# All Questions

15 views

### Observationally distinguishing a galaxy of antimatter from a galaxy of matter [duplicate]

I was just wondering how, observationally, we would distinguish a distant galaxy of "normal" matter from one of antimatter. Maybe there is a simple answer but I don't see it. Once I started thinking ...
16 views

### Physics and baseball

In the following scenario lets just use approximate values. Johnny is a pitcher and he throws 50mph. Johnny climbs on the roof of a car who is traveling 50mph. Johnny throws his normal pitch to his ...
25 views

17 views

### Is it possible that cosmic ray particles and neutrinos account for a significant portion of dark matter?

Sounds a bit naive but I read somewhere that neutrinos were thought to account for dark matter to an extent and I think Zwicki came up with the idea before cosmic rays were announced (not sure ...
47 views

### Combining two data points with different uncertainties

I have two separate algorithms (call them "A1" and "A2") which reconstruct the $(x, y)$-position of an event in a particle detector. I can test both of these algorithms on simulated events from a very ...
16 views

### Can someone explain what the wave function is? [duplicate]

I've been doing she research I don't really understand what a wave function is. I used to think it was the de Broglie's wavelength but I've soon found otherwise. Just a simple and clear explanation ...
44 views

### Rigorous derivation of general relativity from first principles

What is the minimal set of axioms required to derive the mathematical formulation of General Relativity from first principles? What are these first principles? What are good references that detail ...
30 views

### What REALLY happens when matter and antimatter colide? [duplicate]

I've read on numerous websites that when 1 kg of matter collides with 1 kg of antimatter all the mass gets anhilated and converted into energy according to E = mc^2. I know this is the right ...
12 views

### What is the effect of volume on liquid temperature during evaporative cooling?

Imagine two evaporative cooling experiments: In the first, a beaker contains a known volume of liquid. The liquid starts at ambient temperature. The temperatue of the body of liquid remaining in the ...
28 views

### why cannot longitudinal waves travel through space (vacuum)

'The reason sound can't travel through a vacuum is that sound needs a medium (solid, liquid or gas with real vibrating molecules) and not because it is a longitudinal wave' How does this make sense as ...
12 views

### Photoelectric effect and evanescent fields

We all know that if light impinges upon a metal and if it's the energy of the photon (hv) is greater than the work function of the metal then an electron will become unbound from the metal and be ...
19 views

### Variation with respect to the metric and other tensors

When varying an action with respect to tensors and the metric, I'm afraid I get confused as how to one organizes the Lagrangian and then performs the variation. Take for example, the following example ...
25 views

### What is the significance of the possible new 17-MeV “protophobic X boson?”

Krasznahorkay et al ...Possible Signature of a Light, Neutral Boson published results in 2015 of a possible new particle at 16.7 MeV+/-0.35. The researchers suggest "such a boson might be a good ...
172 views

### Universal Gravitation Vector Notation

I'm learning about astrodynamics on my own and I was wondering why the $r_{ij}$ is cubed in the vector notation for of Newton's Law of Universal Gravitation: $$F_g=(Gm_1m_2/(r_{ij})^3)r_{ij}$$ Here ...
33 views

### Fastest Speed of Sound

Obviously, sound (like every other causal phenomena) may not travel faster than the speed of light. I know that materials with a high bulk modulus and low density will typically have faster speeds of ...
42 views

### Need help with Schwartz's QFT: Problem 3.9 in Page 44

This is the problem: I have worked about questions (a), (b) and (c). I do not quite understand question (d), and therefore (e) (in the next page). What does "instantaneous" mean in this case? In ...
20 views

### How to convert detection (of say metal/movement) to electricity?

Is there a way (using (magnetic?) sensors to detect?) , converting movement/speed of for example cars into electricity or other form of energy?
12 views

### Would a celestial body in a horseshoe orbit be tidally locked to the planet?

A smaller celestial body like a moon or an asteroid can be in a horseshoe orbit around a planet. Since it might go inside the tidal lock radius (but does not stay inside it all the time), during the ...
9 views

### Measurement of Quantum Hall Effect

I'm somewhat confused about the original measurement of the QHE in a Si-MOSFET, which might stem from my not understanding MOSFETs very well. In their measurement, von Klitzing et al. measure the ...
31 views

### Wave as a source of electricity?

I was studying a high-school physics textbook and I came across a rather interesting and confusing example. I've been reading about rectification, and it all made sense until it came to the part where ...
38 views

### Poincare Group (Wald, Chapter 4 Page 59)

In Wald's text on general relativity, he mentions that in special relativity, many different global inertial coordinate systems are possible and can be put into one-to-one correspondence with elements ...
49 views

### “Measure of time in general relativity”

Suppose to be in an arbitrary gravitational field and you are moving in it arbitrarily with a clock in your hand. In this general situation I ask: if I read the positions of the hands of the clock, ...
22 views

### How do I load a CIF file in Maud on OSX? [on hold]

I am trying to load a CIF file in Maud, but when I go to edit->load object from CIF and then select my file, nothing shows up but the name. No graph appears. In the edit selected object on the ...
25 views

### How many orientifold planes does one need?

According to the book ''String Theory in a Nutshell'' by Elias Kiritsis (page 204, linked here), the tension of an $Op$-plane is $$T_{Op} = -2^{p-4}T_{Dp}$$ With this picture, one gets -- for type ...
44 views

### Conservation of electromagnetic energy-momentum tensor

I'm trying to show that $\partial_\mu T^{\mu \nu}=0$ for $$T^{\mu \nu}=F^{\mu \lambda}F^\nu_{\; \lambda} - \frac{1}{4} \eta^{\mu \nu} F^{\lambda \sigma}F_{\lambda \sigma},$$ with the help of the ...
19 views

### How to estimate a drag of flow reversing cylinder?

I am assuming a turbulent flow and a full reversal of the flow (in the picture the arrows are at non-straight angle but please consider the skew as negligible). Assuming we are talking about ...
18 views

### Determine the angle the mass leaves a smooth semi cylinder

A small body of mass m rests at the top of a smooth hemisphere with radius r. It is released from rest. At the moment it is released $\theta=0$ with zero initial velocity. There is no friction between ...
20 views

### How do flames neutralize a static charge?

I was working with a demonstration galvanometer recently when it accidentally picked up a static charge from me lightly rubbing my hand across it. The case was made of a clear plastic. My attempts ...
31 views

### Why doesn't the gravitational force have a permittivity in its formula? [duplicate]

We know that the electrostatic force between two charges depends on the medium between the charges and its permittivity. Why, then, doesn't the gravitational force depend on the medium?
24 views

### Amplifying an electromagnet's generated field

Given an electromagnet like this: which is essentially an iron casing with coils inside: Is it right that the centre disc would have one polarity whilst the outer housing another? To boost the ...
25 views

### What is the difference between reflection and total internal reflection (TIR)?

I look at what TIR is and then what reflection is - when the angle of incidence and the angle of election are the same. And TIR is when the angle of incidence is greater than the critical angle. TIR ...
13 views

### compressed air thrust calculation [on hold]

I am working on a project in which there is an article of 100 kg hanging on a wire rope supported by wheels at a height of 12 m from ground at one end and 25 m at the other end. The article needs to ...
75 views

### Can quarks have anti-colors?

I get the basics of QCD (or I think I do). What I was wondering is whether up and down quarks, and their heavier cousins, absolute HAVE to carry color charges, or if they can have anti-color charges ...
28 views

### How does radiative cooling between the earth and space work?

On a chilly (but above freezing) temperatures on a clear night, you can freeze water outside because of radiative cooling. By what mechanism do warm bodies on earth actually lose heat to space? ...
27 views

### Expectation value in second quantization

I am stuck calculating a simple expectation value for an operator, which is expressed in second quantization. I know the result, but I fail to proof it. Lets say I have one-particle wave function ...
16 views

### Find voltage at a point on infinite ladder of resistances [duplicate]

Find voltage at point C. What I have tried. Find the effective ladder of resistances. Which comes out to be R(sqrt.3 - 1) Then find the current. 48/Reff Then split the current and find current ...
10 views

### Energy Transformation BLDC Motor

I would like to know what kind of Energy Transformation happens in a BLDC Motor. I googled it and read that the electric energy is transformed to magnetic energy, and then from magnetic energy to ...
40 views

### Selection rules and type of photon?

Let us consider the following matrix element: $$\langle n',m',l'|x| n, m, l \rangle$$ For the corresponding radiative transition we have the selection rule that $\Delta m=\pm 1$. But will the photon ...
63 views

### What is y(x,t) = Acos(kx - wt)? [on hold]

Im very new to wave mechanics and I've come across the following wave equation. I know this is asking too much,but I wanted to know what is the significance of this equation? what does it tell us / ...
27 views

### Do satellite galaxies have the same proportion of dark matter as “ordinary” galaxies

My question is relatively straightforward: Do we know if satellite/ dwarf galaxies contain the same proportion of dark matter to ordinary matter as "regular" sized galaxies? The Milky Way, for ...
In E Poisson "A relativist tool kit" p.75 it says that the Gauss-Weingarten relation is: $$e^{\alpha}_{a;\beta}e^{\beta}_{b}=\Gamma^{c}_{ab}e^{\alpha}_{c}-\epsilon K_{ab}n^{\alpha}$$ We have the ...