Schrodinger equation is $$ i\hbar \frac{\partial\Psi}{\partial t} = -\frac{\hbar^{2}}{2m} \frac{\partial^{2}\Psi}{\partial x^{2}} + V \Psi $$ Why is the coeffient on the left-hand side ##\hbar##, not ##\frac{\hbar}{2}## or ##i\frac{\hbar}{3}## or something like these Besides, in quantum...

The first thing we need for this is to define what a particle is... It is an object that has specific intrinsic properties and is described by a wave sign How to measure it? This is done by the interaction of the particle to be measured with the measurement system. When measuring, the wave...

Reading Einstein Relativity. The trains experiences lightening strikes differently to the embankment. So the two events are experienced differently from the two reference points because they have different distances to the events. Light from the events taking different times to reach each...

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I am reading the following thesis: https://www.kip.uni-heidelberg.de/Veroeffentlichungen/download/6387/pdf-6387.pdf Specifically, I am confused about equation (2.5), where they give the generic form of the matrix ##\mathcal{M}## due to the Hermiticity of ##\mathcal{H}## and the commutation...

Hi all, I am reading the following post: https://physics.stackexchange.com/questions/336336/from-euclidean-correlator-in-cft-to-time-ordered-correlator-in-real-time-how-do and was confused regarding the portion in the accepted answer where they made a distinction for the ##t>0## and ##t<0##...

It appears to misunderstand the notion of the detectors D3, D4 providing path information and affecting the results D0. My question is: When there is no path information being provided, why does the experiment still call for moving D0? I understand that by moving D0 we can modify the arrival...

Hi I am using the textbook "Modern Particle Physics" by Thomson. Working from the K-G equation and comparing with the continuity equation he states that the probability density is given by ρ = i ( ψ*(∂ψ/∂t) - ψ(∂ψ*/∂t) ) He then states that the factor of i is included to ensure that the...

In the 2001 paper Attosecond Metrology, the authors presented the first attosecond pulse setup and explained the measurements. This paper cites another paper quite often to mention similarities in the setup, but emphasized that they chose different observables: and only the choice made by...

Hey all, I am having trouble relating probabilities with the density matrix of multiple qubits. Consider we have a system of 3 qubits: the first qubit is in the state ##\ket{\psi} = \frac{1}{\sqrt{2}}(\ket{0}+\ket{1})## and the remaining 2 qubits are prepared in the state described by the...

In Classical Physics, when a charged particle oscillates, it emits an electromagnetic wave, and the frequency of the wave depends on the frequency with which the particle oscillates. But in Quantum Physics, when an excited atom emits a photon, the energy of the photon depends on the amplitude of...

Can you explain to me the reason why Thomson Scattering can not explain what happens when light meets an electron at low intensity, and what does that have to do with light being a wave or particle or relativistic/QM effects? https://en.wikipedia.org/wiki/Compton_scattering

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So the double slit experiment. If I understand correctly when electrons are shot through 2 slits and no one is monitoring, measuring or watching they create an interference pattern if they are being measured they create a 1 to 1 pattern. I keep seeing it be said that the mere act of monitoring...

In think the popular science discussion about multiverse theory has problems. For example, they suggest that in another universe, coffe is green. Problem, coffe in our universe is brown for a reason, it reflects brown light. If it would reflect green light, it must contain other chemical...

Niels Bohr famously said --and I paraphrase-- that QM is an abstract description of nature and that it can only prescribe what we can say about nature rather than what nature is. What does QM say about the movement of a particle? Is this movement positively ascertained to be smooth and...

Unfortunately one of the threads about entanglement and Bell tests has again been closed prematurely. It has not been clarified what "locality" means. In the physics community, not involved in philosophical arguments about foundations of QT, it's clearly defined as the property of a...

Hello all, I have a question about the relationship between resp. and the Bloch equations . Are these upper equations special solutions of the Bloch equations? If yes, under what condition(s) do the solutions hold? Thanks in advance for helpful support!

I do not think that true emergent properties -- as defined by behavior of matter that cannot be reduced to fundamental physical law -- exist. Yet I have been told that the Fractional Quantum Hall Effect is an example of an emergent property. What is the consensus?

The first look at a scattering process is something like this: We define an initial state |\textrm{in}\rangle = \int dp_1dp_2 f_{\textrm{in,1}}(p_1) f_{\textrm{in,2}}(p_2) a_{p_1}^{\dagger} a_{p_2}^{\dagger} |0\rangle Here f_{\textrm{in,1}} and f_{\textrm{in,2}} are wavefunctions that define...

I have an idéa how to run the double slit experiment that could give new insight to whats hapening. As I understand, when the photons are observed by someone the wawefunction colapses and the photons become particles, this can be seen as the interference pattern dissapears in the experiment...

It is known that in the double slit experiment, when successive photons are fired, the photons are statistically distributed on the target screen, as if it were a wave. What is the variable that changes between one shot of and another? Probably the slits change places as a function of time.

[Mentor Note: Two similar thread starts merged] The questions are from MIT OCW. First of all, I cannot understand what is the meaning of the measurement outcome being 0. How can an eigenvalue be 0? I tried doing the problems guessing that by 0 they mean the posterior state will be |0>. The only...

A while back I was watching an interview with Sean Carroll and for some reason his explanation of the Everett's interpretation at that particular interview clicked for me and I became a proponent of it. But I always thought it made more sense to look at it at as the universe being a giant...

Hi. I am not being able to understand how we are getting the following spectral decomposition. It would be great if someone can explain it to me. Thank you in advance.

Hello. I have a question about the two photons emitted from a radiative atomic cascade (such as the calcium radiative cascade used by Aspect et al. in their tests of Bell's theorem). The short version of my question is this: Do the two photons have any directional correlation (any correlation...

Hi all, I am having trouble understanding the argument below equation (3.5) in https://arxiv.org/pdf/cond-mat/9605145.pdf where they claim that "Upon antisymmetrization, however, a term with ##k## factors of ##(z_{i}-z_{j})## would have to antisymmetrize ##2k## variables with a polynomial that...

Hello! I am confused about the derivation in the screenshot below. This is in the context of a diatomic molecular potential, but the question is quite general. Say that the potential describing the interaction between 2 masses, as a function of the radius between them is given by the anharmonic...

Does this mean that everything, including us, will turn into photons/electromagnetic waves some day? Sounds fun! A universe of matter converted entirely to light! I couldn't find any papers yet on this on scholar.google.com Here are some good articles though...

I'm just an ordinary person who's very interested in physics. I'm posting a question because I'm curious about quantum mechanics. The wavelength of the material wave that can be obtained when a baseball with a mass of 150 g is thrown at 40 m/s is 1.1×10^-34m by the h/mv formula. As you can see...