5
votes
5answers
2k views

What are the forces acting between two air bubbles in water?

The exact question is Two air bubbles in water attract each other repel each other do not exert any force on each other may attract or repel depending upon the distance between ...
2
votes
1answer
347 views

Non-equilibrium Green functions

How do we use non-equilibrium Green's functions (NEGF) or the Keldysh formalism in the theory of quantum transport? Please take a simple example like the Hopping model with a non-equilibrium ...
6
votes
1answer
860 views

Adiabatic theorem and Berry phase

As far as I can check, the adiabatic theorem in quantum mechanics can be proven exactly when there is no crossing between (pseudo-)time-evolved energy levels. To be a little bit more explicit, one ...
7
votes
1answer
2k views

Riemann invariants…Any physical interpretation?

I am really new to the CFD simulation, and started some simple algorithms recently. I then got introduced to the Riemann Invariants. Can any one provide some physical interpretation? Also, why is ...
1
vote
1answer
95 views

What is *uplift* in respect to extra dimensions and their stability?

What is uplift in respect to extra dimensions and their stability? It's notoriously hard to find something on this, as all possible keyword combinations pull up plethora of unrelated Google hits.
13
votes
1answer
10k views

Will high power laser penetrate mirror?

Many nations are developing hi-energy laser weapon. My question is, what if target is coated with mirror like coating? Can laser (since laser is still light) penetrate mirror? If it can then how is it ...
1
vote
4answers
6k views

Transformer: primary side & secondary side current 180 degree out of phase

I am a novice in electrical engineering. I notice that in transformer the secondary side current & current referred to as primary are 180 degree out of phase from each other. But why it is so, I ...
2
votes
1answer
390 views

How “wide” is a beam of light? What is its half-diameter?

If you believe ray optics where a light ray is a straight line, a light beam is infinitely thin. If you think of fiber optics, you can guide a whole lot of photons down a fiber only one or a few ...
4
votes
3answers
280 views

Basic Question About General Relativity?

I'm a layman that loves Physics. I'm also horrible at math. Having said that I have many, many questions in regards to physics and General Relativity in-particular. I will try to keep my question(s) ...
30
votes
1answer
6k views

Why is the Ampere a base unit and not the Coulomb?

I always thought of current as the time derivative of charge, $\frac{dq}{dt}$. However, I found out recently that it is the Ampere that is the base unit and not the Coulomb. Why is this? It seems to ...
-4
votes
2answers
1k views

Is electron volt an alternate unit for electric potential? [closed]

My question is: Can an electron volt be considered an alternate unit for electric potential?
1
vote
0answers
325 views

Kubo formalism application

Suppose I have some pertubative Hamiltonian on the Hubbard Hamiltonian and I want to calculate the change in current in linear response using the Kubo formalism. Now the kind of perturbative ...
-4
votes
1answer
636 views

Why an electron “rotate” around the nucleus at a speed close to the light one? [closed]

Why an electron "rotate" around the nucleus at a speed close to the light one? I mean where he gets all this energy? One DOES NOT simply approach the speed of light AFAIK.
8
votes
1answer
341 views

What are renormalons from a physics point of view?

This is again a question in the context of this paper about the Exact Renormalization Group. On p 23 and the following few pages, it is explained that for a $\lambda \phi^4$ bare action at the bare ...
0
votes
1answer
527 views

Understanding Charles's Law

Charles's law states that, The volume of a given mass of an ideal gas is directly proportional to its temperature on the absolute temperature scale (in Kelvin) if pressure and the amount of gas ...
2
votes
2answers
3k views

Stress in a thick-walled pressure vessel

I can find many references that give the stress in the walls of a pressure vessel for spheres and tubes, but they all seem to be limited to a thin-wall approximation. I'll limit my writing here to ...
3
votes
2answers
916 views

What is the physical interpretation of the Fourier transform $(\mathcal{F}Z)(t)$ an impedance?

If I compose a impedances out of smaller ones in series and parallel configurations, e.g. $$Z(\omega)=i\omega L_2+\tfrac{1}{\tfrac{1}{R_1}\ +\ i\omega C_1+\ \tfrac{1}{i\omega L_2}},$$ then I get a ...
2
votes
4answers
416 views

Calculating the torque at a point when a motor is stopped? [closed]

So, I'm trying to solve for the torque $\tau_A$ of a motor. I have attached a strong stick to the motor, like so: I apply a force $F$ on the stick which stops the motor. The distance from the ...
2
votes
2answers
306 views

Biggest experimental validations of postulates of Quantum Mechanics

What are some experimental results that validate postulates of Quantum mechanics completely beyond any doubt ? Since there are alternate theories being used by various physicists to describe the same ...
2
votes
3answers
4k views

Cylinder rolling down an inclined plane

I've been trying to solve this problem: A cylinder is rolling down an inclined plane (angle between plane and horizon α). Coefficient of friction is µ. What is the translational and angular speed of ...
3
votes
2answers
762 views

current operator in Hubbard model

How to derive the particle current operators for the non-interacting and interacting Hubbard model ? Hubbard Hamiltonian is given here with the interaction term: ...
0
votes
2answers
74 views

Photon propagation direction prediction possible after interacting with neutral hydrogen?

My current line of research deals a lot with hydrogen's Lyman-alpha emission and subsequent interactions of the Lyman-alpha photons with the surrounding hydrogen gas. My question is whether ...
1
vote
1answer
209 views

Number operator and Dirac field (with anticommutation relations)

Before using anticommutation relatives the energy, momentum, charge and number operators of the Dirac field have following expressions: $$ \hat {H} = \int \epsilon_{\mathbf p}\left( \hat ...
3
votes
2answers
479 views

Tensor Product of Hilbert spaces

This question is regarding a definition of Tensor product of Hilbert spaces that I found in Wald's book on QFT in curved space time. Let's first get some notation straight. Let $(V,+,*)$ denote a set ...
4
votes
3answers
1k views

Direct Sum of Hilbert spaces

I am a physicist who is not that well-versed in mathematical rigour (a shame, I know! But I'm working on it.) In Wald's book on QFT in Curved spacetimes, I found the following definitions of the ...
14
votes
1answer
5k views

How do they measure the rotational speed of cricket balls?

The 2013 Ashes series (a cricket thing between England and Australia) are underway as of today and it seems they have a new (to me) gimmick. It appears that they are now able to measure almost ...
0
votes
2answers
260 views

What is the required prerequisite knowledge of QM, for starting QFT?

As a physics bsc student, I have a very limited knowledge of QM: Dirac formalism, Schrodinger equation and simple solutions (oscillators, particle in a given potential, hydrogen-like atom etc). There ...
2
votes
1answer
128 views

Is the Higgs field really disconnected from gravity?

I would like to understand better the role of Higgs mass in the gravitational interaction. I read here in some posts, that the Higgs mass has nothing to do with gravitation. Is it justified to say ...
3
votes
3answers
298 views

spectral function in condensed matter physics

What is the importance of deriving the results of perturbation theory in condensed matter physics in terms of spectral functions ?
5
votes
0answers
73 views

How to rearrange the fermions in CohFT?

A simple question about notation of Moore Nekrasov and Shatashvili which makes me confused. Page 3, the authors rearranged the action into a novel form. For D=3+1,5+1,9+1 respectively, the path ...
2
votes
0answers
90 views

Intensity of particles with zenith angle dependence

The intensity of particles come to sea level depend to the zenith angle $I=Iv (\cos(z))^n$, where $z$ is the zenith angle and $n$ depend to the particle. I know the vertical intensity depend to the ...
2
votes
1answer
321 views

periodic boundary conditions for vortex in a square lattice

I am trying to follow this paper and track the dynamics of vortex motion on a discrete (square) lattice. The idea is to simulate the time evolution of the Gross-Pitaevskii (GP) equation, which reads ...
2
votes
1answer
862 views

Why some particles interact with the Higgs field and others don't?

Why some particles interact with the Higgs field and others don't? Higgs doesn't explain that much: why some particles have mass and others don't? is like why some particles interact with the Higgs ...
2
votes
2answers
434 views

Finding the illuminance from a triangular light source

Since most light sources in games are point-like, it's pretty difficult to approximate area light sources with point sources. As triangles are a universal form to represent 3D models (thus area light ...
3
votes
1answer
432 views

Deriving the transformation under Weyl rescaling in Polchinski eq. (1.2.31)

I have another question in Polchinski's string theory book volume 1, namely how to derive Eq. (1.2.32)? $$(-\gamma')^{1/2} R'=(-\gamma)^{1/2} (R-2 \nabla^2 \omega) \tag{1.2.32}$$ I have awared his ...
2
votes
0answers
116 views

Prof Lewandowski's paper about different space time for different particles

I read here http://www.sciencedaily.com/releases/2013/07/130709115344.htm that Prof. Lewandowski's work suggests that different particles may experience different spacetimes. His work is in the ...
1
vote
0answers
89 views

Preservation of a scalar along geodesic trajectory

Let $u^\mu$ be the velocity of a particle , and $\xi^\mu$ be a killing vector. would taking a contravariant derivative of to scalar product $\xi_\mu u^\mu$ , and showing that it equals to 0 shows that ...
6
votes
2answers
689 views

Why can't I slap an insect into unconsciousness?

So it is summer, which means I have to deal with insects. Today one has made it into my room and while I tried to work this insect was constantly in my face, which caused me waving my hands in the ...
1
vote
1answer
806 views

How does rolling resistance of rail wheel depend on diameter?

Freight train is more efficient than truck due to lower rolling resistance. And I wonder which one has lower rolling resistance<>, small diameter or larger one or it doesn't not depend on diameter ...
10
votes
2answers
902 views

Movie airplanes and suction

Having watched a recent action movie (with zombies in it) I wondered whether the suction from a hole in the airplane's hull would really be able to rip out luggage, persons and even seating benches. ...
0
votes
3answers
636 views

LHC vacuum tube diameter [closed]

What's the diameter of the tube where the protons fly around? I dont mean the overall diameter (~1-10 m), instead the inner most ultra-high vacuum tube where the protons are. The inner pipe, the ...
1
vote
2answers
732 views

Particle current operator in general vs Particle current operator for tight binding Hamiltonian

I am referring Mahan Many-Particle Physics. There are 2 particle current operators -one in general and one for the tight binding Hamiltonian. How do we go from the general current operator (1.195 in ...
1
vote
0answers
30 views

Where does the reverse current come from in a organic semiconducting diode?

Unlike traditional inorganic diodes, organic semiconducting materials hold few thermally exited free carriers under room temperature, so the reverse current should be small if pinholes of the films is ...
2
votes
0answers
165 views

Definition of a 'tunneling lifetime'

I'm given a one-dimensional potential with two wells, one local minimum at some higher energy and one deep global minimum next to it, separated by a barrier of own shape and height (phase qubit). I ...
3
votes
2answers
3k views

work done by tension

The problem statement, all variables and given/known data Consider the following arrangement: Calculate the work done by tension on 2kg block during its motion on circular track from point $A$ to ...
3
votes
1answer
64 views

What is the minimum time period for constellations to change shape?

All stars are revolving around the center of galaxy. And so I wonder how long it takes before we can see any noticeable change in the shape of any constellation? What are the factors one will use to ...
-1
votes
2answers
57 views

Paraphrasing speed and heat capacity [closed]

Can we call speed the change in distance per time? Further, would you phrase heat capacity as change in heat per temperature or change in temperature per heat? The reason for this question is that ...
26
votes
6answers
3k views

Canonical everyday-life example of a technology that could not work without humans mastering QM in analogy to the application of GR in GPS?

The GPS is a very handy example in explaining to a broad audience why it is useful for humanity to know the laws of general relativity. It nicely bridges the abstract theory with daily life ...
6
votes
1answer
309 views

Naturalness arguments and dimensional regularization?

How do issues of naturalness arise when regularizing QFT using dimensional regularization? I can only recall ever seeing naturalness arguments (hierarchy problem, cosmological constant problem, etc.) ...
2
votes
1answer
157 views

Weyl symmetry and Polyakov action

I have a question in reading Polchinski's string theory volume 1. p12-p13 Given the Polyakov action $S_P[X,\gamma]= - \frac{1}{4 \pi \alpha'} \int_M d \tau d \sigma (-\gamma)^{1/2} \gamma^{ab} ...

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