# All Questions

127 views

### How fast does space/time travel?

It is said that before the big bang there was no before, as space/time was created in the big bang. The big bang creates a rapid expansion of space/time... so how fast is rapid?
232 views

### Gauss' law in differential form and electric fields

I know Gauss' divergence theorem, according to which $$\iiint_D\nabla\cdot\boldsymbol{F}\text{d}x\text{d}y\text{d}z=\iint_{\partial D}\boldsymbol{F}\cdot\boldsymbol{N}_e\text{d}\sigma$$ where $D$ is a ...
40 views

### What is the equation of motion for multiple simultaneous pressure waves in a medium? (In the context of stimulated Brillouin scattering)

My overall motivation is to derive the behavior of Brillouin scattering in a birefringent fiber. Brillouin scattering is a nonlinear interaction between light and sound. In classic Brillouin ...
Knowing that, in the $\mu$-canonical (or micro-canonical) and canonical ensembles, the number of particles is held constant and usually reflects the actual number of particles, in which case $N_{MC} ... 0answers 53 views ### Inuitive analogy for localization? I'm looking for a plain English analogy for electron/wave localization. And in particular weak localization and Anderson/strong localization. Is it possible to describe these phenomena in simple terms ... 2answers 98 views ### See the opposite wall on a mirror [closed] I'm in tenth standard. This is a higher-order-thinking-skills Q I found in a book. One is supposed to use laws of reflection ($\angle i = \angle r$). You can also use mathematical concepts like ... 0answers 100 views ### Bogoliubov de Gennes Hamiltonian, Expansion of electron operator Suppose we are given a particle-hole symmetric Hamiltonian H. This type of Hamiltonian is usually diagonalized by a Bogoliubov transformation. In this context in the literature the local electron ... 1answer 139 views ### Boundary conditions in holomorphic path integral Consider the holomorphic representation of the path integral (for a single degree of freedom): $$U(a^{*}, a, t'', t') = \int e^{\alpha^{*}(t'') \alpha(t'')} \exp\left\{\intop_{t'}^{t''} dt \left( ... 2answers 189 views ### Does increasing pressure, increase gravity? This question might be a little misguided, given that I have never studied GR or quantum field theory, but... As I understand it, dark energy is caused by negative pressure in the gravity field. ... 4answers 87 views ### the relation between frequancy and energy of EM waves In quantum theory,$$E=\hbar w$$In classical theory, we have the Poynting vector:$$\vec{S_\space}=\frac{1}{\mu_0}\vec{E_0}\times\vec{B_0}{\cos}^2(kr-wt)$$given S is energy flux density (the ... 2answers 89 views ### Velocity of photon when changing direction [duplicate] We know the speed of a photon is constant. Yet when any other object changes direction, its velocity must go to zero. Thus, my question is how does a photon avoid this? I assume that to change ... 0answers 167 views ### Scattering, Perturbation and asymptotic states in LSZ reduction formula I was following Schwarz's book on quantum field theory. There he defines the asymptotic momentum eigenstates |i\rangle\equiv |k_1 k_2\rangle and |f\rangle\equiv |k_3 k_4\rangle in the S-matrix ... 1answer 48 views ### Capacitors, Formula for Energy [closed] I had a quick question; As you know there are three formulas for calculating the Potential Energy of a Capacitor U=Potential Energy Q=Charge C=Capacitance V=Voltage/Potential Difference 1) ... 2answers 179 views ### How to calculate force / torque on non-flat lever, i.e. dolly See attached image. The mass is being rotated on a lever where the pivot point (P) is a certain distance (L_2) from the right angle at the bottom. How do I calculate the force necessary to apply ... 0answers 52 views ### Derivation of non-local conserved charges Consider a 2D sigma model with a symmetry group G and whose generators obey [T_A, T_B] = f^C_{AB} T_C and whose conserved currents are Lie algebra-valued, i.e. j_{\mu} = j_{\mu}^A T_A and ... 2answers 66 views ### Gravitational Lensing by Sagittarius A in the center of the Milky Way It was suggested that there is a black hole in the center of the Milky Way, called Sagittarius A. The whole region is heavily obscured by dust, although some very long wavelengths can break through. ... 1answer 55 views ### When can a Hilbert space with a given Hamiltonian be decomposed into non-interacting tensor product factors? Let's say I have a Hilbert space \mathcal{H} (either finite-dimensional or with a countably infinite basis) with a specified Hamiltonian \hat{H}, representing some quantum system. Under what ... 2answers 223 views ### Gauss Law - Infinite line charge I came across a sentence which made no sense to me as I was studying the electric field of an infinite line charge: The flux through the FLAT ENDS of our cylindrical Gaussian surface is zero ... 1answer 100 views ### Yang and Mills' (and others') justification for local gauge invariance In most physics textbooks, local gauge invariance is simply postulated---you start with a global symmetry, e.g. the global phase, then allow it to depend on the spacetime point, make the necessary ... 0answers 65 views ### QFT: What does “finiteness” mean? As above: what is the definition of a QFT to be "finite"? That all UV corrections are finite and there are no divergences at all? That there are divergences, but these divergences can be absorbed ... 1answer 174 views ### Synthesis of Fe-based superconductors Polycrystal cuprate superconductors are generally prepared by solid state reactions: Starting reagents are in powder form, they are mixed to each other and placed into furnace on high temperature ... 3answers 768 views ### What does it mean to “bin” in a spectroscopy context In the following online article http://www.star.le.ac.uk/~sav2/stats/a.html I see the word "bin" used, in relation to x-ray spectroscopy, both as a verb and as a noun (people both "bin" things and ... 1answer 48 views ### What laws of physics apply when a flow of air is divided? The question is rather simple, but I'm not sure if there is a simple answer to it. Let`s say I've got a flow of air (5 bar, 25 l/min) running through a 6mm hose. Then this flow hits a branch dividing ... 3answers 174 views ### Are there any well-known large body objects moving at a significant fraction of the speed of light relative to earth? Introduction I've been reading a book called PsychoPhysics, part of the "Illuminati God Series" by Mike Hockney (a pseudonym). Without getting into too much detail, they basically believe that the ... 2answers 144 views ### How to prove \mathrm{Tr}\left[|\alpha\rangle\langle\alpha|\hat{A}\right]=\langle\alpha|\hat{A}|\alpha\rangle For a coherent state |\alpha\rangle=e^{-\frac{|\alpha|^2}{2}}\sum\frac{\alpha^n}{\sqrt{n!}}|n\rangle, please show me how to prove,$$ ... 1answer 66 views ### Why does the stiffness of organic polymers (plastic) change so much with small changes in temperature? This is on the borderline between Physics and Chemistry, but I would like a Physics perspective. I am guessing that plastics are a glass-like phase, rather than a true solid. 2answers 83 views ### How energy of a photon or EM wave has something to do with frequency? While I was thinking what exactly meant by energy in quantum mechanics, I was quite shocked by the fact that it is proportional to frequency. $$E=hf$$ Given the fundamental definition of energy in ... 1answer 37 views ### Why is metal stiffer at lower temperatures? Each morning I cycle to school and lock my bike with a thick steel wire (about 8 mm thick). I noticed that it's much harder to change the shape of the wire in the morning when it's much colder than ... 3answers 188 views ### Angular velocity of a fluid element If we have a fluid element that is subjected to: Translation Rotation Extensional strain (dilatation) Shear strain As in this picture starting from time$t$it can be shown that angles ... 2answers 46 views ### Is there a formulation for (self-)accelerating fluid flow through permeable medium? I have a permeable system where is an accelerating fluid flow. Imagine a sponge that is squeezed. The fluid starts at rest, accelerates and flows out from the sponge. How to calculate the fluid speed? ... 0answers 82 views ### Numerical Calculation of the Second Harmonic Generation in the Nonlinear Media Recently I am working on the Numerical Calculation of the Second Harmonic Generation in the Nonlinear Media. I refer to the Book ... 3answers 299 views ### Sign of torque when rolling an object down an incline Suppose you have an object rolling down the incline at 30 degrees. Given the point of contact is instantaneously at rest, I decided to analyse torques at that point. Therefore, the only force ... 1answer 80 views ### When to use a whole vector approach vs an energy approach? My professor has just introduced two new ways to solve projectile motions. One approach involve using trigonometry and vectors and the other involves using the idea of conservation of energy. My ... 2answers 35 views ### Filament lamp$(V,i)$behavior between 0 and 1V I was wondering why the behavior is more "chaotic" at 0~1V when compared to 2V+ (which is when it starts to glow). 0answers 30 views ### Approximation for rotational drag on a box? The drag equation gives us an approximation for the drag on an object caused by its velocity in a fluid:$F = \frac{1}{2} \rho A v^2 C_d$Is there any similar approximation for the rotational drag ... 2answers 76 views ### Configuration manifold of a rigid body As I know, a rigid body is a set of$N$particles in three-dimensional space subject to the following constraint: if$b_1,\dots,b_N\in \mathbb{R}^3$are the initial positions of the particles and if ... 2answers 50 views ### Stored Energy in My Badge Reel Finds Equilibrium With Centrifugal Force? Probably a most horrible question title, please respond with suggestions for a better one. In the example of my badge reel, I grab hold of my badge itself and pull on the reel until the string is ... 3answers 147 views ### How do shock waves form in space? Today my aerodynamics professor mentioned that the equations we were learning for atmospheric shock waves can also be applied by astrophysicists to study black hole-related shock waves in space. How ... 1answer 52 views ### How much time would have to pass for a blackbody object at 0.1 K to emit a single infrared photon? [closed] My understanding is that blackbody radiation occurs on a curve that depends on the temperature of the object - as the temperature increases, the area under the curve increases, and the peak of the ... 3answers 97 views ### Equal and opposite reaction at light speed My knowledge of physics is very limited. And this question is only really to help me put to rest a question i have had for some time. I would like the answer to be a simple one as i am not really in ... 2answers 83 views ### Is it possible to mix a drink with a non-standard phase of ice? Would it be possible to safely cool and drink a glass of water with anything else than the Ih form of ice? Here and here you can see that some alternative forms of ice have a higher density than ... 1answer 68 views ### State of academic fields of analytic mechanics and Multi Body Dynamics (MBD) My question essentially contains two parts: one practical question and one philosophical question. Background I am an engineering student working in the field of Multi Body Dynamics (MBD). My ... 2answers 2k views ### Why do voltmeters and ammeters have high and low resistance respectively? I understand why voltmeters are connected in parallel and ammeters are connected in series, but why is it that to measure voltage, you must have high resistance, and to measure current, you must have ... 0answers 66 views ### Funnel theory exists? I have a funnel and I notice when I try to pour powdered sugar through it, it blocks up unless I tap it. If I put sand in the funnel, it goes through. Is there a theory of funnels that can predict ... 0answers 67 views ### Correspondence between variational method and Feynman diagrams There are two ways to look at the Hartree or Hartree-Fock equations. One method relies on the variational method for a particular type of the probe function and the second one originates from ... 2answers 86 views ### cm^-1 and ns^-1 relationship In my last Phys.SE question Emilio Pisanty mentioned this relationship$1 cm^{-1} = 10^n c^{-1} ns^{-1}$. I was wondering where this relationship came from. Does anybody know? Second of all, I am not ... 2answers 148 views ### Rabi problem for a spin-1 particle I'm trying to solve, analitically, the transition probability of a spin-1 particle in a magnetic field $$\vec{B}=B_0\hat{k}+b(\cos{\omega t}\hat{i}+sin{\omega t}\hat{j}).$$ In particular I want to ... 3answers 252 views ### Perturbative series for bosons I have recently read that ... the perturbation series ... is valid only when the perturbed state is qualitatively similar to (or ‘has the same symmetry as’) the unperturbed state. This means ... 0answers 63 views ### semiconductors extrinsic and intrinsic carriers concentrations equations I get why$n \cdot p = n_i^2$in intrinsic semiconductors, but why is that equation also true in the case of extrinsic semiconductor? Here,$n\$ is the charge density of the negatively charged ...
If I'm not mistaken, the state equation $$PV=nRT$$ also known as the 'ideal gas law' serves as a mathematical model relating pressure-volume-mass-temperature states of an assumed ideal gas. In other ...