question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>How do I verify homogeneity and isotropy of space, for example for hyperbolic space? My idea is to verify that Lorentz transformations in 4-vectors can move any desired point on the hyperboloid into the origin, and can rotate the hyperboloid about the origin by an arbitrary angle. The problem is I do not know how to... | g12856 | [
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<p>I'm trying to build a mathematical model of C. <em>elegans</em> locomotion under overdamped viscous environment. </p>
<p>In order to simplify this problem, I use six discrete pieces of rigid cylinders to represent worm body, and this chain of rigid pieces are connected by torsional springs this way:</p>
<pre><code... | g12857 | [
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<p>As far as I know the lightest gas is hydrogen due to low mass of its nucleus, but what if we were to somehow strip hydrogen atoms of electrons and enclose protons in a container made of teflon (high electronegativity).</p>
<p>Would such a proton-only gas be stable? And what would be its density under atmospheric pr... | g12858 | [
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<p>Assuming that the black hole starts grow in the exact center of the planet and that the general structure of the planet does not degrade as it is eaten from the inside, would the gravity on the surface of the planet be affected by the black hole growing.</p>
<p>My uneducated guess is that as the matter becomes more... | g12859 | [
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<p>The gist of my query is in the title. </p>
<p>I have been searching for papers and studies that could provide transmission curves (in nematic liquid crystals) across the visible to near-IR range (or from 300 nm to 2200 nm). I would thus like to know if anyone has encountered, or happens to know, of how a layer of n... | g12860 | [
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<p>As a Mathematician reading about the Dirac equation on the internet, leaves me with a great deal of confusion, about it. So let me start with its definition:</p>
<p>The <strong>Dirac equation</strong>, is given by</p>
<p>$
i \hbar \gamma^\mu \partial_\mu \psi = m c\cdot \psi
$</p>
<p>where the Dirac matrices $\ga... | g12861 | [
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<p>This question is based on an odd phenomenon I noticed last week. Since then, I've been collecting more data to try to get an accurate picture of the facts.</p>
<p>Each morning, I go to my bus stop. It's on a main road, and when I get there, there is a lot of traffic. Every couple minutes a large truck goes by, and ... | g12862 | [
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<p>Can someone please tell me why
$$4\int d^4x \, x^\mu x^\nu ~=~\int d^4x \, g^{\mu\nu}x^2 $$
by some rotational symmetry argument?</p> | g12863 | [
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<p>What are the most popular algorithms used to obtain a binodal curve
for the ternary mixture (starting from Flory-Huggins theory)? I would
like to obtain a plot similar to the one calculated here
<a href="http://physics.stackexchange.com/a/112972/46158">http://physics.stackexchange.com/a/112972/46158</a>.</p>
<p>I ... | g12864 | [
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<p>When showing that the magnetic field derived from the curl being proportional to current and the divergence being zero is unique, Purcell says that at some sufficiently remote enclosing boundary the difference of two magnetic fields (two possible fields from the curl), D, would take a constant value. Why is this tru... | g12865 | [
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<p>With hyperopia, the focal point is behind the retina, shouldn't this mean that the image is flipped on the retina itself from what is usual?
I must be drawing my ray diagrams wrong.</p>
<p>A little something different from: <a href="http://physics.stackexchange.com/q/63883/24082">Optics, lenses and our eyes</a></p... | g12866 | [
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<p>I saw this picture on one of my social media sites with the caption, "I'd do this in a heart beat! Who's with me!"</p>
<p><img src="http://i.stack.imgur.com/GMzJi.jpg" alt="enter image description here"></p>
<p>I was about to go balls to the walls and say, "I'm in! When and where??" But then I got to thinking, how... | g12867 | [
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<p>My data set of upper atmospheric cloud occurrences $N$ versus their thickness (or optical brightness, say $B$) show an exponential variation over more than two orders of magnitude - that is $N$ varies as $\mathrm{e}^{-cB}$. The quantity $c$ can vary from location to location. More than one first-principle model show... | g12868 | [
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<p>The optimum wheel diameter of cars and bikes appear to be roughly the same, certainly well within an order of magnitude. This is despite very different average speeds and propulsion mechanisms.</p>
<p>Can anyone come up with a dimensional analysis-type argument explaining why this is? </p> | g12869 | [
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<p>If we have a configuration of metal concentric spheres (each of negligible thickness) of radii $r_1,r_2,r_3$ respectively and $r_1<r_2<r_3$, and we are given the potentials of the spheres to be $0, constant,0$ respectively. How might we find the capacitance of the configuration?</p>
<p><em><strong>EDIT: My q... | g12870 | [
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<p>The thought randomly occurred to me that a circular particle accelerator would have to exert a lot of force in order to <em>maintain the curvature of the trajectory</em>. Many accelerators move particles at fully relativistic speeds, and I want to ask how that affects things.</p>
<p>Why does this matter? Well, if... | g119 | [
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<p>I've noticed that snow seems to reflect red light more than any other wavelength at night. It's seen in the sky, as well as the snow itself, both of which take on a reddish tint. Why exactly does it do this? Additionally, why is this only noticeable under moonlight? If it's the moonlight that's tinted red, then alte... | g12871 | [
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<p>"When a current passes through an electric motor, the magnetic force on the motor causes a torque on the loop of wire causing it to turn". However, when the loop rotates, there should also be a change in magnetic flux causing an induced emf (Faraday's law). However, my book makes no such reference. </p>
<p>my ques... | g12872 | [
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<p>Does energy of light change when a light travels from a medium to another of different optical density?</p> | g12873 | [
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<p>If the quarks in a neutron are (up,down,down), why isn't it negatively charged? Excuse the silly question, just wondering.</p> | g12874 | [
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<p>In cavity QED for a 2-level atom, the revival time for oscillation b/w the states $\left|\ e\ 0\right\rangle$ and $\left|\ g\ 1\right\rangle$ (absorbing the same photon that is emitted) is said to be infinity because in general, Rabi oscillation revival time is proportional to the square root of the mean of the occu... | g12875 | [
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<p>I am trying to understand a calculation using Wick's theorem. Let $T(z)$ be the analytic part of a stress-energy tensor, and $\phi(z)$ a free boson field. </p>
<p>Now, $$T(z)\partial_{w}\phi(w)=-2\pi:\partial_{z}\phi(z)\partial_{z}\phi(z):\partial_{w}\phi(w).$$ Using Wick's theorem, we know that $$:\partial_{z}\phi... | g12876 | [
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<p>This question is on the construction of the Einstein Field Equation.</p>
<p>In my notes, it is said that </p>
<blockquote>
<p>The most general form of the Ricci tensor $R_{ab}$ is $$R_{ab}=AT_{ab}+Bg_{ab}+CRg_{ab}$$
where $R$ is the Ricci scalar.</p>
</blockquote>
<p>Why is this the most general form (involv... | g12877 | [
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<p>A scammer got a hollow gold bar and fills it with a combination of lead and air, with the same average density as gold. </p>
<p>What's the simplest way of discovering the fraud? I know that x-rays will see it, but are there simple means for analyzing it? (without destroying it)</p> | g12878 | [
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<p>What would be an intuitive way to damage objects in a physics game using impulses? Since momentum is conserved, so is impulse (the change in momentum for any two time periods) in a closed system.</p>
<p>So if I have the impulse of a collision (that is, the change in momentum during a time interval for one of the tw... | g12879 | [
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<p>I've read many facts from NASA's webpage.. Sometimes they tell, (for example) "NASA's Chandra X-Ray Observatory discovered this ultra-luminous X-Ray source (about 15 million LY) which shows an extraordinary outburst..."</p>
<p>My question is.. Is it possible to obtain those electromagnetic radiation (Infra-red, Vis... | g12880 | [
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<p>In Heaviside-Lorentz units the Maxwell's equations are:</p>
<p>$$\nabla \cdot \vec{E} = \rho $$
$$ \nabla \times \vec{B} - \frac{\partial \vec{E}}{\partial t} = \vec{J}$$
$$ \nabla \times \vec{E} + \frac{\partial \vec{B}}{\partial t} = 0 $$
$$ \nabla \cdot \vec{B} = 0$$</p>
<p>From EM Lagrangian: $$\mathcal{L} = ... | g12881 | [
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<p>I read that the definition of the chemical potential is, that it is the partial derivative of the Free energy with respect to the number of particles, $$\mu=\frac{\partial F}{\partial N}.$$ Unfortunately, I don't get this definition, wherefore I wanted to choose an example: Let me think of a system for example a wat... | g12882 | [
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<p>One of the postulates of quantum mechanics is that every physical observable corresponds to a Hermitian operator $H$, that the possible outcomes of the measurements are eigenvalues of the operator, and that after a measurement the state collapses into the eigenfunction corresponding to the observed eigenvalue. </p>
... | g12883 | [
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<p>The Hamiltonian for this system is given by
\begin{equation}
\mathcal{H} \{S\} = -H\sum_i S_i - \frac{J_0}{2} \sum_{ij} S_i S_j,
\end{equation}
where $H$ is the external magnetic field and there is no restriction to nearest neighbour interaction. The spin at each site, $S_i$, may take value $+1$ or $-1$. </p>
<p>No... | g12884 | [
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<p>i've problem with the last terms of uncertainty principle:
1. are these equalities true:
$$\langle \{\Delta \hat A,\Delta \hat B\} \rangle=\langle \{\hat A, \hat B \}\rangle$$
$$\langle [\Delta \hat A,\Delta \hat B] \rangle=\langle [\hat A, \hat B] \rangle.$$</p>
<ol>
<li>what is result of product commutator with a... | g12885 | [
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... |
<p>this doubt is related to Generalized Hamiltonian Dynamics paper by Dirac.</p>
<p>Consider the set of $n$ equations : $p_i$ = $∂L/∂v_i$,</p>
<p>(where $v_i$ is $q_i$(dot) = $dq_i/dt$, or time derivative of $q_i$)($L$ is the lagrangian, $q$ represent degrees of freedom in configuration space)</p>
<p>Now Dirac says ... | g12886 | [
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<p>I came across this problem. Please help me solve it. </p>
<blockquote>
<p>A radioactive sample S1 having an activity of $5\ \mathrm{\mu Ci}$ has twice the number of nuclei as another sample S2 which has an activity of $10\ \mathrm{\mu Ci}$. The half lives of S1 and S2 can be</p>
<p>(A) 20 years and 5 years,... | g12887 | [
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<p>I am trying to figure out how large of a mass and how quickly I need to spin said mass to keep a two-wheeled robot stable. Ideally, I am looking for a formula that relates <i>m</i><sub>1</sub>=mass of robot, <i>m</i><sub>2</sub>=mass of spinning disk, <i>v</i>=rotational speed of disk (rpm), and some sort of stabili... | g12888 | [
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<p>As we all know that light travels in rectilinear motion. But can we bend light in parabolic path? If not practically then is it possible in paper? Has anyone succeeded in doing that practically ?</p> | g806 | [
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<p>I have been reading about quantum entanglement, and I was wondering what quantum states can be "sent" using entanglement. I know you can measure the spin of one of the particles, and know the spin of the other, but can you also do this with other quantum states, such as polarization, or energy?</p>
<p>Also, could y... | g12889 | [
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<p>I was wondering if it would be possible to create an amount of positive energy out of a vacuum, in addition to an equal amount of negative energy, thus not violating the first law of thermodynamics, since the total amount of energy in the universe would remain the same. It could be like an "energy loan," with the p... | g12890 | [
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<p>Were there physics teaching aids used in the past? Or did professors just basically follow the textbook the students read?</p>
<p>In other words: How was physics was taught in the past?</p> | g12891 | [
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<p>I suspect this question has a simple answer, but I can't get my head around it.</p>
<p>The classic example of a person in orbit around the Earth at high speed experiencing a slower passage of time than the people on Earth is well known. I don't understand, however, what dictates that the person orbiting experience... | g84 | [
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<p>What is the origin of induced emf in a conductor which is produced by changing magnetic flux.I mean why changing magnetic flux causes the production of induced emf.
Is there some kind of energy involved?Is it.</p> | g12892 | [
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<p>I know I can use Gauss's law to find the Electric Field inside and outside the cylinder very easily. We can select Gaussian surfaces for different cases (i.e. $r \lt R$ and $r \gt R$, where $R$ is the radius of the cylinder and $r$ is the radius of Gaussian surface). </p>
<p>But I want to know Electric fields at th... | g12893 | [
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<p><a href="http://en.wikipedia.org/wiki/Ehrenfest_theorem" rel="nofollow">Ehrenfest theorem</a> is usually dubbed as the quantum mechanical equivalent of Newton's law and Griffiths states, in the first chapter of his textbook, that Ehrenfests theorem enables us to work with expectation values in quantum mechanics as <... | g12894 | [
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<p>I am not clear on that. </p>
<p>Is gravity repulsive during inflation?</p> | g12895 | [
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<p>It is well known that there exist mappings between operators in N = 4 Super Yang–Mills and spin chain states making the theory Bethe Ansatz integrable. Is integrability a necessity for the Amplituhedron? Most QFTs are not integrable.
Does Nima Arkani-Hamed, et al. claim to be able to extend the Amplituhedron to gen... | g12896 | [
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<p>I am familiar with the basic ideas of quantum field theory but I feel uncomfortable when I have to derive Feynman rules by myself for a given action (for example in non-linear sigma models or electrodynamics). Could you provide a list of good books, articles, lecture notes and/or any other sources (except Peskin), ... | g12897 | [
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<p>The overly simplified explanation I'm giving myself right now is <a href="http://en.wikipedia.org/wiki/Dark_energy" rel="nofollow">dark energy</a> causes the opposite of what gravity does, that's why the universe is expanding. Now where gravity is a force, why dark energy is "energy"? Why it's not called "Dark Force... | g859 | [
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<p>I am quite familiar with duality transformations for lattice spin systems (i.e. systems with global $O(n)$ symmetry) and pure gauge systems (i.e. local $SU(n)$). However, after searching for a bit, I can't seem to find literature on dual variables for lattice fermions (systems with inner products of grassmann field... | g12898 | [
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<p>Joule heating happens every time when the conduction electrons transfer kinetic energy to the conductor's atoms through collisions, causing these conductor's atoms to increase their kinetic and vibrational energy which manifests as heat. Then, why wouldn't it happen when no current is flowing through the conductor? ... | g12899 | [
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<p>In many textbooks an illustration of a vibrating string at a fundamental mode shown (and wikipedia) shown like this <a href="http://en.wikipedia.org/wiki/File%3aStanding_waves_on_a_string.gif" rel="nofollow">http://en.wikipedia.org/wiki/File:Standing_waves_on_a_string.gif</a>). However if you watch youtube videos of... | g12900 | [
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<p>I've thought I had a good understanding how resolution enhancement tricks works for projection lithography, until I tried to understand if it's possible to get sub-diffraction performance for focused laser beam:</p>
<p>Let's assume we have a laser with nice and shiny perfect aspherical focusing lens (with performan... | g12901 | [
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<p>This is a continuation to my <a href="http://physics.stackexchange.com/questions/111063/casimir-forces-due-to-scalar-field-using-path-integrals">previous question</a>, in which I began an attempt solve the Casimir Force problem using path integrals. As one of the <a href="http://physics.stackexchange.com/questions/1... | g12902 | [
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<p>What is a simple bare-bones description of exchange interaction between two electrons?</p>
<p>For instance, it seems to me that the only necessary ingredients are the Coulomb interaction and the requirement that the total wavefunction be antisymmetric.</p> | g12903 | [
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<blockquote>
<p>Three masses $m_1 = 3\text{ kg}$, $m_2 = 9\text{ kg}$ and $m_3 = 6\text{ kg}$ hang from three identical springs in a motionless elevator. The elevator is moving downward with a velocity of $v = -2\text{ m/s}$ but accelerating upward with an acceleration of $a = 5\ \mathrm{m/s^2}$. (Note: an upward acc... | g12904 | [
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<p>In the winter I am in the habit of opening my oven door once I am done baking so that I can add the heat to the house. However I recently thought about it and it would seem that even if the door is closed, and the oven insulated, the only way for the oven to cool would be to heat the house. So this would mean that... | g12905 | [
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<p>I am a senior in High School who is taking the course AP Physics Electricity and Magnetism.</p>
<p>I was studying Gauss's laws and I found this problem: </p>
<blockquote>
<p>A solid insulating sphere of radius R contains a positive charge that is distrubuted with a volume charge density that does not depend on a... | g12906 | [
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<p>So, I understand that quantum teleportation is the transfer of a quantum state from one particle or system of particles and its correlations to another receiving system consisting of particle(s). Can't this be used in a way to achieve teleportation as depicted in sci-fi?</p>
<p>You start out with a translationally ... | g12907 | [
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<p>To measure <a href="http://en.wikipedia.org/wiki/Velocity" rel="nofollow">velocity</a> we use derivative </p>
<p>$$v=\frac {dr}{dt}.$$ </p>
<p>Is the any other mathematical tool to do this?</p> | g12908 | [
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<p>I am trying to get my head around a few concepts related to frame dragging and related physics. In regards to black holes that have no charge and all their mass is tied up in rotational kinetic energy then the ergosphere is a maximum size. By definition, as I see it defined, the outer boundary of the ergosphere is ... | g12909 | [
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<p>I am interested simulating the evolution of an electronic wave packet through a crystal lattice which does not exhibit perfect translational symmetry. Specifically, in the Hamiltonian below, the frequency of each site $\omega_n$ is not constant.</p>
<p>Suppose the lattice is specified by a certain tight-binding Ham... | g12910 | [
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<p>I understand that adding/sprinkling, say NaCl, on a highway depresses the freezing point by making any moisture on the road harder to freeze as the NaCl molecules get in the way of phase transition. So now we need lower temperatures to freeze the moisture on the road.</p>
<p>However, why can I cool my drink "better... | g12911 | [
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<p>Lenny Susskind explains in <a href="http://www.youtube.com/watch?v=-BleG7PBwEA&list=ECA2FDCCBC7956448F&index=2" rel="nofollow">this</a> video at about 40min, as an extended object (for example a relativistic string) is boosted to the infinite momentum frame (sometimes called light cone frame), it has no non-... | g12912 | [
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<p>On a sunny day an outdoor swimming pool will heat up fairly quickly. My question is, what is the exact mechanism for this and can we put numerical figures on it?</p>
<p>Given that water is clear and colorless, it seems that it will absorb very little direct radiation from the sun. The sides and the bottom of the p... | g12913 | [
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<p>Let's say there's a static fluid in an arbitrarily deep vessel with constant non-zero compressibility ($k=\frac{1}{B}=-\frac{dV}{Vdp}=\frac{d\rho}{\rho dp}$). Let $z$ be the vertical distance below the surface of the fluid. Then $dp=g\rho dz$ ($g$ here is positive and assumed to be constant). As $k$ is constant, $\r... | g12914 | [
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0.04855429008603096,
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-0.00457525672391057,
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0.02522066980600357,
0.008396588265895844,
-0.003... |
<p>Water usually comes to mind when thinking about freezing. Once it reaches a certain temperature, water freezes, becoming a solid.</p>
<p>However could you make the same statement about a rock? Is a rock at room temperature considered frozen because it is in a solid state?</p> | g12915 | [
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... |
<p>In a gauge theory like QED a gauge transformation transforms one mathematical representation of a physical system to another mathematical representation of the same system, where the two mathematical representations don't differ at all with respect to observables. Gauge transformations are therefore manifestations o... | g12916 | [
0.0412481315433979,
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0.0397... |
<p>My question is the title of a 1991 paper by Richard Gott and Li-Xin Li:
<a href="http://arxiv.org/abs/astro-ph/9712344" rel="nofollow">http://arxiv.org/abs/astro-ph/9712344</a>
and is also a subject of his popular book, "Time Travel in Einstein's Universe"
Ultimately with cosmology, the chicken and egg question redu... | g12917 | [
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0.04... |
<p>This just started to bother me after reading yet another entangled particle question, so I hate to ask one myself, but...</p>
<p>If we have two entangled particles and take a measurement of one, we know, with certainty, what the outcome of same measurement of the other particle will be when we measure it. But, as w... | g12918 | [
0.025452814996242523,
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0.00810405146330595,
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-... |
<p>It is well know that we can measure the spinning of the Earth with a <a href="http://en.wikipedia.org/wiki/Foucault_pendulum">Foucault pendulum</a>.</p>
<p>But, is there a similar experiment for the rotation of the Earth around the Sun? I would like to know if we could prove that the Earth rotates the Sun without a... | g12919 | [
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0.005328469909727573,
... |
<p>Where is the Higgs Field? I get the idea of how it works but I couldn't find anything explaining where it is.</p> | g12920 | [
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<p>I am trying to derive the out-of-plane <a href="http://en.wikipedia.org/wiki/Phonon" rel="nofollow">phonon</a> <a href="http://en.wikipedia.org/wiki/Phonon#Dispersion_relation" rel="nofollow">dispersion relation</a> for a membrane. As far as I can tell, one of the simplest ways to do so is with a Lagrangian of the f... | g12921 | [
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-0.001692783902399242,
... |
<p>According to multiple sources the SI units for angular momentum are kg * m$^2$ / sec</p>
<p>I am confused about the derivation for this. Here is what I have done:</p>
<p>$$L = I \cdot \omega \\
= m \cdot r^2 \cdot r \cdot v(translational) \\
= kg \cdot m^3 \cdot m/s \\
= (kg \cdot m^4)/s$$</p>
<p>What are ... | g12922 | [
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<p>I read this post, <a href="http://physics.stackexchange.com/questions/86769/is-the-symmetry-group-of-two-spin-1-2-particles-su2-times-su2-or-su4">Is the symmetry group of two spin 1/2 particles $SU(2) \times SU(2)$ or $SU(4)$?</a></p>
<p>If the picked answer is correct, can I believe that an $N$-degenerate system r... | g12923 | [
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0.02516940049827099,
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0.... |
<p>Newtonian gravity can be described by the equation:
$$ \nabla^2 \phi = 4 \pi \rho G $$
where $\rho$ is the mass density, $\phi$ is the gravitational potential, and G is the universal gravitational constant.
Of course, one of the shortcomings is that it is not consistent with special relativity. General relativity ha... | g12924 | [
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<p>According to <a href="http://en.wikipedia.org/wiki/Hund%27s_rules" rel="nofollow">Hund's second rule</a>, the spin tends to be maximal.
That would, in my understanding, imply that, regarding the Stern-Gerlach experiment, the important electron in a silver atom has spin 1/2 (or "up"). This should hold for all atoms.... | g12925 | [
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<p>I already learned the Physics' basics like Newton Laws, Electricity, and Optics.
Can you recommend me a good book for advancing with my learning of physics(with some math)?</p> | g98 | [
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<p>What is the exact $SU(2)$ representation to which these Hermitian generators belong?
\begin{equation}
t_a=\{t_1,t_2,t_3\}=\left\{\frac{1}{\sqrt{2}}\begin{pmatrix}
0 & 1 & 0 \\
1 & 0 & 1 \\
0 & 1 & 0
\end{pmatrix},\,\frac{1}{\sqrt{2}}\begin{pmatrix}
0 & -i & 0 \\
i & 0 & -i ... | g12926 | [
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0.053849052637815475,
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0.012722605839371681,
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... |
<p>I am looking at Griffiths introduction to Electrodynamics 3rd ED. Problem 2.10 asks for the flux of $E$ through the right face of the cube, when a charge $q$ is in the back left corner of the cube. The solution manual says it should be $\frac q {24\epsilon _0}$ but I think it should be $\frac q {6\epsilon _0}$ bec... | g12927 | [
0.048944633454084396,
0.005412225611507893,
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0.016133176162838936,
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0.05146941915154457,
0.011864074505865574,
0.011... |
<p>I have a random point defined by:</p>
<pre><code>int x = rand.nextInt(9)-4;//-4 to +4
int y = rand.nextInt(4)+2;//+2 to +5
int z = rand.nextInt(9)-4;//-4 to +4
</code></pre>
<p>Y happens to be the "up" vector, but I suspect that's irrelevant.</p>
<p>I want to rotate this point so that it ends up relative to a vec... | g12928 | [
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0.011782560497522354,
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0.030105246230959892,
0.02444523759186268,
-0.03... |
<blockquote>
<p>The resistance of a bulb filament is $100\Omega$ at a temperature of $100^\circ \text{C}$. If its temperature coefficient of resistance be $0.005 \space \text{per} ^\circ \text{C}$, its resistance will become $200\Omega$ at a temperature of </p>
<ol>
<li>$300^\circ\text{C}$</li>
<li>$400^\cir... | g12929 | [
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0.008040395565330982,
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0.04183683544397354,
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0.02622... |
<p>When grabbing a typical tree branch of at least two feet, it's so easy to snap with a less than one inch circumference that even a toddler can do it.</p>
<p>However, after breaking it, the smaller halves with multiply from the center, going like this:</p>
<p>1 - BB (before break)</p>
<p>2 - AB (after break)</p>
... | g12930 | [
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0.0028262226842343807,
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<p>Here's my question: if atoms have well defined energy levels and those differences correspond to the frequencies of light that can be absorbed, how is it that opaque objects absorb all or most visible light frequencies?, frequencies get absorbed and you basically don't have any visible light coming out on the other ... | g407 | [
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<p>The terminology "topological charge" is frequent in lots of research papers related to optical vortex or optical OAM, it is used to represent the optical OAM. Why? How to comprehend it?</p> | g12931 | [
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<ol>
<li><p>Will Linear momentum be conserved in a non-inertial frame of reference? </p></li>
<li><p>In other words what is the fundamental condition for linear momentum to be conserved?</p></li>
<li><p>Also which is more fundamental- Newton's Third Law or Conservation of Linear Momentum? (as in Newton's third law does... | g12932 | [
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<p>consider two identical charges moving with uniform velocity. There will be a magnetic force of attraction between them as two currents in the same direction attract each other. If I sit on one of the charges, according to me the other charge is not moving. So there wont be any magnetic attraction. How does changing... | g12933 | [
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<p>My question is:
It is easy to balance a bicycle when it is moving at a fairly high velocity, say 7 m/s or 25 km/hr.
But when a bicycle slows down, it is hard to keep it upright, and the person riding it may thus fall down! Why?</p> | g408 | [
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<p>I'm just brushing up on a bit of CFT, and I'm trying to understand whether conformal symmetry is local or global in the physics sense.</p>
<p>Obviously when the metric is viewed as dynamical then the symmetry is local, because essentially then we're dealing with a change of variables under which the metric transfor... | g12934 | [
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0.0043250396847724915,
0.001123302150517702,
... |
<p>What is
\begin{align}
\sum_{\mu=0}^{3}
\langle \sigma_{\mu} \rangle^2
=
?
\end{align}
$\sigma_{\mu}$ are the Pauli matrices. The Bra-Ket notation is used in this question:
\begin{align}
\langle \sigma_{\mu} \rangle
=
\langle \Psi \lvert \sigma_{\mu} \lvert \Psi \rangle
,
\end{align}
where $\Psi$ is the Pauli spinor... | g12935 | [
-0.007456333842128515,
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0.059654753655195236,
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0.04647085443139076,
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-0.... |
<p>In a holography set-up, as shown in the figure below, Illumination beam and reference beam both are in phase. The interference pattern generated at the detector contains the whole information about the object. When illumination beam will incident on to the object its phase and amplitude will change and further obj... | g12936 | [
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0.005169472191482782,
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... |
<p>Given the electromagnetic Lagrangian density
$$
\mathcal{L}~=~-\frac{1}{4}F_{\mu\nu}F^{\mu\nu}~=~\frac{1}{2}(E^2-B^2)
$$
is a Lorentz invariant, how many other electromagnetic invariants exists that can be incorporated into the electromagnetic Lagrangian?</p> | g12937 | [
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<p>Electrical wires are relatively inefficient in transferring energy--especially when the place of production is quite far from communities.</p>
<p>Would it be possible to transfer that energy via photons? I realize that to some degree photons react with atoms, but might there be a specific condition in which this wo... | g12938 | [
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0.03277353569865227,
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0.04606566205620766,
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-0... |
<p>In the <a href="http://en.wikipedia.org/wiki/Gupta%E2%80%93Bleuler_formalism" rel="nofollow">Gupta-Bleuler formalism</a> we have a problem with two states (scalar photons and longitudinal photons), because here $\langle \vec{k}_a|\vec{k}_b\rangle $ is negative or zero. However, I thought that only $|\langle \vec{k}_... | g12939 | [
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0.0332... |
<p>I am self-studying this <a href="http://www.google.dk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&ved=0CEMQFjAC&url=http://ocw.mit.edu/courses/mechanical-engineering/2-25-advanced-fluid-mechanics-fall-2005/study-materials/06_viscous_flow.pdf&ei=W_W9Uqr4EsKY4wThyoBg&usg=AF... | g12940 | [
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<p>So I was curious about, what if we make a tunnel from one side of the earth, to the other side of the earth? Gravity is ofcourse always negative, which makes you "fall" and not "float".</p>
<p>If we take this image, let's imagine that the BLACK LINE in the midle of the earth, is a tunnel from one side of the earth,... | g76 | [
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<p>What makes Liouville theory subject to relatively intense research field?</p> | g12941 | [
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<p>On <a href="http://en.wikipedia.org/wiki/Quantum_chromodynamics" rel="nofollow">Wikipedia</a> it says that the two peculiar properties of quantum chromodynamics (QCD) are: <a href="http://en.wikipedia.org/wiki/Color_confinement" rel="nofollow">confinement</a> and <a href="http://en.wikipedia.org/wiki/Asymptotic_free... | g12942 | [
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<p>I was trying to do linear stability analysis of spring pendulum. I arrived at the differential equations which describe the system. But I am unable to proceed to linear stability analysis. Is it possible to do linear stability analysis on 2nd order differential equations by finding eigen values of Jacobian matrix? T... | g12943 | [
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0.025161493569612503... |
<p>First of all I'd like to point out that I'm a complete layman when it comes to physics and I apologise if this isn't the correct forum for this.</p>
<p>If I were to sit on an office chair on a laminated floor and fired a shotgun out in front of me roughly how far backwards would I <a href="http://en.wikipedia.org/w... | g12944 | [
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<p>I have seen in the in the Dirac equation $$\gamma_0,\gamma_1,\gamma_2,\gamma_3.$$ Then I have seen the definition of a new matrix $$\gamma_5=i\gamma_0\gamma_1\gamma_2\gamma_3.$$ Now my question is why the name of the new matrix has not been given as $$\gamma_4.$$ Is there any historical reason behind this or it is s... | g12945 | [
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<p>I was playing around with a battery, an ammeter, and a light bulb. The ammeter originally read 1.99 A, but after reversing the leads going into and out of the ammeter the ammeter read -1.98A. I know this was because the leads were reversed but I am not sure why reversing the leads led to the ammeter readind a negati... | g12946 | [
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<p>What happens to a photon shot straight into a black hole? Does it gain infinite momentum before it crosses the horizon? If it has a finite momentum going in, then it would seem that a photon of the same momentum could escape the BH by time reversal.</p>
<p>Does the photon enter the BH at the speed of light as we ... | g12947 | [
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0.0006613573059439659,
0.03790971264243126,
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-0.002... |
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