1
vote
1answer
50 views

Lorentz Transformations in Minkowski space

If $\Lambda$ represents the Lorentz transformation matrix, then the transformation of contravariant components $x^\mu$ is given by $$x'^\mu=\Lambda^{\mu}{}_{\nu} x^\nu$$ and that of the covariant ...
5
votes
1answer
198 views

Hidden momentum

I'm trying to learn about hidden momentum. After reading what I could find with a google search, I understand that it is equal to the momentum carried by radiation, calculated with the Poynting vector....
1
vote
3answers
166 views

Why we do calculus of variation instead of finding maxima or miniama of function? [on hold]

Why we do calculus of variation instead of finding maxima or minima of function? What is the difference between finding maxima or mimima i.e. critical point of a function and calculus of variation?
3
votes
2answers
29 views

How to calculate absorbed dosage

Lets say you have some unstable isotope which decays via beta decay. For example, lets say you want to calculate the amount of beta radiation that is emitted from the decay of $^{87}\textrm{Kr}\to ^{...
-1
votes
0answers
26 views

Virtual Particles in QCD Vacuum [on hold]

I would like to ask what are the virtual particles in the QCD vacuum. Are they QCD bound states or they are just quarks? In fact, I am puzzled by the logics explained in Peskin and Schroeder book, ...
3
votes
1answer
481 views

Stokes' theorem in complex coordinates (CFT)

I am studying CFT, where I encounter Stokes' theorem in complex coordinates: $$ \int_R (\partial_zv^z + \partial_{\bar{z}}v^{\bar{z}})dzd\bar{z} = i \int_{\partial R}(v^{z}d\bar{z} - v^{\bar{z}}dz). $$...
3
votes
0answers
88 views

Optics: finding total power of a system, including wavelength

Specifically I need to create an ABCD matrix given 2 radii of curvatures, the thickness of the lens, type of glass (BK7) and a given wavelength. I will have to figure this out eventually, so help ...
0
votes
3answers
179 views

Treating matter waves as light waves?

Is it valid to treat a matter wave as a light wave with wavelength equal to the de Broglie wavelength of the matter wave? Either way please can you explain why?
0
votes
3answers
269 views

What determines the magnitude of force of an electromagnet?

Let's assume that there is an electromagnet with $B = 1 T$ , what is the magnitude of the force it could generate on other dipoles equal to it's surface area? Can the magnitude of force (in newtons) ...
0
votes
1answer
46 views

Deriving invariance of interval

I'm reading a book called A First Course In General Relativity, and when I got to the invariance of the interval, the book didn't derive it. Also, when the book derived time dilation and Lorentz ...
1
vote
1answer
61 views

Is the MWI symmetric in time?

Reading the blog of Sean Carroll (I recognize he isn't the only voice) has made me more sympathetic to the notion of many worlds, but reading Susskind (also not the only voice) has made me think that ...
4
votes
3answers
80 views

How can I show that inversion is continuously connected to a reflection?

From Ex 3.1 in the TASI lectures on the conformal bootstrap: http://arxiv.org/abs/1602.07982 the problem is the inversion map (with Euclidean signature) $$ I\colon x^\mu \mapsto \frac{x^\mu}{x^2} $$ ...
4
votes
2answers
113 views

What is the black hole information paradox really? [on hold]

Preliminaries What is the black hole information paradox really? Is it a sophisticated way to ponder and debate the existence of an operator on the boundary that can tease out the interior of a ...
0
votes
0answers
43 views

Potential Energy = mgh. What is the name of this principle and did Newton discover it?

Somebody I know keeps trying to convince me that Potential Energy $PE=mgh$ is one of Newton's Laws (as well as the closely related idea, Work Done = force x distance moved). I am fairly certain that ...
7
votes
2answers
790 views

Special relativity paradox and gravitation/acceleration equivalence

One of the features of the black hole complementarity is the following : According to an external observer, the infinite time dilation at the horizon itself makes it appear as if it takes an ...
-6
votes
0answers
28 views

quantum mechanics problem orthonormal function [on hold]

$$\psi_n(x)=\sqrt{\frac{2}{a}}\sin\left(\frac{\pi n}{a}x\right)$$ is this wave function orthonormal or not? If yes then prove it.
0
votes
1answer
61 views

Practical way of expressing the $\delta$-function [on hold]

I have got a problem in using the $\delta$-function. As we know, this function is often used to define a 'density'-related quantity. Such as the density of states or some correlation function. Take ...
1
vote
1answer
288 views

Tight binding in the limit of large system size

Suppose that one has a continuous Hamiltonian with spin-orbit interaction, for example $H=-\dfrac{\mathbf{p}^2}{2m} +\kappa({\boldsymbol\sigma}\times\mathbf{p}) + U(x)$ and want to approximate this ...
2
votes
0answers
65 views
+50

Objective time derivative that is no Lie derivative

Summary Led by an interest into the concept of "Material Objectivity", I am asking myself: Are there objective time rates that are not Lie derivatives? The long read I am trying to understand the ...
1
vote
1answer
17 views

Modeling rubber foams

Are there any good papers/texts on the subject of modeling the dynamics of rubber foams? So far I haven't found any good papers/texts that cover this particular subject and I've done some searching. ...
-2
votes
0answers
15 views

Rigid 3d boby angular momentum about fixed and moving point [on hold]

do the angular momentum of a 3d rigid body about fixed vs moving point differ(assuming both points coincide at this instant) what changes occur when i calculate angular momentum of a rigid body about ...
-1
votes
0answers
17 views

How do I create a new voice out of multiple sources [on hold]

Hey guys, If 50 people with different accents read the same sentences in same tempo and I would record it. Could I create a new voice and accent based on all the recordings? How would the process ...
1
vote
1answer
59 views

R-matrix and S-matrix in QFT

In the study of quantum field theory, one may encounter S-matrix a lot. Recently, in the study of integrability, I encountered R-matrix formulation which I am not familiar with. First of all, the S-...
-1
votes
0answers
16 views

How may I compare the pressure between fluid with relative motions? [on hold]

According to Bernoulli's principle, the air flows faster has lower pressure. However, this does not quite explain when it comes into relative motions. I had the following question. The air in Ring 1 ...
1
vote
1answer
104 views

What does degrees of freedom mean in the context of vibrations?

If you have an $N$ degrees of freedom system what does this mean? What is the difference between a 1 and a 2 degrees of freedom system?
1
vote
2answers
152 views

Fluid speed and fluid density

How does fluid density affect fluid speed? Basically I am trying to figure out if, with all other quantities remaining constant, would an increase in fluid density cause the fluid speed to increase/...
0
votes
1answer
184 views

Two concentric conducting shells connected by wire with modified Coulomb law?

This question is related to this one, but is a bit different. Consider two concentric and conducting spherical shells connected by a wire. Inner shell has radius $b$ and (unknown) charge $q_b$ while ...
0
votes
0answers
12 views

Calculate velocity $v$ [on hold]

A lorry of mass $3.5\times10^4\text{ kg}$ attains a steady speed $v$ while climbing an incline of $1$ in $10$ with its engine operating at $175$ kW. Find $v$. $g=10ms^{-2}$. Neglect friction. The ...
-1
votes
1answer
66 views

What happens if you fly a drone inside moving passenger airplane? [duplicate]

I have a little question. If I mark an X in the center of a passenger airplane, and try to fly a drone from the X mark in the moving passenger airplane (800-900 km/h), what will happen to the drone? ...
1
vote
2answers
51 views

Torque on electric dipole placed in non uniform electric field

When electric dipole placed in non uniform electric field, what is the approach to calculate torque acting on it? Can it be zero?
5
votes
2answers
113 views

Intuitive analysis of gradient, divergence, curl

I have read the most basic and important parts of vector calculus are gradient, divergence and curl. These three things are too important to analyse a vector field and I have gone through the physical ...
-1
votes
0answers
19 views

Numerical problem [on hold]

A spacecraft weighing 500 kg is drifted of its course at a point 60,000 km from the impact point by 100km perpendicular to the actual trajectory. Explain the course correction manoeuvre, giving the ...
0
votes
0answers
14 views

Impedance of a series RLCcurcuit [on hold]

what is the impedance of a series RLC circuit at resonance? What is the current in the circuit resonance?
0
votes
0answers
23 views

Floating block with objects that are thrown in a liquid [on hold]

I'm confused about the following kind of situation. Consider a block of density $\rho_b$, mass $M_b$ and section $S_b$ that floats on a liquid of density $\rho_l$, in a tank of section $\mathcal{S}$. ...
4
votes
1answer
56 views

An ideal black - body at room temperature is thrown into a furnace. It is observed that? [on hold]

An ideal black - body at room temperature is thrown into a furnace. It is observed that? (A) initially it is the darkest body and at later times the brightest. (B) it is the darkest body at all ...
3
votes
2answers
1k views

Does a pivot exert an additional force, normal to the centripetal force, on an object rotating about it?

A thin plank of mass $M$ and length $l$ is pivoted at one end. The plank is released at 60$^{\circ}$ from the vertical. What is the magnitude and direction of the force on the pivot when the plank is ...
14
votes
1answer
283 views

Do Franck-Condon oscillations have natural lineshapes?

I recently found a paper (for the curious, this one) that talks about observing the motion of a nuclear wavepacket in H2O, as initiated by tunnel ionization. This wavepacket should be thought of as a ...
30
votes
5answers
2k views

Do free-electron lasers actually lase?

Free-electron lasers are devices which use the motion of highly energetic electron beams to produce bright, coherent radiation in the x-ray regime. More specifically, they start with a high-energy ...
3
votes
1answer
71 views

Gaussian surface question: arbitrary wire in a plane

I have a question about the setup in the image below. For the wire with $Q+$, if I draw a Gaussian Cylinder that just encloses the wire (as shown), where $s_1$ is the top surface of the cylinder and $...
0
votes
2answers
34 views

“Simple” air resistance question

Two balls with exactly the same size and shape, but different mass, are launched at the same velocity 90 degrees to a flat plane. When air resistance is considered, the object with the larger mass ...
2
votes
3answers
55 views

Two qubits system in polar co-ordinates

I know that I can write a single qubit state in terms of polar co-ordinates $(r,\theta,\phi)$ on a Bloch sphere. \begin{equation} \rho = \begin{pmatrix} \frac{1+r \cos\theta}{2} &\frac{r \exp(-i\...
0
votes
1answer
226 views

Tight binding Hamiltonian for 2D finite dimensional lattice and nanowire

The Hamiltonian of a 1D lattice having finite $N$ atoms, (if we consider one basis per atom) is given by the following $N\times N$ matrix- Here $E$ is the onsite energy and $t$ is the hopping ...
2
votes
1answer
98 views

What is the significance of the Inverse-square law? [duplicate]

Considering its occurrences in various fields like Electrostatics, Gravitation, Acoustics etc. how does the law bind these topics together?
0
votes
0answers
31 views

Calculate displacement of one body connected to an other by spring and forced in a direction [on hold]

Two bodies of mass $M_1$ and $M_2$ are connected by a spring but are otherwise free to move along a horizontal line. A periodic force $F\cos\omega t$ is exerted on the body of mass $M_1$ along the ...
40
votes
5answers
5k views

If a lens focuses all incoming light to a point, how do we get 2D images?

How do lenses produce 2-dimensional images, if a lens bends all incoming rays of light to intersect at the focal point? Shouldn't this produce a single dot of light on a screen placed at the focal ...

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