question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
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<p>I'm looking for the complete set (x,y,z component) of the Navier-Stoke equations under the Eddy Viscosity hypothesis to model turbulent fluid flow. </p>
<p>I found the <a href="http://www.calpoly.edu/~kshollen/ME554/RANS.pdf" rel="nofollow">following</a>, but I have a really hard time believing the transition from ... | g14256 | [
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<p>I have some galaxy kinematic maps (velocity maps) extracted from Integral Field Spectroscopy (IFS) datacubes. Like any observation data coming from a ground telescope, these velocity maps include (ie: are convolved with) a Point Spread Function (PSF).</p>
<ul>
<li>How can someone obtain the PSF for some data? Can s... | g14257 | [
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<p>Is it possible to have a Bose gas with $T = 0$ and $\mu < 0$ ?</p>
<p>I think that there is a problem, because all the states $k$ are such as</p>
<p>$$\langle n_k \rangle = \dfrac{1}{e^{\beta \{\epsilon_k - \mu\}}-1} = 0$$</p> | g14258 | [
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<p>In an answer to my previous question about <a href="http://astronomy.stackexchange.com/questions/1539/what-is-the-best-evidence-theory-about-the-first-generation-of-stars">the first stars</a> it was stated they probably formed at Z=20 to Z=60 and may have had a mass between tens to 100s to 1000s of times the mass of... | g14259 | [
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<p>I'm trying to understand the proof of Noether's Theorem in my Classical Mechanics class. We formulated it as follows:</p>
<p>A continous symmetry is defined as a flow $\phi^{\lambda}(q(t))$ which leaves the Lagrangian invariant, i.e.</p>
<p>$$L(\phi^{\lambda}(q(t)), \frac{\partial}{\partial t}\phi^{\lambda}(q(t)),... | g14260 | [
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<p>I couldn't find any other questions regarding <a href="http://en.wikipedia.org/wiki/Dark_flow" rel="nofollow">dark flow</a> on Wikipedia, so I'm curious. As far as I can tell, <a href="http://en.wikipedia.org/wiki/Dark_energy" rel="nofollow">dark energy's</a> existence seems to be hard to explain. Since there may be... | g14261 | [
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<p>Please can you help me understand the <a href="http://en.wikipedia.org/wiki/Spin%E2%80%93statistics_theorem" rel="nofollow">Spin-Statistics Theorem</a> (SST)? How can I prove it from a QFT point of view? How rigorous one can get? Pauli's proof is in the case of non-interacting fields, how it will be in the presence ... | g14262 | [
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<p>Einstein's relativity tells us that light always travels at the speed of light relative to me, no matter how fast I'm going. Right? This really confuses me though. If light travels from A to B in one second while I'm standing at A, then where does light travel to when I'm going from A to B for one second as well, at... | g14263 | [
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<p>I have some questions about the Kitaev toy model for Majorana fermions (<a href="http://arxiv.org/abs/cond-mat/0010440" rel="nofollow" title="arXiv:cond-mat/0010440">arXiv:cond-mat/0010440</a>). First of all, his proof for the definition of the 'Majorana number' is not so clear to me.</p>
<p>$$P(H(L_{1} + L_{2})) =... | g14264 | [
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<p>There have been many <a href="http://en.wikipedia.org/wiki/Very_Large_Array">radio telescope arrays</a> which are capable of resolving stars significantly fainter than can be resolved with a single "small" radio telescope.</p>
<p>Why don't they do the same with light telescopes?</p> | g14265 | [
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<p>Why is $\pi^0$ created in the high-energy collision $p+p\to p+p+\pi^0$?</p> | g14266 | [
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<p>In my research, I found that my system has the following dissipation relation:
$$\omega^2=k^2+k_0^2\ , $$
where $k_0^{-1}$ is an intrinsic lengthscale of the system and the units are chosen so that the wave speed is unity. This is what it's looks like (blue curve):</p>
<p><img src="http://i.stack.imgur.com/ktK1V.pn... | g14267 | [
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<p>I'm reading Zweibach's introduction to string theory, and don't understand one of his claims.</p>
<p>He defined the mass shell to be the set of points in momentum space s.t. $p^2+m^2 = 0$. Then the physical mass shell is the subset of points where $E=p^0>0$. He claims that in light cone coordinates this may be p... | g14268 | [
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<p>One of the surprises for me in working out the answer to this question: <a href="http://physics.stackexchange.com/questions/8074/why-is-the-earth-so-fat">Why is the Earth so fat?</a> , is that the core is more elliptical than the surface, the extra ellipticity builds up gradually to about 10% (this is false, I misca... | g14269 | [
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<p>Suppose an electron is accelerating along $x-$axis. How does it produce electromagnetic radiation? </p>
<p>Which of Maxwell's equation comes in play and how do I derive the wave equation from it? </p>
<p>For oscillating charge the emerging EM wave is explained in <a href="http://ocw.mit.edu/courses/physics/8-02-el... | g14270 | [
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<p>In basic scattering theory, $d\Omega$ is supposed to be an element of <a href="http://en.wikipedia.org/wiki/Solid_angle" rel="nofollow">solid angle</a> in the direction $\Omega$. Therefore, I assume that $\Omega$ is an angle, but what is this angle measured with respect to? None of the textbooks I am referring to gi... | g14271 | [
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<p>A heavy chain with a mass per unit length $\rho$ is pulled by the constant force $F$ along a horizontal surface consisting of a smooth section and a rough section. The chain is initially at rest on the rough surface with $x=0$. If the coefficient of kinetic friction between the chain and rough surface is $\mu$, dete... | g14272 | [
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<p>The centrifugal force acting on a revolving particle with negligible size is $\frac{mv^2}{r}$. What if the size is not negligible? Say we are talking about a large homogeneous circular disc, so its radius is not negligible when compared to the radius of its revolution.</p>
<p>For the sake of simplicity, assume the... | g14273 | [
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<p>So on the net (one forum) I stumbled upon <a href="http://resonance.is/wp-content/uploads/2013/05/1367405491-Haramein342013PRRI3363.pdf" rel="nofollow">this</a>, and article by Nassim Haramein. Now, I researched this guy a bit, and I couldn't find any peer reviewed article (other than this in 'Physical Review & ... | g14274 | [
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<p>I'm trying to do the problem below, but it seems like there is incomplete information. </p>
<p>PROBLEM STATEMENT:
In a scattering experiment, $10^6$ $\alpha$ particles are scattered at an angle of $4^{\circ}$. Find the number of $\alpha$ particles scattered at an angle of 6$^{\circ}$.</p>
<p>The reason it seems in... | g14275 | [
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<p>Generally, metals are usually fairly conductive, but their oxides aren't. I know conductivity is just one attribute, but in general, should you expect a, say, diatomic bulk crystal's properties to be anything like the bulk properties of each element it's composed of? Or do all those properties get thrown out the win... | g14276 | [
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<p>If I have a quantum system which I prepare in a certain state, this state then evolves unitarily via a Hamiltonian. Suppose an observer provokes a collapse of the wave function by a certain measurement, this means that it must be in an eigenstate of the measurement. </p>
<ol>
<li><p>What happens subsequent to that?... | g14277 | [
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<p>Bill borrows his dad's gun and when he gets outside he fires it straight up into the air. If the bullet left the gun at 400 m/s: How long does it take the bullet to reach its highest point.</p>
<p>I know that I have to use the acceleration of gravity which is 9.81 m/s2. Also, I suspect this question will use quadra... | g14278 | [
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<p>Blackbody radiation is thermal radiation from a hot object emitted over a continuous range of wavelengths. But why are spectral lines, lines (i.e., you only get certain wavelengths when an element is heated up and you look at it's emission spectrum)? Is it because the light from the heated element is passed through ... | g445 | [
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<p>I'm following an introduction to statistical mechanics and have seen the following definitions for fundamental temperature, pressure and chemical potential respectively:</p>
<p>$$\frac{1}{\tau} := \left( \frac{\delta \sigma}{\delta U} \right)_{V,N}$$
$$p := \tau \left( \frac{\delta \sigma}{\delta V} \right)_{U,N}$$... | g14279 | [
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<p>If you float two coffee beans in a cup of water that is being vibrated by your subwoofer (go to a sin wave generator online), the two beans will either be attracted to the sides of the cup or if positioned right will be attracted to each other in an accelerating manner. What phenomenon is this, and could it be rela... | g14280 | [
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<p><strong>What are the equations for $\Delta S_{mix}$, $\Delta G_{mix}$, and $\Delta H_{mix}$ for a real gas?</strong></p>
<hr>
<p>Explanation:</p>
<p>In one of the most well known experiments in thermodynamics, two chambers are placed beside each other with a membrane in between. The thought experiment is used to... | g14281 | [
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<p><a href="http://en.wikipedia.org/wiki/Speed_of_electricity" rel="nofollow">Wikipedia writes</a>, that</p>
<blockquote>
<p>Propagation speed is affected by insulation, so that in an unshielded copper conductor ranges 95 to 97% that of the speed of light, while in a typical coaxial cable it is about 66% of the spee... | g14282 | [
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<p>We all know that there are analytic formulae to solve <a href="http://en.wikipedia.org/wiki/Quadratic_equation" rel="nofollow">quadratic</a>, <a href="http://en.wikipedia.org/wiki/Cubic_function" rel="nofollow">cubic</a> and <a href="http://en.wikipedia.org/wiki/Quartic_equation" rel="nofollow">quartic</a> polynomia... | g14283 | [
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<p>In the paper <em>The geometry of free fall and light propagation</em> by Ehlers and his colleagues (<a href="http://dx.doi.org/10.1007/s10714-012-1353-4" rel="nofollow"><em>Gen. Relativ. Gravit.</em> <strong>44</strong> no. 6, pp. 1587–1609 (2012)</a>), when the authors introduce the differentiable structure on page... | g14284 | [
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<p>I'm just wondering what are the differences (advantage and disadvantage) of different telecommunication bands ? </p>
<p>For example: </p>
<ul>
<li>Why does satellite TV use Ku band? </li>
<li>Is it true the rule is that greater band frequencies (10 ghz vs 1 ghz vs 800 mhz) can transport more data?</li>
<li>Why mob... | g14285 | [
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0.023730816319584846,
0.02245057374238968,
0.0... |
<p><img src="http://i.stack.imgur.com/V2GdM.png" alt="enter image description here"></p>
<p>The net circulation should be zero if the net current is zero. However, I don't see how the circulation in the Amperian loop above can be zero. All the lines interact the loop only once and all their circulation contributions a... | g14286 | [
0.03593090921640396,
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0.042... |
<p>If I connect a zinc half cell (Left hand side) and a copper half cell (right hand side), I get an E cell value of 1.1V taken from the voltmeter. If I replace the voltmeter with a light bulb, I will get light and heat energy of the bulb.</p>
<p>My question:
Will the voltage across the cell remain the same? I mean, t... | g14287 | [
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0.0... |
<p>Recently I read Astrophysics Notes where I found a statement that young stars are classified as Population I stars and relatively older one as Population II stars. Population I stars contain heavier elements or "Metals" - that means elements other than helium and hydrogen. While Population II contain older stars ... | g14288 | [
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0.0291006937623024,
0.022521307691931725,
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<p>If for a quantum mechanical particle $\langle p \rangle(t)=0$ at all time t, in any state $|\psi(t)\rangle$, can I interpret this as the center of mass of the system remains stationary?</p>
<p>EDIT: Is $\langle p \rangle(t)$ somehow related to the group velocity of the particle? If the probability current density $... | g14289 | [
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0.05194511637091637,
0.024879908189177513,
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-0.... |
<p>Ok,</p>
<blockquote>
<p><em>If a disturbance is localized to a small part of the space for a certain time we get a wave pulse and if the source is active for time greater than to complete a single pulse we get a wave packet.</em></p>
<p>H.C.Verma, Concept of physics, Part I.</p>
</blockquote>
<p>However I d... | g14290 | [
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0.036868005990982056,
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<p>I have the following questions.</p>
<ol>
<li><p>Why can't matter pass the speed of the light?</p></li>
<li><p>What happens to an object when it comes close to the speed of the light?</p></li>
<li><p>What makes the object unable to surpass this speed limit?</p>
<p>And finally, I need to know whether this speed limi... | g14291 | [
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0.01956923119723797,
0.004637703765183687,
0.022267524152994156,
0.025... |
<p>Why do you get electric field of a light wave in following form: $E(x,t)=A cos(kx-\omega t- \theta)$?( look at: <a href="https://public.me.com/ricktrebino" rel="nofollow">https://public.me.com/ricktrebino</a> -> OpticsI-02-Waves-Fields.ppt, p. 18)</p> | g14292 | [
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<p>I've been searching the web and many references without much success. My question is how do we know that, in the <a href="http://en.wikipedia.org/wiki/Schwarzschild_metric" rel="nofollow">Schwarzschild black hole solution</a>, the surface with coordinate $r=2M$ (in the geometric unit system) defines an event horizon... | g14293 | [
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0.05522306263446808,
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<p>In most helicopters there is the anti-torque tail rotor to prevent the body from spinning in the opposite direction to the main rotor. </p>
<p>What's the equivalent mechanism in horizontal takeoff single engine propeller, and jet aircrafts, where the air or the jet coming out back from the turbine or propeller is s... | g14294 | [
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0.02306576818227768,
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0.004262509755790234,
0.051957301795482635,
0.03300... |
<p>i know that vermiculite is used in insulation applications.
i found this notion of the R-value of <a href="http://www.doityourself.com/stry/table2" rel="nofollow">vermiculite</a>, that i don't know if its true.</p>
<p>basically i want to know if the attribute of the vermiculite, of expanding in a temperature of 870... | g14295 | [
0.06348727643489838,
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0.01836896874010563,
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0.0202214065939188,
-0.03377603739500046,
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0.00231025367975235,
0.05... |
<p>I was wondering whether there is any equation for viscous liquids (probably derived from energy and mass conservation principles) relating the pressure, rate of volume flow, area of cross section and height of the liquid (or any other parameters if required).</p>
<p>NOTE : I am aware of <a href="http://hyperphysic... | g14296 | [
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... |
<p>During my QFT study I faced a problem of calculating amplitude of 3 to 3 scattering in massless $\phi^4$ theory in zero momenta limit at tree level. One of topologically distinct diagrams corresponding to this process gives a contribution of $\sim \frac{1}{(p_1+p_2+p_3)^2}$ (up to symmetry coefficients and coupling ... | g14297 | [
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0.01048338133841753,
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0.019... |
<p>I watched this <a href="http://www.youtube.com/watch?v=dEaecUuEqfc" rel="nofollow">video</a> on <a href="http://en.wikipedia.org/wiki/Relational_quantum_mechanics" rel="nofollow">Relational Quantum Mechanics</a> yesterday and my brain has been trying to comprehend it since.</p>
<p>The interpretation I currently hav... | g14298 | [
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0.02298060804605484,
0.0... |
<p>This might be a silly question. When the mass of a spin 1 meson is measured, is there a way to check whether it is the same for all polarizations? Is such a meson mass the same for longitudinal and for transverse polarizations?</p> | g14299 | [
-0.01284751296043396,
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0.026737425476312637,
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... |
<ol>
<li><p>Why is gravitational force always an attractive force? </p></li>
<li><p>And is the <a href="http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation" rel="nofollow">Newtonian formula of gravitational force</a> applicable for very small particles like electrons and protons etc.?
From <strong>For... | g81 | [
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0.02825096994638443,
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0.030709246173501015,
0.039601556956768036,
0.03224054351449013,
0.00466... |
<p>The Standard Model contains at least three sources of CP violation. The first of these, involving the Cabibbo–Kobayashi–Maskawa matrix in the quark sector, has been observed experimentally and can only account for a small portion of the CP violation required to explain the matter-antimatter asymmetry. </p>
<p>The s... | g14300 | [
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-0.041571129113435745,
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... |
<p>Is it a coincidence that there are four fundamental forces and four spacetime dimensions ? Does a universe with three spacetime dimension contain four fundamental forces? Can magnetism be realized in three dimensional spacetime?</p>
<p>Edit 1:
Thanks alot for the answer. I'm a first year undergrad student so this ... | g14301 | [
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<p>The question is in the title: are there more bosons or fermions in the universe? Or is there the same number of bosons and fermions? </p>
<p>I think there is the same number but I don't know why exactly.</p> | g14302 | [
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... |
<p>I'm trying to understand intuitively the geometry as it would look to an observer entering the event horizon of a schwarszchild black hole. I would appreciate any insights or corrections to the above.</p>
<p>inmediately after your enter the event horizon, if you look back and try to reach again for the horizon, it ... | g14303 | [
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0.05760771408677101,
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<p>The big bang theory proposes that equal amounts of matter and antimatter were created in the beginning. Shortly afterwards most of it annihilated. Could that have produced enough energy to drive cosmic inflation? </p> | g14304 | [
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<p>I'm a science writer and I'm having difficulty understanding what a non-perturbative approach means. I thought I understood what <a href="http://en.wikipedia.org/wiki/Perturbation_theory" rel="nofollow">perturbative</a> meant, but in looking for explanations of <a href="http://en.wikipedia.org/wiki/Non-perturbative"... | g14305 | [
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<p>If I have a dielectric (a piezoelectric, to be exact) sandwiched between two platinum plates (electrodes), and connect one end to positive potential of a battery and the other to ground (I don't even know why one has to connect the top to ground), and also connect the negative potential of a battery to ground, what ... | g14306 | [
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0.0... |
<p>How <strong>far</strong> would a 100mph wind blow a 71kg, 160cm woman? Is there a formula?</p> | g14307 | [
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0.017857078462839127,
... |
<p><strong>Background</strong></p>
<p>In many textbooks on electromagnetism, the sequence in which topics are introduced are generally as follows -- electrostatics, magnetostatics, electrodynamics, wave propagation in free space, and wave propagation in confined space, in that order. As an example, consider Griffiths'... | g14308 | [
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... |
<p><img src="http://i.stack.imgur.com/RKL4O.png" alt="enter image description here"></p>
<p>What does the Quantization Surface mean here?</p>
<p>Reference: H. Latal W. Schweiger (Eds.) - Methods of Quantization</p> | g14309 | [
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0.0... |
<p>As we know, the Higgs boson gives mass to other particles. But here is onething which is not clear for me. I mean, I do not understand how the Higgs boson gives mass to other particles? Does anyone could explain me more explicitly how procedure goes that the Higgs boson gives mass to other particles. </p> | g14310 | [
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<p>The postulate says that if we want to build the compound state of two sub-systems, we just take the tensor product $\otimes$ of the respective state vectors. This means that if one of the vectors has an overall phase $e^{i \phi}$, that can be interpreted either as the phase of any of the two vectors in the subspaces... | g14311 | [
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<p>I'm studying Feynman's Lectures on Physics, and I'm not really understanding his reasoning here:</p>
<p><em>Consider weight-lifting machines $\overline{}$ machines which have the property that they lift one weight by lowering another. Let us also make a hypothesis: that there is no such thing as perpetual motion wi... | g14312 | [
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0.... |
<p>What are the most recent values of the mixing angles associated with the 3 neutrinos flavors and what are their mass differences? Have they been measured in the first place?</p> | g14313 | [
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<p>What physics departments offer recitation sessions for their physics courses on the undergraduate level?</p>
<p>If there is a site which has such info it will be greatly appreciated.</p> | g14314 | [
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<p>To take an example,</p>
<blockquote>
<p><a href="http://www.feynmanlectures.info/docroot/III_13.html#Ch13-S1" rel="nofollow">Feynman Lectures Vol 3 13-1</a></p>
<p>Let's think of an electron which ban be in either one of two positions [...] There are two possible states of definite energy for the electron. ... | g14315 | [
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<p>It obviously requires single- and multi-variable calculus and linear algebra, but what else? And where do you suggest to get that background from?this isn't a duplicate because I'm for the math needed precisely and only for the feynman lectures.</p> | g185 | [
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<p><a href="http://www.youtube.com/watch?v=EnrwrwMfNSs">This video ("rotor panorama")</a> was captured by a camera attached to the rotor head of a radio-controlled helicopter, with the frame rate set to the rotor's frequency. During a long segment of the video, the horizon looks distorted:</p>
<p><img src="http://i.st... | g14316 | [
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<p>Given a Lagrangian density $\mathcal L(\phi_r,\partial_\mu\phi_r,\phi_n,\partial_\mu\phi_n)$, for which we find out that for some $\phi_n$ <strong>its conjugate momentum vanishes</strong>: </p>
<p>$$\pi_n=\frac{\partial\mathcal L}{\partial(\partial_0\phi_n)}=0.$$ </p>
<ol>
<li><p>What does this imply? </p></li>
<l... | g14317 | [
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<p>In the expression of the density matrix of a (Electron-Spin) Qubit
$$
\rho=\frac{1}{2}(I + x \sigma_x + y \sigma_y + z \sigma_z)
$$
where $\tau=(x,y,z)$ is unit vector in the <a href="http://en.wikipedia.org/wiki/Bloch_sphere" rel="nofollow">Bloch sphere</a>, which is the physical Interpretation of $\tau$? Can it be... | g14318 | [
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<p>Can I say that, matter generally when cooled decreases in volume because, when it is cooled,i.e., we lower the temperature of the surroundings, then the avg. energy of our sample will be higher than the surroundings and hence, the molecules coming out will be more, therefore, the volume decreases...And when equilibr... | g14319 | [
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<p>My question refers to the fact, that, say if our universe were to be sitting on a 4 (spatial) dimensional plane, that we cannot see, then within our universe could there be a point mass in which 1 or 2 (spatial) dimensions could exist. We may not be able to see them, this could be a result of our world only consisti... | g14320 | [
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<p>All materials have same molar specific heat, cf. the <a href="http://en.wikipedia.org/wiki/Dulong%E2%80%93Petit_law" rel="nofollow">Dulong–Petit law</a>. Is it an evidence of atoms? What other examples are there for atomic theory?</p> | g14321 | [
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<p>Let's say I stay at point $x=0$ in vacuum. When I move my arm forward such that it will have a positive $x$ position (say $x=5$) will the rest of my body move backward such that it will have a negative $x$ position (like $x=-1$)?</p> | g14322 | [
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<blockquote>
<p>A heavy sphere of radius r = 1.00 meter is fixed with respect to the ground. A small uniform solid sphere is placed at the top of the larger sphere. After a slight disturbance, the smaller sphere begins to roll downward without slipping. How high h is the small sphere above the ground at the instant i... | g14323 | [
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<p>Inspired by <a href="http://physics.stackexchange.com/questions/4652/how-do-we-know-that-dark-matter-is-dark">How do we know that dark matter is dark?</a> and <a href="http://physics.stackexchange.com/questions/519/what-is-the-temperature-of-the-surface-and-core-of-a-neutron-star-formed-12-billi">What is the tempera... | g14324 | [
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<p>Perturbation theory presumes we have a valid family of models over some continuous (infinitely differentiable, in fact) range for some parameters, i.e. coupling constants. We have some special values for the coupling constants characterizing the unperturbed model, which presumably, is relatively easy to solve. We al... | g14325 | [
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<p>There has been some heated debate as to whether the laws of physics allow for traversable wormholes. Some physicists claim we require exotic matter to construct wormholes, but then others counter the Casimir effect with ordinary matter is sufficient. But these same physicists seldom come up with an explicit solution... | g14326 | [
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<p>I am just starting to look into how we understand the data from particle collisions.</p>
<p>My question is, what are the algorithms or ways that these detectors interpret the data? Are there standard approaches? Or if not, what are some good papers or places to look to get started in learning more about the impleme... | g14327 | [
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<p>Imagine I take a polymer like polyethylene, of length $L$ with some number of Kuhn lengths $N$, and I tie into into a trefoil knot. What is the difference in entropy between this knotted polymer and a circular polyethylene unknot? Is there an approximate way of calculating this for knots of a given crossing number... | g14328 | [
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<p>The top frame of the image below shows an image formed on the screen (at right) of an object (pencil on the left) located at some distance $D$ from the lens. The lens focuses all the light rays hitting it from the top of the pencil onto the corresponding point on the screen.</p>
<p>The bottom frame shows the same s... | g14329 | [
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<p>My question can be summarized as:</p>
<p><strong>Given a potential with a symmetry (e.g. $z\rightarrow-z$), should I expect orbits in that potential to exhibit the same symmetry?</strong> Below is the full motivation for this question.</p>
<p>A while back I came across an intriguing orbit (or rather class of orbit... | g14330 | [
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<p>Although it appears that the universe is not closed (since it is accelerating) I have been reading different websites on how the universe would look like if it was closed and the big crunch did occur. </p>
<p>I believe that I understand that from a big bang singularity, the universe expanded into a low entropy plas... | g14331 | [
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<p>I have a problem understanding the ideas behind a basic assumption of cosmology. The Friedmann equations follow from Newtonian mechanics and conservation of Energy-momentum $(E_{kin}+E_{pot}=E_{tot})$ or equally from Einsteins field equation with a Friedmann-Lemaitre-Robertson-Walker metric. In a radiation dominated... | g14332 | [
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<p>Is it possible to find how gravity changes with time? That is, height is not part of the equation. </p>
<p>For example, $g$ can be expressed as a function of height that does not involve time with this relation:</p>
<p>$g_i=g_0 (r /(r+h))^2$</p>
<p>where $g_i$ is $g$ at any height and $g_0$ is on the Earth's surf... | g14333 | [
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<p>Having read <a href="http://physics.stackexchange.com/questions/630/is-it-possible-to-observe-interference-from-2-independent-optical-lasers?rq=1">this question</a> and answers to it, I've learned that somehow two light beams from independent sources can actually produce interference pattern, if the properties of th... | g14334 | [
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<p>A friend and I were discussing this, and couldn't decide. Since it takes energy to physically raise something up above the planet's surface, does it continue to require energy in order to <em>keep</em> it above the surface? </p>
<p>While gravity is extremely weak relative to atomic/molecular bonding forces, we we... | g14335 | [
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<p>How to measure distances to stars by means of <a href="http://en.wikipedia.org/wiki/Spectroscopic_parallax" rel="nofollow">spectroscopic parallaxes</a> on practice?
What is the accuracy of measuring distances using this method compared with distances based on HIPPARCOS trigonometric parallaxes?</p> | g14336 | [
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<p>When you apply pressure to an incompressible fluid the pressure is transferred. If you stop applying the pressure there is no motion. Volume and density are integral parts of calculating how much energy is required to achieve certain useful motion like lift a truck or press a part. Why don't certain fluids compress... | g14337 | [
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<p>I never understood why one would calculate the <a href="http://www.google.com/search?as_q=%22infinite+plane%22&as_epq=electric+field" rel="nofollow">electric field surrounding an infinite plane</a>, if such thing does not exist. Is there physical motivation for using this model? Are the results applicable to rea... | g14338 | [
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<p>What equation accurately describes the movement of a photon? Does this equation also apply to the movement of an electron?</p> | g14339 | [
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<p>"Cathode rays (streams of electrons) produce mechanical motion of a small paddle wheel placed in their path indicating that they are material particles."$_1$ Isn't this statement wrong? </p>
<p>Assuming electrons to be material particles, "if we use light to observe the position of electron, photons can transfer m... | g14340 | [
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<p>If $P$ is an idempotent operator, $P^2 = P$ and we have a vector $|\psi\rangle$, $P\neq1$, and the relation</p>
<p>$$PL |\psi\rangle = L|\psi\rangle,$$ what conclusions can we draw about $L$, which is a Linear Operator?</p>
<ol>
<li>$L = P$.</li>
</ol>
<p>Are there anything else? If not how does one prove this?</... | g14341 | [
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<p>As a child, I loved to watch the <a href="http://en.wikipedia.org/wiki/Bubble_light" rel="nofollow">bubble lights</a> at Christmas time. The colorful bubbling was hypnotic. However, there was always something about them that bothered me. With all of that boiling happening within the glass vial, why didn't the ent... | g14342 | [
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<p>I'm studying Gauss' Law, and I came across a section where we're supposed to find the electric field of various shapes (like an infinite line of charges, etc), and for an infinite plane with a uniform positive charge per area, it says here in my notes:</p>
<p>Planar symmetry => Charge distribution doesn't change if... | g14343 | [
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<p>This is something I don't ever seem to hear about, except regarding QCD ("lattice QCD"). What about QED? Is numerical integration always inferior to hand-calculating Feynman diagrams in perturbation theory? What about numerical simulation of the full time-evolution of a quantum field (rather than just calculating co... | g14344 | [
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<p>I'm a mathematician studying Arnold's Mathematical Methods of Classical Mechanics. </p>
<p>On p. 83 the following definition is given.</p>
<blockquote>
<p>Let $M$ be a differentiable manifold, $TM$ its tangent bundle, and $L:TM \to \mathbf{R}$ a differentiable function. A map $\gamma: \mathbf{R} \to M$ is called... | g14345 | [
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<p>It is known that in F-theory compactifications on CY 4-folds one can get gauge groups with very large ranks. The largest single factor* gauge group for compact CY 4-folds I found in the literature is the mind-blowing SO(7232) in this paper: <a href="http://arxiv.org/abs/hep-th/9706226" rel="nofollow">http://arxiv.or... | g14346 | [
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<p>I was thinking the other day about what sort of exotic materials one would find in the depths of a planet. I have heard theories about how an enormous diamond might be found in the centre of gas giants.</p>
<p>Current theories have a ball of iron at Earth's core - would that be like the iron up here on the surface ... | g14347 | [
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<p>I learned today in class that photons and light are quantized. I also remember that electric charge is quantized as well. I was thinking about these implications, and I was wondering if (rest) mass was similarly quantized.</p>
<p>That is, if we describe certain finite irreducible masses $x$, $y$, $z$, etc., then ... | g14348 | [
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<p>Trained as a pure mathematician, I see claims about the mass of a galaxy and other such huge measurements that are arrived at experimentally, and I just have to scratch my head.</p>
<p>I know this is a bit of a vague question--but does anyone have a good resource for something like "measurement in astrophysics" or ... | g14349 | [
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<p>First, I want to say upfront that this question need not dissolve into arguments and discussion. This question can and should have a correct answer, please don't respond with your opinions.</p>
<p>GNUplot is very pervasive in Physics research. Many of the plots appearing in things like PRL and JPB are made in GNUpl... | g14350 | [
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<p>What would be the implications to the Standard Model if the Higgs Boson hadn't been found with the LHC?</p>
<p>Also, if the Higgs Boson had not been found with the LHC, would it have been successfully proven as non-existent? Or would we just wait for an experiment with higher energy ranges?</p> | g14351 | [
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<p>Any state analytic in energy (which includes most physical states since they have bounded energy) contains non-local correlations described by the Reeh-Schlieder theorem in AQFT. It is further shown that decreasing the distance between wedges will increase the entanglement as measured by a Bell-type inequality, unti... | g14352 | [
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