# All Questions

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### What is Diffraction?

Diffraction is just light interacting with small objects, and bending, but this seems like a very imprecise definition to me. What is diffraction, actually? I was confused because there are at least ...
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### Potential energy from opposing magnets repelling each other with a gap of 1 mm

I have two powerful rare earth magnets, that are separated by a distance of 1 mm. I applied energy to bring them closer to each other, hence increasing the potential energy. Now, when one of the ...
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### Why does adding milk to my coffee produce a fractal?

Whenever I add milk to my morning coffee I often enjoy watching the patterns which are created. These patterns have a striking resemblance to certain fractals and my question is, "Why?" Oh dear, that ...
982 views

### How do I figure out the momentum of a water balloon when it reaches the person I am throwing it at?

Recently an experiment was performed in which myself and a partner filled a water balloon and threw it back and forth at each other without breaking it. We gradually increased the distance at which it ...
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### If an electron can't go faster than light, how is energy conserved here?

I came across a physics question recently about accelerating electrons with potential differences and it got me wondering. Here is the question that brought the concept to mind. First off, imagine ...
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### Null lines and degenerate plane

Can anyone explain me what null lines are and degenerate plane? I don't know anything about it, I don't have physics background and I am a mathematics student and please tell me if there is any good ...
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### Gravitational collapse and free fall time (spherical, pressure-free)

A very large number of small particles forms a spherical cloud. Initially they are at rest, have uniform mass density per unit volume $\rho_0$, and occupy a region of radius $r_0$. The cloud collapses ...
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### Where does this derivation of the equation of motion for a simple pendulum go wrong?

I am attempting to derive the following equation of motion for a simple pendulum: $$\theta''(t) = - \frac{g}{l} \sin(\theta).$$ For background, see this Wikipedia article. I understand this page ...
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### Is Newton's third compatible with retarded Lorentz force?

In Griffiths Introduction to electrodynamics it is said that Newton's third law is not valid in electrodynamics, but, in the example given, the it does not consider the retarded values for the fields ...
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### Spinor inner products

The spinor inner product in particle physics is given by $\overline{\psi} \psi = \psi^{\dagger} \gamma_0 \psi$, where I take the convention that the zeroth gamma matrix is hermitian while the rest ...
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### Calculating axle load in cargo container (pics are included)

I'm current developing cargo loader software, but i have a little challenge with calculating load for each axle when a cargo is placed on container. I know that all axles will be affected (the nearest ...
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### Gauge Invariance of the Hamiltonian of the electromagnetic field

The Hamiltonian for an electron of mass $m$ and charge $e$ in an exterior electromagnetic field is $$H=\frac{1}{2m}(p-(e/c)A)^2+e\varphi.$$ The corresponding (via canonical quantization) quantum ...
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### Understanding spherically symmetric metric

In these lecture notes the static isotropic metric is treated as follows (p. 71): Take a spherically symmetric, bounded, static distribution of matter, then we will have a spherically symmetric ...
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### Function describing an adiabatic process

I understand that an isothermic process, plotted on a PV diagram, can be described by a rectangular hyperbola. However, now I am wondering, what about an adiabatic process, what function best ...
87 views

### Unable to resolve 2 equivalent geodesic equations

A free particle moves along geodesics, one form being \begin{split} \ddot x^\mu &= -\Gamma^{\mu}_{\sigma \rho} \dot x^\sigma \dot x^\rho \\ &= -\frac{1}{2}g^{\mu \nu}(\partial_\sigma g_{\rho \...
124 views

### What is a degenerate Fermi gas?

In ultracold atoms, it is generally talked of a degenerate Fermi gas. What does degenerate mean here?
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### Simulating how comets travel with Newtonian physics

Imagine that a comet enters the solar system from Oort cloud, what concepts and laws does one need to take into consideration to simulate how it travels? I'm about to start researching how to write ...
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### If the universe could emerge from nothing, what about physical laws? [closed]

I've watched Lawrence Krauss's lecture and read his book. I think I got what he was saying, and I don't have any problems with that; however, what I can't get is how the laws of physics that makes ...
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### How can one calculate the distance a particular sound will travel?

What do you need and how to calculate a distance traveled by sound? For example if you hit a bell with a specific amount of power how far will it travel?
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### Is the Copenhagen interpretation merely an approximation to quantum mechanics?

So, I'm reading Max Tegmark's Our Mathematical Universe (Knopf edition, p. 229). He's discussing Everett/MWI for a bit and I'm not really paying attention and then I wake up to this: [I]t's time ...
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### How should I interpret an article about light being slowed and compressed?

I've found an article describing a method that allowed the light to be "slowed and compressed"... That sounds really strange, which made me google a lot and find this PDF document. However for me, ...
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### Coherence effect on collecting sun light by mirrors

There are systems that collect sun light in one spot. I wonder, why does it accumulate additively, without destructive interference?
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### Chiral Spin Liquid(CSL), Chern number, and the ground state degeneracy(GSD)

Consider a 2D gapped CSL with a nonzero Chern number $m$, then is the GSD of the system on a torus directly related to the Chern number $m$? For example, see this article, in the last paragraph on ...
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### Invariants of Connection Form

I am somewhat going out "on a limb" here, since I am much more grounded in the physics side of things than I am in mathematics. Nonetheless, I am wondering if someone is able to comment on the ...
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### Proofs on Quantum Inequalities?

Are there any experimental proofs witch support The Quantum inequalities and The Quantum interest conjecture?
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### Work of an adiabatic/ compresion: sign?

As far as I am concerned the adiabatic work it is given by: $$W_{adi}=\frac{P_{f}V_{f}-P_{i}V_{i}}{\gamma -1}$$ While I have seen in other examples that it is used with $1-\gamma$ in the ...
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### Earthing or nothing

Sometimes, mostly in winter, when somebody touches the tap it passes current (especially when the hands are soaked in water or soap water). I read 3-4 years back in physics book that the tap circuit ...
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### Coleman-Mandula theorem in mathematical language

Every supersymmetry text starts off mentioning the Coleman-Mandula theorem. Often it is introduced using rather colloquial terminology. I was wondering if anyone knew a precise mathematical ...
318 views

### Dielectric constant of water

I need the dielectric constant of water from $10^{-2}$ Hz to $10^4$ Hz. As stupid as it may seem, I cannot find much info. I've googled for days. All I can find is close to GHz. And the only info ...
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### Reunion condition in the Twin Paradox

In the Twin Paradox two twins initially at rest in the same reference frame are separated and take different journeys through spacetime. Eventually they are reunited. What is the condition for the ...
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### Is it possible to sky dive without a parachute and land safely?

Let's assume an averaged sized man (1.8 meters height 80 kg) who's sky-diving from a 5000 m height. Let's also assume he's using tight clothes and no parachute. The idea is: Is it possible for him ...
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### Spoon released out of a satelite moving with orbital velocity

Suppose we are in a satelite revolving around the earth with a velocity equal to orbital velocity. Now if we release a spoon from the satelite then will it reach the earth or will it just revolve ...
205 views

### Understanding voltage

I understand that water flows because of height difference and current flows because of potential dfference. Now, in this analogy, I can understand height but I fail to realise what potential is. Is ...
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### Frequency of Light Near a Massive Object

We know that near a massive object, light's wavelength and frequency changes, but its speed remains same. I'd like to know how it is possible for frequency to change, unless we change the light source?...
1k views

### To which state of matter does the flame belong to?

I had this question from the day(9 years old, now 16) that i learned about states of matter. I have asked so many of my teachers some of them told me gas some as plasma etc. can anyone answer my ...
### The force exerted on a magnetic dipole $m$ in magnetic field $B$
How can one prove that: The force exerted on a magnetic dipole $m$ in magnetic field $B$, in addition to $F=\nabla(m\cdot B)$, can be expressed by $$F=(m\times \nabla)\times B.$$