0
votes
1answer
4 views
Electric fields problem
Why is this the case..
I would have thought it would have followed the E field`?
http://snag.gy/yEUDu.jpg
0
votes
0answers
14 views
Commutator of Lorentz boost generators : visual interpretation
I have always struggled to visualize the correctness of the commutation relation for the generators of the boost in the Lorentz group. We have $$[K_i,K_j] = i \epsilon_{ijk} L_k$$ I fail to picture ...
-1
votes
0answers
8 views
The physics of Hartree-Fock convergence or non-convergence [closed]
I'm new here so please forgive me if I lack proper stack exchange etiquette.
So, I was wondering if anyone here could provide insight on a problem that I am running into with with a Hartree-Fock ...
1
vote
0answers
38 views
Two balls falling one above the other
Two balls, first with the mass $m_1$ and the second with the mass $m_2$ are falling from the heigh $h$. Suppose all the collisions are perfectly elastic and do not consider the size of the balls. $m_1 ...
0
votes
0answers
12 views
Isn't Estakhr's equation of force at high-energy physics proportional to mass flow rate times speed of light?
This is Estakhr's equation of force at high energy physics (i.e., at speeds comparable to that of light):
$$F=m^2B$$
where the $B$ is known to Estakhr's constant and m$^2$ denotes mass square.
first ...
3
votes
1answer
95 views
why can't you use your cellphone on an airplane? [closed]
why airplanes are banned with use of cellphones? What were the impacts while we answer an call or make an call ,what was the physical reason behind the ban of using cellphone inside airplanes ?
7
votes
2answers
101 views
+50
Stability of rotation of a rectangular prism
I've noticed something curious about the rotation of a rectangular prism. If I take a box with height $\neq$ width $\neq$ depth and flip it into the air around different axes of rotation, some motions ...
2
votes
1answer
59 views
Anti-symmetric 2 particle wave function
Suppose we want to construct a wave function for 2 free (relativistic) fermions. As we are dealing with fermions the total wave function has to be antisymmetric under interchange of the coordinates,
...
1
vote
1answer
27 views
Normalizing a continuous distribution
I work at a help/tutoring center at my university. Today a kid came in with this problem. I've only studied math and haven't drifted into physics, but he had this problem:
Let ...
2
votes
1answer
38 views
Applications to the Van der Pol equation?
What are some applications to the Van der Pol equation? Are there any physical examples?
1
vote
1answer
21 views
Relation between Radiance and Irradiance
I know the radiance is expressed as
$$\text{[Radiance]} = \frac{W \cdot m^2}{\text{sterad}}$$
and
$$\text{[Irradiance]} = W \cdot m^2$$
But what's the relation between theese two quantities?
Is the ...
0
votes
0answers
17 views
system of tubed colliders and desired direction of movement
Imagine tube with mass M and inside it an object with mass M2 that we can force into direction x or -x. Let us say that desired direction is x. Collision between the wall at the end of the tube and ...
5
votes
4answers
587 views
Is energy the ability to do work?
Here was my argument against this, the second law of thermodynamics, in effect says that, there is no heat engine that can take all of some energy that was transferred to it by heat and do work on ...
6
votes
3answers
98 views
What happens with the force of gravity when the distance between two objects is 0?
so I had my first approximation to the gravity equation
\begin{equation}
F=GmM/r^2
\end{equation}
and some questions arose that my teacher couldnt respond:
if r approximates to 0 with mM being ...
0
votes
0answers
14 views
adhesion(bonding etc.) of materials and frictional force
I am studying material physics.
I heard in the class that every adhesion or bonding can occur only with frictional force.
Today, I tried to find a proper reference for this statement but I could ...
6
votes
1answer
146 views
Noether's identities
I have some questions about the Noether's second theorem (generally not covered by field theory books):
What is the most general Noether identity for (classical) field theories?
Why are Noether ...
2
votes
0answers
19 views
Powercounting in String Frame?
if I consider the low energy effective action of type IIB string theory in the string frame, i.e. with an $e^\phi$ prefactor, is it possible to do standard powercounting with this action? I.e. how do ...
0
votes
1answer
127 views
Opposing forces on an air cylinder
If an air cylinder is pushing two platens apart with a force of 100lbs, do the platens need to push back at 100lbs or 50lbs each to keep the cylinder from moving? Assume no friction and both platens ...
0
votes
0answers
50 views
Lagrangian of three connected rods
There is free falling chain - 4 connected rods with mass m and length l:
...
0
votes
0answers
22 views
Does passing large amount of current(via large potential difference) for a large amount of time decrease resistance
So, I just learnt that by developing potential between two points of a conducting wire, the electrons rush towards the point with high potential due to the electric field generated by it. Also that ...
-4
votes
0answers
36 views
Why is it that the atom is considered invariant?
The building of physics rely on the assumption that the atom is invariant through time (*).
IMO the invariance cannot be measured in the laboratory because what happens with the atom also happens with ...
27
votes
2answers
3k views
Why can Hiroshima be inhabited when Chernobyl cannot?
There was an atomic bomb dropped in Hiroshima, but today there are residents in Hiroshima. However, in Chernobyl, where there was a nuclear reactor meltdown, there are no residents living today (or ...
5
votes
1answer
34 views
Electromagnetic waves produced by a charged pith ball
Maxwell’s equations appear to have no limitation as to the length of an electromagnetic wave that can be produced by an accelerating electric charge. So in theory, if I use a charged rod to oscillate ...
1
vote
1answer
340 views
Electric field and insulator or dielectric
I have a uniform electric field $E$ projected from left to right and I placed an insulator or dielectric right in the middle of it. To the left of the insulator, the $E$ is the original $E$. Inside ...
3
votes
1answer
79 views
Why is the stress-energy tensor symmetric?
The relativistic stress-energy tensor $T$ is important in both special and general relativity. Why is it symmetric, with $T_{\mu\nu}=T_{\nu\mu}$?
As a secondary question, how does this relate to the ...
21
votes
3answers
670 views
Why is gravity such a unique force?
My knowledge on this particular field of physics is very sketchy, but I frequently hear of a theoretical "graviton", the quantum of the gravitational field. So I guess most physicists' assumption is ...
0
votes
0answers
35 views
Perturbed stress-energy tensor in a cosmological context?
In the theory of cosmological pertubations, we can write the metric of a null-curvature expanding Universe as :
$ds^2 = -c^2\left(1+2\frac{\psi}{c^2}\right)dt^2 + a^2 ...
2
votes
0answers
64 views
Solving the equation of relativistic motion
How does one solve the tensor differential equation for the relativistic motion of a partilcle of charge $e$ and mass $m$, with 4-momentum $p^a$ and electromagnetic field tensor $F_{ab}$ of a constant ...
1
vote
1answer
249 views
Potential Energy in General Relativity
I often hear about how general relativity is very complicated because of all forms of energy are considered, including gravitation's own gravitational binding energy. I have two questions:
In ...
1
vote
2answers
145 views
Finding the correct units for the energy-momentum tensor?
I'm trying to understand the energy-momentum tensor $T^{\mu\nu}$ but I'm confused about the units. My textbook says the components of $T^{\mu\nu}$ are $\mathrm{Jm^{-3}}$. Four-momentum is is given ...
2
votes
1answer
245 views
Double gyroscope: Can a spinning pencil tumble on only one axis?
Picture an object such as item 7 on this page .. http://en.wikipedia.org/wiki/List_of_moment_of_inertia_tensors
Call that the x axis and z is in to the distance. See diagram below.
We are in deep ...
2
votes
1answer
82 views
Energy-momentum conservation without translation symmetry?
As I checked, the energy-momentum tensor defined as ${T^\mu}_\nu=\frac{\partial {\cal L}}{\partial(\partial_\mu \phi)}\partial_\nu \phi-{\cal L}{\delta^\mu}_\nu$ at the solution $\phi$ of equation of ...
3
votes
0answers
160 views
Symmetrizing the Canonical Energy-Momentum Tensor
The Canonical energy momentum tensor is given by
$$T_{\mu\nu} = \frac{\delta {\cal L}}{\delta (\partial^\mu \phi_s)} \partial_\nu \phi_s - g_{\mu\nu} {\cal L} $$
A priori, there is no reason to ...
3
votes
1answer
75 views
General parameters of the stress energy tensor in local inertial frame
A general 4x4 symmetric tensor has 10 independent components. How many components are we free to prescribe in the local inertial frame?
For example, relativistic dust is $\mbox{diag}(\rho c^2, 0, 0, ...
4
votes
1answer
137 views
Flow of momentum is pressure
In the diagonal terms of the energy-momentum tensor, the flow of $x$-momentum in the $x$-direction is the $x$-pressure. Why the flow of momentum is pressure?
5
votes
1answer
231 views
Geodesic Equation from energy-momentum conservation
I've been reading the excelent review from Eric Poisson found here.
While studying it I stumbled in a proof that I can't make... I can't find a way to go from Eq.(19.3) to the one before Eq.(19.4) ...
5
votes
1answer
124 views
Source term of the Einstein field equation
My copy of Feynman's "Six Not-So-Easy Pieces" has an interesting introduction by Roger Penrose. In that introduction (copyright 1997 according to the copyright page), Penrose complains that Feynman's ...
5
votes
1answer
150 views
Confused about indices of the Ricci tensor
In an intro to GR book the Ricci tensor is given as:
$$R_{\mu\nu}=\partial_{\lambda}\Gamma_{\mu \nu}^{\lambda}-\Gamma_{\lambda \sigma}^{\lambda}\Gamma_{\mu \nu}^{\sigma}-[\partial_{\nu}\Gamma_{\mu ...
6
votes
2answers
211 views
Einstein tensor in Friedmann equations : where is the missing $c^2$?
I would like to demonstrate the several forms of the Friedmann equations WITH the $c^2$ factors. Everything is fine ... apart that I have a missing $c^2$ factor somewhere.
In all the following $\rho$ ...
1
vote
1answer
204 views
Scalar field stress energy tensor
Can anyone explain why $T_{\mu \nu} = \frac{2}{\sqrt{-g}} \frac{\delta \mathcal{L}_M}{\delta g^{\mu \nu}} $, other than justifying it from the einstein field equations?
3
votes
3answers
146 views
Having trouble seeing the similarity between these two energy-momentum tensors
Leonard Suskind gives the following formulation of the energy-momentum tensor in his Stanford lectures on GR (#10, I believe):
$$T_{\mu \nu}=\partial_{\mu}\phi \partial_{\nu}\phi-\frac{1}{2}g_{\mu ...
10
votes
2answers
247 views
The derivation of the Belinfante-Rosenfeld tensor
It seems me that there is a "difference" (at least apparently) in how the Belinfante-Rosenfeld tensor is thought of in section 7.4 of Volume 1 of Weinberg's QFT book and in section 2.5.1 of the ...
0
votes
0answers
45 views
Why is the Maxwell Stress Tensor symmetric?
What is the physical meaning of the Maxwell Stress tensor symmetry?
1
vote
1answer
201 views
Does a negative line tension in a 3-fluid immiscible interface make sense?
This question is inspired by this question/answer pair: Is this formula for the energy of a configuration of 3 fluids physically reasonable?
Consider three immiscible fluids forming contact surfaces, ...
1
vote
1answer
31 views
How does color of galaxies explain their distance?
Why do distant galaxies have different colors than closer ones?
3
votes
3answers
66 views
Commutator with a square root
How to find the commutator $[a, \sqrt{a^\dagger a}]$? Here $a$ is a usual bosonic annihilation operator, and $[a, a^\dagger] = 1$.
The first thing I tried is
$$
[x,A] = [x, \sqrt{A}]\sqrt{A} + ...
1
vote
1answer
22 views
Must string models that describe 4d effective field theories always have D-branes that extend in the 4 non-compact spacetime dimensions?
In string theory the D-branes give those directions that the strings are allowed to move along. The string excitations give the fields that we detect. Is it correct to think of a particle propagating ...
4
votes
3answers
390 views
Is photon emission possible without electrons changing energy levels?
Does molecular vibrational transition and consequent emission of infrared radiation involve electrons changing energy level? In wikipedia, about vibronic transitions it says "Most processes leading to ...
-1
votes
0answers
17 views
What are the *necessary* conditions to deterministic chaos?
What are the necessary conditions (not saying sufficient conditions) in mathematical terms that a deterministic dynamic system can transit to deterministic chaos?
We collected yet:
A positive ...
2
votes
1answer
37 views
Excitation probability for 2 Level Atom
I currently started learning Quantum Optics using the book "Atom Physics" of Christopher Foot. Currently I am working through Chapter 7 and a question occured concerning the intuitively physical ...
