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": "Given sufficient number of qubits starting in $|0\\rangle$ states, and number of operations of universal gates, how to prepare a qubit of a specific state? A state such as$ \\frac{3}{5} |0\\rangle + \\frac{4}{5} |1\\rangle$",
"role": "user"
},
{
"message": "Using only the $X, Y, Z$ and... | How to prepare a qubit into a particular superposition state? | physics | 2018-01-28T22:21:27.943 |
[
{
"message": "I am doing a homework problem on spin precession. Constant magnetic field $\\vec{B}$ is in the z-direction. Initially, the electron is in an eigen state of $\\vec{\\sigma}.\\vec{n}$, where $\\vec{\\sigma}$ is the Pauli matrix vector. $\\vec{n} $ is some vector in the $x-z$ plane. I am asked to fin... | Is the expectation value of spin in magnetic field independent of time? | physics | 2018-01-29T03:02:06.390 |
[
{
"message": "Say we are finding eigenvectors of $\\sigma _z$, the eigenvalues are $1,-1$ so filling into the eigenvalue equation $\\sigma _z (a,b)=(a,-b)=1(a,b)$ and we find that $b=0$. I am confused about why we only have (1,0) as the eigenvector, why note have $(e^{i\\theta},0)$ for all $\\theta$ real. And t... | Eigenvectors and eigendecomposition of Pauli matrices, why isn't there many? | physics | 2018-01-29T17:55:51.657 |
[
{
"message": "Suppose the quantum system has state $$a|0_A\\rangle|0_B\\rangle + b|1_A\\rangle|1_B\\rangle.$$I want to prove that system $A$ has reduced density matrix of $a^2|0_A\\rangle\\langle 0_A| + b^2 |1_A\\rangle\\langle 1_A|$ when $0_A\\rangle$ and $|1_A\\rangle$ are orthogonal (and assume that $0_B\\ra... | How to derive reduced density matrix of $a|0_A\rangle|0_B\rangle + b|1_A\rangle|1_B\rangle$ | physics | 2018-01-29T18:38:57.230 |
[
{
"message": "The uncertainty principle is usually given as an example: $$\\Delta Q\\Delta P \\geq \\hbar.$$ The product of $\\Delta Q$ and $\\Delta P$ is a measure of uncertainty, and amounts to an integral of probability density over phase space. Is it correct that in a closed system the total uncertainty can... | Conservation of uncertainty | physics | 2018-01-30T15:08:26.890 |
[
{
"message": "BackgroundSuppose we have a Hamiltonian $H(\\mathbf{R})$ which depends on some parameters $\\mathbf{R}$. For each value of $\\mathbf{R}$, the Hamiltonian will have some set of eigenvectors $\\{ | \\phi_{j} (\\mathbf{R}) \\rangle \\}$.If we change the parameters $\\mathbf{R}$ around some closed loo... | When is the Berry phase only dependent on path topology? | physics | 2018-01-30T20:10:56.723 |
[
{
"message": "Suppose a piece of metal at 0 C° is exposed to thermal radiation (350 °K or thereabouts, EMR = 7 x 10^12 Hz)), its temp will raise , they say, because the KE of its atoms (or of the lattice as a whole) will increase. But how is the incoming radiation trnsformed into KE?The only way I can think o... | How is thermal radiation transformed into KE in a body (atom/lattice)? | physics | 2018-01-31T07:50:39.297 |
[
{
"message": "I’ve seen many books considering reversible isochoric processes performed by an ideal gas. But is that possible? Suppose I’ve a general isochoric process going from A to B, suppose I’m having a decrease of internal energy so that $$\\Delta U = Q < 0$$, so I’m letting $Q$ get out of my system, n... | Example of reversible isochoric process for an ideal gas | physics | 2018-01-31T11:20:44.250 |
[
{
"message": "Does coulomb field contain photons?As 1 THz and 1 kHz fields differ only in frequency,how does 0 Hz field differ from them?",
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},
{
"message": "No, it does not (admittedly, arguably). The Coulomb fields of charges set the part of the electric field that is irrotational (h... | Photons in coulomb field | physics | 2018-01-31T17:21:33.400 |
[
{
"message": "I'm trying to understand the following description in Classical Mechanics by John Taylor:<a href=\"https://i.stack.imgur.com/aFXOS.png\" rel=\"nofollow noreferrer\">He goes on to write:<blockquote> The vectors $\\hat{\\mathbf{x}}, \\hat{\\mathbf{y}}$ are the same at all points in the plane, wher... | Description of unit vectors in Cartesian vs. Polar Coordinates | physics | 2018-01-31T20:11:46.470 |
[
{
"message": "Starting from special relativity, here I see the de Broglie approximation is valid only if $m_0=0$.Derivation:$E^2=P^2C^2+m_0^2C^4$. Here we put Plank-Einstein relation $E=h\\nu=h\\frac{C}{\\lambda}$. Finally, $\\lambda=\\frac{h}{\\sqrt{P^2+m_0^2C^2}} \\hspace{2cm} (1)$.If $m_0=0$ then $\\lambda=\... | Why is the de Broglie equation as well as the Schrodinger equation is correct for massive particle? | physics | 2018-01-31T23:42:26.253 |
[
{
"message": "When I throw a ball (system) up in the air, gravity (of the earth [external]) does negative work on the system. The system's total energy remains constant as it moves in the conservative field, potential and kinetic energies interplay. Here, work done by gravity is negative and we say that work is... | Work done on/by the system | physics | 2018-02-01T06:07:26.683 |
[
{
"message": "I am currently reading <a href=\"https://www.nobelprize.org/nobel_prizes/physics/laureates/1998/laughlin-lecture.pdf\" rel=\"nofollow noreferrer\">Robert Laughlin's Nobel lecture. In the part where he uses gauge invariance to explain integer quantization of the Hall conductivity, he has a 2D recta... | Boundary Conditions giving Gauge Transformation a Physical Meaning? | physics | 2018-02-01T06:20:29.193 |
[
{
"message": "I am told that in relativity, there is no \"photon's frame\", simply because there is no frame from which a photon can be seen to be at rest.But, I stumbled upon something:Suppose we two photons are emitted along opposite directions. Now, using velocity composition formula to find the speed of one... | Is there a photon's frame in relativity when more than one photon in involved? | physics | 2018-02-01T14:19:00.677 |
[
{
"message": "Hale Bopp has a period of 2400 days, with a mean distance from the sun at 174 AU and 1AU away in perihelion and 357 AU away in aphelion, what is its speed at the perihelion?what formulas do you need in order to get this? We have not even tackled kinetic and potential energy in class but that was t... | Speed at the perihelion | physics | 2018-02-03T14:18:02.943 |
[
{
"message": "Maxwell's equations are\\begin{align} \\nabla\\cdot\\mathbf{E} & = \\frac{\\rho}{\\epsilon_0} & \\nabla\\times\\mathbf{B} &= \\mu_0\\epsilon_0 \\frac{\\partial\\mathbf{E}}{\\partial t} + \\mu_0 \\mathbf{J} \\\\ \\nabla\\cdot\\mathbf{B} & = 0 & \\nabla\\times\\mathbf{E} &... | What are the details behind causality and Maxwell's equations? | physics | 2018-02-03T19:39:39.360 |
[
{
"message": "In quantum field theory, the Feynman/time ordered Green's function takes the form$$D_F(p) \\sim \\frac{1}{p^2 - m^2 + i \\epsilon}$$and the $i \\epsilon$ reflects the fact that the Green's function is not unique, but must be fixed by boundary conditions. Different choices of boundary conditions yi... | Where is the Feynman Green's function in quantum mechanics? | physics | 2018-02-03T23:22:36.673 |
[
{
"message": "From Zee's Quantum Theory in a Nutshell he explains how the classical limit of quantum mechanics can be recovered from the path integral formalism.It can be shown that the path integral formalism is:$$ \\langle q_F | e^{-iHT} | q_I \\rangle = \\int Dq(t)e^{i \\int_0^T dt L(\\dot{q},q)}$$He then re... | Classical Limit of Quantum Mechanics recovered from the Path Integral Formalism | physics | 2018-02-04T11:34:06.583 |
[
{
"message": "<blockquote> A simple pendulum having a bob of mass m is suspended from the ceiling of a car used in a stunt film shooting. The car moves up along an inclined cliff at a speed v and makes a jump to leave the cliff and lands at some distance. Let R be the maximum height of the car from the top of ... | Problem regarding concept of weightlessness | physics | 2018-02-04T16:46:53.540 |
[
{
"message": "Why does the work done on a particle is equal to W=F.dx but not W=x.dF ? Where F is the applied force on the particle",
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},
{
"message": "The required explanation is given below, if you have some doubt in this please revert back !!<a href=\"https://i.stack.imgur.com/2o2Y9... | Work done on a particle | physics | 2018-02-04T17:10:45.580 |
[
{
"message": "How do I calculate the effective resistance of a large piece of metal between two small electrical contacts? The thermal image (from an interesting but unrelated article called An Exception to Ohm's Law) better demonstrates what I'm asking.From the temperature distribution, it is obvious where the... | Resistance of a Large (relative to lead contact) Conductor | physics | 2018-02-04T17:55:01.210 |
[
{
"message": "I'm struggling to understand(in an 'intuitive way') how it is possible to have, in a control volume, a stagnation pressure loss(adiabatic and isentropic slow down to zero velocity) without a change in the stagnation temperature, across a normal shock wave over an airfoil - for example(considering ... | Thermodynamics - 'Internal irreversibilities' in a fluid (gas) across a shock wave | physics | 2018-02-05T17:39:39.770 |
[
{
"message": "The physicist Frank Tipler has argued that spacetimes which end in a single point at what is called an \"Omega Point\" could have no event horizons. In which case, what would stop us seeing naked singularities? He claims our universe can end up in such a point?",
"role": "user"
},
{
"m... | Tipler and omega point spacetimes | physics | 2018-02-05T21:17:24.770 |
[
{
"message": "From reading the answer below (regarding the mechanics of an induction coil) and the wikipedia article, it is clear that induction coils produce AC. However, the following excerpt from the \"Crookes tube\" wikipedia article states that induction coils produce DC. My intuition in regards to this is... | Why does a Crookes tube work when connected to an induction coil? | physics | 2018-02-06T02:53:20.840 |
[
{
"message": "I was going through the book High Energy Physics by DH Perkins.In the decay $\\pi^{0}\\rightarrow 2\\gamma \\ (photon)$, the possible spin components of the system of photons $S_z$ can be $0$ or $2$. A statement in the book says, if the spin of of the pion is $1$, then $S_z$ has to be zero. Why is... | Spin of a neutral pion | physics | 2018-02-06T12:34:17.217 |
[
{
"message": "During the scaling of the Navier-Stokes equations it is often made use the viscous time scale $L^2/\\nu$, where $L$ is the characteristic length and $\\nu$ the kinematic viscosity. What is the physical interpretation of the definition term $L^2/\\nu$?Like $U/L$ might be the \"the large eddy turnov... | Interpretation of the time scale $L^2/\nu$ | physics | 2018-02-06T14:42:07.373 |
[
{
"message": "I read the total equation for the standard model is something like this <a href=\"https://www.symmetrymagazine.org/sites/default/files/images/standard/sml.png\" rel=\"nofollow noreferrer\">https://www.symmetrymagazine.org/sites/default/files/images/standard/sml.pngIt's quite complicated. My unders... | What is the mathematically simplest particle field? | physics | 2018-02-06T17:47:54.507 |
[
{
"message": "Studying ensembles in Statistical Mechanics I have found some formulas such as $$ S = k\\frac{\\partial}{\\partial T} (T\\ln Z) $$ or $$ \\langle E \\rangle = -\\frac{\\partial}{\\partial\\beta} \\ln Z $$ or $$ \\langle \\Delta E ^2 \\rangle = - \\frac{\\partial \\langle E \\rangle}{\\partial \\be... | Magical equations in statistical mechanics | physics | 2018-02-06T22:44:13.350 |
[
{
"message": "The hamiltonian form of the path integral for the time evolution of a single particle in one dimension (in non-relativistic quantum mechanics) is:$$\\langle x|\\hat U(t_2,t_1)|x'\\rangle=\\int \\mathcal Dx \\mathcal Dp \\ e^{\\frac i {\\hbar} \\int_{t_1}^{t_2} dt \\ \\big(p(t) \\dot{x}(t) - \\math... | Deriving the Lagrangian form of the Feynman path integral through Gaussian integration | physics | 2018-02-07T03:03:38.197 |
[
{
"message": "We know that the moon rotates around the earth due to gravitation. But both moon and earth attract themselves towards them. So why doesn't earth goes round the moon? It is feeling a force too. So it should also accelerate around the moon.",
"role": "user"
},
{
"message": "It does. The... | The Earth around the Moon | physics | 2018-02-07T09:29:42.230 |
[
{
"message": "When concentrated external forces is applied to an object, the integration of infinitesimal forces (df) distributed on the whole object due to these concentrated forces is equal to the sum of these concentrated external forces by magnitude and direction. Is there any proof for this fact? many peop... | The proof of the fact that the summation of infinitesimal forces distributed on an object equal to the sum of concentrated external forces on it | physics | 2018-02-07T12:22:28.873 |
[
{
"message": "Are there any general theorems which fix the possible symmetry breaking patterns of <a href=\"https://en.wikipedia.org/wiki/Lie_group\" rel=\"nofollow noreferrer\">Lie groups (such as $SU(3)$) by <a href=\"https://en.wikipedia.org/wiki/Vacuum_expectation_value\" rel=\"nofollow noreferrer\">vacuum ... | Breaking of $SU(3)$ symmetry by bi-fundamental representation | physics | 2018-02-08T13:49:26.743 |
[
{
"message": "We know that we get displacement if we integrate velocity with respect to time. But from my curiosity I am now wonder, what will I get if I integrate position/displacement with respect to time?",
"role": "user"
},
{
"message": "I think what you are talking about is <a href=\"https://en... | Integrating position/displacement with respect to time | physics | 2018-02-08T16:42:14.007 |
[
{
"message": "I am trying to determine the donor/acceptor activation energy for the semiconductor InGaP with the Arrhenius equation. However it contains only one parameter for the activation energy, which turns out to be around $55$meV when fitted on my measurements. Does that mean that this is the activation e... | How to determine the activation energy for donors/acceptors using the Arrhenius eq.? | physics | 2018-02-08T18:45:38.330 |
[
{
"message": "In a fission reaction a neutron collides with another atom. In this process, is the momentum conserved? Some masses turn into energy, so can't it happen that the momentum is not conserved?",
"role": "user"
},
{
"message": "Momentum conservation is a consequence of a fundamental symmetr... | Momentum conservation in fission | physics | 2018-02-09T08:34:14.403 |
[
{
"message": "I am modelling the scattering of hydrogen atoms against each other.In this model, one hard sphere scatters elastically off another hard sphere, they are identical with radius $r$. They meet with impact parameter $b$ and $b<r$. They scatter with angle $\\theta$ and we have $b=R \\cos \\frac{\\th... | What's the distribution of scattering angles for hard spheres with random impact parameter? | physics | 2018-02-10T15:36:38.057 |
[
{
"message": "I've been trying to solve a problem recently:<blockquote> An object with mass $m$ is thrown at the angle $\\alpha$ from the ground with initial velocity $v_0$. At the point of impact vertical projection of velocity turned out to be two times less than initial vertical projection. The resistive fo... | What is the time of flight of an object thrown at an angle from the ground with resistive force proportional to the velocity? | physics | 2018-02-11T20:02:39.933 |
[
{
"message": "I'm want to know how are the transformations of the operators $\\mathbf\\nabla$ and $\\partial/\\partial and $\\partial/\\partial t$ when the transformation of the Galilean relativity is applied. This is what I've tried:Galilean Transformation\\begin{cases} \\mathbf {r}' = \\mathbf r - \\mathbf Vt... | Transformation of the operators $\mathbf\nabla$ and $\partial/\partial t$ under Galilean transformation | physics | 2018-02-12T03:55:37.760 |
[
{
"message": "Let's consider an observer $O$ associated with an inertial referential frame $R$ in a flat spacetime (no gravitational field). This observer is looking at an uniformly accelerated particle $P$ moving in a constant direction (no Thomas precession). The coordinates of the particle in $R$ are ($x$,$t... | How does an uniformly accelerated particle see the world in 1+1D? | physics | 2018-02-12T17:00:58.450 |
[
{
"message": "I heard on the TED Radio Hour that the LIGO team was able to make an independent measurement of the Hubble constant and the increasing expansion rate of the universe when they detected the gravitational waves coming from a neutron star collision. How does measuring those waves tell you about the e... | Determine expansion rate of universe from gravitational waves LIGO? | physics | 2018-02-12T19:21:12.530 |
[
{
"message": "I do not recall where, but I have read someting about the light approaching a blackhole. In the writing, it was said that an object would gain kinetical energy due to the blackhole's gravity, and would increase its speed. On the other hand, when the light approaches than an object, it would gain k... | Is it possible to change the frequency of the light by magnetic fields? | physics | 2018-02-12T20:52:15.133 |
[
{
"message": "Let's say, when x-ray hits a molecule, an electron from an inner shell absorbs the energy and flies away, so there's a hole waiting for an electron to come down and fill it. How long does this hole last? I know its lifetime may depend on the species of the molecule, but is there a general value fo... | What's the lifetime of the excited state when you shine light on molecules? | physics | 2018-02-12T21:13:59.407 |
[
{
"message": "I am reading Sakurai's Modern quantum mechanics and at some point it's trying to draw a parallel between classical and quantum mechanics.It says<blockquote> An infinitesimal translation in classical mechanics can be regarded as a canonical transformation, $$ \\mathbf{x}_{\\mathrm{new}} \\equiv ... | Generating function of infinitesimal translation - classical mechanics | physics | 2018-02-12T22:58:47.010 |
[
{
"message": "An <a href=\"http://www.sciencemag.org/news/2018/01/physicists-are-planning-build-lasers-so-powerful-they-could-rip-apart-empty-space\" rel=\"nofollow noreferrer\">article in Science says:<blockquote> According to the theory of quantum electrodynamics (QED), which describes how electromagnetic fi... | Lasers so powerful they could rip apart empty space | physics | 2018-02-13T01:48:09.473 |
[
{
"message": "Suppose you have some material which has a low reflectance and is opaque to an incomming photon when the angle of incidence is small.Now take that material and make a narrow (20:1 width:length ratio or narrower) wedge with a fine edge - 1/10th the wavelength of said photon or less, the very edge i... | How do photons interact with a very fine edge? | physics | 2018-02-13T07:49:17.680 |
[
{
"message": "I am trying to understand a kinematics problem (that intuitively appears trivial at first glance), with the ultimate goal of simulating it.Consider the following triangle (from Wikipedia).<a href=\"https://i.stack.imgur.com/8NtTT.png\" rel=\"nofollow noreferrer\">Assume $a$ and $c$ are rigid edges... | Describing a singly articulated system with forces on the joints | physics | 2018-02-13T13:18:44.850 |
[
{
"message": "In the description of a neutron interferometer <a href=\"https://www.univie.ac.at/physikwiki/images/0/06/Neutron_Interferometry.pdf\" rel=\"nofollow noreferrer\">here, it says:<blockquote> In an interferometer the incident beam is split into two (or more) separate beams. The beams travel along di... | Why do we use superposition instead of tensor product in interferometer? | physics | 2018-02-13T18:40:19.363 |
[
{
"message": "When a mass connected to a spring is in simple harmonic motion and somewhere between the mean and extreme positions the mass is cut from spring. Then instantaneously after cutting the mass will only have its kinetic energy right? (Or it will have total energy kinetic+potential?)Or I mean to say th... | Energy in simple harmonic motion ─ where is the kinetic energy stored, and where is the potential energy? | physics | 2018-02-14T11:31:50.473 |
[
{
"message": "The following statement is from University Physics (chapter \"Electric Potential\"):<blockquote> \"In particular, at any point just inside the surface the component of E vector tangent to the surface is zero. It follows that the tangential component of E vector is also zero just outside the surfa... | The consequence of electric field lines not being perpendicular to conductor (assuming a spherical one) | physics | 2018-02-14T12:24:06.713 |
[
{
"message": "The following statement is from University Physics (chapter \"Electric Potential\"):<blockquote> \"In particular, at any point just inside the surface the component of E vector tangent to the surface is zero. It follows that the tangential component of E vector is also zero just outside the surfa... | The consequence of electric field lines not being perpendicular to conductor (assuming a spherical one) | physics | 2018-02-14T12:24:06.713 |
[
{
"message": "We can all agree empirically that a stream of photons encountering an arrangement of atoms forming a lens bends.But do the photons actually make contact with the atoms in the process of bending?I think this question maps neatly onto the similar question, do photons passing through swirling water c... | Do photons make contact with the atoms in a lens? | physics | 2018-02-14T12:48:41.113 |
[
{
"message": "I like to think of QFT Weinberg-style: particles come first, and fields come later; and the latter are constructed so as to describe the former. Fields are not to be thought of as fundamental, but only as convenient tools to study particles.In this setting, what justifies the postulate that a ferm... | What justifies the use of Clifford spinors to describe higher dimensional fermions? | physics | 2018-02-14T15:20:57.367 |
[
{
"message": "If I plot a 'time elapsed' versus 'distance traveled' graph of a photon travelling in a vacuum (time on the vertical axis and distance on the horizontal) and then take the volume of revolution about the time axis, will it somehow correspond to what we call a light cone?",
"role": "user"
},
... | Volume of revolution of a time versus distance graph | physics | 2018-02-14T17:14:18.227 |
[
{
"message": "It is intuitive to me (correct me if I am wrong), that the direction of the wave vector in (real, not modeled) laser beam is not aligned with the direction of propagation everywhere in space. It means, that spherical-wave-like behavior gets integrated into plane-wave-picture, and the error, associ... | Bending of laser beam wavefront | physics | 2018-02-14T20:05:37.537 |
[
{
"message": "It is intuitive to me (correct me if I am wrong), that the direction of the wave vector in (real, not modeled) laser beam is not aligned with the direction of propagation everywhere in space. It means, that spherical-wave-like behavior gets integrated into plane-wave-picture, and the error, associ... | Bending of laser beam wavefront | physics | 2018-02-14T20:05:37.537 |
[
{
"message": "I want to know why there is a maximum and then decrease.",
"role": "user"
},
{
"message": "In nuclear and particle physics the only peaked energy spectra come from two body decays (spontaneous fission is a decay). One has a peak with a width due to measurement errors.The neutrino was d... | Why are neutron spectra continuous, with maxima? e.g 252Cf | physics | 2018-02-15T00:03:23.330 |
[
{
"message": "I am uncomfy with the calculation of the neutral pion’s decay rate via the triangle anomaly diagram, which gets touted as evidence of three colors. The calculation invokes PCAC in the guise of the occult Goldberger-Treiman relation, which says that ${{f}_{\\pi }}$ (the charged pion’s proportional... | Old-fashioned approach to neutral pion’s decay rate | physics | 2018-02-15T21:08:44.447 |
[
{
"message": "Imagine empty infinite universe with just a single resting electron - let's ask the question about configuration of electric field in such empty universe.The standard answer would be $E\\propto 1/r^2$.However, if calculating energy $(\\propto E^2)$ of such electric field, due to singularity in $r=... | The problem of infinite energy of electron as point charge? | physics | 2018-02-16T14:56:53.300 |
[
{
"message": "Regarding Gravity Time Dilation: We know that, in a stronger gravitational field, time passes more slowly. As one descends below the Earth's surface, how does the intensity of gravitational pull change, and if it is neutralized at the Earth's center (because of it being equally attractive from ... | Gravity Time Dilation interior to a planet | physics | 2018-02-16T17:34:47.323 |
[
{
"message": "According to the beautiful Noether’s theorem, a conservation law is associated to a specific symmetry such as for example energy $\\leftrightarrow$ translation in time, momentum $\\leftrightarrow$ translation in space, etc... In the context of the Standard Model a lot of new conservation laws come... | What are the symmetries associated to the conservation of quantum numbers? | physics | 2018-02-16T17:54:28.740 |
[
{
"message": "I was able to derive the formula for root-mean-squared speed of a molecule, using a basic method taught at school. However, in my textbook, there's a point that:<blockquote> It can be shown that $\\bar v = \\sqrt{\\frac{8RT}{\\pi M}}$</blockquote>where $R = 8.314\\text{ J mol}^{-1}\\text{ K}^{-1}... | Why is the mean speed of a gaseous molecule $\sqrt{\frac{8RT}{\pi M}}$? | physics | 2018-02-17T05:54:37.183 |
[
{
"message": "I was re-reading my old Relativity book (by Rindler) and taking a look at some of the problems. He asks: Using a Minkowski diagram to establish the following result: <blockquote> Given two rods of rest lengths $l_1$ and $l_2 (l_2 < l_1)$, moving along a common line with relativity velocity $v$... | Rods of two different lengths have same length in unique inertial frame | physics | 2018-02-17T13:14:17.057 |
[
{
"message": "It was a fine morning and I was cleaning up my room. I found a green laser inside my cupboard. I just kindled a spark on my candle and it started to glow. I took my laser and pointed on the candle and it didn't allow the laser to hit the back wall. And I know that it's because the wax is opaque... | Lasers path through flames | physics | 2018-02-17T16:08:20.493 |
[
{
"message": "I am trying to understand why normal shocks are caused in Converging Diverging Nozzles. My textbook explains that since the exit pressure must match the back pressure, the normal shock is the only mechanism that can make this happen, which I understand.However, why does the exit pressure have to m... | Why does exit pressure matches back pressure in a converging diverging nozzle? | physics | 2018-02-17T17:17:12.667 |
[
{
"message": "I am quite new in nuclear physics instrumentation. So my question may seem silly.We are performing timing experiment in our lab to obtain better time resolution of NaI(Tl) detectors in conjunction with its relevant electronics. Modules are-Na22 source; NaI(Tl) followed by preamp (gain set at X6, m... | Nuclear physics timing experiment without Amplifier | physics | 2018-02-18T11:50:03.143 |
[
{
"message": "I recently asked <a href=\"https://physics.stackexchange.com/q/386773/179462\">a question about the measurement of time, and it has become apparent that I'm really asking a set of related questions, the premises of which need to be shored up and articulated clearly, particularly against the contex... | What do clocks measure? | physics | 2018-02-18T14:38:53.777 |
[
{
"message": "Let's assume that the existance of the graviton is proved theoretically or it is detected by the LHC and there is found a way for Pilot Wave Theory for it to account to special relativity via the constructive quantum field theory. Is there a way for any interpretation to be proven theoretically(be... | Quantum mechanics interpretations | physics | 2018-02-18T15:00:23.500 |
[
{
"message": "The text says :The source connected to the transmission line’s sending end may be any circuit generating an output voltage, such as a radar transmitter, an amplifier, or a computer terminal operating in transmission mode.Doesn't radar emit EM waves and not voltage?",
"role": "user"
},
{
... | Electromagnetic Waves as Voltage Signals | physics | 2018-02-18T16:04:10.893 |
[
{
"message": "In SR, velocity addition allows one to get relative speeds for three objects, and they're all perfectly self-consistent.The three objects are all moving collinearly, but differently. Any of the three relative speeds (between pairs of objects) can be derived by combining the other two speeds, using... | Velocity addition leaves velocities consistent, but not time rates | physics | 2018-02-18T19:40:52.250 |
[
{
"message": "I am following the semi-empirical relationship of mass loss rate of a star with mass M, radius R and luminosity L.$log(\\dot{M}v_{inf}R^{1/2})=−1.37+2.07log(L/10^{6})$where $v_{inf}=\\frac{\\sqrt{(2GM/R)}}{2.6}$M, R and L are in solar unit.I simplified the relationship for plotting purpose:$log(\\... | Relationship betwen stellar mass loss and mass of stars | physics | 2018-02-19T05:16:15.980 |
[
{
"message": "Taking the battery as an example. Intuitively, I understand that without the chemical forces that continuously maintains a voltage difference between the two terminals, the current will stop because, eventually, the two terminals of the battery will obtain the same voltage.However, most textbooks ... | The Significance of Electromotive Force Being Non-Conservative in Origin | physics | 2018-02-19T23:55:15.633 |
[
{
"message": "My textbook states <blockquote> When electrified rods are bought near light objects induction by charge occurs. The rods induce opposite charges on the near surfaces of the object and similar charges move to the farther side of the object . (This happens event when the light objects is not a cond... | Why does attractive force predominate repulsive force in induction by charge of an insulator? | physics | 2018-02-20T13:45:31.123 |
[
{
"message": "Pressure cause attractive gravity so we think that negative pressure should be repulsive.And I've read that negative pressure means constant energy density in an expanding universe which mean energy can be created out of nothing and total energy increases.Is it certain that accelerated space expan... | Is it certain that accelerated space expansion is caused by negative pressure? | physics | 2018-02-20T14:55:24.917 |
[
{
"message": "From physics courses, I'm told that (for Hydrojen atom only), for a given $\\vec L$ of the electron, $$|\\vec L| * cos(\\theta) = m_l,$$where $\\theta$ is the angle between the magnetic field and the orbital angular momentum $\\vec L$.Therefore, for example, if $\\vec L$ lies on the $x-y$ plane, $... | A confusion about the naming of orbitals and the values of magnetic quantum number | physics | 2018-02-21T03:54:32.747 |
[
{
"message": "I am currently taking an engineering course in statics and my teacher is insisting that an off-center force will NOT cause a free object to rotate. This goes completely against my intuition. Here is the problem we are working on:<a href=\"https://i.stack.imgur.com/ylaLo.jpg\" rel=\"nofollow norefe... | Will an off-center force cause an object to rotate if there is no resistance? | physics | 2018-02-21T16:25:45.850 |
[
{
"message": "I am rather confused with the interference of waves that must occur in a string with different densities.<br>Say for example we have a string of length 2L. And the first L part has mass per unit length u, while the second part has mass per unit length 9u.A wave is continuously propagated from the ... | Standing waves on string with different densities | physics | 2018-02-22T05:14:25.633 |
[
{
"message": "I am rather confused with the interference of waves that must occur in a string with different densities.<br>Say for example we have a string of length 2L. And the first L part has mass per unit length u, while the second part has mass per unit length 9u.A wave is continuously propagated from the ... | Standing waves on string with different densities | physics | 2018-02-22T05:14:25.633 |
[
{
"message": "I've been told things such as the sum of the rest masses times c^2, or sum of the relativistic masses times c^2 (I forgot which), were actual equal to things such as the enthalpy of the system rather than the energy. For a system of $n$ particles, having respective relativistic masses $m_0$ and th... | Does the Summation of Rest Masses equal the Total Rest Mass of the System? | physics | 2018-02-22T14:49:43.140 |
[
{
"message": "Imagine a situation in which I introduce a photon of frequency w, described by the state $|1\\rangle$ in a cavity previously filled by radiation in a coherent state $|\\alpha\\rangle$. My question is, which the final state of the system?My problem to answer it, is because the $|\\alpha\\rangle$ st... | which is the state if I add a photon to a coherent state of the light? | physics | 2018-02-22T15:42:57.023 |
[
{
"message": "I am confused with the symmetry group and the representation of spin-$N$ particles, and will appreciate any help or suggestions of reference.There are $2N+1$ internal states for a (massive) spin-$N$ particle. These internal states define a $2N+1$-dimensional Hilbert space. It seems to be resonable... | The symmetry group and representation of spin-$N$ particle | physics | 2018-02-22T17:36:46.637 |
[
{
"message": "As a hobby physicist I don't understand how the double slit experiment is performed in terms of single photons. Is the experiment really conducted by sending and measuring single photons? Is there not always interference of some kind (except for some device like the LHC)?If yes: what would happen ... | Is the double slit experiment performed measuring single photons? | physics | 2018-02-22T17:52:40.400 |
[
{
"message": "The proton anti-proton annihilation at <a href=\"https://en.wikipedia.org/wiki/Annihilation\" rel=\"nofollow noreferrer\">Wikipedia describes the process as not as simple as the electron-positron annihilation. Especially it states:<blockquote> Reactions in which proton-antiproton annihilation pro... | Why is the sum of quarks in proton anti-proton annihilation different in the produced mesons? | physics | 2018-02-22T18:42:04.200 |
[
{
"message": "Does it make sense to speak about the speed of electrons in a current-carrying wire (non perfect conductor)? If so, what is their speed?Here are my thoughts:On the Internet (<a href=\"https://en.wikipedia.org/wiki/Drift_velocity\" rel=\"noreferrer\">Wikipedia, physicsforums, <a href=\"https://phys... | Speed of electrons in a current-carrying metallic wire: does it even make sense? | physics | 2018-02-22T19:34:07.907 |
[
{
"message": "I was reading Griffith's book Introduction to Quantum mechanics and found that for the case of a free particle, we can diregard solutions of the form $e^{kx}$, where $k$ is real (positive or negative).The book uses separation of variables to find possible solutions. I can see those solutions are n... | How to prove an expression is not a solution in 1D Schrödinger equation? | physics | 2018-02-23T01:50:29.500 |
[
{
"message": "This might be a silly question. If we're considering The closed Radiation dominated Friedmann-Lemaitre-Robertson-Walker universe It's common to consider the radiation as a perfect fluid and solve the Friedmann equations from there. My issue is that the trace of the Stress energy Tensor of the free... | Tracelessness of the EM tensor in a closed radiation dominated FRW universe? | physics | 2018-02-23T05:35:19.047 |
[
{
"message": "I am having some basic questions about how to interpret Lagrangians, lets start with Dirac:$L = \\bar{\\Psi} (i \\gamma^{\\mu} \\partial_{\\mu} -m) \\Psi$, where $\\Psi$ is a Dirac-Spinor, $m$ is the mass, $\\gamma^{\\mu}$ is a gamma matrix and $\\partial_{\\mu}$ is the derivative.1) Is $m$ a vect... | How to understand the Dirac Lagrangian? | physics | 2018-02-23T15:49:11.493 |
[
{
"message": "I know that there are some other discussions on this on physics stack exchange, but the other day I was playing with the expression for the Lagrangian and thinking about it's connection with relativity and I thought I may have come up with something.The principle of stationary action states that t... | In classical mechanics, can the Lagrangian be thought of as a metric? | physics | 2018-02-23T21:43:23.810 |
[
{
"message": "In a microscopic classic picture, ohmic heating occurs due to collisions. Let's start with the simplest case: a cooper wire and a battery that induces a current. Electrons start flowing and the energy gained by the field that is accelerating them is lost through collisions with the cooper atoms. I... | Ohmic heating in a fusion plasma | physics | 2018-02-23T23:44:10.663 |
[
{
"message": "My teacher posed this question in class and it stumped me, primarily based on my lack of fundamentals of thermodynamics:Peter Griffin was sitting around when all of a sudden he spontaneously combusts (Based on an episode). Is this possible and if not, under what circumstances is this possible?My r... | Spontaneous Combustion | physics | 2018-02-24T00:04:35.680 |
[
{
"message": "Is it possible to define quantum mechanics in real vector spaces instead of complex vector spaces and what would the dimensionality be of such a vector space?Can anyone referee me such a treatment of a 2 state system, say the spin system.",
"role": "user"
},
{
"message": "It depends on... | QM formulation in real vector spaces | physics | 2018-02-24T09:13:25.893 |
[
{
"message": "How exactly do we justify that two parallel forces $\\vec{\\mathbf{F}}_1$ and $\\vec{\\mathbf{F}}_{2}$ having the same magnitude, acting on a rigid body, do not produce a moment?Yes, if we consider any point $O$ exactly in between the the lines of action of the forces, the net moment is $0$. Since... | Why exactly do two parallel equal magnitude forces acting on a rigid body produce no rotational motion? | physics | 2018-02-24T13:28:53.287 |
[
{
"message": "The image from the wiki article on the stress energy tensor gives $T_{00}$ as $1/c^2$ times the energy density. I believe this is incorrect and that the $1/c^2$ factor should be dropped. Am I missing something?",
"role": "user"
},
{
"message": "You are correct and Wikipedia is wrong.... | Definition of stress-energy tensor | physics | 2018-02-24T16:28:24.120 |
[
{
"message": "Since Electromagnetic waves used for Young's double slit interference experiment are coherent travelling waves, they have temporal variation as well. At the position of the minima the two wavelets always interfere destructively so the intensity is zero there, but at the position of the maxima, the... | Double Slit Experiment Temporal Behaviour | physics | 2018-02-25T06:17:15.680 |
[
{
"message": "According to Wave-Particle Duality, in a normal double-slit experiment, the quantum object behaves as wave and many stripes are displayed on the screen. But if we measure which way the quantum object passes (the measurer being the \"observer\"), the object becomes a particle upon being observed an... | Double-slit experiment on cats makes inconsistency? | physics | 2018-02-25T15:09:42.787 |
[
{
"message": "I've been reading up about Lyapunov exponents for a university group project on chaos theory and I'm a little confused as to how they are calculated for a system of multiple dimensions.From what I can tell, the maximal Lyapunov exponent $\\lambda$ for some 1-d map $f(x_{n})=x_{n+1}$ is:$\\lambda \... | How to calculate the maximal Lyapunov exponent(s) of a multidimensional system? | physics | 2018-02-25T20:36:26.673 |
[
{
"message": "Let's say you have a sound source, in the ocean, at a frequency around 50 kHz. It's a point source, so the sound waves emanate in all directions. Some will go straight up. Do these straight up sound waves reflect off the surface and go straight down?If no, then where does the sound energy go? Into... | Do sound waves in the ocean reflect off the surface at 90 deg incidence? | physics | 2018-02-26T09:10:16.320 |
[
{
"message": "Apologies for using an absolute term (vacuum) in a relative way, but the question revolves around such a contradiction.If we could suck matter out of a region with the full power allowed by the laws of physics, would there be much or any difference between such a vacuum and the vacuum of a weaker... | Is there a limit to how strong a vacuum can be? | physics | 2018-02-26T12:41:05.420 |
[
{
"message": "As we know there are three well known ways in which heat is transmitted:conductionconvectionradiationI suppose fire and sun make us warm in the same way: radiation. Of course, near the fire there should be convection and conduction, but about $1$ meter away, heat should be tranferred to our body m... | Fire and sun: transmission of heat | physics | 2018-02-26T17:58:18.597 |
[
{
"message": "As we know there are three well known ways in which heat is transmitted:conductionconvectionradiationI suppose fire and sun make us warm in the same way: radiation. Of course, near the fire there should be convection and conduction, but about $1$ meter away, heat should be tranferred to our body m... | Fire and sun: transmission of heat | physics | 2018-02-26T17:58:18.597 |
[
{
"message": "If either position or velocity of a particle is measured: Does one usually measure many particles and obtain a frequency distribution for either position or velocity? Or does one just measure one single particle?How does \"the probability distribution\" phycisists sometimes talk about enter in?Is ... | Measuring either Position or Velocity of a Particle - Relation to Probability Distribution | physics | 2018-02-26T19:33:16.197 |
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