# All Questions

3k views

### Dirac, Weyl and Majorana Spinors

To get to the point - what's the defining differences between them? Alas, my current understanding of a spinor is limited. All I know is that they are used to describe fermions (?), but I'm not sure ...
152 views

### Speed of light in teflon

I was wondering why some websites are talking about the speed of light in teflon because it seems (tell me if I'm wrong) that it is an opaque material. Or are they talking about the light as an ...
2k views

### What are the implications of the Holographic principle?

What are the implications for the Holographic principle? I understand the basics of the principle, the relationship with black holes and string theory but what this is going to tell us? Does it help ...
198 views

### Why is charge constant in series connections?

Why does each capacitor in a series connection hold the same charge? I understand that voltages and capacitances across capacitor plate pairs in series vary, but why is it a necessity that charge be ...
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### Equation of Motion from Vlasov Equation

From the Vlasov equation $$\frac{\partial f}{\partial t} + \vec{v} \cdot \nabla f + \vec{a} \cdot \frac{\partial f}{\partial \vec{p}} = 0$$ we can obtain the momentum transport equation by ...
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### Size of meteor-made crater [duplicate]

I am reading "What if" piece https://what-if.xkcd.com/20/ and I am very interested in how can one ever estimate the size of a crater? How can you ever know, it must depend on energy of a falling body ...
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### Evidence for standard model description of neutrons

as I don't know too much about experiments I have the following question concerning the evidence about the quark constituents in neutrons. After a quick google investigation I found some sentences ...
104 views

### Fresnel diffraction approximation (parabolic waves)

The Huygens-Fresnel principle (Introduction to Fourier Optics, Goodman), $$U(x,y)=\frac{z}{i\lambda}\int_\Sigma U(\xi,\eta)\frac{e^{ikr}}{r}d\xi d\eta\,,$$ where $\cos \theta=\frac{z}{r}$, shows ...
192 views

### What does it signify that electrostatic mass cannot exist without inertial mass?

From Coulomb's law and Newton's second law we can state that if there is electrostatic mass (charge) at any point of space then there has to exist inertial mass also at that point of space. Otherwise, ...
44 views

### Average distance between molecules

What is the average distance betweeen the gas molecules in a classroom? I want to use that $$l= \frac{kT}{p \pi d^2 \sqrt{2}}$$ where $k$ is Boltzmann's constant and d the diameter of the molecule. ...
57 views

### Is the electric current a flow of charges?

Wikipedia defines charge as the fundamental property of forms of matter that exhibits electrostatic attraction or repulsion in the presence of other matter.Strictly speaking, while defining an ...
111 views

### What are the 1PI correlations functions?

This question may be a little strange, but I'm currently going through my lecture notes and in the construction of the 1PI effective action there is a constant reference to 1PI correlation functions. ...
150 views

### What is the velocity relative to earth?

I'm having trouble distinguishing between different relative velocities. Example: Say I'm riding my bike at a constant speed of 5 m/s in a straight line. While riding my bike I throw a baseball with ...
130 views

### Angular momentum and the Units

I'm just curious about why many physical identities build relationship with the same units as angular momentum like the action, Lagrangian$\cdot$time, Hamiltonian$\cdot$time, phase space area etc?
228 views

### Splitting a light wave

What I want to ask is what would happen if you split a single light ray (a photon) into two using a half silvered mirror? Would the emerging light rays have half the amplitude as the original one or ...
27 views

### Index Placement for Spinors in Relativity

This may ultimately be a silly question, but a pedantic mind like mine gets tied into knots over differing notation. (Disclaimer: I'm a mathematician.) Let $\mathbb{W}$ be a complex two-dimensional ...
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### Can we weld two dissimilar metals without using LASER welding? [closed]

It is said that one of the many advantages of LASER welding over ordinary welding is that it can be used to weld two dissimilar metals, what problems do we face while welding two dissimilar metals by ...
2k views

### Can quaternion math be used to model spacetime?

Quaternions are commonly used to model 4 dimensional systems where the quaternion consists of a real 3 dimensional vector and an imaginary scalar. So on the surface Quaternions seem well suited to ...
51 views

### Are white noises always Markovian?

Are white noises always Markovian? I am a bit confused about it. As white noise always has a constant power spectrum， its auto correlation function must contain a delta function of time. Thus the ...
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### Ask for Frank Wilczek's notes on entropy and entanglement

I know that I am not asking a question on physics. My question is just about learning physics. I am studying entanglement entropy and find some lecture notes by Frank Wilczek But I can only find the ...
57 views

### phase velocity in dielectric lined waveguide

I want to simply calculate the phase velocity of a mode in a dielectric loaded waveguide but am having difficulty arriving at a sensible answer. Here I consider only a TM mode. The problem is ...
82 views

### Four-momentum and Dirac equation in curved spacetime

Norm of four momentum in Minkowski spacetime is proportional to the square of rest mass as $$|P|^2= P^\alpha \eta_{\alpha\beta}P^\beta= (E/c)^2 - p^2 = (mc)^2$$ While in ...
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### Why do we use alcohol as active medium for dye laser?

Why do we use alcohol or ethylene glycol as active medium for dye lasers, they are not liquid organic dyes right? Or do they(alcohol etc.) serve some different purpose here?
49 views

### Explanation and quantum gravity [closed]

Is spacetime accelerating? Is it accelerating within an acceleration ?Or is it a constant? (are different objects or particles influenced by other rates in spacetime compared to one-another and if ...
37 views

### How does the barrier potential in a pn juction diode vary as we move across the diode from p side to n side?

If the drift current has to flow the graph should look like the first one, that is when the electrons (minority carriers in p-type) will be able to flow to the n-type side of the diode, but judging ...
53 views

### How fast does a surface cool depending on the distance to the source of heat

Say for example I have a sheet of metal which is 1 $m^2$. If I would heat the center of this sheet of metal to a 100 degrees celsius. How warm will a point be which is removed 10 cm from the center? ...
332 views

### HOW is electric field inside a conductor placed in an external field zero?

Lets assume that there is an external field +E acting on a conducting plate from left to right. Due to this, some electrons will drift from right to left to cancel out this electric field inside the ...
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### Problem - Demonstration of electromagnetic energy conservation

I want to solve the following problem from my Classical Electrodynamics' book: Consider a pontual mass body, $m$, with charge $q$, which moves with an uniform velocity $v$ in a region where an ...
329 views

### 2D Mass Spring System - Where does $m_3$ go to?

I am struggling to frame proper equations for the following two-dimensional mass spring System: 1D Model Basics first: I started with a simple one-dimensional mass-spring System to model something ...
116 views

### How to solve difficult resistance puzzles? [closed]

How does one solve really difficult resistance puzzles? These are presented in the form of a diagram with multiple resistances in various combinations. I have done some puzzles by reducing the puzzle ...
110 views

### Can the Earth be seen to transit the sun from the outer planets?

Transit of Mercury as seen from Mars The Curiosity rover on the planet Mars observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a ...
30 views

### How to produce sine wave heat generator?

I have taken a lab course this semester and in the experiment of finding thermal diffusivity of brass they are using square wave heat generator(by a simple circuit which gets on and off at regular ...
3k views

### How can the unstable particles of the standard model be considered particles in their own right if they immediately decay into stable particles?

How can the unstable particles of the standard model be considered particles in their own right if they immediately decay into stable particles? It would appear to a layman such as myself that these ...
145 views

### How to determine the trace and determinat of a differential operator?

How to determine the trace and determinant of the operator like $\Box$ or $\nabla^2$ etc. But first of all how to find the same for the simpler operator $\frac{d}{dx}$? I proceeded as follows. What ...
35 views

### Elastic material with exponential behavior?

I know some constitutive models for elastic materials like Neo-Hooke or Mooney-Rivlin, which give a relation between elongation $\lambda=y/y_o$ (where $y$ and $y_o$ are the length of the elastic ...
100 views

### Are the position eigenkets $\lvert x \rangle$ really a basis for the space of states?

In my current understanding, matrix formulation and wave-function formulation of QM are basically the same because $\left|\psi\right>$ and $\psi(x)$ are really the same mathematical object: A ...
568 views

### Physically what does warping (of space-time) mean?

So there's general relativity and Einstein's field equations that tell us "mass(or equivalently energy) warps space-time, and the warping tells mass how to move", but I'm still having trouble ...
5k views

### Difficulties with bra-ket notation

I have started to study quantum mechanics. I know linear algebra,functional analysis, calculus, and so on, but at this moment I have a problem in Dirac bra-ket formalism. Namely, I have problem with "...
204 views

### Complete renormalization in $\phi^4$-theory?

In the one-loop renormalization of $\phi^4$-theory, only 1PI vertex functions $\Gamma^{(2)}$ and $\Gamma^{(4)}$ are regularized and renormalized. But they do not exhaust all the irreducible connected ...
1k views

### Is it possible to produce gravitational waves artificially?

Why don't they make a ball with irregularities, say the size of a tennis ball, then spin it very rapidly, so it would produce gravitational waves like a spinning star with irregularities on it? Is ...
4k views

### Is it possible to use “negative sound waves” to “cancel out” a sound to create silence?

I saw youtube videos that claimed to do this, although I'm quite certain the videos just excluded sound and lied. However, I am wondering if the physics of this is actually possible - to create a ...