4
votes
1answer
134 views

Protoplanetary disks, angular momentum and prograde orbits

So you've got a protoplanetary disk and you're going to gravitate yourself some planets together. The disk is made up of the usual planetary system stuff, dust and gas and whatnot, orbiting a common ...
0
votes
0answers
6 views

How come a greater normal force doesn't make us feel lighter?

If I were to push an object rightward with my hand from rest, the object would receive a net force in the right direction and accelerate. If we created a free-body diagram for this situation, it would ...
2
votes
0answers
19 views

Effective mass approximation

When we calculate the band structure of some solid then we often find that in the bottom of the conduction band the dispersion looks approximately quadratic with some new effective mass: $$E(k) = ...
0
votes
0answers
5 views
-3
votes
3answers
19 views

Displacement of an object dropped from a cliff

An object is dropped from a cliff. How far does the object fall in the third second? Assume that $g = 10 \, \mathrm{m/s^2}$. A: 10 m B: 20 m C: 25 m D: 45 m
0
votes
1answer
51 views

Torsion-free, symmetric connection and non-coordinate basis

The torsion tensor is defined as (Hawking p.34) \begin{equation} \mathbf{T}(\mathbf{X},\mathbf{Y}) = \nabla_{\mathbf{X}}\mathbf{Y} - \nabla_{\mathbf{Y}}\mathbf{X} - [\mathbf{X},\mathbf{Y}]. ...
2
votes
3answers
16 views

Work done on an object by the internal forces

How is the work done by the internal forces acting in a rigid body zero? Actually I read in a book an example for the same. Let me present that example here. Consider a rigid body having two ...
0
votes
2answers
24 views

Discrepancy between the rate of heat flow

I'm in a introductory college physics class, and my professor recently gave us this formula for the rate of heat flow: $\frac{dQ}{dt} = \frac{A \Delta T}{R_{factor}}$ where $R_{factor}$ is ...
5
votes
1answer
286 views
+50

Antiparticles, charge conjugation and chirality

(Why/how) are antiparticles and charge-conjugates different things? I am trying to understand the effect of discrete symmetries on spinor fields (neutrinos in particular). In the article, Dirac, ...
16
votes
3answers
4k views

Deriving the speed of the propagation of a change in the Electromagnetic Field from Maxwell's Equations

I've been told that, from Maxwell's equations, one can find that the propagation of change in the Electromagnetic Field travels at a speed $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$ (the values of which can ...
0
votes
0answers
35 views

How can there be an infinite number of universes? [on hold]

Many proponents of the multiverse (Greene, Krauss, perhaps Hawking) have posited that there may be an infinite number of alternative universes. How could this be possible, or even understood? ...
0
votes
1answer
18 views

Ampere's law and Stokes' Theorem for current density

In both Griffiths and Jackson, Ampere's Law (or conversely the curl of the magnetic field) is derived by applying Stokes' Theorem to the surface integral of the current density J. The argument relies ...
3
votes
1answer
78 views

How to write the Clebsch-Gordan decomposition in tensor notation

Let be $G$ a Lie Group and $\textbf{N}$ its complex representation. It is known that any state $|\ ab\ \rangle\in \textbf{N}\otimes\textbf{N} = \oplus_I\textbf{r}_I$ may be decomposed through the ...
5
votes
4answers
935 views

Lagrangian for relativistic massless point particle

For relativistic massive particle, the action is $$S ~=~ -m_0 \int ds ~=~ -m_0 \int d\lambda ~\sqrt{ g_{\mu\nu} \dot{x}^{\mu}\dot{x}^{\nu}} ~=~ \int d\lambda \ L,$$ where $ds$ is the proper time of ...
1
vote
3answers
32 views

Rotation of a vector

Is a vector necessarily changed when it is rotated through an angle? I think a vector always gets changed because its projection will change, and also its inclination with axes will always change. ...
0
votes
1answer
114 views

Can we embed 2+1 space-time of GR in a 3 Dimensional Euclidean space? [on hold]

Wikipedia says that inflation is the exponential expansion of space in the early universe.I'm trying to have a physical picture of this.Given that I can't visualize 3+1 pseudoriemannian manifolds,I'm ...
2
votes
0answers
17 views

Why don't the colors in an oil slick change as I walk past?

After it rains, I often see bands of color on the asphalt roads. This is usually explained as an thin film interference. Wikipedia has a picture of it which looks by inspection to be the same ...
23
votes
7answers
2k views

Quantum random numbers from a laser — simplest setup?

I'm a software / maths guy who would like to build a physical setup for generating quantum random numbers. I have no physics background, so bear with me. Background The project is for a public ...
4
votes
3answers
66 views

How do I find the relation between the accelerations of the ring and the disc (see image)?

What is the relation between the accelerations of the ring and the disc (see image)? Both the ring and the disc have mass $M$. The ring has a radius $R$ and disc has radius $2R$. They are connected ...
0
votes
1answer
15 views

Under what conditions does a beam splitter entangle two input photons?

There is a dispute on PhysicsForums related to what are the conditions necessary for two photons to be entangled by a beam splitter. Lots of references given by the forum users but they never arrive ...
0
votes
0answers
17 views

Relating Initial velocity and acceleration to distance traveled up a ramp

This is the situation: I am conducting research on the ability of fin-fish to scale short wetted ramps for use alongside sea-lamprey barriers to allow non-target species to pass these barriers. We ...
1
vote
2answers
65 views

Derivation of Euler-Lagrange equation from principle of least action

When deriving the Euler-Lagrange equation for a field $\phi$ the term $$ \int\textrm{d}x^{\mu}~\partial_{\mu}\left( \dfrac{\partial \mathcal{L} }{\partial(\partial_{\mu}\phi)}\right)\delta\phi $$ is ...
1
vote
2answers
1k views

Why is Curl of Current Density $\nabla \times \vec{J}$ equal zero?

I am revisiting the derivation for $\nabla \cdot \vec{B} = 0$ in magnetostatics for the field $\vec{B}(\vec{r})$ of a charge $q$ at position $\vec{0}$ with velocity $\vec{v}$. It proceeds like ...
1
vote
0answers
20 views

How do I model the motion of a particle changing acceleration vector (2D)?

I want to model a particle with an arbitrary initial velocity, and estimate the time it takes to reach a final point given a constant magnitude of acceleration. It should take the quickest path to the ...
-1
votes
1answer
18 views

Net force equal to mass multiplied by acceleration of centre of mass

$F=ma$ if $a=0$ then definitely the net force applied will be also zero (0). then, in this case $m=F/a$. If both net force and acceleration(of centre of mass) is equal to zero (0). then, isn't mass ...
1
vote
2answers
190 views

Rigid Body Dynamics - Calculating Rotation of an Object

I've looked everywhere and I can't quite find a concise answer, especially due to my limited knowledge and understanding of terminology. I found something called Rodrigues' Rotation Formula that ...
5
votes
2answers
103 views

Bell's theorem and fluid-mechanical experiments using droplets: are local hidden-variable theories possible after all?

Recent fluid-mechanical experiments by the groups of Couder in Paris and Bush at MIT, mimic a surprisingly wide range of quantum effects. The essential ingredient of these fluid-mechanical systems is ...
0
votes
0answers
13 views

How can I prove that the Euler-Bernoulli beam PDE is Hamiltonian?

How can I prove that the Euler-Bernoulli beam PDE is Hamiltonian? I'm having trouble with the above. I have the Hamiltonian: how can I prove this is Hamiltonian in structure?
2
votes
1answer
29 views

EM: relative permittivity

After having defined the relative permittivity in lectures, I have problem understanding why it is a complex quantity. Thank you for your help,
5
votes
3answers
578 views

Why don't metals damp vibrations?

Fluid damps vibrations due to viscous dissipation. Does anyone has any insight on a molecular, microscopic level about the reasons that vibration damping of metals is negligible?
0
votes
2answers
200 views

Does a normal torch emit entangled photons?

I was reading a sciencenews.org post about three photons being entangled. My question here is, why is the chance of producing an entangled pair once in a billion times? Isn't every particle produced ...
3
votes
3answers
129 views

What happens after an electron and hole combine?

OK so I understand that an LED is a pn junction where when a forward bias is placed across the junction, the electric field causes the electrons to combine with the holes and release a photon through ...
2
votes
2answers
27 views

How does change in medium affect object distance/image distance?

Say,we have a container filled with a liquid of refractive index $7/5$ upto a height up $H$. There exists a plane glass mirror at the bottom of the container. Now if a fish were placed at a height of ...
0
votes
0answers
22 views

EM: Maxwell relations, waves

I have found the following formula in my lecture notes but I don't know from where it is derived: $i\vec{k} $ x $ \vec{E} = iw\vec{B}$
0
votes
0answers
13 views

Superficial degree of divergence for scalar theories

I have a few questions regarding the derivation of the degree of divergence for feynman diagrams. The result is $$D = [g_E] - \sum_{n=3}^{\infty} V_n [g_n]$$ (following notation in Srednicki, $P118$) ...
0
votes
0answers
12 views

EM: Conducting media

I have a question concerning the following exercise: The physical electric field associated with a plane wave in a material has the form: $\vec{E_{phys}}= E_0e^{-a_1z}\cos(a_2z-a_3t)\vec{y}$ ...
0
votes
1answer
33 views

Does a photon follow a straight trajectory in a thin transparent film?

Does a photon follow a straight trajectory in a thin transparent film if the source has emitted a photon inside the film with impulse parallel to the film surface. (lambda greater than thickness)? ...
-2
votes
1answer
18 views

Formation of nucleus

Is it even remotely possible for a nucleus to be formed in the following way? Consider two nucleons approaching each other at extreme speeds. Could the relativistic increase in mass be so high to ...
4
votes
2answers
387 views

Is gravity the current limitation to laser cooling?

This is something I've been wondering for a while. I've read about the experiment with NASA to reach unprecedented cold temperatures in space because of the microgravity environment not being such a ...
2
votes
0answers
23 views

How fast should 6 liters flow out of the sink through a 6cm pipe?

I had 6 liters in the sink when I pulled the plug. It took the water about 30 seconds to get through a pipe which has about 5cm in diameter. I think the pipe might be blocked a bit, causing it to have ...
0
votes
0answers
18 views

What is quantum foam?

Can someone please explain me what quantum foam is? Is it the space-time fabric or just any other field? Also please explain this image
3
votes
2answers
838 views

Exact definition of momentarily comoving reference frame (MCRF)

Consider a particle $P$ in the framework of special relativity with position $r(t)=(ct,x(t),y(t),z(t))$ respect to an inertial reference frame $\Sigma=(ct,x,y,z;O)$. I need to know if the following ...
0
votes
1answer
14 views

Decompression sickness

In decompression sickness why does nitrogen gas come out as bubbles? Is it because under high pressure the nitrogen gas was converted into liquid nitrogen and during the process of coming to the ...
-5
votes
0answers
24 views

Can anyone explain that “the human body is made of energy (photons)”? and how many photons it contains?

The modern research has shown that our body is basically made of energy not matter in a sense that atom itself is basically consists of energy waves, so it is nothing but energy. How we can explain ...
0
votes
0answers
6 views

Deep Inelastic Scattering - electromagnetic current

When one tries to compute the deep inelastic scattering for the process: where $l$ is a lepton with incoming momentum $k$ and outgoing $k'$, $h$ is an hadron with momentum $P$, $q$ denotes some ...
0
votes
0answers
37 views

Entanglement in Quantum field theory [duplicate]

How is entanglement represented in a field theory? For instance how can I represent a maximally entangled state such as a Bell state? Would such an approach also apply in a Conformal field theory ...
0
votes
0answers
7 views

Have I found all of the zero order laue zone points for an FCC crystal being imaged along the [111] zone axis?

Since the equation for laue zones is hu+kv+lw=0 and the electron beam is incident in [uvw]=[111] can I say that the first order laue zone points are [hkl]=[-110],[1-10],[-101],[10-1],[0-11], and ...
0
votes
1answer
17 views

Where can I learn about band theory?

I'm studying quantum mechanics and I want to understand perfectly where the bands of the electronic sturcture come from. I've read that it is related with the periodic potential, Bloch waves and ...
24
votes
3answers
3k views

How is 6W equal to 40W in LED bulbs adverts?

Every advert I come across for LED bulbs advertise them as the equivalent of a higher W regular bulbs. This makes no sense to me, if the room requires 40W to lighten it up then it'll always require ...
0
votes
1answer
21 views

What kind of other light sources can disturb the detection of LASER

My project objective is to detect different lasers , for example Laser Range Finder: pulse width 10ns and repetition rate is ...

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