question stringlengths 37 38.8k | group_id int64 0 74.5k |
|---|---|
<p>I know that the maximum shear stress </p>
<p>$$\tau = \frac{T}{J}\rho$$</p>
<p>where $\rho$ is the radial distance from the center of the cross section. I have also determined the torsional constant $J$, which is equal to $\frac{\pi}{2}(R_o^4-R_i^4)$ for this particular cross section. The following boundary condit... | 1,590 |
<p>Frequently when trying to solve cosmology questions physicists turn to computer simulations of the universe (albeit massively simplified) in order to verify or disprove their hypotheses. This got me thinking.</p>
<p>My question is about the theoretical maximum possible complexity of these systems.</p>
<p>Let me gi... | 1,591 |
<p>I'm reading through Novotny and Hecht's book on Nano-Optics (Principles of Nano-Optics), and I've come across a subtlety in the boundary conditions for evanescent wave generation (via total internal reflection) that I don't understand.</p>
<p>In the 2nd edition of the book the equation I'm struggling with is labell... | 1,592 |
<p>As far as I understand it the photon propagator, $P(A\rightarrow B)$, described in Feynman's QED book, gives the amplitude that a photon moves from spacetime point A to spacetime point B.</p>
<p>I was wondering if in quantum field theory terms $P(A\rightarrow B)$ is made up of the product of the following two ampli... | 1,593 |
<p>Although I have informed myself in string theory through reading books and watching videos, I do not know or understand what superstrings are made of. History has definitely shown us that when we think something is fundemental, there is always something smaller.</p>
<p>What are superstrings made of?
Can we go small... | 24 |
<p>2+1D lattice gauge theory can emerge in a spin system through fractionalization. Usually if the gauge structure is broken down to $\mathbb{Z}_N$, it is believed that the fractionalized spinons are deconfined. However in general, $\mathbb{Z}_N$ gauge theory also have a confined phase. The question is how to determine... | 1,594 |
<p>I read that the field that have simultaneously minimum divergence and minimal transversal extension is the fundamental Gaussian mode. What are the good references in this subject, to prove the assertion above?</p> | 1,595 |
<p>I know how to derive below equations found on <a href="http://en.wikipedia.org/wiki/Creation_and_annihilation_operators#cite_note-4" rel="nofollow">wikipedia</a> and have done it myselt too: </p>
<p>\begin{align}
\hat{H} &= \hbar \omega \left(\hat{a}^\dagger\hat{a} + \frac{1}{2}\right)\\
\hat{H} &= \hbar \o... | 1,596 |
<p><a href="http://www.google.com/search?as_q=invariable" rel="nofollow">Invariable</a> means which is not variable i.e. can't be changed.</p>
<p>Recently I have seen a sentence when reading a chapter based on measurement:</p>
<blockquote>
<p><em>The accepted standards must be accessible to those who need to calibr... | 1,597 |
<p>Studying quantum mechanics, I've found an exercise I don't know how to solve it. Given the radial Schrödinger equation, </p>
<p>$$\left [ \frac{d^2}{dr^2}+k^2-\frac{2m}{\hbar^2}\lambda U\left ( r \right ) \right ]\psi_\lambda {\left ( r \right )}= 0$$</p>
<p>and doing whatever integrals are required, I have to sho... | 1,598 |
<p>Let's say you have two dipole type antennas. Antenna A has a gain of 2.15 dBi, a horizontal beam width of 360 deg and a vertical beam width of 45 deg. Antenna B is similar to antenna A, but has a horizontal beam width of 360 deg and a vertical beam width of 42 deg. Can you use the ratio of the vertical beam widths t... | 1,599 |
<p>How much memory would we need to represent a human? How would each atom be stored as? Bytes? Something more complex?</p> | 1,600 |
<p>The Feynman lectures are universally admired, it seems, but also a half-century old.
Taking them as a source for self-study, what compensation for their age, if any, should today's reader undertake? I'm interested both in pointers to particular topics where the physics itself is out-of-date, or topics where the ped... | 997 |
<p>Ok, although this question arises out of the global warming debate, this is a question purely for physicists and not intended to branch into that particular debate.</p>
<p>We are told that LWIR from the Earth to the atmosphere is absorbed by greenhouse gasses and then re-radiated (in the form of backradiation) part... | 25 |
<p>If I look through the microwave window I can see through, which means visible radiation can get out. We know also that there is a mesh on the microwave window which prevents microwave from coming out.</p>
<p>My question is how does this work? how come making stripes or mesh of metals can attenuate microwave radiati... | 1,601 |
<p>My apologies if this question has already been answered. Using the notation of Sakurai, we have an energy eigenket N and an eigenvalue n.</p>
<p>$$N | n \rangle = n | n \rangle$$</p>
<p>For the number operator $N = a^\dagger a $. My question is precisely why does n have to be an integer. I do understand that it mu... | 1,602 |
<p>Can someone show me, without glossing over anything, why $F = E - TS$ is minimized when $p_i = e^{-U_i/k_bT}/\sum_ie^{-U_i/k_bT}$? I understand it conceptually, but am having difficulty showing it formally.</p> | 1,603 |
<p>On my textbook is written that gravitational force is the force that attracts bodies with mass. But I've seen on a book that It actually attracts bodies with energy. I'm having a class tomorrow and I would like to know some argumments to use with - against my professor.</p> | 1,604 |
<p><a href="http://physics.stackexchange.com/questions/8626/if-all-conserved-quantities-of-a-system-are-known-can-they-be-explained-by-symme">This question</a> resulted, rather as by-product, the discussion on how to count degrees of freedom (DOF). I extend that question here:</p>
<ul>
<li>Are necessary<sup>1</sup> de... | 1,605 |
<p>Is there a formal definition of <a href="http://en.wikipedia.org/wiki/Drag_%28physics%29" rel="nofollow">drag</a>, say, as some surface integral of normal and shear forces? There seem to be a lot of formulas for specific cases, but is there a general one?</p>
<p>I need to accurately calculate the drag of three cyli... | 1,606 |
<p>The world is full of nuclear warheads being stockpiled. Controlled fusion power seems a long way away. Could we put these warheads to better use by exploding them in a controlled way and capturing the energy they produce?</p>
<p>By useful work I mean the power is then available for the national grid to boil kettl... | 1,607 |
<p>At present the only way we can produce anti-matter is through high powered collisions. New matter is created from the energy produced in these collisions and some of them are anti-matter particles such as positrons, anti-protons etc.</p>
<p>My question is, normal matter and anti-matter are so similar. They both a... | 1,608 |
<p>It is easy to observe that on a windy day, the wind does not blow for several hours at constant speed, then gradually subside. Instead, on a time scale of seconds or tens of seconds, there are stronger gusts of wind followed by lulls.</p>
<p>Presumably this is an effect of turbulence. If so, is this turbulence du... | 1,609 |
<blockquote>
<blockquote>
<blockquote>
<blockquote>
<p>Here the problem states to find the steady state charges on the condensers.<<<<
<img src="http://i.stack.imgur.com/5MwH2.png" alt="enter image description here"></p>
</blockquote>
</blockquote>
</blockquote>
</blo... | 1,610 |
<p>I’m trying to convince my boss that the mixers we are using are too much. I’m trying to prove that we are over-mixing our product. Our product is ink…just your basic ink found in your printer at home. We mix in a 23 inch diameter 50 gallon vessel using a 2.75 inch diameter axial flow impeller going at 1050 rpm with ... | 1,611 |
<p>I would like to write a Gaussian state with density matrix $\rho$ (single mode) as a squeezed, displaced thermal state:
\begin{gather}
\rho = \hat{S}(\zeta) \hat{D}(\alpha) \rho_{\bar{n}} \hat{D}^\dagger(\alpha) \hat{S}^\dagger(\zeta) .
\end{gather}
Here,
\begin{gather}
\rho_{\bar{n}} = \int_{\mathbb{C}} P_{\bar{n}}... | 1,612 |
<p>If we describe spacetime with a Lorentzian manifold, it is always possible to choose a coordinate system such that at any particular point $x^\alpha$, the components of the metric are:
$$ g_{\mu\nu}(x^\alpha) =
\left( \begin{array}{cccc}
1 & 0 & 0 & 0 \\
0 &-1 & 0 & 0 \\
0 & 0 &-1 &a... | 1,613 |
<p><em>[Again I am unsure as to whether this is appropriate for this site since this is again from standard graduate text-books and not research level. Please do not answer the question if you think that this will eventually get closed. Since otherwise the software doesn't allow me to delete questions once it has answe... | 1,614 |
<p>First off, let me just say that I am unsure if this question is appropriate for this site, and if the community deems it necessary, the question should be closed.</p>
<p>So right now I am a fourth year mathematics and physics major whose research interests lie in quantum field theory / quantum gravity. In particul... | 1,615 |
<p>In probability theory, <a href="http://en.wikipedia.org/wiki/Schramm%E2%80%93Loewner_evolution" rel="nofollow">the Schramm–Loewner evolution</a>, also known as stochastic Loewner evolution or SLE, is a conformally invariant stochastic process. It is a family of random planar curves that are generated by solving Loew... | 1,616 |
<p>Are we fooled into thinking that expansion of the universe is accelerating, when in fact, time itself is slowing?</p>
<p>Or if dark energy does exist?</p> | 26 |
<p>I am a little bit perplexed as to how to compute the three-point correlation function for a massive scalar field, I know that it should be equal to zero.</p>
<p>I need to show that: $\lim_{T\rightarrow \infty (1-i\epsilon)} \int D\phi \phi(x_1) \phi(x_2) \phi(x_3) = 0$</p>
<p>How to show this?</p>
<p>Edit: I forg... | 1,617 |
<p>Why are the strong and weak nuclear forces short range?</p>
<p>Are quarks confined or welded together?</p>
<p>Why are elementary particles confined at short range?</p>
<p>Or is color confinement color welding?</p>
<hr>
<p>the quarks are glued together by strong nuclear force, (but not at high energy) it seems t... | 1,618 |
<p>I think of an NMR experiment, but with a single spin half nucleus initially set to the excited state.</p>
<p>When the nucleus finally returns to its ground state, it will emit a photon. An observer in the lab frame will see a photon in the Larmor frequency. An observer in a rotating frame, very close to the Larmor ... | 1,619 |
<p>These facts are taken for granted in a QM text I read. The purportedly guaranteed non-normalizability of eigenfunctions which correspond to a continuous eigenvalue spectrum is only partly justified by the author, who merely states that the non-normalizability is linked to the fact that such eigenfunctions do not ten... | 1,021 |
<p>A 2+1D topological field theory (topologically ordered states), implies that the topological ground state degeneracy (GSD) on $T^2$ torus (2D manifold <strong>without boundary</strong>). For example a level k U(1) Chern-Simons theory implies a GSD$=k$.</p>
<p>If we put the topological field theory on a 2D manifold ... | 1,620 |
<p>Related: <a href="http://physics.stackexchange.com/q/79074/2451">What is meant by boiling off electrons in a heater coil?</a></p>
<p>In the Thomson tube we used in our class to produce an electron beam, the lab manual stated that the tube was filled with a low pressure argon gas. When the electrons collided with th... | 1,621 |
<p>Imagine an eruption of energy/mass $E$ from a singularity $O$, as in a Big Bang. After the energy/mass $E$ is all at more than a distance $d$ from $O$, is it for some value of $d$ possible that there could be a new eruption of energy/mass from $O$, i.e. a new Big Bang? If yes, is there an upper limit to the number o... | 1,622 |
<p>I'm writing my first MD simulation (ever) for liquid Argon. The code is up and running. I am supposed to do the calculations in the <a href="http://en.wikipedia.org/wiki/Microcanonical_ensemble" rel="nofollow">NVE ensemble</a>. Having implemented a 4th order <a href="http://en.wikipedia.org/wiki/Symplectic_integrat... | 1,623 |
<p>I have the following problem to solve:</p>
<blockquote>
<p>A particle of mass $m$ and charge $e$ moves in the laboratory in crossed, static, uniform, electric and magnetic fields. $\mathbf{E}$ is parallel to the $x$-axis; $\mathbf{B}$ is parallel to the $y$-axis. Find the EOM for $|\mathbf{E}|<|\mathbf{B}|$ ... | 1,624 |
<p>Let the free electromagnetic current $J_\mu(x)$ be = $:\bar{\psi}(x)\gamma_\mu Q \psi(x):$ where $::$ is the normal ordering. </p>
<ul>
<li>In this expression why is $Q$ thought of as a "charge operator" instead of just a number?...its quite pesky to keep track of this operator while doing the current-current OPEs ... | 1,625 |
<p>I have this question that I dont know how to solve correctly :</p>
<p><img src="http://i.stack.imgur.com/HsYdc.jpg" alt="enter image description here"></p>
<p>My question is, how do I find $V_B$ ? I will find the angular velocities myself, but I want to know the method to get $V_B$ ?</p>
<p>I know I can start by ... | 1,626 |
<p>I'm confused about the terminology in the two contexts since I can't figure out if they have a similar motivation. Afaik, the definitions state that quantum processes should be very <em>slow</em> to be called <a href="http://en.wikipedia.org/wiki/Adiabatic_theorem">adiabatic</a> while <a href="http://en.wikipedia.or... | 1,627 |
<p>So I was writing a lab report for Physics I, where I was describing projectile motion. I had written </p>
<blockquote>
<p>Since gravity on Earth does not have a horizontal force, the object
will not lose horizontal velocity (velocity in the x direction).</p>
</blockquote>
<p>Then this made me think, is there a... | 1,628 |
<p>As the title suggests, Can the effects of a person's mass upon the local gravitational field be detected and measured remotely?</p>
<p>I am aware any mass produces and effects gravity but couldn't find anything in my searching if it is possible or theoretically possible to detect this effect remotely.</p> | 1,629 |
<p>For my research work i am trying to calculate band gap of zinc oxide theoratically and found this <a href="http://dx.doi.org/10.1002/adma.200304904" rel="nofollow">paper (Determination of the Particle Size Distribution of Quantum Nanocrystals from Absorbance Spectr)</a>. What i am doing is I am making a small matlab... | 1,630 |
<p>If I had a semi infinite, 1-D object and a finite 1-D object, both heated at the same constant rate at one end each for the same time period and both begin at the same initial temperature, is it physically meaningful for me to integrate along the length of the object and consider this integral as a function of time ... | 1,631 |
<p>The discussion on <a href="http://www.physicsforums.com/showthread.php?t=178383" rel="nofollow">this</a> webpage mentions that shining a laser beam at a hair produces an effect like that of the double-slit experiment. Does classical physics predict the effect you observe when you do this (since light is a wave)?</p... | 1,632 |
<p>I am trying to create a machine that moves on two points (Wheels or legs). Because of the extremely difficult nature of perfectly balancing the parts, I am wondering is there any way to create a mechanical mechanism to balance it. I know that it can be done with many different electric circuits, but I am wondering i... | 1,633 |
<p>If you take a giant whale out of the water and put it on land for long enough, <a href="http://books.google.com/books?id=8bFcJgX0lIUC&pg=PA185&lpg=PA185&dq=orca%20crush%20itself%20under%20its%20own%20weight&source=bl&ots=M0cCc-fMyD&sig=gRvN795BpgGYwLIMd9GCEmbvzog&hl=en&sa=X&ei=M36... | 1,634 |
<p>After studying the definition (& derivation) of the potential to an electric field and the Poisson equation I'm currently wondering whether the following is possible:</p>
<ul>
<li>Can one give an example of a physical setup where the Poission equation fails to provide the electric field?</li>
</ul>
<p>I have t... | 1,635 |
<p>I heard at least three claims about the development of the heat of the sun.
In an old book, I read, that nothing dramatically will happen in the next
few billion years. Wikipedia states, that the average temperature will hit
30° in a billion years, 100° a billion years later, and in an internet forum,
someone claime... | 1,636 |
<p>Lorentz length contractions states that the length of any moving object gets divided by the <a href="https://en.wikipedia.org/wiki/Lorentz_factor" rel="nofollow">Lorentz factor</a> equal to the Lorentz factor for that object (always $\geq 1$), equal to
$$
\gamma=\frac{1}{\sqrt { 1-\frac { { v }^{ 2 } }{ { c }^{ 2 } ... | 1,637 |
<p>The most simplified version of the energy equation (which is also the most known) is $E=mc^2$<br>
However, I understand that this <em>only</em> applies to objects with non-zero mass and zero velocity. I also read that "relativistic mass" is equal to the "rest mass" multiplied by the Lorentz factor:
$$
m=\gamma m_0 =... | 1,638 |
<p>I'm quite lost what $B$ and $H$ is. It seams to me that most of the texts I read do quite poor job in explaining them properly. They are explained only in cases when magnetic susceptibility is constant in the space. </p>
<p>First <em>law</em> says that $M = \chi H$, $M$ is magnetization and to my understanding $H$ ... | 1,639 |
<ol>
<li><p>If any configuration of matter can fall into a black hole and hit the singularity, and ditto for the big crunch, and there is time reversal CPT invariance, does it mean anything can pop out of the big bang singularity, and there is unpredictability at the big bang? </p></li>
<li><p>How do we explain the ver... | 1,640 |
<p>I am reading David J. Griffiths and have a problem understanding the concept of discontinuity for E-field.</p>
<p>The E-field has apparently to components. (How does he decompose the vector field into the following?) The tangential and the normal component. The normal component is discontinuous? (Question 1: Why?) ... | 1,641 |
<p>dewitt claimed in his paper </p>
<blockquote>
<p>Bryce S. DeWitt. Quantum theory without electromagnetic potentials, <em>Phys. Rev.</em> <strong>125</strong> no. 6 (1962), pp. 2189-2191, <a href="http://dx.doi.org/10.1103/PhysRev.125.2189" rel="nofollow">DOI: 10.1103/PhysRev.125.2189</a>,</p>
</blockquote>
<p... | 1,642 |
<p>Understanding <a href="http://en.wikipedia.org/wiki/Atmospheric_refraction" rel="nofollow">atmospheric refraction</a>, particularly of ultraviolet, and into the blue part of the visible spectrum is of great interest to me. Although, I have a strong background in trigonometry and geometry, I am very much interested ... | 1,643 |
<p>This is a problem from school. I will show my attempt.</p>
<p>The question:</p>
<p>"The gas constant for dry air R is 287 $\frac{m^2}{s^2*K}$. Assuming the temperature is 330 K and the pressure is 1050 hPa, what is the atmospheric density."</p>
<p>The professor said DO NOT produce an answer by finding a formula, ... | 1,644 |
<p><a href="http://en.wikipedia.org/wiki/Poynting%27s_theorem" rel="nofollow">Poynting's theorem</a> is given by</p>
<p>$$\frac{\partial}{\partial t}\int_{v}Udv
+ \oint_{A}\vec S\cdot \vec {dA}
+\int_{v}\vec E\cdot\vec J dv
=0
$$</p>
<p>Where,</p>
<ul>
<li>the total electromagnetic energy inside the volume v is $U =... | 1,645 |
<p>Can particles emit a virtual Higgs boson in a similar manner to the way a virtual photon is emitted?</p> | 1,646 |
<p>Why did they used to make the mill chimneys so tall?</p>
<p>This question was asked in an Engineering Interview at Cambridge University.</p> | 1,647 |
<p>The context is that I'm building a simulation of a starfield, as seen from a relativistic spaceship. (EDIT: the simulation can now be found <a href="http://tinyurl.com/pt8h6j4" rel="nofollow">here</a>.)</p>
<p>One reference that I'm using is this paper by John M. McKinley (1980) "Relativistic transformation of soli... | 1,648 |
<p>Car batteries are usually 12 V. What is the difference between buying a car battery and hooking up a bunch of cheap household batteries in series? Both would register at 12 V. I assume that cars need much more current to start an engine then regular household things like lamps and toys. Does that mean that a car bat... | 1,649 |
<p>So, we're fairly sure that the universe is infinitely large. We're also fairly sure that it is expanding. My question is how do these two facts relate to the multiverse? My favorite interpretation was given by Michio Kaku, who stated that in the multiverse each individual universe is like a bubble, and all of these ... | 1,650 |
<p>I'm mostly wondering about radio frequencies. I understand that voltage is the movement of electrons, and that the antenna acts as a light bulb, emitting at radio frequencies, following the reverse square law, some materials are opaque, some are transparent. Yet, at the receiver end, it's almost the same as having ... | 1,651 |
<p>I am learning about nuclear fissions and learned about the fission fragment distribution.
It was interesting to see that the fission fragments have unequal masses.</p>
<p>I was wondering as to what governs which fission products are generated in each fission?</p>
<p>What parameters decide which fission products wi... | 1,652 |
<p>The name of the question is rather contradictory and counter-intuitive since sound is <em>produced</em> by vibration. However, very low frequencies around 32Hz and receding are bass. </p>
<p>From what I have read online, you can feel frequencies below 20Hz which is what we normally hear, yet you can hear as low as ... | 1,653 |
<p>So in this Youtube video <a href="http://youtu.be/ZjaaSUHG7Xo" rel="nofollow">http://youtu.be/ZjaaSUHG7Xo</a>
The guy says, around 6:15, that the total energy of the universe is zero because of gravitational potential (or something like that i dont understand completely which is why I'm asking this question) so sinc... | 27 |
<p>Before I am inundated by myriad and vociferous claims that conservation of energy is the single most well-attested and experimentally verified principle in all of science, let me say that I am well aware of the ever-growing body of evidence which seemingly bears this principle out. However there seems to be, in my v... | 1,654 |
<p>This <a href="http://arxiv.org/pdf/hep-th/9303048v2.pdf" rel="nofollow">paper</a> describes a way to find the entanglement entropy of $N$ entangled harmonic oscillators, after tracing out the first $n$. A few statement made within have royally confused me, and I haven't been able to find a solution.</p>
<p>It start... | 1,655 |
<h2>Background</h2>
<p>Let me start this question by a long introduction, because I assume that only few readers will be familiar with <a href="http://en.wikipedia.org/wiki/Coherence_theory" rel="nofollow">the theory of partial coherent light</a> and concepts like a mutual coherence function or a mutual intensity. The... | 1,656 |
<p>I've been trying to find papers that list various properties of idealized cigarette smoke( ex. the molar mass, gas compressibility factor, density of smoke exiting a cigarette) but I have had very little luck.</p>
<p>Does anyone know what these properties are or where I should be able to find them?</p>
<p>I'm buil... | 1,657 |
<p>I would like some help to find</p>
<p>good and detailed books on the history of physics.</p>
<p>Which are the classics in this domain? Which are your favorite?</p> | 1,658 |
<p>I've just begun learning capacitance, and my lecture notes have a section on calculating capacitance for capacitors in vacuum of various shapes, e.g. two parallel plates and concentric spherical shells.</p>
<p><img src="http://i.stack.imgur.com/0s7wd.png" alt="the cylindrical capacitor"></p>
<p>For a cylindrical c... | 1,659 |
<p>I think of <a href="http://en.wikipedia.org/wiki/Coulomb%27s_constant" rel="nofollow">Coulomb's constant</a> as a conversion factor (not sure if this is correct).
Kind of like how you would do calculations in kg and then times it by the conversion constant to convert your answer to pounds.
The conversion factor woul... | 1,660 |
<p>Suppose you have a quantum system with a Hamiltonian having some number (greater than 2, possibly infinite) of eigenfunctions, and that the system is prepared in the ground state.</p>
<p>When can you approximate it as by two-level system (using just the ground state and first excited state)? Is there some property ... | 1,661 |
<p>I'm trying to calculate the 'instantaneous' semi-major axis of a binary system with two equal (known) mass stars for an $N$-body simulation. I know their velocities and positions at a given time, but am unsure how best to calculate the semi-major axis. I tried using the <a href="http://en.wikipedia.org/wiki/Vis-viv... | 1,662 |
<p>Are there any ways to convert an <a href="http://en.wikipedia.org/wiki/Autocorrelation" rel="nofollow">autocorrelation function</a> to a <a href="http://en.wikipedia.org/wiki/Radial_distribution_function" rel="nofollow">pair correlation function</a>, and vice versa?</p> | 1,663 |
<p>If a gas, such as hydrogen, is pressurized into an air tight container, a force in terms of pascals (or whatever unit you want to use) is exerted, correct? That is what pushes against every surface within the container. But what I don't understand is how the gas can constantly push against the walls without being su... | 1,664 |
<p>I worked in my masters thesis with $^{87}Rb$ and $^{40}K$, really small beta emitters. But there are so many other things around in the lab, that I want to keep track on all the things I might get in contact with.</p>
<p>Is there any computer program to calculate the dose of the whole decay chain to get a picture o... | 1,665 |
<p>So I know re-normalization has bean "beaten to death". I want to understand something a bit specific which might seem trivial. Independence of the bare parameters on $\mu$ and relevance to the beta function derivation. This seems to be an important assumption or rather fact that is built upon in making further reno... | 1,666 |
<p>Just a simple question regarding $\epsilon$, the absolute permittivity. I read that it measures the resistance to of certain medium to 'permit' the formation of an electrical field. Does this mean that given a electrical field $E$, one needs more energy to produce it in a medium with a higher $\epsilon$ ? Thanks in ... | 1,667 |
<p>The atoms in my table "stick together" to form a rectangle. Why? What makes them stick together?</p>
<p>I know about ionic/covalent bonding etc., but consider a sheet of pure iron. Just atoms of one element. The atoms still stick together to form a sheet instead of being "everywhere".But if I place ten balls on the... | 1,668 |
<p>Say you have a neutral rod, and you bring a positively charged rod beside it (call the side the charged rod is brought near side A and the other side side B). The electrons from the side B will start moving towards side A and the positively charged nuclei in side A will start moving to side B. </p>
<p>After a large... | 1,669 |
<p>I would like to have a general interpretation of the coefficients of the stiffness matrix that appears in <a href="http://en.wikipedia.org/wiki/Finite_element_method">FEM</a>. For instance if we are solving a linear elasticity problem and we modelize the relation between a node $i$ and a node $j$ as a spring system,... | 1,670 |
<p>I've seen the Kaluza-Klein metric presented in two different ways., cf. Refs. 1 and 2. </p>
<ol>
<li><p>In one, there is a constant as well as an additional scalar field introduced:
$$\tilde{g}_{AB}=\begin{pmatrix}
g_{\mu \nu}+k_1^2\phi^2 A_\mu A_\nu & k_1\phi^2 A_\mu \\
k_1\phi^2 A_\nu & \phi^2
\end{pmatr... | 1,671 |
<p>Tensor equations are supposed to stay invariant in <em>form</em> wrt coordinate transformations where the metric is preserved. It is important to take note of the fact that invariance in form of the tensor equations is consistent with the fact that the individual components of the tensor may change on passing from o... | 1,672 |
<p>My general premise is that I want to investigate the transformations between two distinct sets of vertices on n-dimensional manifolds and then find applications to theoretical physics by:</p>
<ul>
<li><p>Minimalizing the change in the fundamental shape of the vertices (I want a convex polytope to stay a convex poly... | 1,673 |
<p>I am looking for some references which discuss Fourier transform methods in GR. Specifically supposing you have a metric $g_{\mu \nu}(x)$ and its Fourier transform $\tilde{g}_{\mu \nu}(k)$, what does this tell you about the Fourier transform of the inverse metric $\tilde{g}^{\mu \nu}(k)$ or the Riemann tensor $\tild... | 1,674 |
<p>A symmetry is anomalous when the path-integral measure does not respect it. One way this manifests itself is in the inability to regularize certain diagrams containing fermion loops in a way compatible with the symmetry. Specifically, it seems that the effect is completely determined by studying 1-loop diagrams. ... | 1,675 |
<p>What lessons do we have from string theory regarding the fate of singularities in general relativity?</p>
<p>What happens to black hole singularities? What happens to cosmological singularities?</p>
<p>Which points of view on string theory yielded results in this respect? String field theory? AdS/CFT? Matrix theor... | 1,676 |
<p>Consider the following scenario: On earth, I pulse a laser focused at a far away mirror such that the time it takes for the light to reflect off the mirror and arrive back at me is at least a few seconds. Immediately after the laser pulse, I place a second mirror where the reflected beam will arrive. In the photo... | 1,677 |
<p>A few months ago <a href="http://physics.stackexchange.com/q/78442/25794">I asked about phonons</a>. I got some very good answers but I still have difficulty getting an intuition for phonons, while somehow photons, which in many ways are similar and which I realize I hardly understand anything about, seem more acces... | 1,678 |
<p>In quantum mechanics, we talk about (1) vectors, (2) states, and (3) ensembles (e.g., a beam in a particle accelerator). Suppose we want to translate this into mathematical definitions. If I'd never heard of the von Neumann density matrix, I'd approach this problem as follows. Two vectors can represent the same stat... | 1,679 |
<p>I recently came across two nice papers on the foundations of quantum mechancis, Aaronson 2004 and Hardy 2001. Aaronson makes the statement, which was new to me, that nonlinearity in QM leads to superluminal signaling (as well as the solvability of hard problems in computer science by a nonlinear quantum computer). C... | 1,680 |
<p>Einstein postulated that the speed of light in free space
is the same for all observers, regardless of their motion
relative to the light source, where we may think of an
observer as an <em>imaginary entity</em> with a sophisticated set
of measurement devices, <em>at rest with respect to itself</em>,
that perfectly ... | 1,681 |
<p>We can see diffraction of light if we allow light to pass through a slit, but why doesn't diffraction occur if we obstruct light using some other object, say a block? Why are shadows formed? Why doesn't light diffract around the obstruction as it does around the slit?</p> | 1,682 |
<p>I have a friend who has just show me his medical prescription for hyperopia (farsightedness) correction and he needs glasses with 4,25 diopters for that, which seemed to be weird for me because I had learned, from the mirror equation, that the maximum correction possible for hyperopia is 4 diopters:</p>
<p>$$
\frac... | 1,683 |
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