question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>I'm guessing the answer is no ;) but figured I'd ask anyway. Suppose I have one partner in an entangled pair and my friend on Alpha C has the other. I measure an observable (spin?) and the wave function collapses to a single eigenvalue. Does this cause the wave function for that observable to collapse for the rem... | g14995 | [
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<p>What is the most general form of the metric for a homogeneous, isotropic and static space-time?</p>
<p>For the first 2 criteria, the Robertson-Walker metric springs to mind. (I shall adopt the (-+++) signature)
$$ds^2=dt^2+a^2(t)g_{ij}(\vec x)dx^idx^j$$</p>
<p>Now the static condition. If I'm not mistaken, it mean... | g14996 | [
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<p>Let's say a light clock consists of two parallel mirrors, some photons bouncing between the mirrors, and a spring that pulls the mirrors together with the same force that the photons push them apart.</p>
<p>Now we start accelerating the mirrors and the spring, in the direction parallel to the mirrors. Whenever a ph... | g14997 | [
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<p>Initially, there is a single capacitor $A$ attached to a power source that charges it to a certain voltage $V$. Once it is charged, it is instantaneously removed and placed in a circuit with an uncharged capacitor. How do the charges and voltages redistribute over these two capacitors?</p>
<p>It seems that these ... | g14998 | [
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<p>Does heat always rise in a gravitational field?</p>
<p>I recently read that heat could be traveling down to the deep part of the ocean. Is there some new or old physics that makes heat sink?</p> | g14999 | [
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<p>In Zee's Quantum field theory in a nutshell, 2nd edition, p. 74 he claims that:</p>
<p>$$
\sum_a c_a \Lambda_a \sum_n \frac{\omega_n}{\omega_n + \Lambda_a} = - \sum_a c_a \Lambda_a \sum_n \frac{\Lambda_a}{\omega_n + \Lambda_a}
$$</p>
<p>can be proved using </p>
<p>$$
\sum_a c_a \Lambda_a = 0
$$</p>
<p>This doesn... | g15000 | [
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<p><strong>Definition 1:</strong></p>
<p>The notion of independent systems has a precise meaning in probabilities. It states that the (joint) probability or finding the system ($S_1S_2$) in the configuration ($C_1C_2$) is equal to the probability of finding the system ($S_1$) in the configuration ($C_1$) times the pro... | g15001 | [
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<p>What is the work done by an ideal gas when final pressure and volume are both different from its initial pressure and volume or when both pressure and volume changes ?</p> | g15002 | [
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<p>Last week the IceCube South Pole Neutrino Observatory published a <a href="http://icecube.wisc.edu/news/view/121">press release</a> reporting the possible discovery of two neutrinos with energies of over 1 PeV.</p>
<p>Would anyone here be willing to help me understand the significance of this announcement? Althoug... | g15003 | [
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<p>I am now studying quantum teleportation. I get what the process is like but I'm wondering why it happens this way.</p>
<p>You've got two entangled particles A and B whose wavefunctions are entangled. You also have a third particle C, which is the one you want to teleport. You get C entangled with either of the two ... | g15004 | [
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<p>I have a simple doubt. The textbook I'm working with says that if $E_0$ is the electric field in some region without diletric, then if we introduce a dieletric of constant $k$ the new field will simply be $E=k^{-1}E_0$. Is it always true? I mean, suppose we have concentric spheres of radii $a$ and $b$ being $a$ the ... | g15005 | [
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<p><a href="http://www.getbuckyballs.com" rel="nofollow">Buckyballs</a> are a desktoy that consist of 216 tiny spherical magnets. You can makes all kinds of interesting shapes and structures out of them (do a Google search).</p>
<p>I want to model them in a physics simulation, but I'm not sure how to do it.</p>
<p>Ri... | g15006 | [
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<p>On the <a href="http://en.wikipedia.org/wiki/Alcubierre_drive" rel="nofollow">Wikipedia article on the Alcubierre drive</a>, it says:</p>
<blockquote>
<p>Since the ship is not moving within this bubble, but carried along as the region itself moves, conventional relativistic effects such as time dilation do not ap... | g15007 | [
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<p>It has been more than a decade since I did all vector related math and physics so pardon me if my question does not make sense. I am reading some article that says it was inspired from Coulomb's law and gives the following expression to calculate the partial force that one particle is exerting on another at time $n$... | g15008 | [
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<p>While I know (and can observe) that solids don't settle easily in a turbulent flow, I struggle with understanding why exactly this is the case. Here' my problem:<br>
Conceptually, turbulence means high Re-number (or vice ersa), and that means the inertail forces are far stronger than friction/viscous forces. But fri... | g15009 | [
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<p>Watching <a href="http://en.wikipedia.org/wiki/Discovery_Channel" rel="nofollow">Discovery's</a> first episode of the first season of <a href="http://en.wikipedia.org/wiki/Curiosity_%28TV_series%29" rel="nofollow">Curiosity</a> (entitled "Did God Create the Universe?" by Stephen Hawking), I heard this information:</... | g15010 | [
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<p>I read <a href="http://universe-review.ca/R03-01-quantumflu.htm" rel="nofollow">here</a> that particles/antiparticles appear and annihilate each other spontaneously in empty space.</p>
<p>Since particles appear and disappear in empty space, it would seem that empty space has some form of energy. Otherwise these pa... | g15011 | [
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<p>I have a very basic question. What exactly is meant by "regular" initial data in general relativity? Does it mean smooth? at least $C^{2}$?</p>
<p>All literature on the subject just uses this term without exact explanation, so it seems that it's something very trivial, but I just wanted to know for sure.</p> | g15012 | [
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<p>Let's say I have a tube, of large radius (about 5 - 7 meters in diameter), with traversable wormholes at the ends. The wormholes are arranged as such that if something falls inside one hole from inside of the tube, it will come out at the other end still inside the tube. Now, let's say I empty all air from the tube ... | g15013 | [
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<blockquote>
<p>You release a billion protein molecules at position $x = 0$ in the middle of a narrow capillary test
tube. The molecules’ diffusion constant is $10^{−6} \ cm^2 s^{−1}$. An electric field pulls the molecules to the
right (larger x) with a drift velocity of $1\ μms^{−1}$. Nevertheless, after $80 \ s... | g15014 | [
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<p>Suggested here: <a href="http://theoreticalphysics.stackexchange.com/questions/2/what-are-the-justifying-foundations-of-statistical-mechanics-without-appealing-to/33#33">What are the justifying foundations of statistical mechanics without appealing to the ergodic hypothesis?</a></p>
<p>I was wondering about <em>goo... | g15015 | [
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<p>I'm taking a course in string theory and have encountered renormalization for the first time (and I suspect it isn't the last). </p>
<p>Specifically, while quantizing the bosonic and spinning strings, an ordering ambiguity in the mode expansion gives us a constant (called 'a' in the notes I'm reading) which is a di... | g612 | [
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<p>Quantum statistics of particles (bosons, fermions, anyons) arises due to the possible topologies of curves in D-dimensional spacetime winding around each other</p>
<p>What happens if we replace particles by branes? It seems like their quantum statistics should be described by something like a generalization of TQFT... | g15016 | [
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<p>I had a motion detector record the position of a dynamics cart and automatically plot Position vs Time and Velocity vs Time plots in Logger Pro on the computer. If the instrument uncertainty in position $x$ was given by $±0.05$ (or more generally any value $±e$) what will be the propagated instrument uncertainty on ... | g15017 | [
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<p>Let $M$ be the magnetic moment of a system. Below are the <a href="http://en.wikipedia.org/wiki/Bloch_equations">Bloch equations</a>, including the relaxation terms.</p>
<p>$$\frac{\partial M_x}{\partial t}=({\bf M} \times \gamma {\bf H_0})_x-\frac{M_x}{T_2} $$
$$ \frac{\partial M_y}{\partial t}=({\bf M} \times ... | g15018 | [
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<p>Suppose you stand in a basket and you try to pull the handles of the basket. Will you be able to lift the basket from ground?</p>
<p>Similarly, <a href="http://en.wikipedia.org/wiki/M%C3%BCnchhausen_Trilemma" rel="nofollow">Baron Münchhausen</a> allegedly pulled himself and the horse on which he was sitting out of ... | g15019 | [
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<p>I'm a chemical engineering student (just completed BS and am started the PhD program), but I'm very interested in particle physics as a hobby. I'm dismayed though with the sheer amount of information and math that is required to understand this stuff. I feel like a physicist could learn all of my engineering knowle... | g15020 | [
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<p>1)A sphere of a given mass is put in a non viscous liquid. The sphere is released, and it moves down in the liquid. My doubt is, is the mechanical energy of the system decreasing?</p>
<p>The answer given in the book is no, but I dont have access to the reason. I could not think why, because, according to me, when a... | g15021 | [
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<p>I have followed two courses on QFT, which both involved renormalization by dimensional regularization. My confusion is that one of the professors claimed that dimensional regularization can only be used to regularize logarithmically divergent integrals, whereas the other professor claimed that the scheme can renorma... | g15022 | [
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<p>I was reading a thread on Reddit about "what is a skill someone can learn in 6 months that will impress employers on a resume". I pose you the same question, but physicist wannabe oriented. Which are those real world useful skill for physicists that just aren´t taught on college?</p> | g15023 | [
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<p>Why do Calabi-Yau manifolds crop up in String Theory? From reading "The Shape of Inner Space", I gather one reason is of course that Calabi-Yaus are vacuum solutions of the GR equations. But are there any other reasons?</p>
<p>Also, given those reasons, what tend to be the most physically suggestive and useful or a... | g470 | [
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-0.011136948131024837,
0.004435880575329065,
0.059793490916490555,
... |
<p>A point particle is usually thought of as structureless and without dimension. However, given that Heisenberg's uncertainty principle prohibits us from knowing the position of a particle exactly, what is the significance of the concept of a <em>point particle</em>? Are point particles equivalent to elementary partic... | g15024 | [
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<p>A common way to show that anyons exhibit fractional statistics in 2D is by arguing that the paths of two anyons winding round each other cannot be continuously deformed to zero. This seems to assume that the particles cannot pass through each other. Why is this assumption valid?</p> | g15025 | [
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<p>Is it possible to get higher load of energy into a battery if you charge it when its cold eg. +9C?
the battery is emptier, but does it let the power in?
Is it safe?</p> | g15026 | [
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<p>Firstly, I assume prior knowledge/experience with the game <a href="http://en.wikipedia.org/wiki/Portal_%28video_game%29" rel="nofollow">Portal.</a></p>
<p>Should it be possible that two openings (circles/unidentified-constructions) would work as interchangeable gates? With one opening analyzing the molecular struc... | g419 | [
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<p>In order to define the fermi surface we must need to know about the momentum space. But I found a little bit about momentum space. Can you elaborate it pleaese? what is the meaning of the line </p>
<blockquote>
<p>the k-vector has dimensions of reciprocal length, so k is the frequency analogue of r, just as angu... | g15027 | [
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<p>What type of PDE are <a href="http://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations" rel="nofollow">Navier-Stokes equations</a>, and <a href="http://en.wikipedia.org/wiki/Schr%C3%B6dinger_equation" rel="nofollow">Schrödinger equation</a>?</p>
<p>I mean, are they <a href="http://en.wikipedia.org/wiki/Paraboli... | g15028 | [
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<p>A few questions about a magnet and a paperclip:</p>
<ol>
<li>What do you call a material that attracts another material via magnetism? (i.e. the magnet)</li>
<li>What do you call the material that is attracted in #1? (i.e. the paperclip)</li>
<li>What properties make #1 (the magnet) and #2 (the paperclip) get along... | g15029 | [
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<p>I was thinking about the following scenario:</p>
<p>Consider a particle which causes a metric $g_{\mu\nu}$ on an otherwise Minkowski spacetime (or any manifold). Now, consider another particle, somewhere in the vicinity of the first particle, which causes a metric $h_{\mu\nu}$ on a spacetime which would have been M... | g15030 | [
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<p>My question is about web diagram in<a href="http://arxiv.org/abs/hep-th/0206164" rel="nofollow"> paper of Aganagic et al.</a>
How to get the $\alpha-\beta$ line rather than $-\alpha+\beta$ line of rightmost diagram of fig.3?
More precisely, how to show the charge conservation at each vertex? </p>
<p><img src="http... | g15031 | [
0.029487188905477524,
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<p>This question is a continuation of
<a href="http://physics.stackexchange.com/questions/62034/how-to-calculate-a-scalar-curvature-fast">How to calculate a scalar curvature fast?</a> .</p>
<p>Let's have Lorentz-Fock spacetime with an interval
$$
d \hat {s}^{2} = \frac{t_{0}^{2}R^{2}}{\hat {t}^{4}}\left( d \hat {t}^{2... | g15032 | [
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<p>Recently I read an article that there is dark matter around the sun but if it is so, than why can we see it clearly.</p>
<p>If it is called matter than it shall show some hindrance in radiation we receive and the solar flares must bounce back against them though they are coming straight to us.</p>
<p>To stay close... | g15033 | [
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<p>Lets say we have a finite square well symetric around $y$ axis (picture below). </p>
<p><img src="http://i.stack.imgur.com/3PGLK.png" alt="enter image description here"></p>
<p>I know how and why general solutions to the second order ODE (stationary Schrödinger equation) are as follows for the regions I, II and II... | g15034 | [
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<p>From my house there are two wires, i.e. a live and a ground wire which are placed on the power supply lines by a hook. They are not tightly jointed because of some reasons. These two wires are also in contact with air. So, after a few days some material gets deposited on the wire. As a result of which, when wind blo... | g15035 | [
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<p>The molecules are closely arranged in solids, loosely arranged in liquids and are free to move in gases... But, Why are liquids <strong>(especially water)</strong> exhibiting these distinctive properties such as <em>incompressibility</em>, <a href="http://en.wikipedia.org/wiki/Surface_tension" rel="nofollow">surface... | g15036 | [
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<p>In this experiment we only consider the force at z direction, but $\vec B$ field gradient doesn't exclusively exist at z direction according to Maxwell's equations. So why don't we see the splitting in other directions?</p> | g15037 | [
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<p>I never understood why <a href="http://en.wikipedia.org/wiki/N%3D8_Supergravity" rel="nofollow">supergravity</a> $\mathcal N=8$, nor why the spin of a <a href="http://en.wikipedia.org/wiki/Graviton" rel="nofollow">graviton</a> is 2.</p>
<p>I've been reading around but I still don't have a back-of-the-envolope under... | g458 | [
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<p>I am reading Witten's paper on <a href="http://arxiv.org/abs/hep-th/9112056" rel="nofollow">topological string</a>, and I found some mathematical notation is hard to understand for me.
Consider the nonlinear sigma model in 2 dimensions governed by maps
$\Phi : \Sigma \rightarrow X$ with $\Sigma$ being a Riemann sur... | g15038 | [
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<p>I have took a discrete mathematics course this summer and there we talked about power of groups and functions,and yesterday I though and realize that if we can map all the 3D coordinates with a one on one and surjective function from $(x,y,z)$ to $(a)$, we can prove that our planet is 1D, that's mind blowing. Is tha... | g15039 | [
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<p>We have known for quite some time now that light travels at a finite speed of 3x10^8m/s (approx) through vacuum. But why is that? Why can't the speed be infinite? Or at least higher than the current value? What is limiting its speed? I mean, the speed of sound is limited by the density of the medium. Then, what limi... | g122 | [
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<p>This is a problem from my problem sheet but we haven't yet covered the material or received our books. There is a small box of mass $m_{1}$ resting on a plank, of mass $m_{2}$ and length $L$, which itself rests on a frictionless, horizontal surface. Both box and board are stationary when a constant force $F$ is appl... | g15040 | [
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<p>I'm learning about torque on a conductive coil in a magnetic field. I have been taught that $\vec\tau = \vec\mu \times \vec{B}$, where $\vec\mu$ is the magnetic dipole moment. Also, $\mu = I\vec{A}$, where $\vec A$ is the area vector of the loop.</p>
<p>To find the direction of the area vector, I am told to use t... | g15041 | [
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<p>$F=\frac{a}{2}m^2+\frac{u}{4}m^4+\frac{v}{6}m^6-hm$, where F is the free energy, m is the order parameter, h is the external field, $a=a_0(T-T_c)$, and $a_0>0,u>0$ and $v>0$.We know this free energy expansion describes a second order phase transition. How to write down a free energy such that the transition... | g15042 | [
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<p>momenergy = mass * spacetime displacement/proper time for that displacement</p>
<p>What I dont understand is how can the momenergy be frame independent? The unit-4 vector always points in the direction of the worldline?</p>
<p>Agreed, the magnitude of momenergy will be invariant wrt to any frame. But wont the worl... | g15043 | [
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0.003562474623322487,
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<p>I'm trying to do a simple simulation of a 1D charged quantum particle, which gets irradiated by an electromagnetic wave — in context of non-relativistic QM.</p>
<p>The Schrödinger equation for such a particle would be
$$i\hbar\frac{\partial \Psi(x,t)}{\partial t}=\frac1{2m}\left(-i\hbar\frac{\partial}{\partial x}-q... | g15044 | [
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<p>Hubble's law states that the universe is expanding with a velocity equals Hubble's constant*distance from earth. But, recent findings show that the Andromeda galaxy is actually blueshifting towards us and nearby stars and galaxies do show motion with respect to the Earth with so called peculiar velocities. What's t... | g389 | [
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<p>I plan to build a DIY heat pipe! For more info on what a heat pipe is, see <a href="http://en.wikipedia.org/wiki/Heat_pipe" rel="nofollow">Wikipedia</a>.</p>
<p>I would like to use it to cool a desktop graphics card I have. I am having trouble deciding how to choose the working fluid for the pipe. I plan to build i... | g15045 | [
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<p>Suppose you use a rope to hoist a box of tools vertically at constant speed v. The rope exerts a constant upward force of magnitude F_up on the box, and gravity exerts a constant downward force (the weight of the box). No other forces act on the box. To raise the box twice as fast, the force of the rope on the box w... | g15046 | [
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<p>(Transferred as a separate question from comments in Scott Aaronson’s gravitational decoherence question)
<a href="http://physics.stackexchange.com/questions/34993/reversing-gravitational-decoherence">Reversing gravitational decoherence</a> </p>
<p>The modern answer seems to be that they never occur, and that ther... | g15047 | [
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<p>If that’s too hard, how many collapses in 100 cc’s of boiling water in one second?</p>
<p>In biology, the very first robin that is scientifically described is preserved and called the “type robin”. The “type robin” for collapse was described by Einstein in 1905, and won him the Nobel prize. It is called the photo... | g15048 | [
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<p>In a scattering reaction, there are many possible final states for the products, each with different production rates. </p>
<blockquote>
<p><strong>Question:</strong> Is there a way in which we could in general supress certain rates of products of the reaction and enhance others? can we
use coherence and interf... | g15049 | [
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<p>It seems, one could exploit the Casimir effect to send messages across arbitrarily-large distances with carefully-tuned Casimir plates. </p>
<p>Obviously, relativity would preclude FTL information transfer, <em>but</em> as long as one did not try to <em>measure</em> the signal timing and impose an external referenc... | g15050 | [
0.013760254718363285,
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0... |
<p>In regular open circuits with either a capacitor or inductor element, (when capacitor is uncharged) with a battery, when a switch is closed to complete the circuit the current is said to be 0 because current doesn't jump immediately. </p>
<p>But in a circuit with just resistors, as soon as a switch is closed the cu... | g15051 | [
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<p>I found the following anamorphic image on <a href="http://www.moillusions.com/2011/12/an-anamorphic-medusa-painting.html" rel="nofollow">this site</a></p>
<p><img src="http://i.stack.imgur.com/c2f3G.jpg" alt="enter image description here"></p>
<p>Can anyone explain the physics behind the working of such images? Ca... | g15052 | [
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<p>It is tempting to assume the origin of the hydrogen is water vapor from the comet, in which case, what process "breaks up" the water molecule (or perhaps ammonia molecule?)? Is the hydrogen actually hydrogen ions? If the hydrogen is hydrogen ions, then could the source actually be solar plasma brought by the solar w... | g15053 | [
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<p>In my physics book (and <a href="http://en.wikipedia.org/wiki/Thermal_mass#Background" rel="nofollow">wikipedia</a>) it states that the thermal energy transferred to an object is: $Q = c \ m \ dT$, where $Q$ is the transferred thermal energy, $c$ is the specific heat capcity of the material, $m$ is the object's mass... | g15054 | [
0.02246057614684105,
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0.027274485677480698,
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<p>I want to calculate the 3D position $T_x, T_y, T_z$ of an object with respect to a coordinate system if I have the mean velocity (norm) $v$ and its 3D rotation $(\omega, \phi, \kappa)$ with respect to the same coordinate system.
I want to use a naive motion model, as simple as possible. </p>
<p>So, I assume linear ... | g15055 | [
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<p>On Wikipedia, <a href="http://en.wikipedia.org/wiki/Voltage" rel="nofollow">voltage</a> is defined to be the electric potential difference. However, I am still not certain as to whether voltage is the electric potential ($PE/q$) or electric potential change ($\Delta PE/q$). </p>
<p>My confusion partially comes fr... | g15056 | [
0.03825169801712036,
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<p>When a model rocket is launched straight up with an angle of 90degrees to the ground it will do a 180 flip when it reaches the apex of its flight and then dive straight down to Earth.</p>
<p>If I were to launch a rocket at 0degrees to the ground it would fly straight and eventually angle down until the nose hits th... | g15057 | [
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<p>How can the rotational speed, or the length of a day be determined or estimated in a planet which is composed entirely of non homogeneous fluids? There must be internal forces (pressure gradients, Coriolis, and others) that move the gases around, so what do they use as a point of reference in a planet which has no s... | g15058 | [
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<p>Assume a hypothetical 3 media/ 1 layer structure with the following indices of refraction:
$$n_1 = 2, n_2 = 2+i0.5, n_3 = 1$$
where the thickness of the layer is 100 nm and wavelength = 1000 nm.</p>
<p>Using Fresnel equations to derive the total transmittance and reflectance I get that about ~38% of incident power ... | g15059 | [
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0.03313170000910759,
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<p>I have a few related questions:</p>
<ul>
<li>Where is the <a href="http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation" rel="nofollow">CMB</a> coming (emitted/reflected/remitted) from? </li>
<li>When CMB hits the earth, is that the first thing those photons hit since they were emitted 400 thousand ye... | g15060 | [
0.02657221257686615,
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<p>According to the second law of thermodynamics, the total entropy of the Universe must always increase after any interaction (as I understand). So in the hydrogen atom, the electron has a high probability of being found in some region around the nucleus specified by its quantum number. My question is, if the hydrog... | g15061 | [
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0.027852337807416916,
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<p>Say I was sampling a sound incorrectly and it produced a constant signal as below:</p>
<p><img src="http://i.stack.imgur.com/PzXjp.png" alt="enter image description here"></p>
<p>What would this signal sound like? In Matlab, it plays nothing. Is this correct?</p> | g15062 | [
0.0072618876583874226,
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0.027839982882142067,
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0.01859975978732109,
0.007705485448241234,
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<p>I was doing an experiment over the last couple of days to try to crystallize alum using a thermal gradient. The idea was that solute at the bottom of my container would be dissolved at a higher percentage (I used a coffee mug warmer) because of the solubility curve...</p>
<p><a href="http://nshs-science.net/images/... | g15063 | [
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0.048014722764492035,
0.0955989807844162,
-0.01... |
<p>From our planet we can only see so far on any direction.</p>
<p>If a galaxy farther out than ours had a planet would its view extend beyond our measurements?</p> | g15064 | [
-0.03347590193152428,
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-0.0745965912938118,
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0.020293... |
<p>I remember in my highschool chemistry classes, they taught that an atom can be ionized when it loses a valence electron and becomes positively charged. In quantum mechanics, if electrons aren't really particles orbiting the nucleus, what is it that happens during the ionization (is the wavefunction of the electron d... | g15065 | [
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-0.000679883174598217,
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0.03408738598227501,
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<p>The Wikipedia article on <a href="http://en.wikipedia.org/wiki/Moseley%27s_law" rel="nofollow">Moseley's law</a> seems to show that the screening of heavy atoms is by 1 electron charge <em>exactly</em> (in the limit of large Z, experimental precision, within nonrelativistic limits, and).</p>
<p>But why is this exac... | g15066 | [
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0.023386986926198006,
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0.020213019102811813,
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<p>In order for a body to move with uniform velocity in a circular path, there must exist some force towards the centre of curvature of the circular path. This is centripetal force. By Newton's Third Law, there must exist a reactive force that is equal in magnitude and opposite in direction. This is the reactive centri... | g15067 | [
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<p>When i was sitting outside my house yesterday, I heard a strange sharp sound. I could not point out it's exact direction but apparently looked like coming from left but later my friend told me that an accident had occurred (due to which the strange sound came) and it totally came from different direction? How did th... | g15068 | [
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<p>In radiation gauge ($\gamma=0$), the Einstein field equation in vacuum for a perturbation $\gamma_{\mu\nu}:=g_{\mu\nu}-\eta_{\mu\nu}$ is given by $$ \boxed{ \partial^\alpha\partial_\alpha \gamma_{\mu\nu}=0 }$$
where we use the Lorentz gauge condition $ \partial^\mu\gamma_{\mu\nu}=0$.</p>
<p>This equation is equal t... | g15069 | [
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0.06105303019285202,
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0.01256490033119917,
0.... |
<p>Does a reciprocal lattice have a common point with its direct lattice?
(possibly at the origin $\vec{K}=\vec{R}=\vec{0}$)</p> | g15070 | [
0.04380400851368904,
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<p>Weinberg says, page 299, <em>The quantum theory of fields,</em> Vol 1, that </p>
<blockquote>
<p><em>The Lagrangian is, in general, a functional $L[\Psi(t),\dot{\Psi}(t)$], of a set of generic fields $\Psi[x,t]$ and their time derivatives [...]</em> </p>
</blockquote>
<p>My hang-up concerns the use of the word <... | g15071 | [
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0.0... |
<p>I need correctly define covariant derivatives on the coset space $G/H$, where a group $G \equiv \{X_i, Y_a\}$ ($X$ and $Y$ are generators) have a subrgroup $H \equiv \{X_i\}$
Lie algebra of $G$ has the following structure:
$$[X, X] \sim X, \quad [X, Y] \sim Y, \quad [Y, Y] \sim X + Y.$$
The coset space $G/H$ is para... | g15072 | [
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0.012247538194060326,
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<p>The collective field theory (see <a href="http://www.ncatlab.org/nlab/show/collective+field+theory">nLab</a> for a list of main historical references) which came up as a generalization of the Bohm-Pines method in treating plasma oscillations are often used in the study of large N asymptotics (e.g. of matrix models)... | g15073 | [
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0.025279084220528603,
0.04256683215498924,
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0.007968094199895859,
0.04092499241232872,
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0.03... |
<p>Entropy has two definitions, which come from two different branches of science: thermodynamics and information theory. Yet, they both are thought to agree. Is it true?</p>
<p>Entropy, as seen from information theory, measures our ignorance about a system. Entropy, as seen from thermodynamics, measures the amount of... | g15074 | [
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... |
<p>Can someone tell me a reference which proves this? - as to how does the bulk partition function of Chern-Simons' theory get completely determined by the WZW theory (its conformal blocks) on its boundary..</p>
<hr>
<ul>
<li><p>I see that Toshitake Kohno's book seems to do a thorough study of this but its not beginn... | g15075 | [
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0.021656496450304985,
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<p>The symmetrisation postulate is known for stating that, in nature, particles have either completely symmetric or completely antisymmetric wave functions. According to these postulate, these states are thought to be sufficient do describe all possible systems of identical particles.</p>
<p>However, in Landau Lifshit... | g15076 | [
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0.030902862548828125,
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0.0044... |
<p>When a body is in circular motion, realistically, it experiences only the centripetal force, created by gravitational pull, tension etc. , which gives it acceleration towards the centre. Now, assuming we were to fill a bucket with water, and whirl it in the vertical plane. There will be a certain velocity when the w... | g15077 | [
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0.0331... |
<p>What's the equivalent resistance in this circuit (between points A and B)?
<img src="http://i.stack.imgur.com/vyGJV.jpg" alt="enter image description here"></p> | g532 | [
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<p>I am reading <em>Introduction to Quantum Computing</em> by Kaye, Laflamme, and Mosca. Here is a question I am struggling with:</p>
<blockquote>
<p>Exercise 3.5.6: Prove that the transpose map, which maps $\rho \mapsto \rho^{T}$ is positive<sup>1</sup>, but not completely positive<sup>2</sup>. </p>
</blockquote>
... | g15078 | [
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... |
<p>Apart from thermo-nuclear reactions in the Sun, (and in hydrogen bombs and other methods developed by scientists) does nuclear fusion occur naturally in the planets? If not, why?</p>
<p>Also, do the Sun and the Outer planets - Jupiter through Neptune - have rocky cores similar to the Earth (consisting of iron, nic... | g15079 | [
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0.010287... |
<p>The Hamiltonian for a single electron in a magnetic field reads
$$H=\left(\frac{{\bf p}^{2}}{2m_{e}}+q_{e}\phi\right)+\mu_{B}\left({\bf \hat{L}}+g{\bf \hat{S}}\right)\cdot{\bf B}+\frac{e^{2}}{8m_{e}}\left({\bf B}\times{\bf r}\right)^{2}$$ where symmetric gauge ${\bf A}=\frac{1}{2}{\bf B \times r}$ is used and where ... | g15080 | [
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<p>It has happened many times and i have ignored it everytime.
Yesterday it happened again .<br>
I was travelling in a train and saw a fly (insect) flying near my seat.</p>
<p>Train was running at a speed of around of 100 km/hr.
So according to the physics rules , my speed will also be 100 km/hr as i am sitting insid... | g432 | [
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<p>I did an experiment on pendulum motion and in the method it says use the following equation to measure the velocity of the pendulum:
$$v=\frac{W}{t}$$
where $W$ is the width of the pendulum bob and t is the time.
Why is this? Is it correct?</p>
<p>Edit: the time is for the bob to cross the photogate.</p> | g15081 | [
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<p>Some time ago I came across a secondary web source on measurement of light speed in water made by Foucault around 1850. I append its redrawn scheme below (light is reflected from the rotating mirror to the fixed one and from the later again to the rotating one which slightly rotates in the meantime so the light end... | g15082 | [
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<p>In physics text books, it has only stated that Earth cooled down and so, water vapour condense and other life form develop and et cetera. But, why did Earth cooled down in the first place? What caused it to cool down? </p>
<p>In addition, when the Earth cooled down, did the water vapour condense to possible 'raw' m... | g866 | [
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0.043... |
<p>In particle physics, where does the term 'neutral current' originate? An example would be an electron exchanging a Z boson with another electron. I understand that the Z boson itself is neutral, but surely 'current' refers to electron and its associated amplitude in this case?</p> | g15083 | [
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<p>I am from the northern hemisphere and as far as I remember, the Milky Way was "single lane", just one stripe. At least, this is what I recall the Milky Way to look like when seeing it from near Irkutsk, Siberia, in late August 1997.</p>
<p>Yesterday I went to Blackbutt, Australia (somewhere near 27° South, 152° Eas... | g15084 | [
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<p>Two person, $A$ and $B$, each holding one end of a long solid rod. </p>
<p>Now person $A$ pushes the rod on one end.</p>
<p><strong>Question</strong>:
Is it correct that the information that the rod has been pushed will travel to the other end at the speed of light whereas the actual 'push' will travel at the spe... | g4 | [
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