# All Questions

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### Error propagation of statistical error

I have a pulse profile (binned photon counts versus phase) of a star, and for each count rate I have its statistical error. I want to calculate the so-called pulsed-fraction ...
16 views

### 't Hooft duality

Approaching the study of QCD, I've read about the 't Hooft duality that is similar to the Meissner effect for superconductors: magnetic condensation implies electric confinement and vice versa. At ...
14 views

### Large-$N$ Yang Mills

I've bumped into the study of the $SU(N)$ theory in the large-$N$ limit. I'm wondering in which way the study of this Yang-Mills theory, can give contribution to QCD with gauage group $SU(3)$, i.e. ...
5 views

### Algorithm for distribution of radiation

Unstable atomic nuclei will spontaneously decompose to form nuclei with a higher stability. What is the algorithm for deciding what sort of it is? (alpha, beta, gamma, etc. Also, given that alpha and ...
21 views

### 3D ballistic trajectory with quadratic drag. Calculating position and velocity at time $t$

A particle starts at the origin and has an initial velocity represented by a 3D vector. The particle experiences gravity and air resistance with quadratic drag (based on velocity^2). What I've been ...
8 views

### How to determine shear force in this scenario? [on hold]

Here is a 60 lb computer, resting on a shelf. When the entire unit is rotated 90˚, a velcro cinch strap is used to keep the computer from sliding off. Velcro straps are rated in shear strength, so ...
10 views

### How to calculate tortuosity of a signal?

Not sure which stack exchange forum to put this in. Moderators feel free to migrate it. One definition of the tortuosity $T$ of a curve is: $$T=\frac{C}{L}$$ Where $C$ is the chord length ...
29 views

### What make silver, copper, and gold different from other metals?

Silver, copper and gold are special because they have the highest electric conductivity of any other elements, they have a unique color and appearance, and the three are sometimes mixed together to ...
17 views

### Heating effects of microwaves

It is also very popular misconception in India that the mobile towers cause cancer(especially skin cancer). People are so scared that they protest if some company tries to implant mobile tower near ...
91 views

### What does periodicity of $e^{-iHt/\hbar}$ mean in physical terms?

The unitary time evolution operator $U(t)=e^{-iHt/\hbar}$ has some distinct flavour of periodicity to it because of $e^{x+2\pi i}=e^x$. Is this periodicity reflected in any way in physical systems? ...
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### Is there a way to quantify how similar a polycrystal should behave to a single crystal?

So in solid state classes we learned about phenomena like band structure and others arising from a periodic potential. Then we get to doing actual experiment and find out that materials being single ...
18 views

### General formula to compute the redshift (first order perturbations)

Consider an expanding universe with the following metric in conformal time/co-moving coordinates: ...
15 views

### need data-point: count rate of APD (avalanche photo-diode) for specific aperture and stellar magnitude

I hope lab / experimental physics is fair game for this web-site. If not, sorry! I'm designing a sensor system to perform specialized [astronomy and space-sciences] experiments, and need a "reality ...
30 views

### Is it possible the acceleration of the universe creates the micro gravity that holds the universe together? [on hold]

Is it possible the acceleration of the universe creates the micro gravity that holds the universe together? Is it also possible this contributes to the "dark energy" of the universe as well?
28 views

### Induced current and Ohm's law

I am trying to explain this problem: A circular conducting loop composed of N turns of wire has a radius of r and a total resistance of R. Perpendicular to the plane of the loop is a magnetic ...
25 views

### Magnetic Shielding?

Good day people of SE I will be short. Can magnetic fields be "blocked"? For example, in the game, TitanFall, a robot stop bullets with ( presumably ) a magnetic shield. I wish to calculate the ...
11 views

### Electrical resistance of a metallic conductor in a gas

A long time ago I learned that the electrical resistance of a metallic conductor, when surrounded by a gas, varies with the pressure of said gas. • What is the name (if any) of the law involved? • ...
18 views

### Kinetic Energy of a Symmetric Dumb-Bell

The problem is as follows: Consider the system consisting of the symmetric dumb-bell (two particles each of mass $m$ connected by a light, inextensible rod of length $l$) moving in the plane. Use as ...
47 views

### Do black holes exist?

Do black holes exist? From our point of reference nothing actually goes inside the event horizon right? So is there anything inside the event horizon from our reference? If not then no black holes ...
21 views

### Express Maxwell's equations in terms of dipole field equations?

Given Maxwell's equation of the form \begin{align} \bar{\nabla}\times \bar{B} = \dfrac{4\pi}{c} \bar{J} + \partial_0 \bar{E} \\ \bar{\nabla}\times \bar{E} = -\partial_0 \bar{B} \\ \bar{\nabla} \cdot ...
36 views

### Energy/Momentum required to deflect earth from its orbit

This question occurred to me when thinking about the firepower people have on earth. How hard is it to change the characteristics of the orbit of the earth when an explosion happens on its surface, ...
21 views

### Use of fission products for electricity generation

Why can't we use fissions products for electricity production ? As far has I know fissions products from current nuclear power plants create enough 'waste' heat to boil water; and temperature ...
15 views

### Internal resistance of an inductor? [on hold]

In an experiment we did in class, we connected an inductor(0.1156H) and a resistor (1000Ω) in a series circuit which has an AC voltage. I know that the inductor has an internal resistance which we ...
22 views

### What is the perturbative vacuum?

Will someone please supply me with a good definition of the perturbative vacuum vis a vis quantum cosmology?
46 views

### How do I properly write Newton's second law for a particle with drag?

A heavy particle is projected at speed $U$ at an angle $\alpha$ to the horizontal. The particle is subject to air resistance which is experimentally found to vary proportionally to the square of ...
26 views

### Physical meaning of isobaric line in T-v diagram

Hi, I was just reviewing some slides from my thermodynamics class, and in one of the slides the professor asks what is the "physical meaning of the slopes" of the isobaric line in a T-v diagram. ...
22 views

### Is passing on knowledge/information from one body to another and the resultant state is ever studied as any branch of physics?

Does any branch of physics, study the passing off of information/knowledge from one body to another ( body would obviously be living beings here, as they only can do so ). And resultant changes in the ...
24 views

### How do cathode ray tubes (CRT) synthesize colours?

I just read about how LCD displays were based only on black&white contrasts, and only colour filters on each subpixel generated the colour of a pixel. However, how does that happen for a CRT? Is ...
19 views

### Emission of energy at certain frequencies [on hold]

Is it possible to build one tool for emitting electromagnetic waves at arbitrarily chosen frequencies (from 10^4Hz to 10^30Hz)? I know how building such a tool works for certain frequencies but I ...
20 views

### Is every ordinary coloured transparent paper an optical filter?

Can any red coloured transparent paper be used as an optical filter that only allows red colours through it?
42 views

### Very specific type of GR paper hunt [duplicate]

My General relativity skills suck. I need a good paper that does not start with equivalence principle and pages of elevator experiments derives principles mathematically, not by physical intuition ...
21 views

### significance of magnetic translation operator defined in fractional QHE's description

What is the significance of the magnetic translation operator used in describing the Fractional Quantum hall effect ? I was following Anthony Leggett's lecture video in which he defines these ...
29 views

### Why cannot we apply perturbation theory in Born-Oppenheimer approximation

In Weinberg's Lectures on Quantum Mechanics, he mentions Unfortunately, we cannot simply use first-order perturbation theory, with $T_{nuc}$ taken as the perturbation and the state vectors ...
31 views

### Max. speed that a space probe can communite with the Earth?

Question: Which is the average or maximum speed of a telematically sent message that a space probe can send? Is as simple as radio waves travel at the speed of light in a vacuum? Context: I have been ...
46 views

### Definition of entropy

In my textbook there is written $$\Delta S = \int_R \frac{\delta Q}{T}$$ where the $R$ means calculated along a reversible transformation. The variation of entropy only depends upon the ...
54 views

### Force acting on center of mass

Pictured above is a biker (traveling in a curve) whose center of mass has two forces acting on it. These two forces - a horizontal centrifugal force F(centripetal)* and a vertical gravitational ...
33 views

### Is it possible to incorporate the Lorenz gauge term into the electromagnetic fields?

I noticed that the Lorenz gauge term is represented by partial derivatives acting on the four-potential. Is it possible that the Lorenz gauge term could somehow be a similar object that belongs to the ...
15 views

### Magnetic field inside and outside cylinder with varying current density

I am reading through Introduction to Electrodynamics by David J. Griffiths and came across the following problem: A steady current $I$ flows down a long cylindrical wire of radius $a$. Find the ...
28 views

### Quantum Excitations

In the context of quantum field theory, is the schrodinger or dirac equation actually describing some sort of an actual wave in some field like light in EM field ? So all particles are actually waves ...
13 views

### Why leaf of electrometer doesn't repel each other in water?

Why leaf of electrometer doesn't repel each other in water? Normal electrometer filled with air will repel like it should do for electrostatic demonstration, but what if filled with water instead or ...
62 views

### What is $\langle \sigma_\mu \rangle$ $\langle \sigma_\mu \rangle$ for the Pauli Matrices?

What is \begin{align} \sum_{\mu=0}^{3} \langle \sigma_{\mu} \rangle^2 = ? \end{align} $\sigma_{\mu}$ are the Pauli matrices. The Bra-Ket notation is used in this question: \begin{align} \langle ...
5 views

### Diffraction between two beams of right- and left-circularly polarized light

If we have an optical / microwave two-way relay, my understanding is that one strategy to avoid destructive interference-based signal degradation is to use orthogonal linear polarizations. Here, ...
169 views

### What is the physical consequence of the Lorenz Gauge Term not equaling zero?

What happens to the physics of the electromagnetic field if the Lorenz gauge term does not equal to zero? \begin{align} \partial_{\mu}A^{\mu} \neq 0 \end{align}
62 views

### All possible electromagnetic Lorentz invariants that can be built into the electromagnetic Lagrangian?

Given the electromagnetic Lagrangian density $$\mathcal{L}~=~-\frac{1}{4}F_{\mu\nu}F^{\mu\nu}~=~\frac{1}{2}(E^2-B^2)$$ is a Lorentz invariant, how many other electromagnetic invariants exists that ...
53 views

### What happens to gravity in the middle of the earth? [duplicate]

So I was curious about, what if we make a tunnel from one side of the earth, to the other side of the earth? Gravity is ofcourse always negative, which makes you "fall" and not "float". If we take ...
10 views

### Can an inhomogeneous magnetic field be built to slow down and catch a neutron?

Neutrons have a measureable magnetic dipole momentum from their intrinsic spin. Is it possible to slow down and catch the neutron by imposing a force by an inhomogeneous magnetic field. I think the ...
149 views

### Did the singularity really existed in the begining of the universe?

I read in Hawking's brief history of time the following: The final result was a joint paper by Penrose and myself in 1970, which at last proved that there must have been a big bang ...
46 views

### Can the two electromagnetic field tensors be combined into a more general tensor?

Given the electromagnetic field tensor \begin{align} F_{\mu\nu} = \begin{pmatrix} 0 & -E_{x} & -E_{x} & -E_{x} \\ E_{x} & 0 & B_{x} & -B_{x} \\ E_{x} & -B_{x} & 0 ...
Near a magnetar with $B>B_\rm{QED}$, the strong B field will suppress the Compton scattering cross section of photons with a specific polarization (E-mode). Some references I know deal this problem ...