dialog listlengths 4 30 | title stringlengths 15 150 | category stringclasses 14 values | dialog_time stringdate 2016-01-21 01:59:30 2018-07-01 01:34:46 |
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{
"message": "The Berry connection, following the derivation of the Berry phase for a non degenerate system, is $\\mathcal{A}_{k}(\\lambda) = i \\langle n|\\frac{\\partial}{\\partial \\lambda^{k}}|n\\rangle$This result is gauge-dependent, as seen making an analogy with the Aharonov-Bohm effect. What I don't see... | Why is Berry connection a connection? | physics | 2017-11-24T12:22:29.867 |
[
{
"message": "The Density matrix of a quantum system in the microcanonical ensemble is given by the\\begin{equation} \\rho_S = \\frac{1}{dim \\mathcal{H}_S} \\mathbb{1},\\end{equation}which is a mixed state.This system can be thought of as a part of a bigger system $S-\\epsilon$ (i.e. the purification of $\\rho... | Quantum Micro-Canonical Ensemble and Maximal Entanglement with Environment | physics | 2017-11-24T15:01:18.487 |
[
{
"message": "If I have a space of uniform electric field, let's say always pointing upwards and of strength 5 N/C, if I place a ball of 2 C in that field. It should experience a force of 10 N and accelerate upwards, right? (The space is also free from gravitational field)I have a few questions here,Would the f... | Question regarding charge in a field | physics | 2017-11-24T20:55:39.710 |
[
{
"message": "Using Maxwell's Equations, namely,$\\vec{\\nabla} \\cdot \\vec{E} = \\frac{\\rho}{\\epsilon_{0}}$$\\vec{\\nabla} \\cdot \\vec{B} = 0$$\\vec{\\nabla} \\times \\vec{E} = \\frac{\\partial \\vec{B}}{\\partial t}$$c^2 \\vec{\\nabla} \\times \\vec{B} = \\frac{\\vec{j}}{\\epsilon_{0}} + \\frac{\\partial ... | Expressing $\vec{B}$ and $\vec{E}$ in tensor components | physics | 2017-11-25T03:21:25.533 |
[
{
"message": "I came across the following lines that appear after the derivation of equation of continuity for the steady flow of an ideal liquid in Resnick, Halliday, Kranes's Fundamentals of Physics:<blockquote> The equation of continuity states that if within any volume element of space (not volume of fluid... | Equation of continuity | physics | 2017-11-25T10:49:21.420 |
[
{
"message": "Adiabatic process is a process in which there is no heat exchange between the surrounding and the gas.So for example if an expansion happens really fast this process is adiabatic because there is no time for the heat to get transfered.My professor said that we can describe this situation with idea... | Ideal gas going though an adiabatic process | physics | 2017-11-25T18:23:20.097 |
[
{
"message": "How to calculate electric power losses to radiation of electromagnetic waves in copper wire with high-frequency current?For example, I have a copper wire with known diameter and length and I send inside current I ampers with frequency f Hz. How much energy will go to electromagnetic waves? Is the... | Power losses for electromagnetic waves radiation in copper wire with high frequency current | physics | 2017-11-25T23:08:16.637 |
[
{
"message": "An electron possesses ‘spin’ even at absolute zero. But how are the spin-states identified? It couldn’t be in terms of ‘energy’ states because well, the electron does not possess any energy at all.",
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{
"message": "The explanation of the electrons spin in the 20th of t... | How is spin-state of an electron defined? | physics | 2017-11-26T03:48:16.013 |
[
{
"message": "The Vaidya Metric is the metric that can be used to describe the spacetime geometry of a varying mass black hole. This metric reads $$d\\tau^2=\\bigg(1-\\dfrac{2M(\\nu)}{r}\\bigg)d\\nu^2+2d\\nu dr - r^2 d\\Omega_2^2$$For simplicity, I will assume $d\\Omega_2=0$. Now, the condition for any interval... | Nothing falls into an evaporating black hole? | physics | 2017-11-26T17:36:58.840 |
[
{
"message": "Why do these histograms correspond to a simulated SUSY event? What kind of superparticles do we actually see <a href=\"http://atlasexperiment.org/photos/events-simulated-supersymmetry.html\" rel=\"nofollow noreferrer\">here? We observe 3 histograms, but what do they tell us?How do they simulate su... | How does CERN simulate and interpret a simulated SUSY event of the Atlas detector? | physics | 2017-11-26T21:47:02.913 |
[
{
"message": "I have read answers stating that light is not able to be transmitted through a wall whereas sound is easily transmitted due to difference in wavelength. But, why? I am looking for detailed and satisfactory answer. Please also include the reason of WIFI signals easily transmitting through any media... | Why light cannot pass through wall whereas Sound can? | physics | 2017-11-27T12:38:40.227 |
[
{
"message": "For RF applications, we are usually considering of impedance matching to choose load impedance as the conjugate of the source input impedance for the maximum transferred power.But how about design the source having zero input impedance? Is it even possible or good? A naive thought is that all the ... | Is zero input impedance good or possible when considering impedance matching? | physics | 2017-11-27T19:02:30.533 |
[
{
"message": "Imagine two bodies (very, very small compared to the size of the earth), 1(km) separated from each other (in the direction of the earth) at infinity. After they reach earth after an infinite long trip how far will they be separated due to the tidal force? Can you use the tidal force to calculate t... | How far apart are two initially $1000m$ separated bodies that fall to earth from infinity, when the closest body reaches earth? | physics | 2017-11-28T00:21:10.643 |
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{
"message": "I attempted to modify a classic experiment to demonstrate Archimedes principle to my 5th graders and I created an error I can’t explain. We started with a graduated cylinder containing 20 mL of water.We added 10 pennies to the cylinder forcing a rise of about 5 mL to the column, or a total of 25 ... | Displacement and mass of water in a column | physics | 2017-11-28T00:25:44.640 |
[
{
"message": "I was watching a documentary and they were talking about causing fusion with laser( \" over simplified \" ) they have over 100 different lasers all low power like the power of a dvd player as they said but as the beams merge they are filled with a large amount of energy. That hits a hydrogen atom... | If you merge lower power laser beams together do it increase the power? | physics | 2017-11-28T03:05:16.300 |
[
{
"message": "I have a question regarding ideal gasses and pressures. The end goal is to determine the amount of gas (volume) evolved when a specimen within the vessel has been introducted to heat.The specimen is kept in a fixed container, so the volume is fixed. As the specimen heats up in a closed system, i... | Determining volume of gas evolved when a specimen is heated | physics | 2017-11-28T17:19:08.433 |
[
{
"message": "I was trying to explain something about Brewster's angle and realized I don't completely understand how reflection and refraction work on the microscopic, classical level. Consider a plane polarized light wave impinging on glass. The charges inside the glass oscillate in some way so that the origi... | How can reflection and refraction be explained classically and microscopically? | physics | 2017-11-29T20:00:27.740 |
[
{
"message": "If ordinary and extraordinary rays emerge out of calcite crystal at different points(double refraction) then why do we consider perpendicular electromagnetic wave components shifted by a phase in case of wave plates emerge at the same point?",
"role": "user"
},
{
"message": "This is be... | How does ordinary and extraordinary rays split inside calcite crystal in context of electromagnetic waves? | physics | 2017-11-30T12:56:57.897 |
[
{
"message": "Sorry if it has been asked before I am sure. Anyways, if I hold a ball in the air, I do no work but I do spend energy (\"chemical\", heat... whatever happens in my muscle) and get tired. Where does the energy or whatever to cause me to get tired come from? For example how is an electrical charge c... | Where does potential energy come from? | physics | 2017-11-30T22:15:33.470 |
[
{
"message": "This may be a really simple question: If electrons only absorb specific wavelengths of light and then emit them and that’s how we see color, but what happens to the other wavelengths?Without them being absorbed they would just pass through in my mind, but no light does get through a sufficiently t... | Emission and Absorption in color | physics | 2017-12-01T04:17:27.310 |
[
{
"message": "More Details:Is there a spot with antimatter where the repulsive force of its nucleus and the attractive force of the positrons cancel out? As in if I took a heavy antimatter atom, ionized it, would there be a scenario in which electrons get attracted to the positrons around the anti-atom but cant... | Is there an antimatter sweet spot? | physics | 2017-12-02T01:01:08.613 |
[
{
"message": "Referring to <a href=\"https://en.wikipedia.org/wiki/Poisson_bracket#Hamilton.27s_equations_of_motion\" rel=\"noreferrer\">Wikipedia we have that the equation of motion for a $f(q, p, t)$ comes from the formula\\begin{equation}\\frac{\\mathrm{d}}{\\mathrm{d}t} f(p, q, t) = \\frac{\\partial f}{\\pa... | Hamilton equations from Poisson bracket's formulation | physics | 2017-12-02T12:37:29.257 |
[
{
"message": "My lecture notes state that if gas particles sit in an insulating container, with a partition separating the gas from vacuum is removed, no change in internal energy is done, and it is stated that it is because the thing separating the gas is vacuum. I'm confused as to why this implies that the ga... | Why does expansion in a vacuum cause gas particles to not do any work? | physics | 2017-12-02T18:44:39.313 |
[
{
"message": "We have a 25 ft race hull with Jet drive pump (we race 30+ mile legs), we are hitting sppeds @150mph. lowering the hydronic drag is a challenge, i am thinking about installing a air injection system into the bottom of the boat. If i install 40 holes (20 ) on each side facing down and flush with th... | air pressure to assist lift on a Race boat | physics | 2017-12-02T22:21:58.973 |
[
{
"message": "For generic two level system $H = d(R) \\cdot \\mathbf{\\sigma}$.$$C_{-} =\\frac{1}{2\\pi} \\int_{S^2} d\\theta d\\varphi \\Omega_{\\pm,\\theta\\varphi}=+1$$$$\\gamma _{n}=\\int _{\\mathcal {S}}d\\mathbf {S} \\cdot \\mathbf {\\Omega } _{n}(\\mathbf {R} ).$$However ccording to Stokes’ theorem, the ... | Why does calculating Berry phase with stokes theorem give you a different result from a direct integration? | physics | 2017-12-03T00:05:20.087 |
[
{
"message": "Is it possible to arrange a setup in which a convex mirror forms a real image(i.e an image that can be obtained on a a screen.)Imagine a setup in which light from infinity falls on a concave mirror and the mirror converges it on the focus.(f would be negative)However, if we put a convex mirror be... | Can a convex mirror form a real image? | physics | 2017-12-03T15:08:04.833 |
[
{
"message": "In chapter 23.6, Schwartz's quantum field theory book defines renormalizability as follows, paraphrasing a bit for brevity:<blockquote> Consider a given subset $S$ of the operators and its complement $\\bar{S}$. Choose coefficients for the operators in $S$ to be fixed at a scale $\\Lambda_L \\ll ... | What is the Wilsonian definition of renormalizability? | physics | 2017-12-03T16:37:03.753 |
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{
"message": "I was going through this arxiv paper by Magnano: <a href=\"https://arxiv.org/abs/gr-qc/9312008\" rel=\"nofollow noreferrer\">gr-qc/9312008For modeling non linear and repulsive gravity he takes the generic lagrangian $$L = f(R)\\sqrt{-g}.$$Varying this $L$ with $g^{\\mu\\nu}$, I get$$f'(R)R_{\\mu\\... | Repulsive gravity formulation: a query | physics | 2017-12-04T09:27:33.920 |
[
{
"message": "I found this interesting question on some MIT problem sheet. Say a rope with mass is attached to a shaft and experiences angular velocity so that it is horizontal, find a its tension as a function or distance $r$, ignoring gravitational effects.The two initial conditions that I found were, $T\\lef... | Differential tension in a rope experiencing constant angular velocity | physics | 2017-12-04T11:51:35.227 |
[
{
"message": "Almost every text on topological insulators have the Bloch sphere example of a two level system showing the non triviality of the bundle of an eigenvector over the sphere: we can't define an eigenstate over the whole Bloch sphere, instead of this, we must construct two local trivializations, namel... | Is the non-trivial topology on the torus reflected on the Bloch sphere? | physics | 2017-12-04T12:35:29.070 |
[
{
"message": "Course tutorial questions on faradays law often give numerous examples of rates of changing angle between the A and B vector, or rate of changing Area. However for a dB/dt case you are always given dB/dt in Teslas per second.If you only know the velocity of a permanent magnet, moving towards a con... | How can dB/dt be defined as a function of the velocity of a permanent magnet (with a known B value) moving towards and then through a conducting loop? | physics | 2017-12-04T14:52:50.203 |
[
{
"message": "Why is a wave function time dependent? And what does it physically mean when the probability density density function is varying with time. I'm not getting how to physically interpret that. Will taking example of a particle in 1d box with superposition state two energy levels help? Because it's si... | Why is a wave function time dependent? | physics | 2017-12-05T14:04:30.440 |
[
{
"message": "Current understanding:Nonlinear optics is classified by the effect of an electric field applied to a medium. In nonlinear crystals, when a field is applied the dielectric polarization does not respond proportionally to it. Nonlinear mediums are employed to change the wavelength of light. When two ... | How does frequency mixing work in a non-linear crystal? | physics | 2017-12-05T22:37:59.400 |
[
{
"message": "I've been reading the book, <a href=\"https://rads.stackoverflow.com/amzn/click/0262550210\" rel=\"nofollow noreferrer\">World Enough and Space-Time, and I came across a rough list of classical space-times with varying structural significance. <a href=\"http://faculty.poly.edu/~jbain/spacetime/lec... | How can I interpret or mathematically formalize Maxwellian, Leibnizian, and Machian space-times? | physics | 2017-12-06T02:03:16.533 |
[
{
"message": "So I understand that when you take, let's just say a plate, and you spin it, any objects on it would be \"pulled\" outwards (Because they want to follow a straight line rather than a circular one). But if I stood on this plate it would look like the world is spinning, but this spinning world cause... | Do centrifugal forces occur when the world around me is spinning but I am at rest? | physics | 2017-12-06T04:22:39.603 |
[
{
"message": "I have a set of displacement-time graphs from an experiment to convert to the frequency domain. Both the Fourier and Laplace transform seem to do this, so what's the difference between them (difference in end result, not the mathematical difference) Also, is there even a way to perform a Laplace t... | Difference between Fourier and Laplace transforms in analyzing data | physics | 2017-12-06T04:25:21.933 |
[
{
"message": "A light clock is a mechanism that produces (and counts) equal time intervals.Yes, but, do we understand correctly what the light clock is showing?Since we don't know what the time exactly is, we can say that time is represented by a series of events (this event is not a point in Minkowski space-ti... | Time is defined operationally to be that which is measured by clocks | physics | 2017-12-06T08:29:30.340 |
[
{
"message": "I want to calculate the deflection and maximum stress of two beams that hang parallel under each other, as shown in the image below.<a href=\"https://i.stack.imgur.com/G6dto.png\" rel=\"nofollow noreferrer\">With some simulations I have found out that the beam that is perpendicular to both paralle... | Deflection and maximum stress of two beams parallel under each other | physics | 2017-12-06T14:31:38.443 |
[
{
"message": "I am looking for raw data of the (normalized) neutron flux of a $^{241}$Am-$^9$Be-source, or in other words, the data of <a href=\"http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2016/05/20/08.17-75260-AmBeSpec_ISOrecomm.jpg\" rel=\"nofollow noreferrer\">this graph. Where can I find th... | Where can I find raw data of neutron energy spectra? | physics | 2017-12-06T16:27:09.513 |
[
{
"message": "Let $\\phi_a$ and $\\chi_{\\dot{a}}$ be two component commuting spinors, where $\\chi$ is an anti-spinor.In terms of some spinor basis, these can both be written in some arbitrary frame as$$\\phi_a(P) = \\sqrt{E - |p|}\\xi_a^-,~~~~~\\chi_{\\dot{a}}(P) = \\sqrt{E+|p|}\\tilde{\\xi}^-_{\\dot{a}}$$Whe... | Weyl Spinors and Lorentz Invariance | physics | 2017-12-06T17:09:19.653 |
[
{
"message": "Can one think of Wigner D-matrix $\\mathcal D_{mm''}^{(l)}$ as a spherical tensor even in case when $m''\\neq0$ ?In other words does relation like \\begin{equation} \\tag{1} \\mathcal D^\\dagger(R_1)\\mathcal D^{(l)}_{mm''}(R_2)\\mathcal D(R_1) = \\sum_{m'} D_{mm'}^{(l)*}(R_1)D^{(l)}_{m'm''}(R_... | Is Wigner D matrix also a spherical tensor? | physics | 2017-12-06T18:52:58.957 |
[
{
"message": "My title question relies on the answer to this question: Under the standard model and assuming baryon symmetry, is it possible (however unlikely) that some small local pocket of the universe might have had a little more surviving matter than antimatter?If that's possible, than by the weak anthropi... | Why does the lack of antimatter in the observable universe suggest a baryon asymmetry? | physics | 2017-12-07T03:41:31.417 |
[
{
"message": "The displacement operator generates the coherent state out of the vacuum.$$\\hat D(\\alpha)|0\\rangle = |\\alpha\\rangle$$but I am wondering what the meaning of a displacement operator acting upon a number state with $n \\neq 0$ is.i.e $$\\hat D(\\alpha)|1\\rangle$$Is this meaningful? I have not b... | Displacement operator acting on a number state | physics | 2017-12-07T09:21:09.673 |
[
{
"message": "I am a beginner to tensors. For many days,I have been stuck with the contravariant and covariant tensors. I searched everywhere in Internet for its meanings and examples but everywhere there are mathematics for transformation of one form to other. It has become very difficult for me to understand ... | How to visualize the physical examples of contravariant and covariant vectors? | physics | 2017-12-07T09:28:37.113 |
[
{
"message": "The kinetic energy equation$E_k=\\frac{1}{2}mv^2$seems to fit as an integral of the momentum equation$p=mv$But i can't figure out how to relate the meaning of the area under the momentum equation line to the meaning of energy.",
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{
"message": "That's not actually the c... | What does the area under $p=mv$ mean? | physics | 2017-12-08T01:51:29.610 |
[
{
"message": "I have a very simple circuit, composed of a capacitor and a battery with constant $V$.The wires that connect these two components allow the capacitor to gain a voltage V ( the same V of the battery).Do the wires that connect the capacitor and battery maintain a neutral charge density? And why?",
... | Charge density in wires | physics | 2017-12-08T13:43:52.520 |
[
{
"message": "Is Ch. 6 of Birrell & Davies book on QFT in curved space and in particular the 1-loop effective action that they derive up-to-date with the current state of the art in (effective) quantum gravity? I have no background in string theory and only in condensed-matter and I'm quite liking the appro... | Modern treatment of effective QFT in curved spacetime | physics | 2017-12-09T19:10:35.977 |
[
{
"message": "Here is the problem I was trying to solve:<blockquote> Find the potential difference between the points A and D <a href=\"https://i.stack.imgur.com/cOoHp.png\" rel=\"nofollow noreferrer\"></blockquote>I used Kirchhoff's voltage law for the left loop and right loop and found out the current thro... | Will current flow if there's no return path? | physics | 2017-12-10T01:42:50.250 |
[
{
"message": "For example, if a single beam of light was directed directly at the tangent of a semi circular mirror, would it be considered bending or redirecting many times to form a near circular pattern? When I say bend I mean in a curved trajectory, not at an angle.",
"role": "user"
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{
"messag... | For all practical purposes can light be bent (without the help of gravity) or just reflected? | physics | 2017-12-10T03:42:58.273 |
[
{
"message": "I saw a answer for <a href=\"https://physics.stackexchange.com/a/136166/169288\">Why are these two definitions for symmetries in the Lagrangian equivalent?I have an example has $\\delta \\mathcal{L}=0$ but $\\delta S \\neq 0$ and $$\\delta S \\neq \\epsilon \\int_{\\partial\\Omega} d^3 x f .\\tag{... | Transformation has $\delta \mathcal{L}=0$ but $\delta S \neq 0$ and $\delta S \neq \epsilon \int_{\partial\Omega} d^3 x f $ | physics | 2017-12-10T04:35:36.603 |
[
{
"message": "Physicists always ask why the cosmological constant is not exactly zero!I would ask here, what if cosmological constant was zero? The universe wouldn't expand and matter would exert gravitational force and shrink the universe into a big crunch!So, why physicists want the constant to be zero then? ... | What if cosmological constant was zero? | physics | 2017-12-10T16:51:46.290 |
[
{
"message": "When looking at the map of the anisotropies of the CMB, do slightly hotter (red) spots correspond to denser or less dense regions at the time of decoupling? I am getting confused, because I have read arguments in both directions: In some places it is said that photons coming from denser regions ar... | Do hot spots in the CMB anisotropy map actually correspond to denser or less dense regions? | physics | 2017-12-10T18:25:10.330 |
[
{
"message": "Most of Hilbert spaces appearing in real life are separable ones: even such large for the first spaces as Fock space, spaces of functions in infinite number of variables and so on. However I heard that when dealing with quantum loop gravity one encounter non-separable Hilbert space. <blockquote> ... | Non-separable Hilbert space in LQG | physics | 2017-12-11T17:02:05.050 |
[
{
"message": "If we launch an object on one side of the moon (the velocity being tangent to the surface of the moon), how big must the velocity be to let the object arrive at a position on the moon diametrically opposite to the launching site (in some device to catch the mass)? The mass is very, very small comp... | What velocity has a mass when launched tangently on one side of moon, to let it arrive at a position diametrically opposite on the moon? | physics | 2017-12-12T00:17:33.823 |
[
{
"message": "Say the initial moment of inertia of the (physical) pendulum was (ML²)/12.How does that result change once you put a mass anywhere on the pendulum?",
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},
{
"message": "The initial moment of inertia $ \\frac{ML^2}{12} $ of the pendulum is about an axis/center of rotation. ... | How does the moment of inertia of a pendulum changes if you add a mass on the free spinning end? | physics | 2017-12-12T03:06:00.767 |
[
{
"message": "Suppose that I accept that there is wave function collapse in quantum mechanics, and that the probabilities associated with each orthogonal subspace are a function of the wave function $\\psi$ before the collapse. I've seen some references that claim that in this case, <a href=\"https://en.wikiped... | Does Gleason's Theorem Imply Born's Rule? | physics | 2017-12-12T07:50:05.290 |
[
{
"message": "Upon building up supersymmetric Supergravity in 4D, is it necessary to modify the Einstein's vacuum field equations (apart from adding the Rarita-Schwinger Lagrangian for the gravitino) in order to obtain Supergravity 4D or can it be built up by simply adding the Hilbert-Einstein Lagrangian and th... | For making gravity supersymmetric is a modification of Einstein's vacuum field equations necessary ( apart from adding the R-S Lagrangian)? | physics | 2017-12-12T12:40:08.070 |
[
{
"message": "I just want to make sure I am thinking clearly about canonical coordinates and transformations in Hamiltonian mechanics.Suppose we have a Hamiltonian system $(M, \\omega, H)$ — where $M$ is the phase space ($\\dim(M)=2n$, although $M$ is not necessarily a cotangent bundle globally), $\\omega... | Can any symplectomorphism be called a canonical transformation? | physics | 2017-12-12T22:42:54.450 |
[
{
"message": "I'm studying a system formed by multiple rotors non-linearly coupled to each other. If I want to look for phase synchronization, I could just look at the angle for predefined points around the rotational axis of each rotor and test for phase locking or evaluate the synchronization index. The probl... | When can raw data can be used to look for phase synchronization between time series? | physics | 2017-12-13T00:55:54.580 |
[
{
"message": "I came across a question: <blockquote> Find $f(r)$ and prove the centre of mass formula:<br> $$\\vec{r_{cm}} = \\frac{1}{V} \\int f(r) \\vec{dS} $$<br> Where $V$ is the total volume and our surface integral is over a body with uniform density.</blockquote>I'm not even quite sure where to ... | Proving the centre of mass formula with integral | physics | 2017-12-13T11:38:05.130 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/KJZIF.png\" rel=\"nofollow noreferrer\">This is my first time posting anything, but I could not figure out the derivation of the amplitude of the spring for the 4 first oscillations. I am not acquainted with the effect and calculations of damping since I am only... | The amplitude of a damped spring with a weight during the 4 first oscillations | physics | 2017-12-13T19:28:44.397 |
[
{
"message": "I am currently writing a report on the basics of quantum Gauge Invariance and there is a concept I am struggling with. An early part of my discussion in the report concerns charge conservation in QM and I want to briefly explain how it arises from invariance under a Global Phase transformation $e^... | Why do Global phase transformations lead to the conservation of charge? | physics | 2017-12-14T13:46:17.397 |
[
{
"message": "(just a quick thought question for my journal) So if you took a magnet and somehow increased the density would there be a definite increase in the force of the pull or would there be a way for that force to increase without human help. The planets ,when close enough, begin to pull each other towar... | Does density affect Magnetism | physics | 2017-12-14T16:37:16.873 |
[
{
"message": "Setup: Consider the Feynman propagator as it appears in Peskin and Schroeder eq. 2.59:\\begin{align}D_F(x-y)\\equiv i\\int_{\\mathbb{R}^4} \\frac{d^4p}{(2\\pi)^4}\\frac{e^{ip\\cdot(x-y)}}{p^2-m^2+i\\epsilon},\\end{align}where $p,x,y$ denote 4-vectors; $p^2\\equiv \\eta_{\\mu\\nu}p^\\mu p^\\nu$, $p... | Is the Feynman propagator integrable over Minkowski spacetime? | physics | 2017-12-14T20:24:12.630 |
[
{
"message": "Is the following logic correct?:If you have an Hamiltonian, that has time has a variable explicitly, and you get the Lagrangian $L$ from it, and then you get an equivalent $L'$, since $L$ has the total time derivate of a function, both Lagrangians will lead to the same equations Euler-Lagrange equ... | Hamiltonian and Lagrangian classical mechanics | physics | 2017-12-14T20:28:16.570 |
[
{
"message": "Von Neumann entropy is defined as$$ S=-\\mathrm{Tr}\\left(\\rho \\ln\\rho\\right)$$It can be used to measure the entanglement between two sub-systems, provided that the total system is in pure state. 1) What information does the Von Neumann entropy give if the total system is in a mixed state? As ... | What information does the Von Neumann entropy give for mixed states? | physics | 2017-12-14T20:59:08.873 |
[
{
"message": "Assuming that the fibre is flawless, does the index of refraction of an optical fiber cumulatively increase with increasing length of travel of light within the fiber? Some descriptions of refraction lead to a strong inference that this must be true but at the same time the values are usually give... | Does the index of refraction of an optical fibre increase with the light path length? | physics | 2017-12-15T03:00:05.060 |
[
{
"message": "Instantaneous action at a distance may occur due to an experiment being performed within a train. A particle may decay in the middle of the train, and when decaying it may split into 2 entangled particles that fly apart in opposite directions. Let's say that these particles are photons. When meas... | How is instantaneous action at a distance governed by a specific frame of reference? | physics | 2017-12-15T12:03:51.993 |
[
{
"message": "So, this has been bugging my since my youth. When looking at a map of The Earth.In the Southern Hemisphere, both Africa and South America are tall and slim, they get thinner towards the pole. Then there's Australia, and a huge amount of scattered smaller islands.The Northern Hemisphere appears to ... | Why is there more land-mass in The Northern Hemisphere? | physics | 2017-12-15T13:43:04.367 |
[
{
"message": "This question is about the intuition on one end of the Fraunhofer double-slit <a href=\"http://nptel.ac.in/courses/117101055/cdeep%20demo%20ppt/double.html\" rel=\"nofollow noreferrer\">diffraction pattern. It is not about the Fourier transform that connects a pair of rectangles to their sinc func... | Intuition behind the "rect" function in Fraunhofer double-slit analysis | physics | 2017-12-15T16:00:48.157 |
[
{
"message": "\"Change in internal energy is zero if temperature is constant because, internal energy is a function of temperature only.\"Well, this is what I read everytime I study thermodynamics. I came across a question where some heat (Q) was provided and due this heat supplied, a few moles of the gas disso... | Change in internal energy if temperature is constant and some moles of the gas dissociate | physics | 2017-12-16T07:21:49.060 |
[
{
"message": "Electromagnetic radiation from the Sun heats the air –manifested in air currents- which eventually powers, for example, windmills. Schematically, Sun->Air->Windmills.Question: Why isn't Sun->Windmills? What´s the need of intermediary like a air/gas to perform work?",
"role": "user"
},
{
... | The Work of Sunlight. Why intermediaries? | physics | 2017-12-16T18:02:17.890 |
[
{
"message": "Suppose their are 3 objects(O1, O2 and M) in a region of space with large variations in the momentum stress tensor.They all have 3 different trajectories through a very inhomogeneous spacetime manifold, given by O1($\\lambda$), O2($\\lambda$), M($\\lambda$).I understand that the manifold is define... | How are frames of reference releated to each other in general realitivity? | physics | 2017-12-17T16:09:34.087 |
[
{
"message": "Somewhere I found the explanation that the EM fields create and destroy each other during the oscillation (I suppose by Faraday's law) and this makes the wave \"move\". I can't imagine this because unitary vectors in E,B and k directions are a right hand ordered set of vectors and by the fact of E... | How does an electromagnetic wave move? | physics | 2017-12-18T07:19:29.537 |
[
{
"message": "If electrons move from the negative side to the positive side in a battery(thus creating electricity when they flow), why doing the same circuit with two separate batteries doesn't create electricity? <a href=\"https://i.stack.imgur.com/Acbvf.png\" rel=\"nofollow noreferrer\">As you can see in the... | Why two separate batteries in a circuit don't work? | physics | 2017-12-18T12:06:08.270 |
[
{
"message": "When physicists say that a quantum particle's position is uncertain, does that mean specifically that the location where the wave function may collapse on a detector is uncertain? Or can a wave function itself be centred on multiple points in space at once?",
"role": "user"
},
{
"messa... | Can quantum particle wave functions be located on multiple points in space at once? | physics | 2017-12-18T16:58:25.797 |
[
{
"message": "I'm an electronics student, and don't know much about some physics concepts.I was studying superconductivity and came to the London equation, Meissner effect and BCS theory.<br>I kind of understood these things, but am still wondering: what is <a href=\"https://en.wikipedia.org/wiki/Superconductin... | What is superconducting coherence length? | physics | 2017-12-18T17:17:05.713 |
[
{
"message": "The article \"Quantum Logic and Probability Theory,\" by Wilce, has the following in <a href=\"http://plato.stanford.edu/archives/spr2017/entries/qt-quantlog\" rel=\"noreferrer\">section 1.4:<blockquote> 1.4 The Reconstruction of QM From the single premise that the “experimental propositions” ... | Probability measure implies quantum mechanics? | physics | 2017-12-18T21:38:26.090 |
[
{
"message": "In the <a href=\"http://www.feynmanlectures.caltech.edu/I_28.html\" rel=\"nofollow noreferrer\">Feynman Lectures, Volume I, 28-1, Feynman promulgates thisexpression for the electric field due to a moving charge. My notationdiffers from the original.$$\\mathfrak{E}=-\\frac{q}{4\\pi\\varepsilon_{o}}... | Is Feynman correct about this inverse square relationship in Vol I-28? | physics | 2017-12-19T00:42:58.927 |
[
{
"message": "Dark energy/matter is assumed to exist based on calculations that don't include terms for radiation energy in transit thru the system under consideration. Could the energy in transit thru a given system which is not detectable (orthogonal) by line of sight to the detector (telescopes either groun... | Regarding dark energy/matter calculations | physics | 2017-12-19T00:53:52.593 |
[
{
"message": "If rotation was relative, we could say that the Universe is revolving around the earth. But how could this be true, since at some distant point the speed of a star would be greater than the speed of light ? How does Mach answer to this question ?",
"role": "user"
},
{
"message": "One a... | Mach's principle and infinite speed of distant stars | physics | 2017-12-19T01:36:06.153 |
[
{
"message": "Is there any way to tell how the energy levels of a potential well depend upon the quantum number $n$, looking at the potential, before solving?I mean, for harmonic oscillator, $E\\sim n$, for infinite square well, $E\\sim n^2$, so, can it be guessed how it would be for a power law potential $\\si... | Dependence of energy $E$ with principal quantum number $n$ | physics | 2017-12-19T09:08:52.413 |
[
{
"message": "Let us consider a 3D box of volume $V$, containing $N$ identical Brownian particles. The diffusion coefficient of the particles is noted $D$. Inside this box there is a square surface of area $L^2$.To keep everything as simple as possible, the box dimensions are much larger than that of the surfac... | Collision rate of Brownian particles with a surface | physics | 2017-12-19T18:21:33.723 |
[
{
"message": "The link at question is here: <a href=\"http://www.einsteins-theory-of-relativity-4engineers.com/twin-paradox-graphical-solution.html\" rel=\"nofollow noreferrer\">http://www.einsteins-theory-of-relativity-4engineers.com/twin-paradox-graphical-solution.html<a href=\"https://i.stack.imgur.com/rdGh6... | Is this spacetime diagram for the twins paradox correct? | physics | 2017-12-20T19:44:09.977 |
[
{
"message": "The Standard Model of particle physics can be constructed by specifying its gauge group $G$ and the representations of the fields (plus some extra information: Lorentz invariance, values of the coupling constants, etc.). Using this model we can predict many experimental facts, such as the cross se... | Determination of the global structure of the SM gauge group | physics | 2017-12-22T16:46:51.057 |
[
{
"message": "Can anyone explain how Wien's distribution law seen in Wien's original \"On the Division of Energy in the Emission-Spectrum of a Black Body\":$$ \\phi_\\lambda = \\frac{C}{\\lambda^5} e^{-\\frac{c}{\\lambda \\theta}},$$where (using Wien's notation) $\\phi_\\lambda$ is the intensity, $\\lambda$ is ... | Planck's Form of Wien's Distribution Law | physics | 2017-12-22T20:52:27.900 |
[
{
"message": "To generate three light neutrino mass eigenstates via type-I seesaw, we include a set of $n$ (need not be equal to 3) heavy$^1$ right-handed fields $N_R$ in addition to three $\\nu_L$ fields to the Standard Model. Then type-I seesaw mechanism gives 3 light and $n$ heavy mass eigenstates which are ... | How are the sterile neutrinos $\nu_s$ different from the heavy right-handed fields $N_R$ (or $\nu_R$)? | physics | 2017-12-23T19:09:26.253 |
[
{
"message": "It seems that the quark pair superconductor can be odd or even parity pairing respect to the parity $P$.Say that the even parity has the form:$$\\langle\\psi C \\gamma^5 \\psi\\rangle$$the odd parity has the form:$$\\langle\\psi C \\psi\\rangle$$There is no difference at perturbative computation... | Quark pair superconductor: Even parity is favorred than odd parity | physics | 2017-12-25T01:15:26.563 |
[
{
"message": "The question has now been amended to:\"Is there a systematic causal link between the direction of a photon and the direction the mass from which it emerges is moving in? I'm not talking about the direction of the electron involved in radiating the photon, but the direction of the overall mass (ato... | Photon momentum components | physics | 2017-12-25T11:22:06.030 |
[
{
"message": "<a href=\"https://i.stack.imgur.com/sfhKC.jpg\" rel=\"nofollow noreferrer\">A couple of days ago I was in a friend's car, and I noticed this pattern on the windows; I took a picture of the sun through the window to make it clearly visible. The night before had been quite cold, but I don't think th... | Strange pattern on car windows | physics | 2017-12-25T11:25:33.143 |
[
{
"message": "When a quantum of light arrives at a double slit, it passes through both slits as a wave and arrives upon a second screen with the interference pattern of a single wave that has been split into two waves, that have then interfered with each other.If a detector is placed at one of the slits and the... | Double Slit Experiment. What effect does the detector actually cause? | physics | 2017-12-25T23:18:59.560 |
[
{
"message": "Hi everyone this is my first post so please be nice :)Okay so imagine this setup:Let's say that we do the classic double-slit experiment with a twist.We all know how the double slit experiment works (if not please look it up)If we leave the detector in place, but \"turn it off\", then it goes back... | Wheeler's delayed-choice experiment as a time machine? | physics | 2017-12-26T06:43:50.927 |
[
{
"message": "In sound waves, amplitude affects loudness. But just as a single person can sing loud, a choir can sound loud, even though each individual isn't singing loud. Can lights brightness be affected by amplitude as well as intensity?",
"role": "user"
},
{
"message": "The energy transported b... | Does the amplitude of an electromagnetic wave affect brightness? | physics | 2017-12-26T11:16:49.577 |
[
{
"message": "I understand that when I have two separate states that their combination state increases the Hilbert Space to $|\\psi_1\\rangle \\otimes |\\psi_2\\rangle$For example, looking at a simple example where we are considering two possible states, this can be expanded to: $(a|H_1\\rangle+b|V_1\\rangle)\... | How are entangled states created? | physics | 2017-12-26T23:36:13.003 |
[
{
"message": "I'm working on a magnetic system and in the calculations this function appears\\begin{equation}\\gamma(\\vec{k})=\\frac{1}{Z}\\sum_{\\vec{\\mu}}e^{i \\vec{\\mu} . \\vec{k}}\\end{equation}where $\\vec{\\mu}$ are the primitive vectors of a Bravais Lattice. In the book Spin waves: theory and applica... | When does the structure function have parity symmetry? | physics | 2017-12-27T02:07:33.320 |
[
{
"message": "Let's say we have two ships moving at, for example, .8c. Let's put them 1 light-year apart and give them exactly the same velocity. I want to point a laser from one ship and hit the other ship.My initial thought is that since the speed of light is constant I'll have to aim the light ahead, so that... | Communication between two observers moving at the same relative velocity | physics | 2017-12-27T18:30:46.520 |
[
{
"message": "Why it is that high altitude areas are colder than low altitude areas? Is it because the air pressure or any other?If we consider it with the sun rays, high altitudes receive sun rays earlier than low altitude so low altitudes must receive rays with less amount of energy relative to high altitude... | What factors affects the cold temperatures in high altitudes? | physics | 2017-12-28T04:48:37.250 |
[
{
"message": "If you had a perfectly still pool of water and dipped your finger in it, the disturbance would cause waves to ripple through the water, away from you finger. If you measured how curved the waves are, could you find the distance from the measuring point to where the waves started? If this is possib... | Finding the source of a wave based on its curvature | physics | 2017-12-28T07:31:37.983 |
[
{
"message": "Or the lack of them. If DM is much more abundant of common baryonic matter, why its gravitational effect becomes evident only if the dynamic of huge systems such as galactic rotation is considered?I do not understand why DM should not contribute to our weight (assuming that it permeates Earth or i... | On dark matter distribution and effects at terrestrial (or solar system) scale | physics | 2017-12-28T20:35:06.403 |
[
{
"message": "As is well known, the Heisenberg uncertainty principle states that the position and momentum satisfy an uncertainty relation, which follows from the canonical commutation relation \\begin{equation}[\\hat{x}_i,\\hat{p}_j]=i\\hbar\\delta_{ij}.\\end{equation}There is also the well known energy-time u... | Is there a natural operator that is canonically conjugate to the Hamiltonian? | physics | 2017-12-28T23:01:30.097 |
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