# All Questions

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### Electromagnetic induction inside cylinder

Suppose we have a hollow cylinder whose walls conduct some alternating current. We also have a round wire inside cylinder with same axis as that of cylinder so my question is that can there be ...
70 views

### Conservation of spin angular momentum at the vertex having two fermions and graviton

Consider an electron positron scattering/annihilation. This process can also take place via gravitons (as a propagator) because fermions have mass. But the graviton has spin angular momentum of 2 ...
198 views

### Why is the displacement current term needed in the Maxwell's equations?

Why did Maxwell believe that a displacement current term needed to be added to Ampere's circuital law? I have found loads of answers online about the plates acting as capacitors but i don't ...
107 views

### Why $E=mc^2$ formula does not include time?

For $E=mc^2$ formula, if an object of mass $m$ kg goes with speed of light (in theory), it transforms energy according to $E=mc^2$ but if there is no time complexity this will not happen. So I think ...
47 views

### How do I prove that frequencies that are irrationally related lead to quasi-periodic motion?

Consider the equation: $$\dot{x} = Mx,$$ where M = \begin{pmatrix} i\omega_1 & 0 & \cdots & 0 \\ 0 & i\omega_2 & \cdots & 0 \\ ...
103 views

### Electromagnetic wave and an electron

My question is: what happens if I place an electron in the path of an electromagnetic wave ? (a classical picture) I think the electron will first oscillate with the electric fields of the wave and ...
229 views

### How does a longer wavelength penetrate deeper with Rayleigh waves?

I'm struggling slightly to understand this idea. I've slowly been building up an explanation, so at this stage it might be just some confirmation I'm looking for, but also some guidance if I'm off ...
82 views

### Voices of Animals And Humans

What are the differences between voices of animals and humans? (Not by ear but by a program or similar...frequency, other things) Does each animal in each species have a unique voice? Is there a ...
81 views

### Why is a tensor times its dual a derivative?

Why is a term like $$F \tilde{F}$$ or $$G \tilde{G}$$ considered to be a derivative? Where are the derivatives? This is said in a PRD article. Check section IV page 5.
187 views

### Helium Nucleus as boson

I am rather perplexed with this fact that though Helium Nucleus is a boson, the particles insides it: protons and neutrons are essentially fermions. How the nucleus which is made by fermions can be a ...
89 views

### Can Parallel Universes exist? [closed]

I saw Stephen Hawking say that there are an infinity of parallel universes. How did he get to that theory, and is it possible?
171 views

### Is there a deterministic observable that has only single eigenvalue?

Is there an observable in quantum mechanics which has only one eigenvalue and an eigenspace associated with that single eigenvalue? This observable is deterministic in the sense that it gives same ...
206 views

### How do we know that gravity is a fundamental force, rather than an emergent one?

How do we know that gravity is a fundamental force, rather than an emergent one? Also, what are the smallest masses and length scales for which we have measured it?
34 views

### Why do the gauge bosons fix the couplings of the SM?

In chapter Chapter 16: Grand Unified Theories in the very first paragraph, the author states Note the SM gauge interactions of quarks and leptons are completely fixed by their gauge charges. ...
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### Behavior of maximum entropy states in a closed system?

As I understand it, entropy is a measure of the number of permutations of microstates of a system possible, without changing the observed 'macrostate variables' / measurements or properties of a ...
187 views

### $F=ma$ contest problem on angular momentum [closed]

Problem #25 from the 2009 $F=ma$ contest: Two discs are mounted on thin, lightweight rods oriented through their centers and normal to the discs. These axles are constrained to be vertical at all ...
38 views

### Need verification: Hamiltonian formulation states that an atom is bounded to an eigenenergy state due to opposing kinetic and potential energy

I received an explanation from someone who said that electrons in an atom are trapped in an eigenenergy state $E_n$ as per Hamiltonian mechanics is because the KE and PE of the atom balances ...
176 views

### Optics: how can I see 180 degree relative phase shift between internally and externally reflected light?

Guys this is an example from chapter 4.3 of Hecht's Optics. Top picture shows a light beam moving through a glass medium The second picture shows that when the glass medium is cut into two halves ...
200 views

### Time coordinate inside black hole horizon [duplicate]

I am new to physics and was trying to learn more especially about general relativity. The Schwarzschild metric, changes the sign of the time and radial parts of the metric once we cross the event ...
219 views

### Clearing up confusion about calculating torque

Suppose you have a shape consisting of two perpendicular rods (the whole shape is a rigid body) which stands upright so the plane of the rods is perpendicular to the plane of the ground, and the ...
76 views

### When everything follows strict laws in the universe, where does probability come from? [duplicate]

I am told that we can't predict whether we shall get a head or tail. We can only say that for an unbiased coin there is 50% probability for either. But coin is not case of Quantum Physics! I have ...
48 views

### faraday's rotation for revealing hidden text

I'm trying to come up with a twist on the known faraday's rotation experiment involving a laser projecting into a polarizer. the polarized light then passes in a selenoid containing a glass rod. the ...
172 views

### Expanding universe - Creation of Space [duplicate]

Is the expansion of the space between the galaxies caused by stretching of existing space or the creation of new space? The fact that the energy content remains constant, and is therefore not being ...
213 views

### Is the principle of least action fully equivalent to the Euler-Lagrange equations?

I am citing from Landau and Lifschitz, this statement that will seem to you well-known, trivial, etc: "Between these positions, (i.e. $q_1$ and $q_2$) the system moves then in such a way that the ...
62 views

### Where does this viscosity formula come from?

I was reading a scientific article and i came across this formula: $$\frac{\tau_{xy}}{V_{/x}} = \eta_{0}+\eta_{2}V_{/x}^{2}+\eta_{4}V_{/x}^{4}$$ which refers to the calculation of the viscosity of ...
4k views

### Why does the speed of light totally prevent instantaneous information exchange?

Based on the classical light-cone approach it's easy to see you can't transmit information faster than $c$ but why does the speed of light (as far as I know) treat information transmission in this way ...
163 views

### What would happen if you open a bottle of fizzy drink in a weightless environment?

On Earth, pouring a fizzy drink into a glass or opening a bottle, you see the gas start to condense out into bubbles which rise upwards. You can't pour a Coke into a glass on the ISS but you could (I ...
526 views

### Does coordinate time have physical meaning?

I have always been a little confused by the meaning of the "$t$" which appears in spacetime intervals or metrics in general relativity. I concluded that $t$ was just a mathematical thing which allow ...
189 views

### Which one is inertial frame?

I'd like to ask a question which made me confused. I think that, e.g., a man falling to earth could think himself as stationary and the Earth as accelerating towards him. But I, as an observer, I ...
97 views

### How is optical depth (opacity) related with gas pressure?

In stellar atmospheres, if we are given the gas pressure on the surface level of a star $P_{0}$ and we are expected to calculate the gas pressure on a deeper level (say 10000km) of the star, what kind ...
289 views

### More detail on why pressure increases at the bottom of a column of gas

The question Why does the air pressure at the surface of the earth exactly equal the weight of the entire air column above it asks why the air pressure at any elevation is equal to the weight of the ...
114 views

### Two superimposed sounds traveling through low- and high-density matter: is separation possible?

Two superimposed sounds (at source: $s(t) = s_1(t) + s_2(t)$; the two sound components overlap completely in time, partially in spectra) travel through low- and high-density matter and are recorded ...
697 views

### Defining Reference Directions for Voltage and Power (sign convention)

My professor decided to use the above reference directions when calculating power in circuits. He says that when power > 0, power is consumed. When p < 0, power is generated. This definition is ...
184 views

### Voltage calculation and reference voltage in simple linear circuit [closed]

For the diagram above, the following voltages are specified: $V_A = 4V$ $V_D = 2V$ $V_a = 0V$ Here, $V_{ab}$ indicates the voltage referenced from node b to node a. In other ...
89 views

### What is a stochastic process in a physics context? [closed]

In my mind, a stochastic process is simply a "random" process, one where the outcome is informed by initial conditions but not in a deterministic way. Is this a correct definition? What are some ...
224 views

### Number conservation of bosons and fermions

Why is the number of bosons not conserved while the number of fermions is conserved? Does it have something to do with the Pauli exclusion principle?
825 views

### Does the energy of ground and/or excited states have uncertainty?

In this question about absorption of continuous energies by discrete atom states, one of the reasons given to explain the width of spectral lines is the uncertainty principle (natural broadening): the ...
66 views

### Chemical Potential and Blackbody Radiation

What is the physical significance of the fact that the chemical potential of the photons in a box of volume V is zero ?
217 views

### How to calculate the tree-level probability amplitude for the electron-positron to muon-antimuon process?

Consider the following process: $e^+ + e^- \rightarrow \mu^+ + \mu^-$. I'm trying to calculate the probability amplitude of such a process in leading order. In leading order the amplitude is given ...
While applying linear momentum principle, namely that if force is zero linear momentum of the system is constant, in textbook they don't count for $N$ force from $M \to m.$ This force have component ...