What is In quantum mechanics: Definition and 256 Discussions
In quantum physics, a measurement is the testing or manipulation of a physical system in order to yield a numerical result. The predictions that quantum physics makes are in general probabilistic. The mathematical tools for making predictions about what measurement outcomes may occur were developed during the 20th century and make use of linear algebra and functional analysis.
Quantum physics has proven to be an empirical success and to have wide-ranging applicability. However, on a more philosophical level, debates continue about the meaning of the measurement concept.
Hi,
I'm working on a problem where I need to find the different energies allowed for a potential, and I found this link https://quantummechanics.ucsd.edu/ph130a/130_notes/node151.html,
which is similar of what I'm doing. I'm using mathematica to find the values of E.
However, I'm not sure how...
As per title and the TL;DR, I'm curious if there could be some truth in these statements of the headlines I had read recently or are they just sensationalist fluff.
Personally, I find these statements very hard to believe. In fact, impossible to believe. But I'm not a QM expert, not even an...
Just earlier today i was practicing solving some ODEs with the power series method and when i did it to the infinite square well i noticed that my final answer for ##\psi(x)## wouldn't give me the quantised energies. My solution was
$$\psi(x) = \sum^{\infty}_{n=0} k^{2n}(\cos(x) + \sin(x))$$...
Very basic question here, about statistical independence in quantum mechanical experiments. The quote from PD below is what prompted the question.
When we talk about "some kind of pre-existing correlation" are talking about a simple correlation in the sense of the correlation of sunglasses and...
To solve a particle on a sphere problem in quantum mechanics we get the below equation :##\left[\frac{1}{\sin \theta} \frac{d}{d \theta}\left(\sin \theta \frac{d}{d \theta}\right)-\frac{m^{2}}{\sin ^{2} \theta}\right] \Theta(\theta)=-A \Theta(\theta) ##
To solve this differential equation, we...
Let ##|l,m\rangle## be a simultaneous eigenstate of operators ##L^2## and ##L_z## and we want to calculate ##\langle l,m|cos(\theta)|l,m'\rangle## where ##\theta## is the angle ##[0,\pi]##. It is true that in general ##\langle l,m|cos(\theta)|l,m'\rangle=0## ##(1)## for the same ##l## even if...
* The general formula for the magnetic moment of a charge configuration is defined as ##\vec{\mu} = \frac{1}{2} \int \vec{r} \times \vec{J} \,d^3r##* For an electron it's said that the correct equation relating it's spin and magnetic moment is is
##\vec{\mu} =g\frac{q}{2m}\vec{S}##
* It's...
According to the uncertainty principle, when we measure a micro-object with a measuring device, we cannot predict what value the device will show. But if we knew exactly the wave function of this device, together with the wave function of the micro-object, could we exactly predict the result of...
Take a simple case: A system is prepared in state ##\rho_i## at time ##t_0##, and a projective measurement is performed at time ##t_2## with an outcome ##b##. We can retrodict a projective measurement outcome ##a## at time ##t_1## where ##t_0<t_1<t_2##$$p(a|b) =...
I had two questions in the field of physics:
We know that in quantum mechanics there is an electron in a certain distance from the distance to the nucleus as a cloud or a cover. But is motion for the cloud defined by the electron moving around the nucleus?
And the main question is, can the...
Hello! If I place a particle with more energy levels (of the order of kT) in a well defined state, in a thermal bath at temperature T, how will the blackbody radiation affect the internal state of the particle i.e. will the distribution be classical or QM? Basically, if I prepare that particle...
If we prepare a macroscopic system (something like Shrodinger's cat) in a known quantum-mechanical state and we let it evolve for a very long time completely isolated, for what I understand the position of all it's particles will become more and more spread in space.
But if the evolution of the...
The non-normalized wavefunction of a general qubit is given by:
$$|\psi\rangle=A|0\rangle+B|1\rangle.$$
The complex amplitudes ##A## and ##B## can be represented by two arrows in the complex plane:
Now the wavefunction can be multiplied by any complex number ##R## without changing the...
Consider the Schrödinger equation for a free particle:
\begin{equation}
-\frac{\hbar^2}{2m} \partial_i^2\psi = i\hbar\partial_t \psi.
\end{equation}
Let us be interested in the motion of a free particle in quantum mechanics. We say ok, we have a solution to the Schrödinger equation for a...
hi guys
i was thinking about the inner product we choose in quantum mechanics to map the elements inside the hilbert space to real number which is given by :
$$\int^{∞}_{-∞}\psi^{*}\psi\;dV$$
or in some cases we might introduce a weight function dependent on the wave functions i have , it seems...
This is by far the hardest undergraduate class I have ever take.
The majority of class got less than 40% on the midterm. Unfortunately, I was sick during the exam hours too ,so it's hard for me to concentrate and think clearly
Thank god,the professor uses the norm-referenced grading and My...
I learned that the energy operator is
##\hat{E} = i\hbar \frac{\partial}{\partial t} ##
and the Hamiltonian is
##\hat{H} = \frac{-\hbar^2}{2m}\nabla^2+V(r,t)##
If the Hamiltonian represents the total energy of the system. I expect the two should be the same. Did I misunderstand the concept of...
I'm interested in a book which treats scattering in quantum mechanics aimed at the research-level. I'm particularly interested in a text which focuses on mathematical details such as the analytic structure of the S matrix, the relation between the S matrix and various green's/two-point...
In quantum mechanics in books authors discuss only cases ##E<V_0## and ##E>V_0##, where ##E## is energy of the particle and ##V_0## is height of the barrier. Why not ##E=V_0##?
In that case for ##x<0##
\psi_1(x)=Ae^{ikx}+Be^{-ikx}
and for ##x\geq 0##
\psi_2(x)=Cx+D
and then from...
So I am trying to understand and solve the problem mentioned in the title.I found a solution online:
https://physics.bgu.ac.il/COURSES/QuantumMechCohen/ExercisesPool/EXERCISES/ex_9011_sol_Y09.pdf
The problem is, I can't understand this step :
I relly can't find out how the two expontential...
Suppose the unitary operator ##e^{-\frac{i}{\hbar}\hat{H}t}## acts on ##|\psi (0) \rangle##, does it make sense for one to think of the time-evolved state as some sort of time-keeping device? If not, why? If so, is such a notion useful?
Thanks in advance!
The momentum operator for one spation dimension is -iħd/dx (which isn't a vector operator) but for 3 spatial dimensions is -iħ∇ which is a vector operator. So is it a vector or a scalar operator ?
Could one come to think that time is irrelevant in quantum mechanics? we know that the QM equations are written with the time variable, (schrodinger equation). Yet everything suggests that time is irrelevant, as the search for loop quantum gravity seems to indicate
In the double-slit experiment when a detector was placed before the two slits, a 2 strip pattern was produced after the two slits. When there was no detector placed before the two slits, a different pattern was produced after the two slits. Why does the presence of a detector before the two...
I think the effective action should make sense also in Quantum Mechanics, not only in QFT. But I have never seen described in a QM book as such. Could there be a QM book that uses effective actions? Or maybe in QM effective actions are called another name?
I think effective actions in QM could...
In a book it says that "we know of quantum phenomena in the electromagnetic field that represents a failure of superposition,seen from the viewpoint of the classical theory."
I want to about what quantum phenomena is he talking about?
This was from the page 11 of the book Electricity And...
Hello,
I know we have the parity operator for inversion in quantum mechanics and for rotations we have the exponentials of the angular momentum/spin operators. But what if I want to write the operator that represent a reflection for example just switching y to -y, the matrix in real space...
By the Wigner theorem, symmetries transformations are implemented by operators ##\hat{U}## that are unitary or antiunitary. This is what is written in most books. But I have read somewhere that, to ##\hat{U}## represent a symmetrie, it's necessary that ##\hat{U}^{\dagger} \hat{H} \hat{U} =...
When we make a symmetrie transformation in a quantum system, the state ##|\psi \rangle## change to ## |\psi' \rangle = U|\psi \rangle##, where ##U## is a unitary or antiunitary operator, and the operator ##A## change to ##A'##. If we require that the expections values of operators don't change...
Recently, I've been told I was wrong concerning the nature of stationary states and diffusion being related. Even though I pointed out to the people involved that I was merely paraphrasing Max Born, who was apparently quoting the same idea as Linus Pauling.
No one has been able to tell me...
Hi everybody,
While reading some quantum mechanics book, I met the resolvent formalism which is presented as more powerful than the pertubative approach. For a system with a hamiltonian ## H = H_0 + H_{int} ##, when the interaction part ## H_{int} ## is no more a pertubation but rather having...
Hi,
I'm an undergrad, following my very first serious course in QM. We're following Griffith's book, and so far we're staying close to the text in terms of course structure.
Griffiths starts out his book by postulating that each and every state for any system \Psi must be a solution to the...
I study on quantum mechanics and I have question about operator.
In one dimension. How do we know ## \hat{x} = x## and ## \hat{p}_{x} = -i \bar{h} \frac{d}{dx} ##
When schrodinger was creating an equation, which later called "the schrodinger equation".
How does he know momentum operator equal...
Homework Statement
What will momentum measurement of a particle whose wave - function is given by ## \psi = e^{i3x} + 2e^{ix} ## yield?
Sketch the probability distribution of finding the particle between x = 0 to x = 2π.
Homework Equations
The Attempt at a Solution
The eigenfunctions of...
Hello! I got a bit confused about the fact that the whole the description of spin (and angular momentum) is done in the z direction. So, if we are told that a system of 2 particles is in a singlet state i.e. $$\frac{\uparrow \downarrow -\downarrow \uparrow }{2}$$ does this mean that measuring...
Hello. I've been struggling for a day with the following problem on Quantum coherent states, so I was wondering if you could tell me if I'm going in the right direction (I've read the books of Sakurai and Weinberg but can't seem to find an answer)
1. Homework Statement
*Suppose a Schrödinger...
Supposing the Many Worlds interpretation of QM is true... If a branching occurs during what we perceive is a wave function collapse, why would this be perceptible to us as probabilties? Wouldn't we just branch, leaving it just as imperceviable as the passage of time? That is, it just happens...
In Special/General Relativity invariance of a space-time interval is just so important. But in Quantum Mechanics, be it non-relativistic or QFT, there seems to be no such parallel. I have always noticed this.
I have some ideas about the reason:
1 - it's not part of the theory to have a...
Consider the following examples:
1) combine a spin 1/2 state (with 2D Hilbert space and three spin 0 states (each with 1D Hilbert space). The resultant state is in 3D Hilbert space.
2) combine the same spin 1/2 state (with 2D Hilbert space and one spin 0 state.
The states in (1) and (2)...
I would be in college in 2019 (currently I'm in standard 11). I'm greatly interested in Quantum mechanics, QFT, QCD and Quantum Geometerodynamics. Of these, I want to do an internship on the first, because I don't think I'll be able to touch the others till the 2nd year in college.
I'm living...
If I have a general (not a plain wave) state $$|\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...
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...
Homework Statement
A particle of mass m and spin s, it's subject at next central potential:
##
\begin{equation*}
V(\mathbf{r})=
\begin{cases}
0\text{ r<a}\\
V_0\text{ a<r<b}\\
0\text{ r>b}
\end{cases}
\end{equation*}
##
Find the constants of motion of the system and the set of...
In discussing stuff in another thread I used the standard Dirac notion expanding a state in position eigenvectors namely |u> = ∫f(x) |x>. By definition f(x) is the wave-function. I omitted the dx which is my bad but the following question was posed which I think deserved a complete answer. It...