8
votes
7answers
8k views

Why is the decibel scale logarithmic?

Could someone explain in simple terms (let's say, limited to a high school calculus vocabulary) why decibels are measured on a logarithmic scale? (This isn't homework, just good old fashioned ...
1
vote
1answer
252 views

Classical mechanics from Quantum mechanics

I'm looking at a way to prove that one recovers, under ad hoc assumptions, classical mechanics from quantum theory. Usually, we can find in textbooks that the propagator $K(x,x_0;t)=\langle x|e^{-i ...
3
votes
2answers
84 views

Third order optical mixing

It's pretty straight-forward to visualise second-order non-linear optical mixing processes in which two waves of frequencies $\omega_{1}$ and $\omega_{2}$ mix up to generate new waves of different ...
4
votes
4answers
4k views

What determines the direction of precession of a gyroscope?

I understand how torque mathematically causes a change to the direction of angular momentum, thus precessing the gyroscope. However, the direction, either clockwise or counterclockwise, of this ...
3
votes
2answers
1k views

Higgs-Boson/Graviton [duplicate]

The Higgs boson gives particles mass. And the graviton is the theoretical force-carrier of gravity. Gravity depends on mass. So if the Higgs Boson gives things mass, it therefore gives them gravity. ...
13
votes
2answers
293 views

Normalization of the Chern-Simons level in $SO(N)$ gauge theory

In a 3d SU(N) gauge theory with action $\frac{k}{4\pi} \int \mathrm{Tr} (A \wedge dA + \frac{2}{3} A \wedge A \wedge A)$, where the generators are normalized to $\mathrm{Tr}(T^a ...
1
vote
1answer
510 views

How to apply the WKB approximation in this case?

I'm trying to learn how to apply the WKB approximation. Given the following problem: An electron, say, in the nuclear potential $$U(r)=\begin{cases} & -U_{0} \;\;\;\;\;\;\text{ if } r < ...
3
votes
1answer
222 views

Is there a Hamiltonian for the (classical) electromagnetic field? If so, how can it be derived from the Lagrangian?

The classical Lagrangian for the electromagnetic field is $$\mathcal{L} = -\frac{1}{4\mu_0} F^{\mu \nu} F_{\mu \nu} + J^\mu A_\mu.$$ Is there also a Hamiltonian? If so, how to derive it? I know how ...
0
votes
0answers
77 views

Torque in an accelerated frame of reference

I'm taking a mechanics course this year and are currently studying rotational motions for my finals. The book we have explains it mostly fine, but there is one question I can't seam to find an answer ...
1
vote
1answer
118 views

Photon propagator in terms of creation/annihilation?

As far as I understand it the photon propagator, $P(A\rightarrow B)$, described in Feynman's QED book, gives the amplitude that a photon moves from spacetime point A to spacetime point B. I was ...
3
votes
1answer
848 views

Static flow of water in the presence of a loudspeaker

The title, I don't know whether it's correct or not, but I came across a video in youtube, http://www.youtube.com/watch?v=_PkgQQqpH2M. The author of video used the title and hence I used the same. ...
1
vote
2answers
214 views

Is the spin-singlet state also a Resonating-Valence-Bond(RVB) state?

The spin-singlet state of a lattice spin-1/2 system is defined as $S_x\Psi=S_y\Psi=S_z\Psi=0$, where $S_\alpha=\sum S_i^\alpha(\alpha=x,y,z)$ are the total spin operators, in other words, a ...
3
votes
2answers
173 views

Why does $\ell=0$ correspond to spherically symmetric solutions for the spherical harmonics?

In quantum mechanics why do states with $\ell=0$ in the Hydrogen atom correspond to spherically symmetric spherical harmonics?
2
votes
1answer
319 views

Do you regret being a Physicist? [closed]

I'm a high school student and physics is my passion and of course I want to major in physics, but I get a lot of negative responds from my parents, friends and some teachers when I say I want to be a ...
4
votes
1answer
318 views

How can we derive the Feynman rule for the ordinary QED 3-vertex?

I have checked some Quantum Field Theory texts that include basic QED and they all include the Feynman rule that each vertex bring with it a factor of $$\pm i e \gamma^\mu$$ but I have yet to find a ...
2
votes
1answer
152 views

Black Hole evaporation through virtual couples at the horizon

I read about vacuum energy. It explains the Hawking radiation, the black hole necessary radiation: Physical insight into the process may be gained by imagining that particle-antiparticle ...
5
votes
1answer
252 views

M(atrix) theory and things other than D0-branes? And is it non-peturbative M-theory or non-peturbative Type IIA theory?

When I first read the BFSS Paper on M(atrix)-theory, I was under the impression that it was a non-peturbative formulation of M-theory. But recently, upon reading this paper of Nathan Seiberg's, I ...
2
votes
1answer
145 views

Time Dependent Position Operator

How does one find the time dependent position expectation value for a wave function? I thought we could simple take the time dependent wave and apply the position operator like normal, but this gave ...
0
votes
1answer
636 views

Young and freedman vs resnick halliday [closed]

I am a grade 12 student with a very basic knowledge of calculus. Which is a better book - fundamentals of physics by resnick halliday or university physics by young and freedman? For understanding the ...
1
vote
0answers
113 views

Holographic principle and holograms

Holographic principle or Maldacena's duality is a theory that says that the volume of space can be described by just looking the information encoded on a boundary to the region of that space. ...
8
votes
2answers
238 views

What is the mechanism by which magnetic fields do work?

I've seen some conflicted answers to this question in texts and on the web, in the case of a dipole, for example. Do magnetic fields do work directly, or is it their induced electric fields that do ...
2
votes
1answer
88 views

Do I need to take elastic potential energy into account? [closed]

Let's say I have a vertical spring with end points $A$ and $B$ and length $a$ and a mass attached to the endpoint $B$. The mass is dropped from the point $A$ and I need to find the kinetic ...
0
votes
1answer
279 views

Three-mass, two springs copled oscillator NOT attached to walls

Int he three-mass coupled oscillator problem, we often see it stated that you have three masses, (they can be equal or not, but we'll assume they are equal here) connected by two springs and then ...
18
votes
7answers
3k views

Why do people still talk about bohmian mechanics/hidden variables [closed]

I was reading the Feynman lectures in physics and after thinking about it for a while it seems particularly unreasonable to talk about hidden variables. Let us say that the electron has some internal ...
0
votes
2answers
113 views

Would a black hole created on the surface burrow through the crust?

If scientists created a microscopic black hole with an initial mass of one ton on the surface of the earth, would the gravitational attraction to the center be enough for it to "burrow" until it eats ...
5
votes
2answers
257 views

Is it normal for physical functions to lack a 2nd derivative?

My question is about the appearance of a non-analytic function in the formula for the resistive force in air or other medium. Considering the 1-dimensional case as covered by Walter Lewin in his 8.01 ...
1
vote
3answers
253 views

Is there any solution to the potential flow around a square cylinder?

Potential flow around a circular cylinder is a classic solution. But I am wondering if there is any solution similar to this for the flow past a square cylinder?
1
vote
2answers
81 views

Transmit DC power with one cord?

My DC devices have two wires connecting them to the power source, but what would be needed to transfer DC power using just one? I depict it as a kind of headphone, which can work with just one wire.
2
votes
2answers
240 views

Force and Torque Question on an isolated system [duplicate]

If there's a rigid rod in space, and you give some external force perpendicular to the rod at one of the ends for a short time, what happens? Specifically: What dependence does the moment of inertia ...
3
votes
3answers
314 views

Understanding the Ampere's Law

We want to study the magnetic field at point $P$. So, from the figure we take that: $\oint_{L_1} B\cdot dl=\mu_0 I_1$ $\oint_{L_2} B\cdot dl=\mu_0 I_2$ $\oint_{L_3} B\cdot dl=\mu_0 I_2$ The ...
1
vote
0answers
47 views

Information and Black holes

From Youtube: A Thin Sheet of Reality: The Universe as a Hologram (Full) I have a few questions: Why there are not layers of information on a black hole? If the information is stored in the ...
4
votes
1answer
482 views

Why there is a flat band for Kagome lattice?

For the nearest neighbor hopping model on the Kagome lattice, there is a flat band among the three energy bands. Is there some reason, such as symmetry or the special structure of the model, to ...
3
votes
3answers
615 views

Can we apply Schrodinger equation in Newton Gravitational potential and derive the deterministic Newton's gravitation as a special case of it

We know the solutions for wave functions of a an hydrogen atom, and the energy values as given by spectral analysis of radiation emitted by Hydrogen, confirms the possible energy states as predicted ...
1
vote
2answers
120 views

Potential Energy of a Particle in One Dimension?

Since a particle in one dimension can only move in a straight line. Is it possible to have potential energy? And how would the kinetic energy and potential energy differ in higher dimensions?
2
votes
0answers
79 views

Commutators with function

I have following exercise: If $[C,D]$ is a c-number and $f(x)$ is a well-behaved function (i.e. all derivatives exist and are finite), show that: $$[C, f(D)]=[C,D]f'(D)$$ where $f'(D) = ...
1
vote
0answers
77 views

$E$ and $B$ fields in Axial Gauge

I am trying to compute the $\vec{E}$ and $\vec{B}$ fields in the Axial gauge ($n \cdot \vec{A}=0$) where $n^2=1$, but I'm having trouble seeing the usefulness/how it simplifies the equations.
-5
votes
1answer
158 views

Faster than light possibility? [duplicate]

According to general relativity, speed is relative, so for example if you are running at 20 km/h and a car passes you at 30 km/h, the runner is actually moving at 50 km/h relative to the car. Now ...
1
vote
1answer
110 views

Observables still commute even if fields only anti-commute

In Peskin & Schroeder page 56, after introducing anti commutation relations for the fields instead of commutation relations (in order to fix the negative energy problem as well as to have proper ...
2
votes
2answers
4k views

Why exactly does current carrying two current wires attract/repel?

When to parallel wires carrying currents in same direction I1 & I2. http://www.youtube.com/watch?v=43AeuDvWc0k this video demonstrates that effect. My question is, why exactly does this happen? ...
4
votes
0answers
52 views

What are the reasons why dark matter above the TeV scale appears to be ignored experimentally?

I understand that sub-TeV-scale dark matter would fit nicely with TeV-scale SUSY, but are there any other reasons for the large experimental blind spot between TeV-scale WIMPs and the much larger ...
2
votes
1answer
124 views

Why do we get the same result using different ensembles?

There are different kinds of ensembles: microcanonical, canonical, grand-canonical... But for a particular system, no matter whether the system is isolated or closed, they just give the same result of ...
0
votes
2answers
505 views

Between a solid and a hollow cylinders of the same mass, which one has the greater rotational kinetic energy?

I know that rotational kinetic energy is $\frac{1}{2}I\omega^2$. Therefore, the rotational kinetic energy will depend on the moment of inertia. I came to the conclusion that since both have the same ...
0
votes
0answers
48 views

String Theory- Are strings the end? What are they made of? [duplicate]

Although I have informed myself in string theory through reading books and watching videos, I do not know or understand what superstrings are made of. History has definitely shown us that when we ...
1
vote
0answers
43 views

Boundary Condition for Generating Evanescent Waves

I'm reading through Novotny and Hecht's book on Nano-Optics (Principles of Nano-Optics), and I've come across a subtlety in the boundary conditions for evanescent wave generation (via total internal ...
0
votes
0answers
36 views

How do I determine $\phi$ in a hollow circular cross section using torsion equivalence?

I know that the maximum shear stress $$\tau = \frac{T}{J}\rho$$ where $\rho$ is the radial distance from the center of the cross section. I have also determined the torsional constant $J$, which is ...
2
votes
0answers
111 views

How do we derive force/acceleration vectors from Einstein's field equations?

I'm new here and I don't have any formal experience in physics beyond A-level. I've been exploring an idea for a space sim game someone else is developing in which propulsion of a spacecraft is ...
1
vote
0answers
66 views

Action of conformal generators on fields

I am calculating the action of the conformal generators on fields, to be more precise on wavefunctions. For now, I'm classical. I will just paste the part of my report on this to show what I am ...
1
vote
1answer
125 views

Ensemble average of product of spin operators?

How do you evaluate the canonical ensemble average of a product of spins, e.g.: $$[S_zS_x]$$ Where: $$S_x = \frac{\hbar}{2} \begin{pmatrix} 0 & 1\\ 1 & 0\\ \end{pmatrix}$$ $$S_y = ...
0
votes
3answers
683 views

What cosmic event would cause Sun rising from the west?

I'm wondering what kind of astronomical or geological events would cause the reversal of Earth's rotation. For instance, Is a meteorite passing very close to Earth able to reverse its rotation? Can ...
1
vote
1answer
152 views

How is a Higgs boson created?

I have read a lot on Higgs bosons, yet I do not fully comprehend how they are created and how they are "flicked off" the Higgs field. I have also had trouble comprehending why a Higgs boson quickly ...

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