question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<blockquote>
<p>Perpetual motion describes hypothetical machines that operate or produce useful work indefinitely and, more generally, hypothetical machines that produce more work or energy than they consume, whether they might operate indefinitely or not.</p>
</blockquote>
<p>(Source:<a href="http://en.wikipedia.or... | 5,097 |
<p>I think now is a good time to introduce my son to theoretical physics. He asks so many questions about the universe, black holes, gravity, atoms, molecules, light, etc. He's borderline obsessed with the idea of more than 3 space dimensions. And he tries to apply math to everything. So I'd love to find a great in... | 98 |
<p>From Wikipedia <a href="http://en.wikipedia.org/wiki/Spin_triplet" rel="nofollow">here</a> and <a href="http://en.wikipedia.org/wiki/Triplet_oxygen" rel="nofollow">here</a>:</p>
<p><em>''Almost all molecules encountered in daily life exist in a singlet state, but molecular oxygen is an exception.''</em></p>
<p><em... | 5,098 |
<p>How many percent of the whole visible light reaching the Earth are from other stars than the Sun?</p>
<p>Is it maybe 0,5 - 1% or is my guess already too much?</p>
<p>I am interested mainly in visible light, but if you have knowledge about other parts you can drop it too ;)</p> | 5,099 |
<p>ive been given a problem in physics, its to prove if a lump of metal we have been given is real gold or not. one way to to do it would be to work out its density, which is fairly easy, mass/volume. (its not real gold by the way, we just have to show it). But as this is the route that all the other groups are gonna g... | 5,100 |
<p>I'm still struggling a bit with some ideas around double slit experiments. One that keeps cropping up for me is the role of observers. </p>
<p>Imagine a classic double slit experiment with a hidden observer who has arraigned an apparatus to detect which slit the electron's are passing through. This person and their... | 5,101 |
<p>If my understanding is correct, the temperature of space (as defined by the temperature that a black-body will reach) has been decreasing since the big bang. It has never increased. Additionally, because of the existence of dark-energy, it seems most likely that the temperature never <em>will</em> increase.</p>
<... | 5,102 |
<p>In the end, are they not a condensed form of energy?</p>
<p>I want the 'Why' to go as far as it takes. Where it ends?</p>
<p>What are the constituents of the electron and proton, that make them attract each other?</p> | 176 |
<p>When we go to bed at home, we started to put a bowl of water on the radiator (the air gets a bit dry).</p>
<p>By instinct I put a soaked piece of paper (e.g. toilet paper) into the bowl and let it touch the radiator. The next morning all the water in the bowl was gone. My wife was not so sure that the paper actuall... | 5,103 |
<p>In a lab experiment, we connected a simple circuit: an AC voltage source, connected (in series) to a variable resistor and an inductor. We measured the current in the circuit, and the voltage that falls over the inductor.
We calculated the phase difference between the voltages and used it to calculate $V_L$, and use... | 5,104 |
<p>A neutron can decay into a proton, an electron, and neutrino. Could an antiproton, a positron, and a neutrino combine into a neutron? Could this be where much of the "missing" antimatter is?</p> | 5,105 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/6108/comprehensive-book-on-group-theory-for-physicists">Comprehensive book on group theory for physicists?</a> </p>
</blockquote>
<p>I'm having a hard time trying to get my head around the fundamentals o... | 177 |
<p>I heard the term <em><a href="http://www.google.com/search?q=conformal+quantum+mechanics" rel="nofollow">Conformal Quantum Mechanics</a></em> used today.</p>
<ol>
<li><p>What exactly does this mean? </p></li>
<li><p>Why would one want to study this?</p></li>
</ol> | 5,106 |
<p>In the classic experimental physics text "<a href="http://rads.stackoverflow.com/amzn/click/B0000CKKGJ" rel="nofollow">Statistical Theory of Signal Detection" by Carl. W. Helstrom</a>, Chapter II, section 4 concerns Gaussian Stochastic Processes. Such a process is observed at times $t_1, t_2, t_3, ... t_n$ to obtain... | 5,107 |
<p>I think most physicists mostly model physical systems as some kind of Hilbert space. </p>
<p>Hilbert spaces are a strict subset of <a href="http://mathworld.wolfram.com/BanachSpace.html" rel="nofollow">Banach spaces</a>.</p>
<p><strong>Questions:</strong></p>
<ul>
<li><p>Can physical systems really have non-compa... | 5,108 |
<p>A Newtonian homogeneous density sphere has gravitational binding energy in Joules $U = -(3/5)(GM^2)/r$, G=Newton's constant, M=gravitational mass, r=radius, mks. The fraction of binding energy to gravitational mass equivalent, $U/Mc^2$, is then (-885.975 meters)(Ms/r), Ms = solar masses of body, c=lightspeed.</p>
... | 5,109 |
<p>Is it true that the whole galaxy is actually revolving, and powered by a black hole?</p>
<p>Has it been proven, and if it is true, how can our solar systems actually keep up the momentum to withstand the pull?</p> | 5,110 |
<p>Why GPS/GLONASS/Galileo satellites are on low earth orbit?</p>
<p>Why geostationary orbit is so bad? Sattelites might be placed there 'statically' and more precise...</p>
<p>The only problem I can see is navigation close to poles, but they have this problem anyway.</p> | 5,111 |
<p>this is my mental picture for how they travel without a medium, how (like water waves) some can't stay still, why they have wave and particle properties, energy/mass equivalence, conservation, etc. it might capture uncertainty too -- i've heard that all waves have an uncertainty relation (say in their power spectru... | 5,112 |
<p>This is the problem I am dealing with:
"One of the functions of an automotive air bag is that it lengthens the collision time. Without an air bag, suppose the 100Ns impulse of a collision is taken up by a fairly rigid dashboard, requiring a time of 15 ms. Find the amount of force exerted on the passenger."</p>
<p>U... | 5,113 |
<p>When an electric dipole of moment $\mathbf{P}$ is located in a non-uniform electric field $\mathbf{E}$, there is an net force exerted on it.</p>
<p>However, the formula of the force in some books is read $\mathbf{F}=\nabla(\mathbf{P}·\mathbf{E})$, while in other books, it is $\mathbf{F}=(\mathbf{P}·\nabla)\mathbf{... | 5,114 |
<p>I'm reading a review about membranes properties and I have reach a section about fluid membranes. The section discuss the principal curvatures ($c_1, c_2$) and the spontaneous curvatures ($c_0$). After stating some properties of $c_0$, the following two sentences appear:</p>
<blockquote>
<p>The membrane also is a... | 5,115 |
<p>So there appears to be quite a bit of misinformation on the web as to why people should stay in their cars during a thunderstorm. So I'd like to clear some things up. One such non-nonsensical answer is that cars have rubber tires which insulate you from the ground. I believe this contributes little to nothing to the... | 5,116 |
<p>I'm wondering why the weak interaction only affects left-handed particles (and right-handed antiparticles).</p>
<p>Before someone says "because thats just the way nature is" :-), let me explain what I find needs an explanation:</p>
<p>In the limit of massless fermions, chirality (handedness) becomes helicity $(\ve... | 5,117 |
<p>The canonical momentum is a fundamental conserved quantity from Noether's theorem for translational invariance of the Lagrangian. Yet I'm finding it very difficult to see its derivation, or even a statement of what it is for something as fundamental as a relativistically moving charge in a static Coulomb electric fi... | 5,118 |
<p>In <a href="http://physics.stackexchange.com/a/18721/2451">this</a> Phys.SE answer
Ron Maimon stats: </p>
<blockquote>
<p><em>there is no relativistic particle formalism in which the particles have postive energies and casual propagation. You can either deal with fields in which case the particle notion is non l... | 5,119 |
<p>My question is in reference to Landau's Vol. 1 Classical Mechanics. On Page 6, the starting paragraph of Article no. 4, these lines are given:</p>
<blockquote>
<p>If an inertial frame $К$ is moving with an infinitesimal velocity $\mathbf\epsilon$ relative to another inertial frame $K'$, then $\mathbf v' =... | 5,120 |
<p>The Liouville-von Neumann equation for the <a href="http://en.wikipedia.org/wiki/Density_matrix" rel="nofollow">density matrix</a> is:</p>
<p>$$ i\hbar\frac{\partial\rho}{\partial t}=[H,\rho],$$</p>
<p>while in the Heisenberg picture:</p>
<p>$$ \frac{d}{dt}A(t)=\frac{i}{\hbar}[H,A(t)] +\frac{\partial A(t)}{\parti... | 5,121 |
<p>In reading a lot of articles about HEP and what the LHC could detect or what it has excluded (like low-mass superpartners) it seems every author essentially assumes that things like low-mass superpartners and dark matter could be produced at the LHC.</p>
<p>How reasonable is it to assume that these particles are pr... | 5,122 |
<p>I have seen many questions on SE on the dual nature of electrons behaving in certain circumstances as particles and as waves in some other circumstance. There is one thing I couldn't get a clear answer on.</p>
<p>When making double slit experiment, we all agree that the electrons behave as waves. The same is true i... | 5,123 |
<p>I read some material in this forum and realize that entanglement entropy does not correspond to long range entanglement. Then what quantity can be used to characterize the topological order in 1+1D topological superconductor that can be obtained numerically? </p> | 5,124 |
<p>Does it make sense to introduce Faddeev–Popov ghost fields for abelian gauge field theories? </p>
<p>Wikipedia says the coupling term in the Lagrangian "doesn't have any effect", but I don't really know what that means. If it doesn't work at all (probably because structure constants are zero?) then <em>why</em> doe... | 5,125 |
<p>A particle of mass $m$ moves freely in the interval $[0,a]$ on the $x$ axis. Initially the wave function is:
$$f(x)=\frac{1}{\sqrt{3}}\operatorname{sin}\Big( \frac{\pi x}{a} \Big)\Big[1+2\operatorname{cos}\Big( \frac{\pi x}{a} \Big) \Big]$$</p>
<p>I let the normalised wave function in one dimension be:</p>
<p>$$\P... | 5,126 |
<p>A hepa vacuum cleaner will pick up fine dust from the floor, filter it and send the clean air out through the exhaust. However with movement in the room fine dust will also be goinng up in the air and so the vacuum will not take it in and this fine dust will settle hours later.</p>
<p>As far as i can see, a vacuum... | 5,127 |
<p>To add to <a href="http://physics.stackexchange.com/questions/884/books-that-every-physicist-should-read">Books that every physicist should read</a>:<br>
A list of popular physics books for people who aren't necessarily interested in technical physics. </p>
<p>(see also <a href="http://physics.stackexchange.com/... | 1,025 |
<p>The friction term in Navier-Stokes equation assumes that the viscosity coefficients are the same for the longitudinal and transverse directions. This doesn't seem intuitive, because the former is essentially a bulk modulus while the latter doesn't involve any compression of the fluid. How is the assumption justified... | 5,128 |
<p>In Jackson's text he says that Faraday law is actually:
$$
\oint_{\partial \Sigma} \mathbf{E} \cdot \mathrm{d}\boldsymbol{\ell} = -k\iint_{\Sigma} \frac{\partial \mathbf B}{\partial t} \cdot \mathrm{d}\mathbf{S}
$$
where $k$ is a constant to be determined.<strong>(page 210, third ed.)</strong>.He claims that $k$ i... | 5,129 |
<p>The second Bianchi Identity is
$$
\nabla_{[a}R_{bc]de}=0
$$</p>
<p>As far as I know, the proof (say, <a href="http://mathworld.wolfram.com/BianchiIdentities.html" rel="nofollow">Walfram Mathword</a>) start by stating the representation of <a href="http://en.wikipedia.org/wiki/Riemann_curvature_tensor" rel="nofollow... | 5,130 |
<p>I'm refering to the Paper:
PHYSICAL REVIEW B 80, 195310 (2009)
"Möbius graphene strip as a topological insulator"
Z. L. Guo, Z. R. Gong, H. Dong, and C. P. Sun. </p>
<p>The paper is also available as a preprint version via:
<a href="http://arxiv.org/abs/0906.1634" rel="nofollow">http://arxiv.org/abs/0906.1634</... | 5,131 |
<p>The quaternion Lorentz boost $v'=hvh^*+ 1/2( (hhv)^*-(h^*h^*v)^*)$ where $h$ is $(\cosh(x),\sinh(x),0,0)$ was derived by substituting the hyperbolic sine and cosine for the sine and cosine in the usual unit quaternion rotation $v'=hvh^*$ and then subtracting out unwanted factors in <em><a href="http://visualphysics.... | 5,132 |
<p>In one of the documentaries hosted by Morgan Freeman, a reference was made that just like an ordinary three-dimensional object like a ruler has scratches and cracks, in the same way there might be minuscule loops in space-time. My question is this: If many bodies of extremely high densities are brought close togethe... | 5,133 |
<p>I once tried to make a magnetic wheel using different shaped magnets and placing them in different orientations and of different shapes.But it did rotate making a small angle ie it didnt rotate even a full circle.I still think I can do this with strongest magnet with a magnet with required north south pole arrangem... | 5,134 |
<p>I tried calculating this, but it gets too complicated.</p>
<p>Assume, we have a Moon orbit station and ISS on Earth orbit. We have a Moon base. We want to send a tourist for a week on the Moon and back. We need to launch only the oxygen/fuel cells and the fuel for trans-lunar and trans-earth injection.</p>
<p>The ... | 5,135 |
<p>During a lecture that I missed, I was trapped when the lecturer uses the relation</p>
<p>$$dp_x~ dp_y ~dp_z ~=~d^3\mathbf{p} ~=~ 4\pi p^2 dp.$$</p>
<p>Can I know how is this relation derived please?</p> | 5,136 |
<p>After reading <a href="http://physics.stackexchange.com/questions/16090/how-do-we-resolve-a-flat-spacetime-and-the-cosmological-principle/">How do we resolve a flat spacetime and the cosmological principle?</a> I still remain perplex.<br>
Please excuse my ignorance and try explaining to me : </p>
<p>I thought that... | 5,137 |
<p>I am reading a book about wave mechanics. There are two different cord (one light and one heavy) connected together, one person waving the lighter one, the wave transverse to the right from the lighter one to the heavy one. The frequency and the wavelength is given so the speed and the tension of the lighter cord ar... | 5,138 |
<p>I am planning a series of science fiction novels that take place on an imaginary binary planet system. Both planets have a lower surface gravity than the Earth and one has slightly more mass than the other. </p>
<p>If average temperature is the same as earth or cooler and the radius of the larger planet is allowed ... | 5,139 |
<p>I'm trying to learn theoretical physics up to string theory. I know linear algebra, calculus 1+2, complex analysis. I know the basics of homology, homotopy, group theory and differential geometry. Now I'm starting to read a first introduction to QM, which is this PDF: <a href="http://arxiv.org/abs/1007.4184" rel="no... | 5,140 |
<p>At a certain instant in AC generator, when the normal of the plane (rectangular coil) makes an angle of 270 degrees with with the magnetic induction B, the value of emf is:</p>
<p>$E = -NAB\omega$</p>
<p>My teachers would usually say that this is the minimum value of emf that a generator produces. Does it really m... | 5,141 |
<p>Consider the "half BHZ" Hamiltonian</p>
<p>$${\cal H}=\sum_{\mathbf{k}}\left(A\sin(k_{x})\sigma_{x}+A\sin(k_{y})\sigma_{y}+{\cal M}(\mathbf{k})\sigma_{z}\right)c_{\mathbf{k}}^{\dagger}c_{\mathbf{k}}$$ where $M(\mathbf{k})=M-2B\left(2-\cos(k_{x})-\cos(k_{y})\right)$</p>
<p>For simplicity, take A=1 and B > 0. M <... | 5,142 |
<p>I have a professor that is fond of saying that vorticity cannot be destroyed. I see how this is true for inviscid flows, but is this also true for viscous flow? The <a href="http://en.wikipedia.org/wiki/Vorticity_equation">vorticity equation</a> is shown below for reference. From this equation, it looks as if vortic... | 5,143 |
<p>If I want to punch a person inflicting maximum damage, what do I need to care about?</p>
<p>My force of punching, i.e, do I need more acceleration? Or do I need momentum, i.e my velocity for punching?</p> | 5,144 |
<p>I am using a thermoelectric cooler from a pc's heatsink to produce electricity. Its size is 30mm by 30mm. I will cool it on one side at -10 degree Celsius and the other at 24 degree Celsius. Can anyone tell me how much electricity in watts will be produced. I'm using this heatsink cooler because it's the most easily... | 5,145 |
<p>The <em>back motor effect</em> (see <a href="http://en.wikipedia.org/wiki/Counter-electromotive_force" rel="nofollow">Counter-electromotive force</a>) is the counter torque which opposes the rotational motion of the coils in a generator when the generator is under load.</p>
<p>The back motor effect depends upon the... | 5,146 |
<p>For a charged quantum particle, say, an electron or a quark, how in the particle's wavefunction is the electric charge represented? Is it truly possible to represent electric charge using the wave mechanics formulation of quantum mechanics, or is that something only matrix mechanics can satisfy? And does the same ... | 5,147 |
<p>Perhaps it is some free moving spinner attached to the wheel, but as opposed to this question: <a href="http://physics.stackexchange.com/questions/41006/why-does-the-wheel-of-a-car-appear-to-be-moving-in-opposite-direction">Why does the wheel of a car appear to be moving in opposite direction?</a>
I have seen car wh... | 5,148 |
<p>Follow by the Michelson–Morley experiment, What happen if we use something that has a speed lower than light instead of light in Michelson–Morley experiment? How about the result?</p> | 5,149 |
<p>Currently, my experiment involves the three elements: <code>Au, Ga, Bi</code>, and I need <code>Au-Ga-Bi ternary phase diagram</code> to explore the underlying reason for my experimental phenomenon. Unfortunately, I can't find any information about this ternary phase diagram both in <code>Web of Knowledge</code> and... | 5,150 |
<p>Einstein described his discovery of the equivalence principle as the "happiest thought of my life". Why? What, in broad conceptual terms, is the logical chain of reasoning that leads from the equivalence principle to general relativity?</p> | 5,151 |
<p>How can you calculate the amount of energy an object produces that falls from a particular height?
Or water.</p>
<p>In an eg i would like to know how can you calculate the amount of electricity water can produce at a certain height?</p> | 5,152 |
<p>In my acoustics books I see</p>
<p>$$c^2 = \frac{\mathrm{d}P}{\mathrm{d}\rho}$$</p>
<p>where $c$ is the speed of sound, $P$ is the pressure and $\rho$ is the density. Where does this equation come from? In my books it appears almost as a definition. Could you explain this, or at least point me to an article or a b... | 5,153 |
<p>I notice that the larger the truck the greater the magnitude of the lurch. Can anyone give a physical explanation to this?</p> | 5,154 |
<p>According to the Wikipedia article on <a href="http://en.wikipedia.org/wiki/Black_hole#Gravitational_collapse">black holes</a>:</p>
<blockquote>
<p><em>Even though the collapse takes a finite amount of time from the reference frame of infalling matter, a distant observer sees the infalling material slow and halt ... | 178 |
<p>Could someone please explain the difference between a <a href="http://en.wikipedia.org/wiki/Wave_packet" rel="nofollow">wave packet</a> and a wave train? I have rummaged around online but have not been able to find a definitive definition. </p> | 5,155 |
<p>In <a href="http://www.tapir.caltech.edu/~chirata/ph236/2011-12/lec19.pdf" rel="nofollow">this PDF</a> [1], is made reference to specific energy and angular momentum of a particle. If the particle has no mass, like a photon, how should I define these terms in the equations further down for the path of the particles?... | 5,156 |
<p>I've been experimenting with radiative cooling lately and in my mind <a href="http://en.wikipedia.org/wiki/Convective_heat_transfer" rel="nofollow">convection</a> is always an enemy (especially forced convection since the modules are outdoors).</p>
<p>Looking at <a href="http://en.wikipedia.org/wiki/Lumped_capacita... | 5,157 |
<p>As I understand it, gravity is inherent to mass and therefore even a small rock has its own gravitational pull. It seems entirely plausible then that a rock 1" in diameter could orbit a bigger rock, say 10" in diameter. Is this actually possible? Are there any practical limits on the size of objects that could have ... | 5,158 |
<p>Systems of charged particles (such as atomic nuclei and electrons) can be described by nonrelativistic quantum mechanics with the Coloumb interaction potential. A fully relativistic description is given by quantum electrodynamics which is much more complex.</p>
<p>Is it possible to expand various quantities in QED ... | 5,159 |
<p>Quantum entanglement links particles through time, according to this study that received some publicity last year:</p>
<p><ul>
<li><a href="http://www.technologyreview.com/blog/arxiv/26270/" rel="nofollow">New Type Of Entanglement Allows 'Teleportation in Time,' Say Physicists</a> at <a href="http://www.technologyr... | 431 |
<p>Is it possible to disregard drag force of water with no viscosity that affects lightweight pop-up ball (its weight is assumed to be zero)? There is a discussion, on which I consider that although weight is small, the influence of drag force does not depend on ball but on water, therefore this force must be taken int... | 5,160 |
<p>For a countable sequence of positive numbers $S=\{\lambda_i\}_{i\in N}$ is there a construction producing a Hamiltonian with spectrum $S$ (or at least having the same eigenvalues for $i\leq s$ for some $s$)?</p>
<p>Here by 'Hamiltonian' I understand a polynomial of $p_i$ and $q_i$ (or equivalently - $a_i$ and $a_i^... | 5,161 |
<p>When an electron emits a photon from changing energy levels, the frequency of the photon depends on the difference between the energy levels.</p>
<p>But if someone is moving with respect to the atom, the frequency will be apparently red shifted or blue shifted.</p>
<p>Does this mean that the energy levels of the o... | 5,162 |
<p>In statistical mechanics, we usually think in terms of the Hamiltonian formalism. At a particular time $t$, the system is in a particular state, where "state" means the generalised coordinates and momenta for a potentially very large number of particles. (I'm interested primarily in classical systems for the sake of... | 5,163 |
<p>Assuming Higgs is found at 125 GeV.Is there any direct or indirect consequence on string theory ? Will it be a blow to string theory or models employing string theory ? </p>
<p>Thanks</p>
<p>Ps - I am just a curious pure maths student, so forgive me if my question makes no sen... | 5,164 |
<p>This might be hard to ask, but here goes nothing.</p>
<p>I recently poured a cup of water into a black coffee cup. There was a light source--not very bright--above the cup. Anyways, I was squeezing a lemon into my water mindlessly, and to make sure I got every last drop of lemon juice into the water, I watched th... | 5,165 |
<p>For this circuit (a and b are connected by a battery), </p>
<p><img src="http://i.stack.imgur.com/CVtE3.png" alt="circuit diagram"></p>
<p>Will I be able to find the total resistance of the circuit by adding resistors that are in series and combining resistors that are in parallel without using the method of Kirch... | 5,166 |
<p>I have some questions about the construction of $\mathcal{N}=2$ supermultiplets <strong>for chiral matter</strong>. I know that the supermultiplet should not include spin one states since they are always in the adjoint representation. So my first question is: <strong>why are spin-one modes always in adjoint represen... | 5,167 |
<p>A ball is thrown straight up. The height of the ball above the ground in (m) is given by the function:</p>
<p>$$h(t)=-5t^2 + vt + c$$</p>
<p>What is the initial velocity if you want the ball to reach a max height of 100 m?</p> | 5,168 |
<p>Let's say you have a spherical charge distribution of radius R. This distribution has some charge density as a function of radius. I know that I can determine the electric field outside of the charge density by forming a spherical gaussian surface around the charge distribution and apply gauss' law. But what if I wa... | 5,169 |
<p>It is written in my quantum physics book that the K shell contains only 2 electrons due to the Pauli principle.</p>
<p>I know that if $n = 1, l = 0, m = 0$, then the Hilbert space associated to the spin is of dimension $2$. I also know that Pauli principle says that if we have a vector $\lvert\psi\rangle$ which rep... | 5,170 |
<p><img src="http://i.stack.imgur.com/lVTfu.jpg" alt="Witten's Dog"></p>
<p>In an old episode (<a href="http://en.wikipedia.org/wiki/Mars_University" rel="nofollow">"Mars University"</a>) of Futurama which is a TV show, a character named Professor Farnsworth was trying to lecture "Superdupersymmetric String Theory" an... | 5,171 |
<p>Can anyone help me Convert a 200 mm linear stroke into 90 degrees motion with as much <a href="http://en.wikipedia.org/wiki/Mechanical_advantage" rel="nofollow">mechanical advantage</a> as possible or into two 90 degrees motions with as much mechanical advantage as possible? </p>
<p>Essentially I am trying to <em>c... | 5,172 |
<p>For a topological field theory to be a true “extension” of an Atiyah-Segal theory, the top two levels of its target (<em>ie</em> its $(n-1)^{\text{st}}$ loop space) must look like $\text{Vect}$. What other (physical) considerations constrain the choice of target category? The targets of invertible field theories ar... | 5,173 |
<p>I'm having some trouble with Exercise 5.1 in Shapiro's BH,WD&NS book, which goes as follows:</p>
<blockquote>
<p>Consider two particles of mass $m$ at distance $r$ and $r+h$, such that $h\ll r$, on the same vertical line from the center of earth. The particles fall freely from rest a the same time $t=0$ towar... | 5,174 |
<p>Why do physical bodies in universe are bound to follow a rule ( eg. laws of physics ) ? Is their any imaginary condition possible, where they stop following any rule. And everything becomes random ? ( For example, a flying butter fly moves randomly, diseases like cancer occur randomly, then why physical bodies too c... | 179 |
<p>In classical integrable models, in the discrete case we have the <a href="http://www.google.com/search?as_epq=Sklyanin+algebra" rel="nofollow">Sklyanin algebra</a>, $$\lbrace T_{a}(u),T_{b}(v)\rbrace =[r_{ab}(u,v),T_{a}(u)T_{b}(v)].$$ How to prove that the conserved quantities are generated from $\ln(\tau(u))$ in th... | 5,175 |
<p>I know the equation for displacement as a function of time is $$\vec{s} = \vec{v_i} \Delta{t}+\frac{1}{2}\vec{a}\,(\Delta t)^2$$
I need to solve for $\Delta t$ I'm having problems rearranging to do this. As of now, I created a Python program to run through the displacement formula, adding a set amount of time in ea... | 5,176 |
<p>In an electron double slit experiment, let's put two charged plates behind the slits in an attempt to move the pattern up and down on the the screen.</p>
<p>What will happen? Will it just shift the interference pattern on the screen or washes it out completely? </p>
<p>If it washes it out, what's the minimal field... | 5,177 |
<p>For a spinning top, the linearised equation in the angle $\theta$ when the top is spinning about its axis of symmetry, which is vertical, is of the form $$A\ddot\theta+\left(\frac{C^2n^2}{4A}-Mgh\right)\theta=0.$$</p>
<p>Why should we require that the bracket coefficient be positive if we want the top to be stable,... | 5,178 |
<p>The title, I don't know whether it's correct or not, but I came across a video in youtube, <a href="http://www.youtube.com/watch?v=_PkgQQqpH2M" rel="nofollow">http://www.youtube.com/watch?v=_PkgQQqpH2M</a>.</p>
<p>The author of video used the title and hence I used the same.</p>
<p>The video doesn't seems to be fa... | 5,179 |
<p>This question relates to <a href="http://physics.stackexchange.com/questions/74567/how-do-i-show-the-existence-of-a-conserved-ghost-number-with-brst-in-bosonic-str">this post</a></p>
<p>I tried to verify Eq. (4.2.7) in Polchinski's string theory book vol I p. 127 but I miserably miss a sign</p>
<blockquote>
<p>$... | 5,180 |
<p>in wikipedia, the weak interaction coupling constant is said to be 10^-13 times weaker than that of the strong interaction but hyperphysics (<a href="http://hyperphysics.phy-astr.gsu.edu/hbase/forces/couple.html" rel="nofollow">http://hyperphysics.phy-astr.gsu.edu/hbase/forces/couple.html</a>) says it's 10^-6. It se... | 5,181 |
<p>You have three separate glass rods, and you know one is positive, one is negative and one is neutral. You also have an electroscope that is positively charged.</p>
<p>How can you determine which rod is positive, negative and neutral?</p>
<p>I think you will be able to determine which rod is negatively charged, as ... | 5,182 |
<p>consider a system of three identical particles, A B ,and C.
Assume that each particle can be in one of three possible quantum states, 1,2 and 3.
For the following statistics listed below, enumerate the possible states of the system and calculate the probabilities that the state 1 has zero,one,two, and three particle... | 5,183 |
<p>How are photons being watched in the double slit experiment? What exactly does being observed mean, as it is obviously changes the state of the photon somehow - it must be depriving the photon of something or emitting something that interacts with the photon.</p> | 5,184 |
<p>I want to measure the avg. inner cross-section area of a flexible tube of outer diameter 5mm. Since the cross-section isn't a circle exactly, use of vernier caliper to measure inner diameter fails. The idea that I wish to try is, to take a tube of known length, fill it with water, then pour the water on to something... | 5,185 |
<p>These days I was studying the quantum theory.I found that some theories about that is similar to Fourier Transform theory.For instance, it says "A finite-time light's frequency can't be a certain value", which is similar to "A finite signal has infinite frequency spectrum" in Fourier analysis theory.I think that a c... | 5,186 |
<p>it is well known that the intrisic spin is closely related to the rotation in space. However, in 1d , it is impossible to define rotation, therefore it is meaningless to talk about spin in 1d.However, one can see many papers talking about electrons in 1d with spin Is this meaningful?</p> | 5,187 |
<p>So I'm trying to do this problem where I'm given the Lagrangian density for a piano string which can vibrate both transversely and longitudinally. $\eta(x,t)$ is the transverse displacement and $\xi(x,t)$ is the longitudinal one. (So a point at $[x,0]$ at some later time t would be at $[x+\xi(x,t),\eta(x,t)]$). The ... | 5,188 |
<p>When we study electricity in high school we examine the resistance of conductors and its relation with temperature. Diagrams show the relationship at the beginning is pretty much a linear with temperature but at very low temperatures its starts to curve in such why it never reaches zero but it is close and the boo... | 5,189 |
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