question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>I'm trying to understand the classical derivation of Friedmann equation but I'm missing one step.</p>
<p>So, I start with accelerations, where $a$ is a scale factor</p>
<p>$\ddot{a}=-\frac{GM}{a^{2}}$</p>
<p>$\ddot{a}=-\frac{4\pi G}{3}\rho a$</p>
<p>Now multiply both sides by $\dot{a}$, and here is the question:... | g8519 | [
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<p><img src="http://i.stack.imgur.com/O9S2S.png" alt="enter image description here"></p>
<p>Say I have two boxes which both contain, say, 25 red particles (as shown in picture). These particles are randomly placed in a 2d grid, and in one the total area $A_{1}=20000$ and the other has area $A_{2}=10000$. </p>
<p>If t... | g8520 | [
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<p>I have a puzzle I can not even understand.
A graviton is generally understood in $D$ dimensions as a field with some independent components or degrees of freedom (DOF), from a traceless symmetric tensor minus constraints, we get:</p>
<ol>
<li><p>A massless graviton has $D(D-3)/2$ d.o.f. in $D$-dimensional spacetim... | g8521 | [
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<p>I've seen the preferred basis problem referred to in many places, but have not seen a clear explanation of what the problem is. For example, <a href="http://physics.stackexchange.com/questions/65177/is-the-preferred-basis-problem-solved">this question</a> asks whether the problem has been solved, but I wasn't able t... | g8522 | [
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<p><img src="http://i.stack.imgur.com/PxLaK.png" alt="enter image description here">Two particles, $P_1$ and $P_2$, of equal masses $m$ are linked by a spring of stiffness $k$ and natural length $a$. They are sliding freely without friction along a horizontal fixed hoop of radius $R$. </p>
<p>I need some help in writ... | g8523 | [
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<p>I'm reading this <a href="http://arxiv.org/abs/hep-th/9410093" rel="nofollow">paper</a> where to determine the coefficient $C^{\phi\phi O}(x_{12},\partial_2)$ of the OPE (p.10)
$$\phi^\alpha (x_1)\phi^\beta (x_2)=C_\phi \frac{\delta^{\alpha\beta}}{x_{12}^{2\eta}}+C^{\phi\phi O}(x_{12},\partial_2)O(x_2)\delta^{\alpha... | g8524 | [
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<p>Due to electron "spin", a small magnetic field is produced. Maxwell's equations imply that magnetic fields are due to changes in electric fields. Is the magnetic field produced then because the electric field is "spinning" with the "spinning" electron, in the quantum sense of "spinning" and this change in electric... | g8525 | [
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<p>I am looking for a good explanation and/or reference quotation explaining why infrared absorption technique is essentially nonlinear (eg. for carbon monoxide quantification).</p>
<p>When using UV/visible/near-IR absorption technique, Beer-Lambert Law often is valid and then you have quite straight way to convert tr... | g8526 | [
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<p>I guess it depends on the heat or the type of the material but can you give some examples or formulas to calculate it ?</p>
<p>The best example would be the average speed of the air molecules (all types in the air) at room temperature or water molecules at human body temperature.</p> | g8527 | [
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<p>An example of a test: Ask each variant whether its estimate of the electron mass lies within $\pm\,x\%$ of the known value. This surely can't take long per theory. Although $10^{500}$ is huge, whittling them down could be essentially a background task for a few thousand computers. But is it being done, and if not, w... | g8528 | [
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<p>Consider general pure state
$|\psi\rangle$
in some hilbert space $\mathcal{H}$ (which could be a tensor product of other Hilbert spaces)</p>
<p><strong>I would like to know whether there is a way to systematically find the group of all operators on $\mathcal{H}$ under which $|\psi\rangle$ is invariant.</strong></... | g8529 | [
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<p>Flow rate is calculated using only the parallel component of the velocity vector to the area vector. Why is this? How can I mathematically prove this? Namely, how do I prove any perpendicular component of the velocity vector is not contributing to any volume output? </p>
<p>I know this is the result of the dot prod... | g8530 | [
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<p>According to NASA </p>
<blockquote>
<p>a black hole is anything but empty space. Rather, it is a great amount of matter packed into a very small area</p>
</blockquote>
<p>According to the documentary <a href="http://www.imdb.com/title/tt3443616/">Space Unraveling The Cosmos</a> about Black Holes</p>
<blockquote... | g8531 | [
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<p>Does the strong (nuclear) force ever contribute to decay ?
Or is the weak nuclear force the only decaying force ?</p> | g8532 | [
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<p>Little background first:<br>
I take part in preparing environmental reports for wind turbines/farms and now I'm exploring shadow flickering effect of moving blades. </p>
<p>From some sources (in Polish) I got to know that the shadow casted by turbine blades is solid, black only to some distance and above this dist... | g8533 | [
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<p>I was given a graph of square of the wave function of a hydrogen atom, against the distance of the electron from the nucleus (denoted by r).</p>
<p>What I know is that the square of the wave function gives the probability of finding an electron at a particular position, but could anyone explain to me that why does ... | g8534 | [
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<p>I am using cross correlation for sound localization. I have a four microphone array (arranged linearly). My problem is, sometimes the time diff found by cross correlation between a given pair of mics jumps to some eratic value. For example, if for the first three times it gives a value of 14 then the next time it su... | g8535 | [
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<p>I have a question regarding hadron fluctuations. </p>
<p>For instance on page 85 in Feynman's "Photon-Hadron Interactions" equation 15.2 reads:</p>
<p>$$\tag1\vert \omega\rangle = \vert \omega_0\rangle+\frac{\delta}{m_\omega-m_\rho-\frac{\mathrm{i}\Gamma_\omega-\mathrm{i}\Gamma_\rho}{2}}\vert \rho_0\rangle$$
where... | g8536 | [
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<p>Concerning fiber optics, I'd like to know how a 2D <a href="http://en.wikipedia.org/wiki/Microelectromechanical_systems" rel="nofollow">MEMS</a> based optical cross-connect is able to switch optical signals. I've read that these mirrors are controlled with electrical signals to be either "on" or "off" to redirect li... | g8537 | [
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<p>This is about Polchinski's eq(2.3.11).</p>
<p>It says that
$$Res_{z\rightarrow z_0}j(z)\mathcal A(z_0,\bar z_0)+\bar{Res}_{z\rightarrow z_0}\tilde j(\bar z)\mathcal A(z_0,\bar z_0)=\frac1{i\epsilon}\delta \mathcal A(z_0,\bar z_0)$$
from (2.3.10),
$$\oint_{\partial R}(jdz-\tilde jd\bar z)\mathcal A(\sigma_0)=\frac{2... | g8538 | [
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<p>I have several confusions regarding <a href="http://en.wikipedia.org/wiki/Escape_velocity" rel="nofollow">escape velocity</a>. I am sure I am missing something(s) obvious or maybe I am taught wrong. </p>
<ol>
<li><p>Lets say we throw an object of any mass at exactly escape velocity of earth calculated from $v^2=\f... | g8539 | [
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<p>I am studying Quantum electrodynamics (QED) and the core part of this theory is the light-matter interaction (quantized light and matter). I referred many papers and books (Molecular quantum electrodynamics, Interaction Hamiltonian of quantum optics etc), and in all the places the interaction Hamiltonian is derived ... | g8540 | [
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<p>I was having one of those obnoxious conversations with a friend where he was arguing that a fat skydiver would reach the ground faster than a skinny skydiver. To me it seemed as obvious that the world is round (that is: not very obvious unless you have some education).</p>
<p>We weren't getting anywhere so I punche... | g8541 | [
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<p>I'm familiar with the ideal gas law $$PV=nRT$$ but I don't think it applies to liquids like water. If I'm wrong, please correct me! If I'm right, then what equation of state applies to liquids such as water?</p> | g8542 | [
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<p>I learned that the wavefunction for the hydrogen atom can be solved analytically (we did the derivation in class), but that for more complicated atoms it is "impossible" to solve and that only approximations can be obtained.</p>
<p>Well, I'm not sure I like the vague wording with "impossible".</p>
<p>Basically, a ... | g8543 | [
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<p>Along the idea of frame-dragging; </p>
<p>Will the rotation of a black hole, which has some velocity <em>v</em> and angular momentum, influence its path in 3D space?</p>
<p>I've seen the fact that depending on the direction of the rotational velocity, the Kerr metric can give different revolution periods for orbit... | g8544 | [
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<p>i'm working on a paper about symmetron cosmology. symmetron is a scalar field that by its symmetry breaking can explain the dark energy.</p>
<p>the action is:
<img src="http://i.stack.imgur.com/5WGnl.jpg" alt="enter image description here"></p>
<p>ans A , V are assumed to be:</p>
<p><img src="http://i.stack.img... | g8545 | [
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<p>I am sure there is a simple explanation for my confusion, but I am a little puzzled:</p>
<p>We are dealing with two parallel electron cannons that each produces a straight beam of electrons. They are placed at a distance d next to each other, which means there will be a repulsive electrostatic force between the ele... | g8546 | [
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<p>The Hamiltonian for one-dimensional Ising model is given by,
\begin{equation}
\mathcal{H} = -J\sum_{<ij>} S_iS_j; \quad i,j=1,2,...,N+1
\end{equation}
where $<ij>$ denotes that there is nearest neighbor approximation. The partition function is given by,
\begin{equation}
\mathcal{Z}=\sum_{\{S_i\}} e^{-\be... | g8547 | [
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<p>Whereas I have seen frogs fly in high magnetic fields and Sumo ringers stand on high-temperature superconductors it seems that these are not the only forms of levitation.</p>
<p>Recently a group of physicicsts managed to levitate coffee without a magnetic field (<a href="http://www.pnas.org/content/early/2013/07/10... | g8548 | [
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<p>Does speed of writing data on an optical disk ( like DVD ) affect the speed of reading data from that disk ? even a little?</p> | g8549 | [
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0.0069990577176213264,
-0.04288918152451515,
-0.015971986576914787,
0.06300435215234756,
-0.009093069471418858,
0.026237545534968376,
... |
<p>Say we have a bar immersed in liquid, and that the bar in being rotated at a given constant angular speed. What force is exerted on the liquid by the faces of the bar moving through the liquid (so the force is tangent to the circular motion of the bar). My guess is that the expression involves the distance from the ... | g8550 | [
0.06331489235162735,
0.008079255931079388,
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... |
<p>The wave equation is often written in the form</p>
<p>$$(\partial^2_t-\Delta)u=0,$$ </p>
<p>involving the <a href="http://en.wikipedia.org/wiki/Laplace%E2%80%93Beltrami_operator" rel="nofollow">Laplace-Beltrami operator</a> $\Delta$. However, the Laplace-Beltrami operator $\Delta$ is defined only in the presence o... | g8551 | [
0.03996359556913376,
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<p>I'm trying to understand simple things about electricity reading allaboutcircuits.com web. <a href="http://www.allaboutcircuits.com/vol_1/chpt_9/2.html" rel="nofollow">This chapter</a> includes image which´s principle I don't understand. Here it is:
<img src="http://i.stack.imgur.com/gizaD.png" alt="enter image desc... | g8552 | [
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0.0076... |
<p>I <a href="http://www.independent.co.uk/environment/sunflowers-to-clean-radioactive-soil-in-japan-2303086.html">read yesterday</a> that sunflowers were used and to cleanup radioactivity at Chernobyl and the Atomic Bomb sites in Japan and may be used as part of a campaign to clean up the Fukushima area. </p>
<p>But... | g8553 | [
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<p>According to impulse principle the impulse is the same as the change in the object's momentum:
$\bar I = \delta p$</p>
<p>Because the momentum can be calculated like this:
$\bar p = m\bar v$. If we solve speed from that: $\bar v = \frac{\bar p}m$</p>
<p>We can conclude that a certain impulse causes certain change ... | g8554 | [
0.05379744991660118,
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<p>I don't know how to say it, but in the <a href="http://www.youtube.com/watch?v=xE7xRgfPjAI">TV dominatrices</a> and the <a href="http://rads.stackoverflow.com/amzn/click/039333810X">popular science books</a> we see the <a href="http://en.wikipedia.org/wiki/String_theory">string theory</a> as "the best theory to exp... | g8555 | [
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<p>Can you give me examples where the standard physics curriculum in high school or college is inaccurate? When did you realize you had been fed lies?</p> | g8556 | [
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<p>Super string theory establishes that at each point of our 4 dimensional spacetime, there is attached a 6-dimensional Calabi-Yau (CY) manifold which encodes the geometry of the additional compactified spatial dimensions needed to formulate the theory. It is postulated that the size of these compactified dimensions is... | g8557 | [
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<p><a href="http://books.google.com/books?id=hFqxLgPol4EC&printsec=frontcover&dq=equations%20of%20states%20of%20fluids&hl=en&sa=X&ei=lvy5U6jOCe-X0AXd-4GQAg&ved=0CBwQ6AEwAA#v=onepage&q=equations%20of%20states%20of%20fluids&f=false" rel="nofollow">Sengers</a> describes the deviation functi... | g8558 | [
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<p>So, I've been reading about the photoelectric effect for my modern physics class, and I was confused about how Ohm's law works in relation to it. Say we have a photoelectric apparatus that simply generates some current and then applies a stopping potential V to bring the current to zero. If we increase the intensity... | g8559 | [
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<p>Autorefractors are being used by eye opticians for eye diagnosis. I searched internet for articles and wiki page as well but I wasn't satisfied. I am interested to know how do they actually work. How the machine is able to focus sharp on retina automatically. How do they figure out spherical/cylindrical aberration f... | g8560 | [
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<p>Can someone outline the difference between <a href="http://en.wikipedia.org/wiki/Supersymmetry" rel="nofollow">supersymmetry</a> and <a href="http://en.wikipedia.org/wiki/Supersymmetric_quantum_mechanics" rel="nofollow">supersymmetric quantum mechanics</a>? I often hear the two used interchangeably but I'm almost ce... | g8561 | [
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<p>I recently saw this picture posted on <a href="https://twitter.com/makeusabrew/status/355995584190091265" rel="nofollow">Twitter</a> which shows a so-called <a href="http://en.wikipedia.org/wiki/Ice_spike" rel="nofollow">ice spike</a> rising from an ice cube tray.</p>
<p><img src="http://i.stack.imgur.com/DwBoZ.jpg... | g8562 | [
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<p>This is a technical (may be trivial?) question about this <a href="http://arxiv.org/abs/1208.1030" rel="nofollow">sigma scalar field advertised by Chamseddine and Connes</a> to improve the electroweak vacuum stability involved by the weak mass of the already found Higgs-like particle. </p>
<p><strong>Is it a scalar... | g8563 | [
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<p>In Polchinski's string theory vol 1, p. 23, it is said
"It is a complicated question whether the bosonic string has any stable vacuum, and the answer is not known."</p>
<p>The book was published on 1998. What is the current answer of this question? Is bosonic string a realistic theory nowadays? </p>
<p>In addition... | g8564 | [
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<p>I am a first year and general student. I Am just curious about the vacuum or space.
I just don't get what actually space is.
Well, I know our scientists are still working on it but still I just want to know what is the accepted and best definition of space or a point in space that is the basic building block of this... | g8565 | [
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<p>I have two questions, which are connected with each other.</p>
<p><strong>The first question</strong>.</p>
<p>In a classical relativistic (SRT) case for one particle can be defined (in a reason of "antisymmetric" nature of angular momentum) 4-tensor
$$
L_{\alpha \beta} = x_{\alpha }p_{\beta} - x_{\beta }p_{\alpha... | g8566 | [
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0.022762203589081764,
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-0.0... |
<p>I can see that $a_{0}$ is not an independent field and Gauss law is a constraint on the theory arising from field equations. But, I don't get the geometrical picture.</p>
<p>Let $A$ be the space of all field configurations and let $G$ be the group of gauge transformations. Then, the actual configuration space $C=A/... | g8567 | [
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<p>So if light travels from one media to another with a different refraction index, what may happen happen? Refraction, reflection or total reflection? I am quite confused as to the differences between these three concepts. Does 1 of these always happen in this situation? What exactly is total reflection and how/why do... | g8568 | [
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... |
<p>When a permanent magnet attracts some object, lets say a steel ball, energy is converted into for instance kinetic energy and heat when attraction happens, and they eventually collide. Does this imply that energy is drawn from the magnetic field and the magnet is depleted, making it weaker and weaker for each magnet... | g325 | [
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<h1>Setup</h1>
<p>In the typical treatment of time-dependent perturbation theory in quantum mechanics, one arrives at the set of equations</p>
<p>$$
i \dot{a}^{(r + 1)}_m(t) = \sum_n \langle m |H_1(t)|n \rangle e^{i \omega_{mn} t} a^{(r)}_m(t)
$$
for integers $r > 1$ and $\dot{a}^{(0)}_m(t) = 0$. Here, the Hamilto... | g8569 | [
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<p>The orbit of satellites around Earth eventually decays, or so I read. This is typically caused either by atmospheric drag, or by tides. I would assume most satellites have a limited service life in orbit. Hence the question - What is the typical orbital life of a satellite? How long before it's successor must be lau... | g8570 | [
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<p>I think my pure-math head is messing with me on the question below: my physics and CS friends both seemed to think it was a simple computational thing, and my program says the method works, but now I've confused myself about rotations vs. angular velocities.</p>
<p>The problem: you have data from gyroscopes on a ri... | g8571 | [
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<p>Let's imagine a diesel engine's cylinder head would be heated by external means, to some hundred degree celsius, and the fuel would be replaced by water.</p>
<p>We've made modifications so that water would'nt destroy the fuel system and the cylinders would sustain the high temperature.</p>
<p>Now the injector woul... | g8572 | [
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<p>Learning about the fourier transform and its connection to filtering/convolution got me curious about naturally occurring filters.</p>
<p>Why/how is it that brick walls naturally act as a low-pass filter (which requires something as seemingly complicated as convolution with the <code>sinc</code> function) to sound... | g8573 | [
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<p>Imagine that two electrons interact by exchanging a virtual photon.</p>
<p>I know that the total energy and linear momentum of the two electrons is conserved by the interaction.</p>
<p>Is the total (orbital) angular momentum of the electrons conserved as well?</p>
<p>If the photon transfers energy and momentum be... | g8574 | [
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<p>This question is the continuation of <a href="http://physics.stackexchange.com/questions/122980/lsz-reduction-theorem-derivation-in-weinberg-qft">this one</a>.</p>
<p>For simplicity, let's use $(1)$ from the linked question (it is called n-point Green function and in particle case coincides with internal diagram),
... | g8575 | [
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<p>In quantum computation there are several principal quantum gates that have corresponding matrix representations. One of these is the Z gate, whose matrix is $\left[\begin{smallmatrix} 1 & 0 \\ 0 & -1 \end{smallmatrix}\right]$.</p>
<p>... anyway, I've found the eigenvalues (equal to +1, -1) using the charact... | g8576 | [
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<p>The tachyonic string mode in perturbative bosonic string theory indicates that the "vacuum", flat Minkowski $\mathbb{R}^{25,1}$, is not really a vacuum. What is conjectured about tachyon condensation in this theory? Do we expect the theory to have a vacuum? Is there any way the condensate might generate fermions d... | g8577 | [
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<p>As many cosmologists have pointed out, the universe is extremely fine-tuned for life in so many respects, and life, especially intelligent life, is extremely unlikely to emerge. The strong anthropic principle tries to explain it statistically by an extremely huge multiverse ensemble. However, are there alternative e... | g8578 | [
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0.0... |
<p>Sometimes when I'm filling up a container with water from the tap, bubbles will form on the top of the water. They look a little like soap sud bubbles, but there's no soap suds present. I notice, sometimes this will happen more than others.</p>
<p>Why does water sometimes form bubbles when I use the tap to fill con... | g8579 | [
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0.065782... |
<p>What is the difference between <a href="http://en.wikipedia.org/wiki/Proper_time" rel="nofollow">proper time</a> and the observer time?</p>
<p>Whilst thinking about Black holes, when we see the Schwarzschild metric </p>
<p>$$c^2\tau ^2 = \left ( 1 - \frac{r_{s}}{r} \right )c^2t^2 - \frac{r^2}{1-\frac{r_{s}}{r}} - ... | g8580 | [
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<p>When we connect 2 light bulbs to a voltage source in parallel, the equivalent Resistance of the bulbs is R/2 (1/Req = 1/R + 1/R). If we connect these lights in parallel we have Req = 2R; </p>
<p>I= V/Req is valid for both circuits, and P=I*V (with V being the same for circuit 1 and 2)
This means the parallel circui... | g8581 | [
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<p>I'm using modal decomposition to predict the steady state response of a beam structure to harmonic loading. The structure itself is very lightly damped, but we know from experiments that the aerodynamic drag force as it moves through the air is very significant.</p>
<p>I have the mass and stiffness matrices for the... | g8582 | [
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<p>The Sun's magnetic field goes all the way out to the heliopause which is a very long distance to travel and magnetic fields consist of virtual photons so how long do these virtual photons exist for to reach the heliosphere?</p>
<p>Edit
My problem is that if virtual photons travel at light speed then they must live ... | g8583 | [
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<p>If there are two 10 x 2 x 1 foot rectangles in space and they are lined up so if they hit each other there will be no spots that are not hit in the front of the rectangle. Then they are pushed forward with the same exact same force, (this is space and there is no resistance) and they hit, will the tiny shockwaves of... | g8584 | [
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<p>A DC solenoid creates a magnetic field B.
A highly permeability magnetic core is added to solenoid to increase B.</p>
<p>Now, there is change in flux, due to the increase in magnetic field. What would be the result for the circuit? Would more power drawn support the new stronger field? </p>
<p>Or P would be the s... | g8585 | [
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<p>I don't know how to ask this more clearly than in the title.</p> | g8586 | [
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<p>To maintain the surface tension which formed our original bubble (in order to keep the bubble from breaking), we may change the temperature/pressure of air on both sides of the bubble varyingly, with different rates.</p>
<p>Is such a configuration possible? Examples of any such system?</p> | g8587 | [
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<p>Whenever I read about the curvature of spacetime as an explanation for gravity, I see pictures of a sheet (spacetime) with various masses indenting the sheet to form "gravity wells." Objects which are gravitationally attracted are said to roll down the curved sheet of spacetime into the gravity well. This is troubli... | g13 | [
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<p>In three dimensions, the density of states of a free electron is the square root of the energy of the electron. Can somebody explain the relationship between this dependence and the shape/formation of sub-bands in k-space? In other words, how does the shape and quantity of bands always lead to this same dependence a... | g8588 | [
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<p>I am trying to figure out where is the focal point and where is the image. I read some information online about the point where you see the image is the focal point, but however, my supervisor mentioned the point where you see the image is on "point 2" on the drawing. But as I remembered from school, the "point one"... | g8589 | [
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<p>I was recently reading a popular science book called <a href="http://www.amazon.co.uk/The-Canon-Beautiful-Basics-Science/dp/0571239722" rel="nofollow">The Canon - The Beautiful Basics of Science</a> by Natalie Angier, and it talks about subatomic particles like protons, neutrons and electrons in chapter 3. I came ac... | g8590 | [
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<p>I am working on a point charge simulator, and I was wondering what forces can affect point charges (assuming that they operate in a closed system, with no externally generated light, and initial velocities). I have thought of the following "classes" of forces so far:</p>
<ul>
<li>Electric Field</li>
<li>Magnetic Fi... | g8591 | [
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0.0... |
<p>On bicycles.se a question came up about whether one cyclist drafting another causes the lead cyclist to be slowed down. A contributor suggested that the opposite might be the case, that the leading cyclist would be 'helped' too. Clearly, in the real world of cycling there are winds, potholes, traffic and other varia... | g8592 | [
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<p>I have two vectors $\vec{A}$ and $\vec{B}$ and I need to find the x- and y-components of $\vec{C} = \vec{A} + \vec{B}$. Here's what I have so far:</p>
<p>$$|\vec{A}| = 50.0 \mathrm{m}, \theta = -20.0^\circ$$
$$|\vec{B}| = 70.0 \mathrm{m}, \theta = 50.0^\circ$$</p>
<p>$$C_x = |\vec{A}| \cos (\theta) + |\vec{B}... | g8593 | [
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0.027375... |
<p>Suppose there is a material that interacts with 50% of high-energy neutrinos. To what temperature such material will heat up naturally here on Earth? What power can be extracted from it?</p> | g8594 | [
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<p>If Lagrangian of the motion is </p>
<p>$$\mathcal{L}=\frac{1}{2}m\left(a^2\dot\phi^2+a^2\dot\theta^2\sin^2\phi\right)+mga\cos\phi,$$ </p>
<p>how can I show that total mechanical energy is conserved? I've read this: </p>
<blockquote>
<p><em>If the time $t$, does not appear [explicitly] in Lagrangian $\mathcal{L... | g8595 | [
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0.039988525211811066,
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0.02042471058666706,
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<p>A speedway turn, with radius of curvature $R$, is banked at an angle $\theta$ above the horizontal. If there is a coefficient of static friction, $\mu_{s}$ between the tires and the track, what are the maximum and minimum speeds at which turn can be taken? </p>
<p>My solution:
Since coefficient of kinetic friction,... | g8596 | [
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<p>I recently read (sorry but I don’t have a reference) that interference is not only about destructive and constructive interference but moving energy from destructive to constructive regions according to conservation of energy. The place where this really bothers me is when light is being transmitted through glass. A... | g8597 | [
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<p>This is a question of topological insulator.</p>
<p>Liang Fu and C. L. Kane proposed a method to judge whether an inversion symmetric insulator is a topological insulator or not in their article(<strong>L. Fu and C.L. Kane, Phys. Rev. B 76, 045302 (2007)</strong>).
The method is just to determine the parity of the ... | g8598 | [
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<p>Given the following 16 matrix multiplications of the Dirac gamma matrices
\begin{align}
G_{\mu\nu}
=
\dfrac{1}{2}
\begin{pmatrix}
I && \gamma_{0} && i\gamma_{123} && i\gamma_{0123}
\\
i\gamma_{23} && i\gamma_{023} && i\gamma_{1} && \gamma_{01}
\\
i\gamma_{31} &&... | g8599 | [
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0.07441127300262451,
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0.01370739471167326,
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0.03753719851374626,
-0.008731572888791561,
-0.... |
<p>I've heard that special relativity makes the concept of magnetic fields irrelevant, replacing them with relativistic effects between charges moving in different velocity frames. Is this true? If so, how does this work?</p>
<p>[Temporary note, to be deleted later: This is my first attempt to compose a self-answered ... | g8600 | [
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<p>In the Standard Model, the Higgs mass doesn't really have any theoretical constraints. It could have basically any value and nothing 'breaks'.</p>
<p>However, in MSSM models, we often see the tree level constraint
$$m_h < m_z \cos(2\beta)$$
which is modified by 1-loop stop corrections.</p>
<p>What is happeni... | g8601 | [
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0.0206013061106205,
0.10302... |
<p>Inspired by <a href="http://physics.stackexchange.com/q/6419/3816">this question</a> and specifically <a href="http://physics.stackexchange.com/questions/6419/electric-engine-transmission/6425#6425">this answer to it</a>.</p>
<p>From my experience there's always some very specific limit to how much torque an electr... | g8602 | [
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<p>I am trying to learn about radioactive energies and wonder if, because these also seem to come under the topic of radiation, can these energies become electromagnetic. I'm pretty much a beginner, so if you could keep the answer pretty simple, it would help.</p> | g8603 | [
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<p>In Arnold's <em>Mathematical Methods of Classical Mechanics,</em> he derives the Hamilton-Jacobi equation (HJE) using a generating function $S_1(Q, q)$ to get</p>
<p>$$
H\left(\frac{\partial S_1(Q, q)}{\partial q}, q, t \right) ~=~ K(Q, t).
$$</p>
<p>However, this is different from what I've seen in other physics ... | g8604 | [
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<p>I was reading the lecture notes from</p>
<p><a href="http://nic.desy.de/sites2009/site_nic/content/e44192/e62778/e91179/e91180/hmc_tutorial_eng.pdf" rel="nofollow">http://nic.desy.de/sites2009/site_nic/content/e44192/e62778/e91179/e91180/hmc_tutorial_eng.pdf</a></p>
<p>I am a little bit confused how points on the ... | g8605 | [
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-0.... |
<p>Quick question regarding superficial degrees of freedom and Ward identities.</p>
<p>For instance in Peskin and Schroeder it is stated that the photon-self energy is superficially quadratically UV divergent but due to the Ward identity it is only logarithmically divergent. I don't see this argument.</p>
<p>The self... | g8606 | [
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<p>A black hole collapses and emits Hawking radiation with average wavelength 1/M. The observer at I^+ sees it, so do the local observers (infalling or hovering). None of these cases should violate the equivalence principle. What is the size of the 'local' patch associated with the observer, the size beyond which he is... | g8607 | [
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0.0... |
<p>Just what the title states please... and perhaps naive too... </p>
<p>Wikipedia pegs the mass of our Earth at 5.9722 × 10^24 kg.</p>
<p>Does this figure include the mass of the Atmosphere?</p> | g8608 | [
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0.009976739063858986,
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-0.019457755610346794,
-0.03422558307647705,
-0.08626540005207062,
-0.053328730165958405,
0.0... |
<p>Newton's theory explained the changes in sea level by the
effect of gravity (moon/sun).
Now we are trying to detect gravitational waves from distant
cosmic surces.
But, apart from the differences in "signal frequency"
(extremely small in case of sea effects (10-4 / 10-5 Hz or so),
maybe higher in case of "signals... | g8609 | [
0.02811599150300026,
0.02210230566561222,
-0.005147070158272982,
-0.005549419205635786,
0.02191590517759323,
0.05020999535918236,
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-0.02191803604364395,
0.005495550110936165,
0.04948804900050163,
0.07175637036561966,
0.06261... |
<p>During my visit to the doctors', I recollect having seen the sign which says </p>
<blockquote>
<p>Please remove all metallic objects from the body before the commencement of the X-Ray</p>
</blockquote>
<p>How do metallic objects interfere with X-Rays? Don't they have metallic implants for certain kind of surgeri... | g8610 | [
0.04763282090425491,
0.09127433598041534,
0.03452448546886444,
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0.024147825315594673,
0.031377099454402924,
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0.048263248056173325,
0.007172227371484041,
0.02154836244881153,
0.03753485903143883,
0.09097596257925034,
-0.025396008044481277,
-0.0471... |
<p>Are there any applications of quantum information theory to physics?</p> | g8611 | [
0.022540012374520302,
0.05269269645214081,
0.009509338065981865,
0.00210983376018703,
0.02546263299882412,
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-0.01610567979514599,
0.0338752381503582,
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-0.049290210008621216,
0.0056... |
<p>It is known that when a conductor is placed in an electric field, the charges redistribute themselves such that $E=0$ inside the conductor. I was also told that the same is NOT true for the 2D and 1D analogues. Why is this true? And what happens in these cases?</p>
<p>Thank you.</p> | g8612 | [
0.0499308779835701,
0.015488740056753159,
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0.09258418530225754,
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0.0012436240212991834,
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0.004957635886967182,
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0.0048891580663621426,
-0.022079885005950928,
0... |
<p>I was wondering if there's an underlying physical reason why detection in microphones and antennas is a cardioid, or if it's just that a cardioid happens to be the mathematical object that best approximates the shape of the detection pattern.</p>
<p>In other words, is there an "ab initio" reason, or it's an empiric... | g8613 | [
0.06396395713090897,
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0.010388714261353016,
0.05442609637975693,
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-0.03528216481208801,
-0.021534590050578117,
0.037812791764736176,
0.04405934736132622,
... |
<p>I'm reading about the auto-refrigeration effect and can't find a really good explanation. Is the idea that when you have your condensed liquid, and then release it into a low pressure environment, some of it evaporates instantly, leaving behind a cold liquid?</p>
<p>Let me explain what I'm thinking again: pressuriz... | g8614 | [
0.08235251903533936,
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0.010115571320056915,
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0.04986340552568436,
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0.057661786675453186,
0.030066020786762238,
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-0.01613798178732395,
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-0.021883340552449226,
-0.... |
<p>I have been reading the article review for Optical Quantum Memory (<a href="http://arxiv.org/abs/1002.4659" rel="nofollow">http://arxiv.org/abs/1002.4659</a>) when I came across this section about <a href="http://www.google.com/search?as_q=dlcz+protocol" rel="nofollow">DLCZ protocol</a>. I don't understand about Bea... | g8615 | [
-0.02552635222673416,
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0.006450786720961332,
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0.008959664031863213,
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-0.0... |
<p>Imagine $\xi_{\nu}$ is a <a href="http://en.wikipedia.org/wiki/Killing_vector_field" rel="nofollow">Killing vector field</a> on a manifold. Does $\xi_{\nu}\xi^{\nu}$ remain constant along any isometric curve defined by the Killing vector field? </p>
<p>My guess is that yes since as you move along an isometric curv... | g8616 | [
0.03065609745681286,
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0.0005797467310912907,
0.028737250715494156,
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-0.028694355860352516,
0.021292978897690773,
0.022106783464550972,
-0... |
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