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<p>I am confused in one question in general relativity, why we can always express a space-time geometry only by metric. It means a metric, which is just about distance in tangent space, can tell us all the information about the manifold.</p>
<p>I know there are standard proofs, for instance, we can express connection ... | 4,094 |
<p>Does the universe obey the holographic principle due to Stokes' theorem?</p>
<p>\begin{equation}
\int\limits_{\partial\Omega}\omega = \int\limits_{\Omega}\mathrm{d}\omega.
\end{equation}</p>
<p>Can this theorem be enough proof of our Universe being a hologram – the choice of $\omega$ and $\Omega$ is comp... | 4,095 |
<p>Lets say, I have a fluid in a rectangular enclosure (2D). I apply electric potential $U=U_1$ at left boundary and $U_2$ at right boundary. In the lower and upper boundaries, the potential varies linearly from $U_1$to $U_2$.</p>
<p>I am simulating this problem. So what initial condition should I take for potential o... | 4,096 |
<p>In a question I am doing it says:</p>
<blockquote>
<p><em>Show explicitly that the function $$y(t)=\frac{-gt^2}{2}+\epsilon t(t-1)$$ yields an action that has no first order dependency on $\epsilon$.</em></p>
</blockquote>
<p>Also my textbook says that</p>
<blockquote>
<p><em>[...] if a certain function $x_0(... | 4,097 |
<p>In Landau & Lifshitz's book, <em>Classical theory of fields,</em> the action for a free particle is defined as: </p>
<p>$$\tag{8.1} S= \int ^b _a {-mc \ \text d s}=0,$$</p>
<p>where $$\text d s=c\,\text d t\sqrt{1-\frac {v^2}{c^2}}$$
is the the invariant space-time interval beetween points along the particles ... | 4,098 |
<p>Based on the concept that energy can never be destroyed and is only transferred. Does it mean that energy since the formation of literally everything still here today?</p> | 4,099 |
<p>I have been told that $$[\hat x^2,\hat p^2]=2i\hbar (\hat x\hat p+\hat p\hat x)$$ illustrates <strong><em>operator ordering ambiguity</em></strong>.</p>
<p>What does that mean?
I tried googling but to no avail.</p> | 4,100 |
<p>When there is a charged conducting object near an another charged conducting object the charges on these objects accumulate to the sides where the closest points between these objects are. However, since these objects are conductors the charges also have to move on these objects. For a perfect conductor(~superconduc... | 4,101 |
<p>People sometimes talk about KeV mass sterile neutrinos as a warm dark matter candidate. I think they call them <em>KeVins</em> (horrible name btw). Now, In order for it to be a good dark matter candidate it must be quasi-stable, so the mixing with active neutrinos must be quite small. Can anybody give me the upper b... | 4,102 |
<p>A circuit has a self-induction of 1 H and carries a current of 2 A. To prevent sparking when the circuit is witched off, a capacitor of which can withstand a voltage of 400 V is used. The least capacitance of the capacitor connected across the switch must be?</p>
<p>I have no idea how to go about this question.</p... | 4,103 |
<p>Let's say a room is filled with butane, I then throw a cigarette into the room. What happens to the atoms/molecules of the butane when they are in contact with the heat from the cigarette?</p> | 4,104 |
<p>Lets say you have a plank is you hit it once and get t time if you hit is 2x as hard will it travel t/2? will it be the same or will it travel only slightly faster?</p> | 4,105 |
<p>I am totally confused about the Hilbert Space formalism of Quantum Mechanics. Can somebody please elaborate on the following points: </p>
<ol>
<li><p>The observables are given by self-adjoint operators on the Hilbert Space. </p></li>
<li><p>Gelfand-Naimark Theorem implies a duality between states and observables </... | 783 |
<p>I was doing an exercise in physics and I noted that the <a href="http://en.wikipedia.org/wiki/Angular_acceleration" rel="nofollow">angular acceleration</a> was negative. What does that mean?</p> | 4,106 |
<p><strong>A clarification please</strong></p>
<p>The following scheme to measure <strong>linear</strong> polarization states (a single polarizing beam splitter and two photo counters) orientation (as $arctan \sqrt{\frac{v}{h}}$) of <strong>coherent</strong> light pulses cannot discriminate two states which make the s... | 4,107 |
<p>In looking at the answers to this question regarding light from distant galaxies ever being visible to us:</p>
<p><a href="http://physics.stackexchange.com/questions/129980/expansion-of-the-universe-will-light-from-some-galaxies-never-reach-us">Expansion of the Universe, will light from some galaxies never reach us... | 4,108 |
<p>I know that there is theory that strings are the most fundamental particles. But if it is a string, then it can be 'cut' into pieces, and if it can be 'cut', then it can be cut at infinitely many places. Then does this makes the strings fundamental? If not, then what is ultimately <em>the</em> fundamental piece of m... | 4,109 |
<p>In string theory, it is assumed that all particles can be described as quanta corresponding to the excitations of only one kind of fundamental string. </p>
<p>How can in principle the different motion patterns of one kind of string give rise to the whole particle zoo in the standard model? How are the different pro... | 4,110 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/1030/what-is-the-evidence-for-inflation-of-the-early-universe">What is the evidence for Inflation of the early universe?</a> </p>
</blockquote>
<p>I am reading some public science books on inflationary u... | 123 |
<p>What is the electric current $I_{12}$ and voltage $U_{12}$ in following electric circuit? $I_{12}$ and $U_{12}$ are between points 1 and 2. </p>
<p><img src="http://i.stack.imgur.com/FldEO.jpg" alt="picture of electric circuit of the question">. </p>
<p>I have used <a href="http://phet.colorado.edu/en/simulation/c... | 4,111 |
<p>Recently, I was pondering over the thought that is most of the elementary particles have intrinsic magnetism, then can gravity be just a weaker form of electromagnetic attraction? But decided the idea was silly. </p>
<p>But I then googled it and found <a href="http://theuniverseandman.wordpress.com/2012/09/04/how... | 4,112 |
<p>The usual Lagrangian for a relativistically moving charge, as found in most text books, doesn't take into account the self force from it radiating EM energy. So what is the Lagrangian for a relativistic charge that includes the self-force?</p> | 4,113 |
<p>Is it really possible to walk on water or levitate in air. If not then how do some magician,s like dynemo do it simply while walking on street? And i have asked for possibility of walking on water so please exclude the high velocity concept.</p> | 4,114 |
<p>As I understand, to an observer well outside a black hole, anything going towards it will appear to slow down, and eventually come to a halt, never even touching the event horizon. </p>
<p>What happens if you (theoretically) wait long enough? Would you see the black hole vaporize before you see the object even reac... | 4,115 |
<p>Question: How can I find physics papers, articles and preprints decades ago?</p>
<p>Description: The search of articles and papers is for the purpose of finding interesting phenomenon for a science project. Where can I find a website where physics papers, articles and preprints in the 80's or older could be found?<... | 4,116 |
<p>What's so ingenious in diffractive lenses?</p>
<p>To my naive eye they seem to be just Fresnel lenses with smaller features.</p>
<p>What makes it so magic and why all the fuss about it?</p> | 4,117 |
<p>muon neutrino momentum distribution </p>
<p>I have read the public data of T2K ,KEK to find this subject, I'm curiously that it's coincides with my prediction perfectly: The neutrino get its momemtum by its effect-partner, which is obvious in the reactional formula, especialy that by me. You can find the figures<a ... | 4,118 |
<p>If lasers are collimated, what causes them to decollimate? Their production system seems to suggest a completely linear, collimated light source, but they do spread out over large distances. The same holds for synchrotrons. Why does this happen?</p> | 4,119 |
<p>I am referring to <a href="http://physics.stackexchange.com/questions/30105/is-qcd-free-from-all-divergences">this question</a>, and especially <a href="http://physics.stackexchange.com/a/30150/605">this answer</a>. </p>
<blockquote>
<p>In addition, QCD has - like all field theories - only an asymptotic
perturb... | 4,120 |
<p>Consider the particle in a box problem in QM. The crux of the reason why QM is able to explain the physical phenomenon is not just the theory but also able to impose boundary conditions which eventually result in quantization. Now in the particle in a 1-d box problem, the wave function is assumed to be zero at the b... | 4,121 |
<p>The most basic situation, water at room temperature, has dissolved oxygen, CO2, and more. Why is this? How would one calculate how much gas <em>should</em> be dissolved in any given liquid?</p> | 4,122 |
<p>Why we don't see any <a href="http://en.wikipedia.org/wiki/Gibbs_phenomenon" rel="nofollow">Gibb's phenomenon</a> in quantum mechanics?</p>
<p><strong>EDIT</strong></p>
<p>At sharp edges (discontinuities), we usually find ringing. This cane be observed in many physical phenomenon (eg. shock waves). Naturally, when... | 4,123 |
<blockquote>
<p>A heavy particle is projected at speed $U$ at an angle $\alpha$ to the horizontal. The particle is subject to air resistance which is experimentally found to vary proportionally to the square of the speed. Show that</p>
<p>$$\vec{\dot{v}} = -\frac{g}{V^2}\lvert\vec{v}\rvert\vec{v} - g\vec{j},$$</... | 4,124 |
<p><strong>The problem statement</strong></p>
<p>Vehicle A and Vehicle B are moving in opposite directions on the NJTP. Vehicle A is heading south toward atlantic city while vehicle B is heading north towards Hoboken. In situation 1 and 2 described below, at t = 0s both vehicles are at a distance of separation of 400m... | 4,125 |
<p>I'm studying for a course in electromagnetism, and I've been given an electric field for which I need to find the associated scalar potential. The field is the field generated by a sphere of radius $R$ with constant charge density $\rho$ throughout its volume, so that the total charge $Q=\dfrac{4\pi r^3 \rho}{3}$con... | 4,126 |
<p>I have the following circuit:</p>
<p><img src="http://i.stack.imgur.com/Ss7Iv.jpg" alt="enter image description here"></p>
<p>It is subject to a steady, time-invariant magnetic field which points out of the paper. At $t = 0$, the switch closes.</p>
<p>I thought that the magnetic flux would decrease at the moment ... | 4,127 |
<p>When i read about how the concept works, it looks pretty solid. But in reality accelerometers are just so imprecise that they need to be configured with alternative ways to correct their positions.</p>
<p>I am talking about using accelerometer to measure position. Sounds very solid, but then fails to be practical f... | 4,128 |
<p>I'm redoing the calculations of "Point Canonical Transformations in the path integral", by Gervais and Jevicki; while doing so I stumbled in integrals like
$$
\int \mathrm{d}t \, \Delta_F^3(t) = -\frac{1}{12} \frac{i}{\omega^4}, \\
\int \mathrm{d}t \, \dot{\Delta}_F^2(t) \Delta_F(t) = \frac{1}{12} \frac{i}{\omega^2}... | 4,129 |
<p>This comes from Electromagnetic Fields and Waves by Lorrain et al, page 77 on a Hollow, <em>ungrounded</em> conductor enclosing a charged body:</p>
<blockquote>
<p>The surface charge density at a given point on the <em>outside</em> surface of the conductor is independent of the distribution of Q in the cavity. It... | 4,130 |
<p>I am curious what causes the charge build up in a cloud before lightning occurs. I have seen in a few places such as <a href="http://www.regentsprep.org/Regents/physics/phys03/alightnin/default.htm" rel="nofollow">this</a>, that the process is not fully understood. Is this some form of static electricity? Here are a... | 4,131 |
<p>I want to calculate the matrix elements of the operator $\hat{x} \hat{p}$ in momentum and position basis, that is the two quantities ($p,q$ - momenta, $x,y$ - positions):</p>
<p>$$\langle p|\hat{x} \hat{p}|q\rangle$$
$$\langle x|\hat{x} \hat{p}|y\rangle$$</p>
<p>I don't know how to do this. I write $\hat{p}|q\rang... | 4,132 |
<p>Why can't we use fissions products for electricity production ?</p>
<p>As far has I know fissions products from current nuclear power plants create enough 'waste' heat to boil water; and temperature decreases too slowly for an human life. So why can't we design a reactor to use this energy.</p> | 4,133 |
<p>I used to live in Italy, and when it was sunny, well, as expected it was pretty hot. Both under the direct light of the sun through the atmosphere, and in the shade, where the temperature was of course lower. I would formalise this saying that in Italy I experienced a certain $\Delta T_{it}$ between shady and expose... | 4,134 |
<p>Let's say that there is a circular conical section that has diameter $D=.25x$ without any heat generation and I need to find the temperature distribution.</p>
<p>Originially I thought I could use the heat diffusivity equation at steady state to find the temperature distribution. The differential equation would be:<... | 4,135 |
<p>I am developing a numerical algorithm to find the ground state of a Hermitian matrix. Obvious applications are quantum many-body systems and particles in various potentials. I am a little stuck with the comparison for the latter example. Could anyone recommend a numerical package to find a ground state of a particle... | 4,136 |
<p>If I have an optical transparent slab with refractive index $n$ depending on the distance $x$ from the surface of the slab, the refractive index can be described by:
$$n(x)=f(x)$$ where $f(x)$ is a generic function of $x$. so, we can write:
$$\dfrac{dn(x)}{dx}=f'(x)$$
The snell law of refraction states:
$$n_1\sin(\t... | 4,137 |
<p>The question is: if I were to insert a brass plate between two charges, what will happen to the force between the charges? Would it increase, decrease or stay the same?
<br><br>
Does the brass plate increase the value of permittivity of the medium and therefore the force decreases?
<br><br>
The correct answer is tha... | 4,138 |
<p>I'm a math student who's dabbled a little in physics, and one thing I'm a little confused by is separation of variables. Specifically, consider the following simple example: I have a Hamiltonian $H$ which can be written as $H_x + H_y + H_z$ depending only on $x,y$, and $z$ , respectively, and I want to find the eige... | 4,139 |
<p>My problem is the following: I'm trying to model a dust (pressure-less relativistic gas) in the presence of electromagnetic field using colisioness vlasov-equation (relativistic version of boltzmann equation). Please note that I'm in flat minkowski spacetime with signature $(+,-,-,-)$.</p>
<p>So, I have the followi... | 4,140 |
<p>General relativity tells us that there is no absolute frame of reference (actually, it tells us that all frames are relative, which is close but not the same as there is no absolute frame).</p>
<p>Special relativity demonstrates that there <em>is</em> an absolute: the speed of light.</p>
<p>Notwithstanding the imp... | 4,141 |
<p><a href="http://en.wikipedia.org/wiki/Future_of_an_expanding_universe#Matter_decays_into_iron" rel="nofollow">Wikipedia</a> says that all matter should decay into iron-56. But it also says <a href="http://en.wikipedia.org/wiki/Nickel-62" rel="nofollow">Nickel-62</a> is the most stable nucleus.</p>
<p>So could this ... | 4,142 |
<p>I am teaching myself basic mechanics from a standing start. I am trying to understand Angular Acceleration and have set myself a problem to solve. My answer 'feels' wrong, so I'd like some help to understand if I've misunderstood, or miscalculated anything. I've taken many liberties with rounding, please ignore, thi... | 4,143 |
<p>I still don't know what mathematicians mean by <a href="http://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations" rel="nofollow">Navier-Stokes</a> existence and smoothness. Since there is a <a href="http://www.claymath.org/millennium/Navier-Stokes_Equations/" rel="nofollow">reward</a> for proving it, it seems im... | 4,144 |
<p>I always heard that three atomic force in small scale structure (Strong, weak and electromagnetic) are unified through the <a href="http://en.wikipedia.org/wiki/Standard_Model" rel="nofollow">standard model</a>, but I've never seen its unified equation. What is single unified equation that describes these three forc... | 4,145 |
<p>Apologies if this is a little vague. It might not have a good answer.</p>
<p>Given the interpretation of $|\psi(x)|^2$ as a probability distribution it's unsurprising that a wave function that is concentrated around a point $x$ should behave at least a little like a classical particle at the point $x$.</p>
<p>Is t... | 4,146 |
<p>I'm a bit confused on the topic of refraction, some starting premises:</p>
<p>1) When light refracts from exiting a medium with a low n and entering a medium with high n the light bends. </p>
<p>2) The index of refraction, n, supposedly is lower for longer wavelengths than others (why exactly this is, is difficult... | 4,147 |
<p>Suppose I have a large 2D flat rubber sheet of a given stiffness. I mark a point on the sheet at polar coordinates $(\rho, \theta)$. Then I insert a pencil in the center of the sheet and I leave the pencil there. The pencil creates a round hole of radius $r$ centered at $(0, 0)$. Assuming the sheet does not fold int... | 4,148 |
<p><strong>EDIT</strong>: Additional question at the end</p>
<p>I am trying to illuminate how the "unphysical" gauge bosons $W^{1}_{\mu},W^{2}_{\mu},W^{3}_{\mu},B_{\mu}$ will be the "physical" $W^{\pm},Z_{\mu},A_{\mu}$ when diagonalizing the mass matrix. Notice that it is in Euclidean time, so we do not have to care ... | 4,149 |
<p>I'm having some trouble doing an easy computation with the AdS space. I'm considering $\text{AdS}_3$ space with the Poincaré coordinates, so the metric reads</p>
<p>$$ds^2 = \frac{R^2}{z^2}(dz^2 - dt^2 + dx^2)$$</p>
<p>I want to compute the geodesics for a $t=\text{const.}$ slice, in order to obtein the holographi... | 4,150 |
<p>I'm beginner and amateur interested by the quantum physics.</p>
<p>I would like to know if it exists entanglements systems at natural state or it requires mandatory an human intervention ?</p>
<p>Is it possible ? Either no or yes, Why ?</p>
<p>Thanks</p> | 4,151 |
<p>If you see through small enough aperture, you can see things without glasses. </p>
<p>How does this trick work?</p> | 4,152 |
<p>I have been looking in to alternating current and I am confused. If the voltage reverses doesn't the flow of electrons also reverse? I am aware of another fair answer on this site <a href="http://physics.stackexchange.com/q/28036/">here</a>. That answer, however, confused me just a bit. I gathered that the electrons... | 124 |
<p>The theory of Inflation explains the apparent consistency of the universe by proposing that the early universe grew exponentially for a 1E-36 seconds. Isn't a simpler explanation that the universe is just older and so the homogeneousness comes from a slower more steady growth? Is there any evidence that rules out a... | 123 |
<p>I understand how telescope, microscope and glasses work.<br>
But how do contact lenses work?</p> | 4,153 |
<p>I've been working with some dust solutions in General Relativity, practicing calculating the Riemann curvature tensor, and I came across an odd metric: the Tolman-Bondi-de Sitter metric. A quick internet search (to supplement the book I'm reading) can tell you that it describes spherical dust, while accounting for a... | 4,154 |
<p>This is a very cool, and highly beneficial problem in my opinion. I feel as though truly understanding this proof would broaden anyone's conceptual understanding of electric potential.</p>
<p>My textbook asks me to utilize the identity: </p>
<p>$\bigtriangledown(\phi\bigtriangledown\phi) = (\phi\bigtriangledown)^2... | 4,155 |
<p>This question concerns the physics behind the implementation of electronic compasses to find the orientation of a device.</p>
<p>In the robotics community, <strong>3-axis magnetometers</strong> are often used for this purpose. After some calibration, these provide a three-dimensional vector indicating the direction... | 4,156 |
<p>When describing the defining characteristics of bosons and fermions, I have a problem with the idea of "label switching" - whereby you have the wavefunction for two particles and the particles' labels in the wavefunction are switched and we look at the effect. The wavefunction remains the same: particles are bosons... | 4,157 |
<p>Take a gravitational field (with all the field lines pointing inwards) and a perfectly circular curve as an object's trajectory. To find the work exerted by the force on the object, compute the line integral $\oint_C\overrightarrow{F} \cdot \overrightarrow{v}=Work=0$; which is to be expected as there is no tangent... | 4,158 |
<p>I'm learning some applications for equation of motion. But I'm failing to relate velocity, acceleration and position.</p>
<p>If $v=\frac{dr}{dt}$ and $a=\frac{dv}{dt}$, why $a$ is $\frac{d^2r}{dt^2}$ instead of $\frac{dr}{dt^2}$? Probably i'm lacking basic calculus or physics.</p> | 4,159 |
<p>In Feynman's simple QED book he talks about the probability amplitude P(A to B) ,where A and B are events in spacetime, and he says that it depends of the spacetime interval but he didn't put the expression. I would like to know what expression was he referring to.
He said then that the amplitudes for a photon to g... | 4,160 |
<p>It is known that there is a famous quantum factorization algorithm by Peter Shor. The algorithm is thought to be suitable only for quantum gate computer.</p>
<p>But can a an adiabatic quantum computer especially that which is capable of quantum annealing be used for factorization?</p>
<p>I am asking this because i... | 4,161 |
<p>In the <a href="http://en.wikipedia.org/wiki/NEAR_Shoemaker" rel="nofollow">wikipedia article about NEAR Shoemaker</a> it is mentioned that the craft stopped operating under these conditions:</p>
<blockquote>
<p>At 7 p.m. EST on February 28, 2001 the last data signals were received from NEAR Shoemaker before it w... | 4,162 |
<p>There is a <a href="http://io9.com/5811236/this-is-the-coolest-science-experiment-youll-see-all-week" rel="nofollow">video</a> of an experiment from University of Mexico using corn syrup (highly viscous) and water. They are "mixed together" in a container by turning a crank but when the crank is turned in the opposi... | 4,163 |
<p>I've heard mentioned in various classes that neutron stars, like superconductors, are described by BCS theory. I know that in superconductors a key element in forming cooper pairs is a net attractive force between the electrons which would normally repel one another. That attractive force is accounted for via lattic... | 4,164 |
<p>In the <a href="http://www.bipm.org/en/CGPM/db/3/2/" rel="nofollow">Resolution 2 of the 3rd meeting of the CGPM</a>, defining the <a href="http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kilogram.html" rel="nofollow">kilogram</a>, “the International Service of Weights and Measures” is mentioned (the <a href="http... | 4,165 |
<p>I was blown away by the abundance of good info here, and I thought maybe you could help me out with this query. I am in the process of developing a product, and I need to find a clear plastic resin (visually clear) that is also transparent to x-rays (medical application). The only only other characteristic that I ne... | 4,166 |
<p>Consider an electron described by a wave packet of extension $\Delta x$ for experimentalist A in the lab. Now assume experimentalist B is flying at a very high speed with regard to A and observes the same electron. The extension of the wave packet will appear contracted, and the uncertainty on momentum will increase... | 4,167 |
<p>How can we define simply that velocity is a vector quantity without mentioning that velocity has vector properties. How can we simply say it needs both magnitude and direction for its complete description? Well for magnitude it is clear but for direction being a necessary for its complete description I found it hard... | 4,168 |
<p>After watching a few videos on <a href="http://en.wikipedia.org/wiki/Light" rel="nofollow">light</a> and <a href="http://en.wikipedia.org/wiki/Electromagnetic_radiation" rel="nofollow">electromagnetic radiation</a>, I am a little confused.</p>
<p>The way things are explained, is that light is just the same as elect... | 125 |
<p>I'm looking for guidance to choose material for a self-study of <a href="http://en.wikipedia.org/wiki/Photonics" rel="nofollow">photonics</a>, biased towards <a href="http://en.wikipedia.org/wiki/Optical_computing" rel="nofollow">optical computing</a> recent advances.</p>
<p>My background is of undergraduate level ... | 4,169 |
<p>Why is the anticommutator actually needed in the canonical quantization of free Dirac field?</p> | 4,170 |
<p>I had a left over coffee cup this morning, and tried to wash it out. I realized I always instinctively use hot water to clean things as it seems to work better. </p>
<p>A google search showed other people with similar results, but this yahoo <a href="https://answers.yahoo.com/question/index?qid=20120808093022AAeM... | 4,171 |
<p>Consider the scenario where you measure the time it takes for light to travel to the left 10 meters and to the right 10 meters. Both measurements will take the same time, even though we are moving through space at crazy speeds. This must mean that light is not moving relative to "space" as a whole. What does it move... | 4,172 |
<h1>The Question</h1>
<p>If the three vector electric and magnetic fields come from the four component four-potential, then is there a fourth component to the electric and magnetic field?</p>
<h1>Related Question</h1>
<p>I posted the following question: <a href="http://physics.stackexchange.com/questions/103664/expr... | 4,173 |
<p>Mathematically, I'm having trouble understanding where we can use what with light. I read somewhere on this site that Huygen's Principle is effectively just taking an expansion of a wave onto the spherical harmonics, (<a href="http://physics.stackexchange.com/questions/89884/is-huygens-principle-axiomatic">Is Huygen... | 4,174 |
<p>What would the properties of a particle be that would allow light to orbit it?</p>
<p>Light travels fast. <em>Really</em> fast. Almost to the point where we consider that it's instantaneous, and moves only in a straight line.</p>
<p>We know that's not true though. Light can be influenced in such a way so as to cau... | 126 |
<blockquote>
<p><em>Unlike mass, the charge on an object is said to be unaffected by the motion of the object.</em> </p>
</blockquote>
<p>This statement in my text book, is not yet understood by me. I don't know theory of relativity. On expressing my inability in understanding this particular line. My sir said that... | 4,175 |
<p>When we define $\mathcal{N}=4$ SYM on flat Minkowski space, the supersymmetric vacua are parametrized by scalars living in the cartan subalgebra of the gauge group. A generic point in the moduli space breaks the SU(N) to $U(1)^r$ where r is the rank of the cartan subalgebra. This is called the Coulomb branch for obv... | 4,176 |
<p>The stability group $G_\Sigma$ is a subgroup of the Poincare group $P(1;3)$. Its generators $X$ in the <strong>front form</strong> leave the hypersurface $\Sigma: x^+ = 0$ invariant. Phrased differently they satisfy the condition: $x'^{+} = x^+ + [x^+, X] = x^+ = 0$. So, for the generators $X$ of the stability group... | 4,177 |
<p>I am trying to gain an intuitive picture of what is referred to by "electron-shell energy". I have read that outer electron shells have higher energy than inner electron shells, and this seemed to make sense to me by analogy to a wheel — a point on the rim of a wheel moves faster than a point on the hub. However, ... | 4,178 |
<p>What are <a href="http://en.wikipedia.org/wiki/Coherence_%28physics%29" rel="nofollow">coherent</a> and incoherent radiation?</p>
<p>I am a mathematician who is self-learning physics. In reading Jackson's electrodynamics and other books, I often hear that radiation is incoherent or coherent. What does this mean? Do... | 4,179 |
<p>I'm studying a circuit in AC. I use a function generator and set a waveform.
I have a solenoid and I put a small solenoid inner it.
Could you tell me if there is a relation between frequency and emf inducted on the second solenoid?</p> | 4,180 |
<p>I want to know what is the smallest device that can work on a few milli amps? I know a NE-2 neon bulb works on less than 10 mAmp but on 90 volts at least.</p>
<p>I need to find a small device that:
1- Consumes electricity
2- works independently (just by hooking it to a battery)
3- Operates on a few volts
4- Operate... | 4,181 |
<p>When light from an object passes through a convex lense, it gets reconverged at a single real image. From there, the light rays presumabely begin diverging again, exactly as if there was a light-emitting object sitting at the real image point.</p>
<p>Why is it that if we look at the real image point, we don't see a... | 4,182 |
<p>In the semi-classical model of a crystal in solid state physics, electrons and holes are assigned effective masses that account for their different mobilities. E.g. in silicon, holes have a bigger mass than electrons. This results in different electron/hole densities as the temperature increases.</p>
<p>Why do thos... | 4,183 |
<p>Has anyone done any research about the upwind vehicle <a href="http://www.popsci.com/cars/article/2012-07/wind-powered-car-travels-upwind-twice-speed-wind" rel="nofollow">http://www.popsci.com/cars/article/2012-07/wind-powered-car-travels-upwind-twice-speed-wind</a>?</p>
<p>I think it is impossible but get a surpri... | 127 |
<p>Would this be a valid equation to calculate kinetic energy created from a drop from a height:</p>
<p>$$E_{kinetic} ~=~ v_{vertical}tmg$$</p>
<p>Velocity multiplied by time gives distance. Distance multiplied by gravitational force acting on it provides kinetic energy. Would this equation be valid?</p> | 4,184 |
<p>I'm following the proof of <a href="http://electron6.phys.utk.edu/qm2/modules/m4/projection.htm" rel="nofollow">Wigner-Eckart projection theorem</a> which states that:</p>
<p>$$\langle \bf{A} \rangle ~=~ \frac{\langle \bf{A} \cdot \bf{J} \rangle}{\langle {\bf{J}}^2 \rangle} \langle \bf{J} \rangle $$</p>
<p>if ${\b... | 4,185 |
<p>I have two sets of data (globular clusters), one for the Andromeda Galaxy and one for the Milky Way.</p>
<p>I want to compare the distribution of globular clusters between the two galaxies according to their distance from the galaxy's centre, but the measurements they use in each are different.</p>
<p>Andromeda ha... | 4,186 |
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