question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>To make a superfluid rotate in an annulus shaped container, we start with a normal fluid, rotate the container, then cool it to below critical temperature to get a rotating superfluid. </p>
<p>The allowed values of circulation in a superfluid rotating with non-zero velocity are n*h/m where n could be 1,2,3.. or -1,... | 3,364 |
<p>Is it really possible in the foreseeable future to create a gamma ray laser? I've read these two articles on Wikipedia:</p>
<p><a href="http://en.wikipedia.org/wiki/Hafnium_controversy" rel="nofollow">the Hafnium controversy</a></p>
<p><a href="http://en.wikipedia.org/wiki/Induced_gamma_emission" rel="nofollow">In... | 3,365 |
<p>After reading an recent news "<a href="http://www.dailymail.co.uk/sciencetech/article-2300651/Stargazers-capture-picture-planet-suns--just-like-Luke-Skywalker-s-home-planet-Tatooine-Star-Wars.html" rel="nofollow">Stargazers capture first picture of a planet with two suns – just like Luke Skywalker’s home planet of T... | 3,366 |
<p>What will be the consequence (severe ones) on laws of physics if a particle that travels <a href="http://en.wikipedia.org/wiki/Faster-than-light">faster than light</a> is discovered?</p>
<p>I am looking for a more general answer so that a high school student would be able to understand. Or is it not possible to exp... | 3,367 |
<p>Suppose engineers built a large circular room in a rotating space station where if one looked directly up from any location, one could see the floor.</p>
<p>If one used a ladder to reach the center of the room, could they balance an object in the center of the room's rotation, such that the object floated unsupport... | 3,368 |
<p>If a laser beam is looked at from the side versus a dark background, a sparkling effect can be seen caused by dust particles in the air hit by the beam. </p>
<p>Is there any simple model or coarse estimations how often that would happen, how bright the particles flash and how long a single flash will be?</p> | 3,369 |
<p><em>Let $x^\mu$ be the coordinates of a reference frame, $K$, where all bodies feel the same constant and uniform acceleration $\textbf{a}=\textbf{g}=-\nabla\varphi$; let $\xi^\mu$ be the coordinates of a Locally Inertial Frame, $LIF$. Using the <strong>Principle of Equivalence</strong>, show that the linear part in... | 3,370 |
<p>A Starship is going to accelerate from 0 to some final four-velocity, but it cannot accelerate faster than $g_M$, otherwise it will crush the astronauts.</p>
<p>what is the appropiate equation to constraint the movement so the astronauts never feel a gravity higher than $g_M$? for a moment i thought the appropiate ... | 3,371 |
<p>In talking about production/decay processes, I've heard people speaking of decay modes or cross sections being 'phase space' suppressed. For example, a two body final state is more likely to occur than a three body final state, since in the later case the three particles must share the initial energy momentum where... | 3,372 |
<p>Here is a <a href="https://www.youtube.com/watch?v=YnuDx8RsraU" rel="nofollow">Video</a> showing the honey accelerating in the hot water. As you can see, there are also dynamics. Since the water started stationary, I guess the dynamics arise because of convection flow. What is the explanation for the fast accelerati... | 3,373 |
<p>What i really want to ask how much has the Milky Way moved, relative to where it was "at the big bang" or the soonest time that makes sense (since i doubt "at the big bang" makes much sense in this question). I suppose the galaxies have non-zero impulse, otherwise we wouldn't see things like galaxy collisions. So, r... | 3,374 |
<p>I asked this question because I supposedly did last year, Stanfor Klein which belongs to the Solar Energy Laboratory of the University of Wisconsin says that "the color of a car does not affect its internal temperature".</p>
<p>I wonder why Metals with different colors perhaps do not absorb different doses of tempe... | 3,375 |
<p>Wikipedia says, </p>
<blockquote>
<p>A <a href="http://en.wikipedia.org/wiki/Black_hole" rel="nofollow">black hole</a> grows by absorbing everything nearby, during its life-cycle. By absorbing other stars, objects, and by merging with other black-holes, they could form <a href="http://en.wikipedia.org/wiki/Superm... | 3,376 |
<p>We know that space cannot spread a sound wave as there is no "air" or a medium that would support the spread of a sound wave. However if we put ourselves in the vicinity of an exploding star, would it be possible to hear something?</p>
<p>The question arises from the idea that within the explosion of a star (first ... | 3,377 |
<p>It´s usual to read in QFT books of how it is "easier" to have a canonically normalized kinetic term. So, for instance:</p>
<p>$${\cal L} = {1 \over 2 }\partial_{\mu} \phi \partial^{\mu}\phi - {1 \over 2 } m^2 \phi \phi - {\lambda \over 4!} \phi^4$$</p>
<p>is canon. And:</p>
<p>$${\cal L}_2 = \partial_{\mu} \ph... | 3,378 |
<h2>Residual Resistivity</h2>
<hr>
<p>I saw that the graph of resistivity to temperature of alloys like nichrome is like so<img src="http://i.stack.imgur.com/KkiDU.png" alt="enter image description here"></p>
<p>Meaning that even at 0 K it has some resistivity just like copper :</p>
<p><img src="http://i.stack.imgu... | 3,379 |
<h2>Superconductivity</h2>
<p>I read in a book "Physics - Resnik and Halliday" the explanation of Type-I Superconductors{cold ones} that:</p>
<blockquote>
<p>The Electrons that make up current at super-cool temperatures move in coordinated pairs.One of the electrons in a pair may electrically distort the molecular ... | 1,018 |
<p>Can we transfer energy from one place to another separated by arbitrarily large distances without any time lag?</p>
<p>For instance, if Alice and Bob are two observers making measurements having a singlet state, they synchronize their clocks and then go to sufficiently large distances both of them apply a magnetic ... | 3,380 |
<p>Consider the following scenario:</p>
<ul>
<li><p>I get in a spaceship, and travel really close to the speed of light for a while, and then come back.</p></li>
<li><p>A lot of time has passed on the Earth, but since I was traveling so fast, I only experienced a few years passing.</p></li>
<li><p>So, my friends on Ea... | 84 |
<p>I remember once, as a child, thinking that objects do not really "move," but that at a very small scale they would have to "disappear" and then "appear" again at their newly shifted position, just the way computers render moving particles based on refresh rates. This relates to <a href="http://en.wikipedia.org/wiki/... | 3,381 |
<p>Density of ice is much higher than air. Then how can hail stone remain in earth atmosphere before they fall down ? </p> | 3,382 |
<p>Is <a href="http://en.wikipedia.org/wiki/Wormhole" rel="nofollow">wormhole</a> a practical concept? If not what did the scientists do, to theorize it? Does it have any limitations pertaining to speed of light?</p> | 3,383 |
<p>I know that materials behave differently in the nanoscale. One of the reasons that I have heard as to why this happens is due to the increased surface area of the atoms. But, are there other factors? Are the potentials that act on particles different? Do other effects emerge/become insignificant that cause the diffe... | 3,384 |
<p>Inspired by the thunderstorm overhead, and after reading the question and answers <a href="http://physics.stackexchange.com/questions/28560/voltage-and-current-of-positive-lightning">Voltage and current of positive lightning</a> - what is an effective means to explain this phenomenon to a layman?</p>
<p>Also, as de... | 3,385 |
<p>I want to know why some semiconductors band gap decreases after doping with elements. <strong><em>Burstein-Moss band-filling effect</em></strong> can be useful to explain band gap widing in a semiconductor materials but i was unable to find any logical explanation for band narrowing effect. Can you please explain th... | 3,386 |
<p>I ask this question, because at the end of this long day I'm just too dazed to derive the proofs myself (even though I know that I should feel ashamed for this). </p>
<p>So, the question: </p>
<p>Given two <a href="http://en.wikipedia.org/wiki/Pendulum_%28mathematics%29#Simple_gravity_pendulum" rel="nofollow">sim... | 3,387 |
<p>I was wondering what is the group theoretic way to find the resulting charges of matter fields after a scalar field is given a vev.</p>
<p>In the case of the EW symmetry breaking, one can directly read the charges from the Lagrangian by setting the Higgs field $H=v+h'$ and going in the unitary gauge.</p>
<p>Given... | 3,388 |
<p>Among the other ways, one way to prove the Earth is round is the <strong>triple-right triangle</strong>.</p>
<p>The idea is simple: </p>
<ol>
<li>Starting from point A you move in a straight line for a certain distance.</li>
<li>At point B, turn right 90° degrees, move along the line for the same distance.</li>
<l... | 3,389 |
<p>When two balls of different masses, thrown from equal height they reaches the ground at the same time. Can anyone explain this in terms of laws of Physics(or with mathematical equations)?</p> | 85 |
<p>What is the output of a CNOT gate if both inputs are in superposition?</p>
<p><img src="http://i.stack.imgur.com/GF1HJ.png" alt="CNOT gate"></p>
<p>For example, what happens if:
$\left|x\right>=\alpha_x\left|0\right>+\beta_x\left|1\right>$
and
$\left|y\right>=\alpha_y\left|0\right>+\beta_y\left|1\ri... | 3,390 |
<p>I'm a high school student. I'll start self learning of physics and maths. I want to be theoretical physicist. I'm especially interested in gravitational physics. I wonder which website advice below is better to follow? Why? </p>
<p><a href="http://www.staff.science.uu.nl/~Gadda001/goodtheorist/index.html" rel="nofo... | 86 |
<p>I can't really imagine the way the magnetic field would be created due to electron flow in an inductor. We say that in a straight current carrying conductor the magnetic field follows the thumb rule. So according to that shouldn't the magnetic field get nullified inside the coil as<br>
every loop will of the coil wi... | 3,391 |
<p>I tried to look up some facts regarding sunspots and its relation between the brightness of the sun, only to find information that are intriguing yet not what I am looking for.</p>
<p>My understanding is that sunspots are darker compared to the brightness around it, so I am thinking ... the more sunspots there are ... | 3,392 |
<p>Here is an example of Cassegrain telescope: Parallel rays from a distant object get reflected by the concave mirror forming an image at its focus behind the convex mirror. This image acts as a virtual object for the convex mirror, and it forms a real image in front. Let's apply mirror equation for this convex mirror... | 3,393 |
<p>I wanna become a scientist, so wanna improve my skill like problem solving etc. Could you suggest me some more skills which I need to improve.</p> | 40 |
<p>I am not sure if I understand how forces work during hammerthrow. From my understanding athlete works with Fmuscle force on a ball and rotate it and a ball works with equal opposite force on an athlete causing rotation in a opposite direction. The mass of athlete is greater than the mass of ball so athlete rotates w... | 3,394 |
<p>Let's imagine a boat on a lake. Observer A is sitting on the shore. Observer B is sitting in the boat on the bow. Observer B has a ball attached to the end of a string which he holds in his hand.</p>
<p>Observer A sees that the boat is not moving. Boat B has some mass. So does the ball. Let's ignore water/air frict... | 3,395 |
<p>As in most tight-binding atomic model, the Hamiltonian $H$ is sparse and in my problem, it is a <a href="http://en.wikipedia.org/wiki/Banded_matrix" rel="nofollow">band matrix</a>.</p>
<p>How to compute the partition function $Z = Tr\left( {{e^{ - \beta \hat H}}} \right)$ efficiently since $H$ is so large.</p>
<p>... | 3,396 |
<p>When a muon decays from rest, typically what fraction of the energy is carried off by the electron? I tried looking into some papers, but I wasn't sure how to interpret the graphs they displayed. </p> | 3,397 |
<p>I was reading a paper that described how the force a low-thrust torsion pendulum was measured. In it, the paper states a laser is bounced off a mirror and the displacement is "...based upon the beam reflection time." The paper states that the device can measure sub-micrometer displacements.</p>
<p>Conceptually as I... | 3,398 |
<p>I once read a paper, in which:</p>
<ul>
<li>a fluid in a container was heated from below,</li>
<li>after reaching temperature $T_1$, a circular motion (convection) was clearly distinguishable, in form of <strong>cylinder</strong>,</li>
<li>after reaching temperature $T_2$, the circular motion splitted into <strong>... | 3,399 |
<p>I'm looking for an introduction to the treatment of <a href="http://en.wikipedia.org/wiki/Piezoelectricity" rel="nofollow">piezoelectricity</a>, specially in semiconductors emphasizing the dependence on the orientation of the unit cell and the interactive effects with the electron-holes pairs.</p> | 3,400 |
<p>What is actually a resonating vibration and <a href="http://en.wikipedia.org/wiki/Resonance" rel="nofollow">resonance</a>?</p>
<p>I have searched many books and made Google search too but couldn't understand it clearly.</p> | 3,401 |
<p>I have tried a google search and checked my condensed matter books but I can't find out what pulsed neutron diffraction is and how it differs from inelastic neutron scattering.</p> | 3,402 |
<p><a href="https://en.wikipedia.org/wiki/Ionocraft" rel="nofollow">Ionocraft</a> or "lifters" are lightweight devices that produce thrust by ionizing the air around an electrode, and then accelerating the ions toward another electrode with an electric field, during which the ions push against neutral air molecules and... | 3,403 |
<p>In high pressure physics what is the difference between <a href="http://en.wikipedia.org/wiki/Diamond_anvil_cell" rel="nofollow">diamond anvil cells</a> (DAC) and Bridgman cells? My understanding is that they are both forms of anvil cell but in the scientific literature I'm reading they are referred to separately.</... | 3,404 |
<p>Just from reading <a href="http://rads.stackoverflow.com/amzn/click/0071636412" rel="nofollow">this</a> slightly technical introduction to supersymmetry and watching <a href="http://itunes.apple.com/itunes-u/supersymmetry-grand-unification/id384233338#ls=1" rel="nofollow">these</a> Lenny Susskind lectures, I thought... | 3,405 |
<p>When experiencing alpha decay, atoms shed alpha particles made of 2 protons and 2 neutrons. Why can't we have other types of particles made of more or less protons?</p> | 3,406 |
<p>An electric field in a conductor causes charges to redistribute so as to cancel out the original field, bringing the field to zero. This is, I think, a common argument for why conductors are generally opaque to EM waves.</p>
<p>But some conductors are transparent, including various electrolytes and indium tin oxide... | 3,407 |
<p>I was going through Mark Srednicki's book on QFT. It says in the relativistic limit the Schrodinger equation becomes something like :</p>
<p>$$ i\hbar\frac{\partial}{\partial t} \psi(\vec x,t) = \sqrt{-\hbar^2c^2\nabla^2+m^2c^4}\psi(\vec x,t) $$
Now he says that if I expand the square root (say binomially) it will... | 3,408 |
<p>Apart from other reasons, recently my interest in this area got piqued when I heard an awesome lecture by Seiberg on the idea of meta-stable-supersymmetry-breaking. </p>
<p>I am looking for references on learning about phase transitions/critical phenomenon in supersymmetric field theory - may be especially in the c... | 3,409 |
<p>I am working on an model of a permanent magnet synchronous machine. Right now I am stuck with calculating the eddy current losses caused by the harmonics of the stator magnetic field in the electrical steel of the rotor. Or to put it differently. How do I calculate the eddy current in electric steel at high frequenc... | 3,410 |
<p>Is it only the spin of a particle that can be entangled with another particles spin?
Also is there any good physical interpretation of the spin of a particle? because the rotational invariance of entaglement almost literally blows my mind.</p> | 3,411 |
<p>In their celebrated work, Capelli Itzykson and Zuber established an
<a href="http://arxiv.org/abs/0911.3242"><i>ADE</i>-classification of modular invariant <i>CFT</i>s</a> with chiral algebra $\mathfrak{su}(2)_k$.</p>
<blockquote>
<p>How much of that classification can one see using the tools of perturbative qua... | 3,412 |
<p>I'm just trying to understand this problem from a qualitative perspective. The Doppler effect is commonly explained in terms of how a siren sounds higher in pitch as it is approaching a particular observer. I understand this is because the velocity of the wave is constant and so the frequency of the waves increase ... | 3,413 |
<p>I'm looking for texts about topics in string theory that are "advanced" in the sense that they go beyond perturbative string theory. Specifically I'm interested in</p>
<ol>
<li>String field theory (including superstrings and closed strings)</li>
<li>D-branes and other branes (like the NS5)</li>
<li>Dualities</li>
<... | 3,414 |
<p>When considering observables and their corresponding operators, would it be correct to believe that discerning discrete values for an observable is possible ONLY when $\psi$ is an eigenfunction of the operator? Alternatively, would it also be correct to believe that the average value of an observable is ALWAYS obtai... | 3,415 |
<p>I am currently facing the problem of calculating integrals that take the general form</p>
<p>$\int_{R} P(\sigma)d\sigma$</p>
<p>where $P(\sigma)$ is a probability density over the space of mixed quantum states, $d\sigma$ is the Hilbert-Schmidt measure and $R$ is <em>some subregion</em> of state space, which in ge... | 3,416 |
<p>Right now, my understanding is that, a mixture of photons of many different frequencies is perceived as white by your eye. While no photons at all, is perceived as black. And photons with the blue frequency only cause you to see blue, etc.</p>
<p>My question is, how is the "brightness" controlled? I think it has to... | 3,417 |
<p>Graphene has two atoms in its primitive unit cell. This makes it intuitive to see that the tight binding Hamiltonian can be constructed as a $ 2 \times 2 $ matrix $H$ acting on a spinor $S$ that consists of the wavefunction from an atom in sublattice A and B.</p>
<p>$H_{monolayer}=\gamma \cdot \begin{pmatrix}
0 &am... | 3,418 |
<p>In <a href="https://archive.org/details/IntroductionToQuantumMechanics_718" rel="nofollow">Griffiths</a>' book page 53, when we derive the solution of the quantum harmonic oscillator by using the power series way, we have: $$a_{j+2} = \frac{2j+1-K}{(j+1)(j+2)}\, a_{j} .$$ And for large $j$, we have: $$a_{j+2}\approx... | 3,419 |
<p>I'm going mad about the problem.</p>
<p>I really don't understand why do electron have 1/2 spin number, why they are not actually spinning.</p>
<p>I can accept that the electrons have their own magnetic field, which is certain, but why do they have $\hbar\sqrt3/2$ of angular momentum, and I don't know what the hec... | 3,420 |
<p><a href="http://physics.stackexchange.com/questions/87123/equation-for-relativistic-electron-and-two-component-spinor">Here</a> I asked about getting an equation for two-component spinor as the alternative for Dirac equation. It was found that it is called Majorana equation. It may be easily derived by using histori... | 3,421 |
<p>I'm not quite sure this question fits the format of this site but I try to word it the best I can to comply the rules.</p>
<p>The question is simple: How far can we go talking about the origin of the universe before admitting that the initial conditions cannot be explained without postulating some kind of god-like,... | 51 |
<p>How does this number get calculated?</p>
<blockquote>
<p>About 380,000 years after the Big Bang the temperature of the universe fell to the point where nuclei could combine with electrons to create neutral atoms.</p>
</blockquote>
<p><a href="http://en.wikipedia.org/wiki/Photon_epoch">http://en.wikipedia.org/wik... | 3,422 |
<p>I am just starting to dig a little deeper into particle interactions, and just have an introductory college physics background (no quantum mechanics). But I am interested in the conditions of the early universe and am looking for a list of all the particles in the standard model, and the relationships between the pa... | 3,423 |
<p>When talking about the wave-particle duality, teachers and books say that when you send a single photon through a slit, it makes a wave pattern. But if you send that particle through the slit and "you observe it directly", then it appears as a single point (a particle).</p>
<p>What is meant by "observe"? Is that li... | 3,424 |
<p>I am quite puzzled with the problem that spectral analysis has been either applied to noisy dynamical systems or to chaotic ones. I was wondering why nobody makes analysis of non-linear dynamical systems based on their autocorrelation? At least for non-linear oscillators which are essentially periodic and seem to su... | 3,425 |
<p>Does anybody know the status of the problem to define the wave function (non-relativistic Quantum Mechanics) of a particle localized at a definite point? </p>
<p>Landau-Lifshitz says in chapter 1 that this function is $\Psi(x)_{x_o} = \delta(x-x_0)$ and gives an explanation that it produces the correct probability ... | 3,426 |
<p>I was just wondering why Fresnel Lenses are not widely used in the production of solar electricity. Their use there would mean that you could produce heat within a fraction of a second, up to a few minutes and run a turbine to produce electricity.</p>
<p>Though it is used in welding, I am not sure what are the prob... | 3,427 |
<p>I know that when electrons encounter photons, they become excited and move to an orbit farther away from the nucleus of an atom as a result. What I want to know is exactly why the photons <em>cause</em> the electrons to enter this state.</p>
<p>Edit: Sorry, I wasn't being very clear. What I mean is, why do photon... | 3,428 |
<p>Have you ever noticed that when you are filling a container with fluid. As it approaches the top, it makes a different sound? You can tell by listening when your about to reach the top. Why is this?</p> | 3,429 |
<p>I was wondering whether there exists a theory that describes Mie Scattering for spheres that have a constant dipole moment. Since there are theories that describe Mie scattering in the case of a charge, this might exist too?</p> | 3,430 |
<p>Just starting to learn physics. Now reading about constant motion and after this paragraph
I have a task:</p>
<p>Boy with growth of 1.5 m runs at a speed of 3 m / s in a straight line, passing under a street lamp, hanging at a height of 3 m. Show that the shadow of his head moves uniformly, and find the speed of th... | 3,431 |
<p>The proof of the No-Cloning Theorem states "By the linearity of quantum mechanics, ..." -- Could someone please give me a rough sketch/outline of what this means. Does it have to do with the Hilbert Space that wave functions live in? </p>
<p>I apologize if this question isn't specific enough, I just wanted to fully... | 3,432 |
<p>Consider a container with some fluid of density $\rho_l$ and volume $V_l$. This is kept on a measuring device and has weight $\rho_l V_lg$. Now, consider a block of density $\rho_b$ and volume $V_b$. This block is put into the fluid and here, its apparent weight equals $\rho_b V_bg - F_b$ , where $F_b$ equals the bu... | 3,433 |
<p>Can anybody cast some physical insight into this? I've been studying differential equations on my own and don't understand how you can have a whole host of general solutions. It seems like a rather curious situation which we don't come across in other areas of mathematics. Is there anything more to the discussion th... | 3,434 |
<p>According to wikipedia, here are the Cygnus X-1 vital stats:</p>
<pre><code>Mass 14-16[7] M☉
Radius 20–22[8] R☉
</code></pre>
<p>A radius of 10 R☉ means a volume of 10^3 = 1000 Sols. Divided by 16 M☉ that means that Cyg X-1 is 60 time <em>less</em> dense than Sol. So how could it be a black hole?</p> | 3,435 |
<p>The equations of motions for a simple pendulum is given by </p>
<p>$$\ddot{\theta} ~=~ -\frac{g}{\ell}\sin(\theta),$$</p>
<p>where $g$ is acceleration due to gravity and $\ell$ is the length of the pendulum's string. Notice that the differential equation is of second order, does this mean that if I solve this equa... | 3,436 |
<p>I had self-studied Griffiths(~ 4 chapters), and Sakurai (~2.5 chapters) for quantum mechanics some months ago. Now, I have to take a course in QM this sem, and I want to further my understanding of basic QM. I am looking for a textbook that is more advanced than the one mentioned above, possibly more mathematically ... | 87 |
<p>How can two seas not mix? I think this is commonly known and the explanation everyone gives is "because they have different densities".</p>
<p><img src="http://i.stack.imgur.com/KpmVf.jpg" alt="enter image description here"></p>
<p>What I get is that they eventually will mix, but this process takes a long time.</p... | 3,437 |
<p>Schwinger has on his grave (it seems) the relation between the g-factor of the electron and the fine structure constant:</p>
<p>$$g~=~2+\frac{\alpha}{\pi}+{\cal O}(\alpha^2)$$</p>
<p>Did Schwinger or somebody else ever give a simple explanation for the second term of the right hand side? The 2 appears from Dirac's... | 3,438 |
<p>I've been thinking about refrigeration technology and am a bit confused about two common answers. Specifically, the part where the expansion valve releases the pressurized fluid and stuff gets real cold.</p>
<p>One is that refrigeration works by lower pressure = lower temperature. This makes sense to me because if ... | 3,439 |
<p>I'm just curious highschooler beginning an interest in electronics and this concept of light detecting resistance component is really intriguing to me. I assume it's an application of the Photoelectric effect but how exactly does a CdS <a href="http://en.wikipedia.org/wiki/Photoresistor" rel="nofollow">photoresistor... | 3,440 |
<p>Imagine driving in a straight line on a ice lake, when you hit the brakes, if your goal is to stay in straight path with no spinout, which wheels would you choose to have locked: front or rear? Assuming the steering wheel is kept fixed in both cases, I learned that it's better to have front wheels locked in this cas... | 3,441 |
<p>I'm confused about the physical interpretation of the four-velocity $U^\mu=\frac{dx^\mu}{d\tau}$ in General Relativity. I know that it is a tangent vector to a particle's "worldline", but what does this mean more physically?</p>
<p>For example, I am comfortable with what $U^\mu$ means in Special Relativity. In your... | 3,442 |
<ol>
<li><p>First of all, why wasn't the sextant ever used for land navigation? The horizon is easier to see at sea, but land based sextants could be used in conjunction with artificial horizons (as at sea when horizon is hidden by fog).</p></li>
<li><p>Parallax has been used by both the US army and navy to measure di... | 3,443 |
<p>The use of renormalisation constants often puzzles me. A good example is the use of $Z_2$ in the equation (7.58) of Peskin Schroeder. $Z_2$ is defined in equation (7.26).
as $Z_2^{-1} = 1-\frac{d\Sigma}{dp}$. Later in equation (7.31) it is said: </p>
<p>$Z_2-1 = d\Sigma/dp$ although this term is supposed to be inf... | 3,444 |
<p>We are trying to emulate <a href="http://www.dailymotion.com/video/xv1j0n_the-secret-life-of-chaos-2010_shortfilms&start=2091" rel="nofollow">the chaotic system Jim Al-Khalili demonstrate</a> (3 min video).</p>
<p>In our chaos lab, we are trying to research the chaotic system shown in the video.
We are using ju... | 3,445 |
<p>Consider a curved space, e.g. Schwarzschild:
\begin{align*}
ds^2 = -\left(1-\frac{2M}{r}\right)dt^2+\left(1-\frac{2M}{r}\right)^{-1}dr^2+r^2d\theta^2+r^2\sin^2\theta d\phi^2
\end{align*}
Now, the energy of a photon is $E = \hbar \omega$, and $|\mathbf{k}|= \frac{2\pi}{\lambda}$, but am I correct in assuming that $\o... | 3,446 |
<p>If you think of a one-parameter group of transformations <a href="http://books.google.ie/books?id=2mc9e04FzMgC&lpg=PA36&ots=Pxfs8GcFvW&dq=bluman%20symmetry%20and%20integration%20%22if,%20in%20addition%20to%20satisfying%20axioms%22&hl=vi&pg=PA37#v=onepage&q=bluman%20symmetry%20and%20integratio... | 3,447 |
<p>we already know that if we plot speed vs time of free falling object it will be
y=gx graph because we know the acceleration is gravity.</p>
<p>If it have air-resistance acceleration will be change right?.</p>
<p>So how we can plot the speed vs time graph of free falling object with air resistance ?</p>
<p>It hav... | 3,448 |
<p>Knowing some about thermodynamics and reactions, I do understand how it can be shown that a change is reversible. But irreversible? Why can't it be that a change that was deemed irreversible thousands of years ago via new changes perhaps developed by physicists or changes, processes and reactions from some other par... | 3,449 |
<p>I believe I am missing something simple here. My question concerns flywheel energy storage.</p>
<p>Say we have stored some amount of energy in a spinning flywheel. The flywheel is attached to a generator. So the flywheel (and therefore the generator) would be spinning rapidly at first, and then gradually slow do... | 3,450 |
<p>Does anyone have any suggestion on a handbook of instrumentation?
In particular <a href="http://en.wikipedia.org/wiki/Nuclear_Instrumentation_Module" rel="nofollow">NIM</a> instrumentation...
I would like an approach which will explain the techniques and how to use NIM modules.</p>
<p>In particluar I would like an ... | 3,451 |
<p>I think this is an interesting question, to which I don't really know the answer to. (Also, not a homework question.)</p>
<p>Say you have an uncharged metal sphere constrained to move in the z-axis. There is a charged ring lying in the x-y plane centered at the origin. Two cases: 1) the diameter of the ring is larg... | 3,452 |
<p>Can someone give a qualitative description on why the density of states for a two dimensional free electron gas is independent of energy while it is not in one and three dimensions? In one dimension it goes as $E^{-1/2}$ and in three $E^{1/2}$.</p> | 3,453 |
<p><a href="http://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_statistics" rel="nofollow">Quoting Wikipedia</a>:</p>
<blockquote>
<p>In <a href="http://en.wikipedia.org/wiki/Statistical_mechanics" rel="nofollow">statistical mechanics</a>, Maxwell–Boltzmann statistics describes the statistical distribution of mat... | 3,454 |
<p>The question follows from <a href="http://en.wikipedia.org/wiki/Xkcd" rel="nofollow">xkcd</a> cartoon <a href="http://www.xkcd.com/941/" rel="nofollow">"Depth Perception (941)"</a>. I've isolated the frames that describe the concept here.</p>
<p><img src="http://i.stack.imgur.com/vNeDp.png" alt="Credit: Randall Mun... | 3,455 |
<p>A cone standing on its tip is considered to be in unstable equilibrium as a slightest force could topple it. So, if the cone is stood on its tip with no other force other than gravity (and the corresponding <a href="http://en.wikipedia.org/wiki/Ground_reaction_force" rel="nofollow">ground reaction force</a>), will i... | 3,456 |
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