question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>I have a two-component field:</p>
<p>$$\phi(\vec{x}) = \left( \begin{array}{c} \phi_1(\vec{x}) \\ \phi_2(\vec{x}) \end{array} \right)$$</p>
<p>with $\phi^T = (\phi_1, \phi_2)$. And I am trying to evaluate:</p>
<p>$$(\vec{\nabla}\phi)^T(\vec{\nabla}\phi)$$</p>
<p>I'm pretty sure the result of that expression is ... | 6,374 |
<p><strong>Observation:</strong></p>
<p>So, I know that all computer screens are able to project many different colors by varying how they display the RGB (Red, Green, Blue) pixels.</p>
<p><strong>Question</strong></p>
<p>What's the difference between say, yellow light (575 nm) from sunlight vs. the same yellow ligh... | 308 |
<p>Optical coatings designed for reflection or anti-reflection are made of many thin layers which will expand when heated. What will the effect be on the wavelengths the coating will reflect when the coating is heated? </p>
<p>For a first guess, magnesium fluoride has a thermal expansion coefficient of about 10 µm/m/K... | 6,375 |
<p>Are there any other particle predictions by the <a href="http://en.wikipedia.org/wiki/Standard_Model" rel="nofollow">standard model</a>? </p> | 222 |
<p>Based on John Isaacks' question, "<a href="http://physics.stackexchange.com/questions/11940/if-you-view-the-earth-from-far-enough-away-can-you-observe-its-past">If you view the Earth from far enough away can you observe its past?</a>" and the responses, it appears that we could use mirrors to see into the past. Usi... | 6,376 |
<p>I was reading through the Wikipedia article on <a href="http://en.wikipedia.org/wiki/Quasar" rel="nofollow">Quasars</a> and came across the fact that the most distant Quasar is 29 Billion Light years. This is what the <a href="http://en.wikipedia.org/wiki/Quasar#Overview" rel="nofollow">article</a> exactly says </p>... | 10 |
<p>We are a group of rocket amatures who build rockets using LOX and ethanol as propellent. We normally buy the LOX from supplier. But lately we have been discussing if there is an easy way to produce it in house. </p>
<p>I have looked at several methods:
<a href="http://en.wikipedia.org/wiki/Gas_separation" rel="nofo... | 6,377 |
<p>What exactly is a <a href="http://en.wikipedia.org/wiki/Frame_of_reference" rel="nofollow">frame of reference</a>? Does it have an objective existence and if so what is it? </p>
<p>What's the size of a reference frame? Is a reference frame the same size for a stationary frame of reference and one that's acceleratin... | 6,378 |
<p>I have a couple of questions about how <a href="https://en.wikipedia.org/wiki/Railgun" rel="nofollow">railguns</a> actually work, and the mathematics behind them. I understand that the projectile is driven forward by the Lorentz force caused by high currents traveling through the projectile. Because of these high cu... | 6,379 |
<p>I understand that the higher your velocity the slower light will move. But how does time itself slow down while you are moving faster?</p> | 6,380 |
<p>In <a href="http://arxiv.org/abs/quant-ph/0202122" rel="nofollow">quantum information theory</a>, one can adopt the basic formalism where every system is given by an operator algebra, state preparation procedures correspond to linear functionals on that algebra (macroscopic systems preparing quantum systems), yes-no... | 6,381 |
<p>Expectation values of $(x,y,z)$ in the $| n\ell m\rangle$ state of hydrogen?</p>
<p>Does anyone know of a quick way of finding this (if there is even one)? Can I somehow use the relation that:</p>
<p>$$\langle r\rangle ~=~ \frac{a_0}{2}(3n^2 - \ell(\ell+1)),$$ </p>
<p>or do I just have to brute force and use prop... | 6,382 |
<p>In the Standard Model, I understand that the mass of the electron is assume to arise from two effects:</p>
<ol>
<li><p>A bare mass given by Yukawa interaction with the Higgs field, and</p></li>
<li><p>A mass correction from mass renormalization effects</p></li>
</ol>
<p>In this framework, why do we need to assume ... | 6,383 |
<p>When we push something it moves due to the disturbance in it's molecular arrangement causing waves. How do I calculate the speed of push/waves?
<a href="http://www.youtube.com/watch?v=Dnv-Pm4ehFs" rel="nofollow">http://www.youtube.com/watch?v=Dnv-Pm4ehFs</a>
The push actually depends on the amount of the force appl... | 6,384 |
<p>Is the <a href="http://en.wikipedia.org/wiki/Gravitational_constant" rel="nofollow">gravitational constant</a> $G$ a fundamental <a href="http://en.wikipedia.org/wiki/Physical_constant#Table_of_universal_constants" rel="nofollow">universal constant</a> like Planck constant $h$ and the speed of light $c$?</p> | 6,385 |
<p>Alpha Centauri Bb is an exoplanet orbiting Alpha Centauri B. It is asserted that given the close distance to the star the planet should be tidally locked. </p>
<p>The orbiting period of the planet is about 3.2 days.</p>
<p>If the planet has no atmosphere (which is very possible due to proximity to the star), its d... | 6,386 |
<p>Here is a question which frequently occurs on my school exam paper:</p>
<p>"Prove that Faraday's law of electromagnetic induction is consistent with the law of principle of conservation of energy." What does this actually mean? Any help would be better...</p>
<p>P.S. I apologize if the question is too elementary.<... | 6,387 |
<p>What is a good resource to learn about higher degree <strong>degenerate</strong> <a href="http://en.wikipedia.org/wiki/Perturbation_theory" rel="nofollow">perturbation theory</a> - one that involves mathematics that isn't much more advanced than first order perturbation theory? I've looked around and I've only found... | 6,388 |
<ol>
<li><p>Can some one please explain in simple words that what is <a href="http://www.google.com/search?hl=en&as_q=effective+refractive+index" rel="nofollow">effective refractive index</a>? </p></li>
<li><p>How it is different from the <a href="http://en.wikipedia.org/wiki/Refractive_index" rel="nofollow">refrac... | 6,389 |
<p>In relation to mass/gravitational/centrifugal force. Is the increased gravitational force due to the increase in mass of a planet (i.e. earth) from meteorites, etc.. directly proportional to the centrifugal force of the planetary body?</p> | 6,390 |
<p>When calculating in a rest frame, doesn't one assume both, definite velocity (zero) and position (origin)? Why is Heisenberg okay with that?</p>
<p>Edit: E.g. For a decay we can do calculations in which we say that the particle is truly at rest with 0 (three vector) momentum. Wouldn't that automatically mean that t... | 6,391 |
<p>As pressure is increased, do we require more energy to increase the temperature from given temperature for the same mass. For example, if we heat water at 1 atm to raise its temperature by 20 degree Celsius, will the heat required be the same when pressure is 3 atm?</p> | 6,392 |
<p>I'm not a physicist but I majored it at high school (a long time ago) and I study university math.</p>
<p>Me and my roommate discussed whether the performance of a Thermos bottle is influenced by how full it is. So if it is not full do the contents cool down faster, slower or equally?</p>
<p>Thanks.</p> | 6,393 |
<p>I'm trying to understand what is the <a href="http://en.wikipedia.org/wiki/S-matrix" rel="nofollow">S-matrix</a> in QFT. People say that it has to be a unitary matrix, but that I guess will change with a different normalization of the incoming and outgoing states. My question is, how do we choose the normalization o... | 6,394 |
<p>Let's assume a close room with 1-2 people who only one of them smoking cigarette.
What is the equation describe the smoking spreading? is it diffusion? what are the parameters is so? </p>
<p>Is there a way/ device I can build or put that make the smoke go in some specific way, like a vacuum that will suck all the ... | 6,395 |
<p>I understand that the objects acceleration is determined by the force exerted <em>on it</em>, and that the force exerted <em>on it</em> is <a href="http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion" rel="nofollow">determined</a> by its acceleration.</p>
<p>But, does an object's (named A) acceleration (and mas... | 6,396 |
<p>Two objects go in against each other, and then they collide, will object 1, exerting force 1, necessarily get on it a reaction equal in magnitude and opposite in direction?</p>
<p>EDIT:-
In my book it only says "for every action there is always opposed an equal reaction". But after reading on Newton's third law on ... | 6,397 |
<p>Suppose they have the same contact area with you.</p>
<p>The term "painful" is a bit ambiguous.
Let me ask in another way:</p>
<p>Two cars with same momentum but different masses are going to hit a wall. You must stand right in front of the wall. Which car will you choose to be hit by? </p> | 6,398 |
<p>If i have a small toy boat with a sail, and i attach a fan onto it, FACING THE SAIL, which runs by solar. Once the fan turns on will the boat move or will it remain at rest. Apparently it wont because according to Newtons third law the sail will give an equal force back on to the air pushed by the fan. But that does... | 6,399 |
<p>This is asserted by Trinh Xuan Thuan in his book "Chaos & Harmony" chapter String theory.</p> | 6,400 |
<p>This question follows from a <em>schooling</em> I received in <a href="http://physics.stackexchange.com/questions/45387/why-cant-missing-mass-be-photons/45389#45389">this thread.</a> </p>
<p>I figured that photons do not interact with gravity, except when they've spontaneously converted into a particle-antiparticle... | 6,401 |
<p>Sorry if this is a naive question, not being even a part qualified physicist in any way shape or form.</p>
<p>I've read that the universe is expanding and the rate of expansion is increasing. The assumption being that it will continue expanding indefinitely.</p>
<p>However isn't there another possibility.</p>
<p>... | 6,402 |
<p>We wouldn't have computers if we didn't know about quantum physics. I understand understanding of general relativity is needed to make GPS work well. Has knowledge of quarks or string theory resulted in useful things? </p>
<p><strong>EDIT:</strong> I suspect quarks and string theory could be used by people who stu... | 6,403 |
<p>I'm studying perturbation theory in the context of quantum mechanics. My lecture notes say that in order to calculate the first-order correction of eigenfunction $\psi_n$, that is $\psi_n^{(1)}$, I shall use the formula:</p>
<p>\begin{equation}
\psi_n^{(1)}=\sum_{m,m\ne n} \frac{V_{mn}}{E_n^{(0)} - E_m^{(0)}}\psi_m... | 6,404 |
<p>Suppose that a fluid is flowing parallel to and over a flat plate. Obviously, a boundary layer develops in which the velocity ranges from 0 to 99% of the upstream velocity U. Could somebody please show me how the streamlines would look in this boundary layer? What do they typically look like in boundary layers?</... | 6,405 |
<p>I have seen it remarked in some problem sets that if you have an electromagnetic wave traveling in the $x$-direction with it's $y$-coordinate given as</p>
<p>$y(x,t)=y_0\sin (\omega t +kx)$</p>
<p>and you want a build an antenna to receive the wave, the antenna must be of length $2y_0$. I want to know how much tru... | 6,406 |
<p>It makes sense, since gravity tends to push the surface of a body towards it's center. Unless I'm mistaken, everything with mass has it's own gravity, every atom and for instance, our own bodies should also have their own gravity. The question is: how strong is our own gravitational pull? I know it must be extremely... | 6,407 |
<p>As i understood , if you have a point charge in the center of a hollow conducting sphere then the electric field inside it, is zero because the charge distribution is spherically symmetric. </p>
<p>But what's going on if the point charge isn't in the center of the sphere?</p>
<p>Will then be an Electric field insi... | 6,408 |
<p>I always see $\vec j\cdot \vec e$ as Joule's dissipation and I don't understand why. For example, if we have a uniform electric field $\vec e=e_o\vec u_x$ and we release an electron in it, it will start moving, accelerating in the direction of $-\vec u_x$, so the we will have a current $\vec j$, and this product $\v... | 6,409 |
<p>I know that the sum of vacuum bubbles can be related to the Vacuum energy, but I'm trying to understand how this follows from the Gell-Mann Low theorem/equation. My question will use equations from Peskin and Schroeder's (1995) text.</p>
<p>I start from the equation just below (4.30) on page 87, reproduced here</p... | 6,410 |
<p>In Feynman's lecture notes, he said that it is not (at his time).</p>
<p>How is the situation today?</p>
<p>Can first-principle calculation accounts the ferromagnetism of iron quantitatively now?</p> | 6,411 |
<p>In 't Hooft's original paper:
<a href="http://igitur-archive.library.uu.nl/phys/2005-0622-152933/14055.pdf" rel="nofollow">http://igitur-archive.library.uu.nl/phys/2005-0622-152933/14055.pdf</a></p>
<p>he takes $N \rightarrow \infty $ while $ g^2 N$ is held fixed. Is this just a toy model? Or is there some reason t... | 6,412 |
<p>It's said in Chapter VI.4 of A. Zee's book <em>Quantum Field Theory in a Nutshell</em>, a theory defined as $L(U(x))=\frac{f^2}{4}Tr(\partial_{\mu}U^{\dagger}\cdot\partial^{\mu}U)$, can be write in the form of a non-linear $\sigma$ model (up to some order)</p>
<p>$L=\frac{1}{2}(\partial\vec{\pi})^2+\frac{1}{2f^2}(\... | 6,413 |
<p>I've already faced this situation several times: given a statement (in area of thermodynamics) I used it to provide an example of some perpetual motion machine (of first or second kind). Therefore, I thought, proving the initial statement wrong. And sometimes I get a response that I haven't actually constructed a pe... | 6,414 |
<p>In condensed matter physics, people always say <a href="http://en.wikipedia.org/wiki/Quantum_fluid" rel="nofollow">quantum liquid</a> or <a href="http://en.wikipedia.org/wiki/Spin_liquid" rel="nofollow">spin liquid</a>. What does liquid mean?</p> | 6,415 |
<p>Why is constant for the conversion of mass to energy square of the ligths speed? is it bedside it's the fastest real matter? </p> | 6,416 |
<p>i saw this Lagrangian in notes i have printed:</p>
<p>$$
L(x,dx/dt) = (m^2(dx/dt)^4)/12 + m(dx/dt)^2*V(x) -V^2(x)
$$</p>
<p>what is it? is it physical? it seems like it doesn't have the right units of energy,</p>
<p>thanks</p> | 6,417 |
<p>I'm not certain that this question will make sense, but here goes...</p>
<p>In most monte carlo generators, when Z events are produced, there is a lower mass cutoff on the Z pole. I've been told that this is because the interference term with the photon would otherwise cause a divergence if you have $\gamma \to ee$... | 6,418 |
<p>I've just found <a href="http://www.youtube.com/watch?v=tu57B1v0SzI" rel="nofollow">Dr Quantum</a> video sample where double split experiment is presented conducted out by researchers. Are there any papers published in peer reviewed journals on that experiment to read in detail?</p> | 6,419 |
<p>Won't it be correct to define a CFT as a QFT such that the beta-function of all the couplings vanish? </p>
<p>But couldn't it be possible that the beta-function of a dimensionful coupling vanishes but it does so at a non-zero value of it - then the scale invariance is not generated though the renormalization flow i... | 6,420 |
<p>Looking at the relevant wikipedia page, one can read that the graviton should be massless. Is it 100 % certain that it is massless or is there room in any "nonstandard" models for a tiny non-zero mass (which could lead to a similar surprise as the detection of the neutrino oscillation) such that the graviton (if it ... | 6,421 |
<p>Can there be some non-electromagnetic radiation which would be perceived by human eye as light? I mean, say ultra-sound or some particle rays etc.</p> | 6,422 |
<p>I have read that string theory predicts (or requires ?) the existence of gravitons.
So, would that make it a quantum theory of gravity ?</p>
<p>If so, I have also read that quantum gravity would allow us to understand what happens at the singularity inside a black hole. What does string theory say about this ?</p>
... | 6,423 |
<p>Other than one having a positive invariant scalar product and the other a negative one, what are the actual physical differences between these vectors?</p> | 6,424 |
<p>I do have a question about an assumption made in the very interesting Hayden-Preskill paper of black holes as informational mirrors. Alice throws her top secret quantum diary which is $k$ qubits long into a black hole hoping to get rid of it. Unfortunately, the top forensic scientist Bob was already maximally entang... | 6,425 |
<p>Lets take a disc that is rolling without slipping which has moment of inertia $I=kmR^2$. It will have total kinetic energy $E=\frac{1}{2}mv^2+\frac{1}{2}I\omega^2=\frac{1}{2}mv^2(1+k)$.</p>
<p>Lets now use the parallel axis theorem to measure the moment of inertia around a different axis, lets say the point of cont... | 6,426 |
<p>Is it correct to state that the elements of Spin(n) fulfill a Clifford algebra and that the Lie group generators of Spin(n) is given by the commutator of the elements?</p>
<p>If not, then what is the relation of the Spin(n) group, the matrices that fulfill the Clifford algebra (that can be used to construct SO(n) b... | 6,427 |
<p>I've recently been to a converence on plasma physics were, to my surprise, a lot of presentations were concerned with <a href="http://en.wikipedia.org/wiki/Plasma_actuator" rel="nofollow">plasma actuator</a>. </p>
<p>Could someone, preferably in the field, tell me how long people have been reseaching this now and, ... | 6,428 |
<p>In a NPR News story from a few years back:</p>
<blockquote>
<p>"A gamma-ray burst from about 13
billion light years away has become
the most distant object in the known
universe."</p>
</blockquote>
<p>I'm a layman when it comes to physics, so cut me some slack if this is an ignorant question, but assuming ... | 6,429 |
<p>In D'Alembert principle forces are classified into constraint and applied forces ? Is this classification different from internal-external forces ?</p> | 6,430 |
<p>In this particular example shown the image below (from engineering dynamics - Meriam), I do not figure out why the direction of friction is in direction of the translation of the car?
Or I just didn't get it right and the car is slipping down in that situation?</p>
<p>(see hint 2 and 3 on the right hand side of th... | 6,431 |
<p>I'm reading page 488 of <a href="http://www.physics.usyd.edu.au/~fvon4093/introgr.pdf" rel="nofollow">Hobson, Efstathiou & Lasenby</a>, and I don't understand something they write... so I came here. </p>
<p>The concept they describe is in linearised general relativity. In particular, they are describing the ave... | 6,432 |
<p>Let us consider a Wightman field theory for the free scalar neutral field $\phi$, and let $O\mapsto\mathfrak F(O)$ be the net of local von Neumann field algebras. If we take a non-empty bounded open subset $O$ of $\mathbb R^4$, then according to the Reeh-Schlieder theorem the vacuum vector $\Omega_0$ of the Fock con... | 6,433 |
<p>I suppose to build a small dehumidifier, but I am narrowing the question down into its logics.</p>
<p>I have relatively powerful computer fans, water cooling radiator and some metal tubings. With the idea of compression and decompression I think air would do the job better than the original liquid coolant in the wa... | 6,434 |
<p>I'm given with these data:<br><br></p>
<p>Lens 1 and lens 2 are convex lenses with different focal lengths<br><br></p>
<p>Distance of object from lens 1: 50 mm <br>
Distance between lens 1 and lens 2: 100 mm <br>
Distance of final image from lens 1: 300 mm<br><br></p>
<p>How to compute for the Distance of first i... | 6,435 |
<p>I have encountered those terms in various places. As I understand it, "soft wall" can correspond to a smooth cutoff of some spacetime, while "hard wall" can be a sharp one, which can be described in terms of D-branes. Could somebody please explain the terminology, and in which context it can occur?</p> | 6,436 |
<p>Data are coming in, and it seems that recent Higgs boson observation is eliminating many SUSY models. If so, what is happening to superstring theories, like M-theory?</p> | 6,437 |
<p>Usually, when asked whether the purple color exists rainbows, an answer similar to this is given:</p>
<p>The purple color is perceived by human eyes via the activation of both red-sensitive and blue-sensitive cone cells. It is known that purple isn't a physically monochromatic light (a light composed of a single wa... | 6,438 |
<p>When material of rest mass M falls from infinity onto a black hole accretion disk, it gets heated and then emits so much light that the energy radiated away can measure up to about 30% or so of M c^2. Let's say that ε is the fraction of rest mass energy radiated away. </p>
<p>My first question is, after this accret... | 6,439 |
<p>From what I've read, the only remaining candidates appear to be either sterile neutrinos or MOND (MOdified Newtonian Dynamics -- it does seem to keep changing.)</p>
<p>Did I miss anything else plausible?</p> | 6,440 |
<blockquote>
<p>An insulated disk, uniform surface charge density $\sigma$, of radius $R$ is laid on the $x,y$ plane. Deduce the electric potential $V(z)$ along the z-axis. Next consider an off axis point $p'$, with distance $\rho$ from the center, Making an angle $\theta$ with the z-axis. Expand the potential at $p'... | 6,441 |
<p>I've been asked to show that both the position-momentum uncertainty principle and the energy-time uncertainty principle have the same units.</p>
<p>I've never see a question of this type, so am I allowed to substitute the units into the expressions and then treat them as variables?</p>
<p>If so, here's my attempt.... | 6,442 |
<p>Consider the following figures. </p>
<ul>
<li><p>The top one shows the construction of a spring where its left end is attached to the wall and its right end is stretched by a force.</p></li>
<li><p>The bottom one is supposed to be the free diagram of the spring. Is it correct?</p></li>
</ul>
<p><img src="http://i.... | 6,443 |
<p>From the perspective of <em>dimensional analysis</em>, in the Laplacian of Gaussian operator
$$LoG(x,y,\sigma)=\frac{\partial^2g}{\partial x^2} +\frac{\partial^2g}{\partial y^2}.$$ I think $x,y$ are variables with dimension $L$, $\sigma$ is a parameter with dimension $L$. But what about $g$? Since $g$ is a function ... | 6,444 |
<p>the spectrum of <a href="http://en.wikipedia.org/wiki/Quarkonium#QCD_and_quarkonia" rel="nofollow">quarkonium</a> and the comparison with positronium potential for the strong force is:</p>
<p>$V(r) = - \frac{4}{3} \frac{\alpha_s(r) \hbar c}{r} + kr$</p>
<p>(where the constant $k$ determines the field energy per un... | 6,445 |
<p>There is an art/science video clip on youtube, by Simon Faithful, called <a href="https://www.youtube.com/watch?v=_wnyp3Nrp0w" rel="nofollow">Escape Vehicle no.6 (chair in space)</a>. It shows a simple steel office chair, tied to a weather balloon, ascending into the upper reaches of the atmosphere. Apparently it re... | 6,446 |
<p>I'm undergraduate and I'm looking for a text about <a href="http://www.google.com/search?q=small+oscillations+matrix" rel="nofollow">Small Oscillations</a> in which matrices are used. Could you suggest me a book or a PDF file?</p> | 6,447 |
<p>The <a href="http://en.wikipedia.org/wiki/Shapiro_delay" rel="nofollow">Shapiro delay</a> was predicted in 1964 and observed by 1966, and is now a tool used to measure the mass of distant binary pulsars. The <a href="http://en.wikipedia.org/wiki/Terrell_rotation" rel="nofollow">Terrell-Penrose rotation</a> was publ... | 6,448 |
<p>My understanding is that in a double-slit experiment, quantum interference disappears if which-path information is <em>available</em>. How is <em>available</em> defined? Consider the following experiment:</p>
<p>SPDC is used to create an entangled pair of photons. The signal photon goes through a double-slit with a... | 6,449 |
<p>The state $$|\Psi \rangle = |0\rangle + \sum_j \int d\omega f_j(\omega)\hat{a}^\dagger_j (\omega) |0\rangle $$ is coming from a far field and incident on a double slit setup. Here j is the index of j-th atom of the detector and $\omega$ is the index of the $\omega$-th mode of the field.</p>
<p>On the other side the... | 6,450 |
<p>Is it theoretically possible for humans to manipulate Atoms to the extent that we can recreate anything we want?</p>
<p>e.g.
'Computer, Milky Way Bar please [or other].' (out pops a Milky Way Bar after some Atom Manipulation). <br>
'thanks computer'.<br>
'no problem'.<br>
'this one's melted'<br>
'sorry i'll do you ... | 6,451 |
<p>A problem I am trying to work out is as follows: </p>
<p>A particle moves in a force field given by
$\vec F =\phi(r) \vec r$. Prove that the angular momentum of the particle about the origin is constant. </p>
<p>I set it up as follows: </p>
<p>$\vec F = m {d^2\vec r \over dt^2}$</p>
<p>$\vec v = \int {\frac {\v... | 6,452 |
<p>i have seen on internet the following, for $ E >> 1 $ the Eigenvalue Staircase can be approximated by $ N(E)= \frac{1}{\pi}argZ(1/2+i \sqrt E ) $</p>
<p><a href="http://books.google.es/books?id=hLEJkA34GMQC&pg=PA35&lpg=PA35&dq=quantum+chaos+bolte%27s+formula&source=bl&ots=jqMMu5KK-L&si... | 6,453 |
<p><img src="http://i.stack.imgur.com/5QlUP.png" alt="enter image description here"></p>
<p>I have to <strong>write the matrix element</strong> $i\mathcal{M}$ given the Feynman rules of QED for the $\gamma \gamma$ scattering as the example above. Suppose that time is the vertical. There are two times, $t, t'$ such tha... | 6,454 |
<p>Consider the following Lagrangian for a massive vector field $A_{\mu}$ in Euclidean space time: $$\mathcal L = \frac{1}{4} F^{\alpha \beta}F_{\alpha \beta} + \frac{1}{2}m^2 A^{\alpha}A_{\alpha}$$ where $F_{\alpha \beta} = \partial_{\alpha}A_{\beta} - \partial_{\beta}A_{\alpha}$ which means $$\mathcal L = \frac{1}{4}... | 6,455 |
<p>In practical transformers there is always a leakage reactance. If it is possible to construct such a transformer (ideally thinking) having zero leakage reactance will the transformer action work? or not.</p>
<p>Edited: Probably my approach was wrong for that question I actually wants to ask. Now I'm trying to say i... | 6,456 |
<p>Consider a disk with a radius $R$ (I'll use $R=1$ at various points here) that has a constant surface charge density $\sigma$. Unlike the similar problem of the <a href="http://physics.stackexchange.com/questions/9830/tension-in-a-curved-charged-wire-electrostatic-force-does-wire-thickness-mat">field in the vicinit... | 6,457 |
<p>According to neural sciences the brain process information and reacts in 1 teen of a seconds the numbers add up to be 10.596674 loest is 30 to 20 years the course of time in reality recorded by our time keeping devices. but whant to know if perhaps i estimated wrong or something or if it really is what it sound like... | 6,458 |
<p>RHIC experiment results of forming Quark-Gluon Plasma and a possible bubble was news few months ago. I cannot find any follow up news after that. Does anybody know anything new in this front? I am especially interested to hear about the bubble formed.</p>
<p>Edit: The following are some links about this story.</... | 6,459 |
<p>Can you give details of a recent experiment of deflection of light by the Sun?</p>
<p>What is the distance from the surface of the Sun and what is the exact value of the angle of deflection?</p> | 6,460 |
<p>The book Nielsen & Chuang "Quantum Computation and Quantum Information" presents the concept of tensor products as follows. </p>
<p>Suppose we have the vectors $|v\rangle$ and $|w\rangle$ which exist in vector spaces $V$ and $W$ respectively. We also define the linear operators $A$ and $B$ which exists in same ... | 6,461 |
<p>I've been reading several papers on Adiabatic Quantum Computation, and I am confused by the form in which the adiabatic approximation condition is presented. For example (<a href="http://arxiv.org/abs/quant-ph/0001106" rel="nofollow">quant-ph/0001106</a> eq 2.8) \begin{align*}
\frac{\mbox{max}_{0 \leq s \leq 1} |\la... | 6,462 |
<p>I am reading paper <a href="http://jasss.soc.surrey.ac.uk/12/1/6/appendixB/Axelrod1997.html" rel="nofollow">Axelrod's model of dissemination of culture </a>, I am unable to understand the transition probabilities of time homogeneous Markov chain for this model. Can some one please explain it clearly how the expressi... | 6,463 |
<p>How would one most appropriately model the magnetic field around each bunch of protons in the LHC at maximum operating speed? Is the assumption of a wire of the same length of the beams (~30cm) with the beam current of 0.58A sufficient?</p> | 6,464 |
<p>In this video: <a href="https://www.youtube.com/watch?v=0jHsq36_NTU#t=55" rel="nofollow">https://www.youtube.com/watch?v=0jHsq36_NTU#t=55</a> at 0:55, it is claimed that "the sun is [...] dragging the planets in its wake".</p>
<p>Is this true? My understanding is that the sun and the planets are all moving toge... | 6,465 |
<p>I'm asking this to get yet another lessson in the inability of QFT and GR to cohabit. Many people believe GR must yield to quantization. The question here is as to why the activity of the vacuum cannot be imagined as having some sort of baseline and the gravitational force having some origin in this baseline. And, o... | 6,466 |
<p>I'd like to build a simple desk; just a single plank of wood (or a few side-by-side) with solid supports on each end of the desk. What I'm trying to figure out is how thick a plank I want to use for the surface. Ideally, the desk will be able to support my weight (since sometimes I stand on my desk to change light... | 6,467 |
<p>Is there some material state that can propagate light indefinitely without dissipation or absorption, like superconductors are able to transmit current indefinitely?</p>
<p>If not, then the question is, why not? would some fundamental principle being violated in such a material?</p> | 6,468 |
<p>I would like to know why $\mu^+$ muons can easily penetrate a solid metal such as Fe with negligible interactions while $\pi^+$ mesons lose their energy a lot faster when traveling through the metal. Is it because the interaction between electrons and muons is forbidden or should I somehow see it from the crossectio... | 6,469 |
<p>Is it possible to calculate the shortest time it takes a body to travel a certain distance if the only information provided is its maximum acceleration, its maximum retardation and the distance it travels (from rest to rest)? I've been trying to work it out but I was wondering if it is necessary to know either its m... | 6,470 |
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