0
votes
2answers
95 views

Helium Balloon Experiment

We tied a helium filled balloon to a counterweight and it floats aimlessly in the room. I asked if the balloon were to be outside the building (all things considered no wind same temperature) would ...
4
votes
3answers
222 views

Using 2D position, velocity, and mass to determine the parametric position equations for an orbiting body

I have a gravity-related question. I am programming an orbit simulator. I have everything up and running, but I would like to render the smaller body's orbital path (the larger body is fixed). To do ...
0
votes
0answers
137 views

Has the Reissner-Nordstrom metric ever been experimentally verified?

In contrast to the solution of the conventional Reissner-Nordstrom problem, where the Schwarzschild metric takes on an additional $1/r^2$ term due to the added electric charge, P. Mannheim has in ...
2
votes
2answers
118 views

What might be a satisfying criterion to qualify something as having physical existence?

I'm a philosophy undergraduate student who has an interest in, and respect for (as most people do) what physicists know about the world. In philosophy, the traditional criterion of physical ...
4
votes
4answers
536 views

Sign of Work and potential energy in electrostatics

Conceptual question: Suppose we have a configuration of point charges. If the potential of the energy of the system is negative, this means work is positive. I'm kind of rusty with my mechanics, ...
3
votes
1answer
113 views

Quick-and-dirty way to integrate out heavy fields

I understand the roughly understand the process of integrating out heavy degrees of freedom of a Lagrangian, namely, taking the action and performing the path integral over the high momentum modes. ...
0
votes
0answers
32 views

P-type MOSFET performance

I was reading somewhere that the (110) plane has the highest atomic density and is best for p-channel MOSFET performance. How high atomic density influence the performance of P-channel crystal?
6
votes
1answer
138 views

Mass corrections to fermions proportional to the mass?

In this post regarding quantum corrections to a massless fermion field, the answerer stated that quantum corrections to the mass will always be proportional to the mass (at least in QED). This point ...
0
votes
1answer
395 views

Limits of Poisson's ratio in isotropic solid

For an isotropic solid, Poisson's ratio can be expressed in terms of stiffness constants as: $$\sigma = \frac{c_{11} - 2c_{44}}{2c_{11} - 2c_{44}}$$ Alternatively we may express Poisson's ratio in ...
1
vote
1answer
63 views

Electric potential and movement of charges

If electric field is not present does that mean that charges achieve condition of electrostatics I.e, charges are stationary ? But we say that stationary charge can produce electric field. I read ...
1
vote
2answers
199 views

Optics Brewster's Angle Reflection Intensity

An incident unpolarised light beam of intensity $I_{0}$ strikes glass plate B at Brewster's Angle. The reflected light travels vertically and strikes a second glass plate A, again at Brewster's Angle. ...
3
votes
2answers
119 views

Showing $K_\pm$ are raising/lowering operators

In this post, I have the following operators defined: $$K_1=\frac 14(p^2-q^2)$$ $$K_2=\frac 14 (pq+qp)$$ $$J_3 = \frac 14 (p^2+q^2)$$ I am given $ J_3|m\rangle = m|m\rangle$ and asked to show that ...
1
vote
0answers
81 views

Moment of Inertia in SR/GR & Calculating it in General

In classical mechanics you want to calculate the moment of inertia for hollow & solid: lines, triangles, squares/rectangles, polygons, planes, pyramids, cubes/parallelepiped's, circles, ellipses, ...
0
votes
0answers
119 views

Why do electric generators need a prime mover?

Commonly electric generators need a mechanical torque provided by a prime mover [gas turbine or steam turbine or diesel engine,etc] which is needed for the rotation of the generator rotor. I would ...
3
votes
2answers
825 views

Capacitors: why is the energy not stored in a magnetic field?

When a capacitor is charging, the rate of change $dE/dt$ of the electric field between the plates is non-zero, and from the Maxwell-Ampère equation this causes a circulating magnetic field. Now, ...
0
votes
0answers
40 views

Proof of uniqueness for holographic assumption

Is it proved that the over-counting of the degrees of freedom in a field theory of gravity has as only solution the "axiom" of holography? While I understand the behaviour of the area vs. volume ...
1
vote
2answers
251 views

Double slit experiment and representation of light waves

Consider the following image from Wikipedia and based on it I have a doubt. I do not understand why are the light waves represented like the waves in water. Shouldn't the waves be like sine waves. ...
7
votes
1answer
465 views

Boosts are non-unitary!

The boost transformations are not unitary unlike rotations, the boost generators are not Hermitian. When this induces transformations in the Hilbert space, will those transformation be unitary? I ...
1
vote
1answer
99 views

Generator of Velocity Transformations - Galilean Transformations

Under a Galilean transformation, the coordinates and momenta of any system transform as: $$ t \rightarrow t',\\ \vec r\:' = \vec r + \vec vt,\\ \vec p\:' = \vec p + m\vec v $$ where $\vec v$ is ...
3
votes
1answer
301 views

Scalar field transformation and generators

When we do a transformation (norm preserving one) for a given quantity, from what I have understood it seems like there is a representation of the group element for each quantity depending how they ...
4
votes
1answer
191 views

Adjoint representation of the Lorentz group

Is it possible to construct an adjoint representation for the Lorentz group?
2
votes
1answer
90 views

Angular momenta of photon

$A^\mu$ can have multipole expansions in classical electrodynamics. This gives rise to dipole photon, quadrupole photon etc. For dipole photon $j=1$ (In electrodynamics books they write it as $l=1$). ...
0
votes
0answers
42 views

Scalar potential in em field task

(Sorry for my English) Task. There is a volume with some arbitrary current or voltage source connected to wires. One wire is buried in the ground. I know values of electric and magnetic fields in ...
-1
votes
1answer
54 views

branes-collision -> big-bang -> 2nd Universe?

If the cosmological model of the Ekpyrotic universe is correct should there not always a 2nd universe be created on the 2nd brane? Pictures and articles I found speak only of one universe. Why is ...
2
votes
1answer
129 views

Seesaws and Dark Energy

Lawrence Krauss and James Dent recently proposed a mechanism for producing the observed scale of dark energy. This proposal was inspired by the see-saw mechanism that produces light yet non-zero ...
0
votes
0answers
58 views

When do wave function collapse in this case?

In relativity, an event A can occur before another event B in one frame while A may occur after event B in another. In quantum mechanics, we may measure the spin of two entangled electrons: If you ...
2
votes
1answer
136 views

Has QFT successfully mediated between QM and Special Relativity?

I understand that QFT is the theoretical framework for combining QM and Special Relativity, but as I understand it, though even without proof or experimental confirmations; has QFT managed to "behind ...
0
votes
1answer
142 views

Original formula of Einstein time dilation

The formula of time dilation at constant speed v is known. The formula $T_0'= T /\gamma$ where gamma is the Lorentz factor. I would be interested, as Einstein's original formula was. I've checked ...
5
votes
1answer
216 views

Orbital angular momentum of photon

People talk about orbital angular momentum (OAM) of photons. Is there some physical example that cannot be explained without assuming that photons have non-zero OAM? Does different photons have ...
-2
votes
3answers
89 views

Are these assumptions correct?

Is it true:From equation $C=Q/V$ 1)If Potential difference is kept constant $C \propto Q$ 2)If charge is kept constant $C \propto 1/V$ Then if 2nd is true I do have a question: Don't we say that ...
3
votes
2answers
104 views

Cause of Magnetic field

According to maxwell-ampere equation, the curl of magnetic field at that point equals the current density at that point + change of electric field at that point w.r.t. time. Now, the second factor ...
2
votes
3answers
79 views

Diagonalization Of $(\sigma_x+\sigma_y)$

Can this matrix $(\sigma_x\pm\sigma_y)$ be diagonalised? Clearly, if $\sigma_x$ is diagonalized by a similarity transformation $S_1\sigma_x{S_1}^{-1}$, then $\sigma_y$ can't be diagonalized by $S_1$, ...
3
votes
1answer
70 views

Eigenvalues of Infinite Dimensional Matrix [duplicate]

If I take a infinite-dimensional square matrix, what can I say about its eigenvalue spectrum? Will they have a discrete infinity of eigenvalues or continuous infinity of them?
4
votes
4answers
211 views

If you suddenly move a piece of metal, will that disturb the free electron density?

If we have a hollow pipe sitting at rest filled with gas and we moved the pipe suddenly along its length to the right, then the gas density will be momentarily higher near the rear of the pipe and ...
2
votes
2answers
185 views

A conceptual question about Green's function's treatment of interaction

Here we have electron gas and some other stuff. We expand the Hamiltonian to the 1st order of one single harmonic oscillator's displacement $\vec{u}$. Its equilibrium position is at the origin. Then ...
0
votes
0answers
104 views

How do I measure the total weight of a container with scales on four corners?

I have a very large movable trash container, and I have mounted four scales, one on each corner. How do I get the total weight of the entire container? Is it simply a sum of the four weights, or is it ...
4
votes
1answer
83 views

Sunrise/Sunset on a planet with an A-Class or B-Class Star

I'm a sci-fi writer, and I have a question regarding a hypothetical planet that orbits an A-class or B-class Star. Assuming Rayleigh Scattering occurs roughly the same way on this planet as it does ...
0
votes
1answer
61 views

Relativity of simultaneity and cause-effect

Relativity says that two events simultaneous in one reference frame might not be simultaneous in another reference frame. Can we extend this idea a little and say that the order of two events might be ...
3
votes
3answers
98 views

Ferromagnets and magnets tend to align in the center. Why is that?

When you bring a large iron plate and a magnet, the magnet attracts the iron plate and it tends to slide itself to the center. When I place it on the edge, it always aligns at the center why is that?
3
votes
2answers
111 views

The speed of light as it approaches a massive body

No matter how fast you go, you will aways perceive the speed of light as constant. Taking that as a fact, the special relativity theory was formulated. Now, for what I understand about general ...
1
vote
0answers
377 views

Electric potential on a uniformly charged tube

A hollow cylinder, with no top or bottom, of radius $R$ and length $L$ is uniformly charged with density $\sigma>0$. I have to find the point on space where a point charge $q>0$ has to be drawn ...
0
votes
2answers
172 views

How does cold fusion work? [duplicate]

I understand that in regular fusion, you use high levels of heat to accelerate atoms to speeds high enough to overcome nuclear force. What method would be used to achieve cold fusion?
12
votes
1answer
222 views

Why do we assume local conformal transformations are symmetries in 2D CFT

The global conformal group in 2D is $SL(2,\mathbb{C})$. It consists of the fractional linear transforms that map the Riemann sphere into itself bijectively and is finite dimensional. However, when ...
1
vote
2answers
297 views

Gaussian Probability Distribution?

The uncertainty principle states that, $$\sigma _{{x}}\sigma _{{p}}\geq {\frac {\hbar }{2}}.$$ It is mentioned from many sources that the probability distribution of the particle position and ...
3
votes
1answer
219 views

Detail of deriving Berry Curvature From Berry Connection

The Berry curvature of the $n^{\mathrm{th}}$ eigenstate of Hamiltonian $H$ for the vector of external parameters $\vec{R}$ can be derived in part by writing the following two lines: $B^n(\vec{R}) ...
4
votes
2answers
4k views

Maximum theoretical efficiency of internal combustion engine

This Wikipedia article states that: Most steel engines have a thermodynamic limit of 37%. Is it correct? If so then it is not clear where this number comes from. Is it derived somehow from ...
2
votes
1answer
89 views

Bravais lattice with sublattices : why multiple bands?

I have a very naive question : given a tight-binding model (with nearest-neighbor hoping) on a lattice defined by a Bravais lattice with a number of sublattices (for instance the honeycomb lattice is ...
5
votes
0answers
99 views

What coordinate system is used to describe planets positions in the universe?

How are planets positions described in the space and in respect to what? For example is Sun the origo and right now at this moment Earth has [coord_X, coord_Y, coord_Z]? or maybe [lng, lat]? ...
1
vote
2answers
208 views

Gauss's Law and Conductors?

At equilibrium, the electric field at and just at the surface of the conductor must be perpendicular to the surface. If it were not, it would have a component parallel to the conductor's surface ...
3
votes
1answer
121 views

Canonical partition sum for two fermions in harmonic potential

In an old exam, I found the following problem: Two Particles in a potential well We look at a onedimensional harmonic potential well that hold two spinless particles that do not interact with ...

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