# All Questions

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### OPE of parity even theories in CFT.

If I consider an OPE of some operators, which belong to a theory where parity is not violated, will I have a constraint on the kind of operators appearing in the right hand side ? For example, I ...
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### Naive question: Do you really get to choose the topic for your PhD and postdoctoral research? [on hold]

Does a student after completing their masters in Physics , get to choose his topic of research for the future?Also, is there active research going on in quantum mechanics and general relativity? How ...
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### Could you explain this flow of calculation?

I am reading this book, Quantum Optics by Walls and Milburn. I am working on Chapter 6 which is about the Stochastic Methods. I don't understand a calculation in this chapter. Let $w(t)$ be the ...
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### Induced emf vs Potential difference

Let a square conductor frame length l, resistance R,be pulled out with a constant velocity v from a magnetic field B perpendicular to plane of the frame .Then an emf e=Blv is produced across the ...
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### Must bound operators must have normalisable eigenfunctions and discrete eigenvalues?

When we have bound states, to my knowledge, we have states that are normalisable and a discrete energy spectrum. However, in the case of scattering states that have a continuous energy spectrum, the ...
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### Is Bekenstein entropy limit for black holes + No-hair conjecture inconsistent with universal continuity?

It is unknown whether the universe is discrete or continuous in its intricate quantum level structure. See for example: Can universal continuity be experimentally falsified? Is the universe finite ...
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### How does the integer $n$ affect the calculation of lattice spacing in the equations from davisson-germer experiment?

Given the equation $n\lambda=2D\sin{\alpha}\cos{\alpha}=D\sin{\varphi}$ (where $\varphi=2\alpha$), to solve for $D$ (the spacing of atoms in the crystal), you get the following equation: ...
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### Derivative of equation of path difference in double slit

p = Path difference r = Distance travelled by the rays x = Perpendicular distance between interference of the rays to the medium point of the incident rays a = Vertical distance ...
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### Engineering Physics - Gravitational and Kinetic Potential Energy

For part 1, I need to 'substitute all unknown numbers into the equation for potential energy' and leave X as a variable. In the picture below I've included the question and details that I've been able ...
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### Why water reflects light?

Please explain clearly: Why water reflects light? Why do we see our image on calm water? What is the Maxwell equation? What is Fresnel reflection? Please answer my questions. My exam is ...
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### Can one make a propagating field along a waveguide only have longitudinal/transverse components?

We know that in free space, a propagating electromagnetic wave is always transverse. However, along a dielectric waveguide, the propagating wave can have longitudinal components. The exist of the ...
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### Finding the range of frequencies to which a receiver should be set

I'm working on a study guide for a test for my Modern Physics class, and ran across a question I'm having trouble with. A radar transmitter emits a pulse of EM radiation with ...
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### Is there any such thing as gravity?

So I was seeing this documentary and ended up knowing that there is no such thing as gravity. It said that we are not being pulled by Earth but being pushed by the space time curvature acc. to general ...
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### How'd that lunar buggy fit inside that lunar lander anyway?

That buggy on the moon was pretty big. Did they pack it in with the two astronauts? Deflate the tires and then pump them up after landing?
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### Why the centered rectangualr lattice is not considered a special case of an oblique lattice

From Wikipedia, there are 5 different kinds of lattice in 2 dimension: But I am wondering how the third type (the centered rectangular) is different from the first kind (the oblique lattice). The ...
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### lattice vibration

I would like to inquire about acoustic and optical branch in Phonons vibration; My question is: what is the physics description when we say what is make acoustic and optical branch appear ...
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### Why is there a change in entropy in a irreversible isothermic process?

Suppose you have a irreversible isothermal process. The net change in entropy from the system to the surroundings should be 0 if the temperature does not change. So why is there an increase in ...
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### What are the gradient lines of the magnetic field of a cylindrical magnet?

The problem is identical to that for a finite-length solenoid. I was interested in finding the family of curves for the equipotential lines, rather than the B field itself, which is more readily ...
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### How does gas spin the turbine in a jet engine?

I am confused about how the energy from the ignited gas spins a turbine in a jet engine, as pictured below ...
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### Derive an equation related to magnetism

Solve the equations for $v_x$ and $v_y$ : $$m\frac{d({v_x)}}{dt} = qv_yB \qquad m\frac{d{(v_y)}}{dt} = -qv_xB$$ by differentiating them with respect to time to obtain two equations of the ...
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### Adiabatic Invariance of 2 mass system

I am trying to solve the second part of Problem 10 from David Tongs CM notes. Specifically we have The neutron star is in a non-circular orbit with E < 0. Give an expression for the ...
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### Trouble Understanding the Concept of Invariance

I am reading the book by Brian Cox & Jeff Forshaw: "Why does E=MC^2". It is getting to the point of explaining what invariance is. According to Cox & Forshaw, invariance states that the laws ...
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### What is a center of gravity?

I have tried to search the definition of it up on Google, but the explanations of center of gravity were very confusing and I had trouble understanding them. Can you explain to me what center of ...
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### How to measure thickness of an ultra-thin metal layer?

I have a sample of a metal (aluminum/oxide) layer with a gradual thickness ranging from monolayer to 100nm. This layer is deposited on a transparent substrate, like glass. How can I measure this ...
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### Inclined plane within a car on a banked turn

Question: Truck is on a banked turn with angle $\theta$ around a circular track with radius $r$. A spider rests on the inside wall of the truck. The coefficient of static friction between the truck ...
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### Is there a special name for thermodynamic process during which no work is performed?

Let $W$ denote the work done on a system during a thermodynamic process. Is there a commonly-accepted, dedicated term for a process during which $W=0$?
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### Noether's Theorem for Hamiltonians and Lagrangians

Looking around I see one version of Noether's Theorem that creates conserved quantities from symmetries that preserve the Lagrangian (e.g. http://math.ucr.edu/home/baez/noether.html), and another ...
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### What is used to measure the spin of a particle?

I was wondering what is the specific system or method that is used to measure spin of a particle? e.g. In a lab what would they use to tell what a particles spin is? P.S. I am new to stack exchange ...
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### Why is sum of Boltzmann factors equal to $1 + \exp(-\epsilon / k T)$?

I am bit confused about how they write $$Z = \sum e^{- \epsilon_i / kT} = 1 + e^{-\epsilon/kT}$$ Where did the subscript $i$ go and where does the $1+e^{-\epsilon/kT}$ come from?
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### Coupled differential equations: how to write in terms of only one coordinate?

I have a mass-spring system, which is as follows: ...
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### clouds reflective property

I am a private pilot and see some beautiful clouds in all forms and patterns up in the sky. I wonder what causes some of the clouds to be so shiny, reflecting such an intense light that it's hard to ...
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### Physical meaning of eigenvectors of mass matrix

What is the physical meaning of the eigenvectors of the mass matrix? If I consider a 2-dof system with one mass linked to two orthogonal springs and I write the equations in any orthogonal system of ...
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### Coma Cluster X-Ray Mass

Does anyone have a reliable reference for the x-ray mass of the Coma Cluster? I've got a value of 3e13 M⊙ from Gursky's 1971 report. Anything more recent would be appreciated.
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### Hilbert space, does $|r\rangle$ satisfy $\langle r |r\rangle = 1$?

Let's say we start with no particles: $\mid0\rangle$. We have $\vec{p}\vert0\rangle = 0$, $H\vert0\rangle = 0$, where we are ignoring $\infty$ vacuum energy. Also, $a(\vec{k})\vert0\rangle = 0$ for ...
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### How to calculate thrust from opening a pressure vessel in space?

One popular trope in science fiction is the astronaut who (deliberately or inadvertently) punctures an air tank or something and goes rocketing off. How would we calculate the amount of force that ...
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### Confusion I have regarding the Carnot cycle as introduced originally by Sadi Carnot

This P-V diagram summarizes the Carnot cycle. Given a working fluid whose state $(P,V,T)$ is $(P_1,V_1,T_1)$ (at point 1) where $PV=nRT$, the working fluid undergoes a cycle of four stages and again ...
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### Einstein Equations

I find out Einstein Field Equations and also I know they depend on energy-momentum tensors. The solution of the field equations is a space-time metric. $$G_{\mu \nu}=8\pi G T_{\mu \nu}$$ Now, I ...
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### Observing light speed

Consider an observer B is moving with speed $0.8c$ relative to another observer A in standard-configuration (I think it is called that in english: that the system B is only moving along the $x$-axis ...
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### Maximizing Ion harvesting for renewable energy

There are hundreds to thousands of ionic charges per centimeter in the lower atmosphere {say at 130 Ft of elevation} to produce a strong capacitor/battery effect with an earth ground. There is a lot ...
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### What is the equation for the elementary-excitation spectrum of superfluid heluim over whole momentum space?

I have a simple question. What is the equation for the elementary-excitation spectrum of superfluid helium over whole momentum space, not just near the origin (i.e., phonon excitations $\epsilon=cp$) ...
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### Pressure variation

Imagine we have a vertically orientated pressurised liquid filled closed pipe. The pipe is shaped like a spring, such that the distance between the two ends can be adjusted vertically, whilst the ...
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### Completeness Relations of Polarization Vectors in QCD

What are the completeness relations of the polarization vectors of (external) particles in QCD amplitude calculation? (I assume the polarization vectors depend on the gauge and even so still have some ...
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### Is entropy violated inside black holes and worm holes?

Do the laws of thermodynamics hold true everywhere in universe ? What about black holes and worm holes ?
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### Book recommendation on Physics of Graphene

The fact that you need Quantum Electrodynamics to describe (massless) charge carriers in graphene instead of ordinary QM in other materials is interesting. I am looking for a book that describe ...
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### Does energy flowing from one place to another in an object cause displacement of that object?

Let's consider two situations: a) A solid metal box (not hollow) where some heat is put on its left side. After some time, the heat will have moved to the right and also have spread out throughout ...
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### Hot bullets and shot cowboys

Please excuse the title of the question, there is physics involved here, but I can't think of a way of writing the title without it being more to the point of the question. A cowboy film is pure ...
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### A cantilever beam is hit with a mass. At what time will the beam loses contact with the mass?

I am currently doing computational physics and was given this question. There is a cantilever beam that is hit with a mass on the free end. We are asked to identify the time the bar loses contact ...
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### Wave nature of an electron

Quantum mechanics tells us that any quantum particle behaves as particle or waves in different situations. Assume that the electron is behaving like wave in some situation, how can I visualise this ...