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": "In non-relativistic quantum mechanics, causality is violated by saying that the amplitude of propagation of a particle $$A=\\langle \\textbf{x}|\\exp{\\Big(\\frac{-i\\textbf{p}^2t}{2m\\hslash}}\\Big)|\\textbf{x}_0\\rangle$$ between any two points $\\textbf{x},\\textbf{x}_0$ is non-zero for any tim... | Why do we take two different approaches to the "meaning of causality" in quantum mechanics and quantum field theory? | physics | 2017-09-24T15:31:35.670 |
[
{
"message": "The only occurrence I see complex numbers used in dynamical systems is to analyse the eigenvalue $\\lambda$ of the linearised approximation to determine the characteristics of equilibrium points. However in classical mechanics when plugging $x=Ae^{i(\\omega t+\\phi)}$ into the equation of the osci... | Is there a relation between phase plane and complex plane? | physics | 2017-09-24T15:32:24.490 |
[
{
"message": "Please tell me if my reasoning is correct. The energy of a dipole in an electric field is given by $\\vec{a}\\cdot\\vec{E}=-aE\\cos(\\theta)$. For a collection of non-interacting dipoles in a thermodynamic system, the angular distribution is just given by the Gibbs-Boltzmann factor:$$f(\\theta)=Ae... | Angular distribution of dipole moments? | physics | 2017-09-24T21:51:59.930 |
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{
"message": "Two quarks can bound to form a meson, and the wavefunction consists of several quantum degrees of freedom:$$\\psi=\\psi(\\text{flavor})\\psi(\\text{color})\\psi(\\text{orbital})\\psi(\\text{spin})$$According to <a href=\"https://en.wikipedia.org/wiki/Meson#Types_of_meson\" rel=\"nofollow noreferre... | Wavefunctions of (Pseudo)-scalar/vector meson and Tensor meson | physics | 2017-09-24T22:25:39.057 |
[
{
"message": "I have often seen the notation $F_{ij} F^{ij} = F^2$, for the term in the Yang-Mills lagrangian, and I was wondering about the justification of this. Is this a positive quantity?For concreteness, let $A = A_i dx^i$ be a $U(1)$ potential on some spacetime $(M,g)$. Then $$F = dA = F_{ij}dx^i \\wedge... | Yang Mills Lagrangian positive? | physics | 2017-09-25T03:07:02.083 |
[
{
"message": "When light enters a refractive medium, is speed changes according to the refractive index $n$ of the medium. The microscopic explanation for this relies on calculations involving waves.There should also be an explanation in terms of photons, no? Why does a photon slow down when it is in a refracti... | Photon in refractive medium | physics | 2017-09-25T06:43:06.890 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/3zDcZ.png\" rel=\"nofollow noreferrer\">I have used this link where I plugged in the numbers. I am having trouble understanding what does it mean to have a frequency infinity when the velocity of the source matches the velocity of the sound wave. This ties with ... | In doppler effect, approaching frequency reaches infinity. What is the physical realization/manifestation of this? | physics | 2017-09-25T14:11:57.623 |
[
{
"message": "This issue has been bothering me for a bit but I fail to find a convincing answer.It all has to do with particles indistinguishability and how they alter the concept of probability.As I understood it, in the microcanonical ensemble all configurations are equiprobable: we find the macroscopic equil... | Entropy and Probabilities: does not "particle indistinguishability" alter probability values? | physics | 2017-09-25T16:01:16.077 |
[
{
"message": "(Note: Those who are already familiar with the notion of OPE's might prefer to skip directly to the Question section below) Setup:Consider, for simplicity, the quantum theory of the Euclidean K-G field. That is, the \"quantization\" of the the classical field which obeys the equations of motion, ... | Operator product expansion (of free field theory) in functional quantization approach | physics | 2017-09-25T16:21:45.860 |
[
{
"message": "Ok, I am having doubt in proving Kepler's 1st Law.<br>The link given below are 2 lectures in which a professor proved the Law:- 1st Lecture: <a href=\"https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=video&cd=2&cad=rja&uact=8&ved=0ahUKEwi-qsW65cDWAhUJqo8KHb... | Deriving Kepler 1st law of Planetary Motion | physics | 2017-09-25T17:50:14.783 |
[
{
"message": "I tried to find the reason by couldn't get the right answer. I wished to know that if both proton and electron have same acceleration, do they radiate same amount/amplitude of EM radiation. Also, since quarks have electric charge, do they radiate photons along with gluons (which I have not heard m... | Charged particles emiting radiation | physics | 2017-09-25T19:03:17.790 |
[
{
"message": "As I mentioned yesterday, Hollywood screenwriter working on a TV pilot about physics trying to get the details right.What empirical evidence is there that tachyons do not exist? I understand that objects with mass cannot accelerate to (much less past) $c$. So anything capable of FTL travel would h... | How do we know tachyons don't exist? | physics | 2017-09-25T22:03:06.417 |
[
{
"message": "I am reading \"Techniques for Nuclear and Particle Physics Experiments\" by William R. Leo. I have some questions about the ionization detectors chapter (chapter 6). In chapter 6, there is a figure like this:<a href=\"https://i.stack.imgur.com/UFX1c.jpg\" rel=\"nofollow noreferrer\">I understand t... | Basic concepts of ionization dectectors | physics | 2017-09-26T18:06:08.637 |
[
{
"message": "I'm working through Griffith's Intro to Quantum Mechanics, attempting to solve problem 2.27. <blockquote> Consider the double delta-function potential $$ V(x)= -\\alpha [\\delta(x+a)+\\delta(x-a)] $$ where $\\alpha$ and $a$ are constants. b) How many bound states does it possess? Find the all... | Parity in the Double Delta-Function Potential | physics | 2017-09-26T21:43:29.953 |
[
{
"message": "Say you connect Circuit A with low output impedance to circuit B with high input impedance. Why does this cause minimal voltage drop compared to connecting them the other way around (high output to low input).",
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{
"message": "<blockquote> Why does this cause minimal ... | Why does having a high input impedance and low output impedance cause minimal voltage drop when you connect two circuits? | physics | 2017-09-27T00:03:34.690 |
[
{
"message": "Water is used as a moderator to slow down neutrons in a light water nuclear reactor. By slowing down the neutrons they have more of a window to strike fission ready nuclei thus promoting the self perpetuating chain reaction when more neutrons are released from the fissions and slowed neutrons.All... | Why fast neutrons for breeder reactor but slow neutrons for regular light water reactor? | physics | 2017-09-27T00:39:01.573 |
[
{
"message": "If we start with the Einstein-Hilbert action with no matter, and consider time independent finite energy field configurations, then any static solution (e.g Schwarzchild metric) seems to be a soliton-like solution of the equations of motion corresponding to the Lagrangian. Now topological solitons... | Are static black holes solitons? | physics | 2017-09-27T02:14:20.257 |
[
{
"message": "I have measured a quantity $b$ as a function of two variables, let's say $x$ and $y$. Then I fitted a 2D polynomial of the shape $$b(x,y) = \\sum c_{j,i} x^i y^j$$to the data (which is of 2nd order).To estimate the goodness of the fit, I calculated the $\\chi^2$ value as $$\\chi^2 = \\sum_i\\frac{... | How to estimate the goodness of a 2D fit using the chi-square distribution? | physics | 2017-09-27T12:23:04.540 |
[
{
"message": "From Wikipedia:<blockquote> The horizon problem is the problem of determining why the Universe appears statistically homogeneous and isotropic in accordance with the cosmological principle. For example, molecules in a canister of gas are distributed homogeneously and isotropically because they ar... | Why is it said that a random energy distribution in the early universe is more natural than a uniform one? | physics | 2017-09-27T13:08:56.483 |
[
{
"message": "In QED, the fine structure constant $\\alpha$ runs upwards in the UV, with a loop calculation (involving a geometric series of the vacuum polarisation diagram) indicating a divergence in $\\alpha$ at $\\sim 10^{286}\\,\\text{eV}$. It is often claimed (see, for instance, Schwartz, QFT and the Stand... | Does QED really break down at the Landau pole? | physics | 2017-09-27T15:37:02.170 |
[
{
"message": "I remember seeing an experiment where Chris Bishop from the Royal Institution activates a mixture of hydrogen gas and chlorine gas using blue light. However, the blue light was in fact from white light passing through a blue filter. Since blue light is only a component of the white light's spectru... | Why does white light have less energy than blue light? | physics | 2017-09-27T18:08:42.040 |
[
{
"message": "Given a rope with both ends free, the general solution is$$\\psi(x,t)=f(x-vt)+g(x+vt),$$such that $$\\frac{\\partial\\psi}{\\partial x}(0,t)=0=\\frac{\\partial\\psi}{\\partial x}(L,t).$$QuestionIf $f(x-vt)=\\cos\\left[k(x-vt)\\right]$ then what is $g(x+vt)$ ? The answer should be $g(x+vt)=\\cos\\l... | Incorrect result for a standing wave on a rope with free ends | physics | 2017-09-27T21:43:17.497 |
[
{
"message": "During the last few weeks, I've spent quite some time thinking about Relativistic Quantum mechanics and have realized that special Relativity and Quantum mechanics can be woven together into a single theory as shown by Paul Dirac. But then one thing that I am confused about is what happens when th... | Why does relativistic Quantum mechanics make sense? | physics | 2017-09-28T18:42:55.023 |
[
{
"message": "I have a question related to the heterotic string theory. I have seen many references that are talking about heterotic tadpole but no one explains why it is an important quantity. The questions are thatWhat is a heterotic tadpole?Why is it important?Does a non-zero heterotic tadpole mean that we c... | Heterotic tadpole | physics | 2017-09-29T05:05:12.047 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/zJWjn.png\" rel=\"nofollow noreferrer\">The question said that 'the floor of the lift exerts forces of magnitude 678N and 452N respectively on Albert and Bella'. This is the normal reaction force. But why are they taken as downwards? Shouldn't they be upwards? D... | Could someone please help me understand why the normal forces are acting downward here? | physics | 2017-09-29T06:08:35.243 |
[
{
"message": "Suppose a cylinder sits upright in \"dry water\" (zero viscosity). The cylinder has half the density of the water, and we'll ignore the dynamics of the atmosphere.If I push the cylinder down some past its equilibrium, the buoyant force pushes back up. If I slowly increase the force I'm using to pu... | What is the oscillation frequency of a buoyant cylinder? | physics | 2017-09-29T07:54:42.607 |
[
{
"message": "When I calculate the fundamental commutator, $[x,p]$, in finite-dimensional Hilbert space (Heisenberg picture), the result is not proportional to identity, e.g. if I put $n=2$ (operators in natural units):\\begin{equation}[x,p]=i[a,a^\\dagger]=i\\left(\\begin{array}{ccc}1 & 0 & 0\\\\0 &... | Truncated harmonic oscillator gives wrong Heisenberg uncertainty? | physics | 2017-09-29T09:01:39.613 |
[
{
"message": "The EM-Field Hamiltonian is, in principle, a functional (with a chosen operator ordering) that is defined on operator-fields $\\hat{A}(x)$ and $\\partial_\\mu \\hat{A}$. If you carry out the calculations and use definitions of $\\hat{B}$ and $\\hat{E}$, you'll arrive at:$$\\hat{H} = \\int d^3x \\f... | Can we always express the EM-Field Hamiltonian as (possibly time dependent) pair of annihilation and creation operators? | physics | 2017-09-29T10:01:21.273 |
[
{
"message": "$K=hc/\\mathrm{wavelength}$ is the usual for finding energy for one particular wavelength. But how would one calculate a range of wavelengths incident on a particular sized surface area? Because no source is ever just one wavelength. It's always a range.For instance, I'd like to calculate what dif... | How to calculate energy per range of nm | physics | 2017-09-29T11:37:24.400 |
[
{
"message": "Apologies if this is the wrong SE site, this came up and my highschool physics was 30 odd years ago.So, lets say I have built a working ring space station with spin to give the effect of 1g artificial gravity.My next task is to build a moonbase, I also want 1g on the moon. So I decided to build m... | Forces acting on a spinning moon base | physics | 2017-09-29T12:00:56.480 |
[
{
"message": "How do we arrive at the conclusion that redshifted light from cosmological sources has 'lost' energy during its transit from source to us? If in fact no actual measurement of the energy of the redshifted light is ever made. Either at source or here on earth.Am I correct in assuming that it is not ... | Tired light question | physics | 2017-09-29T13:43:02.947 |
[
{
"message": "What do we physically mean by term smearing in DFT based codes like VASP or generally in condensed matter?",
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},
{
"message": "The smearing in density functional theory codes means that you occupy the states of the Kohn-Sham system according to a smooth function, e.g., th... | What do we physically mean by smearing in condensed matter? | physics | 2017-09-29T19:01:28.400 |
[
{
"message": "So for the problem, you have a flat mirror, a light source and a point A . The problem is to move the mirror in a way in wich the light source always hits the target point A, with the light source being able to move on an axis.So this is basically the problem I've defined to simplify this situatio... | Adjusting the inclination of a mirror in order for the light to hit a specific point | physics | 2017-09-30T02:16:14.633 |
[
{
"message": "Suppose I am in location X, emitting a sound at some loudness level. Bob, standing at location Y, hears the sound with an intensity reduced by $x$ db. If Bob starts emitting sound, will I also hear it at an intensity reduced by exactly $x$ db? (In general, there may be some general configuration o... | If I can hear Bob, can Bob hear me? (AKA, why does the Green's function for sound propagation have to be symmetric?) | physics | 2017-09-30T03:53:39.067 |
[
{
"message": "My question is do we see in nature any potential which is close to Double Delta Potential barrier/well? If yes then which are those? Thanks in advance.",
"role": "user"
},
{
"message": "This is often used as a <a href=\"https://quantummechanics.ucsd.edu/ph130a/130_notes/node155.html\" ... | What are the real life examples of Double Dirac-Delta Potential barrier/well? | physics | 2017-09-30T17:06:53.800 |
[
{
"message": "Suppose the initial wave function of a free particle is given by $\\psi(x,0)$. Now to find how the wave function evolves with time we generally do the Fourier transform of the wave function at $t=0$.The Fourier transform is given by $\\phi(k)$ and the time dependence of the wave function is given ... | Why is the momentum-space wavefunction for a free particle not a function of time? | physics | 2017-10-01T07:44:09.390 |
[
{
"message": "For a quantum mechanical description of a system (like a small molecule) we can write:$$\\langle\\psi|\\hat {H}|\\psi\\rangle = \\overline E$$Question: Is that energy the same as zero Kelvin energy obtained by statistical mechanics (using $E_n$ energies and partition functions)?",
"role": "use... | Simple question about Schrodinger equation (time independent) | physics | 2017-10-01T09:05:37.283 |
[
{
"message": "I would like to understand the distinction between an axial anomaly in QCD (Theta Vacuum: axion -> 2 gluons) and an axial anomaly in QCD of current (Chern–Simons term: pion->two photons, photon->three pions, ...). A more specific question: is the current axial anomaly related to the topological pr... | Axial anomaly in QCD VS axial anomaly in current algebra QCD | physics | 2017-10-01T13:12:46.833 |
[
{
"message": "I noticed that people who shoot 9 mm pistols on YouTube experience quite some recoil but that I don't experience any recoil when shooting my compound bow, even though the projectiles of both have similar momentum. Unfortunately, I don't own any firearms and it's been a decade since I shot one, I d... | What characterizes recoil? | physics | 2017-10-01T15:44:29.220 |
[
{
"message": "Let $\\mathrm{SO}(1,d-1)_{+}$ be the restricted Lorentz Group in $d$ dimensions. Are there projective irreducible representations of this group that do not descend from a representation of $\\mathrm{C}\\ell_{1,d-1}$?In other words, it is known that any representation of the Clifford algebra induce... | Are there projective representations of the Lorentz Group NOT coming from a Clifford algebra? | physics | 2017-10-01T20:04:36.287 |
[
{
"message": "As far as I understand, photons get 'stretched' as the universe expands, and the expansion rate is not constant. This means that the redshift observed on Earth is the sum of individual redshifts arising from the expansion of space at each instant. Therefore, I'd think the 'velocity' of the object ... | Why does the redshift formula hold true when measuring the expansion of the universe? | physics | 2017-10-01T21:48:15.540 |
[
{
"message": "I understand how Raman scattering works and I understand that it can only be clearly explained if you think about light as a wave instead of a particle (otherwise you have to start talking about \"virtual states\" which I think are not the most clear and accurate way of describing it).But, for som... | How does Raman scattering not break energy quantization? | physics | 2017-10-02T13:53:56.007 |
[
{
"message": "The definition of Strong Force is the force that binds the quarks into the protons and neutrons, and spills over around each proton and neutron and is an attractive one. And Binding Energy is the energy that must be put into a nucleus in order to break it apart. or the energy given out when a nucl... | Relation between Strong Forces and Binding Energy | physics | 2017-10-02T15:17:31.253 |
[
{
"message": "I've been looking for a solid square (non-pourus) that can expand to 12x it's size and back again. This needs to grow and shrink on only the x and y axis, the z doesn't matter whether it grows or not. The expansion and retraction needs to be done smoothly or in small increments and equally (not x ... | Is there a material that can stretch to 12 times it's size on the x & y axis (not concerned about the z), and can still return to it's original shape? | physics | 2017-10-03T00:21:54.227 |
[
{
"message": "For a spin $s$ system, we can define the observable $A=S_z$, such that $A(t) = S_z(t) = U^\\dagger S_z U$, where $U$ is the unitary operator $e^{-iHt}$. We can always define a projector $\\Pi_m = |s,m \\rangle \\langle s, m|$, such that $\\Pi_m~(t) = U^\\dagger(t) |s,m \\rangle \\langle s, m| U(t)... | Observable Vs Projector in case of Neutrinos | physics | 2017-10-03T00:26:54.960 |
[
{
"message": "To be clear here, My question arises from studying Electromagnetic induction and polarisation and photoelectric effect.From here I've gained the knowledge that creation of a photon involves:Electromagnetic Induction - The fact that a time varying spatial magnetic field which creates a closed loop ... | How is light produced? | physics | 2017-10-03T12:08:19.463 |
[
{
"message": "I am facing difficulties in understanding the definition of Takens' delay embedding from a non-physicist point of view....too much technical jargons. Can somebody please provide a simpler way to explain? Thank you.",
"role": "user"
},
{
"message": "The embedding theorem treats 1-dimens... | Part 2 - - Terminology -- phase space embedding | physics | 2017-10-03T16:23:49.047 |
[
{
"message": "In looking at the particle charts I see 3 Sigma baryons with very similar masses and other similar properties, which makes sense since they all have one strange quark plus two others from the (up,down) group. As I understand things, up and down have similar small masses but differ in charge, so i... | Why does the $\Lambda^0$ exist? | physics | 2017-10-03T22:03:27.643 |
[
{
"message": "A skyrmion in a 3-dimensional space (or a 3-dimensional spacetime) is detected by a topological index $$n= {\\tfrac{1}{4\\pi}}\\int\\mathbf{M}\\cdot\\left(\\frac{\\partial \\mathbf{M}}{\\partial x}\\times\\frac{\\partial \\mathbf{M}}{\\partial y}\\right)dx dy$$ where $M$ is the vector field in 3-... | Non-chiral skyrmion v.s. Left/Right chiral skyrmion | physics | 2017-10-03T23:31:30.287 |
[
{
"message": "In General Relativity, time moves slower near massive objects where spacetime is curved stronger. In quantum gravity, the gravitational force is represented by the quantum field that refers to the probability of interactions, which can be in a simplified way described as exchanges of virtual gravi... | Time dilation in quantum gravity | physics | 2017-10-04T00:36:11.980 |
[
{
"message": "How do we know that we measure only the higgs boson and not also its <a href=\"https://en.wikipedia.org/wiki/Superpartner\" rel=\"nofollow noreferrer\">superpartner, the width of the <a href=\"https://atlas.cern/updates/press-statement/latest-results-atlas-higgs-search\" rel=\"nofollow noreferrer\... | How do we know that we measure only the higgs and not 2 or more particles? | physics | 2017-10-04T06:28:02.870 |
[
{
"message": "It is well known that the voltage $V$ across, say, a p-n junction can be related to the electro-chemical potentials (usually and improperly called Fermi levels) at the borders of the junction $$ qV = \\mu_{n}-\\mu_{p}$$I'm trying to find a way to understand this in thermodynamic terms. This formul... | Voltage and Fermi levels | physics | 2017-10-04T09:15:16.970 |
[
{
"message": "From <a href=\"https://en.wikipedia.org/wiki/Sticky_bead_argument\" rel=\"nofollow noreferrer\">Wikipedia: <blockquote> \"Feynman’s gravitational wave detector: It is simply two beads sliding freely (but with a small amount of friction) on a rigid rod. As the wave passes over the rod, atomic forc... | Sudden doubt about the principle behind Gravitational waves detection | physics | 2017-10-04T13:14:51.583 |
[
{
"message": "It is often stated that points in the space of quantum field theories for which all parameters are invariant under renormalisation – that is to say, fixed points of the RG evolution – are scale-invariant field theories. Certainly this should be a necessary condition, since the theory must look the... | Are fixed points of RG evolution really scale-invariant? | physics | 2017-10-04T14:58:51.833 |
[
{
"message": "When light is emitted (or received) one photon at a time, what is the duration of each pulse? Is it related to the wavelength? Is it possible to create a laser pulse (or a radio wave) shorter than the wavelength (period of oscillations)?",
"role": "user"
},
{
"message": "This depends o... | Do photons have a duration? | physics | 2017-10-05T19:07:27.697 |
[
{
"message": "A rubber band that has a 5 cm diameter (Assume a perfect circle). The rubber band is then stretched. The rubber band returns to it's original shape. The rubber band now has a 6 cm diameter. What is this effect called?Is there a material that returns to it's original shape exactly and always or is ... | When a rubber band is stretched, it returns slightly larger. What is this effect called? Is there a material/property that doesn't exhibit this? | physics | 2017-10-05T23:38:26.247 |
[
{
"message": "I'm trying to improve my conceptual understanding of Cooper pairs in a superconductor. Not how they come about such as discussed in this <a href=\"https://physics.stackexchange.com/questions/10174/a-better-conceptual-model-for-cooper-pairs-in-a-superconductor\">question, but more in the sense of w... | How to conceptually think of Cooper pairs in a superconductor | physics | 2017-10-06T15:30:00.297 |
[
{
"message": "How do I determine that a certain space is Euclidean space or not? Are Spherical and Cylindrical Coordinates Euclidean too?<br>This question may be be elementary but I need to understand this.",
"role": "user"
},
{
"message": "The curvature of a space (or spacetime) is described by the... | How to identify an euclidean space? | physics | 2017-10-07T10:20:38.347 |
[
{
"message": "Where is the experimental proof that observers travelling toward or away from a light source, will always find that light from that source measures the same speed, regardless of their own speed?",
"role": "user"
},
{
"message": "It is claimed that on the earths surface the existence of... | Experimental proof of light speed isotropy | physics | 2017-10-07T10:31:43.583 |
[
{
"message": "Let’s say a green laser is shone onto a target ten meters away. Suppose we look at a photon as it attempts to travel to the target. Please correct my assumptions as needed.Assumptions:1. It could get “lucky” and miss all of the nitrogen and oxygen atoms. In this case it would be essentially travel... | What actually happens when light encounters air? | physics | 2017-10-07T17:38:06.290 |
[
{
"message": "I am trying to do problem 2.4 in the book \"Quantum field theory for the gifted amateur\". I have a math background but little training in physics.I am asked to use the identity $$\\langle x \\mid p \\mid \\psi \\rangle = - i h \\frac{d}{dx} \\langle x \\mid \\psi \\rangle.$$My questions: what is... | Quantum field theory for the gifted amateur: problem 2.4 | physics | 2017-10-07T21:01:23.853 |
[
{
"message": "According to the equations, $$P=VI =I^2R\\,\\text{ and voltage } V=IR$$it seems clear that when the resistance is lower by fixing the voltage at constant, the current is therefore, higher, generating high power. But what confused me was when the resistance is higher by fixing the current at consta... | Electric power and resistance dependance | physics | 2017-10-08T03:26:41.263 |
[
{
"message": "When Maxwell's equations are solved, one of the solutions is electromagnetic waves that should move at a certain speed ($c = \\frac{1}{\\sqrt{\\mu_0 \\epsilon_0}}$). Now, one could argue that since Maxwell's equations hold for all observers regardless of their reference frame, they should all see ... | Is my argument deducing from Maxwells equations the exclusion of faster than light travel flawed? | physics | 2017-10-08T03:38:10.547 |
[
{
"message": "When Maxwell's equations are solved, one of the solutions is electromagnetic waves that should move at a certain speed ($c = \\frac{1}{\\sqrt{\\mu_0 \\epsilon_0}}$). Now, one could argue that since Maxwell's equations hold for all observers regardless of their reference frame, they should all see ... | Is my argument deducing from Maxwells equations the exclusion of faster than light travel flawed? | physics | 2017-10-08T03:38:10.547 |
[
{
"message": "I am not well versed in physics and it is being difficult to understand the <a href=\"https://en.wikipedia.org/wiki/Degrees_of_freedom_(physics_and_chemistry)\" rel=\"nofollow noreferrer\">degrees of freedom of a system. QuestionI know that degrees of freedom are movements in which a system - like... | How to understand the concept of degree of freedom? | physics | 2017-10-08T11:26:34.703 |
[
{
"message": "I am not well versed in physics and it is being difficult to understand the <a href=\"https://en.wikipedia.org/wiki/Degrees_of_freedom_(physics_and_chemistry)\" rel=\"nofollow noreferrer\">degrees of freedom of a system. QuestionI know that degrees of freedom are movements in which a system - like... | How to understand the concept of degree of freedom? | physics | 2017-10-08T11:26:34.703 |
[
{
"message": "I am currently doing the <a href=\"https://en.wikipedia.org/wiki/Oil_drop_experiment\" rel=\"nofollow noreferrer\">Millikan oil drop experiment. Where we use an oil atomizer and spray oil between two capacitors of opposite polarity. I have two questions to ask. We have been asked to take a set of ... | Stokes' law for small oil droplet correction factor | physics | 2017-10-08T22:28:05.410 |
[
{
"message": "How a Total Solar Eclipse Helped Prove Einstein Right About RelativityThat historic experiment was carried out on May 29, 1919. British astronomer Sir Arthur Eddington was paying attention to Einstein's outlandish yet powerful new ideas after getting word from Dutch physicist Willem De Sitter (Hol... | Einstein's General Theory of Relativity | physics | 2017-10-10T14:25:06.353 |
[
{
"message": "Take a body having mass $M$ and velocity $v$, it has initial momentum $Mv$.The body radiates two photons of the same \"mass\" in directly opposite directions (one at the body motion direction, one - opposite direction).The momentum of the system gets reduced:$(M-2m)v + mc - mc = Mv - 2mv$.Does it ... | Momentum conservation with photons | physics | 2017-10-10T17:00:48.090 |
[
{
"message": "I read about the cosmic pancakes, filaments and halos and I think I don't have much intuition about them.First, what they are exactly are? are these shapes in which the matter in the universe is ordered?Moreover, I read that pancakes corresponds to 1D, and I can't understand it geometrically. I th... | Cosmic pancakes, filaments and halos | physics | 2017-10-11T06:38:32.257 |
[
{
"message": "It appears that they both describe the upper boundary mass that a cloud in space may inhibit before gravitational collapse. From Wikipedia, on <a href=\"https://en.wikipedia.org/wiki/Jeans_instability\" rel=\"nofollow noreferrer\">Jeans mass/Jeans instability:<blockquote> \"...Jeans instability c... | What is the difference between the Jeans mass and the Bonnor-Ebert mass? | physics | 2017-10-11T12:42:12.073 |
[
{
"message": "How is the position or momentum of a Quantum particle is measured experimentally in laboratory? Suppose we want to know the position or momentum of quantum particle which is kept in a box i.e. an infinite square well, then how to perform the experiment? In some books it is said that we have to use... | How does one actually measure the position or momentum of a quantum object? | physics | 2017-10-12T05:10:13.103 |
[
{
"message": "Obviously when an experimenter measures it, it projects onto the basis that the experimenter was trying to measure. But this process occurs on it's own, all the time. Obviously the answer has to do with the exact situation in which it occurs, but what I'm really struggling with is conceptualizing ... | When a wavefunction collapses on its own what basis does it project onto? (In Quantum Thermo, why do we privilege energy eigenstates?) | physics | 2017-10-12T14:42:52.010 |
[
{
"message": "Suppose a massless, frictionless piston assembly initially has a higher pressure than the external (atmospheric) pressure, and it is pinned so that the piston does not move. Once the pin is removed, the piston would expand until the pressure inside the piston becomes the atmospheric pressure. Duri... | Work done by the piston versus work done by the surrounding | physics | 2017-10-13T04:26:46.810 |
[
{
"message": "Well, we give the same quantity of heat to two objects. If these ones are different, they have different temperature. So the objects absorb different quantity of heat and they release a different quantity. But why this? I know that some are conductors and some not, but why this? What happens in th... | What happens when an object doesn't absorb all the heat? | physics | 2017-10-13T17:47:00.137 |
[
{
"message": "I seem to be puzzled by the importance of using angular frequency $\\omega$ as the frequency scalar to the mathematical model of Simple Harmonic Motion rather than merely using $f$, and by $f$ I mean $f$ such that $f = \\frac{1}{T}$, $T$ being the time/distance (time in this case) for a complete c... | Ordinary vs. Angular Frequency in SHM | physics | 2017-10-14T14:30:59.787 |
[
{
"message": "Is possible to study the dysprosium-BEC only using a mean-field approximation? I know the beyond mean-field brings new properties to BEC, but is it possible to use only the mean-field approximation?",
"role": "user"
},
{
"message": "You may or may not be aware of this work on quantum d... | Dipolar Bose-Einstein condensate | physics | 2017-10-15T00:02:53.743 |
[
{
"message": "Consider an isolated electron and a photon of energy 'hf' suffers collision with it. Then will the electron absorb all the energy of the photon or some amount is absorbed? If so, then does the photon after collision has less frequency than the previous one?",
"role": "user"
},
{
"messa... | Collision between photon and an isolated electron | physics | 2017-10-15T02:29:00.177 |
[
{
"message": "Say that a bead is on a wire that has shape $x^2+z^2/\\kappa^2 =1$, where the wire can rotate about the $z$-axis. Gravity acts upon the bead. I can take $\\theta,\\phi$ as two generalized coordinates. $\\theta$ is the angle from positive $z$, anticlockwise. $\\phi$ is rotation in the $x-y$ plane.I... | Kinetic energy for generalized coordinates? | physics | 2017-10-15T10:22:39.177 |
[
{
"message": "In quantum mechanics, the amplitude of wave function propagation can be found using the Feynman's path integral$$\\langle z'|e^{-itH/\\hbar}|z\\rangle=\\int\\limits_{x(0)=z\\\\x(t)=z'} Dx(t')\\:\\exp\\left\\{\\frac{i}\\hbar\\int_0^t dt'\\:\\left[\\frac{m\\dot{x}^2(t')}2-V(x(t'))\\right]\\right\\}.... | Least action principle in imaginary time | physics | 2017-10-15T11:44:24.333 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/ieikq.jpg\" rel=\"nofollow noreferrer\">I've read several answers on here to similar questions, and I've also looked at several different picture interpretations to no avail. I can't wrap my head around it. I understand that under the assumption of L >> d, both ... | In Young's double slit experiment, why are the two theta values equivalent? | physics | 2017-10-15T15:19:38.803 |
[
{
"message": "It should be possible to compute value from hydrodynamics of Solar prominence. What is the current estimate of Solar surface viscosity?",
"role": "user"
},
{
"message": "<blockquote> What is the viscosity of Sun surface?</blockquote>By \"solar surface\" I am going to assume you mean t... | What is the viscosity of Sun surface? | physics | 2017-10-16T22:12:08.633 |
[
{
"message": "The equation is: $\\lambda = \\frac{h}{mv}$. m here means the mass of the object while moving or while stationary?",
"role": "user"
},
{
"message": "The de Broglie wavelength is given by:$$ \\lambda = \\frac{h}{p} $$where $p$ is the momentum. Under non-relativistic conditions the momen... | What does $m$ mean in the De Broglie equation? | physics | 2017-10-17T05:20:58.107 |
[
{
"message": "Given a plane wave with $\\overrightarrow E$ field parallel to $z$ axis and a square metal plate in $xy$ plane. Suppose we know the power of the incident wave and the size of the plate, how can we calculate the total amount of power absorbed by the plate, knowing its electrical parameters? The thi... | Electromagnetic plane wave interaction with metal plate | physics | 2017-10-17T07:26:30.227 |
[
{
"message": "Say I am working on a problem where I have two generalized coordinates $q_1,q_2$ and my Lagrangian is independent of $q_1$, i.e. I have $L(\\dot{q}_1,q_2,\\dot{q}_2)$. Then we say that $q_1$ is a cyclic coordinate, and that the corresponding momentum $\\frac{\\partial L}{\\partial \\dot{q_1}}$ is ... | Cyclic coordinates don't actually seem to have constant generalised momentum? | physics | 2017-10-17T15:28:14.610 |
[
{
"message": "Suppose a man of mass 50 kg climbs a stair of height 10 m.Clearly it gains 5000 J of potential energy.But since work done by normal reaction on the man is zero does'nt it violates the principle of conservation of mechanical energy because no non-conservative force is acting on the man and thus me... | How is energy conserved when a man climbs a stair? | physics | 2017-10-17T16:08:26.557 |
[
{
"message": "Static friction :Static friction opposes relative motion between two surfaces. The car is moving at some tangential velocity relative to the road. Thus, static friction should act in the opposite direction. Yet it acts down the incline rather than parallel to the motion of the car. If it acts down... | Directions of static & kinetic friction? | physics | 2017-10-17T18:56:52.003 |
[
{
"message": "On page 130 of \"A Modern Introduction to Quantum Field Theory\" by Michele Maggiore they evaluate a divergent four dimensional integral writing: <blockquote> \"We introduce a cuttoff stating that we integrate only over Euclidean momenta with $k^2 < \\Lambda^2$, and we extract the divergent pa... | How to remove the divergent part of loop integrals when employing the cuttoff procedure? | physics | 2017-10-18T03:04:31.017 |
[
{
"message": "Given1) A fluid with known properties2) a gas mixture above the fluid with known properties3) A number-density of gas dissolved in the fluidIs there a way I can find out the rate at which the gas is released from the fluid?",
"role": "user"
},
{
"message": "There are a few different... | Modelling Gas Release From a Fluid | physics | 2017-10-18T15:01:56.097 |
[
{
"message": "To derive the time period $T$ of a <a href=\"https://en.wikipedia.org/wiki/Pendulum\" rel=\"nofollow noreferrer\">pendulum using dimensional analysis it is assumed that it depends upon the mass $m$ of the bob, the length of the string $\\ell$, the acceleration due to gravity $g$ and the amplitude ... | Why the time period cannot depend upon the angular velocity and angular acceleration of the pendulum? | physics | 2017-10-18T19:18:45.060 |
[
{
"message": "I've been staring at a formula for a while now which equates an angle with a solid angle. I can see how the equation is dimensionally consistent, which settles well with me, but I get tripped up at the idea of equating radians with steradians. To me these are two different units.But apparently it'... | Equating a radian to a steradian | physics | 2017-10-18T19:40:00.830 |
[
{
"message": "The chemical potential enters the grand canonical ensemble, in statistical physics, as the Lagrange multiplier ensuring the conservation of particle number. In QFT and relativistic theories in general, the conservation number of particle is not always a requirement. E.g. for a scalar field with U(... | Chemical potential in quantum field theories | physics | 2017-10-18T22:58:33.980 |
[
{
"message": "I've seem sometimes a construction being carried out specificaly in Minkowski spacetime:One picks the standard metric tensor $$g = -dt^2 + dr^2 + r^2 d\\Omega^2$$an introduces two new coordinate functions $T,R$ defined by$$T=\\arctan(t-r)+\\arctan(t+r),\\quad R=\\arctan(t+r)-\\arctan(t-r)$$these ... | Definition of $i^+,i^-,i^0, \mathscr{I}^+,\mathscr{I}^-$ in a general spacetime | physics | 2017-10-19T01:13:37.610 |
[
{
"message": "As an ice skater I cannot understand why I need to sharpen my blade very often since the hardness of steel is higher than that of ice for sure. Since steel is much harder than ice, how can ice change the shape of steel?",
"role": "user"
},
{
"message": "Friction affects both surfaces. ... | Why steel can be blunted by ice? | physics | 2017-10-19T02:01:20.873 |
[
{
"message": "I want to model the 1D trajectory of an alpha particle post-emission. The alpha particle will be emitted in a cloud chamber and the particle will move over a coordinate plane that I will use to measure the distance that it traveled in the x-axis over time t. I will then use this information to sol... | Experimental procedure for predicting post-emission alpha particle path | physics | 2017-10-19T21:07:12.113 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/CcPdo.png\" rel=\"nofollow noreferrer\">We know that in order for a spherical mirror to focus, the equation $$y^2 = 4fx - x^2$$ do behave like the parabola equation $$y^2 = 4fx,$$ so we need $$f>> x.$$However, it is told me that the inequality $u > h_1$... | In order for spherical mirror to focus, why do we need $\mu > h_1$? | physics | 2017-10-20T08:06:06.573 |
[
{
"message": "I was talking with a colleague about Maxwell's Equation, in particular <a href=\"http://hyperphysics.phy-astr.gsu.edu/hbase/electric/maxeq2.html#c3\" rel=\"noreferrer\">Faraday's Law of Induction, and I realised that I did not understand the following. Usually, this equation is expressed as:The in... | How to read Maxwell's Equations? | physics | 2017-10-20T11:43:55.500 |
[
{
"message": "Let's say we have a symmetric needle, and it's standing vertically. Then we apply a symmetric force F from above. What happens as we gradually increase F? Does the needle start to shrink in length? Will the length eventually approach zero? Forgive me if I couldn't be more precise, but I think you ... | How does a symmetric force affect a symmetric object? | physics | 2017-10-21T21:41:26.667 |
[
{
"message": "In phase space formulation of quantum mechanics, the so-called Husimi function can be defined as the convolution of the Wigner function by an appropriate Gaussian. There are apparently alternative definitions and more elegant treatments, but I was unable to find an explicit derivation of the Husim... | Deriving the Husimi Function of Harmonic Oscillator Eigenstates by Convolution | physics | 2017-10-21T22:01:51.527 |
[
{
"message": "In $\\text{Work} = m \\times a \\times \\text{distance}$.From this equation, How can we know the duration of the force that have been appliedin order to move the object over that distance?",
"role": "user"
},
{
"message": "First of all: in general you cannot substitute $F_r=ma$ into $W... | Where is the element of time in the definition of work? | physics | 2017-10-21T22:07:51.400 |
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