All Questions
0
votes
0answers
5 views
In coordinate-free relativity, how do we define a vector?
Relativity can be developed without coordinates: Laurent 1994 (SR), Winitzski 2007 (GR).
I would normally define a vector by its transformation properties: it's something whose components change ...
0
votes
0answers
3 views
photo of reflection in magnifying mirror shows normal magnification. Why?
I just took a picture of myself in one of those magnifying shaving mirrors and in the photo I am unmagnified! Why is this?
Thanks for any insight!
0
votes
1answer
6 views
Can Earth's Magnetic Field Create Electricity?
If the earth has a magnetic field, can it, in theory, be run through a conductive metal coil to create electricity?
0
votes
2answers
37 views
Can we measure “wavefunction” of quantum particles?
We know that there is uncertainty principle, so question: can we ever measure wavefunction of particles? I do not think this is possible, but I am not sure. I guess that everything is probabilistic. ...
0
votes
0answers
12 views
Some questions about the edge states for time-reversal invariant topological superconductors?
Stimulated by my some recent calculations on edge states(ES) for time-reversal invariant(TRI) topological superconductors(TS) as well as many questions concerning the "edge states" in Physics ...
1
vote
1answer
12 views
what is the magnetic quadrupole operator?
To find magnetic or electrical moments in quantum theory we must calculate the expectation value of an appropriate operator. the dipoles operator are similar and is easy to find but the magnetic ...
-1
votes
0answers
10 views
What is the magnetic quadrupole moment of a nucleus in cylindrical coordinates?
What is the magnetic quadruple moment of a nuclei in cylindrical coordinates?
The quadrupole moment of a nucleus is zero in spherical coordinates but in the cylindrical coordinates it can't be ...
0
votes
0answers
27 views
Object $A$ : Object $B$
Object $A$ has the ability to send information via some wavelength.
Object $B$ houses object $A$ and has the ability to detect when wavelengths are transmitted.
What is object $B$?
2
votes
0answers
22 views
Radio antennas that are much shorter than the wavelength
From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall ...
1
vote
1answer
16 views
Can we build a synthetic event horizon?
If we imagine ourselves to be a civilization capable of manipulating very heavy masses in arbitrary spatial and momentum configurations (because we have access to large amounts of motive force, for ...
0
votes
1answer
19 views
Does $\Omega_2 \ d\Omega_2$ equal zero (trying to find the metric onAnti-de Sitter space)?
So, I'm trying to find the metric on AdS space embedded in a flat 5-D space:
$$ds^2=-dX_0^2-dX_4^2+dX_1^2+dX_2^2+dX_3^2=-dX_0^2-dX_4^2+dX_i^2$$
I found earlier that
$$X_0=l \ cosh\ \frac{\tau}{l} \ ...
1
vote
0answers
23 views
The gravitational random walk
When we shoot a single photon out into space, the chance that it will eventually return to our vicinity from a different direction is vanishingly small, even though spatial curvature exists due to the ...
1
vote
0answers
22 views
When (Exactly) Does Bremsstrahlung Radiation Occur?
Regarding the Bremsstrahlung Radiation emitted when a charged particle curves inside a uniform magnetic field; Is there a way of calculating the angle through which the charged particle will precess ...
0
votes
0answers
22 views
What are some novel physics demonstrations that can be done cheaply? [closed]
I have a physics demonstration assignment for high school, and I was wondering if you had any ideas for demonstrations that could be done on a tight budget?
0
votes
1answer
18 views
The nature of resistance at copper wire terminations
Speaker wire consists of twisted copper strands, around AWG 22 thru 10. As good consumers know, one must consider resistance when wiring speakers. How do the wire terminations contribute to the ...
-1
votes
1answer
50 views
Periodic boundary condition on a Wave Function of a Particle in a Box
Until now solving the Schrodinger Equation for a particle in a box was relatively easy because the boundaries conditions imposed zero value on the wave function at the boundaries. But now I must find ...
0
votes
1answer
46 views
How do we know that time and distance are not discrete?
I know that it is believed that energy is discrete, in that it travels in quanta. I was wondering if there is any evidence which either proves or disproves something similar with both time and ...
1
vote
0answers
32 views
Why do some things break when twisted?
Explain at the atomic level why twisting something, like a thin tree branch or arm, will break from twisting, but something else, such as a bowling ball or cinder block, will not break from twisting.
1
vote
1answer
45 views
Question about the Boltzmann distribution
In the derivation of the Boltzmann distribution they consider a system $A$, enclosed by a diathermal wall in a heat reservoir $R$. Then they calculate the probability that the system $A$ is in an ...
-3
votes
0answers
23 views
Is it possable to create a magnitive field genarator, over the magnitive field [closed]
I had this dream that I was working on this machine. It looked like a donut shaped magnet and it was over the poles of the magnetic field. There was no electric motors to get it started , but the out ...
5
votes
2answers
92 views
Quantum Mechanical Operators in the argument of an exponential
In Quantum Optics and Quantum Mechanics, the time evolution operator
$$U(t,t_i) = \exp\left[\frac{-i}{\hbar}H(t-t_i)\right]$$
is used quite a lot.
Suppose $t_i =0$ for simplicity, and say the ...
3
votes
1answer
55 views
Volume element $\mathrm{d}^4k =\mathrm{d}k^0 \,|\mathbf{k}|^2\,\mathrm{d}|\mathbf{k}| \,\mathrm{d}(\cos\theta) \,\mathrm{d}\phi$ in Minkowski space?
Suppose we have an integral
$$\int \mathrm{d}^4k \,\ f(k)$$
we want to evaluate and that we're in Minkowski space with some metric $(+,-,-,-)$.
Is it true that: $$\mathrm{d}^4k = \mathrm{d}k^0\ ...
0
votes
0answers
21 views
The stress in copper and steel parallel compound members [closed]
Came across this question and just needed abit of help undertanding how i should go about this question
I have also attempted to solve it to the best of my ability but im stuck.
A concrete column of ...
0
votes
1answer
56 views
Doubts about the definition of mass
I'm having some problems understanding what are the possible definitons of mass and how they are related to each other.
In Classical Mechanics, we can distinguish between inertial and gravitational ...
0
votes
0answers
21 views
Maxwell-Boltzmann distribution for transport equations
I have to calculate the transport coefficients for the Maxwell-Boltzmann distribution. But I'm not sure what distribution I have to use.
As far as I know it should not be the MB distribution for ...
1
vote
3answers
66 views
Can a diver swim a short distance in great depths without being physically crushed by the pressure?
I recently saw "The Abyss".
Does it make sense that they do dives in these depths (700m) with soft suits?
Also - what is all the depressurization talk about? Why do divers need to depressurize long ...
2
votes
1answer
136 views
What are units actually?
This question is about the concept of units in physics.
Firstly - do units have a formal mathematical definition? How are they different from pure numbers? Are pure numbers defined to be ratios of ...
4
votes
2answers
68 views
Why distinguish between row and column vectors?
Mathematically, a vector is an element of a vector space. Sometimes, it's just an n-tuple $(a,b,c)$. In physics, one often demands that the tuple has certain transformation properties to be called a ...
1
vote
1answer
33 views
How do determine the equation of motion of slinky?
I'm trying to analyze the forces exerted to slinky. First, I've divided both ends.
The red vector is the force exerted by slinky itself, and the green vector is gravitational force.
Force exerted ...
1
vote
0answers
18 views
Explain the Föppl–von Kármán equations
I am a newbe to elasticity.
Could someone please explain to me briefly how the Föppl–von Kármán equations work?
What are we trying to solve for?
Is there some kind of intuition to the way they look?
...
0
votes
0answers
11 views
Kinetic and Thermodynamic Dopant Solubility
I've read that both the "kinetic" and "thermodynamic" solubility of impurities limit substitutional dopant concentrations. Unfortunately, I haven't found a clear explanation of the two types and the ...
2
votes
1answer
66 views
Physics of every-day life: rotating bag of tea
Whilst studying for my physics courses, I like to drink tea. Today, I noticed that if you pull a bag of tea out of a hot cup of water, it gradually starts to rotate, picking up speed as time ...
1
vote
2answers
45 views
Vector $\vec{z}$ and its conjugate transpose $\overline{\vec{v}^\top}$ - is it the same as $\left|z\right\rangle$ and $\left\langle z \right|$
Lets say we have a complex vector $\vec{z} \!=\!(1\!+\!2i~~2\!+\!3i~~3\!+\!4i)^T$. Its scalar product $\vec{z}^T\!\! \cdot \vec{z}$ with itself will be a complex number, but if we conjugate the ...
4
votes
1answer
46 views
Existence and uniqueness of Stokes flow
What are the solution existence and uniqueness conditions for Stokes' flow?
$$\nabla p = \mu \Delta \vec{u} + \vec{f}$$
$$\nabla \vec{u} = 0$$
Maybe you could also provide some articles or books ...
1
vote
0answers
38 views
de Sitter versus Minkowski QFT and cosmological constant
WMAP/Planck results confirm than we live in a de Sitter-like phase, i.e., a Universe with positive acceleration or positive cosmological constant! Therefore, I believe that a way to solve the ...
0
votes
1answer
29 views
Problem with a rotating frame of reference on the South pole
Consider this problem:
A high-speed train is traveling at a constant 150 m/s (about 300 mph)
on a straight horizontal track across the south pole. Find the angle
between a plumb line suspended ...
1
vote
1answer
27 views
How do particles become entangled?
A person asked me this and I'm just a lowly physical chemist.
I used a classical analogy (how good or bad is this and how to fix?)
Basically, light has a net angular momentum of zero, insofar as ...
2
votes
0answers
38 views
Estimating the Kolmogorov Complexity of the Standard Model
The Kolmogorov complexity of a hypothesis / theory / model is the shortest computer program that simulates it, regardless of how inefficient executing that program may be in terms of memory and time. ...
1
vote
1answer
32 views
Finding Hamilton's equations given a Hamiltonian
I am trying to find Hamilton's equations for a general Hamiltonian given by $$H[u]=\int_\mathbf{R} \phi(u,u_x)dx$$
Suppose $$\frac{\delta f[u]}{\delta u(x)}\equiv \frac{\partial f}{\partial ...
4
votes
2answers
67 views
How can a car's engine move the car?
Newton's First Law of Motion states that an object at rest or uniform motion tends to stay in that state of motion unless an unbalanced, external force acts on it.
Say if I were in a car and I push ...
0
votes
0answers
17 views
Showing that the CHSH inequality is not violated
I can usually work out whether CHSH inequality is violated when the observables that we are measuring and the state we are in is given explicitly, but I'm struggling with the generality of the ...
4
votes
2answers
71 views
Can a current carrying loop or wire produces no magnetic field?
A current carrying wire produces magnetic field around it. We can find the direction by Fleming's Right hand rule. We know change in electric filed produces magnetic field and change on magnetic field ...
4
votes
1answer
84 views
Mass gap for photons
I am puzzled by the answers to the question:
What is a mass gap?
There, Ron Maimon's answer gives a clear-cut definition, which I suppose applies to any quantum field theory with Hamiltonian $H$, ...
12
votes
2answers
124 views
Is every quantum measurement reducible to measurements of position and time?
I am currently studying Path Integrals and was unable to resolve the following problem. In the famous book Quantum Mechanics and Path Integrals, written by Feynman and Hibbs, it says (at the beginning ...
0
votes
0answers
36 views
What does the difference in odds for Bell's inequality tell us about quantum mechanics?
Bell's inequality defines a lower bound for agreement/disagreement between entangled particles. When the experiment is conducted it shows lower odds.
What does this tell us? Is it possible that we ...
5
votes
2answers
108 views
Why isn't data lost when sent over large distances?
I was thinking about how information is sent, for example through the atmosphere. There are plenty of obstacles, as well diffraction, etc.
Still, no information is lost. How is information sent to ...
2
votes
0answers
33 views
Intuition behind the notion of reflection positivity
I came across Yuji's question. I'm finding it difficult to parse the meaning behind what's said on Wikipedia. Could someone give an explanation of the concept involved? I would also appreciate ...
1
vote
0answers
9 views
Does phase lead or lag in a plasma?
Plasmas have an index of refraction less than 1. So say you sent one beam of light through a plasma and another one along a path just as long but in free space. Is the plasma beam's phase lead or lag ...
2
votes
1answer
44 views
Derivation of Weyl tensor
I want to derive the Weyl tensor along the lines of this derivation,
but I am unable to complete it. (I am only interested in $4$ dimension for now.)
Every contraction I perform gives either $0=R + 3 ...
5
votes
1answer
50 views
Do all known planets and moons have magnetic field?
In this Wikipedia article it is stated, that magnetic field of Earth is caused by currents in her core. The same origin is for Jupiter magnetic field.
For Moon (article) there is a magnetic field, ...
