What is Quantum mechanics: Definition and 995 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.
I am going to be doing a high school senior research project on how Quantum Physics will change the world in the future. Basically my current idea is to describe what quantum mechanics is, and then give examples that I explain and I state how they will change the world and the estimated time...
Studying QFT on curved spacetimes I've found the algebraic approach, based on ##\ast##-algebras. In that setting, a quantum system has one associated ##\ast##-algebra ##\mathscr{A}## generated by its observables.
Here we have the algebraic states. These are defined as linear functionals...
This ties into this thread https://www.physicsforums.com/threads/i-want-to-know-the-exact-problems-of-merging-gr-and-qm.939509/ , I would like to know SR/GR's opinion of QM/QFT. I need both sides of the story.
This thread is I want a set of experts in the subject to show me the exact math of why Einstein's field Equations along with Special Relativity and Schrodinger's Equation along with deeper QM like QFT cannot be fused with GR. I want to see the exact anomalies in the equations myself from the...
Hi all, so I'm not sure if what I'm asking is trivial or interesting, but is there any general or canonical way to interpret say, The follwing operator? (Specifically in the study of quantum mechanics):
A = 1/(d/dx) (I do not mean d-1/dx-1, which is the antiderivative operator )
How would...
Hi all,
There is this question;
Firstly, I am new to quantum mechanics and there are a lot of terms I am unfamiliar with. So there is this question that asks me to develop a method to reconstruct state and I have no clue how I should start with. Any help or steps to solve this type of...
[Mentors' note: one short paragraph of speculation in violation of the Physics Forums rules has been removed from this post]
Follow the link below to play with my experimental sandbox. The rest of this post includes some instructions for use and then observations and a little bit of theory...
It is the resolution given by my thermal interpretation, and this resolution is valid (independent of the thermal interpretation) even without being accepted.
Bell assumes that measurement outcomes follow strictly and with infinite precision Born's rule for a von Neumann measurement. But the...
Hi, I have two questions concerning the double slit experiment in the scenario where we fire one photon at a time and it interacts with itself to create an interference pattern over time:
- Does the photon actually interact with itself or with the photons fired before/after it?
- What happens...
This problem bothered me many years ago when I was taking a university course in
quantum mechanics, but I assumed it was due to an error on my part which I, however,
couldn’t locate, and I didn’t ask my course instructor about it. Recently, when
researching a q.m. question on the Internet...
Greg Bernhardt submitted a new PF Insights post
The Fundamental Difference in Interpretations of Quantum Mechanics
Continue reading the Original PF Insights Post.
Hi all,
Is there a critical analysis of this work ( https://arxiv.org/abs/1506.00431 ) that claims to develop a new representation of microstate and then using this representation as a reference-and-imbedding-structure, to develop a new foundations of an intelligible reconstruction of the...
I would be in college in 2019 (currently I'm in standard 11). I'm greatly interested in Quantum mechanics, QFT, QCD and Quantum Geometerodynamics. Of these, I want to do an internship on the first, because I don't think I'll be able to touch the others till the 2nd year in college.
I'm living...
Homework Statement
Homework EquationsThe Attempt at a Solution
The energy eigen - value of a particle in a box is given by ## E_n = \frac { n^2 h^2}{8mL^2} ## .
Now, applying classical mechanics , ## \frac { p^2}{2m} = \frac { n^2 h^2}{8mL^2} ## .
## p \propto \frac { 1} L ## ,
So...
Homework Statement
Homework Equations
This is solution of Griffith problem 11.16
The Attempt at a Solution
This is procedure to get a 1-D integral form of Schrodinger equation.
I don't understand why that contour integral include only one pole for each contour?
Hi, I'm a new member. I joined this forum because I'm interested with astrophysics and cosmology. I'm actually in the field of the Humanities, currently taking up my Master's in Art History. I like physics because I think it is where art and science beautifully intersect. I started to see the...
Homework Statement
The answer is as follows: [/B]
However they said that time t=0 so I am confused how the exponent has a t in it surely it should be zero. Thanks
Hi everyone, I just have some confusion regarding Planck's and Einstein's equation.
The following is an explanation of the photoelectric effect using Einsteins theory:
Light is composed of photons. Each photon has energy hf and mass hf/c^2. When ultraviolet photons are brought to rest by zinc...
Homework Statement
A particle is restrained to move in 1D between two rigid walls localized in ##x=0## and ##x=a##. For ##t=0##, it’s described by:
$$\psi(x,0) = \left[\cos^{2}\left(\frac{\pi}{a}x\right)-\cos\left(\frac{\pi}{a}x\right)\right]\sin\left(\frac{\pi}{a}x\right)+B $$
, determine...
Homework Statement
How can we tell that quantum effects do not propagate along forward light-cones that represent two separate events? How do light-cones demonstrate this notion of quantum effects? Homework EquationsThe Attempt at a Solution
We don't know which events occur first given...
If Alice and Bob had a large number of particles entangled together (with state of either 0 or 1), could Alice send information by breaking the entanglement of particles with state of 1 (by flipping the state for example) so that Bob measures 50% of particles to be in the state opposite of 0...
If I have a general (not a plain wave) state $$|\psi\rangle$$, then in position space :
$$\langle \psi|\psi\rangle = \int^{\infty}_{-\infty}\psi^*(x)\psi(x)dx$$
is the total probability (total absolute, assuming the wave function is normalized)
So if the above is correct, does that mean...
In Hartree-Fock method, I saw the Fock operator has two integrals: Coulomb integral and exchange integral. One can define two operator. "The exchange operator is no local operator" why? Whats de diference: local and no local operator?
And why do the operators have singularities?
thanks
"... quantum mechanics isn’t a complete physical theory in its own right, but rather a framework for the construction of physical theories."
I found this in Michael Nielsen's blog and elsewhere. I am perplexed. In what fundamental way does it differ from Newtonian Mechanics (Theory) in not...
Homework Statement
Earlier in the book it was remarked that early in the history of nuclear physics the electrically neutral mass of nuclei now attributed to neutrons was considered to arise from neutral particles composed of combinations of protons and electrons (as opposed to neutrons in...
In the momentum representation, the position operator acts on the wavefunction as
1) ##X_i = i\frac{\partial}{\partial p_i}##
Now we want under rotations $U(R)$ the position operator to transform as
##U(R)^{-1}\mathbf{X}U(R) = R\mathbf{X}##
How does one show that the position operator as...
Homework Statement
[/B]
Consider the hydrogen atom in the 42F5/2 state. Take into account the effects of finestructure (spin-orbit coupling).
(a) Write down the spectroscopic notation of the state that the 42F5/2 is degenerate with, in the absence of an external magnetic field.
(b) Calculate...
Question
(a) Write down the quantum numbers for the states described in spectroscopic notation as 2S3/2, 3D2 and 5P3.
(b) Determine if any of these states are impossible, and if so, explain why. (Please note that these could describe states with more than one electron.)
My Attempt
A)
I came...
Hi everyone! I'm trying to make a list of recommended books (introductory and advanced). So far, what I was able to search are the following:
Particle Physics:
- Griffiths: Introduction to Elementary Particles
- Thomson: Modern Particle Physics
- Nachtmann: Elementary Particle Physics
-...
It seems like every argument in favor of the Simulation Hypothesis revolves around the strangeness of Quantum Mechanics. So can the Simulation Interpretation be just as valid of an interpretation as Copenhagen or Many Worlds? If not why not?
Homework Statement
Given 3 spins, #1 and #3 are spin-1/2 and #2 is spin-1. The particles have spin operators ## \vec{S}_i, i=1,2,3 ##. The particles are fixed in space. Let ## \vec{S} = \vec{S}_1 + \vec{S}_2 + \vec{S}_3 ## be the total spin operator for the particles.
(ii) Find the eigenvalues...
Homework Statement
The Hamiltonian of a certain two-level system is:
$$\hat H = \epsilon (|1 \rangle \langle 1 | - |2 \rangle \langle 2 | + |1 \rangle \langle 2 | + |2 \rangle \langle 1 |)$$
Where ##|1 \rangle, |2 \rangle## is an orthonormal basis and ##\epsilon## is a number with units of...
1. The problem statement
Consider a particle of mass m under the action of the one-dimensional harmonic oscillator potential. The Hamiltonian is given by
H = \frac{p^2}{2m} + \frac{m \omega ^2 x^2}{2}
Knowing that the ground state of the particle at a certain instant is described by the wave...
Homework Statement
Homework Equations
in addition to those provided in the questions, I used the following:
Tr(B) = sigma<x_j|B|x_j>
purity = Tr(rho^2)
The Attempt at a Solution
[/B]
I find calculating trace and purity very confusing. Am I on the right track with question 1? With...
i want an article or video that i can start with at the journy of understanding the basics of quantum physics and quantum mechanics
i don't want a book because in my country i would not find this types of books every where it's unique to find them
thanks
Hi everyone;
A very stupid confusion here. When we want to talk about the most probable radius to find the electron in $1s$ orbital, why do we talk about the radial density and not the probability itself? For instance, the probability of finding the the electron at a radial distance $r$...
Homework Statement
A) Show that <x>=<p>=0
hint: use orthogonality
B) Use the raising and lowering operators to evaluate an expression for < x2 >
Homework Equations
Also A- and A+ will definitely come in handy
The Attempt at a Solution
I tried setting up the equations for <x> and <p> but I...
Homework Statement
Show that the virial theorem holds for all harmonic-oscillator states. The identity given in problem 5-10 is helpful.
Homework Equations
Identity given: ∫ξ2H2n(ξ)e-ξ2dξ = 2nn!(n+1/2)√pi
P.S the ξ in the exponent should be raised to the 2nd power. So it should look like ξ2...
Homework Statement
Show that application of the lowering Operator A- to the n=3 harmonic oscillator wavefunction leads to the result predicted by Equation (5.6.22).
Homework Equations
Equation (5.6.22): A-Ψn = -iΨn-1√n
The Attempt at a Solution
I began by saying what the answer should end...
In an experiment involving electron scattering from a finite rectangular well of depth 4 eV, it is found that electrons of energy 5 eV are completely transmitted. What must be the width of the well? At what next higher energy can one expect to again observe T = 1?
My Attempt:
I used the...
A massless spin 1 particle has 2 degrees of freedom. However, we usually describe it using four-vectors, which have four components. Hence, somehow we must get rid of the superfluous degrees of freedom. This job is done by the Maxwell equations. To quote from Gilmore's "Lie Groups, Physics, and...
Homework Statement
Homework EquationsThe Attempt at a Solution
This is the Solution. I am having trouble understanding parts of it.
The first part I don't get is why the e^i... goes with the -z. Did my professor just choose one at random, or is there a specific reason?
The second part...
Homework Statement
I am trying to understand a solution to a problem. I may not need to post the entire question, I just need to know if ##-i = e^{\frac{-i*pi}{2}}##
Homework EquationsThe Attempt at a Solution
the reason for this question is that one step of the problem has a quantity...
Hi fellow thinkers, my main area of interest is in QM. However I am always interested in new discoveries and experimental proofs of outstanding theories. Looking forward to joining the discussion with others who, like me, enjoy absorbing new information...
S.P.W...
Hi.
From what I read, according to Quantum mechanics "even though it looks like objects are in definite places, when we get down to ridiculously tiny objects (like electrons) they seem to be not he in anyone place. And everything looks like a hazy clouds of probability."...
Homework Statement
This is a question asked in a entrance examination[/B]
A charged particle is in the ground state of a one-dimensional harmonic oscillator
potential, generated by electrical means. If the power is suddenly switched off, so that the
potential disappears, then, according to...