# All Questions

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### What is the maximum current carrying capacity of a Josephson tunnel junction?

In 1962, Josephson predicted that for a sufficiently thin insulating layer, it should be possible for Cooper pairs to tunnel between two pieces of superconductor. With a potential difference $V$ ...
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### Spatial and Temporal Coherence

How is it possible to achieve waves which are spatially, but not temporally, coherent? Can this be done with a bandpass filter? Conversely, how is it possible to achieve waves which are temporally, ...
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### Electric field and magnetic field question here?

Are the energy of the electric field and the energy of the magnetic field concentrated on their sources OR are they scattered in the environment where the fields arent zero? Can you base your answer ...
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### Evaluating the Einstein-Hilbert action

The Einstein-Hilbert action is given by, $$I = \frac{1}{16\pi G} \int_{M} \mathrm{d}^d x \, \sqrt{-g} \, R \, \, + \, \, \frac{1}{8\pi G}\int_{\partial M} \mathrm{d}^{d-1}x \, \sqrt{-h} \, K$$ ...
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### An intuitive proof of Bertrand's theorem

Is there a way to see that Bertrand's theorem is true intuitively. I mean without getting into too much mathematics ?
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### Fate of light ray

This question has been on my mind since I first learned optics. Where will the light ray go? First, I thought of it taking a U turn. But, I don't think that will happen. Now I have studied a bit ...
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### Is refraction sharp or smooth?

Refraction: light changes direction of propagation when entering a material with a different refractive index. Does the direction of propagation of light change sharply and almost instantaneously ...
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### Can an electron and a positron annihilate to form a Z_0 boson?

I guess that, since the Z boson is more massive than both the electrons, they need to go pretty fast (so that their kinetic energy is large enough), but is annihilation a property of the ...
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### How am I able to tell whether it is dark or not while eyes closed? [closed]

How am I able to tell whether there is a light on in front of me even with my eyes closed? The usual darkness whilst eyes closed transforms into a not so bright glow when my closed eyes are in front ...
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### What is the difference between these two Feynman diagrams?

In which direction is time flowing and what reactions do they represent? EDIT for Gigi (I could not add a comment to your answer): 1) Do Feynman diagrams by definition only show fundamental ...
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### Falling metal cylinders more devastating than a nuke?

I just saw G.I JOE - Retaliation. In the movie the bad guys (spoiler alert) Could this device cause more destruction than a nuclear weapon? If so, how?
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### Weak interaction and Neutrino

I have heard that Neutrino is the only particle affected by the Weak Interaction. How is a Neutrino affected by Weak interaction? I would like to ask the detail. Are there any relationships between ...
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### Relation between pressure and particle displacement in an acoustic wave

Consider an acoustic wave in some medium, expressed as particle displacement: $$s(t,x) = e^{j(-\omega t + kx)}$$ I understand that pressure must be at its maximum when the particle displacement is ...
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### Helium and its unique behaviour

It is said that helium is lighter than air. What does that exactly mean ? If it is lighter than air,then there should be absolutely no trace of helium in earth's atmosphere(it should have flown into ...
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### Can we treat $\psi^{c}$ as a field independent from $\psi$?

When we derive the Dirac equation from the Lagrangian, $$\mathcal{L}=\overline{\psi}i\gamma^{\mu}\partial_{\mu}\psi-m\overline{\psi}\psi,$$ we assume $\psi$ and ...
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### Application of Displacement Current

I'm reasonably happy with the derivation and results of displacement current, however, I'd like to be aware of a few practical applications of this idea. So far, the only one I'm aware of is when ...
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### What can cause a change in wave's shape - One dimensional wave

what can cause a change in wave's shape of one dimensional wave moving through a rope? It's velocity ? or the wave's length ? What can cause him change his shape.
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### Why a mono-atomic crystal layer (2D) can't be stable?

According to Peierls and Landau, 2D crystals were thermodynamically unstable. They can't exist! Of course, this theory was disapproved in 2004 (example: graphene). What is the general definition of ...
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### Gauge Higgs Unification [duplicate]

http://arxiv.org/abs/1003.6023 Gauge Higgs unification . Does gauge higgs unification has been proved ？ If so , what is it exactly meaning ?
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### 'Pseudo-Relativistic' behavior in Graphene

I've read that electrons in Graphene behave 'pseudo-relativistically'; what does this mean? how do they behave differently from electrons in other materials?
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### How to derive quantum Fourier transform from discrete Fourier transform (DFT)?

I am interested in Shor's algorithm, and I am reading several papers that related to the quantum Fourier transform (QFT). I know the there is a difference between the output of QFT and DFT (DFT). ...
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### Scalar field divergent mass correction interpretation question (hierarchy problem)

Simple power counting tells you that a scalar field coupled to some fermions at one-loop picks up a correction to the mass of the order $\Lambda^2$. Based on this people say things like "it's natural ...
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### Were Maxwell's equations first formulated by McCullough?

Some years ago, I heard a talk about a an Irish or Scottish physicist named McCullough who had formulated Maxwell's equations several years before Maxwell. This fellow was recognized for his work, ...
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### Are covariant derivatives of Killing vector fields symmetric?

I'm reading the Lecture Notes on General Relativity by Matthias Blau, and in section 9.1 (point 1) he writes: Let $K^\mu$ be a Killing vector field, and ${x^\mu(\tau)}$ be a geodesic. Then the ...
335 views

### Plano-convex lenses

When the spherical side of a planoconvex lens is towards the source we will see reduced aberrations normally we use it for the beam collimation. When we use two planoconvex lenses configuration why we ...
151 views

### Layer thickness measurement

I am interested to measure the thickness of the coating on a mirror (e.g. in the picture below). What are the available and best way of measuring it? What will be the solution in case of multiple ...
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### What happens when two high-speed objects going opposite directions crash into a black hole?

Imagine the following: A-> [] <-B where [] is a black hole, and A and B are two objects of same mass, traveling with the same speed right at the black ...
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### Lippmann-Schwinger Equation with Outgoing Solutions

I'm reading about Green's functions and how the Lippmann-Schwinger equation eventually leads to the textbook expression for the form of wavefunctions in the far radiation zone after scattering by a ...
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### Time dilation factor for the circular orbit at 3/2 Schwarzschild radius

What would the time dilation factor be if a massive (as in rest mass>0) point particle orbiting a Schwarzschild black hole in the photon sphere? If I understand correctly, this is the only possible ...
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### Why does most of the heat transferred on Earth come from the infrared part of the electromagnetic spectrum?

Why does the most of the heat transferred on Earth come from infrared part of electromagnetic spectrum?
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### Spring Constant - Waves

I tried to prove that for the same length of spring , a one dimensional wave that moves faster in spring 2 than spring 1, its spring constant will be lower than the spring constant of spring 1. ...
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### Balloon tethering cable tension model (catenary tether)

If a moving balloon (let's say it's a blimp system with some rotors for movement) is connected to the ground by a cable that describes a catenary curve, how can we model the tension being applied on ...
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### Does a black hole have enough time to actually form a singularity?

I am trying to wrap my head around black holes, singularties and hawking radiation. Physics.se contains many intresting questions and answers, but from none I could so far read about the interaction ...
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### Obtain the eigenfunction of Jz for the wave function of an electron in a hydrogen atom? [closed]

The wave function of an electron in a hydrogen atom is given by Is this wave function an eigenfunction of Jz , the z-component of the electron’s total angular momentum? If yes, find the ...
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First off: I am not familiar with the details of quantum mechanics or relativity. My understanding of Hawking radiation is as follows: Pairs of particles and anti-particles can and will spontaneously ...
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### Is the neutral pion a singlet?

In Griffiths' Introduction to Elementary Particles, it is mentioned p. 179 that the $\pi^0$ is a singlet under $SU(2)$ isospin. But it is also part of the $\pi^-,\pi^0,\pi^+$ isospin triplet. How can ...
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### What are you studying when you study a Harmonic Oscillator in QM?

This probably is a naive question - so please forgive a self-studier. In the text I am studying, one builds a HO by placing a particle in a potential that increases quadratically from the origin. The ...
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