dialog listlengths 4 30 | title stringlengths 15 150 | category stringclasses 14 values | dialog_time stringdate 2016-01-21 01:59:30 2018-07-01 01:34:46 |
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[
{
"message": "The phase trajectory of a one-dimensional simple harmonic oscillator is a closed one (In particular, it's an ellipse). Is closed phase trajectory a generic feature of any periodic motion at least in one-dimension? If it is, is there a simple proof of it?",
"role": "user"
},
{
"message"... | Is closed phase trajectory a necessary feature of any one-dimensional periodic motion? | physics | 2017-08-28T15:43:55.893 |
[
{
"message": "I have kind of a dumb question: what would happen if the color gauge group is $SO(3)$ instead of $SU(3)$, assuming there are still 3 colors and physical states are still color singlets? Will we e.g. get a different zoo of hadrons?",
"role": "user"
},
{
"message": "My sense is that you ... | SU(3) vs SO(3) color gauge | physics | 2017-08-28T20:30:03.483 |
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{
"message": "Prove that $L_i$ ; i = 1, 2, 3 of the ang. momentum operator are related to the Cartesiancomponents of the position and mometum operators by $L_i = \\epsilon_{ijk} x_ j p_ k $ (summation convention: sum over repeated indices).I got to the part following <a href=\"http://www2.ph.ed.ac.uk/~ldeldebb/... | Prove that $L_i = \epsilon_{ijk} x_ j p_ k $ | physics | 2017-08-28T21:40:54.823 |
[
{
"message": "I am studying this paper: <a href=\"https://journals.aps.org/prb/abstract/10.1103/PhysRevB.74.024408\" rel=\"nofollow noreferrer\">Physical Review B 74, 024408 (2006) (<a href=\"https://arxiv.org/abs/cond-mat/0512142\" rel=\"nofollow noreferrer\">arxiv) <blockquote> ABSTRACT We derive a multi-ba... | Units related to chemical potential and orbital magnetization | physics | 2017-08-29T08:17:52.587 |
[
{
"message": "So this problem actually comes from The Classical Electromagnetic Field by Leonard Eyges and says:<blockquote> Find the distribution of charge giving rise to an electric field whose potential is $\\Phi(x,y) = 2(\\tan^{-1}(\\frac{1+x}{y}) + \\tan^{-1}(\\frac{1-x}{y})),$ where x and y are Cartesian... | Calculate the charge distribution given the potential $\Phi(x,y) = 2(\tan^{-1}(\frac{1+x}{y}) + \tan^{-1}(\frac{1-x}{y}))$ | physics | 2017-08-30T00:53:15.313 |
[
{
"message": "At the event horizon of a black hole, time and the spatial direction toward the center exchange places. The direction inside the black hole from the event horizon to the the singularity in the center is the direction in time.Assume a symmetrical non-rotating black hole and also assume that things ... | Time paradox inside a black hole | physics | 2017-08-30T03:37:39.487 |
[
{
"message": "I heard sometimes that the pressure inside separated flow wakes are relatively low. For example, if we place a plate perpendicular to the fluid flow, we have large vortex and wakes behind the plate. The pressure behind the object is usually considered low (like as a suction) while its front pressu... | Fluid pressure within separated area | physics | 2017-08-30T11:20:32.353 |
[
{
"message": "I have a lot of experience with classical scattering theory (acoustic and electromagnetic waves), but I have no experience with quantum scattering theory, so I am trying to understand the similarities between the two fields as I start looking into quantum scattering.Firstly, I don't understand the... | Understanding quantum scattering coming from a classical scattering background? | physics | 2017-08-30T11:55:57.050 |
[
{
"message": "I've been trying to figure out what scientists believe to have been infinitely condensed in initial singularity. It is claimed that all mass existed before the big bang, this leads me to many questions to which I can't find the answer, with all of them relating to one big question: What is the mos... | What was (physically) present within the mass of initial singularity? | physics | 2017-08-30T12:45:08.453 |
[
{
"message": "The force between two atoms takes the generic shape of a Lennard-Jones potential. It has an attractive part caused by dipole-dipole attraction and a short-range repulsive part which is said to be caused by Pauli's exclusion principle. Can some one explain rigorously how the repulsive part of the p... | The Lennard-Jones potential and the Pauli exclusion principle | physics | 2017-08-30T16:06:59.417 |
[
{
"message": "I understand that the object should continue to move in a straight line (relative to the earth) and therefore should move towards the outside of the turn, but will it be pushed to the front or the back of the vehicle? Can the vehicle be considered a frame of reference to apply newtons laws?",
... | How will an object inside a vehicle move (relative to the vehicle) while the vehicle is making a turn at a constant speed? | physics | 2017-08-30T17:44:35.330 |
[
{
"message": "While reading about the Källén-Lehmann representation I came across the definition of eigenstates in general QFT. As $\\vec{p}$ (total momentum) and $H$ commute they can be simultaneously diagonalized, thus one obtains:$$ H | {\\lambda_{\\vec{p}}} \\rangle = E_{\\vec{p}} |{\\lambda_{\\vec{p}}} \\r... | Are there any non-physical eigenstates of interacting QFT? | physics | 2017-08-31T10:25:28.370 |
[
{
"message": "BackgroundKepler's second law says that planets sweep equal areas in equal times in their orbits; the closer a planet is to its parent body, the faster it moves; the farther it is, the slower it moves. This is explained by the conservation of angular momentum, which is $m * v * r$. $m$ is the plan... | Why doesn't the conservation of angular momentum modify mass instead of velocity? | physics | 2017-08-31T18:56:48.730 |
[
{
"message": "The Higgs Field and Higgs Boson are theorized to be responsible for mass, or $m$. If these could be manipulated, altered, or controlled for, if the Higgs Field could somehow be parted, would this result in $m=0$ in all physics equations? Since the units of $c$ are just time and distance, would $c$... | If the Higgs Boson and Higgs Field could be manipulated, could that potentially make $m=0$ in all physics equations? | physics | 2017-08-31T19:38:47.297 |
[
{
"message": "<blockquote> A rigid body is formed by two masses $m_1$ and $m_2$ ($m_1$ is not equal to $m_2$) joined by a rod of negligible mass and length $4L$. The system is initially at rest on a smooth horizontal surface. If a horizontal and perpendicular force is applied to the middle of the rod, then it ... | Rigid body dynamics: Ideal rod with 2 masses | physics | 2017-08-31T20:09:22.317 |
[
{
"message": "I'm no cosmotolicist, so forgive me if I'm completely off the mark here.<br>A few years back, I read that there is not enough matter in the universe to account for the gravitational forces at play, keeping the stars clumped together as galaxies. Then the other day, I saw a show that I'd seen befor... | Is it possible for the Oort cloud to account for the Dark Matter issue? | physics | 2017-09-01T05:48:49.550 |
[
{
"message": "Suppose a force pushes/pulls Earth straightly toward Sun and make earth x kilometers closer/farther from Sun. For what x the Earth remains in a stable orbit, rather than spirally fall to the Sun or going farther away? What math is involved?",
"role": "user"
},
{
"message": "Are we cons... | What is the stable range for orbit of the Earth? | physics | 2017-09-01T08:45:35.657 |
[
{
"message": "Suppose a force pushes/pulls Earth straightly toward Sun and make earth x kilometers closer/farther from Sun. For what x the Earth remains in a stable orbit, rather than spirally fall to the Sun or going farther away? What math is involved?",
"role": "user"
},
{
"message": "For the Sun... | What is the stable range for orbit of the Earth? | physics | 2017-09-01T08:45:35.657 |
[
{
"message": "When I encounter the parity transformation in physics,I feel often it's not really clear what we are doing and I want to understand it in a more rigorous way, can you help me with that?(I want to see it from the point of view of tensors.)I always hear that the electric field changes it's sign, oft... | What exactly is the Parity transformation? Parity in spherical coordinates | physics | 2017-09-01T15:31:42.160 |
[
{
"message": "Considering this classical example:<a href=\"https://i.stack.imgur.com/jDZ5V.jpg\" rel=\"nofollow noreferrer\">The magnetic flux($\\phi$) is increasing over some time(increase in $B$), and it's simple to indicate the direction of the induced current using Lenz law.However, the magnetic field is on... | Varying magnetic field, and a larger area? | physics | 2017-09-01T19:09:58.030 |
[
{
"message": "I was wondering if there is such a thing as electromagnetic induction for non closed loops. As far as I've been taught, Faraday's law applies only to closed loops, which makes me wonder how come an inductor (which is obviously not a closed loop) or a circuit with a capacitor (not really a closed l... | Induction in non closed loops | physics | 2017-09-01T19:45:36.483 |
[
{
"message": "Consider the Chiral Perturbation Lagrangian to first order(quark masses set to zero):$$L = L^{(2)} = \\frac{f^2}{4}tr[\\partial_{\\mu} U \\partial^{\\mu} U^{\\dagger}] ,$$ where U is a $2 \\times 2 $ exponential matrix collecting the pion-goldstone fields and the terms of the Lagrangian come from... | Scattering amplitude calculation using first order Chiral Perturbation Theory Lagrangian | physics | 2017-09-02T09:09:35.127 |
[
{
"message": "I was looking up how Newton derived his Law of Gravitation and came across <a href=\"http://www.relativitycalculator.com/Newton_Universal_Gravity_Law.shtml\" rel=\"nofollow noreferrer\">this website which derives the law from Kepler's laws.It starts off with Kepler's second law and says that:$$\\... | Why is $\vec{v}$ constant according to Kepler's laws? | physics | 2017-09-02T11:02:13.040 |
[
{
"message": "I'm working on a 3D space game with no gravity but multiple engines at the spaceship. My problem is collecting the different rotations and put them together.For example, this is the prototype of my spaceship:<a href=\"https://i.stack.imgur.com/reT4x.png\" rel=\"nofollow noreferrer\">If I put force... | multiple 3D rotations on one object | physics | 2017-09-02T18:20:42.480 |
[
{
"message": "For example, I am trying to calculate the number of 800lm light bulbs required to light a 16sq meter bed room with a 2.5m celling that doubles as a study. Assuming 75 lux is required, this converts to 1200 lumens. Below are what I have calculated so far:Let x be the number of light bulbs required:... | Does the lumen value of a light bulb take into account the inverse square law and volume of a room? | physics | 2017-09-03T01:57:59.637 |
[
{
"message": "I've got the same problem while trying to understand air cooling and heating.My textbook shows a picture like the following:<a href=\"https://i.stack.imgur.com/YkBtH.jpg\" rel=\"nofollow noreferrer\">In this Mollier chart we s that for air cooling we have too cool down the temperature, then dehumi... | Explanation of cooling and dehumidifcation process on psychometric chart | physics | 2017-09-03T17:34:42.990 |
[
{
"message": "I am was reading some answers on does <a href=\"http://There%20are%20some%20ways%20to%20store%20or%20accumulate%20static%20electric%20charge.%20A%20https://en.wikipedia.org/wiki/Va...%20is%20a%20device%20that%20accumulates%20electric%20charge.%20This%20can%20actually%20store%20a%20net%20charge.%20... | does a van de graff generator storea charge | physics | 2017-09-03T20:00:07.470 |
[
{
"message": "Computing the thermodynamic entropy in a macro way (integrating the Clausius entropy from 0 K to 298.15 K) yields the value 3.9 J/g,K. This can also be found from tables on entropy for various substances.My question now is the following: Is it ok to convert this value to Shannon information bits b... | How many (Shannon) bits are there in one gram of 25° water? | physics | 2017-09-03T23:51:52.643 |
[
{
"message": "I am bit confused with process \" $q \\bar{q}\\rightarrow g$\" and \"$q \\bar{q}\\rightarrow Z^0$\". I guess at high energies of LHC, weak interaction is more probable and hence \"$q \\bar{q}\\rightarrow Z^0$\" is more probable. But I guess I am wrong with this. Also, is $ q \\bar{q}$ annihilatio... | Which process(interaction) is more probable? | physics | 2017-09-04T15:44:46.000 |
[
{
"message": "There are some confusions regarding infrared (IR) divergences in gauge theory:What is the primary reason for the appearance of IR divergences in gauge theory? Anything other than the existence of massless particles in the spectrum of the theory. Why there is no IR divergence for massive theories? ... | Infrared Divergences in Gauge Theories | physics | 2017-09-04T23:46:38.217 |
[
{
"message": "I tried to find detailed data at the <a href=\"https://www.xfel.eu/facility/overview/facts_amp_figures/index_eng.html\" rel=\"nofollow noreferrer\">official site and learned that the linear accelerator is 1.7 Km long, the energy of the electrons is 17.5 GeV (35000$\\times$ the electron rest energy... | The physics behind European XFEL | physics | 2017-09-05T06:27:20.620 |
[
{
"message": "At 1 atm and room temperature, is water vapor in a metastable state? As I understand it, water vapor is water in a gaseous state when the stable phase of water under this temperature and pressure is liquid, not gaseous. I understand that water vapor can be the result of evaporation, where water mo... | Is water vapor in a metastable state at 1 atm and at room temperature? | physics | 2017-09-05T20:51:06.053 |
[
{
"message": "At 1 atm and room temperature, is water vapor in a metastable state? As I understand it, water vapor is water in a gaseous state when the stable phase of water under this temperature and pressure is liquid, not gaseous. I understand that water vapor can be the result of evaporation, where water mo... | Is water vapor in a metastable state at 1 atm and at room temperature? | physics | 2017-09-05T20:51:06.053 |
[
{
"message": "I want to do electrostatic shielding but I am not sure if blocking only the line of sight is enough.Imagine that there is an electrically charged sphere, $1 cm$ in diameter, just sitting stationary in free space. Some distance away, for example, $3cm$ is another sphere with neutral charge that I w... | Does electrostatic field "go" around obstacles? | physics | 2017-09-06T01:18:12.263 |
[
{
"message": "I want to know if there exists some mathematical limits to what extent we can perform magnification. Like we can now build devices to see some objects as small as 1 micron(or even smaller than that). Now I want know if we can build a device which can magnify a particle having size equal to electro... | Is there any limit to extent of magnification? | physics | 2017-09-06T04:56:23.523 |
[
{
"message": "Let's suppose we have two particles described by \\begin{equation}|\\psi\\rangle_1 = \\begin{pmatrix} a \\\\ b \\end{pmatrix}_1 \\text{ and } |\\psi\\rangle_2 = \\begin{pmatrix} \\alpha \\\\ \\beta \\end{pmatrix}_2\\end{equation}If the particles don't interact the product state $|\\psi\\rangle$ ... | Product state of two particles | physics | 2017-09-06T15:00:17.153 |
[
{
"message": "Recently I entertained a silly thought about if the earth's rotation can be \"harnessed\" to do work. So my question is:We know that earths rotation is eastward.If i set up 2 mirrors on 2 different points on earth; one \"east\" and the other \"west\" several thousand miles apart. Then, in the cent... | Does light travel faster if fired in the direction of Earth's rotation as opposed to against it? | physics | 2017-09-06T15:23:40.353 |
[
{
"message": "As we know from junior high school physics, when an object is at twice of focal length f, it could form a same size real image. When an object is placed far from the lens, it would form a smaller image. But why, when we taking pictures using a phone, do we need to make something in focus? Does thi... | Why does and object need to be in focus when taking a picture? | physics | 2017-09-06T21:05:07.150 |
[
{
"message": "I'm really confused about Lorentz transformations at the moment. In most books on QFT, Special Relativity or Electrodynamics, people talk about Lorentz transformations as some kind of special coordinate transformation that leaves the metric invariant and then they define what they call the Lorentz... | Lorentz Transformations Vs Coordinate Transformations | physics | 2017-09-07T22:02:28.110 |
[
{
"message": "Given a vector field of the form $\\textbf{B}=\\nabla\\times\\textbf{A}$, can we uniquely find out the direction of $\\textbf{A}$?",
"role": "user"
},
{
"message": "No.If you find any specific $\\mathbf A_0$ such that $\\nabla \\times \\mathbf A_0$ matches your given $\\mathbf B$, then... | Can we find out the direction of $\textbf{A}$ from $\nabla\times\textbf{A}$? | physics | 2017-09-08T07:37:47.630 |
[
{
"message": "The curvature of spacetime has been observed many times from the deflection of light around massive astronomical objects. But has it been observed around small objects in a lab?In the <a href=\"https://en.wikipedia.org/wiki/Cavendish_experiment\" rel=\"noreferrer\">Cavendish experiment, the gravit... | Has the curvature of spacetime been measured at the human scale? | physics | 2017-09-08T15:55:01.060 |
[
{
"message": "This is an example problem from my E&M textbook that I don't quite understand: <blockquote> A metal sphere of radius a carries a charge $Q$. It is surrounded, out to radius b, by linear dielectric material of permittivity $\\epsilon$. Find the potential at the centre (relative to infinity).</... | Determining electric displacement using Gauss's law | physics | 2017-09-08T17:49:53.893 |
[
{
"message": "In the standard picture of gravitational lensing, we have a picture like the one you can see as Figure 1 of <a href=\"https://arxiv.org/abs/0807.3872\" rel=\"nofollow noreferrer\">this paper (I don't know if I can copy/paste it). In it, the source S and the observer O are both at a finite distance... | In gravitational lensing, if the source is infinitely far away how can we have an image on the same side of the lens? | physics | 2017-09-08T18:15:54.973 |
[
{
"message": "Entropy always increases or remains same.But what happens in sedimentation?The two mixtures separate out i.e disorder decreases.This violates the second law of thermodynamics. What all points am i missing?",
"role": "user"
},
{
"message": "You are conflating two different notions of 'e... | Doesn't sedimentation violate the second law of thermodynamics? | physics | 2017-09-09T05:39:37.410 |
[
{
"message": "So this Youtube video has been around for a while now <a href=\"https://youtu.be/mNHp8iyyIjo\" rel=\"nofollow noreferrer\">Veritasium Hydrodynamic levitation! .Also this <a href=\"https://physics.stackexchange.com/questions/354599/fluid-mechanics-explanation/354608#354608\">Fluid Mechanics explana... | Any solutions to Veritasium's hydrodynamic levitation? | physics | 2017-09-09T08:25:26.437 |
[
{
"message": "Suppose a long rigid pipe (e.g., 30m) open at both ends is completely submerged into some deep body of water. The pipe gets completely filled with water. Then, one end gets closed and the pipe is slowly vertically lifted out of the water, with the closed end on top.At some point, the open end stay... | Lifting submerged straw above 10m | physics | 2017-09-10T01:34:54.867 |
[
{
"message": "My textbook says two different things and I'm not sure how to reconcile these two:electric field inside a conductor is always 0.for a conductor with a cavity with a charge q inside it, the field due to the charge q for all exterior points is cancelled by the induced charge on the inner surface.The... | Electric field inside a conductor and induced charges | physics | 2017-09-10T10:05:59.310 |
[
{
"message": "Context : I'm a 3D \"artist\", I do photorealistic renders with <a href=\"https://www.blender.org/features/cycles/\" rel=\"nofollow noreferrer\">Blender Cycles path tracing.<br>I try to understand more in deep about albedo and fresnel effect.<br>Even if I'm interested in science and more specific... | In optics, does the albedo encompass the Fresnel effect? | physics | 2017-09-10T17:03:55.203 |
[
{
"message": "Let me be sort of specific. We consider a Weyl particles on $S^{1}$, with the following Hamiltonian$$\\mathcal{H}=v\\int_{0}^{2\\pi}\\psi^{\\dagger}(x)(-i\\partial_{x})\\psi(x)dx$$Such particles have the property that $$\\rho(x)=\\psi^{\\dagger}(x)\\psi(x)$$is ill defined, in the sense that the ex... | Question on functional bosonization | physics | 2017-09-11T09:10:27.443 |
[
{
"message": "Consider an object colliding elastically with an inclined wall (with much bigger mass than the object) as in picuture. The wall is at $45°$ and it is moving.$(A)$: frame of reference of the wall (the wall is steady)$(B)$: frame of reference with respect to which the wall is moving towards the righ... | Angle of reflection of an object colliding on a moving wall | physics | 2017-09-11T17:55:49.607 |
[
{
"message": "These principles both seem to apply in flat space. Perhaps they are related through the equation for work (Fs=W)?",
"role": "user"
},
{
"message": "No. Newton's law of universal gravitation does not have a signal propagation delay built into it, the way Maxwell's laws of electromagneti... | Do Special Relativity and Newton's Law of Unverisal Gravitation hold in flat space? | physics | 2017-09-11T22:56:14.297 |
[
{
"message": "What I meant to say is that I've always been confused about the idea of \"atomic shells\",\"orbitals\",etc...The main question that I want to highlight is that if we say that orbitals are areas have 90% probability of finding an electron,so how are we so confident about giving precise atomic radiu... | How do we define atomic radius if we know that the actual scenario is not about discrete shells but of an electron cloud? | physics | 2017-09-12T02:39:54.303 |
[
{
"message": "(This question is about the making of a box to enclose a 3D printer and its filament. 3D printer filaments are sensitive to ambient moisture. The model described below is only a simplification of the real design problem: how well sealed must be the box, where to place the door, use chemical or ele... | Mixing in connected gases | physics | 2017-09-12T08:40:07.360 |
[
{
"message": "In this Wikipedia article <a href=\"https://en.wikipedia.org/wiki/Joule_expansion#Entropy_production\" rel=\"nofollow noreferrer\">https://en.wikipedia.org/wiki/Joule_expansion#Entropy_production, three ways to perform an adiabatic expansion are mentioned.Irreversible non-quasistaticIrreversible q... | Different ways to perform an adiabatic expansion | physics | 2017-09-12T16:44:10.797 |
[
{
"message": "An explanation for the gravitational pull on Earth provided by some who claim we live on a flat world is that the Earth is constantly accelerating \"up\" at a rate of $9.81\\frac{m}{s^2}$. That would mean Earth has been gaining speed since the dawn of time. It would be wildly incorrect to assume N... | Special relativity: is "the accelerating Earth" possible as by flat-earth science | physics | 2017-09-12T21:53:55.507 |
[
{
"message": "This might be a \"meta physical\" question with no actual physics content and if such is the case then I apologize. Actually, to figure out whether or not it is an actual physics question is one of the major reasons I am posting this question. Extremal Black Holes are the black holes that have the... | Why Do Extremal Black Holes Not Radiate? | physics | 2017-09-13T06:12:45.253 |
[
{
"message": "I know that a higher color temperature is associated with cool light and a lower color temperature is associated with warm light (green and red)However I'm trying to understand the correlation between color temperature and wavelength (or frequency) of light.Most bulbs are rated with a color temper... | Is Color Temperature and Wavelength the Same Thing? | physics | 2017-09-13T15:37:13.947 |
[
{
"message": "I am interested in working in a problem which has a mixture of discrete and continuous basis functions. In particular, I am trying to work with the finite square well.I am inspired by a similar problem for the infinite square well, where some numerical wavefunction, say x(1-x) for a well of length... | Mixed Basis States | physics | 2017-09-13T19:17:12.097 |
[
{
"message": "I was reading a paper that gives a nice collection of all scalar integrals that crop up in QCD loop calculations. Such integrals are computed in some kinematic region and then the authors say the results may be analytically continued if so desired. I just wonder how is this analytic continuation d... | Analytical continuation of 2,3,4-point integrals | physics | 2017-09-13T19:22:09.993 |
[
{
"message": "The famous signals recorded by the LIGO observatories are characterized as <a href=\"https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves\" rel=\"nofollow noreferrer\">observations of passing gravitational waves, where at <a href=\"https://en.wikipedia.org/wiki/LIGO#Observatories... | Is the LIGO detector acceptance different for gravitational and for non-gravitational waves? | physics | 2017-09-13T20:18:54.900 |
[
{
"message": "I am trying to do some simulations using the de Broglie - Bohm formalism and am wondering if there are computational tools that already exist in this area. I generally use Python, but will consider anything, even if I have to put some numerical analysis theory into code.That is, any theory or know... | Numerical Tools and Related Resoures for Bohmian Mechanics | physics | 2017-09-13T21:26:34.953 |
[
{
"message": "Physical interpretation of the wave function $\\psi (x,t)$ is a probability amplitude for location $x$ and the Fourier transform of $\\psi (x,t)$ can be interpreted as a probability amplitude for momentum $p$.Therefore $x$ and $p$ are conjugate variables, in the Fourier sense, and we can actually ... | Born rule and expectation value integral for $L = x \times p$ | physics | 2017-09-13T22:42:21.743 |
[
{
"message": "I understand the mathematical proof of conservation of momentum. The forces of particles on each other in an isolated group if summed together cancel in pairs. So the vector sum of their momentum stays constant unless an external forces acts on the particles. But, I also have an intuition. A singl... | Intuition for conservation of angular momentum of a group of particles | physics | 2017-09-14T06:30:26.423 |
[
{
"message": "First. The universe is flat and not negativity curved because whatever dark energy is made from represented by the cosmological constant adds to the side of the density in the first Friedman equation:$$\\left(\\frac{\\dot{a}}{a}\\right)^2 - \\frac{8πGρ}{3} -\\frac{Λc{^2}}{3} = -\\frac{kc{^2}}{a^2}... | Does the cosmological constant solve the flatness of the Universe with 2 independent methods? | physics | 2017-09-14T07:36:47.727 |
[
{
"message": "is it possible to create an electrical analogue of jet engines?How is its efficiency compared to ion ejection engines?",
"role": "user"
},
{
"message": "Well I might be about to answer the wrong question here, but assuming for the moment that you mean the thing on a 747 and the thing ... | Feasibility of Electrical jet engine | physics | 2017-09-14T11:37:56.563 |
[
{
"message": "For SU(2), if bond operator is <a href=\"https://i.stack.imgur.com/StPjy.png\" rel=\"nofollow noreferrer\">how can I derive the matrix of bond operator nuder the basis of spin coherent state:<a href=\"https://i.stack.imgur.com/UC65k.png\" rel=\"nofollow noreferrer\"> I just know I should use the o... | How can I derive the matrix value of bond operator? | physics | 2017-09-14T12:38:52.120 |
[
{
"message": "My current understanding of Bragg's experiment is this:A collimated X ray beam was shone to the surface of a metal. If we model a metal as an aggregate of parallel planes, one X ray reflects off one layer, then another X ray reflects off a layer that is deeper. The point is that as a result of thi... | Confusion about interference of light in Bragg's experiment | physics | 2017-09-14T13:03:56.150 |
[
{
"message": "I am looking some papers on quantum key distribution and there is an article based on BB84 phase coding. I am unable to understand how it is possible for them to associate TE and TM mode with different phase. Can anyone help me?The random polarization rotation, together with suitable birefringence... | How to differentiate TE and TM mode with AMZI in this article | physics | 2017-09-14T13:24:15.963 |
[
{
"message": "Is newton's second law a consequence of the principle of conservation of energy? How can we arrive at <blockquote> net force = rate of change of momentum</blockquote>using only the law of conservation of energy?",
"role": "user"
},
{
"message": "It is true that Newton's 2nd law with c... | Derivation of Newtons second law of motion from the principle of conservation of energy | physics | 2017-09-14T13:46:22.100 |
[
{
"message": "I try to understand a physical interpretation of the four components of the Dirac 4-spinor for a moving electron (in the simplest case, a plane wave). There is a very <a href=\"https://physics.stackexchange.com/q/91692/\">good question and answer about the interpretations already at SE. Basically ... | Interpretation of Dirac equation states for moving electron | physics | 2017-09-14T14:32:35.850 |
[
{
"message": "GR predicts no Ricci curvature in vacuum (or at least when we can ignore the cosmological constant). Would theories that violate this lead to observable light dispersion in solar system tests of gravity, or in light lensed around a galaxy?The field equation for light travelling in curved space-tim... | Light dispersion in gravitational theories | physics | 2017-09-14T15:10:11.780 |
[
{
"message": "Is it dangerous to expose the human eye to 3,000 - 10,000+ lux of UV (ultra violet) light in the 380 - 400 nm wavelength? I understand anything below 400 nm is tech classified as UV/UVA radiation but it is very close to the violet spectrum (which begins at 400nm). Further, most people can see ligh... | Danger to Expose Eye to 10,000+ Lux of Ultraviolet Light | physics | 2017-09-14T15:40:08.853 |
[
{
"message": "Why the moment of a force is calculated by the cross product of two vectors,how it is related ? And what does it means ,when I said that moment of a force about a point is for example 6 N.m ? I want simple clear explanation.",
"role": "user"
},
{
"message": "The word torque derives fro... | Why is moment of a force calculated by cross product? | physics | 2017-09-14T17:21:17.487 |
[
{
"message": "Let $\\mathrm{SO}(1,d-1)^\\uparrow$ be the connected Lorentz group in $d$ dimensions. I am looking for a book/article where its finite-dimensional projective representations are studied in detail. Rather surprisingly, I haven't been able to find anything online, so here I am.Some topics that I'd l... | Representations of the Lorentz group in an arbitrary number of space-time dimensions$.$ | physics | 2017-09-15T16:33:17.287 |
[
{
"message": "The following question is from 2016 $F=ma$ exam. My question is with regards to one step in the reasoning of the solution.Original question: A small ball of mass $3m$ is at rest on the ground. A second small ball of mass $m$ is positioned above the ground by a vertical massless rod of length $L$ t... | Two masses connected by vertical rod is displaced | physics | 2017-09-16T05:53:41.600 |
[
{
"message": "Just beginning to learn quantum mechanics and my textbook says that all pure eigenstates of the Schrodinger equation ( with a specified potential function) have a zero standard deviation for the expectation values of energy.So every measurement of the energy yields a fixed value. I've also read th... | Basic quantum mechanics- violation of the time-energy uncertainty principle by pure eigenstates of the infinite potential well? | physics | 2017-09-16T09:50:59.503 |
[
{
"message": "A Semicircular Ring can be considered to be made up of many Semicircular Arcs. A Semicircular Arc has its center of mass at ( 0, 2R/pi). Turns out, a Semicircular Ring also has its center of mass at ( 0, 2R/pi). Is there any intuitive explanation as to why both of their Center of Mass have the sam... | Comparing the Center of Mass of a Semicircular Ring and Semicircular Arc | physics | 2017-09-16T15:18:33.980 |
[
{
"message": "With a gravity water feed system, a header tank is in the attic. Every 1 metre drop from the water tank typically equates to around 0.1 Bar in pressure.There is a 500l cold water tank in the attic, the tank size is approximately 1.7m3. In the bathroom directly below the attic, the water pressure... | Cold water header tank and pressure | physics | 2017-09-16T19:32:24.573 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/zlBi9.png\" rel=\"nofollow noreferrer\">Why is the product of 2X and cosine delta t divided by 2 is the varying amplitude and not the other 2X and cosine function?",
"role": "user"
},
{
"message": "The cosine function varies between $\\pm 1$ and you ... | Varying amplitude of beats | physics | 2017-09-17T06:17:46.413 |
[
{
"message": "A recent question on <a href=\"https://physics.stackexchange.com/questions/357214/light-dispersion-in-gravitational-theories\">Light dispersion in gravitational theories, led to an implication that confuses me. It appears that the field equation for a massless vector field travelling in a backgrou... | Cosmological constant in GR can induce vector field tachyons? | physics | 2017-09-17T07:11:18.000 |
[
{
"message": "I do not understand when two Gaussian distributions are considered separated, given the resolution of the variable.Let us suppose we want to know if the difference between two times of flight is statistically relevant. Kleinknecht's book suggest a 4$\\sigma_T$ separation, where $\\sigma_T$ is the ... | separation between two Gaussian distributions | physics | 2017-09-17T10:11:42.237 |
[
{
"message": "I was looking through some lecture slides and I came across this page:<a href=\"https://i.stack.imgur.com/LIyWN.png\" rel=\"nofollow noreferrer\">I understand that the equation highlighted blue (top right corner) is obtained from the Principle of Least Action. Given a generating function $F(q, Q)$... | Independent Quantities in Canonical Transformations | physics | 2017-09-17T12:55:40.940 |
[
{
"message": "The three traceless Pauli matrices $\\sigma_x,\\sigma_y,\\sigma_z$ are arbitrary in the sense that any three operators with the appropriate commutation relations can be represented with those matrices. This is expressed by the action of the group SU(2) on $\\mathbb{C}^2$ and hence also on the spac... | Symmetry of the Pauli group and its representations | physics | 2017-09-17T14:38:30.020 |
[
{
"message": "I'm having problems with the sign of the electric potential in this situation. If given that <a href=\"https://i.stack.imgur.com/llurt.gif\" rel=\"nofollow noreferrer\">If I choose a path from point A to point B (in the figure below), the angle between the electric field and the path of integratio... | Electric potential from integration sign changes | physics | 2017-09-18T03:20:51.773 |
[
{
"message": "When I encounter a <a href=\"https://en.wikipedia.org/wiki/Deflection_(engineering)#Cantilever_beams\" rel=\"nofollow noreferrer\">single cantilever beam with a uniform load, I know what to do. When I know the uniform load, I can calculate the deflection of the end of the cantilever beam.Now, I ha... | Deflection of a composite beam of two materials | physics | 2017-09-18T11:39:22.177 |
[
{
"message": "The density matrix $\\hat{\\rho}$ for a canonical ensemble is given by $$\\hat{\\rho}=\\frac{\\sum\\limits_{n}e^{-\\beta E_n}|n\\rangle\\langle n|}{Z}\\tag{1}$$ $$=\\frac{e^{-\\beta E_0}}{Z}\\Big[\\sum\\limits_{n=0}|0\\rangle\\langle 0|+\\sum\\limits_{n=1}e^{-\\beta(E_1-E_0)}|1\\rangle\\langle 1|+... | Does a statistical system go into a pure state as the temperature $T\to 0$ (or $\beta\to\infty$)? | physics | 2017-09-18T16:29:27.440 |
[
{
"message": "Closed Kepler orbits are ellipses with the Sun at one focus.The force felt by the planet points in the direction of the Sun. As such, it is not a central force, since the focus is not the center.I am confused. The force should be central. What is my misunderstanding?",
"role": "user"
},
{
... | Why is the pull from the Sun a central force if it is not at the centre of an elliptical orbit? | physics | 2017-09-18T23:54:44.433 |
[
{
"message": "Below the Currie temperature, the Ising model shows first-order phase transition if we consider the external field as the control parameter. The order parameter jumps from e.g., $-m_0$ to $m_0$. The free energy has two local minima with the same depth at $\\pm m_0$, so the mean magnetization must... | Mean magnetization of the Ising model at the first-order transition line | physics | 2017-09-20T14:19:03.793 |
[
{
"message": "I want to know what are the minimal requirements for a physical system that must be satisfied in order for it to make sense to talk about time.For example, there shouldn't be time in a system of a single free particle since nothing ever changes. Another example - if we have two particles that move... | The most simple system that has time | physics | 2017-09-20T14:23:57.237 |
[
{
"message": "I understand that when you make a hole on a bottle filled with water and put the cap on tight the water will not flow out due to the atmospheric pressure acting on the hole.<br>My confusion is that, lets say the there's some space remaining on the top of the water, when you close the tap, there's ... | Hole in a water bottle | physics | 2017-09-20T15:11:16.190 |
[
{
"message": "Recently scientist have proved that gravitational waves travel through spacetime fabric and any mass object distorts the space fabric because of which we fill gravitational pull. My question is if the theory of gravity because of distorted spacetime fabric is correct what is the axis of mass objec... | Where is the pole of distorted spacetime fabric? | physics | 2017-09-20T18:16:52.227 |
[
{
"message": "This is derived from the answer and comments of <a href=\"https://physics.stackexchange.com/q/352268/\">this Phys.SE question concerning the calculation of two-point one-particle-irreducible diagram. On the one hand, according to the discussion on P.236 of An introduction to quantum field theory b... | About the calculation of one-particle-irreducible two-point diagram | physics | 2017-09-20T21:26:45.397 |
[
{
"message": "In the book - The Elegant Universe<blockquote> In quantum field theory, a particle and its antiparticle can momentarily annihilate one another, producing a photon. Subsequently, this photon can give rise to another particle and antiparticle traveling along different trajectories.</blockquote>Does... | Particle-antiparticle annihilation | physics | 2017-09-21T16:16:27.087 |
[
{
"message": "What happen with the Electrons produced in Stars due to the weak force decay? Do they combine with other atoms? Are they just pushed out of the Sun? Do they help with the electron degenerative pressure? OR they are used again to produce another neutron? Or Physicist are not sure yet?",
"role":... | What does happen to electrons in the weak force decay? | physics | 2017-09-22T17:25:06.280 |
[
{
"message": "For example, in the figure given below let the object be at O, the eye of the observer at E and the pole of the convex and concave lenses be P and P' respectively. Assume the object is placed such that, on using the lens formula (with only the convex lens) we get the first image to the right of th... | Does the position of the final image in a multiple lens system found by the thin lens formula always agree with the ray diagrams? | physics | 2017-09-23T07:03:55.697 |
[
{
"message": "For example, in the figure given below let the object be at O, the eye of the observer at E and the pole of the convex and concave lenses be P and P' respectively. Assume the object is placed such that, on using the lens formula (with only the convex lens) we get the first image to the right of th... | Does the position of the final image in a multiple lens system found by the thin lens formula always agree with the ray diagrams? | physics | 2017-09-23T07:03:55.697 |
[
{
"message": "When wind blows, leaves of a tree move and they gain kinetic energy. In this case air molecules loose kinetic energy and transfer it to the leaf.But In case of eddy currents moving inside a conductor, where do these electrons get their energy from for rotating? What is lost? Does magnet provides t... | How do Eddy currents follow conservation of energy? | physics | 2017-09-23T08:34:29.063 |
[
{
"message": "Lets theoreticaly imagine completly stationary particle,like electron for example.Can photon,or photons shake it,or oscillate its position? By that I dont necesarily mean the center position must change,more like jumping up and down,cyclicaly attracted and repelled by electromagnetic field.What I ... | Does photon electromagnetic field "shake" charged particles and atoms? | physics | 2017-09-23T13:01:10.990 |
[
{
"message": "I have some confusions about proper time and the calculations involved in Einstein's famous thought experiment where two lightning strikes occur simultaneously at the ends of a train for an observer observing the train to be moving at velocity v (Observer 1) while an observer stationary with respe... | Calculations in the Relativity of Simultaneity Train Thought Experiment | physics | 2017-09-24T05:03:12.427 |
[
{
"message": "Gravity pulls down the atmosphere thus creating higher Atmospheric Pressure close to the surface, This pressure is adding Force to air molecules, and since Heat is essentially adding force to molecules,will the measured temprature on the surface be higher than the tempratures at higher altitudes?I... | If Earth had gravity and atmosphere, but no Sun, will Temprature change with altitude? | physics | 2017-09-24T08:42:42.397 |
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