# All Questions

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### Can anyone explain me how time can bend according to Einstein in simple way? [duplicate]

I am just 16 and curious to learn about Theory of Relativity. Can any one explain it simple enough for me to understand? I read that it is bending of time-space or space-time that causes gravity. How ...
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### Reversing time for a closed system of particles

For a closed system of particles, the lagrangian in classical mechanics is $$L=\sum \frac{1}{2}mv_a^2 - U(\mathbf{r_1},\mathbf{r_2}, \cdots)$$ For an arbitrary position function $x(t)$, to see the ...
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### Path of light as it travels between two black holes

What would happen to light passing through a narrow space between the event horizons of two equal-mass black holes? Would it deviate or follow a straight path?
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### How to calculate the inertia tensors of an irregular polygonal right prism?

I am attempting to program a physics simulation and need to do calculate the inertia tensors of a mesh constructed of a series of irregular polygonal prisms. Now, I am aware that you can calculate ...
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### Cosmological constants

I've heard that the cosmological constant is 0.[123 more zeros] and then a 1 [in some units]. Does that means that it used to be exactly zero? Is the value of this constant changing or is it fixed at ...
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### Random walk recurrence term and the self-energy

Consider the "first passage problem" A random walk proceeds on a graph of connected points. On this graph, there is one "end" point $j$ meaning that if the random walker lands on this point the ...
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### Can a mouse rotating a wheel violate conservation of angular momentum?

I have been thinking on the following problem. Part I. Imagine we have a mouse enclosed in a rigid box. We placed the box on a frictionless surface (ice might be a good example). Can the box start ...
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### Rotating cone magnetic field [on hold]

how to calculate the magnetic field of a rotating cone with a superficial charge distribution that varies with the altitude? It knows : height of the cone , truncated cone rays bases , there are ...
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### Reflected wave from free end

A wave pulse on a string moving from left to right towards a free end will reflect and propagates from right to left with the same speed and amplitude as the incident wave, and with the same polarity. ...
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### Quantising the magnetic monopoles the make Maxwell symmetric

I don't believe this has already been asked, but I might be wrong; sorry. One can add a magnetic charge density/magnetic monopoles to Maxwell's equations to make the theory symmetric between the ...
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### Understanding sine waveform [on hold]

Hey I want to study basics of propagating periodic signal I.e. sine waveform which is a power signal. Let us write expression as $$y=A*sin(2\pi*ft)$$ There are mainly two types of energies ...
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### Particles are waves, particles are trapped in time and waves are trapped in zero time? [on hold]

So, particles are waves that have been or are being observed? A particle is trapped in time and a wave exists outside time? A wave and particle are one and the same, in time and out of time at the ...
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### Fourier transformation and mode expansions [duplicate]

Sorry as this is a rather trivial question, but I'm stuck with a certain implication. I'm working on exercise 1.7 from Polchinski where we are given an open string with boundary conditions ...
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### Is it true that whenever any non rigid body moves, there must be a delay between the ends? [duplicate]

Very sorry if this is still considered a duplicate to this, but unfortunately the answer to this question cannot address this because mine is a nonrigid body and involve biological locomotion concepts ...
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### Refraction at Hyperbolic surface [on hold]

Could some one provide proof for the application of Snell's law of refraction at a hyperbolic surface? I understand that a ray parallel to the axis of hyperbola passes after refraction, through the ...
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### Period to calculate the universal gravitational constant

To calculate the torsion constant in Cavendish experiment we need the period of it, so my question is if I need to measure the time the rod with two small spheres takes to do an oscillation (with an ...
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### Throwing the object around the Earth [on hold]

You have to throw an object to a point diametrically opposite od Earth surface, take that Earth does not rotate and that it is perfect sphere. What angle to the ground and velocity must object have? ...
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### How does a Venturi Injector comply with energy conservation?

How does a Venturi Injector comply with energy conservation? When the water flow enters the narrower cross section its velocity increases and its pressure decreases. When the pressure gets lower ...
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### 't Hooft–Polyakov and Dirac monopoles, instantons and solitons

What is the difference between a 't Hooft–Polyakov and a Dirac monopole? (I read in the wikipedia article on the 't Hooft–Polyakov monopole: "The 't Hooft–Polyakov monopole is a topological soliton ...
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### How does one solve this problem(IOAA 2014, data anlysis)? [on hold]

http://prntscr.com/7nwjrt http://prntscr.com/7nwjvf This problem is from IOAA 2014, second problem from data analysis. I can't connect height with time without velocity or maximum height, which I ...
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### Importance sampling for Coulomb potential

The integral I have to solve is: $$I=\int\int d \mathbf{r}d \mathbf{r}' \frac{\Phi(\mathbf{r})\Phi(\mathbf{r}')}{|\mathbf{r}-\mathbf{r}'|}$$ It is a six-dimensional integral which I am going to ...
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### The logarithmic decay of WIFI

I have been told that Wi-Fi, LTE etc signal strength fall of as $$\propto \frac1{\log(r)}$$ where $r$ is the distance. I am wondering why this is. I better explain what I mean with this question. ...
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### Stress-energy tensor on spacetime satisfying Klein-Gordon equation

Consider the stress-energy-momentum tensor $$T_{\alpha \beta}=(\nabla_\alpha \phi )\nabla_\beta \phi -\frac{1}{2}g_{\alpha \beta}((\nabla^\nu \phi ) \nabla_{\nu} \phi +m^2 \phi^2$$ where the ...
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### Are there any $f(R, T)$ models of gravity?

For the past few months I have been studying $f(R)$ models of gravity. Recently I came cross $f(R, T)$ modified models of gravity. For the past few days I have tried to work out the difference between ...
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### Where can I find a good theoretical treatment of conductor shielding?

Suppose one has three current carrying cylinders lying on the same plane. Some of the electric field of the first conductor ends up on the second conductor, which creates capacitance C12. But also ...
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### How to find the remaining subgroup after some Higgs field gets a VEV?

Say we have a group $G$ and a set of Higgs fields in a representation $R$ of $G$. One of the Higgs fields in $R$ gets a VEV, how can I determine the remaining subgroup after this symmetry breaking? ...
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### Question about Elon Musk's Hyperloop Suspension [on hold]

So one of the proposed suspension systems that will be used on the hyperloop include the externally pressurised air cushions. These cushions lift (or atleast help lift) the capsule and reduce drag ...
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### I want to test a tunable optical filter that can quickly change from being a red light filter to being a blue light filter [on hold]

I want to test a tunable optical filter that can quickly change from being a red light filter to being a blue light filter. For this purpose, I am wondering about photonic crystal tunable optical ...
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### entaglement experiments in laboratories for atom and photons-how to? [on hold]

My questions come splited like this: 1.How to entangle a spin and a photon in an experiment? 2.How to test for entanglement in the lab?3.There is entanglement for an atom also? 4. What does it mean ...
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### electric field inside hollow conducting bodies

Let's say that there's a hollow conducting sphere placed in the presence of an external electric field and there's a + ve charge placed inside the sphere at a point other than the centre of the ...
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### What is the optimal slope for Archimedes screw?

The Wikipedia article has nice image showing how the Archimedes screw work: As I understand, the red balls do not fall down because they are in minima caused by the screw. Because of material ...
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### How to calibrate an ammeter and voltmeter? [migrated]

How can I calibrate an ammeter and a voltmeter. I just want to know about it. Because it might be helpful and I think it is a vital thing to know. So how can I do this?
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### Question about Majorana spinor's property

I am reading the BBS, Exercise 5.1 This exercise is nothing but showing that two Majorana spinors $\Theta_1$ and $\Theta_2$ \begin{align} \bar{\Theta}_1 \Gamma_{\mu} \Theta_2 = -\bar{\Theta}_2 ...
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### Does measurement change the evolution of wave function?

Basically any measurement is on wave function $|\psi\rangle$ is done by operator $X$ such that $X|\psi\rangle$ results observable $x$ with some probability. But what happens to $|\psi\rangle$? Does ...
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### Is there / What is the technique to detect water on a remote object? [on hold]

For example, before human went to the moon, how could we find out if there are water underneath? How accurate will it be? Or without opening a suitcase, how could we non-destructively find out how ...
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### Firing a blank vs live round from a gun [on hold]

When firing a gun, is there any difference between a blank round and a live round, aside from the obvious presence of a bullet exiting the muzzle? For example, does it sound different? Is there any ...
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### What is supposed to be used as $m$ in $m\lambda = d\sin \theta$

I know that $m$ is the "order number", but I don't understand how this applies to the lab we did. We looked at Hydrogen gas through a diffraction grating (500 lines/cm or something like that) and saw ...
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### Inhomogeneous Maxwell's equations in CGS units [on hold]

What is the basis or the way of writing the given inhomogeneous Maxwell's Equations in CGS units: $$\nabla.E=\frac{\rho}{\epsilon_0}$$ \nabla\times B=\mu_0J+\mu_0\epsilon_0\frac{\partial E}{\partial ...
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### Is there a physical interpretation of the alternating property?

A map from a vector-space to its base field is called "alternating" if each vector with repeated elements is mapped to zero. I've read that symplectic geometry is an important representation of ...
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### SHM by using cos equation [on hold]

A particle start motion according to equation $x=a\cos \left(\omega t+ \frac{\pi}{6}\right)$. Find time required for it to reach mid point. The displacement according to it must be 0 but how?
### Should the (On-shell) (2+1)d $N=2$ Chiral Multiplet Contain Two Scalars and Two Majorana Spinors?
When we use minimal subtraction scheme, for instance, we have a dependence of coupling on a scale $\mu$. Using the $\beta$ function, we can observe the behavior of the coupling at different scale ...