question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>A ball rests on a smooth surface. The ball's particles are in constant motion. So are the particles of the floor. Some of the ball's particles collide with the floor's particles and transfer kinetic energy. But overall, the kinetic energy of the floor and the ball is constant. They are at thermal equilibrium. </p>
... | 3,276 |
<p>Do you know the last works about <a href="http://en.wikipedia.org/wiki/Bystander_effect_%28radiobiology%29" rel="nofollow">Bystander Effect</a> on cells caused by radiations? I have found research papers until 2004.</p> | 3,277 |
<p>Equation for an expectation value $\langle x \rangle$ is known to me: </p>
<p>\begin{align}
\langle x \rangle = \int\limits_{-\infty}^{\infty} \overline{\psi}x\psi\, d x
\end{align}</p>
<p>By the definition we say that expectation value is a sandwich: $\langle \psi|\hat{x}|\psi\rangle$. So: </p>
<p>\begin{align}
... | 3,278 |
<p>Sorry if this is the wrong place to post this but it seemed the most appropriate.</p>
<p>I read a riddle earlier today that mentioned something about shadows underwater and it got me thinking.. Would a shadow under water/ in the ocean technically be "wet"?</p>
<p>I understand that light isn't matter, but something... | 3,279 |
<p>I stumbled across this article <a href="http://blogs.scientificamerican.com/cross-check/2010/12/21/science-faction-is-theoretical-physics-becoming-softer-than-anthropology/" rel="nofollow">http://blogs.scientificamerican.com/cross-check/2010/12/21/science-faction-is-theoretical-physics-becoming-softer-than-anthropol... | 3,280 |
<p>Please note, I am only interested in classical mechanics discussion on this. Please do not involve quantum mechanics.</p>
<p>Inspired by this question: <a href="http://physics.stackexchange.com/questions/822/is-angular-momentum-truly-fundamental/4045">Is Angular Momentum truly fundamental?</a></p>
<p>My question ... | 3,281 |
<p>I am trying to reconcile data that I have found in <a href="http://www.xn--rheinischesmuseumfrphilologie-2bd.com/fileadmin/content/dokument/archiv/silvaegenetica/18_1969/18-1-2-40.pdf" rel="nofollow">one publication (Allen 1969)</a> with data that I found in <a href="http://www.jstor.org/stable/1514306" rel="nofollo... | 3,282 |
<p>I have seen in popular media, claims that Hawking does not believe the Higgs boson exists due to microscopic black holes and even made a bet against it. This is based on something published in journal Physical Review D.</p>
<p>I don't have access to journal Physical Review D, and I can't find a clear detailed expl... | 3,283 |
<p>Does salt water conduct mostly by the ions travelling through the solution, or by electrons collectively flowing or hopping through the solution like in metals?</p> | 3,284 |
<p>When I first heard about the <a href="http://en.wikipedia.org/wiki/Black_hole_information_paradox">black hole information paradox</a>, I thought it had no content. At the time, papers about it had been written for numerous years and they keep on coming. Now that the press got wind of Hawking's latest one, I thought ... | 3,285 |
<p><code>Question</code>:</p>
<blockquote>
<p>Prove that $p^2$ and ${\bf r}\cdot {\bf p}$ commute with every component of ${\bf L}$ using the identity
$$[{\bf p},{\bf e}\cdot {\bf L}]=i\hbar\, {\bf e}\times {\bf r} $$
where ${\bf e}$ is a unit vector given by ${\bf e}=a\hat{i}+b\hat{j}+c\hat{k}$. where $\sqrt{a^... | 3,286 |
<p>Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth?</p>
<p>It's not just a subtle difference, but significant in my opinion. I'm inclined to discredit the notion that it's just a matter of speed because I can blow fast with an open... | 3,287 |
<p>The <a href="http://mathworld.wolfram.com/BrachistochroneProblem.html">brachistochrone problem</a> asks what shape a hill should be so a ball slides down in the least time. The <a href="http://mathworld.wolfram.com/TautochroneProblem.html">tautochrone problem</a> asks what shape yields an oscillation frequency that... | 3,288 |
<p>I am interested in the finite-difference beam propagation method and its applications. I try to solve the Helmholtz equation. At first, i would like to solve numerically it for the easiest case, without nonlinearities. Just to make sure I'm on the right way. But i really don't understand how to wright the boundary c... | 3,289 |
<p>By comparatively examining the operators</p>
<p><img src="http://i.stack.imgur.com/Ox5jp.png" alt=""></p>
<p>a student concludes that `Energy is actually the momentum in the direction of time.' Is this student right? Could he be wrong? </p> | 3,290 |
<p>Could someone experienced in the field tell me what the minimal <strong>math knowledge</strong> one must obtain in order to grasp the introductory Quantum Mechanics book/course?</p>
<p>I do have math knowledge but I must say, currently, kind of a poor one. I did a basic introductory course in Calculus, Linear algeb... | 223 |
<p>Is there a simple mathematical expression for the stopping power of a given thickness of armor, given the thickness of armor plate, the radius of a cannon ball, the density of the cannonball and the armor, the tensile strength and/or toughness of the armor, and the speed of the cannonball? For simplicity assume th... | 3,291 |
<p>I was watching Discovery channel the other night, they were telling that time slows down when you travel at a higher speed. This means there is a difference between the actual speed you travel at, and the perceived speed.</p>
<p>Does anybody know what the perceived speed is, the speed it seems you're travelling at?... | 3,292 |
<p>I've had a discussion with my father today, about the fuel usage of a vehicle at the same rpm, but a different gear.</p>
<p>He claims that the following situations have the same fuel usage:</p>
<pre><code>Gear: 2
rpm: 2000
fuel usage: 5.1l/100km
Gear: 5
rpm: 2000
fuel usage: 5.1l/100km
</code></pre>
<p>I say it ... | 3,293 |
<p>I have a <a href="http://en.wikipedia.org/wiki/Uninterruptible_power_supply" rel="nofollow">UPS</a> of 1000 Volts connected with 2 batteries each of 150 Amp. How much time it will take to consume the whole UPS (after fully charged) when a device of 1Amp is getting electricity form that UPS.</p>
<p>Please also expla... | 3,294 |
<p>I am following the first volume of the course of theoretical physics by Landau. So, whatever I say below mainly talks regarding the first 2 chapters of Landau and the approach of deriving Newton's laws from Lagrangian principle supposing Hamilton's principle of extremum action. Please keep this view in mind while re... | 3,295 |
<p>In Peskin and Schröder's QFT, equation 2.56, could anyone give a list of all the arguments necessary in order to make all the transitions mathematically rigorous?</p>
<p>I tried composing such a list myself and I came up with:</p>
<ol>
<li>$\frac{d}{dx}\theta(x)=\delta(x)$</li>
<li>$\theta(x)\delta(x)\equiv0$?</li... | 3,296 |
<p>I know Hamiltonian can be energy and be a constant of motion if and only if:</p>
<ol>
<li>Lagrangian be time-independent, </li>
<li>potential be independent of velocity, </li>
<li>coordinate be time independent.</li>
</ol>
<p>Otherwise
$$H\neq E\neq {\rm const},$$ or
$$H=E\neq {\rm const},$$ or
$$H\neq E={\rm c... | 3,297 |
<p>This question is about Godel's theorem, continuity of reality and the Luvenheim-Skolem theorem.</p>
<p>I know that all leading physical theories assume reality is continuous. These are my questions:</p>
<p>1) Is reality still continuous according to string theory? what is the meaning of continuous if there is a fi... | 3,298 |
<p>Dark energy is constantly pulling all objects away from each other with increasing speed. This in turn causes a red-shift of the light from the most distant object where this effect is most profound. This red-shift will gradually increase as the objects move away faster. Is there going to be a time where the objects... | 79 |
<p>Newton's work was related to Enlightenment philosophy.</p>
<p>Einstein was influenced by Mach.</p>
<p>The founders of quantum mechanics had strong philosophical opinions.</p>
<p>What is the role of metaphysics in the development of physics?</p> | 3,299 |
<p>In general, when I think of movement through space, I think of this:</p>
<p>$$\frac{dx}{dt}$$</p>
<p>But in special relativity, we also have a concept of relative duration, which means that $t$ must have a rate of change, but with respect to what?</p>
<p>$$\frac{dt}{d?}$$</p> | 80 |
<p>If I stand exactly in front of a colorful wall, I imagine the light waves they emit, and they receive should randomly double or erase out each other. </p>
<p>So as a result, I imagine I should see a weird combination of colors, or a full-black/full-white/very lightly perception of the wall, when all the light waves... | 3,300 |
<p>I know other people asked the same question time before, but I read a few posts and I didn't find a satisfactory answer to the question, probably because it is a foundational problem of quantum mechanics. </p>
<p>I'm talking about the Hilbert space Separability Axiom of quantum mechanics. I'd like to understand why... | 3,301 |
<p>Following Arnold's [1] definition of the universe as an affine space $ A ^4$ with the group $\mathbb R ^4$ acting on it, we may define a galilean transformation as <em>an affine map $g:A^4 \to A^4$ which preserves the galilean structure</em>, i.e. which preserves time intervals and spatial distance beetween simultan... | 3,302 |
<p>The Hamiltonian is given as</p>
<p>$H=-\frac{\hbar^2}{2 m_e r^2}\displaystyle\sum_{n=1}^N \dfrac{\partial^2}{\partial \theta_n^2}$</p>
<p>In the first part we show that the $\psi_k=\frac{1}{\sqrt{2\pi}}\exp(i k \theta)$ is a solution to $H\psi_k = E_k \psi_k$.</p>
<p>$E_k=\frac{k^2 \hbar^2}{2 m_e r^2}$</p>
<p>Al... | 3,303 |
<p>When I perform parametric modeling, if there is significant multicollinearity between variables I think should be independent, but in fact are not, I run into the case where one or more of the coefficients becomes exceeding small (or large) relative to the others. How is that different than what occurs in fine tuni... | 3,304 |
<p>My quantum mechanics textbook says that the following is a representation of a wave traveling in the +$x$ direction:$$\Psi(x,t)=Ae^{i\left(kx-\omega t\right)}\tag1$$</p>
<p>I'm having trouble visualizing this because of the imaginary part. I can see that (1) can be written as:$$\Psi(x,t)=A \left[\cos(kx-\omega t)+i... | 3,305 |
<p>Suppose a magnetic dipole $\mathbf{m} = m \hat{z}$ is falling towards a circular loop of radius $b$ under gravity. Assuming the dipole always stays along the $z$-axis of the loop, determine the following:</p>
<p>In terms of the height $z$, loop resistance $R$, and speed $v$, determine</p>
<p>(a) The EMF induced ar... | 3,306 |
<p><a href="http://www.youtube.com/watch?v=yTeBUpR17Rw" rel="nofollow">This</a> video explains that heat at negative temperatures flows from the negative object to the normal object. If the temperature of the normal object is absolute hot, what happens with the heat? The heat can't be transferred to the absolute hot ob... | 3,307 |
<blockquote>
<p>The difference between a “timelike” spacetime interval and a “spacelike” spacetime interval can be understood in the following way: If the spacetime interval between two events is timelike, there exists a reference frame which measures the proper time between the two events; i.e. it sees the events oc... | 3,308 |
<p>The principle of equivalence - that, locally, you can't distinguish between a uniform gravitational field and a noninertial frame accelerating in the sense opposite to the gravitational field - is dependent on the equality of gravitational and inertial mass. Is there any deeper reason for why this equality of "charg... | 3,309 |
<p>Ok, I am stumped by this question:</p>
<p>In the system in the diagram below, block M (15.7 kg) is initially moving to the left. A force F , with magnitude 60.5 N, acts on it directed at an angle of 35.0 degrees above the horizontal as shown. The mass m is 8.2 kg. There is no friction and the pulley and string are ... | 3,310 |
<p>Mach's principle says that it is impossible to tell if something is accelerating unless there is something else in the universe to compare that motion to, which seems reasonable. However, if you had one detector in the universe, you seem to be able to tell if it is accelerating because an accelerating detector would... | 3,311 |
<p>What does it mean for a field (say, $\phi$) to have a charge (say, $Q$) under the action of a group (say, $U(1)$)?</p> | 3,312 |
<p>How does it work that gravitational forces can affect time and what usable applications could arise from this?</p> | 3,313 |
<p>I'm looking for a simple explanation of how a whistle operates. I know <em>that</em> forcing air over a sharp lip can set up a wave in a resonating cavity, but <em>how</em>? "Most whistles operate due to a feedback mechanism between flow instability and acoustics"--yes, but what <em>does</em> that feedback mechani... | 3,314 |
<p>I would like to support open-access journals by choosing to publish in journals which allow readers free online access. Ideally I would also like to retain copyright instead of signing it over to the journal.</p>
<p>What are some of the better open-access journals in physics? I am particularly interested in jou... | 3,315 |
<p>Inspired by this: <a href="http://physics.stackexchange.com/questions/8937/electrical-neutrality-of-atoms">Electrical neutrality of atoms</a></p>
<p>If I have a wavefunction of the 'reduced mass coordinate' for a hydrogen like atom made from an electron and a positron, what is the spatial charge distribution?</p>
... | 3,316 |
<p>So I was cruising around at YouTube and saw this <a href="http://www.youtube.com/watch?v=im1iNq02Kz0&feature=feedu" rel="nofollow">sweet video</a>, and as I was watching started to wonder: "How is this possible?".</p>
<p>For a little bit of background, in case you decide to not watch the video, what is happenin... | 3,317 |
<p>Cavendish measured the gravitation constant $G$, but actually he measured that constant on the Earth. What’s the proof that the value of the gravitation constant if measured on Neptune would remain the same? What’s the guarantee of its being a constant?</p>
<p>There are many such constants; I just took one example ... | 643 |
<p>IANAP, so feel free to berate me for thinking apocryphal thoughts! Just as magnetism has two charges, in which particles of like-charge repulse and particles of dissimilar charge attract, might gravity have two charges in which particles of like-charge attract and particles of dissimilar charge repulse?</p>
<p>In p... | 81 |
<p>I need to send infrared light from its emitter to a distance of about 10-12 feet. Is this possible?</p> | 3,318 |
<p>What happens microscopically when an electrical current starts to flow? I'd like to understand microscopically what happens in detail when electrons start moving (quasi-classically).</p>
<p>Electrons can have different velocity, they can produce electromagnetic fields, leads have free electrons and rigid atom cores... | 3,319 |
<p>In Ady Stern's review of the Quantum Hall effect, he says of a quantum hall system "The spectrum at $\Phi = \Phi_0$ is the same as the spectrum at $\Phi = 0$..." Can someone explain why this is? It seems like the applied magnetic field certainly changes the hamiltonian, and thus the spectrum, but apparently not when... | 3,320 |
<p>This is freshly ground pepper on water. </p>
<p>Why is there a triangular configuration of the water around the pepper fragment? Surely all these pepper fragments have different shapes? You can clearly see one of these triangles on the lower left edge of the reflection of the kitchen light.</p>
<p><img src="http:/... | 3,321 |
<p>The Situation:</p>
<p>A ball is placed in a beaker filled with water and floats. It is also attached to the bottom of the beaker via a string. </p>
<p>The Question:</p>
<p>The ball is attached to the beaker, thus making the ball and beaker (and string included) a system. However, it is floating. Does the ball add... | 3,322 |
<p>I am stuck with this process of calculating the tree-level scattering amplitude of two positive helicity (+) gluons of momentum say $p_1$ and $p_2$ scattering into two gluons of negative (-) helicity with momentum $p_3$ and $p_4$. </p>
<p>This is apparently $0$ for the diagram where one sees this process as two 3 g... | 3,323 |
<p>Specifically the Hamiltonian takes the form of</p>
<p>$$\hat H = \frac{\Delta }{2}{\hat \sigma _z} + {\omega _1}\hat a_1^\dagger {\hat a_1} + {\omega _2}\hat a_2^\dagger {\hat a_2} + {g_1}\left( {{{\hat a}_1}{{\hat \sigma }_ + } + \hat a_1^\dagger {{\hat \sigma }_ - }} \right) + {g_2}\left( {{{\hat a}_2}{{\hat \sig... | 3,324 |
<p>Suppose you are given an $n$-qubit quantum channel defined as $\mathcal{E}(\rho) = \sum_{i} p_i X_i \rho X_i^\dagger$, where $X_i$ denotes an $n$-fold tensor product of Pauli matrices and $\{p_i\}$ is a probability distribution. The Holevo-Schumacher-Westmoreland capacity of the channel is defined by
$$
\chi(\mathc... | 3,325 |
<p>It is often stated that in quantum gravity only charges coupled to gauge fields can be conserved. This is because of the no hair theorem. If a charge is coupled to a gauge field then when it falls into a black hole the black hole acquires a corresponding field. However if it is not then the black hole doesn't "remem... | 3,326 |
<p>Given that spacetime is not affine Minkowskispace, it does of course not possess Poincare symmetry. It is still sensible to speak of rotations and translations (parallel transport), but instead of</p>
<p>$$[P_\mu, P_\nu] = 0$$</p>
<p>translations along a small parallelogram will differ by the curvature. I have not... | 3,327 |
<p>Let's assume an observer looking at a distant black hole that is created by collapsing star.</p>
<p>In observer frame of reference time near black hole horizon asymptotically slows down and he never see matter crossing event horizon. So black hole is visible as some kind of frozen object - falling matter almost sto... | 3,328 |
<p>Apologies to all if this has been asked before, I searched but was unable to find one similar. </p>
<p>This is a question that has been bugging me for a while that i haven't really been able to find a suitable answer for. </p>
<p>I am aware that the space between an atoms nucleus and its electron cloud is teeming ... | 259 |
<p>If the object in motion gains mass, will it affect the change in mass of Earth if it stops revolving around Sun, since mass is responsible for gravity how will be the gravitational force change? </p> | 3,329 |
<p>I read that the <a href="http://www.astronautix.com/engines/f1.htm" rel="nofollow">F-1</a> engine from the 1st stage of the Saturn V rocket is the most powerful engine ever created by mankind, delivering ~200 gigawatts of power.</p>
<p>Thus, I have got two questions:</p>
<ul>
<li><p>Will future rockets propelled b... | 3,330 |
<p>Is it possible to determine which kinds of 10 basic cloud type will form by reading skew-T graph? Also, can I determine the top and the bottom of the clouds by skew-t graph?</p> | 3,331 |
<p>This is in continuation of my previous question,
<a href="http://physics.stackexchange.com/q/68210/">IR divergence and renormalization scale in dimensional regularization</a>.</p>
<p>Lubos gave a nice answer there but I want to get to a very specific example which is not found in usual QFT - (in QFT all examples I... | 3,332 |
<p>I was watching a documentary entitled <a href="http://www.bbc.co.uk/bbcfour/documentaries/features/atom.shtml" rel="nofollow">"The Atom"</a> and one of the statements made was that Atoms behave differently when we look at them. I wasn't too sure about the reasoning behind this and i'm hoping someone could explain ho... | 3,333 |
<p>I found this problem, and so far I am stumped. I was wondering if anyone wanted to solve it with me, or help me calculate eigenvectors, or just give insight on my questions.</p>
<blockquote>
<p><em>Consider a system of two spin-1/2 particles interacting through the Hamitonian
$$H = A(S_x^2 − S_y^2) + BS_z^2,$$ ... | 3,334 |
<blockquote>
<p><em>A force of 6 Newton and another 8 newton can be applied together to produce the effect of a single force of a) 1 N b) 11 N c) 15 N d) 20 N.</em></p>
</blockquote>
<p>More generally, if I know the magnitude of one force, say $F_1$, and another, say $F_2$, then how do I find the magnitude of the r... | 3,335 |
<h2><strong>Preface</strong></h2>
<p>The definition of <strong>average speed</strong> of an object is defined by the distance travelled over time:
$$v_{avg} = \frac{x_2 - x_1}{t_2 - t1}$$
The interpretation of speed is that when you multiply speed with the time interval, you should get the distance you travelled at t... | 3,336 |
<p>Suppose I want to solve Nonlinear Schrodinger equation using imaginary time propagation to get the ground state solution. I choose $t = - i t$, and then solve the equation using split step Crank Nicholson method. All the excited states will decay faster than the ground state and leave finally the ground state of the... | 3,337 |
<p>So, in one science fiction story, that tries to be realistic as possible apart from few space magics, humanity has contingency plan to blow up Jupiter. As in, totally destroy it in one massive nuclear explosion.</p>
<p>I'd like to know the effects of such event. Would it totally wreck the solar system or would the ... | 3,338 |
<p>I would like to extrapolate my current router wifi radiation from my phone.
If I know that my router is transmitting at 300mW and my phone displays the strength in -dbm (from 0 to -100 scale) if I have -50dbm strength does it mean that the current radiation is 150mW?
If not how can I extrapolate the radiation from -... | 3,339 |
<p>In $\phi^3$ theory, are there any formula for determining the Symmetry factor as that is found for the $\phi^4$ theory in any standard book of Quantum Field Theory?</p> | 172 |
<p>The <a href="http://www.google.com/#q=Gutzwiller+trace" rel="nofollow">Gutzwiller trace</a> is about</p>
<p>$$ d(E)=d_{0} (E)+ \sum_{p.o}A_{p}Cos(S(E)\ell_{p}) $$</p>
<p>and $ \ell_{p} $ are the length of the orbit.</p>
<p>However my question is, how can one derive the length of the orbit from the potential insid... | 3,340 |
<p>Does anybody know of an example were one could derive some important properties of a physical system from a symmetry of said system. </p>
<p>I´m specially looking for simple classical examples, which could serve to illustrate the importance of finding symmetries of a system to non-physicists (high school students o... | 3,341 |
<p>Following Edwin Hubble, it is widely believed that the universe is expanding, which is based on the red-shift of light from distant objects. Can <a href="http://en.wikipedia.org/wiki/Dark_matter" rel="nofollow">dark matter</a> cause light to be red-shifted and make it look like the universe is expanding while in fac... | 3,342 |
<p>I understand from some studies in mathematics, that the generator of translations is given by the operator $\frac{d}{dx}$. </p>
<p>Similarly, I know from quantum mechanics that the momentum operator is $-i\hbar\frac{d}{dx}$.</p>
<p>Therefore, we can see that the momentum operator is the generator of translations,... | 867 |
<p>When you fill a glass with water, water forms a concave <a href="http://en.wikipedia.org/wiki/Meniscus" rel="nofollow">meniscus</a> with constant contact angle $\theta$ (typically $\theta=20^\circ$ for tap water):</p>
<p><img src="http://i.stack.imgur.com/VV6ik.png" alt="enter image description here"></p>
<p>Once ... | 3,343 |
<p>On p. 49 of Polchinski's book, he says: "Incidentally, the free massless scalar in two dimensions has a remarkably large amount of symmetry -- much more than we will have occasion to mention." </p>
<p>Does anyone know what this is referring to? Is he just saying that there are a lot of different choices of stress t... | 3,344 |
<p><a href="https://en.wikipedia.org/wiki/Hooke%27s_law">Hooke's Law</a> tells us that $m\ddot{x} = -kx$. We can apply the <a href="https://en.wikipedia.org/wiki/Chain_rule">chain rule</a> to rewrite $\ddot{x}$ as follows:</p>
<p>$$\frac{\operatorname{d}\!^2x}{\operatorname{d}\!t^2} = \frac{\operatorname{d}\!v}{\opera... | 3,345 |
<p>I am referring to yet another version as the classical twin paradox.</p>
<p>In my version the moving apart of the twins is entirely induced by space expansion between them and they move apart each other at a very high speed, conceivably even higher than the local speed of light.</p>
<p>After some time space itself... | 3,346 |
<p>I need to get a nice picture about how electron moves around nucleus? I find concept of probability and orbitals quite difficult to understand?</p> | 579 |
<p>I've got metal coin : <a href="http://www.worldpeacecoin.org/" rel="nofollow">http://www.worldpeacecoin.org/</a></p>
<p>Ruble/dollar, a coin of disarmament with certificate. But, I am very spleeny person, I fear of it's radiance level and I don't know if I can trust it or check it somehow.</p>
<p>That could look w... | 3,347 |
<blockquote>
<p><em>Show that the hydrogen atomic wavefunction $\psi_{3,1,1}$ is normalized, and that it is orthogonal to $\psi_{3,1,−1}$.</em></p>
</blockquote>
<p>I'm not sure if I'm supposed to consider the radial part. I can show that the spherical part, Y{m=1, l=1}, is normalized and that Y{m = 1, l=1} and Y{m... | 3,348 |
<p><em>EDIT: Further clarification in the context of answers/comments received to 20 Jan has been appended</em></p>
<p><em>EDIT: 21 Jan - Response to the Lubos Expansion appended [in progress, not yet complete]</em></p>
<p><em>EDIT: 23 Jan - Visser's calculations appended</em></p>
<p><em>EDIT: 26 Jan - Peter Shor's ... | 3,349 |
<p>We start with the general case of $AdS_{p+2}$ i.e AdS space in $p+2$ dimension.
\begin{equation}
X_{0}^{2}+X_{p+2}^{2}-\sum_{i=1}^{p+1}X_{i}^{2} = R^2
\end{equation}
This space has an isometry $SO(2,p+1)$ and is homogeneous and isotropic. The Poincare Patch is given by
\begin{equation}
ds^2 = R^{2}\left(\frac{du^2}{... | 3,350 |
<p>An spherical spaceship moving in two dimensions is at position $(x, y)$ and has a velocity $(v_x, v_y)$. It also has a maximum acceleration $a_{max}$. Its goal is to be at position $(x', y')$ with a velocity of $(v'_x, y'_x)$. How long does the optimal path take?</p>
<p>I see that the problem can be reduced to a sp... | 3,351 |
<p>Let me start by saying that my knowledge of physics is almost nil (only high school level and I pretty much forgot everything) so bear with me, if I am asking naive questions the answers of which may be obvious to you. </p>
<p>My question is twofold: </p>
<ol>
<li><p>Is it possible to use the motion of the planet... | 82 |
<p>For positive definite infinite dimensional Hilbert spaces, there is the standard Cauchy norm topology. What if this state space has an indefinite norm or a positive semidefinite one, as in gauge theories or Faddeev-Popov ghosts? Which infinite sums are valid, and which aren't?</p>
<p>Similarly, for the algebra of o... | 3,352 |
<p>Which kind of truss would produce the best result (aka. best distribution of force):</p>
<p>i) An equilateral, isosceles or a scalene triangle<br/>
ii) 3 small triangles or 2 big triangles (the overall masses of both are equal)</p>
<p>The truces are made of fettucini and need to hold a mass of 6 kg.</p> | 3,353 |
<p>I know that the Coulomb potential is logarithmic is two dimensions, and that (see for instance this paper: <a href="http://pil.phys.uniroma1.it/~satlongrange/abstracts/samaj.pdf">http://pil.phys.uniroma1.it/~satlongrange/abstracts/samaj.pdf</a>) a length scale naturally arises:</p>
<p>$$ V(\mathbf{x}) = - \ln \left... | 3,354 |
<p>I was just starting a barbecue fire by blowing on the smouldering coals when I realised I had no idea what the sound was actually caused by. I can make the sound by blowing at almost any flame I can think of, and I guess it is perhaps related to the increased oxygen consumption and a turbulent flow. Why does a distu... | 3,355 |
<p>For a physics issues investigation I chose to investigate what effects lightning could have on an aeroplane while in flight if it was struck and then go on to discuss some possible implications of engineers not taking into account the power of positive lightning.</p>
<p>Just in-case you don't know what positive lig... | 3,356 |
<p>I'm looking for a formula that will return the number of <a href="http://en.wikipedia.org/wiki/Day_length" rel="nofollow">hours per day</a> given a specific location. I was thinking that can be calculated as a difference of sunrise and sunset, but I see that there are some other ways, like in this <a href="http://ma... | 28 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/14968/superluminal-neutrinos">Superluminal neutrinos</a> </p>
</blockquote>
<p>I remember not too long ago hearing very much speculation about a discovery that perhaps neutrinos are faster than the speed... | 83 |
<p>What is each mathematical step (in detail) that one would take to get from:</p>
<p>$E^2 - p^2c^2 = m^2c^4$ </p>
<p>to</p>
<p>$E = \gamma mc^2$,</p>
<p>where $\gamma$ is the relativistic dilation factor.</p>
<p>This is for an object in motion.</p>
<p>NOTE: in the answer, I would like full explanation. E.g. when... | 3,357 |
<p>I'm teaching myself mechanics, and set out to solve a problem determining the optimum angle to throw a projectile when standing on a hill, for maximum range. My answer seems <em>almost</em> plausible, except for one term, which, to be plausible, needs to have its sign switched. But I can find no hole in my reasoni... | 3,358 |
<p>I would like to know how come if <a href="http://en.wikipedia.org/wiki/Dark_matter">dark matter</a> was electrically charged it would reflect light. What are the equations or the logic behind it? </p> | 3,359 |
<p>The Dirac adjoint for Dirac spinors is defined as,
$$
\bar{u} = u^{\dagger} \gamma^{0} \, .
$$
However I have come across this,
$$
\overline{\gamma^{\mu}} = \gamma^{\mu} \, , \tag{1}
$$
(where $\gamma^{\mu}$ are the $4\times4$ gamma matrices). Naively applying the same rules as for the Dirac spinor clearly does not ... | 3,360 |
<p>A friend of mine seems to think that wind affects the ground-speed of a ground-based vehicle in the same way airspeed affects an airborne aircraft. i.e. If faced with a 10mph headwind, your car isn't actually traveling at 60mph, it's traveling at 50mph.</p>
<p>I have tried to explain why that is incorrect, but I'm ... | 3,361 |
<p>Light bulbs, <a href="http://en.wikipedia.org/wiki/Electric_power" rel="nofollow">Wattage</a> meaning?</p>
<p>Two incandescent bulbs (120 V, 25 <a href="http://en.wikipedia.org/wiki/Watt#Confusion_of_watts.2C_watt-hours_and_watts_per_hour" rel="nofollow">Watt</a>) and (120 V, 500 Watt) connected to the same batteri... | 3,362 |
<p>In a simply connected container containing a superfluid and rotating, there is a net circulation of superfluid. This is found due to the vortices formed, around which the superfluid rotates. These vortices have been found to generally form a triangular lattice arrangement. Why is the triangular lattice arrangement p... | 3,363 |
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