# All Questions

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### How to connect the dimension of perturbation constant with renormalizability

Let's have the Lagrangian $$L = L_{0} + \lambda V , \qquad (1)$$ where $\lambda$ is constant which is small in the next senses: if $\lambda$ is dimensionless, it means that $\lambda < 1$; if it ...
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### Can a pool ball change direction mid-roll?

In this youtube video, a pool shark consistently gets the cue ball to drastically change direction mid-roll (i.e. while after he's hit it). Is this theoretically possible without using trick balls? If ...
203 views

### Probability of being in the same initial state

If the Hamiltonian has basis of eigenvectors $\phi _1, \phi_2,..$ with corresponding eigenvalues $E_1,E_2,...$. I then define an observable $A$ by: $$A\phi_1 = \cos(\beta)\phi_1 + \sin(\beta)\phi_2$$...
708 views

### Solving the BCS Hamiltonian via the Bogoliubov Transformation

I was doing a calculation in Giamarchi's Introduction to Many Body Physics, chapter 3, on BCS theory and second quantization, and ran into some confusion with the BCS Hamiltonian. The pdf is here for ...
109 views

### How does porous anti-reflection coating work?

We know ideally that if we stick a coating of thickness $d = \frac{\lambda}{4}$ with refractive index $n_{coat} = \sqrt{n_{air}n_{glass}} \approx 1.2$ there would be no reflected wave. But it's hard ...
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161 views

### Two explanations of non-zero atomic radius

I have came across two separate explanations for why atoms have a positive atomic radius (as opposed to electrons "collapsing" into the nucleus). The first is via Heisenberg Uncertainty Principle, ...
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### Momentum of Light Question [duplicate]

Since light beams carry momentum, why would a person holding a flashlight not feel a recoil similar to that of a rifle being fired?
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### Numerical Computation of Linbald Equation [closed]

Can anybody suggest me a good algorithm for the time evolution of the reduced density matrix using Linbald equation. My Hamiltonian is time dependent. I am aware about Qotoolbox and Qutip. I have ...
2k views

### What is the relative speed of two near-light speed particles headed towards each other?

I understand that nothing can move faster than light due to time dilation. I want to build upon my understanding of Einstein's theory of Special Relativity, so I came up with this hypothetical problem ...
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### What is $\hat{p}|x\rangle$?

Trying to solve a QM harmonic oscillator problem I found out I need to calculate $\hat{p}|x\rangle$, where $\hat{p}$ is the momentum operator and $|x\rangle$ is an eigenstate of the position operator, ...
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### What is Jacobian about the “Jacobian Edge” in $E_\mathrm{T}$ distributions?

Particle physicists often talk of a "Jacobian Edge" in distributions, i.e. when looking at the $E_\mathrm{T}$ distribution of $W \to e \nu$ decays at rest. How is this related to the Jacobian ...
3k views

### In the case of riding a bicycle, why can the system of the bicycle and rider be accelerated if no external net force?

According to Newton, if no external net force acting on a system then the system will keep its initial condition whether at rest or moving uniformly in a straight line. Let's consider a boy riding a ...
471 views

### Reducibility of tensor products of Lorentz group representations

Consider the statement: (34.29 in Srednicki's QFT text) $$\tag{34.29} (2,1)\otimes(1,2)\otimes(2,2)~=~(1,1)\oplus\ldots$$ Where of course, $(a,b)$ label representations of Lorentz group in the usual ...
644 views

### Quantum entropy in term of density matrix

Why in von Neumann expression of quantum entropy we have trace of density matrix expression? Why don't off diagonal term play a role?
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### Feynman Rules in Momentum space

What's the difference between Feynman rules in momentum space for $\phi^3$ theory and for $\phi^4$ theory? I know it's only a slight difference and perhaps found in the vertex factor? But for some ...
106 views

### Enthalpic Excess - Regular Solution Upon Mixing

My professor, in his handout, says the following about this diagram : For $\beta>+2$ there are two minima and phase separation occurs driven by unfavourable enthalpic interactions. I ...
The differential operator for diffusion in three dimensions is given by $\partial_t - k \nabla^2$ where $k$ is a constant. The Green's function is (according to Wikipedia) $$\theta(t)\left( \frac{1}{4\... 0answers 92 views ### Why does strong interaction increase with distance? I read numerous times that strong interaction increases with distance. But how can one actually derive the force-distance relation from the lagrangian (quark field + gluon field + gauge coupling)? ... 0answers 145 views ### Integration of Dirac propagator I've got confused with Dirac (or Spinor) Propagator. Everywhere in books I have examples of integration Klein-Gordon propagators, which are quite easy. But I don't understand how to integrate 4-... 0answers 319 views ### How does one refute a Machian mechanism for inertial emergence? Introduction: Consider the diagrams representing the duality between the weak and strong principles of equivalence. Now based on how these diagrams were taught to us (at least how they were taught ... 1answer 105 views ### \gamma^5 factor in Quantum Field Theory I have a problem with interpretation of \gamma^5 factor in the interaction Hamiltonian. I know that \frac{1\pm\gamma^5}{2} is a helicity projection and it requires helicity conservation in ... 0answers 115 views ### Understanding the concept of cross section The cross section is a tiny area, measured in barns (10^{-28} \mathrm{m^2}), which relates to the probability of a reaction. But I have trouble interpreting that number. For example how can I ... 3answers 165 views ### Is there a table that sums up the parameters, the assumptions/symmetries, and the predictions of the standard model? The title says it all: is there a table that sums up the parameters, the assumptions/symmetries, and the (most important) predictions of the standard model? 0answers 8 views ### Temperature of air effused into tank - different predictions? Consider air from outside at T_0 effusing into an evacuated and thermally isolated chamber. By thermodynamic potentials, the temperature inside the tank is given by:$$T_1 = \gamma T_0 = \frac{5}{...
In paramagnetic-to-ferromagnetic phase transitions, the symmetry spontaneously breaks down from SO(3) to the subgroup SO(2) below $T_\text{crit}$. This implies that there should be two Goldstone modes ...