# All Questions

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### Variable mass dynamics: Particle and Rigid Body

I'm encountering some issues in the understanding of some basic concepts about the dynamics of variable-mass particles and rigid bodies. For what I found, for example reading On the use and abuse of ...
147 views

### Feynman rule for propagator with derivatives

Suppose you have an interaction term of the form $$\mathcal{L}_{int} = \frac{h g}{3!}\phi^3\partial^2\phi$$ where $h$ and $g$ are both couplings. Now if I draw a diagram of the form given in the ...
858 views

### Does the heat equation violate causality?

I've ran across the idea that, besides simply writing partial differential equations in covariant form, they need to be hyperbolic with all characteristic speeds less than the speed of light. A ...
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### Is there a rigorous, explicitly geometric, general characterization for whether a given clock had been “good”, or not?

In MTW, "Gravitation", there is a brief section (p. 26 and Fig. 1.9) discussing that clocks may be characterized, trial by trial, as having been "good" in the trial under consideration, or having been ...
23 views

### Expansion rate in matter dominated era [duplicate]

Do physicists know what the expansion rate of the Universe was at the beginning of the matter dominated era?
72 views

### Why is the value of the action integral in general relativity the same on all regions that are homologous?

In their famous paper Action integrals and partition functions in quantum gravity, Gibbons and Hawking argue that in order to avoid the singularity of a Schwarzschild black hole you can complexify ...
191 views

### Penrose diagram for spacetime which flows to $AdS_{2}$ at infinity

Consider I have the following 2 dimensional spacetime $(t,z)$: $$ds^2=\frac{4}{z^{2}}\left(1+\frac{1}{z}\right)^{-1}(-dt^{2}+dz^{2}).\tag{1}$$ When $z\rightarrow \infty$ we have ...
94 views

### Scalar field in a Schwarzschild metric

I have found this article recently published in Classical and Quantum Gravity giving the exact solution of a scalar field in the Kerr-Newman metric. These authors also derived Hawking radiation for ...
35 views

### Will the particle reach other end?

I place a container at rest in vaccum and filled it with air (STP conditions). Suppose a moving particle(sphere of dia 2mm) is placed at one end of the container without disturbing anything, will ...
1k views

### Hit a bottle of beer on the top with another causes the first to spit all the gas, why?

So, on the other day me and my colleges were discussing the following phenomena: Pick two open bottles of beer. With the bottom of the first, hit the second on the bottleneck, in the following way: ...
58 views

### Integration of $e^{-it\sqrt{\mathbf{p}^2 + m^2}}$ for QM amplitude

My question might be more about maths than physics, but it originated in a Physics context. Take $\hbar$ = $c$ = 1. I was looking at the amplitude for a free particle to propagate from an initial ...
59 views

### Calculating speed of spheres in an asymmetric Newton's cradle?

If we assume there is an asymmetric Newton's cradle with 3 spheres of different masses and the first one is initialized with some certain speed can we calculate the outcome speed of the all the three ...
42 views

### Charge density and space inversion

J. D. Jackson in his book Classical Electrodynamics on page 249 ff. discusses the behaviour of electromagnetic quantities under space inversion (parity operation) and time reversal. He remarks: ...
112 views

### Why isn't all of the dust in a nebula used in the formation of a star?

I was watching a show on discovery and according to it, in a nebula the dust and gases slowly come together and as the gravity increases and the pressure rises in the core the gases fuse together and ...
314 views

### Dimensional Regularization Integral Formula

In the formula $$\int \frac {d^{4-2\epsilon} \ell} {(2\pi)^{4-2\epsilon}} \frac 1 {(\ell^2-\Delta)^2} = \frac i {(4\pi)^{2-\epsilon}} \Gamma(\epsilon) \left(\frac 1 \Delta\right)^\epsilon,$$ how ...
125 views

### Green's function for a dielectric with a charge [closed]

Suppose there are two infinite planes, one in $z=a$ and the other in $z=b$, with $a<b$. Between the planes, there is a dielectric medium with constant $\epsilon_1$. The differential equation for ...
70 views

### Center of mass of two $\gamma$ rays moving in opposite directions

Suppose there are two $\gamma$ rays with frequencies $\nu_1$ and $\nu_2$ moving in opposite directions according to a reference frame $S$. I want to find the velocity of the center of mass of this ...
211 views

### How torque and friction cause wheel to roll

I apologize if this question has been answered before, but I did not find the explanation that I needed. If a torque is applied to a wheel situated on a frictional surface, what forces cause the ...
573 views

### New subatomic particles

In reference to the findings talked about here http://online.wsj.com/articles/two-new-subatomic-particles-found-using-large-hadron-collider-scientists-say-1416409980 and other similar articles ...
83 views

### Abstract concept of wave propagating on a string

I'm a real beginner in physics with a really basic doubt about waves. Suppose i have a string ( perfect elastic material ) whose left-end i can manipulate ( i can change its heigth ) and whose ...
55 views

### How two prove the second sound velocity is $1/\sqrt{3}$ times than the first sound [closed]

From Landau two hydrodynamics model in superfluid, we have the result $c_1^2=\frac{\partial P}{\partial \rho}|_T$ and $c_2^2=\frac{\rho_s s^2 T}{\rho_n c}$. In the zero temperature limit, how to ...
30 views

### The physics behind capacitive touch sensing

Can someone explain the mechanism of capacitive touch sensing? According to http://www.planetanalog.com/document.asp?doc_id=527410&site=planetanalog&print=yes, it mentions "fringing fields", ...
257 views

### How much power would be needed to make a substitute for natural Earth magnetic field?

Imagine the Earth's magnetic field will take a "maintainance pause" for 1000 years and there is gigant 10 years "doomsday countdown timer" ticking in the sky. The magnetic field will be just normal ...
199 views

### Why should multiple versions of a weak measurement preclude it from being a measurement of intrinsic properties of some system

I've been trying to understand Stephen Parrott's criticisms of weak measurement, outlined most concisely here: http://arxiv.org/abs/0909.0295 One of his major criticisms is that weak measurement is ...
19 views

### Is there an applied force acting on the balloon? [duplicate]

A long string is running through a straw with a blown-up balloon taped onto the straw, and a penny taped onto the balloon. There should be a support force going up, gravity going down, friction force ...
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### Do lower dimensional objects actually exist? [duplicate]

Take for instance a arbitrarily chosen 2-dimensional rectangle, having a length of $x$, a width of $y$ and height of 0. (Or take for instance a 1-dimensional line. Or take for instance a 0-dimensional ...
86 views

### Is there a scientific term for star formation?

It might be my stupidity to think that many laymen terms that most people use to describe some physics phenomena usually have a scientifically accepted term or name? The process of star formation, ...
64 views

### Why do silicon solar cells only produce ~0.6v when the band gap of silicon is ~1.1v?

I've been researching into this and can't quite figure out where that lost voltage is going. When silicon is excited by a photon within its absorption spectrum, it will always have an internal ...
75 views

### Why doesn't the temperature of fluid flowing in a pipe increase if the flow speed is increased ?

I have learnt that the temperature of something is a direct measurement of the kinetic energies of the molecules in it. Going by this argument, if the flow speed of fluid is increased in a pipe, and ...
113 views

### Would the rotation of a space ship in vacuum slow it down

Firstly , a few disclaimers : English is not my native language so I may use the wrong technical terms. Please notify me and I will correct the question. As a CS major , I've only taken one class ...
76 views

### How can one be 'certain' about anything that has an “Uncertainty Principle” at its core? [closed]

The Uncertainty Principle, which says that more than one aspect of a particle cannot be measured simultaneously, illustrates one of several major differences between quantum physics and classical ...
51 views

### What is the movement of the material point? [closed]

The parametric formulas for movement of a material point are: $x(t) = A \sin (kt)$ $y(t) = B(1 - \sin(kt))$ Describe the movement of the MP. That is the entire question and I can't find a ...
2k views

### Why do we calculate energy by integrate the Signal squared?

What's the interesting thing in the square of a signal? I know integrating gave us the sum of the differentiated energies, but why the Energy is the square of the signal?
460 views

### Are virtual particles limited by the speed of light? [duplicate]

I have recently been reading about Quantum Electrodynamics which I found very interesting, but even more confusing. I understand photons mediate the electromagnetic force and interactions between ...
247 views

### Why can we see white objects as white under the sun if the sunlight is yellow?

I get that the sun is producing white light which is scattered threw our atmosphere so that the light of the sun reaching our eyes is yellow. So how come if I look to a piece of white paper under ...
111 views

### Mach's principle in empty universe, centripetal force and violation of Newton's laws of motion [closed]

Questions about Mach's principle and empty universe has been asked many times but nowhere can I find it's cross examination with centripetal force. From my point of view the simple logic of ...
74 views

### Will a person experience micro or zero gravity inside an aeroplane in free fall

Let us say that a person is inside a stationary aeroplane being held by a stationary helicopter in the sky. Now, the cable which supports the aeroplane breaks. Thus the aeroplane is now not bound by ...
41 views

### How water and honey behave with increasing pressure?

Is there any relation between pressure and viscosity of water and honey? I hardly found the topic. Is it related to the Laplace law?
39 views

### Is there an analytical expression for the conductivity of the surface of topological insulators?

I have a question about the conductivity on the surface of Topological Insulators (TI): Is it accurate to model the conductivity by the Drude model (I read a paper that modeled the conductivity with ...
50 views

### Is the orbit in Schwarzshild metric a path with maximal proper time?

In curved spacetime particles follow timelike geodesics, which should have maximal proper time (at least locally). I thought this path usually corresponds to a global maximum, and there are only ...
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### Everything moves at the speed of light? [duplicate]

Whatever happened to that idea? Presumably it came from a concept known as Zitterbewegung. As wiki says, a theoretical rapid motion of elementary particles, in particular electrons, that obey the ...
29 views

### Dielectric constant of gases - constant?

A number of questions related to the topic: Is the dielectric constant actually constant as a function of temperature and frequency (when used in a capacitor)? Is it constant as a function of field ...
74 views

### Algorithm for calculating lunar crescent visibility

Is there a simple algorithm for predicting lunar crescent visibility. Simple enough to do with a calculator at least. Those offered in Meeus' Astronomical Algorithms are a bit too involving to do by ...
76 views

### Black Holes and Light [duplicate]

It is said that photons have zero rest mass so how can gravitational force of a black hole affect light? And if photons do have some effective mass while traveling at speed of light then only can a ...