Quantum mechanics describes the microscopic properties of nature in a regime where classical mechanics no longer applies. It explains phenomena such as the wave-particle duality, quantization of energy and the uncertainty principle and is generally used in single body systems. Use the quantum-field-...

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Advanced Quantum Mechanics (Galilean transformations)

I have been reading quantum mechanics textbook by Ballentine, and in the third chapter, he says that the eigenvalues of the transformed operator, A', must remain same as those of A. I am confused ...
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791 views

Does Dirac's argument against classical mechanics stand in contradiction to Bohm's theory?

In his book on Quantum Mechanics, P.A.M. Dirac talks about the stability of the atom as a means of demonstrating the need for quantum mechanics. He writes: The necessity for a departure from ...
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$\frac{d}{dr}=0$ and $\frac{d}{dz}=0$ (cylindrical coordinates) for a 1D ring

In http://ritchie.chem.ox.ac.uk/Grant%20Teaching/2010/Lecture%204%202010.pdf slide 21 of 26, he says "Radius of ring is fixed and so derivatives in $r$ are 0." Presumably this goes for $\frac{d}{...
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58 views

What state is a qubit system in if it is measured by someone, but we do not know the measurement?

Suppose there is a qubit Q in the state $$ \lvert+\rangle = \frac{1}{\sqrt2}(\lvert 0\rangle+ \lvert 1\rangle) $$ Now somebody makes a computational basis measurement on Q, but we do not learn the ...
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64 views

Thermodynamic expectation value at $T=0$

The thermodynamic expectation value for an observable $A$ is defined as $$\langle A \rangle = \frac{1}{Z} \sum_n \langle\psi_n| e^{-\beta H} A|\psi_n \rangle, \qquad (1)$$ where $\beta=1/k_bT$, the $\...
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1answer
48 views

Hydrogen atom nodes and Bohmian trajectories

Wouldn't the fact that the solutions to the hydrogen atom orbital shape have nodes prove that the Bohmian interpretation is incorrect, since in that interpretation, the electrons would have a fixed ...
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1answer
99 views

Blackbody radiation and thermal equilibrium?

I'm new to blackbody radiation and I have been struggling with the following question: The dominant wavelength of light from the sun is 510nm. Estimate the equilibrium temperature of the earth. ...
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2answers
123 views

Physical quantities have definite values?

I don't really know if this question has an anwser but I thought it was worth to try asking. My point here is the following: in Quantum Mechanics, to describe the states of a system we use a Hilbert ...
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2answers
154 views

General formula for expanding wave function in terms of orthogonal states?

Given a wave function $\psi(x) = \langle \psi | x \rangle$. It can be expanded in terms of orthogonal states: $$ \langle \psi | x \rangle = \sum_n \langle \psi | n \rangle \langle n |x \rangle $$ ...
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104 views

Need help understanding delayed choice and entanglement swapping

From delayed choice experiment The results of all four measurements are then compared. If Victor did not entangle his two photons, the photons received by Alice and Bob are uncorrelated with ...
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50 views

Double slit experiment with a single photon stream

I would like to know if the following are true: Each time you fire a single photon, only one dot appears on the back screen. Is there ever a case that while we are watching the experiment, that a ...
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1answer
32 views

Saturation Recovery - Why is $M_z = 0$?

This article, p. 13, describes the evolution of the longitudinal magnetisation in NMR, $M_z$, if a 90° pulse is shot. My question is: why will $M_z$ become zero after every shot, since imediately ...
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Is a single photon always circularly polarized?

While trying to understand polarization in quantum field theory, I wondered how a single photon could go through a linear polarizer. I found a paper which asked "Is a single photon always circularly ...
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1answer
24 views

Lifting an analogy of a pond to question signals at natural or artificial boundaries in space-time [closed]

I conjured up an idea to lift an analogy into the language of QFT and GR. I thought up the universe as a pond with a liquid. If we imagine a liquid poured into some pond (sort of bang and inflation ...
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1answer
58 views

Unitary Transfomation from One Basis to Another [closed]

So we have two orthonormal linearly independent basis $\{ |\phi_1 \rangle, \dots, |\phi_n \rangle \}$ and $\{ |\psi_1 \rangle, \dots, |\psi_n \rangle \}$. We can express the basis vectors of the ...
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3answers
120 views

Any suggestion for a book that includes quantum mechanics principles and smoothly introduces you to QED (quantum electrodynamics)?

I am not a physicist but I am into quantum mechanics and statistical mechanics. In my department, the quantum mechanics we do include only Schroedinger's equation and problems, some approximation ...
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1answer
76 views

What are negative and positive energies? [closed]

So I have a few questions related to the negative and positive energy topic. 1)What do we mean by negative and positive energies in physics? 2)I have seen in a minute physics video that we need ...
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0answers
26 views

possible J quantum numbers for Lithium

Is it correct that the possible J quantum numbers for Lithium (assuming L-S coupling) is given by: $|L-S| \le J \le |L+S|$ With S = $\pm$ 1/2 and L = 0,1 Making J: $|0-1/2| \le J \le |0+1/2|$ , $J ...
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2answers
68 views

Does measurement really affect polarization?

The answer must be yes but I have trouble dealing with the explanation given here. As you can see the light is polarized at an angle theta to x axis. After it passes the polarizer A which is along x ...
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1answer
99 views

Particle in a double delta potential, scatter states

I was studying the scatter states of a particle in a double delta potential given in this link: Double delta function well – scattering states But I dont understand how equations (20) and (21) were ...
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1answer
44 views

Energy of a state of free particles

We know the the normalization of a state of free particle is indeterminate and therefore it doesn't have any physical state or definitive energy. On the other hand, lets consider a free particle of ...
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439 views

What is “spin-orbit torque?”

I am trying really hard to understand the concept of spin-orbit torque. It is a new-ish discovery in the field of spintronics and has many applications for magnetic devices. The information that has ...
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2answers
85 views

Meaning of the general matrix element $\langle x'|O|x\rangle $

In a recent lecture we were told that $\langle x'|\hat{O}|x \rangle = O(x,x') = O(x)\delta(x-x')$ "due to the locality of quantum mechanical observables". I have no idea what this is supposed to ...
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282 views

Is there supersymmetry between Dirac and Klein Gordon solutions?

Usually supersymmetry is explained at the level of the action of a quantum field theory, and there are two ways to go down from QFT to relativistic quantum mechanics: either a non-covariant way where ...
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1answer
277 views

Is long-term weather forecast impossible in principle?

This question can be asked about any chaotic dynamical system, but hydrodynamics of the atmosphere makes it more concrete. Arnold describes his 1966 result as follows: I have calculated the ...
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2answers
119 views

Wavefunction collapse and relativity [closed]

In classical QM, when I measure the wave function of a system, e.g. the position of an electron somewhere in a box, its wave function collapses instantaneously to some classical position. But how fast ...
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1answer
229 views

Is it possible to explain voltage and current using the wave nature of electrons?

Assume a battery and resistor are connected in a series. If we double the resistance, the current gets cut in half. This is a macroscopic effect. How do those electrons know that resistance has ...
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0answers
26 views

Is static gauge quantization of the particle equivalent to covariant quantization?

In the covariant quantization one is able to get directly (from the constraint $p^\mu_\mu+m^2=0$) the Klein-Gordon equation. But if one uses the parametrization $\tau=X^0$ then the Schrodinger ...
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1answer
94 views

Why the visual system does not affect by the quantum zeno effect?

According to the quantum zeno effect, if a measurement is done on a sufficiently high frequency, the system remains stationary. In daily life, human see objects are moving. (i) Is the procedure ...
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173 views

Phase transitions, Landau Ginzburg theory and Symmetry reduction

On one side of critical temperature (usually for $T<T_{c}$), symmetry is reduced w.r.t the symmetry on the other (usually $T>T_{c}$) regime. I heard on the road (near a theoretical physics ...
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2answers
125 views

Maxwell equations and quantized electromagnetic field

When the electromagnetic field is quatized for a single mode, we first take Maxwell equations and proceed to write the electric and magnetic field as a stationary wave, since we consider the ...
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0answers
34 views

What are the exact setups to observe a wave function and its collapsed state?

I was reading this article: http://io9.com/5528321/how-smart-do-you-need-to-be-to-collapse-a-wave-function and I need some clarification on the experiment setup. My current understanding is that if ...
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21 views

Interferometric Entanglement of path and spin

I am looking at a problem where I can have a spin 1/2 particle in either a |1$\rangle$ or a $|0\rangle$ state in an interferometer (0 is one arm and 1 is the other). And the spin 1/2 follows the ...
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1answer
79 views
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1answer
97 views

A Confusion About Energy-Time Uncertainty Relations

In all the textbooks that I have seen, energy-time relation is written in the following way:$$\Delta E \cdot \Delta t \geqslant \frac{\hbar}{2}$$ Here is my interpretation of this principle: The ...
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3answers
703 views

A quantum particle moving from A to B will take every possible path from A to B at the same time

If a quantum particle can take an unlimited number of paths to get from point A to point B wouldn't a quantum particle never get from point A to point B? A quantum particle takes every path at the ...
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1answer
96 views

Do two 1D Rectangular Potential Barriers have Bound States?

This question had me thinking about the related 1D problem of two rectangular potential barriers: The potential is zero everywhere but for the two barriers, where it is $V_0$. As ever I am ...
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1answer
307 views

Specific heat capacity and temperature, 0 K?

I've found similar threads like this, but with no clear answer. I understand that the specific heat capacity of a substance increases with temperature, because the vibrational nodes and rotational ...
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1answer
61 views

What is the 'area law' in the context of matrix product states?

I am trying to get into the topic of matrix product states by reading this: A practical introduction to tensor networks: Matrix product states and projected entangled pair states. R. Orús. Ann. ...
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2answers
319 views

Energy of two spin-1 bosons in 1D harmonic oscillator

I'm taking a QM class and I'm trying to work out a problem given to us by the professor. We have two identical, non-interacting, spin-1 bosons that are located in a one dimensional harmonic ...
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3answers
220 views

Has the double-slit experiment been performed with animals as the observers?

As the title says, has the double-slit experiment been performed with animals (not humans) as the observers? If yes, which animal was used and what was the result?
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1answer
74 views

Quantum States, Hilbert Space and Time

I'm having troubles with the assertion "(normalizable) wave-functions constitutes (projective) Hilbert space". The standard argument I find for this seems to go as following: say $\Psi(\vec{x},t)$ is ...
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1answer
50 views

Does entanglement entropy remain constant?

So I am trying to figure out what the time derivative of entanglement entropy is $$\frac{d}{dt}(-Tr[\rho^A\log_2\rho^A])$$ Where $$\rho^A=\sum_{aa'}|a\rangle\langle a'|\sum_bc_{ab}c_{a'b}^*$$ And $$...
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1answer
256 views

How does one account for the momentum of an absorbed photon?

Suppose I have an atom in its ground state $|g⟩$, and it has an excited state $|e⟩$ sitting at an energy $E_a=\hbar\omega_0$ above it. To want to excite the atom, one generally uses a photon of ...
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1answer
63 views

Why do bosons and fermions exist? [duplicate]

if i have two particles in states: |1> and |2> respectavly , and they are not identical then the combined state is |1>|2> , but if they are identical then the labels 1 or 2 are arbitrary and could be ...
2
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2answers
80 views

Have David Wolpert's findings really “slammed the door” on scientific determinism?

I recently read an article describing how mathematician/physicist David Wolpert's research closed the door on scientific determinism. I have huge doubts about the implied conclusion, considering the ...
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1answer
49 views

Spin $\frac{3}{2}$ representation in Georgi's book?

Georgi's book Lie Algebras in Particle Physics 2ed equation 3.32 lists the spin operators in the spin $\frac{3}{2}$ representation as: $$J_1=\left( \begin{array}{cccc} 0 & \sqrt{\frac{3}{2}} &...
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1answer
89 views

What do the density function and the current vector in the continuity eq. form of the Klein-Gordin eq. mean?

In explaining Dirac's attempt to arrive at a relativistic Schrodinger equation, it is claimed that Dirac has started from $$\left(-\frac{1}{c^2}\frac{\partial^2}{\partial t^2} + \nabla^2\right)\phi = ...
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2answers
39 views

Proportionality of states in quantum harmonic oscillator

What is the justification for $a_{\pm} \psi_{n}$ being proportional to $\psi_{n\pm1}$ in a quantum harmonic oscillator? Here $a_{\pm}$ is the raising/lowering ladder operator.
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2answers
105 views

How to we measure quantum purity experimentally?

Quantum purity $\gamma$ of a general state $\rho$, where $\rho$ is a density matrix, is defined as $\gamma = \text{tr} (\rho^2)$. I'd like to understand how $\gamma$ can be measured experimentally. ...