1
vote
0answers
69 views

1D Hubbard model in the negative U limit

In the 1D Hubbard model at half-filling, is the ground state considered as a charge-density wave (CDW) state in the very negative U limit? Is there a long range order exist in this case? Is a CDW ...
2
votes
1answer
374 views

Trajectory of a photon around a Schwarzschild black hole?

Consider a photon coming from the infinity in a unbounded orbit to a Schwarzschild black hole (Schwarzschild radius $r_{s}$) (see this for illustration). Its impact parameter is $b$ and its distance ...
2
votes
1answer
213 views

How are complex sound waves combined?

Audio is often explained by single frequencies. Typically this is a sound wave: plot sin(x) * 2 from 0 to 10 However we usually deal with more complex sounds, ...
7
votes
6answers
1k views

How do traveling waves pass through a standing wave node, if the node doesn't move?

I'm having trouble with the explanation that a standing wave in a string is the superposition of traveling waves. The nodes in the diagram above are points where the particles of the string's ...
0
votes
1answer
49 views

electrostatics particle charging

When ash particles are charged negatively ( corona charging, ionization, tribo electric ) and then collected on to the fabric; particles arrange themselves in porous dendritic structure whereas ...
1
vote
3answers
244 views

Gas vs electric cooking

I've heard it said many times that you're more likely to burn food on an electric stove than a gas one, but I can't tell a difference. This seems to me to be a fallacy perpetuated by the natural gas ...
5
votes
5answers
532 views

Infinite series of derivatives of position when starting from rest

Suppose you have an object with zero for the value of all the derivatives of position. In order to get the object moving you would need to increase the value of the velocity from zero to some finite ...
1
vote
3answers
170 views

In what applications are lasers necessary vs. just convenient?

I’m trying to understand, given the wide array of current laser applications: Which applications actually require laser characteristics, as opposed to just use lasers because they are the most ...
1
vote
1answer
83 views

Pair Creation and Reference Frames

Ok, so I have a question about an example problem in my textbook (its long but not difficult to follow. My question is at the bottom after I outline the problem). Consider the situation below in frame ...
2
votes
0answers
44 views

Would a large, small mass object in orbit experience induced rotation

Imagine a large (multiple earth radii), very small-mass ring orbiting The Sun. Half of the ring would be closer to The Sun than the outer half. Since orbital velocity decreases with distance, two free ...
4
votes
5answers
582 views

Would dropping large blocks of ice into the ocean mitigate global warming?

This amusing Futurama sketch 'solves' global warming by dropping increasingly larger blocks of ice into the ocean. Presumably the blocks of ice are harvested from a different planet. Would this ...
1
vote
1answer
111 views

About Hilbert and Physics [duplicate]

Was one of Hilbert questions regarding physics to make an axiomatic foundation for physics? Regardless of Godels work could some Physics principles that are 'basic' and 'presently verifiable' be ...
5
votes
0answers
255 views

Book with solved examples and exercise problems for SUSY

I am learning supersymmetry right now. I am mostly following Bailin and Love. I try to connect all the steps from the book and complete the derivations in order to get comfortable with the ...
2
votes
1answer
75 views

Conservation law of equation involving Hilbert transform

I am trying to confirm a conservation law I cam across in a paper (Janssen 1983 "On a fourth-order envelope equation for deep-water waves" Journal Fluid Mechanics), and am having difficulty. In ...
6
votes
7answers
364 views

Effect of wavelength on photon detection

When some photon detector detects a photon, is it an instantaneous process (because a photon can be thought of as a point particle), or does the detection require a finite amount of time depending on ...
3
votes
1answer
333 views

Help calculating field of view for ball lens I just bought

I have a 5 mm (diameter) solid ball lens that I intend to place in front of a 36mm X 24mm camera sensor (I have already made preparations for the image to be focused and for the sensor to not be ...
3
votes
0answers
16 views

Resolution Limit for Energy Transition Imaging

I understand that when imaging the resolution limit can be given by Ewald's sphere which is due to the maximum amount of momentum that can be transferred. But how can we come up with a resolution ...
0
votes
1answer
115 views

Question about the Klein Paradox

I am trying to understand some points about the paradox, what I am doing is solving the for the step potential $$ V = V_0 ~\theta(z) $$ I have two solutions $$ \Phi_I = e^{ik_1 z} + r e^{-ik_1 z}$$ ...
1
vote
0answers
474 views

A general solution to continuity equation

Let us write the standard continuity equation $$\frac{\partial \rho}{\partial t} + \vec{\nabla} \cdot \vec{\jmath} = 0.$$ Should the relation $\vec{\jmath} = \rho \vec{v}$ be considered as a general ...
1
vote
1answer
2k views

In which direction does the current flow in a wire moving north to south along the Earth?

I stumbled upon this question and I can't seem to figure it out. I think you have to use the right hand rule, but it doesn't get me the right answer. The question is something along the lines of: ...
9
votes
2answers
989 views

Quantization of a particle on a spherical surface

Suppose we have a particle of mass $m$ confined to the surface of a sphere of radius $R$. The classical Lagrangian of the system is $$L = \frac{1}{2}mR^2 \dot{\theta}^2 + \frac{1}{2}m R^2 \sin^2 ...
2
votes
1answer
1k views

Why is scale factor inversely proprotional to temperature?

In all lectures, books and papers I have read about, the scale factor of the universe is inversely proportional to temperature $$a \propto \frac{1}{T}$$ What is the reasoning behind this relation? ...
0
votes
0answers
21 views

How did WMAP measure the flatness of space? [duplicate]

I read that WMAP constrained $\Omega_0$ to be $1$ within $1%$. I realize that the baryon acoustic oscillations will produce a preferred scale on the CMB. Knowing $H_0$ one could measure the angle of ...
1
vote
1answer
53 views

Why some electrons in ESR aligned their spin towards external magnetic and some against

Why some electrons in ESR aligned their spin towards magnetic field and some against?
0
votes
1answer
47 views

Is it possible to more cloud-layers go to different ways on the sky

Sometime when I see the sky I see that the cloud-layers go to different ways. But how is it possible?
0
votes
2answers
72 views

Back $emf$ and power?

The formula relating current to back emf is $V-E=IR$ where $V$ is the source pd and $E$ is the back Emf. Thus it can be seen that as the back emf increases the current decreases. But what is the ...
4
votes
1answer
155 views

Derivation of matrix element

I have tried to understand paragraph 10.7 (Kallen-Lehmann Representation) in Weinberg's Quantum theory of fields (vol.1). He calculated matrix element $$\langle0|\Phi(0)|p\rangle ...
3
votes
2answers
290 views

What is the precise definition of state of a quantum system?

On studying quantum states of the hydrogen atom, I came with a question: what is the precise definition of a quantum state? Particularly, when solving the Schroedinger eq for the H using separation of ...
0
votes
1answer
360 views

Bullet hits block. Show that linear momentum is not conserved [closed]

"..." First, we will familiarize us with a collision between a homogeneous beam of length, $L$, mass $M$ attached with a frictionless hinge in point $O$ at one of the endpoints of the beam. The ...
2
votes
1answer
190 views

When photons are emitted, do they accelerate to reach the speed of light? [duplicate]

Photons are considered mass-less particle with a specific velocity but according to the electromagnetic theory, a photon is considered to have both energy and momentum. So what happen when they are ...
0
votes
2answers
131 views

confusion on vector quantities

How can we define simply that velocity is a vector quantity without mentioning that velocity has vector properties. How can we simply say it needs both magnitude and direction for its complete ...
0
votes
2answers
3k views

What is the difference between light and visible light? [duplicate]

After watching a few videos on light and electromagnetic radiation, I am a little confused. The way things are explained, is that light is just the same as electromagnetic radiation I thought this ...
0
votes
1answer
51 views

Learning material on optical computing

I'm looking for guidance to choose material for a self-study of photonics, biased towards optical computing recent advances. My background is of undergraduate level electrodynamics, quantum ...
2
votes
2answers
564 views

Tangential and radial acceleration in projectile motion

I'm currently learing kinematics, specifically projectile motion and as an example in my textbook is a bullet fired at some angle. I understand the derivation of formulas to describe that motion and ...
13
votes
2answers
845 views

Yang-Mills CP violation

Why does a term proportional to $\left(F,\,\tilde{F}\right)\propto Tr\left[ F_{\mu\nu}\tilde{F}^{\mu\nu}\right]$ in the Lagrangian of the pure Yang-Mills theory violate CP?
0
votes
1answer
62 views

Estimating quantum efficiency of gated avalanche photodiode

I have a photon counting system that uses a gated avalanche diode to detect single photons. The repetition frequency of the gates is $f_1$ and the temporal gate width is $\tau_1$ (so the duty cycle is ...
4
votes
2answers
203 views

Momentum in Free Scalar Field

I have a few issues with making the transition between these: $\phi(x)=\int{\frac{d^3p}{2\pi^3}\frac{1}{\sqrt{2\omega_{\vec{p}}}}(a_{\vec{p}}e^{i \vec{p} \vec{x}}+ a^{\dagger}_{\vec{p}}e^{-i \vec{p} ...
7
votes
2answers
277 views

No monopoles in the Weinberg-Salam model

I'm reading Chapter 10.4 on the 't Hooft-Polyakov monopoles in Ryder's Quantum Field Theory. On page 412 he explains why magnetic monopoles cannot appear in the Weinberg-Salam model. I'm I right by ...
2
votes
1answer
55 views

Gravity fed containers

Curiosity got the better of me, and was wondering if you have two containers full of sand each at different heights but both fed in a tube from a with the containers being in Y configuration, is there ...
1
vote
1answer
50 views

Basic Field Calculations

I'm trying to derive the relation: $\phi(x)\phi(y)=:\phi(x)\phi(y):+\langle 0|\phi(x)\phi(y)|0 \rangle$ but struggling to see the first few steps I need to make. I've made the substitutions ...
1
vote
1answer
481 views

How do we know quantum entanglement exists/happens? [closed]

We know that this strange behavior is there but I am not to able grasp, how do we know it exists? Have we ever tried any experiments confirming this? Do we make use of this?
0
votes
1answer
556 views

Charge Distribution On Hollow Sphere

Say we have a hollow conducting sphere (with some finite thickness). If this object has an excess charge amounting to +Q coulomb, and there is no extra electric field in the surroundings (due to ...
1
vote
1answer
64 views

What can you use voice sound for? [closed]

Moreover, how can you make your voice productive? for example, using it to light an LED, charge a battery, purificating water, growing plants in a more efficient way. I want to do a project that ...
4
votes
1answer
111 views

The properity of $\mathbb{R}^4$ that has infinitely many differential structures is related to Yang-Mills field?

I heard a saying that $\mathbb{R}^4$ having infinitely many differential structures which are not diffeomorphic to each other has a relationship with Yang-Mills field. Does anyone can explain it, and ...
2
votes
2answers
200 views

Static wave's equation

In my book equation for two advanced wave which created this static wave are $$y_1=asin{\frac{2\pi}{\lambda}(vt+x)} $$ $$y_2=asin{\frac{2\pi}{\lambda}(vt-x)} $$ and equation for static wave is ...
1
vote
1answer
140 views

Spin FET Transistor

Spin FET Transistor. When a gate voltage is applied to a current of spin polarized electrons, a spin precession will occur. If this spin precession is enough to make the bulk electron spin ...
2
votes
2answers
527 views

Magdeburg Hemispheres

The Magdeburg Hemisphere experiment was the experiment that showed the effect of pressure differences on a vacuumed sphere. We know that the Force caused by pressure is $\Delta p A$ and so you can ...
1
vote
0answers
271 views

Compute and plot satellite position on 2D Earth map

I would like to create a simple satellite tracking software, that will compute the position of a satellite at any time, and show its location on a 2D map of Earth. Let's assume it is possible to ...
7
votes
2answers
483 views

Is there a simple layman way to explain the incompatibilities between quantum mechanics and (general) relativity to high school students?

Is there a simple layman way that I can use to explain the incompatibilities between quantum mechanics and (general) relativity to high school students (people with not much knowledge of the intricate ...
1
vote
1answer
353 views

Determining if a decay is due to strong or weak interaction

How do we determine if a decay is due to weak or strong interaction. Is it true that as long as any one of the 6 flavour numbers are not conserved, then it must be due to weak interaction? Is there a ...

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