question stringlengths 37 38.8k | group_id int64 0 74.5k |
|---|---|
<p>I just found this news article: <a href="http://www.nature.com/nature/journal/v481/n7379/full/nature10695.html" rel="nofollow">http://www.nature.com/nature/journal/v481/n7379/full/nature10695.html</a></p>
<p>What did those researchers actually do? The article itself doesn't sound to me like it can be taken seriousl... | 4,636 |
<p>Is spacetime moving in general relativity?</p>
<p>If not, how does spacetime retain its past, while moving toward future?</p> | 4,637 |
<p>In his <em>Principles of Quantum Mechanics</em> Dirac writes: $$\int \langle \phi \frac{d}{dq}|q'\rangle dq' \psi(q')=\int \phi(q') dq' \frac{d\psi(q')}{dq'}.$$</p>
<p>To me it is rather strange, and it seems as if he was treating the operator $\frac{d}{dq}$ as a function of the observable canonical coordinate $q$,... | 4,638 |
<p>Acceleration due to mechanical force will lessen due to relativistic mass increase as kinetic energy becomes significant near c.</p>
<p>But if acceleration is gravitational, the gain in relativistic mass will produce a corresponding gain in gravitational attraction, so acceleration should be constant (for large M a... | 153 |
<p>I was informed that:</p>
<blockquote>
<p>There is a maximum density at which we can store information. For a sphere with surface area A, the maximum information that can be contained
within is equivalent to the maximum entropy of a sphere of size A, which is given by S_max = A/(4 L^2), where L is the Planck le... | 4,639 |
<p>What about considering the microwave background radiation (2.7K if I remember well) as a reference system with some absolute character? Please explains if this question make sense and possible answers.
Thank you.</p> | 4,640 |
<p>I am looking for an exact or approximate solution to a statistical lattice-particle problem:</p>
<p>Given a lattice of size $L\times L$ where $\rho\cdot L^2$ particles are randomly distributed, calculate the probability distribution function for $M$ defined as number of pair interactions, i.e. number of edges betwe... | 4,641 |
<p>Its difficult to put this into the title.</p>
<p>I was watching the Redbull Jump and noticed that the height of this is at 39 kilometres (24 mi) the atmosphere pressure is at I believe about 0.4% of that at sea-level. I was imagining, what if a large satellite dragged a cable (bungie cord, rope) below it into the a... | 4,642 |
<p>In his book "Einstein's mistakes" H. C. Ohanian, the author, holds that Einstein delivered 7 proofs for $E=mc^2$ in his life that all were in some way incorrect. This despite the fact that correct proves had been published and mistakes in his proofs were sometimes pointed out to him. </p>
<p>The first proof e.g. c... | 4,643 |
<p>Here we have electron gas and some other stuff. We expand the Hamiltonian to the 1st order of one single harmonic oscillator's displacement $\vec{u}$. Its equilibrium position is at the origin. Then we get an effective coupling Hamiltonian $\vec{j}(\vec{r})\times\vec{f}(\vec{r})\cdot \vec{u}$, wherein $\vec{j}(\vec{... | 4,644 |
<p>I saw this <a href="http://www.ted.com/talks/woody_norris_invents_amazing_things.html">TED talk</a> and I am curious as to how the sound is focused on the general level. Can anyone explain this or does anyone have any good articles?</p> | 4,645 |
<p>I am having difficulty of finding more basic information on warped geometries. All the standard textbooks are not covering it.</p>
<p>In the <a href="http://en.wikipedia.org/wiki/Warped_geometry" rel="nofollow">wiki article</a> it's only said that warped geometry is the one which can be decomposed in a certain way,... | 4,646 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/312/particle-physics-getting-started">Particle physics getting started</a> </p>
</blockquote>
<p>I'm looking for a book that introduces the building blocks refer to the standard model for a course in nuc... | 154 |
<p>Question shortly:
How far would a hockey puck slide in two different cases:</p>
<ol>
<li>The puck is sliding (translation) on ice and spinning on its flat surface. </li>
<li>The puck is sliding on ice without spinning. </li>
</ol>
<p>Other conditions are the same in both cases. </p>
<p>Simplifications: no air r... | 4,647 |
<p>If a butterfly did not flap its wings some time ago, but instead decided to slide for that millisecond, can this cause a tornado on the other side of the earth if we just wait long enough? Does this perturbation die out, or average out in short time, or does it propagate and have grander and grander consequences?<br... | 4,648 |
<p>I'm using a game engine that has a ton of physics stuff built in. What I am currently trying to do is to simulation an explosion with F force in an x,y coordinate system and figure out how other items in the world are affected by it (they each have x,y as well).</p>
<p>The ultimate function I call is applyForce and... | 4,649 |
<p>I'm trying to determine the viscous dampening coefficient of a spring $c$. Read about it on Wikipedia <a href="http://en.wikipedia.org/wiki/Damping" rel="nofollow">here</a>.</p>
<p>The two equations which I have are:
$f=-cv$ and $ma+cv = -kx$</p>
<p>I know the spring constant $k=5$, the mass is $50\text{ }\mathrm{... | 4,650 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/556/what-is-the-fallacy-in-this-infinite-motion-machine">What is the fallacy in this infinite motion machine?</a> </p>
</blockquote>
<p>Most of the "troll physics" images I can figure out, but this one h... | 155 |
<p>I'm reading Cardy's notes on CFT. He states the following in section 4.3:</p>
<p>$$\hat L_n\left(\hat L_{-2}|\phi_j\rangle-(1/g)\hat{L^2}_{-1}|\phi_j\rangle\right)=0.$$</p>
<p>I tried to work this out explicitly and I managed to prove it for $n=1$ and $n=2$, but I can't figure this out for general $n$. After some ... | 4,651 |
<p>For example... If I had a 2W green laser, a 2W red laser, and a 2W blue laser, could I combine them using crystals to form a 6W white laser? Or is that now how it works? If not what would be the output in watts of such a laser?</p> | 4,652 |
<p>I had previously asked <a href="https://physics.stackexchange.com/questions/117172/how-to-calculate-gravity-path-integrals-about-an-ads-background">this question</a>. This is kind of a continuation of that. </p>
<p>I recently found this expression which seems to be called the "Fierz-Pauli action" which is apparentl... | 4,653 |
<blockquote>
<p><em>A completely isolated neutral conducting sphere of radius $R$ is kept such that its center is at a distance of $r\left(>R\right)$ from a
point charge $+Q$.</em></p>
</blockquote>
<p>How can I find the force of interaction of the induced charges and the point charge, or at least the energy?
I... | 4,654 |
<p>we recently took a tour to a radio telescope and recorded some spectra, one of them being Cassiopeia A. Looking at the difference in on-source and off-source spectra, we find sharp absorption and emission lines in the spectrum for the 21 cm line. Where do they come from? We have different theories:</p>
<ol>
<li>The... | 4,655 |
<p>I'm currently trying to figure out the following in the simplest possible way:</p>
<p>Say we have a nozzle in a vacuum environment.
A gas of a certain pressure is emitted through the nozzle, which has a certain diameter, int the vacuum. The question is now how high is the pressure at a certain distance away from t... | 4,656 |
<p>How do you tell the difference between a gamma-ray burst and a star just from a picture of a nebula, in which it cannot flash on and off here and there?</p> | 4,657 |
<p>I am trying to get my head around why would and silicon engineer care about the minority life time carrier and how does the minority carrier affect the switching speed of PN junction. Why is it so much about minority and not majority carriers?</p> | 4,658 |
<p>Given 10-1000 Watts of electrical power (and no other consumables), what is the current best way to turn it into thrust? Just running it through a heater on an insulating pad would result in an IR thruster, but has bad focus. A laser has good focus but only for a small percentage of the energy. </p> | 4,659 |
<p>In theories with extended supersymmetry, both short and long multiplets exist. For some reason or other, short multiplets are studied more often. Why? What's wrong with long multiplets?</p> | 4,660 |
<p>Let's say 10 kg block is sliding on a frictionless surface at a constant velocity, thus its acceleration is 0. </p>
<p>According to Newton's second law of motion, the force acting on the block is 0:</p>
<p>$a = 0$</p>
<p>$F = ma$</p>
<p>$F=0$</p>
<p>So let's say that block slid into a motionless block on the sa... | 4,661 |
<p>Eventually, we are going to reach our limits in particle physics, or maybe find the theory of everything. Particle physics isn't easy by any means. It requires at the minimum years of graduate school and more to master. The main motivation of learning is to come up with original research later. But if there is no mo... | 4,662 |
<p>Suppose that an electron with spin up emits a photon in the field of an ion (<a href="http://en.wikipedia.org/wiki/Bremsstrahlung" rel="nofollow">bremsstrahlung</a>). What is the spin of the emitted photon? Is it correct to say that the photon is circularly polarized if the spin of the electron flips down and linear... | 4,663 |
<p>How should we put two converging lens in order for parallel rays passing between both lens to remain parallel?</p> | 4,664 |
<p>I'm working on a 2D He superfluid system with vortices. I was asked to calculate the kinetic energy of vortex-(anti-)vortex pairs and compare the two situations. One finds in literature that the vortex-vortex situation is unstable, and the vortex-antivortex pair can be stable. This is a crucial observation in what f... | 4,665 |
<p>Related to my other misguided question, D-branes are equivalent to p-branes. D-branes are described by a sheet in topologically trivial spacetime. p-branes are extremal black branes. They have an event horizon which is infinitely far away along spatial geodesics. However, we may cross the event horizon in a finite a... | 4,666 |
<p>I am trying to decipher what decibels are:</p>
<p><a href="http://en.wikipedia.org/wiki/Decibel" rel="nofollow">http://en.wikipedia.org/wiki/Decibel</a></p>
<p>It seems to be a log ratio of audio amplitude multiplied by a constant. I am confused by what this means though.</p>
<p>If my original volume is X, what d... | 4,667 |
<p>Taken from <a href="http://www.feynmanlectures.caltech.edu/I_06.html#Ch6-S3" rel="nofollow">Volume 1, Chapter 6, Section 3</a> of the Feynman Lectures on Physics.</p>
<p>Feynman says that in describing random, equally-probable-backwards-or-forwards motion, that,</p>
<blockquote>
<p>We might therefore ask what is... | 4,668 |
<p>I want to solve a problem of plane wave diffraction by a large sphere. There is a formula that I found from papers, but I think needs numerical solution. please help me to solve it The formula is below:
<img src="http://i.stack.imgur.com/98WnS.jpg" alt="enter image description here"></p>
<p><img src="http://i.stack... | 4,669 |
<p>I am currently studying this problem: <a href="http://aforrester.bol.ucla.edu/comprobs/F06prob14.pdf" rel="nofollow">14 b)</a></p>
<p>There you see an integral $$A(r) = \int f(\theta) (-\sin(\phi), \cos(\phi),0) d \Omega$$ where $f$ is the function containing all the rest of the integrand that you see there and I d... | 4,670 |
<p>Why does current return to its source?</p> | 4,671 |
<p>Jupiter has about twice the density of Saturn (1.33 versus 0.69 g/cm^3) because it apparently has a higher mass percentage of rocky core and of metallic hydrogen in its interior. Available density references of this hydrogen state differ widely from 0.3 - 2.8 g/cm^3. Is there a theoretical density of metallic hydro... | 4,672 |
<p>Suppose we have a quantum state, well described by its time-independent wave function $\Psi$. And we have a well-defined Hermitian (self-adjoint) operator $\hat{A}$. We successfully evaluate the expectation value of the operator $\hat{A}$. Next we derive the general formula for the higher moments of $\hat{A}$ (i.e. ... | 4,673 |
<p>What is the meaning of the text quoted below?</p>
<blockquote>
<p>In the physical world, if a system is described by an equation that is
first order in time, the system is general dissipative (has energy
loss). If the equation is second order in time, the system may be non
dissipative. Such a system has tim... | 4,674 |
<p>What factors affects the size of a shadow and how would you derive the diameter of a shadow of a circular object on a flat screen?</p> | 4,675 |
<p>The mass-energy equivalence, first established by Einstein is an important and highly discussed phenomenon in physics. Without claiming much knowledge about high-end discussions on this topic, I would like to ask a question on this. </p>
<blockquote>
<p><strong>Does the equivalence allow "energy" to exhibit gra... | 4,676 |
<p>In Weinberg's <em>Gravitation,</em> </p>
<p>the formula for the volume element in curviliniar coordinates is given by
$$dV=h_1 h_2 h_3 dx^1 dx^2 dx^3.$$ </p>
<p>The metric is given by $ds^2=h_1^2 dx_1^2+h_2^2 dx_2^2+h_3^2 dx_3^2.$</p>
<p>I am totally confused by this, I know that $\sqrt{g^{\prime}}=|\frac{\parti... | 4,677 |
<p>Earth rotates on its axis and revolves around the sun, <a href="http://www.universetoday.com/18028/sun-orbit/">the sun revolves around the galaxy</a>, the galaxy is also moving. So Earth's net rotation as observed from a fixed inertial frame consists of all these contributions (and is rather complex).</p>
<p>Now a ... | 995 |
<p>Why is quantum physics needed to explain photosynthesis?</p>
<p>In what aspect does the corresponding classical theories for photosynthesis fail?</p> | 4,678 |
<p>The below is the sixth question of the very first chapter from halliday and resnicks fundamentals of physics text,which i'am not able to comprehend.</p>
<p><strong>Harward Bridge,which connects MIT with its fraternities accross the Charles River,has a length of 364.4 Smoots plus one ear.The unit of one Smoot is bas... | 156 |
<p>Consider a piece of metal of length $L$ and linear thermal expansion coefficient $\alpha$. We eat the metal $\Delta T$ degrees, causing the metal to increase to length
$$ L' = L + L \alpha \Delta T$$
Now, cool the object back to the original temperature. This causes the metal to decrease in length to
$$L'' = L' - L'... | 4,679 |
<p>I have the following exercise:</p>
<p><strong>Use Heisenberg's uncertainty principle and the relation $\Delta u = \sqrt{\langle u^2 \rangle - \langle u \rangle^2}$ to find the range of energy an electron has in an atom of diameter 1 amstrong.</strong></p>
<p>This is the attempt at a solution:</p>
<ul>
<li><p>From... | 4,680 |
<p>Based on the transfer length method (TLM), one can accurately calculate the contact resistivity for an <a href="http://en.wikipedia.org/wiki/Ohmic_contact" rel="nofollow">ohmic contact</a>, by evaluating the absolute resistance measured through the test structure and plotting it as a function of the gap spacing betw... | 4,681 |
<p>Does entanglement not immediately contradict the theory of special relativity? Why are people still so convinced nothing can travel faster than light when we are perfectly aware of something that does?</p> | 4,682 |
<p>The following <a href="https://www3.amherst.edu/~rhromer/MISC/Spin-Statistics-AJP.pdf">link</a> provides a letter to the editor by Robert H. Romer who writes, </p>
<blockquote>
<p>In a 1994 "question" in this journal,
Neuenschwander asked whether anyone
had yet met Feynman’s challenge of pro-
viding an elem... | 4,683 |
<p>Every space show I watch mentions that anti-matter used to exist, or still does and we just can't detect it. I think some shows even say we can create a small amount of anti-matter. It is not presented as an unproven conjecture like string theory, but rather as a fact.</p>
<p>In terms someone without a PhD might u... | 4,684 |
<p>If the curvature of the universe is zero, then $$Ω = 1$$ and the Pythagorean Theorem is correct. If instead $$Ω> 1$$ there will be a positive curvature, and if $$Ω <1$$ there will be a negative curvature, in either of these cases, the Pythagorean theorem would be wrong (but the discrepancies are only detectabl... | 4,685 |
<p>I have mass, $g$, and luminosity of each of the stars in a binary system, extracted from a model. I calculated the individual radii from $g$ and the mass. I am trying to compute $a$, but I seem to be stuck or I'm just missing something obvious. I can't think any method that does not involve the period.</p>
<p>How d... | 4,686 |
<p>I know roughly <em>how</em> a turbine engine (let's say a gas turbine producing no jet thrust) is supposed to work:</p>
<blockquote>
<p>The compressor forces fresh air into a combustion chamber, where it reacts with fuel to become hot exhaust gas. On its way out of the engine, the exhaust gas drives a turbine, an... | 4,687 |
<p>Is it possible to take a tensor to the other side of the equation, and the tensor becomes its inverse(i.e contravariant becomes covariant and vice versa)? It is a stupid question, but It confuses me.</p>
<p>For example, if</p>
<p>$A_{ij} = B_{ij},$</p>
<p>Can I write</p>
<p>$A_{ij}B^{ij} = \delta_{j}^{i}$, (tho... | 4,688 |
<p>In J.D. Jackson's first chapter, he says the proper equation connecting $\textbf E$ and $\textbf D$</p>
<p>$$ D_\alpha = \sum_\beta \int d^3x'\int dt' \epsilon_{\alpha\beta}(\textbf x',t')E_\beta(\textbf x - \textbf x',t-t') $$</p>
<p>He suggests that this is because the connection between these two can be non-loc... | 4,689 |
<p>I think,</p>
<p>$$\sigma_{ij}\sigma^{ij} = \sigma^2.$$</p>
<p>However,
on the Wikipedia page on <a href="http://en.wikipedia.org/wiki/Raychaudhuri_equation" rel="nofollow">Raychaudhuri equation</a>,
It was mentioned:</p>
<p>$$\sigma^2=\frac{1}{2}\sigma^{ij}\sigma_{ij}$$</p>
<p>I am confused, but I think the firs... | 4,690 |
<p>I am trying to understand how complex numbers made their way into QM. Can we have a theory of the same physics without complex numbers? If so, is the theory using complex numbers easier?</p> | 4,691 |
<p>I'm working on a sample problem and it asks on how steep of an incline can a car park? From what I learned the friction is in the opposite direction if there was motion in said friction-less environment. Is this the right way to think of friction? </p>
<p>In the case of the car; it would move down the hill and the ... | 4,692 |
<p>The Wikipedia says on the page for the <a href="http://en.wikipedia.org/wiki/Uncertainty_principle" rel="nofollow">uncertainty principle</a>:</p>
<blockquote>
<p><em>Mathematically, the uncertainty relation between position and momentum arises because the expressions of the wave function in the two corresponding ... | 4,693 |
<p>Suppose a person is walking in rain carrying an umbrella. He is tilting his umbrella at some angle with the vertical so as to protect himself from the rain. But a neutral observer who is standing still will find it really absurd because he will find the rain falling vertically downward, how come there are two realit... | 4,694 |
<p>I listened to Christoph Weniger present his results at SLAC today. See his paper is here: <a href="http://arxiv.org/abs/1204.2797">http://arxiv.org/abs/1204.2797</a> and also see a different analysis here: <a href="http://arxiv.org/abs/1205.1045">http://arxiv.org/abs/1205.1045</a>. The data seems convincing to me! I... | 4,695 |
<p>In real-space renormalization group how does one find the complete RG flow exactly, (not schematically)? I understand it needs to be done on a computer. For example, I have the ising model on a triangular lattice and I also get the RG recursion relations by blocking ( as done in Nigel Goldenfeld's book). Now how do ... | 4,696 |
<p>I am long been confused by these entropy terms. Would be obliged if an explanation is provided in less technical jargon</p>
<ol>
<li>What are the differences between Shannon's entropy, topological entropy and source entropy?</li>
<li>What exactly is the significane of Kolgomorov complexity. Is it same as Kolgomoro... | 4,697 |
<p>In CFT, when we have an OPE:
$$O_1(z)O_2(w)=\frac{O_2(w)}{(z-w)^2}+\frac{\partial O_2(w)}{(z-w)}+...$$
this holds inside a time-ordered correlation function, so $O_1(z)O_2(w)=O_2(w)O_1(z)$. Does it mean that
$$O_1(z)O_2(w)=\frac{O_1(z)}{(w-z)^2}+\frac{\partial O_1(z)}{(w-z)}+...$$
?</p> | 4,698 |
<p>Here is a problem on diffraction -
Diffraction pattern of single slit of width 0.5 cm is formed by a lens of focal length of 40 cm. calculate the distance between first dark and next bright fringe from the axis. Wavelength of the light used is 4890 Angstrom.</p>
<p>I am confused by following statement of the proble... | 4,699 |
<p>I came upon this while wondering whether the friction of a rolling cylinder on an inclined plane depends on the value of friction coefficient.</p>
<p><img src="http://i.stack.imgur.com/wC0Lz.png" alt="enter image description here"></p>
<p>now,
$$f\leqq\upsilon N$$</p>
<p>Again after calculating I found that the
$... | 4,700 |
<p>I am wondering if an extension of <a href="http://en.wikipedia.org/wiki/Noether%27s_theorem" rel="nofollow">Noether theorem</a> to <a href="http://en.wikipedia.org/wiki/Supergroup_%28physics%29" rel="nofollow">supergroups</a> exists.
In particular the analogy with the usual case should be that supersymmmetries are i... | 4,701 |
<p>I was watching the men's <a href="http://en.wikipedia.org/wiki/Luge">luge</a> ride with my dad. My dad said, the mass of the athlete must be at an optimum level so that he wins. I said, his volume should be minimum, but it has nothing to do with the mass, as the acceleration is independent of mass.</p>
<p>Is it jus... | 4,702 |
<p><img src="http://i.stack.imgur.com/xyeuE.jpg" alt="Image 1"></p>
<p>Imagine that there is a car and it is not moving but its headlights are on. There is a wall in front of the car but is very far away. Right now energy is being used only in switching on the headlights. Now the car starts moving at a very high speed... | 4,703 |
<p>I've read that electrons in Graphene behave 'pseudo-relativistically'; what does this mean? how do they behave differently from electrons in other materials?</p> | 4,704 |
<p>I'm studying quantum mechanics in its most basic level (I don't even know if Physicists call this already quantum mechanics) and I have one doubt in Schrodinger's equation. The book presents the equation for the special case where the solution is of the form $\Psi(x,t)=\psi(x)e^{-i\omega t}$ and says that Schrodinge... | 4,705 |
<p><a href="http://arxiv.org/abs/1003.6023" rel="nofollow">http://arxiv.org/abs/1003.6023</a> Gauge Higgs unification .</p>
<p>Does gauge higgs unification has been proved ？
If so , what is it exactly meaning ?</p> | 157 |
<p>I was researching a question for another post and it occurred to me that you might expect to see Hawking radiation at the mouth of wormholes. </p>
<hr>
<p>Given the mechanism of Hawking radiation at the event horizon of black holes: virtual partial pairs forming at the edge and being separated by the event horizon... | 4,706 |
<p>Some years ago, I heard a talk about a an Irish or Scottish physicist named McCullough who had formulated Maxwell's equations several years before Maxwell. This fellow was recognized for his work, according to the talk, by systematically discounted by other luminaries of the day like Stokes and Hamilton in favor of... | 4,707 |
<p>I am interested in <a href="http://en.wikipedia.org/wiki/Shor%27s_algorithm" rel="nofollow">Shor's algorithm</a>, and I am reading several papers that related to the quantum Fourier transform (<a href="http://en.wikipedia.org/wiki/Quantum_Fourier_transform" rel="nofollow">QFT</a>). </p>
<p>I know the there is a dif... | 4,708 |
<p>Simple power counting tells you that a scalar field coupled to some fermions at one-loop picks up a correction to the mass of the order $\Lambda^2$.</p>
<p>Based on this people say things like "it's natural to expect that the mass of the scalar is roughly the cut-off scale", which in this case is some GUT/Planck sc... | 4,709 |
<p>I am reading a online tutorial about Lagrangian mechanics. In one section, it states that if the kinetic term in Lagrangian has no explicit time dependence, the Hamiltonian does not explicitly depends on time, so $H=T+V$. I just wonder if it is always true that $H=T+V$, why require it has no explicit time dependence... | 4,710 |
<p>According to Peierls and Landau, 2D crystals were thermodynamically unstable. They can't exist!
Of course, this theory was disapproved in 2004 (example: graphene).</p>
<p>What is the general definition of stability of a general system?</p>
<p>What is the thermodynamics' stability?</p> | 4,711 |
<p>I have read most of the supernova article on wikipedia, and there are a lot of numbers and different types of supernovae so I am confused.</p>
<p>What I need to know is how much energy is released from some very powerful supernovae in any form (EM radiation, kinetic energy.. etc) other than neutrinos. </p>
<p>Also... | 4,712 |
<p>I'm looking to compute the pitch and roll of a device fitted with a three axis accelerometer when it is not at rest or moving at a constant velocity. Most applications I've seen so far are for stationary tilt sensing... genuinely stuck on this one! Aiming to use it for a python program I'm working on.</p>
<p>For a ... | 4,713 |
<p>The topological ground state degeneracy(g.s.d.) provides useful information for a topological field theory(TQFT), such as <a href="http://physics.stackexchange.com/questions/92809/topological-ground-state-degeneracy-of-sun-son-spn-chern-simons-theory">this post</a> shows some example.</p>
<p>To <strong>count g.s.d.... | 4,714 |
<p>Related to: <a href="http://physics.stackexchange.com/questions/5456/the-speed-of-gravity">The speed of gravity?</a></p>
<p>In the related question and in many other questions here, it seems as if the propagation speed of the gravitational interaction is $c$. To my understanding, the only axioms of relativity is th... | 158 |
<p>I came across an interesting article by Montesinos (J. Geom. Phys. 2 (1985), no. 2, 145–153.). In it, he finds that spin structures (as lifts of $SO(4)$) are not compatible with all Riemannian metrics on a four-manifold. In other words, he finds there are no "universal spinors" which are compatible with all the poss... | 4,715 |
<p>From what I've learned so far, it appears that all models that attempt to explain the expansion of the universe are either based on Lambda-CDM or quintessence. The former support a big bang with rapid expansion, then deceleration of the expansion and then expansion again (non accelerated expansion) with $w=-1$. The... | 4,716 |
<p>Is there any way to tell how far away a lightning strike is by how its thunder sounds? I thought one way might be by using the fact that higher frequencies travel faster than lower frequencies. Would you have to correct for the fact that thunder may not take a straight path? (If so, this would affect the distance ca... | 4,717 |
<p>My brother and I built a wood burning, convection based, thermal circulating hot-tub. (With and oxyacetylene torch, lots of 1.5" pipe, a brake drum, and a thirty year old jacuzzi). Our design is similar to that of what is manufactured and sold for $7000 by the name of a Dutchtub: </p>
<p><img src="http://i.stack.im... | 4,718 |
<p>When choosing a self-adjoint extension of a Hamiltonian, in general one can obtain domains in which
(i) the probabilities teleport* between points on the boundary
and
(ii) boundary conditions locally conserve probabilities.</p>
<p>The ones which locally conserve probability currents somehow seems nicer to me. Bu... | 4,719 |
<p>The surface of a running track (i.e. cinder or rubber) has an effect on a runner's performance. I would like to get some device for measuring how much energy a runner loses on each surfaces. I've tried to rig up a system with a ball and measuring how high it bounces on both surfaces, but this hasn't worked out well.... | 4,720 |
<p><a href="https://en.wikipedia.org/wiki/Gravitational_wave" rel="nofollow">Gravitational waves</a> are a yet unproven idea... The lack of positive results from <a href="https://en.wikipedia.org/wiki/LIGO" rel="nofollow">LIGO</a> indicates these are still theoretical constructs not yet supported by experimental data. ... | 4,721 |
<p>Vacuum cementing apparently is far more likely in space than on a planetary surface in atmosphere. How long must two surfaces be kept in contact with each other in a vacuum for vacuum weld/cementing to 'take'? </p> | 4,722 |
<p>What is the difference between a battery and a charged capacitor? </p>
<p>I can see lot of similarities between capacitor and battery. In both these charges are separated and When not connected in a circuit both can have same Potential difference <code>V</code>.</p>
<p>The only difference is that battery runs for ... | 4,723 |
<p>I've written a program which simulates the motions of planets and other bodies. I'd like to run it on our own solar system, but to do so I need to know the current positions (preferably in heliocentric coordinates) of the planets as well as their current velocities. Is there a website where I can find this?</p>
<p>... | 4,724 |
<p>By looking at this picture:</p>
<p><a href="http://earthspacecircle.blogspot.com/2013/01/earths-location-in-universe.html" rel="nofollow">http://earthspacecircle.blogspot.com/2013/01/earths-location-in-universe.html</a></p>
<p>The earth is near the center of the universe. I've read that the universe look the same ... | 725 |
<p>The Weyl tensor equates the Riemann tensor in vacuum</p>
<p>$$ C_{\mu \nu \eta \lambda} = R_{\mu \nu \eta \lambda} $$</p>
<p>So it makes me wonder about the tensor</p>
<p>$$T_{\mu \nu \eta \lambda} = C_{\mu \nu \eta \lambda} - R_{\mu \nu \eta \lambda} $$</p>
<p>and how it relates to the 2nd-rank stress-energy te... | 4,725 |
<p>atom has well defined spin(up and down) and orbital(s,p,d,etc) momentum, but when forming crystals, why the spin degree continues to be good quantum number while orbital momentum is quenched?</p> | 4,726 |
<p>When building models people typically gauge $SU(N)$ but rarely try to gauge $SO(N)$ (the only example I know about is $SO(10)$, but even that isn't quite $SO(10)$ but actually its double cover). At least with $SO(2) $ and $SO(3)$ one could choose to gauge these groups or their isomorphisms, $U(1)$ and $SU(2)$. </p>
... | 4,727 |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.