question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>The title largely sums up my question, what does happen if you either x-ray an x-ray, or point two x-ray generators at each other?</p> | g18770 | [
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<p>When considering fundamental particles as waves in fields, it seems like any collision of two particles of some fundamental type could only create energy within that type's field. Why do we expect that this energy is transferred to a different field, creating a different fundamental particle? What is the mechanism o... | g18771 | [
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<p>I am currently working through Brian Greene's "World Science U" course on special relativity, and I have a question regarding one of the calculations performed for an exercise on time dilation (MODULE 12: Time Dilation: Examples - Problem 1). The question is as follows:</p>
<blockquote>
<p>Imagine that today is ... | g18772 | [
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<p>Just what the title says. </p>
<ol>
<li><p>What's the basic definition of an <a href="http://en.wikipedia.org/wiki/Orbifold">orbifold</a>? </p></li>
<li><p>How do they arise in physics and why are they interesting?</p></li>
</ol> | g18773 | [
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<p>Let's say you have a charged, isolated parallel plate capacitor with a certain voltage. My book says that if you insert a dielectric, the voltage drops. However, that would require the energy between the plates to go down. What is this energy converted to? </p> | g18774 | [
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<p>I find Anderson's <a href="http://dx.doi.org/10.1103%2FPhysRev.109.1492" rel="nofollow">original paper</a> too terse. I am looking for something that introduces me gently to the subject so that I can understand Anderson's paper and other literature. What references are out there that introduce <a href="http://en.wik... | g18775 | [
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<p>A small car that we made in class was travelling and its time was recorded at 1,2,3, and 4 meters. The points in the form (seconds, meters) are as follows:</p>
<pre><code>pt1 = (2.13,1)
pt2 = (3.48,2)
pt3 = (5.09,3)
pt4 = (6.75,4)
</code></pre>
<p>The question is to determine the distance traveled after 3.48 s, f... | g18776 | [
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<p>I have come across a weird integration during derivation of relativistic kinetic energy. Our professor states that i can get RHS out of LHS using integration by parts: </p>
<p>$$
\int\limits_0^x \! \frac{d}{d t}\Big[ mv \gamma(v)\Big]\,\, d x = v \!\cdot\! mv \gamma(v) - \int\limits_0^v \! m v \gamma(v)\, dv
$$</p>... | g18777 | [
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<p>Given a spin 1/2 particle in state $|\alpha\rangle=\begin{bmatrix}a \\b\end{bmatrix}$, what is the probability of it being measured in the $S_{y+}$ state. Is this equivalent to, if $S_y$ is measured on this particle, what is the probability the result being $\hbar/2$? I think it's supposed to be like $|\langle S_{y... | g18778 | [
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<p>I have two questions one practical and one theoretical. Even though I have a decent understanding of superconductivity both phenomenological as well as theoretical (i.e. BCS), some things just slipped through the cracks.</p>
<p>First question: Say you have a decent quality superconducting ring completely isolated f... | g18779 | [
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<p>Binary systems emit gravitational radiation. This causes the system to lose energy, which results in a shrinking of the semi-major axis. I have read on countless occasions that this 'inspiral' is <em>adiabatic</em> (<a href="http://arxiv.org/abs/gr-qc/0001013" rel="nofollow">here for example</a>). What does this mea... | g18780 | [
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<p>What is the expression for the period of a <a href="http://en.wikipedia.org/wiki/Pendulum#Compound_pendulum">physical pendulum</a> without the $\sin\theta\approx\theta$ approximation? i.e. a pendulum described by this equation:
$$
mgd\sin(\theta)=-I\ddot\theta
$$</p>
<p><strong>Motivation for my question:</strong><... | g18781 | [
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<p>It has been proposed that the Sun's gravitational lens be used to observe distant objects, but according to relativity a spacecraft would need to be 550 AU away in order to take advantage of the effect.
Would it be possible to instead slow down the light while it is being deflected using, for example, a Bose-Einstei... | g18782 | [
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<p>Proton and electron have a distance equal to the Bohr radius apart in the hydrogen atom. Knowing this, what's the ionization energy of the atom?</p>
<p>So we know $U = -kq^2/a_B$, but this is potential energy, not ionization. How should I proceed?</p> | g18783 | [
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<p>I am currently taking a course in General Relativity, and I've hit a bit of a roadblock with a homework assignment.</p>
<p>We are given the metric for Einstein's universe to be (forgive me, this is meant to be formatted for LaTeX, but I'm new at this whole stackexchange business) $ds^2 = c^2 dt^2 - \frac{1}{1-kr^2}... | g18784 | [
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<p>I understand that <em>information</em> cannot be transmitted at a velocity greater than speed of light. I think of this in terms of the radio broadcast: the station sends out carrier frequencies $\omega_c$ but the actual content is carried in the modulated wave, the sidebands $\omega_c\pm\omega_m$. The modulation en... | g18785 | [
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<p>The shock wave I'm interested in occurs for a special <a href="http://en.wikipedia.org/wiki/Riemann_problem" rel="nofollow">Riemann problem</a>, known as
<strong>1D Noh's problem</strong>.</p>
<p>It means basically that two streams of air (equal density) are colliding at $x = 0$ for $t = 0$; I will show my calculat... | g18786 | [
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<p>I have two (maybe equal) question <br />
1- Is this possible to send a spacecraft to the moon without any acceleration(Constant speed)?
<br>
2-Is this possible to throw a ball on the top of tree without any acceleration(Constant speed)?
<br>
<br>
Thanks</p> | g18787 | [
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<p>I'm wondering what kind of astronomical or geological events would cause the reversal of Earth's rotation. For instance, Is a meteorite passing very close to Earth able to reverse its rotation?
Can the presence of Earth inside extreme magnetic field cause such event? </p> | g18788 | [
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<p>I am interested in whether there is a field theoretic description (there is, so what is it?) of the tensor product (aka density matrix) model of open quantum systems. In particular, I am interested in how QFT might express the model of decoherence where the environment has some probability per time of "measuring" th... | g18789 | [
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<p>I am working with certain input-output maps that can be thought of as large RLC-networks. I thought maybe this might be a place to get some thoughts/ideas/answers.</p>
<p>My basic question is, given some large connected RLC network (all linear and ideal elements) and two ports in the network, say 'a' and 'b', can ... | g18790 | [
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<p>The expectation value of momentum is given by:</p>
<p>$$
\langle p\rangle = \int_{-\infty}^{\infty}\psi^{*}(x)\left(-i\hbar\frac{\partial}{\partial x}\right)\psi(x)dx
$$</p>
<p>How can I show that the above expression is equivalent to this?
$$
\langle p\rangle = \int_{-\infty}^{\infty}p|\tilde\psi(p)|^{2}dp
$$</p>... | g18791 | [
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<p><strong>Background</strong></p>
<p>Ive spent almost 7 hours researching about li-ion batteries because my Galaxy S2 only lasts 2 hours with all turned off and only 40 apps installed (removed system apps and bloatware) and screen brightness below lowest normally possible in settings (I use ScreenFilter and set its o... | g18792 | [
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<p>I am trying to understand diagrams that involve showing phonon scattering processes that contribute to raman spectroscopy peaks.</p>
<p>For example, I drew the one at the bottom of this post. This is supposed to the scattering process responsible for the D peak in graphene. I'm trying to understand exactly what the... | g18793 | [
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<p>I'm trying to computationally find the refractive indices (real and imaginary) for a thin slab suspended in air (so the only indices to deal with are air and my material's). I've experimentally taken transmission and reflection measurements of intensity in a certain wavelength range.</p>
<p>From a textbook chapter ... | g18794 | [
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<p>My understanding of Kolmogorov scales doesn't really go beyond this poem:</p>
<blockquote>
<p><em>Big whirls have little whirls that feed on their velocity, and little whirls have lesser whirls and so on to viscosity.</em> - <a href="http://en.wikipedia.org/wiki/Lewis_Fry_Richardson">Lewis Fry Richardson</a></p... | g18795 | [
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<pre><code>^
|
|
|
| +
| +|
| + |
| + |
| + |
| + |
| + |
| + |
| + |
| + |
|+ |
+----------------------------->
o x1
</... | g18796 | [
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<p>The dielectric constant of water is very high. Then why is it not used as a dielectric in the condenser?</p> | g18797 | [
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<p>All of the physics books that I've seen which discuss General Relativity do so in terms of coordinates - the tensor calculus - even though the naturally relevant entities are invariant under general coordinate change. Modern Differential geometry dispenses with coordinates. </p>
<p>Is there a book out there that di... | g390 | [
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<p>I have been taught that a perpendicular force acting on an object will only change the direction of its velocity, not its magnitude. The explanation that was provided to me is that because there no force in the direction of its velocity, there must be no work done and thus change in the object's kinetic energy. Howe... | g18798 | [
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<p>Similar questions have been asked before, but this one extends the scope of interpretation and applications. </p>
<p>Let us ask the question: What will happen if we have a laser gun that produces a high intensity laser beam, of the order $10^{15} Wm^{-2}$ say, and frequency about $4\times 10^{14}Hz$, and shoot a... | g18799 | [
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<p><a href="http://www.ptcri.ox.ac.uk/research/beam.shtml" rel="nofollow">PAMELA</a> is a particle accelerator which have two concentric rings, protons are accelerated in the inside ring.
At <a href="http://en.wikipedia.org/wiki/ISIS_neutron_source" rel="nofollow">ISIS</a> muons are produced when a 800 MeV proton beam ... | g18800 | [
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<p>What is the fundamental reason to fix the <a href="http://en.wikipedia.org/wiki/Lorenz_gauge_condition" rel="nofollow">Lorenz gauge</a> to $0$?</p> | g18801 | [
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<p>In Einstein's General Relativity we relate the effects of gravity with the curvature of the Levi-Civita connection on the spacetime manifold. Also, when we get the electromagnetic tensor $F = dA$ where $A$ is the $4$-potential, I've heard it's possible to show that $F$ is the curvature $2$-form on a certain principa... | g18802 | [
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<p>From <a href="http://www.hindustantimes.com/HTNext/LifeAndUniverse/Curiosity-sends-data-about-Martian-surface/Article1-918611.aspx" rel="nofollow">this article on hindustantimes.com - Curiosity sends data about Martian surface</a>:</p>
<blockquote>
<p>The resultant flash of glowing plasma is viewed by the system’... | g18803 | [
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<p>I'am reading John Taylor's Classical Mechanics chapter 1 page 20 where he proves the principle of conservation of momentum which states "If the net external force $F^{ext}$ on an $N$-particle system is zero,the system's total momentum $P$ is constant.</p>
<p>I'am not sure if i'am having trouble understanding the us... | g18804 | [
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<p>I am preparing for the exam. And I need to know the answer to one question which I can't understand.</p>
<p>"Give an example of non-Hamiltonian systems: in case of infinite number of particles; for a finite number of particles".</p>
<p>I hope somebody can help me. Thank you.</p> | g798 | [
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<p>Wikipedia indicates the approximate energy released by an earthquake relative to the <a href="http://en.wikipedia.org/wiki/Richter_magnitude_scale" rel="nofollow">destructive power of TNT</a>
With reasonably large quakes (> 4.0 on the Richter Scale), the energy dissipated is of the order of Giga-joules. If a seam (U... | g18805 | [
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<p>I've <a href="http://physics.stackexchange.com/questions/30125/how-do-kolmogorov-scales-work-in-shear-thinning-fluis">asked about microscales</a> before, and thinking more about the problem turned up more basic questions. Consider the length scale of a microscale:
$$ \eta = \left(\frac{\nu^3}{\epsilon}\right)^\frac{... | g18806 | [
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<p>I wanted to know if, since basic chemistry teaches you that states of matter can be changed, I was wondering particularly about plasma.</p>
<p>I know that virtually all of the <a href="http://en.wikipedia.org/wiki/Sun" rel="nofollow">Sun</a> is plasma, so I was wondering if it's possible to de-ionize the sun, and c... | g18807 | [
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<p>I have tried to find a retarded and advanced Green function for Proca field equation.</p>
<h2>$(\Box - \mu^2)A^{\mu}=J^{\mu}$</h2>
<p>where $\mu$ is the mass term.</p>
<p>How I did it:</p>
<p>first: I made Fourier Transformation for $A^{\mu}$ and its inverse, I got it:</p>
<h2>$Ã^{\mu}(k)=\int d⁴x e^{ikx}A^{\mu... | g18808 | [
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<p>Einstein's big revelation was that time and space are inseparable components of the same fabric. Physical observation tells us that distant galaxies are moving away from us at an accelerated rate, and because of the difficulty (impossibility?) of defining a coordinate system where things have well defined coordinate... | g274 | [
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<p>Are length contraction and time dilation real physical phenomena, or are they just a measurement issue? </p> | g18809 | [
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<p>I was wondering why a lightning bolt is coined as "plasma", or a "spark" from an electrical wire/device is as well, yet flares, molten lava, and burning buildings are not(flares are pyrotechnic, exothermic, and very much like flames).</p>
<p>I hear arguments saying that plasma is ionized gas, but how can a flare no... | g18810 | [
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<p>There are two types of perturbation theory corresponding to explicit lorentz-covariance of amplitudes. </p>
<p>The first one is called Rayleigh-Schrodinger perturbation theory. It is based on following ideas. Let's have Hamiltonian in a form $\hat{H} = \hat{H}_{0} +\hat{V}$, where there isn't dependence on time and... | g18811 | [
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<p>I have read a long series of paragraphs on <a href="http://en.wikipedia.org/wiki/Mobile_phone_radiation_and_health" rel="nofollow">wiki</a>. Previously I thought that microwaves are not harmful to living beings but Wiki does not claim this explicitly. while doing an experiment in my college laboratory our teacher c... | g18812 | [
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<p>What calculations were done to discover the size of all the planets?</p> | g18813 | [
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<p>I know that when viewed from infinity (or from a very large distance from the black hole event horizon), an object that falls into the black hole will appear to slow down and will become more and more red-shifted as it approaches the event horizon. To the far away observer the object will never be seen to enter pas... | g18814 | [
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<p>There is Iron in blood. Iron is magnetic. Roughly how strong would a magnet have to be to induce a noticeable attraction? It would be nice to know this for several distances. Also, do electromagnets that strong exist?</p> | g18815 | [
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<p>I'm asked to show that </p>
<p>$\frac{d(\hat{A}\hat{B})}{d\lambda} = \frac{d\hat{A}}{d\lambda}\hat{B} + \hat{A}\frac{d\hat{b}}{d\lambda}$ </p>
<p>With $\lambda$ a continuous parameter</p>
<p>Should I use the definition</p>
<p>$\frac{d\hat{A}}{d\lambda} = \lim_{\epsilon \to 0} \frac{\hat{A}(\lambda + \epsilon) - ... | g18816 | [
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<p>When deriving the Lagrangian for Spin $\frac{1}{2}$ particles we are naturally led to using $\Psi$ and $\bar{\Psi}$. The Euler-Lagrange equations lead us to two wave equations:
\begin{equation} (i\gamma_\mu \partial^\mu - m ) \Psi =0 \end{equation}</p>
<p>\begin{equation} (i \gamma_\mu \partial^\mu + m )\bar{\Psi... | g18817 | [
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<p>I'm implementing a little QM calculation just for fun and to make sure I understand how it works (calculating the helium ground state energy). My problem is that my basis set doesn't seem to be orthonormal. I'm using the spherical harmonics for the angular part and the Slater type radial function. If I integrate ... | g18818 | [
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<p>The propagator of Maxwell theory is different, depending on the gauge fixing procedure used. Then why will the S-matrix elements be the same for the same process in different gauges? </p> | g18819 | [
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<p>We know that some galaxies are moving away from us faster than the speed of light and we know it by measuring the redshift, but how's that possible? If they're moving away say at 2c, how would the light of the galaxy even reach us? How do we measure "redshift" for something faster than light?</p> | g18820 | [
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<p>I was just wondering What would happen if the earth spins the other way around purely interms of Physics ?</p> | g580 | [
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<p>I am starting with Quantum Mechanics, learning online. I can't seem to find the reason for $|\Psi|^2$ being the probability density of finding an electron. They've just taken it for granted everywhere. I am learning all this math but I am not able to fully grasp the intuitive idea behind all of it. If anyone could e... | g18821 | [
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<p>Do all galaxies radiate gravitational waves? What is the origin of these waves, the origin of the Galactic center? If it exists, do two galaxies warp together due to these waves, when they come closer?</p> | g18822 | [
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<p>Someone told me that, in a hamiltonian system, the hamilonian function is the generating function of the canonical transformation given by time translation. However, this statement doesn't make any sense to me. Typically, the generating function is a function of some of the "old" coordinates as well as the new (tran... | g18823 | [
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<p>The LHC results today seem to have two possible Higgs peaks, one at 119 GeV and the other at 125-126 GeV. All the multi-Higgs supersymmetry models I've seen that have multiple Higgs have only one light Higgs, the others being well over 600 GeV.</p>
<p>While I think it's unlikely that there really are two distinct... | g18824 | [
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<p>I want to discretize the wave equation</p>
<p>$$\frac{1}{c^2}\frac{\partial^2\psi\left(\vec{r},t\right)}{\partial t^2}=\triangle\psi\left(\vec{r},t\right)$$</p>
<p>in polar coordinates. I find the following relations:</p>
<p>$$\vec{r}=r\mathbf{e}_r$$
$$\triangle=\frac{1}{r}\partial_r r \partial_r+\frac{1}{r^2}\pa... | g18825 | [
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<p><strong>Q1:</strong>
Lets say a particle with mass $m_1$ and kinetic energy $W_k$ colides with a particle of mass $m_2$ at rest. Can anyone explain why do both particless ($m_1$ and $m_2$) plus an aditionall mass $m$ still exist after the colission?</p>
<p><img src="http://i.stack.imgur.com/wE0yJ.png" alt="enter im... | g18826 | [
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<p>I'm a nurse, so my physics background is about that of a lay person. Please explain on that level. Ordinarily an Intravenous (IV) fluid bag is one liter. An IV piggyback is usually 2500ml. usually the fluid in each is the same 0.45% NaCl. The piggyback may also get medication mixed in with it. such as a Gm of Roceph... | g18827 | [
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<p>What is Quantum Annealing and quantum annealing computing and what are its advantages and disadvantages with respect to quantum circuit quantum computing/computers?</p> | g18828 | [
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0.031207328662276268,
0.02453436143696308,
0.04680904746055603,
-0.0015665274113416672,
0.04049627482891083,
0.017294928431510925,
0.016600999981164932,
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0.0... |
<p>Currently, (classical) gravity (General Relativity) is NOT a gauge theory (at least in the sense of a Yang-Mills theory). </p>
<p><strong>Why should "classical" gravity be some (non-trivial or "special" or extended) gauge theory? Should quantum gravity be a gauge theory?</strong></p>
<p>Remark: There are some cont... | g18829 | [
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... |
<p>In the paper <em>The River Model of Black Holes</em>:<br>
Am.J.Phys.76:519-532,2008, Andrew J. S. Hamilton, Jason P. Lisle<br>
<a href="http://arxiv.org/abs/gr-qc/0411060" rel="nofollow">http://arxiv.org/abs/gr-qc/0411060</a><br>
The authors give a way of describing the action of a rotating (and or charged) black ho... | g18830 | [
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-0... |
<p>I've been told that in countries like Israel the truth of the matter is that the only available work for B.Sc./M.Sc. in physics/math is only to be high school teacher. To do serious stuff you need to be Ph.D.</p> | g584 | [
0.012556967325508595,
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-... |
<p>Consider a cylinder of permanently magnetized material, with uniform magnetization pointing along the cylindrical symmetry axis (the $z$-direction). The magnet is rotating about its cylindrical symmetry axis with angular velocity $\omega$. What electric field does the rotating magnet generate?</p>
<p>Backstory: Mov... | g18831 | [
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0.036184173077344894,
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<p>I'm trying to simulate inclusive $\chi_c$ production in $p \bar p$ collisions at very low energies (~ 5.5 GeV) using <a href="http://home.thep.lu.se/~torbjorn/Pythia.html" rel="nofollow">Pythia8</a> event generator. Leaving aside problems bounded with applicability of parton model at such low energies, I have encoun... | g18832 | [
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<p>$F=ma$. A car strikes a wall at 60 mph. Its acceleration is zero at the time. The force of the car against the wall or vice versa is? To look at the car the force is not zero. Please explain.</p> | g18833 | [
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<p>Is there any way to check whether in a Monte Carlo simulation using Ising model is stuck in any (false) local minima of energy or not, particularly in 3D system ?</p> | g18834 | [
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<p>In my class notes, I have the two types of inelastic scattering described as follows. (Note that the "difference in energy" here is the difference between the incident frequency of the light and the emitted radiation)</p>
<blockquote>
<p><em>Brillouin scattering</em>: The difference in energy generates acoustic p... | g18835 | [
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<p>Gravity and electromagnetism are inverse-square laws. This makes geometric sense -- if you build a spherical shell around a lamp then a shell with twice the radius has four times the surface area and hence a quarter of the intensity per unit area.</p>
<p>The strong interaction has a nontrivial distance-strength rel... | g18836 | [
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<p>Could someone explain in simple terms (let's say, limited to a high school calculus vocabulary) why decibels are measured on a logarithmic scale?</p>
<p>(This isn't homework, just good old fashioned curiousity.)</p> | g18837 | [
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<p>I have been studying fluid mechanics and currently I was trying to understand Euler's equation for fluid flow. For that purpose, I was following <a href="http://www.av8n.com/physics/euler-flow.htm" rel="nofollow">this</a> webpage.</p>
<p>My problem is in understanding Eq.(12). The author states that that is the con... | g18838 | [
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<p>We all know that the universe is governed by four Fundamental Forces which are The strong force , The weak force , The electromagnetic force and The gravitational force .</p>
<p>Now, is there any relationship between Electromagnetism and gravity?</p> | g247 | [
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<p>If I bring together 2 objects with the same charge, they repel. But even after the repulsion has taken place, there is no loss of charge in the objects. Where does the energy come from then?</p> | g18839 | [
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<p>I am given the information that an air parcel undergoes isobaric heating from 0° C to 20° C, and that's all I'm given. I have to determine the work done by the parcel on its surroundings. I know that $dW = pdV$, and that $pV = RT$ ($V$ is the specific volume), but I don't know how to go about solving this in terms o... | g18840 | [
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<p>Take a source of light which gives out infinite number of rays, each ray with finite number of photons and each photon with a finite amount of energy,</p>
<p>Then, Aren't the number of photons become infinite and hence the energy in the beam of light becomes infinite? If this is wrong, then how's this finite?</p> | g18841 | [
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<p>In <a href="http://physics.stackexchange.com/questions/39335/wavefunction-in-quantum-mechanics-and-locality">Wavefunction in quantum mechanics and locality</a>, wavefunction is constrained by $H = \sqrt{m^2 - \hbar^2 \nabla^2} $, and taylor-expanding $H$ results in:</p>
<p>$$ H = \dots = m\sqrt{1 - \hbar^2/m^2 \cdo... | g18842 | [
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0.030705086886882782,
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0.010... |
<p>Just curious, can anyone show how the integral and differential form of <a href="http://en.wikipedia.org/wiki/Maxwell%27s_equations" rel="nofollow">Maxwell's equation</a> is equivalent? (While it is conceptually obvious, I am thinking rigorous mathematical proof may be useful in some occasions..)</p> | g18843 | [
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<p>We use contraction of indices method to manipulate Tensors. However, I cannot relate that manipulation visually. We can change covariant tensor to contravariant tensor and vice versa by contracting indices with the metic tensor. Covariant tensor represent gradient and contravariant tensor basically represent tangent... | g18844 | [
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<p>One of the Episodes of BBC's documentary <a href="http://www.bbc.co.uk/programmes/b00mfl7n" rel="nofollow">Frozen Planet</a> describes an upwell phenomenon in the Antarctic Ocean. I am trying to understand the cause of seawater freezing when this upwell occurs.</p>
<p>Liquid sea water can remain liquid below salt ... | g18845 | [
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<p>As emphasized by Penrose many years ago, cosmology can only make sense if the world
started in a state of exceptionally low entropy. The low entropy starting point is the
ultimate reason that the universe has an arrow of time, without which the second law
would not make sense. However, there is no universally accept... | g18846 | [
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... |
<p>If amps = volts / ohms, and ohms is 0, then what is x volts / 0 ohms?</p> | g18847 | [
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<p>I read somewhere that if we increase the temperature of the material in a <a href="http://en.wikipedia.org/wiki/Laser_diode" rel="nofollow">laser diode</a> the threshold current for lasing also increases. Can someone explain to me why is this happening? What is the mechanism inside the diode behind this increase?</p... | g18848 | [
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<p>This comes from one of the <a href="http://apcentral.collegeboard.com/apc/members/exam/exam_information/2007.html" rel="nofollow">free response questions</a> from the AP Physics B 2014 exam.</p>
<blockquote>
<p><img src="http://i.stack.imgur.com/i3Qo2.png" alt="diagram of rod suspended by springs in magnetic fiel... | g18849 | [
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<p>In Brian Hatfield's book on QFT and Strings there is the following quote:</p>
<blockquote>
<p>In particular $$ [A_i (x,t), E_j(y,t)] = -i \delta_{ij}\delta(x-y) $$
implies that $$ [A_i(x,t),\nabla \cdot E(y,t)] = -i\partial _i \delta(x-y).$$</p>
</blockquote>
<p>I'm not sure how to get between those lines. If... | g18850 | [
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0.014341338537633419,
0.0048216343857347965,
0.0... |
<p>I've seen in the literature the relation:</p>
<p>$\sigma (q,\omega) = \frac{i e^2 \omega}{q^2}\chi(q,\omega)$</p>
<p>where $\sigma$ is the conductivity and $\chi$ the polarizability.
However my attempt to derive it leaves me without the factor $q^2$. I have</p>
<p>\begin{equation*}J_{int} = \sigma E_{tot}\end{equ... | g18851 | [
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-0... |
<p>My physics text says that force is "an interaction between two bodies or a body and its environment." </p>
<p>When an object undergoes acceleration we explain it with a force. But we don't measure force, we measure the change in acceleration. Is force just a helpful construct to be a placeholder in the equation F... | g18852 | [
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0.00613... |
<p>Say I want to pump water from one container to another. The water levels are 3 meters apart, and I want to pump 10 litres per hour. I figure the mechanical power necessary, assuming no losses, is:</p>
<p>$$
\require{cancel}
\dfrac{10\cancel{l}}{\cancel{h}}
\dfrac{kg}{\cancel{l}}
\dfrac{\cancel{h}}{3600s}
= \dfrac{0... | g18853 | [
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<p>is there any mathematical explanation for the UV divergences ??</p>
<p>i have read that in the framework of Epstein-Glser theory :D these UV divergences appear from the product of distributions</p>
<p>anyone does the numbers (divergences) of the form $ \delta ^{(n)}(0) $ have a meaning within the renormalization f... | g18854 | [
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<p>Usually complex lenses are described by their geometric design - usually a sequence of spherical cap sufraces and materials. The lens behavior, ie. how it transfers rays, can be evaluated by ray tracing. On the other hand wavefront aberrations can be computed for a particular lens. Are they enough to represent the r... | g18855 | [
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<p>This is a questions on 't Hooft's beable models (see here: <a href="http://physics.stackexchange.com/questions/32203/discreteness-and-determinism-in-superstrings/32966#32966">Discreteness and Determinism in Superstrings?</a>) for quantum mechanics, and the goal is to understand to what extent these succeed in reprod... | g18856 | [
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0.0... |
<p>Inspired by a recent dialogue from another question:</p>
<p>Given 2 objects moving at some velocity $v$ relative to one another, is it possible to determine whether they are moving or whether the space between them is expanding?</p> | g359 | [
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... |
<blockquote>
<p>Suppose a car travels at 5m/s north for 5 seconds, it then turn east and travel at 7m/s for 10 seconds, finally it turns north east and travel at 10 m/s for 20 seconds. What is the average acceleration over these 35 seconds?</p>
</blockquote>
<p>I know acceleration average is the change in velocity o... | g18857 | [
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<p>I'm learning basic maths for physicist and was wondering what do we use the <a href="http://en.wikipedia.org/wiki/Dirac_delta_function" rel="nofollow">Dirac delta function</a> for? What is the difference between "the Fourier integral form" and the usual way of expressing the delta function? </p>
<p>I know how to us... | g18858 | [
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0.0... |
<p>please could someone check this MIT video (<a href="http://www.youtube.com/watch?v=Lkuo6nZ6nZM" rel="nofollow">http://www.youtube.com/watch?v=Lkuo6nZ6nZM</a>) at 26mins 31secs. He says that if you threw a tomato on a bathroom scale then you would get a certain force indicated by the scales. He then says if you then ... | g18859 | [
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<p>In physics, many problems start with a mathematical relationship of the physical phenomenon at hand, and then, in many occasion, always only leave whatever in the first order to get a nice and solvable differential equation. Then there may be terms of higher order considered later, by as far as I know, it rares goes... | g18860 | [
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... |
<p>Most liquids can be approximated to be incompressible, since the Mach-number is much smaller than 1. That means that the density variations are <strong>negligible</strong> and from the relation between pressure p and density ρ,
$$
p=c_s^2 \rho
$$
we see that the pressure in constant as well. Now, say that I look at ... | g18861 | [
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<p>On page 181 in Peskin & Schroeder they say that we consider the integral (intensity)
$$\tag{1}\mathcal{I}(\mathbf{v},\mathbf{v}') = \int\frac{\mathrm{d}\Omega_\hat{k}}{4\pi}\,\frac{2(1-\mathbf{v}\cdot\mathbf{v}')}{(1-\hat{k}\cdot\mathbf{v})(1-\hat{k}\cdot\mathbf{v}')}-\frac{m^2/E^2}{(1-\hat{k}\cdot\mathbf{v}')^2... | g18862 | [
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0.008... |
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