# All Questions

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### why can't you use your cellphone on an airplane? [closed]

why airplanes are banned with use of cellphones? What were the impacts while we answer an call or make an call ,what was the physical reason behind the ban of using cellphone inside airplanes ? I ...
80 views

### adhesion(bonding etc.) of materials and frictional force

I am studying material physics. I heard in the class that every adhesion or bonding can occur only with frictional force. Today, I tried to find a proper reference for this statement but I could ...
52 views

### Powercounting in String Frame?

if I consider the low energy effective action of type IIB string theory in the string frame, i.e. with an $e^\phi$ prefactor, is it possible to do standard powercounting with this action? I.e. how do ...
976 views

### Why is the stress-energy tensor symmetric?

The relativistic stress-energy tensor $T$ is important in both special and general relativity. Why is it symmetric, with $T_{\mu\nu}=T_{\nu\mu}$? As a secondary question, how does this relate to the ...
537 views

### How does color of galaxies explain their distance?

Why do distant galaxies have different colors than closer ones?
184 views

### Electromagnetic waves produced by a charged pith ball

Maxwell’s equations appear to have no limitation as to the length of an electromagnetic wave that can be produced by an accelerating electric charge. So in theory, if I use a charged rod to oscillate ...
92 views

### Must string models that describe 4d effective field theories always have D-branes that extend in the 4 non-compact spacetime dimensions?

In string theory the D-branes give those directions that the strings are allowed to move along. The string excitations give the fields that we detect. Is it correct to think of a particle propagating ...
388 views

### What are the *necessary* conditions to deterministic chaos?

What are the necessary conditions (not saying sufficient conditions) in mathematical terms that a deterministic dynamic system can transit to deterministic chaos? We collected yet: A positive ...
514 views

### Equations of motion for bob-on-a-string — am I missing some terms?

The dynamics of a type of physical system I am currently working on are modeled in most of the literature by replacing the moving parts of that system with an equivalent set of pendulums. Parameters ...
133 views

### Integral related to particle diffusion

In the context of particle diffusion, I am trying to understand the equations that describe Brownian motion as a macroscopic process. Assume $N(x,t)$ is a number concentration and $D$ is a diffusion ...
148 views

### Heat equation with Neumann boundary conditions

I would like to explore some ideas with different geometries exchanging energy mainly by radiation. This means solving Laplace equation for the steady state. However the boundary conditions are always ...
525 views

### Why is the Maxwell Stress Tensor symmetric?

What is the physical meaning of the Maxwell Stress tensor symmetry?
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3k views

### What happens with the force of gravity when the distance between two objects is 0?

so I had my first approximation to the gravity equation $$F=GmM/r^2$$ and some questions arose that my teacher couldnt respond: if r approximates to 0 with mM being ...
139 views

### Evidence for expansion of space [duplicate]

If I understand correctly, Einstein's theory of General Relativity predicted the expansion of space itself, and Hubble confirmed this prediction by observing the red shift of receding galaxies. I ...
117 views

### Need an intermediate resistivity part/material

I need a part or material for a planned experiment (the experiment is similar to those described in my articles http://arxiv.org/abs/1208.0066 and http://arxiv.org/abs/1109.1626 ). The problem is that ...
310 views

### No well-defined frequency for a wave packet?

There are similar questions to mine on this site, but not quite what I am asking (I think). The de Broglie relations for energy and momentum $$\lambda = \frac{h}{p}, \\ \nu = E/h .$$ equate a ...
5k views

### Can facial surgical mask effectively filter particles of smoke pollution?

Singapore is having an episode of its regular hazy season because of smoke from forest fires. Some people have been using surgical masks like the the one shown at the bottom of this post, presumably ...
1k views

### Charging by induction (and grounding)?

The very familiar phenomenon of charging by induction includes bringing a charged object near an uncharged sphere to induce a separation of charges in it and then grounding the charge (same as the ...
194 views

### Consequences of Quantum Mechanics [closed]

First of all, this question is going to seem a a bit of philosophy but know that vague and purposeless wandering is certainly not what i'm trying to propose here. Also, the reason i didn't post in ...
133 views

### Gas giant evolution over lifetime

In various types of stars, one can learn how they evolve differently, depending on factors such as their size and chemistry. Some stars have a short lifetime and others much longer. But, what is ...
363 views

### polarization of the lower mode gaussian beam

In most introductory analysis of Gaussian beam optics, Helmoltz scalar optics is assumed. Hence polarisation is ignored. But I'm not clear what are the possible orientations for the lower transverse ...
3k views

### According to Hubble's Law, how can the expansion of the Universe be accelerating?

Scientists today think the expansion of the universe is accelerating. According to Hubble's law, objects further away are moving faster than objects closer to us. The further away an object is, the ...
650 views

### Is there any relation between weak and strong fields, similar to electric and magnetic fields?

Is it possible to unify the strong, weak, electric and magnetic field just by Maxwellian type equations? (Maxwell by adding a small change - unified electric and magnetic field, then Einstein's ...
511 views

### Applications to the Van der Pol equation? [closed]

What are some applications to the Van der Pol equation? Are there any physical examples?
239 views

### Anti-symmetric 2 particle wave function

Suppose we want to construct a wave function for 2 free (relativistic) fermions. As we are dealing with fermions the total wave function has to be antisymmetric under interchange of the coordinates, ...
331 views

### Possible implications of Quark Quartet

Today on Nature's website appeared a news about the discovery of a quark quartet (formed from two quarks and two antiquarks). They say that this particle containing four quarks is confirmed. This is ...
291 views

### Notation for Standard Model Charges?

Does anybody know what these following numbers describing an electron $(1, 1, -1)$ represent in $SU(3) \times SU(2) \times U(1)$? Or, these numbers that describe an up quark: $(3, 1, 2/3)$? I'm ...
2k views

### What if the Moon was 100x more massive: Binary planets?

I was reading a couple of Earth-Moon related questions (Mars just collided with Earth! A question of eccentricity, Could the earth have another moon?) and they got me thinking about planet-moon ...
2k views

### Who (and Why) started the “electrons are negative, protons are positive” convention? [duplicate]

For some reason everyone labels electrons using a minus sign and protons using a positive sign, even though the opposite seems more intuitive: Who started the convention that electrons should be ...
1k views

### Under what conditions is a pure fusion nuclear bomb possible?

Is it possible to build a pure fusion and powerful nuclear bomb? (because due to the harsh conditions of enriched uranium, I would guess most of today's world countries will want to build pure fusion ...
187 views

### Why doesn't an electrometer's capacitance influence the measurement of voltage?

I've read on the subject in several books, and none of them mentioned whether we can neglect electrometer's influence on the measurement of the voltage or not. Maybe my question sounds a bit stupid, ...
160 views

### Singularity and Black Hole Complementarity

When looking at a (eternal) Schwarzschild Black Hole, we may identify two worlds. The region $R_1$ (right) - our world -, and the region $R_2$ (left) - an other world. The "black hole interior" ...
288 views

### Method of images tutorial?

I'm having an exam in Electrodynamics soon. I think I have most of it under control, but the method of images I'm not quite sure about. There is not much in my book about, so I was thinking some of ...
2k views

### Materials with Zero Poisson Ratio

Poisson's ratio is defined as negative ratio between transverse and axial strain. So, a material with zero poisson ratio must necessarily exhibit no transverse strain. After checking the wikipedia, ...
293 views

### Is the Green function a prescription for a connection?

I'm trying to learn connection on principal fibre bundle. As far as I can see, the connection is just a given prescription for the displaced field/function on the base space to remains on the ...
245 views

### Why do baked goods “fall” if exposed to sudden cooler air?

This is a know rule when baking: Don't open the oven until your cake is finished, otherwise it will "fall". Why/how does this happen?
186 views

### Is there a non-technical way to understand why proving confinement is difficult?

As far as I understand, the basic reason why we are not able to solve QCD at low energies is because it is a strongly coupled quantum field theory, and a part from exceptional situations in simplified ...