All Questions
234,158
questions
1
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1
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42
views
Are these Transformations of the Green's function equivalent?
The Green's function $G(E)$ can be constructed from the Hamiltonian $H$
$G(E) = [(E+i\epsilon)I - H]^{-1}$
where $I$ is the identity matrix. Say we want to perform a transformation into another basis ...
2
votes
1
answer
374
views
Spacetime as a medium for electromagnetic waves? [closed]
We typically break waves into two separate types mechanical waves and electromagnetic waves and treat them entirely differently based primarily on the observation that electromagnetic waves don't ...
1
vote
1
answer
184
views
Approximation of Stable Orbits as Harmonic Oscillators
A textbook on classical mechanics I am currently reading considers the stable orbit (at $r_0$) of a body subject to the power law:
$$\mathbf{F}(r)=-Kr^n\mathbf{\hat{r}},\quad n\in\mathbb{Z}$$
$$\...
0
votes
1
answer
117
views
Does electric current through gases emit light?
Not electric spark or electric arc, just normal current through gases created by heating the air between cathode and anode as shown in the following video at 1:25.
https://www.youtube.com/watch?v=pg0-...
0
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1
answer
96
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Is it possible for a particle to have all the energy of the Isolated System of particles?
We have read the Fundamental postulate of statistical mechanics which says that :
In a state of thermal equilibrium, All the accessible microstates of the system are equally probable.
Suppose a ...
0
votes
0
answers
494
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Wick's time ordering operator
I am working on Wolfgang Nolting's . And I feel confused about the time ordering operator.Below is how he introduced the Wick's time ordering operator.
My understanding is that the $\epsilon$ in the ...
-2
votes
2
answers
50
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Projection of ball. find initial velocity [closed]
A cannonball is fired at an angle of 33 degree relative to the horizontal as shown in the figure bellow.
It is 1930 meters in the horizontal direction from the point of launch to the point where the ...
3
votes
2
answers
572
views
If $T dS = dq$ only for reversible processes, why can I use it for non-reversible processes?
Assume some system with constant heat capacity $C_V$ at initial temperature $T_0$ is in thermal contact with a reservoir at temperature $T$. As is typically done, the net entropy increase in the ...
1
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1
answer
148
views
Where is the observer in AdS-Schwarzschild coordinates?
for an AdS-Schwarzschild black hole in 4d, the metric is
$$
ds^2 = -f(r)dt^2 + \frac{dr^2}{f(r)} + r^2d\Omega^2
$$
where $f(r) = 1 + r^2/l^2 - C/r$. $l$ is the AdS length scale and $C$ is some ...
1
vote
3
answers
113
views
Postulate of a priori probability and harmonic oscillator
According to the fundamental postulate of a priori probability in Statistical Mechanics:
An isolated system in equilibrium is equally likely to be in any of its accessible states.
But for a ...
-4
votes
1
answer
85
views
Gravity between antimater and matter [duplicate]
Always considered antimater as negative mass so:
$$m1=10kg(matter)$$
$$m2=-10kg(antimater)$$
$$displacement=r=10m$$
$$gravity =\frac{Gm1m2}{r^2}=-1N<0!HOW?$$
1
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1
answer
560
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With air resistance, why does the horizontal range decrease and why is it steeper during the descent phase?
IN RELATION TO PROJECTILE MOTION
I've been trying to grasp this concept but cannot figure it out.
From what i understand, the x-component only has air resistance acting on it so it slows it down ...
7
votes
5
answers
2k
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Is information conserved in quantum mechanics (after wave function collapse)?
I have heard in popular science that there is a law of "conservation of information." Some times this is described as: for any event that happens, there is enough information to reconstruct ...
0
votes
1
answer
410
views
Why ferroelectric material needs to be insulating?
Is it necessary for a ferroelectric material to be insulating?
Is it possible for a metal to be ferroelectric?
3
votes
3
answers
768
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How do we know neutrons and electrons are distinct particles on the same scale as protons?
I'm aware my question may not even make sense when considering quantum physics, but please excuse my ignorance. We are taught in school that there are basically 3 particles that make up atoms (protons,...
1
vote
0
answers
25
views
Is there one photon per wavelength of light? [duplicate]
This makes intuitive sense to me but I have never heard anyone confirm it. Is it one photon per wave length or does it approximate one wavelength or something else?
0
votes
2
answers
713
views
Confusion regarding calculation of Miller indices for planes
Let's calculate the Miller indices of the yellow plane.
The intercepts are $(2,2,1)$ . Taking the reciprocals we get $1/2,1/2,1$ .
Clearing the fractions I get the indices as $(1,1,2)$. Why then does ...
16
votes
3
answers
4k
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"To come back to Earth...it can be five times the force of gravity" - video editor's mistake?
I've watched The Truth About Gravity With Professor Jim Al-Khalili | Gravity And Me | Spark where astronaut Chris Hadfield says at 3:55:
To come back to Earth is violent
Then after several seconds ...
1
vote
0
answers
166
views
Non-markovian random walk. Elephant random walk
When one does the numerics for the usual random walk, one might use the transition matrix in order to get the probability ar time $t$ of the process. As expected, the asymptotic behavior yields a ...
1
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1
answer
346
views
Why is there no friction at the top and bottom of loops? [duplicate]
A beginner's question: Why does friction vanish at the top and bottom of loops? If friction works in the opposite direction of motion, then surely there must be some element of friction that is in the ...
3
votes
0
answers
34
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Hydrogen gas tube and the spectrum [duplicate]
Generally to detect the hydrogen spectrum people uses the hydrogen gas tube as a light source.
My question is, since the gas in the tube is the hydrogen molecule $H_2$ why is the spectrum equal to the ...
-2
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1
answer
80
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Is it feasible to measure one-way speed of light this way? [closed]
I know that this subject is pretty saturated even thought i waited a few days.Its a generalized theoretical expiremental set up to try and measure one way speed of light,i just want any input on this ...
2
votes
1
answer
69
views
Can we improve transmission of light through water?
Is there any way to add chemicals/elements to water to increase the penetration of light (em waves)?
0
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0
answers
302
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What is the internal energy of water vapor? OR How many degrees of freedom does $\rm H_2O$ have?
Here are the "knowns" that I'm working with.
Internal Energy for a monatomic gas is
$$U = (3/2)nRT$$
For a diatomic gas its
$$U = (5/2)nRT$$
All of my textbooks and online sources indicate ...
2
votes
3
answers
148
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Measuring relative speeds in SR
Suppose I have a galactic ruler spanned from earth to Alpha Centauri. With marked units, so one can read off the distance starting with 0 at earth until 4 light-years at Alpha Centauri.
Now according ...
-1
votes
1
answer
73
views
Trying to solve second equation of motion without substituting $v=(at+v_i)$ [closed]
The derivation of $s = s_0 + v_0t + ½at^2$ starts with $v = dx/dt$, which is rewritten as $v dt = dx$ and then $v$ is replaced with $at+v_i$. Then it is integrated.
Why is this substitution done, ...
0
votes
1
answer
63
views
What is the probing depth of the XAS spectroscopy at Iron edge?
I want to find the probing depth of the XAS spectroscopy at Iron edge, I found this paper: Magnetic circular dichroism study of Fe/Co/Cu (001) using electron yieldx-ray absorption spectroscopy with ...
1
vote
3
answers
99
views
Why does the light beam move with the lightclock?
A flat platform is moving in a tunnel with velocity V relative to the tunnel. A stationary reference frame S1 is attached to the tunnel and a reference frame S2 is attached to the moving platform.
On ...
2
votes
1
answer
122
views
Entropy and time asymmetry
I have learned in Lagrangian Mechanics that the existence of time symmetries makes energy conserved. But from thermodynamics, we know that entropy introduces the forward arrow of time, i.e., the time ...
2
votes
1
answer
693
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Connection between particle physics and weight diagrams
I have a hard time combining two topics that are often discussed in physics in a coherent way.
In a lot of Introduction to particle physics-classes one will hear about "multiplets", which ...
1
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0
answers
107
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Is the "preferred basis problem" about why things entangle in specific ways?
I know there are various definitions of the "preferred basis problem." I'm trying to understand what Wikipedia is saying here.
... a quantum state can equally well be described (e.g.) as ...
1
vote
0
answers
33
views
Transition between vibronic states and the meaning of transition dipole moment
Let's say we have a molecule which belongs to $D_{3h}$ point group. The IRREPs of inital and final states are as follows $A'_{1}$, $E'$. Hence, the IRREP for the dipole moment has to be $E'$ so that ...
0
votes
2
answers
71
views
Energy of photon from superposition to ground state
Let an atom is in superposition of two eigen energy states E1 and E2. If it returns to the ground level E0 what energy should have the photon released in the process?
0
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1
answer
130
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Multiple-Slit Interference Clarification
Source: https://openstax.org/books/university-physics-volume-3/pages/3-3-multiple-slit-interference
$dsin(\theta) = m \lambda$ (1)
where
$d \mbox{= spacing between slits}$
$\theta \mbox{= ...
0
votes
1
answer
209
views
Hamiltonian on qubit: necessarily induces rotation. What about the reflections? Are they impossible to implement?
Any Hamiltonian dynamic on a qubit is necessarily a rotation.
I consider a two level system having a Hamiltonian $H$. Decomposing this Hamiltonian on the Pauli basis and using the fact it is Hermitian,...
0
votes
1
answer
856
views
Scalar field energy density
Considering a classical scalar field theory, I can find the canonical energy momentum tensor and if I calculate the $00$ component I get:
$$T^{00}= \frac{1}{2} \dot \phi^2 + \frac{1}{2} (\partial_i) \...
3
votes
1
answer
66
views
Is the every translationally invariant operator constructed from powers of the Laplacian operator?
Consider for simplicty 1d discrete chain - $x_1 \ldots x_n$, and let the field $\phi(x_i)$ reside on the sites of this chain.
Operators $\mathcal{O}$ in this this theory are hermitian $n \times n$ ...
2
votes
1
answer
291
views
Is a quasiparticle just an eigenstate of the Hamiltonian?
The description of quasiparticles seems to come in two flavors: Completely qualitatively, where it is simply said that different (quasi-)particles interact to "form" a quasiparticle, or ...
0
votes
1
answer
280
views
Lorentz transformations and rest frame
So a Lorentz transformation leaves the internal product invariant in the Minkowski spacetime. Let's say I do a Lorentz transformation, either a boost or a spatial rotation and I choose the reference ...
1
vote
1
answer
135
views
Invariant of supersymmetry?
Given two vectors in 3D superspace $(x_1^\mu,\theta_1^\alpha,\overline{\theta}_1^\alpha)$ and $(x_2^\mu,\theta_2^\alpha,\overline{\theta}_2^\alpha)$ I am trying to find a polynomial invariant under ...
1
vote
1
answer
85
views
Heterodyne interferometry, an alternative?
I am trying to setup an interferometer to monitor displacements and tilts between two surfaces. I am quite struggling because I never studied optics deeply.
I have found in this paper (https://...
2
votes
2
answers
1k
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How much can water be compressed?
How much can liquid water be compressed?
I mean the maximum achieved in practice and the maximum achievable in theory.
Say for example one liter of water at 4 degrees Celsius.
Can it be compressed to ...
0
votes
2
answers
552
views
How to compare several experimental measurements and their uncertainties?
I have a basic question about how to make the comparison of different measurements. For example, suppose I have measured the focal length of a convergent lens using three different methods, $A$ and $B$...
0
votes
0
answers
46
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Is temperature a Lorentz scalar [duplicate]
If I see a body at temperature $T$, will I see the same temperature in another frame under a Lorentz boost. And will the internal energy of a body also remain invariant under a Lorentz boost or not..
0
votes
2
answers
169
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Why is the frame attached on the Earth non-inertial in General Relativity? ( neglecting rotations etc..)
The EEP says that freely falling frames are inertial. But suppose the freely falling frame is at some spacetime point. At that point you can by a suitable coordinate transformation reduce the metric ...
0
votes
3
answers
528
views
Why is the pressure in quantum infinite wells the derivative of total energy, not potential?
Consider some quantum well of infinite depth, that is, for some region $R$,
$$
V(r)=
\begin{cases}
0, \quad r \in R \\
\infty, \quad r \notin R
\end{cases}
$$
My professor said, without much ...
0
votes
1
answer
263
views
Why is an RG fixed point scale invariant?
I cannot understand why people say RG fixed point is scale invariant.
Scale invariant means the action $S[\phi]$ of the theory is invariant under scale transformation like $\phi(x)\to\lambda^{-\Delta}...
0
votes
1
answer
74
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How to transform coordinates from heliocentric to a view from earth?
I'm trying to find and model the apparent motion of the retrograde motion of Mars as viewed on Earth, incorporating Lagrangian or Hamiltonian mechanics.
For the lagrangian I have:
$L = \frac{1}{2}m_{...
0
votes
1
answer
318
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Does discharging of capacitor create sound?
Today I found a wierd thing that as I removed the pin of the echo dot
And after this I removed the adapter from the power supply and found a wierd buzz sound coming from the adapter but as I ...
0
votes
2
answers
66
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Waves and guitar frets
I have a problem to solve for Physics III at university and I can't seem to understand how to solve this question.
I have the fundamental frequency at $440$ Hz, $L=1$ m, and $ρ=0.002$ kg/m.
After ...