# All Questions

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### Repulsive component of intermolecular interaction

Intermolecular interaction mainly consists of 2 components: (a) Dipole vs dipole (permanent & induced), which is more likely to be attractive (b) Pauli exclusion, which is always repulsive The ...
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### Is rigid body rotation an integrable problem?

Is there chaos? The configuration space is 3d. There are four constraints, namely, the energy, the three components of the angular momentum. So there are still two degrees of freedom.
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### Relativistic probability amplitude of a particle to be in certain position

In the book “The story of spin” by Tomonaga on page 110, it says They insisted that a concept like "the probability of a particle to be at $x$ in space" is meaningless for relativistic particles—...
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### Compactly generated vs. compactly constructed causality violating region?

I am currently trying to grasp the nuance between a compactly generated future Cauchy horizon (as per Hawking's chronological protection conjecture) and a compactly constructed causality violating ...
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### Electic potential due to finite rectangular plate

I am trying to find the potential at any point (x,y,z) due to a rectangular plate with a constant surface charge density. Let's assume the plate is centered on the X-Y plane and extends from -n to n ...
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### Thermodynamics with explicit math

Nathaniel adeptly answers a question on thermodynamics by making the variable dependencies explicit: Determine the Dependence of S (Entropy) on V and T Is there a thermodynamics text that is written ...
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### How does fluid velocity affect the dissolution of some solute in the fluid?

Suppose we have a perfect sphere of some solute such as sugar and we place it in a fluid such as water, at a certain temperature, that is not moving. It will dissolve and diffuse into the water due to ...
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### If charge is carried by ions, is there a limit to the amount of charge that salt water can conduct? [duplicate]

The reason salt water is so conductive is dissolved ions (e.g. $\mathrm{Cl^-}$ and $\mathrm{Na^+}$). If I put an anode and a cathode in water, will ions eventually build up and create an opposing ...
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### Gas Cooled Fast Reactor Coolants

I know that helium and CO2 are used as fast reactor coolants, but how about other inert gasses with low neutron absorption, like argon, why do I see no papers on that? Oh and bonus question. Why is ...
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### Single story, hotter outside, some shade from porch, enough to cool house with/without air flowing?

So, I know this question, without shade or one-story, has been answered many times on the 'net, so I have some actual physics for the one-story. I am fairly certain my roommate mixed up thermal ...
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### What is crossover?

It is known that EW and QCD phase transitions in SM are so-called "crossovers". What is the difference between crossover and phase transition of the second kind?
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### Instantons and Fivebranes

What is the general relationship between instantons and fivebranes? In the paper Magnetic Monopoles in String Theory'' by Gauntlett, Harvey and Liu, the authors state the fivebrane ansatz of ...
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### Excitation spectrum of heisenberg model

I understand that ferromagnetic Heisenberg model (lattice of spin variables that can point in any direction) spectrum can be deduced by a $\lambda\phi^4$ theory with $\phi$ being complex. This model ...
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### EMF and Distance

As a professional underground service locator, EMF and Inductance are at the very core of what we do. Very few of us truly understand the detailed physics behind what makes it possible for us to ...
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### Supersymmetric transformation of general Wess-Zumino Lagrangian

I suspect that I might have understood something wrong here. I'm trying to show that the general Wess-Zumino Lagrangian \begin{align} \mathcal{L} &= \int d^2\theta d^2\bar{\theta} K(\Phi^*, \Phi) +...
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### How does a magnet cause magnetic force and its magnitude/direction?

Magnetic fields are created due to electron spin in the magnet. But how exactly does an electron "spinning" create forces around the magnet? Also, the magnetic force on a charge moving in the magnetic ...
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I have some particular questions about the nature of light...cause all I know from all sources is that it is a wave and a particle and stuffs, but if light "COMES" from all directions, and is in every ...
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### Why do practical fusion reactions need more than one product?

The Wikipedia article about nuclear fusion says that To be a useful energy source, a fusion reaction must ... have two or more products: This allows simultaneous conservation of energy and ...
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### What are the longest half-lives we can detect experimentally? What stops us going further? Are we trying to?

Xenon 136, apparently, has a half-life of 2.11×1021 years. This strikes me as a humongously long time to run an experiment, clocking in at about 11 orders of magnitude longer than the age of the ...
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### Can i use the $y^*$ representation of the chiral covariant derivatives on a superfield that contains both $y$ and $y^*$?

Imagine I want to compute this $$D^{\dagger2}D_{\alpha}(\Phi^*\Phi)$$ where the $D$-s are super-covariant derivatives and $\Phi$ is a chiral superfield. Following the notation of this review on ...
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### About the rotating speed of a light beam which has been reflected off a rotating mirror [closed]

A light beam generated from a source reaches a rotating mirror (x m away from the source), and is reflected off to a fixed mirror ( x m from the rotating mirror ), and again back to the rotating ...
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### How is it possible to combine various techniques in cold atom experiments?

I’ve been reading about laser-trapped cold atoms (6Li in particular, which is a fermion) and was amazed at the number of things to keep track of in the experiments, just to gain that degree of control ...
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### Layer of graphene on reflector of an antenna

First of all I have no idea how well graphene "sticks" to other metals, but let's suppose it does (well, if I may please ask you to reply to the "stick" question too...). Given the material great ...
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### How can all quantum measurement statistics be seen just as projective measurements on pure states?

Let $\rho$ be the density matrix for a system and let the POVMs be $\{E_m\}$ such that $\sum_i {E_m} = I$. The probability of getting the outcome $m$ is $\operatorname{Tr}(E_m \rho)$. The source I ...
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### Is there any evidence from observational cosmology to say Dark Energy dominated era begins 5 billion years ago

People say that Dark Energy Dominated era begin 5 billion years ago. Do we have evidence for that from observations
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### Normal of a null surface and null junction conditions in general relativity

I am trying to use the null junction formalism in general relativity (as explained in eg http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.43.3763&rep=rep1&type=pdf, "Junctions and thin ...
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### Determine the necessary radii of a double equiconvex lens, such that BFL=0

Sounds like a simple problem, but I have no training in optics: I need to work out the optimal radius to use for all four surfaces of a double equiconvex lens, so that a point light source close to ...
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### Why are most antennas in cellular networks +/- 45° polarized?

I've just been asked a strange question that I cannot find an answer to (even on the internet it seems I can't find any explanation for this) and I ended up wondering why most of the antennas which ...
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### Pentaquark spin prediction

Is there a straightforward way to see what the spin of the recently-discovered pentaquark states should be, from the representation theory of $SU(3)\times SU(2)\subset SU(6)$? I can see that from the ...
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### Does center of mass affect how an object falls?

Suppose you drop an object which has two ends, of which one is heavy and the other is pretty light. Will the object fall with its heavier end downward or with the lighter one? Why does it happen?
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### A way to determine if a body accelerates or loses speed at a certain time

With given vectors for acceleration and velocity, is there a way to determine if a body accelerates or decelerates at a certain time-interval? Can this be determined, for instance, by simply observing ...
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### How to describe frequency spectrum with samples?

I posted this same question in math, but nobody is answering, so I thought I'd give a try here too... To me the question is pretty straightforward, but nevertheless I have to explain its background ...