# All Questions

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### Light's oscillation in time

Electromagnetic waves have electric (and magnetic) fields that oscillate spatially and with time. But light, moving at the universal speed limit, is a "space-like" object according to relativity since ...
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### A reference frame is any coordinate system or just a set of cartesian axes?

In Physics the idea of a reference frame is one important idea. In many texts I've seem, a reference frame is not defined explicitly, but rather there seems to be one implicit definition that a ...
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### Coincidences in diffractograms through thin metal foils

Diffraction through thin foils has always been considered evidence for the wave behavior of the incident particles, $X$-photons at first, then electrons, protons, neutrons, $α$-particles, etc. After ...
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### Massless boson in 2D and its (retarded) propagator

I have the retarded propagator for a free scalar field in 1+1 dimensions. Inside the light cone, this looks like $J_0(m \sqrt(t^2-x^2))$, J being a Bessel function. When I take the massless limit, ...
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### Branch cuts in two-point function

The propagator of a QFT is known to have a branch cut as a function of the (complex) external momentum. The branch point (as done by, say, Peskin & Schroeder in eqn.7.19 section 7.1) is ...
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### Dipole moment of a specific charge distribution

A negative charge cloud distribute as a ellipsoid and a positive point charge situated at the centre of the ellipsoid.What should be the dipole moment of charge distribution.I mean, if the negative ...
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### The limited Computing Capabilities of Space, Increased quantized info leads to time-dilation?

Are there any approaches to Special and General Relativity using space as a computing medium? With space having a maximum computing capability and time dilation as lag? Could this idea describe the ...
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### Hartree Fock equations

I don't understand how the Hartree Fock equations define an iterative method! For this discussion, I am referring to the HF equations as described here: click me! Basically if you guess a bunch of ...
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### What is the top speed of the SR-72 aircraft? What affects would traveling at this speed have on the human body?

What is the top speed of the SR-72 aircraft? What affects would traveling at this speed have on the human body?
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### Given the quadrupole moment of the nucleus as Q, is it possible to find the order of magnitude of the dimensions of the nucleus?

Given the quadrupole moment of the nucleus as Q, is it possible to find the order of magnitude of the dimensions of the nucleus? ( order of magnitude means approximate value to the powers of ten)
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### Can magnetic fields be redirected and focused at one point?

I know that magnetic fields can be redirected, but... given a situation where you have static magnetic field over a large area, and you want to quickly change the magnetic field strength. Is it ...
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### Solving for the density operator in the quantum Brownian motion master equation

I want to solve for the density operator in the quantum Brownian motion master equation, \begin{align} \begin{aligned} ...
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### What is the simplest way to perform a Cavendish experiment?

The Cavendish experiment is cool, but seems complicated to experimentally perform. What is a simple way to recreate the experiment?
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### Electric field 0 everywhere inside Gaussian surface

Gauss's Law shows that the electric field everywhere inside a spherical shell of uniform charge density is $0$. Suppose we have a surface which divides space into two disjoint regions (an interior and ...
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### Is there a typology of different fundamental physical objects?

As I understand it (and of course, I may be wrong!).... In classical mechanics, all objects are basically the same in the sense that They are composed of atoms bunched together. These atoms occupy ...
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### Finding the Expectation Value of basis states [on hold]

I am a Mathematician and I am taking a Quantum Computing class. We have been asked to find the expectation value of $X$ tensor $Z$ and $H$ tensor $H$. X is the not operator and switches the state the ...
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### Boiling water and salt

I would like to have a good understanding of what is happening when you add salt to boiling water. My understanding is that the boiling point will be higher, thus lengthening the process (obtaining ...
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### How is extremely low frequency (ELF) radiation collected by a submarine antenna?

The U.S. Navy Project ELF managed to generate extremely low frequency (ELF) radiation at down to $\approx 76$ Hz (implying a wavelength of $\approx 3,945$ km!). I was curious, what kind of receiving ...
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### How does the radius of a pipe affect the rate of flow of fluid?

Poiseuille's law states that the rate of flow of water is proportionate to $r^4$ where $r$ is the radius of the pipe. I don't see why. Intuitively I would expect rate of flow of fluid to vary with ...
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### What does a 'complex displacement' of phonon mode mean?

I'm trying to interpret the phonon mode output file from a phonopy/vasp calculation, and this webpage talks about the 'complex displacement' of atoms in the system, and I'm not quite sure what it ...
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### Thickness of Si-substrate

we fabricate pn Heterojunction on p-Si(100). Is there any difference if the thickness of si-substrate become 260 micro.m. or 570 micro m. And which is better for the Heterojunction performance?
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### What does the gradient of a graph of 1/current against resistance graph represent?

I did an experiment to investigate how current varied with changing resistance and plotted my results on a graph of 1/current against resistance.The graph is a straight line showing $1/I$ to be ...
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### How does vasoconstriction/vasodilation of arterioles change blood pressure?

Background: I am coming at this question from an electrical engineering background, and I feel like I am missing certain assumptions that are going into the statement found in my physiology textbook, ...
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### How is $P = P_{ext}$ derived, for reversible processes?

I know that for a reversible process the inexact differential of $Q$ must be equal to $TdS$ because the total entropy of the universe must remain constant. Is there a way to formally derive that the ...
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### The Frequency of a Trill

My music teacher recently pointed out to me that, when performing a trill (alternating between two notes very quickly) the finger for the higher note should be placed slightly lower on the string than ...
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### Torque vs Moment

I was wondering, why in Newtonian physics torque is called "torque" while in static mechanics they call it "moment"? I prefer by far the term "torque", for not only it sounds strong, but also ...
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### Length contraction alongside acceleration

Let's have a ship, a target and a ship traveler which we use as point of view. Assuming no other object are observed and we are so far from any other object that gravity distortion are negligible, I'm ...
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### Confused about the “negative” - “positive” direction in this case [on hold]

If we have a graph on a pc screen of velocity versus time and the line on that graph is pointing downward implying a slope with a negative value which is a.k.a acceleration here. Now, you have a ...
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### Calculating the mass of a wormhole

What would for an observer be the mass of an isolated Wormhole (meaning that there is no gas and no mass of stars arround it) if the wormhole mouth oposite to the observer reflects the light of a ...
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### Orbital Velocity Question

I have a satellite in a stable trajectory around Earth, of a Mass $m$. I know that its velocity is: $$v = \sqrt{mG/r}\,$$ But now it begins accelerating directly against the gravity vector (i.e. ...
I just read that a permutation operator $P_{i,j}$ acts on a product state $|a_1,...,a_n \rangle \in H^n$ by $$P_{i,j} |a_1,...,a_i,a_j,...a_n\rangle = |a_1,...,a_j,a_i,...a_n \rangle .$$ Now my ...