Quantum Definition and 999 Threads

Suppose we have a quantum system ##S##, an apparatus ##A##, and an observer, say Alice, ##O##. WLOG let ##S## be a spin-##1/2##particle in a state ##0.6|\uparrow\rangle+0.8|\downarrow\rangle##. The apparatus measure it in the ##\sigma_z## basis. Then the observer sees the result. According to...

LSU Physics grad, 30+ years as a Health Physicist for the State of Louisiana, retired guardsman of the military intelligence branch. Interested in approaches to quantum gravity, e.g., Maldaxena, Swingle, Preskill, Van Raamsdonk et al. Convinced the cosmological constant problem is rich with...

In relation to travel back in time, we've all heard of the grandfather paradox, whereby killing your grandfather before they sired offspring would preclude your future existence. This contradiction leads to the conclusion that time travel to the past must be impossible. But it doesn't quite end...

I don't have good references yet and am hoping you all can help me understand my pop-sci reading. I have read that quantum mechanics implies that there is a very small probability that I can fall right through my chair, or suddenly appear on the moon among other crazy things. I subscribe to the...

"We present, for the first time, the full history of the evaporation of a large charged black hole." https://arxiv.org/abs/2411.03447 The evaporation of charged black holes Adam R. Brown, Luca V. Iliesiu, Geoff Penington, Mykhaylo Usatyuk Charged particle emission from black holes with...

The internet has a question for the PF members who are also musicians. Is quantum physics easier than playing this guitar piece?

In Peskin's textbook, the coupling factor is given by a constant in the interacting field theory. The scattering matrix ##S## is given by the time-evolution operator, ##\exp(-iHt)##, in the limit of very large t, i.e., ##t\rightarrow \infty##, as expressed in Eq. (4.71). In my mind, the...

Consider an infinite well between ##0## and ##a##, the energy eigen functions are: $$\phi(x)=\sqrt{\frac 2 a}\sin{\frac{n\pi x}a}$$ Since the Hamiltonian of this system is only a function of momentum operator ##(\hat H=\hat p^2/2m)##, we should be able to find a common energy and momentum...

Hi All, We say that the Schroedinger equation stipulates a smooth and unitary evolution for the wave function, and that the measurement causes the wave function to collapse into one of the eigenstates of the operator that represents the observable parameter being measured by the apparatus. My...

In the following thread: https://www.physicsforums.com/threads/particle-movement-in-quantum-mechanics.1054807/ the discussion established/confirmed that the matter of a particle’s continuity of movement in Quantum Mechanics QM is not a scientific question in QM, refer to PeroK post # 6...

As this has been a key point, in several threads in particular the last close one https://www.physicsforums.com/threads/local-mechanism-for-nonlocal-anticorrelations-inside-spin-theory.1066463/ I though I would clarify what this means becuase it is a pity to confuse it with classical (newtons)...

In the standard model fermion field components collect quite a few labels. The basic fermion field has 4 components that obey anti commutation relations. If one has two types of fermions, say electrons and muons. Do these commute or anti commute? Same question for other labels like gauge group...

This page is Quantum mechanics by bransden. My homework is explain why there is no regulation of quantum number n in selection rule. Also explain that by solving that integral of radial part is always non-zero. ∫∞0[rRnl(r)]Rn′l′(r)r2dr (n is different with n') I tried to solve it by just...

Suppose in the presence of an electric field we solve electronic Hamiltonian with eigen energies and corresponding eigenstates ##|\psi\rangle##. The action of momentum operator on the stationary states ##|\psi\rangle## doesn't change by time. So, momentum-change (or acceleration) seems...

https://www.amazon.com/Student-Friendly-Quantum-Field-Theory/dp/0984513957?tag=pfamazon01-20

I want to create a thread about quantum ethology, and don't know whether such thread will be ok for moderators. Can I ask somebody whether I can post it? I wrote a PM to Greg Bernhardt, but he didn't respond, maybe he receives too much messages to read all? Whom of the moderators can I ask such...

My question is about a statement in the book, Many-Body Quantum Theory in Condensed Matter Physics: An Introduction, by Henrik Bruus, Karsten Flensberg in appendix A: In a translation invariant system, any physical observable ##f(\vec r,\vec r')## of two spatial variables ##\vec r## and ##\vec...

It is a giveaway. https://www.amazon.com/Quantum-Field-Theory-Standard-Model/dp/1107034736/?tag=pfamazon01-20

Classics ! James D. Bjorken, Sidney D. Drell - Relativistic Quantum Mechanics (1964) James D. Bjorken, Sidney D. Drell - Relativistic Quantum Fields (1965) Sold as a pair (reasonable infraction on the 1 book/thread guideline) Mint condition

Free electrons can not absorb photons. Otherwise, the 4-momentum will not be conserved. That is what I understood from the very first lecture of QED. However, we do have the semi-classical picture for HHG - electrons are kicked out from the atom and get accelerated by the radiation field. I...

I was pondering the notion of fiction in the multiverse and remembered my fluids dynamics lecturer many years ago telling us all something about quantum mechanics and how the air molecules in the room could all spontaneously move to one side and we would all suffocate. Slightly disturbing and...

While reading something recently that mentioned the Casimir Effect, it occurred to me that the plates used in an experiment to measure the force between them would be close enough to allow quantum tunnelling if the plates were transparent, which led me to wonder whether (and how much) the force...

Cost me a fortune when the dollar was so high in the eighties :rolleyes: ##\ ##

Hello everyone, I'm trying to recall the title and author of a quantum mechanics textbook I read many years ago, likely a couple of decades or more. My local library had what I thought was a very good quantum mechanics book, which I worked through during the summer holidays after completing my...

Roderich Tumulka (a long-time Bohmian) demonstrates in his newest preprint, R. Tumulka, A Vision for a Bohm-Style Theory of Quantum Electrodynamics, arXiv:2409.07784. that for the interpretation QED, the Bohmian interpretation has nothing to offer (apart from speculation called vision), and...

TL;DR Summary: Funny (but very well informed) video about QM. Hope this is not considered off-topic, but anyone so inclined and with 4+ mins to spare might enjoy this (intentionally funny) QM video by eigenchris:

I am happy to announce my book A. Neumaier and D. Westra, Algebraic Quantum Physics, Vol. 1: Quantum mechanics via Lie algebras, de Gruyter, Berlin 2024. It features a mathematically rigorous but still physically lucid account of quantum mechanics from the point of view of symmetries...

Assuming the situation where there is an infinite inflation stream, from which bubble universes are appearing in locally slowed regions. There could be many many such bubble universes and so very rare occurrences in some bubbles can sometimes happen. Also assumed, as is often postulated, that...

Hello to everyone. I have some doubts about one problem of quantum mechanics. My attempt. I need to calculate the coefficient ##W_{ij}=<\psi_i | H' |\psi_j>## where ##H' = -eE(t)z## is a perturbation term in the hamiltonian and ##|\psi_i> = |\psi_{nlm}>##. We have four states and sixteen...

So one of the major topics in physics is trying to reconcile quantum mechanics and general relativity. And the classical expression of electromagnetism is Maxwell's equations, and its linked to quantum models of electromagnetism. And then there's also a gravitational analogue of Maxwell's...

How does QFT treat the Young’s DSE? Is there a wave function (wave packet) attached (and created at the moment of launching of the photon) or the modes of the EM quantum field are pre-existing due to experimental configuration (including the screen) and do they play the role the wave function is...

How did you find PF?: Wandering on the internet looking for answers about Bell's theorem I teach adult education at 2 universities near Denver and I'm writing a book for non-scientists interested in quantum mechanics and its foundations. The challenge is to present the concepts digestibly...

https://physics.stackexchange.com/questions/824561/two-element-theory

For this, The solution is I have doubt where they got the reflection probability formula from. Someone may know how to find it. I think that ##R + T = 1##. But I'm not sure where the transmission probability formula comes from either. Kind wishes

For this problem, The solution is, I have a doubt about Step number 3 about boundary conditions. Someone maybe be able to solve that doubt? Kind wishes

So basically assume both the star and earth have high-tech machines and a mapping of natural changes in orbit such that the detectors will always be the same settings on star and earth, and the ship in between earth and star will move in such a way that the alignment will be exactly halfway in...

Suppose that Bob and Alice decides to do a quantum experiment, more precisely, The Shroedinger Cat Experiment. Alice goes to the Andromeda Galaxy, and wait for Bob’s results. According with the MWI, the wave function never collapses, suppose that in one world Bob found the cat alive, in the...

in an interview Michio Kaku was asked about Loop Quantum gravity and he replied that it is only a theory of pure gravity, but that the universe also contains particles, the particles of the standard model, and only superstring theory unifies both quantum gravity and the standard model. the...

Recently I’ve been learning more about quantum physics, and Physics Forums has had a lot of good answers to questions I had. I’m looking forward to learning even more, and asking questions that I’ve beenconfused about.

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hi, I'm solving this statement, We split into parts Can some expert in Quantum say that my working is professional? Kind wishes to you

Hi, I try and solve this problem I have solved the problem in different parts But me not sure how to plot the graph. Maybe someone knows? Merci

How did you find PF?: Good Need documents to help me understand quantum computing!

A quantum dot is placed in a line between two optic fibers The dot can emit a photon in every direction which is unknown. If the fibers are combined at a beamsplitter would there be interference if the photon is not observed outside the fibers? Do you know if somebody made such experiment...

I have come across the following paper (arxiv preprint link, it looks like it is published in Phys Rev D) that claims to have found a consistent quantum field theory of tachyons, overcoming three issues which have been said in previous literature to make such a theory impossible...

I am currently a research scholar specializing in open quantum systems.

Hello, I am interested in the following model: $$ H = \sum_{<i,j>} -t (c_i c_j^{\dagger} + \text{H.C.}) + U (n_i n_j) + \sum_{<<i,j>>} -t' (c_i c_j^{\dagger} + \text{H.C.}) + U' (n_i n_j) $$ where \( <i,j> \) indicates nearest neighbors, and \( <<i,j>> \) indicates next-nearest neighbors...

My first "active" encounter with the transactional interpretation happened when I tried to react to Ruth Kastner's criticism of consistent histories. My next encounter happened when I was fighting with reciprocity occuring for crystal effects in electron scattering, and hoped for help from time...

I am a second-year combined honour in physics and math with prerequisites for 4th-year math courses. I am hoping for a direct entry (if possible) into a top US PhD physics program in quantum with a current unofficial cGPA of 3.9/4 (88%). Since I am in Canada, we only have percentage grades that...