10
votes
1answer
79 views

Probability Conservation in WKB Tunneling

Suppose we have quantum mechanical plane waves of energy $E$ incident upon a one-dimensional potential barrier $V(x)$ with sloping sides. One can compare the WKB solutions in the three relevant ...
2
votes
4answers
1k views

Could submarine SONAR kill a diver?

Could a diver swimming next to a submarine be killed or seriously injured by its SONAR? What physical aspect of SONAR affects the human body in a potential harmful way?
0
votes
4answers
63 views

Does measurement change the evolution of wave function (quantum mechanics)?

Basically any measurement is on wave function $|\psi\rangle$ is done by operator $X$ such that $X|\psi\rangle$ results observable $x$ with some probability. But what happens to $|\psi\rangle$? Does ...
2
votes
3answers
899 views

Which way does a black hole spin?

As far as I understand, and from what I have been shown in renderings of black holes, they spin (like water going down a drain). My question is, firstly, does the matter being pulled into a black ...
0
votes
0answers
9 views

Traveling wave on a transmission line encountering a mass

I would like to know the solution to this problem (or at least some very helpful tips): On a transmission line with a constant line density $\rho$ we have a traveling wave with wavelength lambda. On a ...
0
votes
0answers
6 views

Need some intuition behind spontaneous dimensional reduction

In quantum gravity there is the notion of spontaneous dimensional reduction. Namely, at small scales, the dimension of spacetime is less than the ostensible four. In the causal dynamical ...
0
votes
3answers
42 views

Path of light as it travels between two black holes

What would happen to light passing through a narrow space between the event horizons of two equal-mass black holes? Would it deviate or follow a straight path?
5
votes
1answer
48 views

Are the electrons at the centre of the Sun degenerate or not?

Trying to find an answer to this question, I came across two different methods of determining whether electrons at the center of the sun are degenerate or not. The first method, used here, calculates ...
0
votes
0answers
8 views

In discrete models of spacetime, what are some implications of the Einstein equation

We have several models of discrete spacetime. Sorkin has a causal growth dynamics, there's spin foams, Panangaden showed a correspondence between interval domains and spacetimes. I am looking for ...
0
votes
2answers
66 views

Proof of conservation of energy with the relativistic definition of force

Professor Susskind proved conservation of energy in one of his lectures by taking the classical definition of force ($F = ma$) and by showing that its time derivative is zero. How can we do that with ...
0
votes
1answer
26 views

Help recognizing partial differential equation

I would be very grateful if someone could tell me something about the following partial differential equation: $$ \frac{\partial U}{\partial t} = K * (\frac{\partial^2 U}{\partial r^2} + ...
1
vote
1answer
104 views

Constant magnetic field attenuation by µ-metal (mu-metal)

I am interested in the magnetic field attenuation by µ-metal. Specifically shielding from earth's magnetic field $\boldsymbol{H}_E$ through closed cylindrical layers of such a metal, yielding an ...
1
vote
0answers
16 views

On the Lorentz Group representation

I am going through the notes on QFT by Srednicki (which is certainly a worth reading on the subject, and can be found online, see http://web.physics.ucsb.edu/~mark/qft.html). When describing ...
0
votes
0answers
17 views

Where do negative powers of $f_\pi$ in the hadronic amplitudes come from?

According to Peskin and Scrhroeder the pion decay constant $f_\pi$ is defined via the following matrix element $$\left\langle0|j^{\mu5a}(x)|\pi^b(q)\right\rangle=-if_\pi \delta^{ab} q^\mu e^{-iqx}$$ ...
35
votes
3answers
6k views

Is the Planck length the smallest length that exists in the universe or is it the smallest length that can be observed?

I have heard both that Planck length is the smallest length that there is in the universe (whatever this means) and that it is the smallest thing that can be observed because if we wanted to observe ...
0
votes
0answers
5 views

2D aerofoil fluid analysis - find velocity ratio

I just wondered if you could help me out on this fluid mechanics question? I have worked through it myself but am very unsure of my answer (not yet found the pressure difference) the question is ...
5
votes
4answers
218 views

Is gravitational Chern-Simons action “topological” or not?

Here are the 2+1D gravitational Chern-Simons action of the connection $\Gamma$ or spin-connection: $$ S=\int\Gamma\wedge\mathrm{d}\Gamma + \frac{2}{3}\Gamma\wedge\Gamma\wedge\Gamma \tag{a} $$ $$ ...
0
votes
0answers
21 views

Coherent state and commutator [on hold]

I got confused when I calculated expectation values of a coherent state $\mid z>$ and would appreciate some help. First: $< z\mid a^{-} a^{+}\mid z> = e^{-\mid z\mid^{2}}\sum_{n}\sum ...
5
votes
0answers
99 views

Why the curvature is related to the Lie bracket of the solutions to the flow equation in this problem?

I've been studying locomotion at low Reynolds number for some time now and it has been a quite tough problem. I've already asked two questions about the problem here, and now there is this question ...
1
vote
1answer
194 views

Does Lyapunov exponent equate to exponential inflation?

Physics can be modeled by dynamical systems $f^t(x)$ as well as by PDEs. The most common dynamical system has hyperbolic fixed point and can be an attractor or a repellor. The dynamics at repellors ...
0
votes
0answers
11 views

Alternative to Naphthalene Sublimation

I work in a wind tunnel that uses Naphthalene for flow visualization. Specifically we use it to see the location of transition from laminar to turbulent flow. We mix it with acetone and spray it on ...
0
votes
0answers
28 views

Is there a physical interpretation of the alternating property?

A map from a vector-space to its base field is called "alternating" if each vector with repeated elements is mapped to zero. I've read that symplectic geometry is an important representation of ...
1
vote
1answer
22 views

Are coefficients of friction invariant by switching the materials?

I don't have the ability to make the experiment to answer these questions : Let $A$ and $B$ be two materials. Is the static coefficient of friction of $A$ on $B$ equal to the static coefficient of ...
0
votes
0answers
25 views

entaglement experiments in laboratories for atom and photons-how to? [on hold]

My questions come splited like this: 1.How to entangle a spin and a photon in an experiment? 2.How to test for entanglement in the lab?3.There is entanglement for an atom also? 4. What does it mean ...
16
votes
6answers
3k views

Does a person inside a falling bus fall to the front of it?

So I was watching Final Destination 5 and something caught my attention. There's a part where a bridge collapses and everything falls apart, so there's this bus that has a person inside (unaware of ...
-1
votes
0answers
17 views

Particle on a hemisphere - lagrange [duplicate]

A particle of mass m is on top of a frictionless hemisphere centered at the origin with radius $R$. It starts sliding down the hemisphere. Set up the lagrange equatinos of the first kind and ...
0
votes
0answers
46 views

Particle on a sphere - lagrange [on hold]

A particle of mass m is on top of a sphere of radius $R$. A small displacement makes the particle slide frictionlessly down the sphere. a)Set up the lagrange equations of the first kind for ...
6
votes
1answer
43 views

Random walk recurrence term and the self-energy

Consider the "first passage problem" A random walk proceeds on a graph of connected points. On this graph, there is one "end" point $j$ meaning that if the random walker lands on this point the ...
1
vote
1answer
176 views

Doppler shift for a uniformly accelerating observer

This was given in textbook as an example. An observer on a spaceship with a four velocity $u$ is approaching from $x = +\infty$ a star at rest in the reference frame $S$ while undergoing constant ...
-2
votes
0answers
24 views

What happens to sound at speeds greater than mach 1?

So what we know about the Doppler effect is that if the origin of a sound is moving, the sound waves will be closer together in-front of the origin, and farther apart behind it. This creates a ...
0
votes
0answers
18 views

How do we measure thermal energy?

My textbook says "The amount of thermal energy is found by measuring the temperature of the substance." However, how is this true? Temperature measures only the average kinetic energy of the particles ...
3
votes
2answers
821 views

Can a mouse rotating a wheel violate conservation of angular momentum?

I have been thinking on the following problem. Part I. Imagine we have a mouse enclosed in a rigid box. We placed the box on a frictionless surface (ice might be a good example). Can the box start ...
0
votes
0answers
13 views

Initial conditions for second order ODE with complex stiffness

I tried this on Math Stack Exchange. I'm trying to find initial conditions to ensure systems of the form stay bounded $$\ddot{x}_i+\sum_{j=1}^N k_{ij} x_j = 0, \quad k_{ij} \in \mathbb{C}.$$ For ...
1
vote
0answers
38 views

(Super)Gauge Fixing in Supersymmetry

I have three questions about gauge fixing in supersymmetry, one is general and the other two explicit: Why gauge fixing seems not important in supersymmetry? By "not important" I mean gauge fixing ...
1
vote
2answers
677 views

Can someone explain LO-TO Splitting?

LO-TO splitting occurs in an ionic (i.e. polar) solid such as GaAs or NaCl. What happens is that the degeneracy of the transverse optical (TO) and longitudinal optical (LO) phonons at $k=0$ is broken ...
0
votes
1answer
26 views

Reversing time for a closed system of particles

For a closed system of particles, the lagrangian in classical mechanics is $$L=\sum \frac{1}{2}mv_a^2 - U(\mathbf{r_1},\mathbf{r_2}, \cdots)$$ For an arbitrary position function $x(t)$, to see the ...
0
votes
0answers
14 views

heavy quark pair spin states

Apparently, heavy quark pair can have "axial-vector" spin state, "vector" spin state, and two different "tensor" spin states. Can anyone explain to me what they are and why this is the case? Thank ...
1
vote
1answer
163 views

Force of a solenoid on a ferromagnetic material

As I was trying to find an answer to another problem, I was informed that the equation I was using would not work. The equation I was using was $$F = (NI)^2\mu_0\frac{\text{Area}}{2g^2}$$ If this ...
0
votes
2answers
2k views

Calculating the coefficient of thermal expansion in liquid

I am trying to write a matlab function that calculates the coefficient of thermal expansion of water from a given temperature. From what I understand the thermal expansion coefficient is calculated as ...
2
votes
1answer
118 views

What really generates time evolution?

A fundamental principle of quantum mechanics, as far as I can tell, states that the Hamiltonian generates time evolution. A common result about generators are the following: let $\mathrm T$ be the ...
3
votes
2answers
87 views

Can anyone explain me how time can bend according to Einstein in simple way? [duplicate]

I am just 16 and curious to learn about Theory of Relativity. Can any one explain it simple enough for me to understand? I read that it is bending of time-space or space-time that causes gravity. How ...
0
votes
0answers
28 views

I want to test a tunable optical filter that can quickly change from being a red light filter to being a blue light filter

I want to test a tunable optical filter that can quickly change from being a red light filter to being a blue light filter. For this purpose, I am wondering about photonic crystal tunable optical ...
1
vote
1answer
38 views

Calculating euler number of disk

I'm trying to do exercise 3.1 from Polchinski, which should be a rather easy differential geometry problem. I have to calculate the euler number defined by $$\chi = \frac{1}{4\pi}\int_{M}d^{2}\sigma ...
0
votes
0answers
43 views

't Hooft–Polyakov and Dirac monopoles, instantons and solitons

What is the difference between a 't Hooft–Polyakov and a Dirac monopole? (I read in the wikipedia article on the 't Hooft–Polyakov monopole: "The 't Hooft–Polyakov monopole is a topological soliton ...
0
votes
0answers
27 views

Moment of inertia of a connecting rod in a crank-slider mechanism [on hold]

PROBLEM I am trying to determine the moment of inertia of a connecting rod (conrod) in a crank-slider mechanism. $r$ is the crank radius. $l$ is the length of the conrod. $I_{rod}$ is the moment of ...
1
vote
1answer
39 views

Cosmological constants

I've heard that the cosmological constant is 0.[123 more zeros] and then a 1. Does that means that it used to be exactly zero? Is the value of this constant changing or is it fixed at a particular ...
-1
votes
0answers
8 views

How to calculate the inertia tensors of an irregular polygonal right prism?

I am attempting to program a physics simulation and need to do calculate the inertia tensors of a mesh constructed of a series of irregular polygonal prisms. Now, I am aware that you can calculate ...
5
votes
2answers
2k views

Why are scattering matrices unitary?

In Griffith's QM book, he introduces scattering matrices as an end-of-the-chapter problem. For a Dirac-Delta potential $V(x) = \alpha \delta (x - x_0)$, I've derived the scattering matrix and ...
-1
votes
0answers
22 views

Rotating cone magnetic field

how to calculate the magnetic field of a rotating cone with a superficial charge distribution that varies with the altitude? It knows : height of the cone , truncated cone rays bases , there are ...

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