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": "Minkowski space has both translational and Lorentz symmetry, which together give Poincare symmetry. (It also has some discrete symmetries like parity and time-reversal that I won't be concerned with.) In some senses, it seems natural to think of the diffeomorphism invariance/general covariance of... | Which global symmetry of Minkowski space (if any) gets gauged to the diffeomorphism invariance of general relativity? | physics | 2017-07-21T07:35:11.453 |
[
{
"message": "Strongly inspired by the current thread <a href=\"https://physics.stackexchange.com/questions/346628/\">Are there necessarily always at least two points where the Earth's magnetic field is vertical?, this is the opposite question:Is there necessarily always at most one point (on Earth's surface) w... | Why would the North Magnetic Pole (dip pole) of the Earth be well-defined (uniquely determined)? | physics | 2017-07-21T07:54:38.827 |
[
{
"message": "There is an integral that I stumbled upon when I saw a calculation related to magnetic field energy (in static current density case) $$U_B= \\int_\\text{whole space} \\mathbf j \\cdot \\mathbf A \\:\\mathrm dV .$$ The integral is expanded using Maxwell's Laws and we reach to something like this $$... | Evaluation of a surface integral in Electromagnetism | physics | 2017-07-21T16:16:09.933 |
[
{
"message": "Say you have a conductive liquid with a changing magnetic field going right through it, causing an electric current. How exactly does the electric current travel and how could you calculate the effect of Joule heating on the liquid?",
"role": "user"
},
{
"message": "You are looking fo... | Joule heating for a fluid | physics | 2017-07-22T22:19:55.590 |
[
{
"message": "I'm trying to figure out what are the theoretical and practical, implications and limitations, when a high-dimensional chaotic process is modeled as a random process. I understand how low-dimensional chaos (callin' it LD chaos from now on) is different from a stochastic process. However I'm unclea... | Stochastic process vs high dimensional chaos in models | physics | 2017-07-23T23:18:06.853 |
[
{
"message": "the rough paint drawing attached is meant to show a sort of standing wave, where there is a 1,2,1,2,1,2,1,2 pattern: same wavelength but every other cycle is double amplitude. Is there a name for this particular type of wave? I did a bit of poking around in discussions & texts about standing w... | Identification of a certain type of standing wave | physics | 2017-07-24T01:10:48.930 |
[
{
"message": "Whenever we boil water there, there comes bubble from the bottom surface of the kettle when the water reaches 100 degree Celsius. Why is that? We also see some bubble sticking to the surface even before the water reaches the boiling temperature. Why is that also. But we do not see the same phenome... | Why is there bubble in the boiling water? | physics | 2017-07-24T18:16:16.977 |
[
{
"message": "I have a sealed, insulated vessel that contains water and air. I start pouring heat via an internal heater at P Watts. I know the mass/volume of air and water at the starting conditions of standard atm and temperature (1 atm, 20C). How do I calculate at what time I will reach 120C?",
"role": "... | How long will it take a sealed vessel containing water and air at standard temp and pressure to reach 120C? | physics | 2017-07-24T20:27:51.527 |
[
{
"message": "I've been reading about <a href=\"https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion\" rel=\"noreferrer\">spontaneous parametric down conversion (SPDC). The Wikipedia article on it says:<blockquote> A nonlinear crystal is used to split photon beams into pairs of photons that, in... | Why is a nonlinear crystal necessary to stimulate quantum fluctuations that entangle photons? | physics | 2017-07-25T00:57:21.563 |
[
{
"message": "It's a small problem that is bothering me for a long time. Suppose I have a system in a state $ \\psi_i$. It is a linear superposition of several states, let's say $$\\psi_i = a\\psi_1 + b\\psi_2$$ with $|a|^2 ≤1$, $|b|^2 ≤1$ and $|a|^2 + |b|^2 =1$ . Now this system has a probability of interactin... | Basic of quantum mechanics | physics | 2017-07-25T07:18:09.093 |
[
{
"message": "When one calculate the amplitude for a particle to propagate between two points, the results seems to violate causality. One book that makes some comments about this is Peskin & Schroeder, chapter 2, page 10. They use a square-root Hamiltonian. Even if we use the local Klein-Gordon equation an... | Do string-wave functions always spread superluminally? | physics | 2017-07-26T23:51:47.570 |
[
{
"message": "In the original Michelson-Morley experiment, white light was used and the interferometer produced colored fringes. Does anyone know why the fringes are colored?",
"role": "user"
},
{
"message": "Because a constant path difference represents a different number wavelengths for the variou... | Michelson-Morley Colored Fringes | physics | 2017-07-27T06:25:41.330 |
[
{
"message": "I am having trouble finding any clear information on <a href=\"https://en.wikipedia.org/wiki/Functional_integration\" rel=\"nofollow noreferrer\">path integrals. I am trying to find the difference between a normal path integral, a Feynman path integral and a Wiener path integral and the conditions... | Difference between a path integral, a Feynman path integral, a Wiener path integral? | physics | 2017-07-27T09:37:16.450 |
[
{
"message": "I have an old Hamamatsu PMT, and I have a scintillator that can detect neutrons, gammas, alphas. But no matter what source I use, I'm only seeing counts at the first few channels (up to about the 15th channel) and then no counts for any of the other channels. I've been using this PMT since a while... | Photomultiplier pulses only at first few channels? | physics | 2017-07-28T21:13:54.360 |
[
{
"message": "My sincere apology if this question is not appropriate for this forum. Recently, I came across the Unruh radiation or Unruh effect. From the Wikipedia page <a href=\"https://en.wikipedia.org/wiki/Unruh_effect\" rel=\"nofollow noreferrer\">https://en.wikipedia.org/wiki/Unruh_effect I see that it i... | Unruh effect and string theory? | physics | 2017-07-28T22:30:13.070 |
[
{
"message": "Gravitational and electrostatic forces are everyday phenomenon, and continue to be tested by sophisticated experiments at various distances.Rutherford measured the electrostatic repulsion of alpha particles from nuclei. At higher incident energies when the projectile energy is sufficient to overco... | Has the weak force ever been measured as a force? | physics | 2017-07-29T03:53:46.657 |
[
{
"message": "It is generally accepted that black hole emits Hawking radiation.I wonder if this radiation is different in case of black hole created by collapsing anti-matter.Consider a thought experiment. At the beginning we have antimatter which collapses into black hole. Then black hole evaporates as a resul... | Anti-matter black holes radiation | physics | 2017-07-29T13:24:09.113 |
[
{
"message": "In the book Quantum Field Theory, Ryder defines the vacuum-to-vacuum transition amplitude in the presence of the source $J(x)$ as$$Z[J] = \\int \\mathcal{D}\\phi \\, \\exp\\left\\lbrace i\\int \\, d^4x \\, \\left[\\mathcal{L}(\\phi) + J(x) \\phi(x) + \\frac{i}{2} \\epsilon\\phi^2 \\right]\\right\\... | Generating Functional for Scalar Fields | physics | 2017-07-29T19:09:31.907 |
[
{
"message": "The equation for conservation of angular momentum in electrodynamics is\\begin{equation}\\frac{\\mathrm d\\vec{L}_\\mathrm{mec}}{\\mathrm dt} + \\frac{\\mathrm d\\vec{L}_\\mathrm{EM}}{\\mathrm dt} = \\int M \\cdot \\mathrm d \\vec{S}\\end{equation}where $\\vec{L}_\\mathrm{EM}=\\int \\vec{r} \\time... | Is there intrinsic angular momentum in the conservation law of EM? | physics | 2017-07-30T16:50:46.540 |
[
{
"message": "In Ch. 1 Derivation 1 of Goldstein's mechanics, we have<blockquote> Show that for a single particle with constant mass the equation of motion implies $$\\frac{dT}{dt} = \\vec{F}\\cdot\\vec{v}$$</blockquote>The first step seems straightforward$$\\frac{dT}{dt} = mv\\dot{v}$$But$$\\vec{F}\\cdot\\ve... | What conditions are required for the derivative of kinetic energy to be F.v? | physics | 2017-07-31T01:47:53.350 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/A2yZY.jpg\" rel=\"nofollow noreferrer\">So I just started reading Goldstein's classical dynamics and I'm having a brain fart trying to understand something. I found a slide that paraphrases a part of the section, which is about systems of particles and center of... | Internal forces and rocket thrusts | physics | 2017-07-31T05:06:32.207 |
[
{
"message": "I'm reading the paper Significance of Electromagnetic Potentials in the Quantum Theory by Aharonov and Bohm (<a href=\"https://doi.org/10.1103/PhysRev.115.485\" rel=\"nofollow noreferrer\">Phys. Rev. 115, 485 (1959); <a href=\"https://jemebius.home.xs4all.nl/Aharonov-Bohm-1959.pdf\" rel=\"nofollow... | Aharonov-Bohm effect: exact solution for scattering problems | physics | 2017-07-31T12:48:28.380 |
[
{
"message": "My understanding that In quantum eraser experiment we can interpret the past only when we got the data from the coincidence counter.My question is why we need coincidence counter? Is this becuase the BBO produce entanglement photon and normal photon together ? i.e. if it is efficent 100% ( entangl... | Quantum eraser and coincidence counter | physics | 2017-07-31T19:27:15.757 |
[
{
"message": "I realize there are answers to my question many places across the internet and many places on this site, but from what I have seen, most of them either involve terms I don't know or assume you can use a test mass or \"accelerometer,\" which I think does not apply in the case I outline below (if it... | To what extent is acceleration relative? | physics | 2017-07-31T21:23:05.227 |
[
{
"message": "Eddy currents are generated in a conducting sheet when it oscillates in a uniform B field. It is stated in books that the sheet will show damping, as in the setup below:<a href=\"https://i.stack.imgur.com/3154d.gif\" rel=\"nofollow noreferrer\">So, I want to ask if I placed a superconductor in pla... | If eddy currents are generated in superconductor what would happen if ..? | physics | 2017-08-01T06:27:42.260 |
[
{
"message": "Hey am new to this forum but I have a question regarding topologically protected states.. Lets suppose we have a 1D gapped system divided two to distinct regions that have different periodicity or different properties and that at the centre, where the two regions 'meet' states appear in the gap. T... | Are edge states always topological? | physics | 2017-08-01T13:46:02.500 |
[
{
"message": "As I understand the Casimir effect is the effect that when two uncharged conducting plates are placed appropriately close to each other, the net pressure b/w the plates due to virtual photons (or particle anti-particle pairs) is less than that on the outside of the plates. Thus, the two plates are... | A query regarding Casimir Effect | physics | 2017-08-01T15:13:53.863 |
[
{
"message": "Am I right that the calculated Schwarzschild radius for the moon, Earth and Sun is also the calculated value for the \"stretching\" of space at the surface of the moon, Earth and Sun (.01cm, .88cm, and 2.96km, respectively)?If so, I would have thought this would have been an interesting \"factoid\... | Schwarzschild radius equals spatial stretch at surface? | physics | 2017-08-01T15:41:16.650 |
[
{
"message": "For a mass point in a local intertial system on which no forces act, we have that$$ \\frac{\\partial^2\\xi}{d\\tau^2}=0$$where $\\tau$, the proper time, is defined through $ds = c d\\tau$and the $\\xi$ denote the Minkowski coordinates in the local intertial system.I have read that for photons the ... | Proper time for a light particle | physics | 2017-08-01T23:00:02.323 |
[
{
"message": "I'm a little confused by the following computation from Baez and Muniain's Gauge Fields, Knots, & Gravity.<blockquote> Let $M = [0,1]\\times S$ where $S$ is any compact oriented manifold. Let $E \\overset{\\pi}{\\to} M$ be a trivial vector bundle over $M$ with connection $A$ in ``temporal gau... | Chern-Simons computation | physics | 2017-08-02T02:10:01.980 |
[
{
"message": "I am currently collecting research about theories to do with accelerated motion, and scientists who were involved in the development of such theories.Intrinsically, <a href=\"https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion\" rel=\"nofollow noreferrer\">Newton's Second Law came up. While th... | ${\bf F}=m{\bf a}$ versus ${\bf F}=\frac{\mathrm d{\bf p}}{\mathrm dt}$ alternate expressions (differential) | physics | 2017-08-02T10:04:23.730 |
[
{
"message": "I am currently collecting research about theories to do with accelerated motion, and scientists who were involved in the development of such theories.Intrinsically, <a href=\"https://en.wikipedia.org/wiki/Newton%27s_laws_of_motion\" rel=\"nofollow noreferrer\">Newton's Second Law came up. While th... | ${\bf F}=m{\bf a}$ versus ${\bf F}=\frac{\mathrm d{\bf p}}{\mathrm dt}$ alternate expressions (differential) | physics | 2017-08-02T10:04:23.730 |
[
{
"message": "I found <a href=\"https://physics.stackexchange.com/questions/88111/smallest-number-of-quantum-gates-to-simulate-other-gates?newreg=e032df9c700243c89535353376774da8\">this answer for how to simulate a Toffoli gate with a Fredkin gate; however, it leaves a bunch of garbage bits. Is there some way t... | Simulating Toffoli Gates with Fredkin Gates with no garbage bits | physics | 2017-08-02T16:59:07.587 |
[
{
"message": "Various textbooks that I am currently consulting (including Spacecraft Dynamics and Control An Introduction - Anton H.J. De Ruiter | Christopher J. Damaren | James R. Forbes Section 1.4, page 32) use $\\delta$, not $d$ or $\\partial$ to express an infinitesimal quantity. In the context of the refe... | $\delta$ differential notation | physics | 2017-08-02T19:47:34.103 |
[
{
"message": "We have a rod, 1m long and negligible height compared to the length, mass 10Kg uniformly distributed (center of gravity in the center). It is tilted theta degrees to the horizontal, and rests on two supports at two extreme ends, with one support higher than the other. What is the weight distributi... | Distribution of weight of an inclined object | physics | 2017-08-03T05:55:40.723 |
[
{
"message": "I started on my own learning about GR and SR two months ago, and I still do not have clear if it is possible or not. The following example was explained to me by someone who affirmed: \"SR applies only on inertial reference frames\":Let's imagine we have two different reference frames : A' and A. ... | Special Relativity: Does non inertial frame of reference work in SR? | physics | 2017-08-04T01:14:30.243 |
[
{
"message": "There are several ways to find that the entropy of thermal photon gas in a blackbody cavity takes the from$$S=V\\int d\\nu \\frac{8\\pi\\nu^2}{c^3} \\left( (1+n_\\nu)\\log(1+n_\\nu) - n_\\nu \\log n_\\nu \\right)$$where $n_\\nu=(\\exp(\\frac{h\\nu}{kT})-1)^{-1}$ is the average number of photons wi... | Blackbody entropy | physics | 2017-08-04T06:23:49.637 |
[
{
"message": "From some simulations I have obtained as an output a signal which roughly looks like a Gaussian with some elevated tails. Note that the input was a Gaussian. Now I would like to quantify the deviation from the Gaussian, or, to be more specific, I would like to quantify the elevation of the tails. ... | How to quantify elevated tails of a Gaussian like signal? | physics | 2017-08-04T10:18:37.360 |
[
{
"message": "Consider there are 2 bodies placed at different locations in space. Now I maintain them at a potential of V and -V respectively. If a conducting wire is placed connecting each other, current starts to pass. The power dissipated is described by $$P=I^2R$$ or$$P=V^2/R$$The questions I have in mind a... | Extremely confused about Ohms law | physics | 2017-08-05T17:36:00.863 |
[
{
"message": "Say you have a circle that's rotating with a linear velocity of $v$ and radius $r$. It's circumference at rest is $2\\pi r$. Speeding the disc up should (in my mind) cause this circumference to contract according to $$2\\pi r \\sqrt{1-\\beta^2}$$However in the book I am reading (The Elegant Univer... | Why does a rotating circle near light speed increase in circumference, not decrease (contract)? | physics | 2017-08-05T19:41:01.073 |
[
{
"message": "Apparently this result is used in many areas physics including the extra dimensions of string theory, which is not the scope of the question. The result is apparently also used to understand the Casimir force and there are many different types of experimental validation for this force. But how d... | Is the fact that the sum of all natural numbers $\sum_{n=1}^\infty n = -\frac{1}{12}$ essential to the understanding of the Casimir Force In QED? | physics | 2017-08-06T00:18:40.603 |
[
{
"message": "I am interested in Jeremy England's non-equilibrium thermodynamic theory of \"dissipative adaptation\". See refs like <a href=\"https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.119.038001\" rel=\"noreferrer\">https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.119.038001.My understanding of ... | Dissipative adaptation vs. principle of least entropy production | physics | 2017-08-06T02:40:52.613 |
[
{
"message": "In mathematical physics, we sometimes encounter situations where we have integrals of the forms:$$\\text{(case 1):}\\ \\ \\ \\ \\int\\limits_{D} f(x,y,z) dx dy dz =k$$ $$\\text{(case 2):}\\ \\ \\ \\ \\int\\limits_{D} f(x,y,z) dx dy dz =\\int\\limits_{D} g(x,y,z) dx dy dz$$where $D$ is an arbitrary... | Integration over arbitrary domains | physics | 2017-08-06T20:59:03.273 |
[
{
"message": "Recently I learned that electrostatic potential energy of a system of charges can be calculated like so:$$E = \\int \\frac{1}{2} \\epsilon_0 \\mathbf E^2 dV.$$However, for a system of two point charges Q1 and Q2, we have$$E = \\int \\frac{1}{2} \\epsilon_0 (\\mathbf E_1 + \\mathbf E_2)^2 dV = \\in... | Confusion about calculating electrostatic energy using the electric field | physics | 2017-08-07T04:37:47.007 |
[
{
"message": "The pressure variation in a static fluid depends only on depth, which is represented by the equation: $$P=P_0+\\rho_{water} gz\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\space\\sp... | Why is static fluid pressure below a submerged object not affected by the presence of that object? | physics | 2017-08-07T05:57:51.067 |
[
{
"message": "This question has been edited a little to clarify the confusion I have.The matter-antimatter asymmetry of the Universe quantified by the baryon asymmetry as$$Y_B=\\frac{n_B-n_{\\bar{B}}}{s}=\\frac{n_B}{s}$$i.e., the difference in the number densities of baryons $n_B$ and antibaryons $n_{\\bar{B}}$... | Quantifying the baryon asymmetry requires normalizing the baryon number density w.r.t comoving entropy density or photon density-Why? | physics | 2017-08-07T09:14:18.017 |
[
{
"message": "If you go on YouTube, you will find a large number of machines that work for almost forever. But why do all of them stop working after some time? Which Law (Other than Conservation of Energy) prevents a machine from running till eternity?",
"role": "user"
},
{
"message": "Most don't wo... | Perpetual Motion Machines | physics | 2017-08-07T11:01:06.167 |
[
{
"message": "Disclaimer: I took High School Physics over a couple decades ago.\"Hypothetically,\" suppose a 12oz (353.8 grams) paper cup from let's say Wendy's, filled with ice and water, was delivered from a car going one direction into the grill of, let's say a 1999 Jeep going the other direction. I'm intere... | Basic(?) question of impact of a cup with a car grill | physics | 2017-08-07T19:51:50.547 |
[
{
"message": "I am not good at Physics so please bear with me. I am trying to understand harmonics and in order to do that I thought its best to ensure my understanding of the equations of motion is correct. To do this I tried to model a mass vibrating on a frictionless surface with restoring force from a sprin... | Mass connected to spring on a frictionless surface vibrating out of control | physics | 2017-08-08T02:33:41.223 |
[
{
"message": "I am not good at Physics so please bear with me. I am trying to understand harmonics and in order to do that I thought its best to ensure my understanding of the equations of motion is correct. To do this I tried to model a mass vibrating on a frictionless surface with restoring force from a sprin... | Mass connected to spring on a frictionless surface vibrating out of control | physics | 2017-08-08T02:33:41.223 |
[
{
"message": "Here is the famous book: <a href=\"http://www.springer.com/gp/book/9783540719304\" rel=\"noreferrer\">fundamentals of many-body physics, written by Wolfgang Nolting. In the third chapter, he talked about the Green's function method to many-body physics and give a brief summary of this theoretical... | Is the many-body Green's function method a mean field theory? | physics | 2017-08-08T13:44:48.223 |
[
{
"message": "It is often stated in texts on general relativity that the theory is diffeomorphism invariant (N.B., I am considering active diffeomorphisms), i.e. if the universe is represented by a manifold $\\mathcal{M}$ with metric $g_{\\mu\\nu}$ and matter fields $\\psi$ and $\\phi:\\mathcal{M}\\rightarrow\\... | How to show that the Einstein-Hilbert action is diffeomorphism invariant? | physics | 2017-08-08T17:08:01.813 |
[
{
"message": "The following is a proof that higher (angular) frequencies have a greater deflection than lower frequencies through a glass prism:<a href=\"https://i.stack.imgur.com/sakMCl.png\" rel=\"nofollow noreferrer\">It was omitted in the image above, but $n_{\\text{air}}$ is taken to be unity, which is the... | Why do high frequencies have a larger deflection through a prism than smaller frequencies? | physics | 2017-08-09T22:03:53.457 |
[
{
"message": "I want to know how difficult it would be for me to observe two-beam interference at home.I have:A laser pointer.A non-polarizing beam-splitter.A mirror.Two concave lenses.An uneven shaky floor, some chairs, and tape. Patience that spans an entire day.This is the sketch of the setup that I have in ... | Observing Two-Beam Interference at Home | physics | 2017-08-10T05:49:05.923 |
[
{
"message": "I am trying to understand the physics behind waves, sound, and music and feel as if I'm missing some important conceptual components.How is it even possible for a string to vibrate at multiple frequencies at the same time? I understand that harmonics exist and how they work but I have yet to find... | Music / Harmonics | physics | 2017-08-10T19:37:29.930 |
[
{
"message": "I'm having trouble with a step in the derivation of the Newton-Euler equations for rigid body translation and rotation when the body frame is not centered at the center of gravity.The Newton-Euler equations in a frame $G$ attached to the center of gravity are$$\\begin{bmatrix}mI & 0 \\\\0 &... | Newton-Euler equations in frame not centered at center of mass | physics | 2017-08-11T17:11:21.387 |
[
{
"message": "Consider electromagnetic cylindrical waves. Cylindrical waves can be derived from the plane waves using energy conservation consideration: since the power must be a constant the amplitude of a cylindrical wave must decrease with $\\sqrt{r}$. Therefore a cylindrical wave expression must be $$\\math... | Different expressions of cylindrical EM waves if derived from one dimensional or three dimensional wave equation? | physics | 2017-08-12T01:27:36.967 |
[
{
"message": "We know that the work done by a force $\\mathbf{F}$, along a path $\\mathbf{x}$, is given by:\\begin{equation} W = \\mathbf{F}^T \\cdot \\mathbf{x} \\end{equation}However, suppose that i apply some change of basis, given by a matrix $A$. So, $\\mathbf{F}$ will become $A\\mathbf{F}$ and $\\mathbf{x... | Shouldn't work be the same in all coordinates? | physics | 2017-08-12T17:43:48.180 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/mkM7W.png\" rel=\"nofollow noreferrer\">I have read from sources that the curves are generated by the phosphors in the bulb, and the spikes are caused by the mercury vapor.However, if the mercury vapor's release of uv particles combine with the phosphors to prod... | What does the spikes and curves in the spectral graph for compact fluorescent lights represent? | physics | 2017-08-12T18:01:57.457 |
[
{
"message": "I came across this with a lecturer while discussing the Gibbs-Adsorption Isotherm. The lecturer insisted that the unit for the concentration parameter in the the formula surface excess concentration should be $\\mathrm{mol}$ as per literature. However, the formula itself would suggest otherwise.Th... | What are the units for the C parameter in the formula for surface excess concentration? | physics | 2017-08-13T13:27:00.397 |
[
{
"message": "I saw these 2 threads: <a href=\"https://physics.stackexchange.com/questions/258278/does-heat-cause-time-dilation\">Does Heat Cause Time Dilation?<a href=\"https://www.quora.com/Does-temperature-affect-time-like-gravity-and-velocity-do\" rel=\"nofollow noreferrer\">Does temperature affect time lik... | Is there a relation between fluctuations, temperature and time dilation? | physics | 2017-08-14T01:35:34.307 |
[
{
"message": "Consider a particle executing 1D simple harmonic oscillation. To formulate Lagrangian, we use $V=(1/2)kx^2$ for potential energy, and via Euler-Lagrange's equation we formulate the equation of motion $F=ma=-kx$. Now, my 1st question is, if we initially did not know anything about the force, then h... | What is the implication of using the Lagrangian to find the equation of motion? | physics | 2017-08-14T08:07:34.850 |
[
{
"message": "The equation for $D_{min}$ is, $D_{min} = 2i - A$($D_{min}$is angle of minimum deviation, $i$ is angle of incidence, and $A$ is prism angle)But as $i$ changes with $D_{min}$ and $A$, I'm not sure if it's possible to plot this on a graph. (Or at least i don't know how to plot this on a graph).The e... | Variation of angle of minimum deviation with prism angle | physics | 2017-08-14T14:35:40.803 |
[
{
"message": "In the paper <a href=\"http://physics.unipune.ernet.in/~phyed/26.2/File5.pdf\" rel=\"noreferrer\">http://physics.unipune.ernet.in/~phyed/26.2/File5.pdf, the author solves the LC-circuit using Euler-Lagrange equation. She assumes that the Lagrangian function for the circuit is $$L=T-V$$ where $T=L\... | Lagrangian method for an $LC$-Circuit | physics | 2017-08-14T16:09:26.087 |
[
{
"message": "Born's statistical interpretation of the wave function says that $|\\Psi (x,t)|^2$ is the probability density of finding the particle at point $x$ at time $t$, then$$\\int_{-\\infty}^\\infty |\\Psi (x,t)|^2 dx = 1 \\tag{1}$$In other words, the particle must be somewhere in space at a certain time ... | Normalize wave function with respect to time instead of space | physics | 2017-08-14T16:43:06.193 |
[
{
"message": "I don't really know much about entropy but I learned recently that the entropy in the universe is constantly increasing and that perhaps someday we will reach maximum entropy and the universe will end in heat death. In order for this to be true there are two prerequisites:Heat must constantly be c... | Don't certain processes decrease entropy? | physics | 2017-08-14T17:33:49.613 |
[
{
"message": "Consider a Photoelectric effect experiment apparatus as shown in Figure 11.1<a href=\"https://i.stack.imgur.com/SC3Gcm.jpg\" rel=\"nofollow noreferrer\">The variation of photocurrent with the voltage applied across $\\mathrm{A}$ and $\\mathrm{C}$ is as shown in Figure 11.3.<a href=\"https://i.stac... | Why do electrons move towards anode in the Photoelectric effect experiment? | physics | 2017-08-15T06:15:06.163 |
[
{
"message": "I was reading about the derivation for group velocity on the wiki page <a href=\"https://en.wikipedia.org/wiki/Group_velocity#Derivation\" rel=\"nofollow noreferrer\">here. As far as I understood, what's done here is based on these stepsGet a basic solution for the wave equation, that is $exp(i(kx... | Group velocity for wave pack | physics | 2017-08-15T15:25:17.033 |
[
{
"message": "The order parameter for liquid to crystalline solid transition is given by the Fourier transform of the density $$\\tilde{\\rho}(\\textbf{k})=\\int\\rho(\\textbf{r})e^{i\\textbf{k}\\cdot\\textbf{r}}d^3\\textbf{r}.\\tag{1}$$ One of the symmetries that get broken in this process is the continuous tr... | Order parameter for liquid to crystalline solid transition under the broken continuous group of translation | physics | 2017-08-16T15:04:58.110 |
[
{
"message": "According to Wightman's theorem, given a set of distributions $\\{W_n\\}$ satisfying a set of axioms, we may conclude the existence of a set of operators $\\{\\phi\\}$ which satisfy some properties, and such that\\begin{equation}W_n(x_1,\\dots,x_n)=\\langle 0|\\phi(x_1)\\cdots\\phi(x_n)|0\\rangle\... | Wightman's theorem, in terms of time-ordered functions$.$ | physics | 2017-08-16T18:07:27.047 |
[
{
"message": "Electromagnetic waves are produced by oscillating electric charges. In quantum mechanics this fact is represented by photons mediating electromagnetic interactions of particles with an electric charge.However, in quantum mechanics, any particle can be viewed as a wave. Can this wave also be viewed... | Oscillations of what cause quantum waves? | physics | 2017-08-16T23:24:50.770 |
[
{
"message": "If sun suddenly disappears then it will take 8 minutes for earth to move out of its orbit due to the presence of gravitational waves.What happens if proton in an atom suddenly disappears, will the electron too takes time to move out of its orbital or will it take place instantaneously?",
"role... | Is there any electrostatic waves for electrostatic force like the gravitational waves for gravity? | physics | 2017-08-17T15:15:37.387 |
[
{
"message": "When the Malus' law is applied to a beam of unpolarized light, I can understand that the incident light has all the possible polarizations, so that I should apply the law for all the angles.If I had three angles $\\theta_1$, $\\theta_2$ and $\\theta_3$, I guess the equation should be$I = I_0\\left... | Average value of $\cos^2(\theta)$ in Malus' law | physics | 2017-08-18T14:04:20.953 |
[
{
"message": "The Wigner function for a wave function $\\Psi(\\vec{r})$ is$$W(\\vec{r},\\vec{k}) = \\frac{1}{2\\pi} \\int dy e^{-i \\vec{k} \\cdot \\vec{y}} \\Psi^{*}(\\vec{r}-\\vec{y}/2) \\Psi(\\vec{r}+\\vec{y}/2) . \\tag{1}$$Recently, I've come to learn of the generalized Cahill phase space representation$$ F... | Can the Wigner function be described using coherent states? | physics | 2017-08-18T17:22:50.233 |
[
{
"message": "Suppose an electron is moving through empty space at speed v. It produces an electric field because it is a charge. But this field changes as it moves. Changing electric field must give rise to magnetic field.However, the moving electron also constitutes a transient current through various points ... | Fields of a moving electron | physics | 2017-08-19T03:06:26.433 |
[
{
"message": "In QFT a path integral involving fermion fields can be expanded out as a series giving the Green's function propagators. $G^{\\alpha \\beta}(x,y)$ and so on. The Fermion fields $\\psi^\\alpha(x)$ are seen as Grassman valued placeholders which don't take on actual values. On the other hand boson fi... | Do boson fields actually have values in QFT? | physics | 2017-08-19T03:54:46.240 |
[
{
"message": "For example, you took a photo of the surface of a table, which is a rectangle. You also know the length of the table, say 50 inches. In the photo, the surface appears as a trapezoid (whose parallel sides correspond to the table's length). Can you calculate the width of the table by just measuring ... | Can you tell the actual size of an object in a photo by just measuring the image (with some reference objects)? | physics | 2017-08-20T05:41:58.003 |
[
{
"message": "I am having an exercise: Deriving the Bernoulli's equation ($\\space p_1+\\frac{1}{2}\\rho V_1^2 = p_2+\\frac{1}{2}\\rho V_2^2 $ ) from the energy equation: $$\\rho \\frac{D(e+V^2/2)}{Dt} = \\nabla(pV)$$To make it clear: $\\rho$ is the density, e is the internal energy of one infinitesimal elemen... | A paradox when I was deriving Bernoulli's equation from energy equation | physics | 2017-08-20T11:16:23.247 |
[
{
"message": "When people say that a photon is a quantum of light, are they talking about 2 different types of quantizations in one phrase? To quantize something, you take something continuous and make it discrete (more or less). Energy in a light wave of frequency $f$ is continuous in (1) value and (2) locatio... | Terminology: Meaning of a Quantum | physics | 2017-08-20T12:24:51.320 |
[
{
"message": "I was wondering if someone could please tell me why this below solution models damping well? In particular its amplitude and frequency of the damped oscillation.$$y=e^{-λt/2m}[A\\cos(Λt)+B\\sin(Λt)],$$where$$Λ=\\sqrt{(ω^2/m)−(λ^2/4m^2)}$$I know that for simple harmonic motion the frequency of osci... | How well does this solution model damping? | physics | 2017-08-20T13:08:01.790 |
[
{
"message": "I want to know whether every kind of energy is converted to mass like kinetic and potential energy?Is it only possible for particles like electrons and protons or even either for big objects like a car or bus.Please consider the minutest / smallest change in mass even 0.000000000000000000000001 kg... | When is energy converted to mass? What are the required conditions to do so? | physics | 2017-08-21T08:35:40.133 |
[
{
"message": "In a book* I was reading, it said that a neutron star is formed when the pressure is so large that the electrons in a white dwarf interact with the protons, forming neutrons. The neutrons then collapse until the Pauli Exclusion Principle stops any further collapse.However, my issue is with why, wh... | Why can a neutron star implode? | physics | 2017-08-21T09:34:11.097 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/iB0op.png\" rel=\"nofollow noreferrer\">The situation sketched above is often used for problems involving tension. Let's say we know the masses $m$ and want to determine the tension $T$ in the rope (no friction, massless rope).Since this is a vector problem, I e... | Determining tension in a rope between two masses using only force arrows (i.e. no calculations) | physics | 2017-08-23T00:47:53.997 |
[
{
"message": "How does amplitude depend on the medium in which wave travels? I came across a lot of questions that asked to find amplitude after transmission of wave in a different medium.Is there any concept relating medium and amplitude?",
"role": "user"
},
{
"message": "You may see <a href=\"http... | Wave motion and sound | physics | 2017-08-23T07:06:41.483 |
[
{
"message": "According to <a href=\"http://www.cs.bham.ac.uk/internal/courses/intro-mqc/current/lecture08_handout.pdf\" rel=\"nofollow noreferrer\">this source, Grover's algorithm works as follows:Begin with uniform superposed state (i.e. Hadamard gate applied to |00000>)Apply the Oracle (flip amplitude of sta... | In Grover's algorithm how do we flip all Fourier transformed states *except* the 0th? | physics | 2017-08-23T16:25:05.037 |
[
{
"message": "I started using <a href=\"http://theory.sinp.msu.ru/~pukhov/calchep.html\" rel=\"nofollow noreferrer\">CalcHEP for some analytical calculations, and in looking at Higgs decays, I noticed that the default model file (as well as some other files available online) mention Higgs coupling to fermion pa... | Higgs-fermions coupling in CalcHEP? | physics | 2017-08-23T19:07:24.183 |
[
{
"message": "The answers <a href=\"https://physics.stackexchange.com/questions/169730/uncertainty-principle-and-measurement\">here about experimentally verifying Heisenberg's uncertainty principle says the following. <blockquote> Step 3, select two operators A and B Step 4a, for some of the systems prepare... | Steps of measurement to verify the Heisenberg uncertainty principle | physics | 2017-08-24T12:24:45.647 |
[
{
"message": "In Griffiths Introduction to Elementary Particles we are asked to draw the Feynman diagram for The decay of The $K^{+}$ Meson into a photon, antimuon and muon-neutrino. I am confused by the Diagram in the solution which seems to be completely false as the photon is just a virtual particle and the ... | Decay of K+ Meson | physics | 2017-08-24T18:59:26.183 |
[
{
"message": "How to call system that only consumes energy/matter, but not give away?I know that there are open and dissipative systems, but they can both consume/produce energy/substance.What about the systems that only consume them?",
"role": "user"
},
{
"message": "Sink (see dictionary definition... | How to call system that only consumes energy/matter, but not give away? | physics | 2017-08-25T03:44:20.997 |
[
{
"message": "Perhaps one of the most important results of the whole of Classical Mechanics is that the volume occupied by an ensemble in the phase space remains constant in time. Another very interesting result is that it is also invariant under canonical transformations (as the Jacobian of the canonical trans... | Physical Interpretation of Phase Space Volume | physics | 2017-08-25T07:27:14.303 |
[
{
"message": "Do the eigenstates of the number-operator in an abritrary Hilbert-space form a complete basis? For simplicity I will restrict myself to the case of just one mode. Given we have 2 Operators $\\hat{a}$ and $\\hat{a}^\\dagger$, which satisfy the commutation relations $[ \\hat{a}, \\hat{a}^\\dagger] =... | Do the eigenstates of the number-operator in an abitrary Hilbert space form a complete basis? | physics | 2017-08-25T08:21:49.217 |
[
{
"message": "As part of my investigation into whether Hohmann Transfer Orbit could become more feasible for long term space travel, I was wondering if it would be possible to reduce the journey time to another planet (half the period) by increasing delta-v beyond the required value for the transfer.If so, is t... | Reducing the journey time of a Hohmann Transfer | physics | 2017-08-25T18:21:17.230 |
[
{
"message": "What textbook would you recommend for a bachelor in theoretical physics to study <a href=\"https://en.wikipedia.org/wiki/Dynamical_systems_theory\" rel=\"nofollow noreferrer\">dynamical systems theory? I don't want to focus too much on chaos, just having a broad view of every interesting character... | Self-study book for dynamical systems theory? | physics | 2017-08-26T12:52:51.480 |
[
{
"message": "According to this diagram:<a href=\"https://i.stack.imgur.com/dXzdH.png\" rel=\"nofollow noreferrer\">And a few articles I have read, the number of photons emitted from a point source is exactly the same, and the reason why we detect fewer photons as we move away from the source is because the pho... | What happens to photons over distance? | physics | 2017-08-27T05:24:05.437 |
[
{
"message": "If I have a $t^2$ vs position graph and a $t$ vs average speed graph (where $t$ is time) for the same data set, shouldn't both their slopes equal and be the acceleration? However, my $t$ vs average speed graph yielded in a slope almost twice as big as the slope from $t^2$ vs position graph. To cl... | determining acceleration from graphs | physics | 2017-08-27T05:45:37.157 |
[
{
"message": "In the linear sigma model, the Lagrangian is given by$ \\mathcal{L} = \\frac{1}{2}\\sum_{i=1}^{N} \\left(\\partial_\\mu\\phi^i\\right)\\left(\\partial^\\mu\\phi^i\\right) +\\frac{1}{2}\\mu^2\\sum_{i=1}^{N}\\left(\\phi^i\\right)^2-\\frac{\\lambda}{4}\\left(\\sum_{i=1}^{N}\\left(\\phi^i\\right)^2\\r... | Where are the delta functions in Peskin & Schroeder eq. (11.67)? | physics | 2017-08-27T09:51:47.360 |
[
{
"message": "In school (at least in Russia) we are taught about the light as EM wave with wave–particle duality.The nature of waves is described as recursive emitting electric and magnetic fields chain. Then it's mentioned that all objects have such duality: could be represented as waves (with help of the de B... | EM vs Matter wave. The lie and confusion of School Physics | physics | 2017-08-27T11:57:01.340 |
[
{
"message": "A pion is a spin-zero composite particle, so $S = S^z = 0$. A $\\pi^-$ pion can decay into an antineutrino $\\bar{\\nu}$ and a negatively charged lepton $l$, each with spin-$1/2$. Let the direction of antineutrino motion be the positive $z$-axis. Since all antineutrinos are right-handed (neglectin... | How is pion decay compatible with conservation of angular momentum? | physics | 2017-08-27T22:27:42.563 |
[
{
"message": "I am currently in my first year of college, and I'm having a hard time understanding how stimulated emission works. I was told by my professor that an excited electron jumps back to the ground state upon interaction with a photon, in other words, the photon stimulated the electron's de-excitation.... | How does stimulated emission work, exactly? | physics | 2017-08-28T07:09:21.810 |
[
{
"message": "Let's say there is a virtual digital pet, who is aging, as a computer simulation. And there is one flying computer, launched from far way, which is now close to the earth and is moving away from the earth to a star. There is another flying computer moving toward the earth from the star. Those 2 fl... | Twin Paradox with a Virtual Digital Pet | physics | 2017-08-28T12:01:55.553 |
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