question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<ul>
<li>vector D = 4 cm North <br /></li>
<li>vector J = 4.5 cm West</li>
</ul>
<p>what is D+J?</p>
<p>In a more general sense, how can two 2D vectors that are perpendicular to each other be added?</p> | 5,190 |
<p>Hoping this is not a silly and stupid question let me ask for help in this problem.</p>
<p>I have a particle in an infinite square well (the box is from 0 to a), in the state described by the function </p>
<p>$\psi (x) = Ax(a-x) \qquad \mathrm{for }\qquad 0<x<a$, $\qquad 0 \qquad otherwise$.</p>
<p>I have t... | 5,191 |
<p>A block of mass $m$ is pushed towards a moveable wedge of mass $\eta m$ and height $h$, with a velocity $u$. All surfaces are smooth. The minimum value of $u$ for which the block will reach the top of the wedge is:
<img src="http://i.stack.imgur.com/vaqFh.png" alt="enter image description here"></p>
<ol>
<li>$\sqrt... | 5,192 |
<p>I have been really staring for a while in a MP-Beiser book and I totally disagree with a statement he does there. On a page 85 he states that photons act as they have a mass $m$. He derives this by stating that:</p>
<p>$$
\begin{split}
p &= m v\\
\frac{h\nu}{c} &= m c\\
m&= \frac{h \nu}{c^2}
\end{split... | 5,193 |
<p>Imagine a large bandgap material which is irradiated by an intense laser beam.
If the photon energy is only high enough for 1/5 of the bandgap, is there a way to approximate the absorption by 5-photon excitation, i.e. the ratio of transmitted to initial Intensity?</p>
<p>All I found is related to 2 or 3 Photon abso... | 5,194 |
<p>While reading this question: <a href="http://physics.stackexchange.com/questions/32120/why-do-we-still-not-have-an-exact-definition-for-a-kilogram">Why do we still not have an exact definition for a kilogram?</a> , I had a crazy thought.</p>
<p>Using <a href="http://en.wikipedia.org/wiki/Polymerase_chain_reaction" ... | 5,195 |
<p>So they say the remote observer will never see anything fallen to the black hole, because any object will slow down as it gets closer to the event horizon and eventually stop to stay there forever. Am I getting it right so far? With this said, it turns out nothing has fallen to the black hole up till now. That is fo... | 5,196 |
<p>I am reading this research paper authored by NS Manton on the <a href="http://www.sciencedirect.com/science/article/pii/0550321377902942" rel="nofollow">Force between 't Hooft-Polyakov monopoles</a>. I have a doubt in equation 3.6 and 3.7. We assume the gauge field for a slowly accelerating monopole to be $A_0 = \e... | 5,197 |
<p>From <a href="http://www.facebook.com/ConstellationEnergy" rel="nofollow">Constellation Energy</a></p>
<blockquote>
<p>Quick energy efficiency tip: To stay cool and manage your energy at
the same time, use ceiling fans to create a “wind chill” in rooms you
are using. The wind chill will help you feel cooler t... | 5,198 |
<p>What is the influence of Hermitian condition ($\psi=\psi^{\dagger}$) of Majorana fermions operators in their statistical behavior?</p>
<p>A Majorana fermion gas must obey the Fermi-Dirac statistics, or their stastistical behavior may be anyonic?</p> | 5,199 |
<p>Wave-particle duality states that a particle has both wave properties and particle properties when one is <em>not</em> observing it.</p>
<p>1) What is an observer? Need it be anything living or can other particles also act as observers?</p>
<p>2) When doing the electron double slit experiment--shooting just one el... | 5,200 |
<p>Both the <a href="http://en.wikipedia.org/wiki/Black%E2%80%93Scholes" rel="nofollow">Black-Scholes PDE</a>{*} and the Mass/Material Balance PDE have a similar mathematical form of the PDE which is evident from the fact that on change of variables from Black-Scholes PDE we derive the heat equation (a specific form of... | 5,201 |
<p>I am asking about an English translation of a Helmholtz paper: </p>
<blockquote>
<p>Ueber die physikalische Bedeutung des Princips der kleinsten Wirkung. <em>Journal für die reine und angewandte Mathematik (Crelle's Journal), Volume 100, Issue 2, 1887, Pages 137-166, and Volume 100, Issue 3, 1887, Pages 213-222.<... | 5,202 |
<p>I'm trying to understand Morin's example of a spring pendulum. What I don't get is his expression for $T$. I can understand the $\dot x^2$ term in the brackets. But I don't understand the $(l + x)^2\dot \theta^2$. </p>
<p>Also, it seems rather strange to break up Kinetic Energy into tangential and radial components... | 5,203 |
<p>Almost everybody knows that light is massless. But where this come from and how it can be proven (experimentally or theoretically)? I actually found <a href="http://open.salon.com/blog/kwatts59/2009/02/18/physics_what_is_the_mass_of_light" rel="nofollow">this article</a> which explains and calculates the mass of lig... | 180 |
<p>If there are no orbitting electrons in a neutron star's makeup to interact with EM, what happens to light that strikes it? </p> | 5,204 |
<p>When we want to figure out the long-term evolution of a planet's atmosphere/orbit, when is perihelion more important than mean distance?</p>
<p>E.g. some processes (like Jeans Escape and escape of atmospheres) are disproportionately affected during perihelion (point of closest approach) rather than during aphelion.... | 5,205 |
<p>Who has succeeded in demonstrating the <a href="http://en.wikipedia.org/wiki/Lense%E2%80%93Thirring_precession" rel="nofollow">Lense-Thirring effect</a>?</p>
<p>This effect is one that describes the rotational motion of the Earth from a space-time structure. This effect is the "drag" of the geometrical structure of... | 5,206 |
<p>I've heard that differential forms are related to densities, however I'm still a little confused about that. I thought on the case of charge density and I came to that: let $U\subset\mathbb{R}^3$ be a region of $3$-space, and let $\rho : U \to \mathbb{R}$ give the charge density at every point of $U$. I can then cre... | 5,207 |
<p>When reading the term <a href="http://en.wikipedia.org/wiki/Superselection#Superselection_Sectors">superselection sector</a>, I always wrongly thought this must have something to do with supersymmetry ... DON'T laugh at me ... ;-)</p>
<p>But now I have read in <a href="http://physics.stackexchange.com/a/56526/2751"... | 5,208 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/36469/are-the-protons-and-neutrons-in-the-nucleus-arranged-in-any-particular-way">Are the protons and neutrons in the nucleus arranged in any particular way?</a> </p>
</blockquote>
<p>Isotopes have diffe... | 181 |
<p>In a tight binding model, we usually start from the atomic orbits and linearly combine them to get the wave function of the crystal energy band.</p>
<p>My questions are:</p>
<ol>
<li><p>Since this kind of tight binding is an approximate method due to using atomic orbits, is it exact to use the <a href="http://en.w... | 5,209 |
<p>Experiments used to observe particle spin properties (such as <a href="http://en.wikipedia.org/wiki/Stern-Gerlach_Experiment" rel="nofollow">Stern-Gerlach</a>) rely on a varied magnetic field and a dipole-like reaction in the particle, deflecting it in one direction or another.</p>
<p>In the case of a point-particl... | 5,210 |
<p>When we are on Earth, we look <strong>UP</strong> in the Sky to see the <strong>Moon</strong>. How do we have to look at Earth from Moon.. Is it the way ? If so, how are these bodies actually placed in the space?</p>
<p>Are all astronomical objects lined up on the same planar level? When we look at stars, are they ... | 5,211 |
<p>It is usually said that the idea of fields was introduced (electric and magnetic fields) in electricity and magnetism after Coulomb's law to cure the conceptual problems of action at a distance.</p>
<p>Could someone explain what are the conceptual and physical difficulties or contradictions that one might have with... | 5,212 |
<blockquote>
<p>An astronomer is trying to estimate the surface temperature of a star with a radius of $5 \times 10^8\ m$ by modeling it as an ideal blackbody. The astronomer has measured the intensity of radiation due to the star at a distance of $2.5 \times 10^{13}\ m$ and found it to be equal to $0.055\ W/m^2$. Gi... | 5,213 |
<p>I remember when I was in primary school, the science teacher put me in charge of a mercury thermometer. I do not quite understand the mechanics behind except that mercury expands when it is hot and contracts when it is cold, and that this could be read off a temperature scale along the stem of the thermometer.</p>
... | 5,214 |
<p>Basis is the ideal gas law: <a href="http://en.wikipedia.org/wiki/Ideal_gas_law" rel="nofollow">http://en.wikipedia.org/wiki/Ideal_gas_law</a></p>
<p>This law only states a mathematical relationship between many variables. It does not state what happens if one of the variables changes.</p>
<p>So we pin down the vo... | 5,215 |
<p>I have a physics question that I need some help with:</p>
<p>"Proxima Centauri is a star in the Alpha Centauri solar system, it’s the
nearest star to our sun (4.24light−years) <a href="http://en.wikipedia.org/wiki/Listofneareststars" rel="nofollow">http://en.wikipedia.org/wiki/Listofneareststars</a>.
How large of a... | 5,216 |
<p>The question:</p>
<p>At what temperature is the RMS speed of Hydrogen molecules equal to the escape speed from the earth's surface? Values of radius of earth($r$) and gas constant $R$ has been supplied only.</p>
<p>We know that the escape velocity of a body on earth is given by $(2rg)^{1/2}$. Putting in the values... | 5,217 |
<p>The following Fierz relation does not seem so obvious to me :</p>
<p>\begin{equation}
\bar{\psi}_1 \gamma^\mu (1+\gamma_5)\psi_2 \bar{\psi}_3 \gamma_\mu (1-\gamma_5) \psi_4 = -2 \bar{\psi}_1 (1-\gamma_5) \psi_4 \bar{\psi}_3 (1+\gamma_5) \psi_2.
\end{equation}</p>
<p>As a first step I would have tried to do somet... | 5,218 |
<p>I thought about posting this question in the Biology StackExchange site but really it is just the application of my question that applies to biology not the core of the question itself.</p>
<p>Can anyone help me find the math involved in modeling acoustic lubrication? My goal is to investigate whether it could be e... | 5,219 |
<p>A <a href="http://en.wikipedia.org/wiki/Supernova_remnant" rel="nofollow">supernova remnant</a> is the structure resulting from the explosion of a giant star. The supernova remnant is surrounded by an expanding shock wave that is formed from material ejected by the explosion and interstellar material swept away duri... | 5,220 |
<p>I am living up north in <a href="http://en.wikipedia.org/wiki/Norway" rel="nofollow">Norway</a>, 300 km above the <a href="http://en.wikipedia.org/wiki/Arctic_Circle" rel="nofollow">Arctic Circle</a>, which gives me six months per year of darkness and cold. I used to have a starter telescope when I was living in Spa... | 5,221 |
<p>A ring placed along $y^{2}$ + $z^{2}$ = 4, x = 0 carries a uniform charge of 5 $\mu$C/m. Find D at P(3,0,0)</p>
<p>Should I be using Gauss's Law to solve this problem? I was considering using a spherical Gaussian Surface, and then using the formula D = $\epsilon_0 $E to find D, but I'm not sure how to set up my int... | 5,222 |
<p>The <a href="http://en.wikipedia.org/wiki/Method_of_image_charges" rel="nofollow">method of image charges</a> is a well-known and very useful tool for solving problems in electrostatics.</p>
<p>Unfortunately, when I was taught this method, it was presented simply as an algorithm. No real physical justification was ... | 5,223 |
<p><img src="http://i.stack.imgur.com/hWGlJ.png" alt="alt text"></p>
<p>This problem is giving me a lot of problems. So $E=k*q/d^2$. We'd want to find the distance from q1 to P, which is .1 meters (not cm) using pythagorean thereom. So we know k, which is just $9x10^9$ times q1 which is $-2.4u$ where $u=10^{-6}$ divid... | 5,224 |
<p>A week or so back I asked a <a href="http://physics.stackexchange.com/questions/21275/trying-to-understand-laplaces-equation">question</a> about the gravitational potential field
$$\phi=\frac{-Gm}{r}, \qquad r\neq 0, $$
and how to show the Laplacian of $\phi$ equals zero for $r\neq 0$? Eventually, (it took a while)... | 5,225 |
<p>Some lecture notes I was reading through claimed that a pressure pulse propagates through a liquid-filled tube (blood in a vein) with the speed</p>
<p>$$c=\sqrt{\frac{A}{\rho}\frac{dP}{dA}},$$</p>
<p>where $A$ is the cross-sectional area, $\rho$ the density and $P$ the pressure (which pressure?).</p>
<p>How can t... | 5,226 |
<p>Below is attached for reference, but the question is simply about whether vectors used in physics in a vector space can be represented by complex numbers and whether they can be divided.</p>
<hr>
<p>In abstract algebra, a <a href="http://en.wikipedia.org/wiki/Field_%28mathematics%29" rel="nofollow">field</a> is an... | 5,227 |
<p>Suppose a car moves with a constant speed of $20 \text{m/s}$ a quarter of a circle, and completes the quarter in $5$ seconds. One way to calculate the circumference is simply $20 \cdot 5 \cdot 4 = 400 \text{m}$. However, I know that $a=\frac{v^2}{R}$ and the circumference is $2 \pi R$. The acceleration is defined as... | 5,228 |
<p>I am trying to understand the effect of effective pressure or effectiveness in a hydraulic system. I have seen different units for effective pressure or effectiveness - e.g. Nm/Bar Nm/L etc. and I am a bit confused how or why this is different.</p>
<p>Any help would be appreaciated.</p> | 5,229 |
<p>This is a question about evaporative cooling as used in residential evaporative cooling appliances. This type of cooling uses the heat in the ambient outside air to evaporate water and remove the heat from the air, then push the cooled air inside. The equation to predict the temperature of the resulting air after it... | 5,230 |
<p>The FRW metric is given by:
$$ds^2=-dt^2+a^2(t)\ dr^2$$
where $ds$ is an interval of proper length, $dt$ is an interval of cosmic time, $dr$ is an interval of co-moving co-ordinate distance and $a(t)$ is the scale factor (also $c=1$).</p>
<p>If I take $dt=0$ then I find that an interval of proper distance $ds$ is g... | 5,231 |
<p>I'm using a book from Griffiths, I got really stuck about how he arrived at the approximate solution, is it just by trying( trial solution method?), I really appreciate any help on this.</p>
<p>$$-\frac{\hbar^2}{2m}\frac{\mathrm{d}^2\psi}{\mathrm{d}x^2} + \frac{1}{2}m\omega^2 x^2\psi~=~E\psi.$$</p>
<p>Change varia... | 5,232 |
<p>Almost in every textbook of condensed matter physics, the standard description of SSB could be formulated as follows: </p>
<p>Consider the lattice Heisenberg model in an external magnetic field $H=\sum_{ij}J_{ij}\mathbf{S}_i\cdot\mathbf{S}_j+hS_z$, where $h$ is the magnitude of magnetic field and $S_z=\sum_iS_i^z$.... | 5,233 |
<p>I've been wondering about this question for a while. If you have alpha and beta particles released from a radioactive core, how do they ionise surrounding particles?</p> | 5,234 |
<p>In several papers (including a recent one by Banks and Seiberg) people mention a "folk-theorem" about the impossibility to have global symmetries in a consistent theory of quantum gravity. I remember having heard one particular argument that seemed quite reasonable (and almost obvious), but I can't remember it.</p>
... | 5,235 |
<p>Let's say I have internal pipe flow with a constant wall temperature. If I evaluate the properties at either the mean inlet temperature of the average of the inlet and outlet, do I always have to account for property variation due to the wall temperature? For example, I would calculate the Nusselt number using a pre... | 5,236 |
<p>What are direction ratios and how are they useful in studying vectors?</p>
<p>How are these different from direction cosines?</p> | 182 |
<p>In the calculation of the Chern number within a 2D lattice model, let's take the <a href="http://prl.aps.org/abstract/PRL/v61/i18/p2015_1" rel="nofollow">Haldane model</a> for example, the Chern number$=\pm1$ has 2 contributions coming from 2 Dirac points described by
$$h_1(\mathbf{q})=q_y\sigma_x-q_x\sigma_y-\sigm... | 5,237 |
<p>Let's suppose we have a particular light frequency emitter and relative sensor array, and that there is no external source of this light.</p>
<p>This emitter has a know angle respect the receiver, and emit a point of line. Because the angle is known, we can determine the distance just by looking at which point in t... | 5,238 |
<p>In the early days of quantum electrodynamics, the most popular gauge chosen was the <a href="http://en.wikipedia.org/wiki/Gupta%E2%80%93Bleuler_formalism" rel="nofollow">Gupta-Bleuler gauge</a> stating that for physical states,
$$\langle \chi | \partial^\mu A_\mu | \psi \rangle = 0.$$</p>
<p>However, this gauge is ... | 5,239 |
<p>I know that it must have something to do with (gauge?) theories with N=2 supersymmetry, BPS states and even black holes, but most papers on the subject are too technical for me. So what is wall crossing?</p> | 5,240 |
<p><img src="http://i.stack.imgur.com/3v6mf.png" alt="enter image description here"></p>
<p>Sorry for using this image, but I thought this was the most convenient way of asking this question. Please zoom in.</p>
<p>I do not understand from the line, "Now, in the body frame $T = (T_{x'}, T_{y'}, T_{z'})\ldots$"</p>
<... | 814 |
<p>The difference between pure and mixed states is the difference in their density matrix structure.</p>
<p>For density matrix $\rho$ of mixed state the trace of $\rho^{2}$ should be less than 1. For pure state corresponding trace $Tr(\rho^{2}) = 1$.</p>
<p>But when I tried to check the Bell two-qubit state, i got:
... | 5,241 |
<p>Let us suppose I am running on a street. When my eyes are open, I can see many things moving backward, and thus it gives me an idea that I am moving wrt those things. Not even this, even if I close my eyes during the run, I can really <em>feel</em> that I am moving. </p>
<p>Now assume that I reach a world where the... | 5,242 |
<p>I can't find any law that states this (maybe the combined gas law does and I'm misinterpreting it?), but Feynman said that if you compress a gas, the temperature increases. This makes sense, for example, a diesel engine (or gas engine with insufficient octane or too high a compression ratio). Also, must thinking abo... | 5,243 |
<p>Neutrons have no charge so they would not, I think, interact with photons. Would a neutron star be transparent?</p> | 5,244 |
<p>I'm in doubt in the application of Gauss' Law to find electric fields when the charge distribution is symmetric. Well, first of all: I know how to find the magnitude of the field - we just enclose the charge distribution with a gaussian surface on which the electric field will not change it's magnitude, and then usi... | 5,245 |
<p>So I know that magnetism can be obtained from the combination of electric fields and special relativity.
I am familiar with the way one can derive the magnetic field of a current carrying wire felt by a moving charge from length contraction.</p>
<p>Now: we all know that if a magnet falls through a coil, the magneti... | 5,246 |
<p>I know when we speak to the microphone, the pitch of our voice cause the vibration of magnet in the microphone, thus causing generation of different voltages of electrical signal.</p>
<p>But my question is: when we speak, our voice not only contains the pitchs, but also the content of our words. How are the words (... | 5,247 |
<p>SO I have a hamiltonian:
$$H=\alpha J_1\cdot J_2$$
And I am asked to find the eigenstates and eigenvalues of this Hamiltonian in terms of products of the eigenstates of the z components of the individual spins.</p>
<p>The wording of this question is kind of confusing me. I could solve it by rewriting it in terms o... | 5,248 |
<p>Even though a viscous fluid has a translation symmetry (invariance) for its Lagrangian , it still 'waste' Linear momentum. How come ?, isn't the rule that every symmetry yields a conservation law ? </p> | 5,249 |
<p>I was reading up on the history of $W/Z$ bosons today and I got a little puzzled. I always assumed that people measured $M_Z$ and $M_W$ and then derived the Weinberg angle. But it appears that they knew what the Weinberg angle was by the late 70's (but not with a lot of precision). How do you measure the Weinberg an... | 5,250 |
<p>As small as it may be, does every 'thing' have a gravitational pull? That is, something with mass or energy. No matter how obsolete or negligible it may be, is it there? If so, how is it calculated? What does 'it' affect?</p> | 5,251 |
<p>I'm musing about how to give students an intuitive feeling about density by letting them lift a same sized volume of different materials, e.g. 1 liter of water, a 10x10x10 cm cube of iron, lead etc. So far, the densest material accessible and affordable to a teacher would probably be mercury (I certainly remember my... | 5,252 |
<p>How do you actually define an <a href="http://en.wikipedia.org/wiki/Orbit" rel="nofollow">orbit</a>?</p>
<p>I believe, Newtonian Mechanics describes an orbit as one object in free fall around another where projectile paths become elliptical. I think, Einstein describes an orbit as an object taking the shortest dist... | 5,253 |
<p>I understand that it has to do with acceleration. Say a pilot does a quick maneuver and experiences a force of 5g. What exactly is happening here? </p>
<p>And what is this force relative to? </p>
<p>If someone can show an example with some calculations that would be really helpful. </p>
<p>Thank you </p> | 5,254 |
<p>We want to solve Legendre's equation:</p>
<p>$$
\frac{d}{dx}[(1-x^2)\frac{d}{dx}P(x)]+l(l+1)P(x)=0,\quad (1)
$$</p>
<p>Jackson writes $P(x)=x^\alpha\sum_{j=0}^\infty a_j x^j$, puts this in eqn. 1 and then comes up with:</p>
<p>$$
\sum_{j=0}^\infty\{(\alpha+j)(\alpha+j-1)a_j x^{\alpha+j-2}-[(\alpha+j)(\alpha... | 5,255 |
<p>I am to find an equation for the time it takes when one falls through a planet to the other side and returns to the starting point. I have seven different sets of values - mass of object falling, mass of planet, radius of the planet, and time. I'm not including friction in the calculations.</p>
<p>I think this qual... | 5,256 |
<p>Is the solar energy coming from the sun infinite and will continue to be radiated to our earth forever? (discarding any outer factors) what's the sun's fuel?</p> | 5,257 |
<p><img src="http://i.stack.imgur.com/lRJAa.jpg" alt="enter image description here"></p>
<blockquote>
<p>Two wires of diameter 0.25 cm, one made of steel and the other made of brass are loaded as shown in fig. The unloaded length of steel wire is 1.5 m and that of brass wire is 1.0 m. Compute the elongation of the s... | 5,258 |
<p>At what angle the projectile should throw with initial velocity v in order to reach distance d? discard the air resistance, only gravitation acts. So far I got the equations for horizontal and vertical velocity. Can someone point me in right direction?</p> | 5,259 |
<p>I was wondering this: suppose you have two oxygen atoms. They will both have 8 protons and 8 neutrons in the nucleus (at least if they are the most common isotope). Now, will all those particles be arranged in the same way in both atoms? If they are, why would that be, and if not, does that affect the element's prop... | 181 |
<p>It is my understanding that when moving near the speed of light, time slows down relative to other things not moving so fast. Based on this principle, would it be theoretically possible to travel a thousand light years in a year, with the thousand years only having passed on the place you're moving relative to? </p>... | 5,260 |
<p>It is fine to say that for an object flying past a massive object, the spacetime is curved by the massive object, and so the object flying past follows the curved path of the geodesic, so it "appears" to be experiencing gravitational acceleration. Do we also say along with it, that the object flying past in reality ... | 5,261 |
<p>Looking at the decay chain, I saw it undergoes double beta decay. How is it feasible for something to undergo a simultaneous double decay?</p> | 5,262 |
<p>I'm looking for analytically solvable models in statistical mechanics (classical or quantum) or related areas such as solid state physics in which
the beta function for a certain renormalization procedure (preferably but not necessarily real space normalization) is exactly known. </p>
<p>I'm looking forward to you... | 5,263 |
<p>How to systematically show that the resulting charges in <a href="http://en.wikipedia.org/wiki/Oil_drop_experiment" rel="nofollow">oil drop experiment</a> are integers multiplied by $e$ in other word how to extract $e$ from the data?</p> | 5,264 |
<p>Can someone explain how <a href="http://en.wikipedia.org/wiki/Isospin" rel="nofollow">isospin</a> and <a href="http://en.wikipedia.org/wiki/Hypercharge" rel="nofollow">hypercharge</a> can be used to label representations? What is the meaning of the term singlet, doublet etc in this context? In particular how can I u... | 5,265 |
<p>In a $N$ dimensional phase space if I have $M$ 1st class and $S$ 2nd class constraints, then I have $N-2M-S$ degrees of freedom in phase space. How can I calculate the degrees of freedom in configuration space?</p> | 5,266 |
<p>In Prof. Xiaogang Wen's theory, photons and electrons are described as quasi-particles appeared as a result of the existence of the string-net liquid, which is the topological order of the qubits that form the space. This is a little confusing to me. Consider when we talk about other quasi-particles, for example, ph... | 5,267 |
<p>Quantum optics all discuss the quantization of free electromagnetic radiation. The result is well established.</p>
<p>But what about an arbitrary electromagnetic field? For example, the simplest case, electric field generated by a point charge. How is that field quantized as an operator?</p> | 5,268 |
<p>I have a question that has been annoying me for a while. Going across many textbooks on quantum mechanics, looking at the postulates they list, we find that the number of postulates vary from one text to another. That means either some the postulates listed in some books are either not sufficient or are redundant.</... | 5,269 |
<p>I got the impression that a regular iPhone charger can charge the iPhone and the iPhone won't become too hot while charging, and the charging time is standard, but if using the 10W iPad charger to charge the iPhone, then it is 2A and it can make the iPhone hotter while charging and the charging time will be less?</p... | 5,270 |
<p>I know that heat flow from higher temperature to lower temperature, but theoretically, is it possible to build heat pump that can move energy from ocean which are at ~295 kelvin to a small boiler which could be at ~500 kelvin. </p>
<p>I am asking out of curiosity that If its at least theoretically possible or not ?... | 5,271 |
<p>Why is it when we look up into the night sky we can see stars. but when you see pictures taken from the ISS you don't see any stars. Why is this?</p> | 5,272 |
<p>I am not talking about any other attributes of particles, vacuum etc ruling out <em>Uncertainty Principle</em> thing. If talking about pure Space (which is continuous, not discrete, cf. e.g. <a href="http://physics.stackexchange.com/q/9720/2451">this</a> Phys.SE post), why is Planck Length lower measurement limit?</... | 5,273 |
<p>I am not so much familiar with the computations tools of conformal field theory, and I just run into an exercise asking to demonstrate the following formula (related to the bosonic field case):</p>
<p>$$\cal{R}j(z_1)j(z_2)~=~\frac{1}{(z_1-z_2)^2}~+~:j(z_1)j(z_2):$$ </p>
<p>with $j$ defined as </p>
<p>$$j(z)~=~\su... | 5,274 |
<p>Suppose that I have a li-ion battery with voltage 10V (and some capacity, say 1000mAh).</p>
<p>Can I charge it completely using 5V voltage?</p>
<p>What will hapen if I charge it with 12V voltage?</p>
<p>Edit:
Found the <a href="http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries" rel="nofoll... | 5,275 |
<p>So, when biking, I noticed that when going up hills, it was less tiring if I went up them more quickly. This is not total Work done as is Force * Distance, as that should be the same.</p>
<p>But the longer one is going uphill, the longer gravity is pulling you backwards. And if you only are providing enough force... | 5,276 |
<p>We all know that predicting tsunami and earthquake is difficult, with too many variables involved.</p>
<p>But with the advent in data collection and computing power and better models, one should be able to predict tsunami better than in the past. How accurate is current tsunami prediction?</p> | 5,277 |
<p>Why do optical fibers usually have a cladding? Ok, if you put make a bundle of optical fibers this prevents that light leaks from one fiber to another fiber in contact. However, are there other reasons to use claddings? Are there applications of optical fibers without claddings?</p>
<p>(I am mainly interested in op... | 5,278 |
<p>I was surprised to read that we don't know how to analyze turbulent fluids. On page 3-9 of <em>The Feynman Lectures on Physics (Volume One)</em>, Feynman writes:</p>
<blockquote>
<p>Finally, there is a physical problem that is common to many fields, that is very old, and that has not been solved. [..] Nobody in ... | 5,279 |
<p>Is the entropy of every system zero at the absolute zero?</p>
<p>Or is it taken to be zero at the absolute zero?</p>
<p>Are there systems that doesn't reach zero entropy even till absolute zero?</p> | 5,280 |
<p>I'm confused about how to normalize the Hartle-Hawking state in 2D quantum gravity. We can compute the HH state for two circles of length $\ell_1$ and $\ell_2$ in the matrix model as $\langle W(\ell_1)W(\ell_2)\rangle$, using the notation of Ginsparg and Moore, <a href="http://arxiv.org/pdf/hep-th/9304011v1.pdf" rel... | 5,281 |
<p>In quantum mechanics, is there an upper bound for the <a href="http://en.wikipedia.org/wiki/Uncertainty_principle" rel="nofollow">uncertainty principle</a>? I know that quantum harmonic oscillator (QHO) has the uncertainty relation $\sigma_x\sigma_p = \hbar(n+1/2)$, but I think the QHO becomes localized at two peaks... | 5,282 |
<p>To be efficient, a phase-matching condition has to be fulfilled in many nonlinear optical processes. For instance, the phase-matching requirement for second-harmonic generation is</p>
<p>$k_{2\omega}=2k_{\omega}$ or $\Delta k = k_{2\omega}-2k_{\omega}=0$</p>
<p>It is often said that this is equivalent to momentum ... | 5,283 |
<p>Im trying to show that the integral over a closed loop of a crossproduct stays the same if I choose a different origin with $\overrightarrow{r}=\overrightarrow{r}\prime+\overrightarrow{r_0}$ and $\oint{d\overrightarrow{F}}=0$</p>
<p>$$ \overrightarrow{N} = \oint{\overrightarrow{r} \times d\overrightarrow{F}} = \oin... | 5,284 |
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