# All Questions

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### Why IR divergences cancel by cross sections of next-to-leading diagrams?

I was reading QFT & Standard Model by Schwartz, Chapter 20 which is about IR divergences. He says that IR divergences only cancel cross sections for processes involving different initial or ...
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### What is the potential associated to a pure torque proportional to one of the principal axis of a particle?

I'm writing a code in molecular dynamics in which a particle is subject to a pure torque around one of its principal axis. E.g., if the particle has principle axis $\hat u$, $\hat v$, $\hat f$, all ...
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### Commutator and Hamiltonian [on hold]

Assume that $[\hat{A},\hat{H}]_-=0$ and $[\hat{B},\hat{H}]_-=0$ but we know that $[\hat{A},\hat{B}]_-\neq 0$. Then there exists degenerate stationary states of $H$. How to prove it?
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### How much charge per unit time passes with the Josephson current

I wonder how does the simple defintion of a current $I = \frac{dQ}{dt}$, i.e. charge per unit time which passes through some area, is applied to the Josephson effect. If I have a Josephson junction ...
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### If one were to have boots made of Vibranium, could one jump from any height? [on hold]

Vibranium is a fictional metal from the Marvel universe, with interesting properties, it "possesses the ability to absorb all vibrations in the vicinity as well as kinetic energy directed at it." It ...
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### Where does air pressure come from?

Where does air pressure come from? I thought it was from gravity or the speed of the gas resulting from its heat. However, analyzing my own hypotheses, I think that my 'heat conjecture' is probably ...
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### Paradox in applying Newton's second law [duplicate]

Suppose I apply two vertical but opposite forces with the same magnitude in a body like is shown in the picture: According to newton's second law, the center of mass shouldn't accelerate, since the ...
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### Derivation of plane wave from inner product of position ket and momentum ket

In textbooks it seems to be taken for granted that $$\langle \mathbf{r}|\mathbf{k}\rangle ~=~ \frac{1}{\sqrt{\Omega}}\exp(i\mathbf{k}\cdot\mathbf{r}).$$ I'm sure it's obvious but is there a ...
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### Continuity equation for charge and current densities of an accelerated point charge

For a point charge that moves with the trajectory $\vec r(t)$, we know that it has the singular charge and current densities $$\rho (\vec x, t) = q \delta^3(\vec x - \vec r(t))$$  \vec J(\vec x, ...
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### Understanding notation regarding particles states and wavefunctions

In the development in my notes of second quantisation I have a problem in understanding notation. We start by considering a basis $\psi_i(\mathbf{r})$ for the Hilbert space of single particle ...
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### Do two bodies spinning about the same axis in the opposite direction repel each other?

I recently read about the GEM equations, which look very much like the Maxwell's equations. Does this mean the behavior of mass is like the the behavior of the electric charge? So for example if you ...
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### Database of experimental cross sections for atomic collisions

Does the atomic-physics community keep a comprehensive database of experimental data (cross sections) for atomic collisions? I am looking for data about Bremsstrahlung, ionization, elastic and ...
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### What technologies exists today that can reduce the effects of air pollution on individuals (eg. carbon capture)? [on hold]

I'm researching the matter of air pollution and I would like to know if there are any technologies that I might not be aware of (beside carbon capture and Sharps's plasma cluster) whose objective is ...
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### What are some current models for neutron star core? [on hold]

How are neutron star cores described today, do we use models of superfluidity/superconductivity for instance?
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### Is an H bomb a better “fusion reactor” than the Sun's core?

A few days ago I asked about a fusion reactor design I was thinking about and got a well-founded answer by #BowlOfRed#. There is the question I posted: Another cold-fusion reactor scheme :) Is it ...
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### What is the importance of reciprocal lattice?

Reciprocal lattice is the diffraction plot of a crystal. Now with the STM instrument we can get the get the topology of the crystal, so what is the importance of reciprocal lattice or the Brillouin ...
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### Determining the path of a light ray

Let's consider a ray in the plane $xy$. Let the refractive index be defined in any point of the plane with the function $n(x,y)$. In time $t=0$ the ray is located in coordinates $(x_0, y_0)$ and its ...
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### Could you make a system that exchanges matter but no thermodynamic energy?

Normally we distinguish between open, closed and isolated systems, assuming that the 4th possibility does not exist, as an exchange of matter presupposes energy exchange. Could it be nevertheless ...
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### value of $n$ in modified gravity model

In the modified gravitational model $f(R)=R+\lambda{R_{0}}\left(\left(1+\frac{R^{2}}{R_{0}^{2}}\right)^{-n}-1\right)$ Are there any restrictions on the value on $n$. Also is this model valid for ...
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### Is the induced EMF dependent on the conventional or actual flow of electrons?

I have been studying Faraday's Law and Lenz's Law and the flow of current to oppose change in flux linkage. My question is whether the actual flow of electrons to the top of a material makes the EMF ...
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### How can we see the electrons?

As you can see the blue glows is the electrons flowing inside the cathode ray tube under vacuum condition. My question is how can we see the electrons in the setup? electrons are everywhere but why ...
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### why cant we use a pair of polarizer ( whose pass axis are oriented at 90 degrees ) to protect from entry of harmful radiation?

If a pair of polarizers can block EM waves passing through it, then can we use them for protecting against harmful radiation (in nuclear reactors and also in spaceships) in thermo flasks to ...
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### Energy, Voltage Drop, and Current

In an electrical circuit consisting of just a source and some load, when current hits the load, the voltage drops. This means that the potential energy of the charge drops. If the potential energy ...
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### Question in rotational mechanics [on hold]

Question number 14. This question is from coaching material for the chapter rotation. For a cylinder, moment of inertia I=1/2MR^2 How do I apply this data? Is it to do with conservation of ...
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### Quantum mechanics and Classical limit(s)

I have tried to make sense of this and i am not sure i get it. What i gather from this page about the classical limit is: You need coherent states something like $\hbar \to 0$ is not really enaugh. ...
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### Physical meaning of quantum interpretations

Do interpretations of quantum mechanics have physical meaning? An argument for no would be the fact that no matter the interpretation, one gets the same measurements. They also do not follow logical ...
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### If the only observer dies, could observer 2 ever have see a different result?

My layman's understanding of QFT is that once a system is observed, any future observation will always yield the same result. Would this hold true if the original-and-only observer, without any ...
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### Apparatus for generating plasma

I'm working on a project which involves superheating nitrogen to a state of plasma. I was considering using my Fusor, but this project has nothing to do with plasma, and I believe that there are way ...
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### Can reduced density matrices of sub systems of an entangled composite system be different?

In a 4-dimensional hilbert space, only 4 entangled states( normalized ) are possible ( if I am not wrong ), the bell basis. In each of the state in bell basis the reduced density matrix is ...
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### Charles Law inverse for cooling?

Is the inverse of Charles law also true? If I double the volume, the temperature will be halved? In the fire service we ventilate a structure by opening up the ceiling to the attic. If the volume of ...
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### Explaining William Herschels Infrared Discovery

First post, and probably a very simple answer that I'm missing, but here goes. Herschel found that red light warmed a thermometer more than blue light, and consequently that the region beyond the red ...
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### Find the acceleration of a stirrer [on hold]

Question: There is a fixed 55 cm long rod (vertical) that is charged uniformly up to +137 nC, Place a 0.5 g plastic 6 cm long stirrer (horizontal) that is charge to +93 nC, 2.3 cm away from the rod ...
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### Is the Physics for extremely large and extremely small particles similar in any aspect?

Do the equations we use to describe natural phenomenon on a very large scale, work as well for describing natural phenomenon on an extremely small scale as well? Is there any correlation at all which ...
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### Dilemma: Fusion space from a direct sum of anyons or NOT

In Preskill's note, 9.1.2 in page 44, concerning the fusion space, it states that: The fusion rules of the model specify the possible values of the total charge $c$ when the constituents have ...
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### Will a thermometer successfully be able to measure the temperature of this?

In a hypothetical experiment, I would be measuring the temperature change over time of a system; however, there would be no liquid (mostly gas). Would a regular thermometer (probably digital, as ...
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### A way to treat uncertainty in intensity measurement

I did an experiment to prove the inverse square law but don't know if my idea to calculate the error is correct. The relationship is $I=const*r^{n}$ with n=-2, we had a computer software to read the ...
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### How to simulate pendulum movement with high amplitude

I need to make a C# simulator for a simple pendulum. I have been searching the web for 3 days and I am stuck. The problem is I have found many equations that would give the angle position as a ...
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### Is this the correct entropy? [on hold]

In order to take a nice warm bath, you mix $50$ liters of hot water at $55^{\circ} C$ with $25$ liters of cold water at $10^{\circ} C$. How much new entropy have you created by mixing the water? What ...
Let $\Omega$ be a domain in $\mathbb{R}^n$. Consider the time-independent free Schrodinger equation $\Delta \psi = E\psi$. Solutions subject to Dirichlet boundary conditions can be physically ...