question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>Ok, so on Science channel was a special about Hawking/Susskind debating black holes, which can somehow remove information from the universe.</p>
<p>A) In stars, fusion converts 4 hydrogen into 1 helium, with a small fraction of the mass being converted to energy. </p>
<p>This happens all the time, and nobody is t... | g9544 | [
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<p>In an infinite plane elastic plate of thickness $d$, it is shown that the modes of oscillation corresponding to a fixed time-frequency $\omega$ have wave-numbers given by solutions of the Rayleigh-Lamb equations</p>
<p>$$\frac{\tan (pd)}{\tan (qd)}=-\left[\frac{4k^2pq}{\left(k^2-q^2\right)^2}\right]^{\pm 1}$$</p>
... | g9545 | [
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<p>I'm trying to understand how the <a href="http://en.wikipedia.org/wiki/Gell-Mann%E2%80%93Nishijima_formula" rel="nofollow">Gell-Mann–Nishijima relation</a> has been derived:
\begin{equation}
Q = I_3 + \frac{Y}{2}
\end{equation}
where $Q$ is the electric charge of the quarks, $I_3$ is the isospin quantum number and $... | g9546 | [
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<p>I am trying to recreate the results of an article (membrane simulation) and I have the following line:</p>
<blockquote>
<p>Both particles have the same soft radius,</p>
<p>$U_{rep} (r)/\epsilon = \text{exp}\left\{ -20 (r/\sigma -1 )\right \}$</p>
</blockquote>
<p>With $\epsilon$ being the energy scale, $\si... | g9547 | [
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<p>I want to have a general solution for calculating orbits. Wikipedia says how to put object on circle orbit. We must give it speed $v_1 = \sqrt{g(h_o +R)} $ Where $h_o$ - is the orbit height, and $R$ - Earth radius.</p>
<p>But how to calculate the speed that the object must have, if we want to place it in an ellipt... | g311 | [
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<p>In a Young’s double slit experiment 10 fringes are observed in a given segment of the screen when light of wavelength 500.0 nm is used. When the experiment is performed in a medium of refractive index 1.2, the number of fringes observed in the same segment remains 10. The wavelength of light used in this case is? </... | g9548 | [
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<p>If <strong>yes</strong>, then how does this accord with relativity: the laws of physics are the same in all reference frames? We can move from a reference frame in which the photon has near zero energy density, to a reference frame where it has near infinite energy density.</p>
<p>If <strong>no</strong>, why are th... | g9549 | [
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<p>I am working on the Hydrogen atom and I was trying to calculate $\frac{d<r>}{dt}$ using $$\frac{d<r>}{dt} = \frac{i}{\hbar} <[\hat{H} , \hat{r}]>.$$ Here $r = \sqrt(x^2 + y^2 + z^2)$ and $H = \frac{p^2}{2m} + V$ where $p^2 = -\hbar^2 \nabla^2 $. Now according to Ehrenfest's theorem should behave c... | g9550 | [
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<p>Firstly I think shades of this question have appeared elsewhere (like <a href="http://physics.stackexchange.com/questions/10088/why-does-gravity-need-to-be-quantised">here</a>, or <a href="http://physics.stackexchange.com/questions/57228/why-is-gravity-such-a-unique-force">here</a>). Hopefully mine is a slightly di... | g9551 | [
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<p>There is a large disk of glass sitting outside, it is pretty thick and wide. Via different sensors hooked up to a controller, I can measure the air temperature, glass temperature (on the surface), and air humidity. Since the disk is fairly large, the temperature difference between it and the air could reach a few de... | g9552 | [
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<p>Let us define a 4D rotation by using two unit quaternions: $$\mathring{q}_l=\frac{a+ib+jc+kd}{\left|a+ib+jc+kd\right|}$$ and $$\mathring{q}_r=\frac{e+ib+jc+kd}{\left|e+ib+jc+kd\right|}.$$ They differ only by the real term (the denominators are only for normalization). And let us consider two cases: the $a,b,c,d,e \i... | g9553 | [
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<p>In physics total energy is extremal for the static solutions of equation of motions.</p>
<p>Can anyone explain this sentence to me?</p> | g9554 | [
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<p>The short story is, that I have to calculate some transport coefficients, but using the the MB distribution as my distribution function.
What I currently need to solve is:</p>
<p>${{\mathcal{L}}^{\,\left( \alpha \right)}}=\frac{1}{2}\frac{{\bar{N}}}{V}{{\left( \frac{{{h}^{2}}}{2\pi m{{k}_{B}}T} \right)}^{3/2}}\fra... | g9555 | [
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<p>I am trying to reproduce a calculation by <a href="http://dx.doi.org/10.1080/00218469508014354" rel="nofollow">Carre et al. (1995)</a> in which they calculate the shape of a droplet on an incline.</p>
<p>My issue is in the derivation of the potential energy (essentially the center of mass) of the droplet. The dropl... | g9556 | [
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<p>There is something I don't really understand about flashes of lightning.</p>
<p>When a flash occurs, how come electricity be transferred at the speed of light since electricity's displacement is very slow ?</p>
<p>It is not that I don't understand anything about that. I've read wikipedia and the others. But on tha... | g9557 | [
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<p>I am trying to understand the workings of a CMOS image sensor. I understand that increasing wavelength results in an increased penetration depth in the silicon often used in <a href="http://en.wikipedia.org/wiki/CMOS" rel="nofollow">CMOS</a> image sensors. What I am in need of assistance in understanding is how th... | g9558 | [
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<p>When we use the quantum fourier transform, for a function, the output is entangled, so if a measurement is made on the output, the result may not be that of the function that one wanted in the first place. For a trivial example, consider addition of two polynomials, using DFT, or
even FFT, this is straight forward,... | g9559 | [
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<p>Given a fixed shell with the mass of $M$ and a radius $R$ ,
what would be the metric tensor for $r<R$?
I do know that using Birkhoff Theorem the metric for $r>R$ should be schwarzschild.
I'm not sure how to solve $G_{\mu\nu}=0$ for the inner part, and I'm not sure if I can demand continuity at $r=R$.</p> | g50 | [
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<p>Inspired by a similar post, I wanted to know what video games, past or present, most accurately employ the laws of physics. </p>
<p>I believe this is an appropriate question for this site, since the users are far more qualified to answer it than anyone on typical gaming sites, who think they know a lot more about p... | g9560 | [
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<p>Let us consider the functional integral:
\begin{equation}
\int \mathcal{D} A e^{iS[A]}
\end{equation}
where $S[A]$ is the action for $U(1)$ gauge field and
\begin{equation}
\mathcal{D}A\equiv \mathcal{D}A_0 \mathcal{D}A_1 \mathcal{D}A_2 \mathcal{D}A_3; \\
\mathcal{D}A_i = \prod_x dA_i(x).
\end{equation}</p>
<p>Now... | g9561 | [
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<p>This is a question motivated by <a href="http://en.wikipedia.org/wiki/Angry_Birds" rel="nofollow">Angry Birds</a>. </p>
<p>When playing the game, I notice that if the initial velocity is constant, the way to land a bird furthest away from the launching point is by launching the bird at 45 degrees from the ground. <... | g9562 | [
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<p>I know a lot about Theoretical Physicists from <a href="http://en.wikipedia.org/wiki/Sheldon_Cooper" rel="nofollow">Sheldon Cooper</a> of <a href="http://en.wikipedia.org/wiki/The_Big_Bang_Theory" rel="nofollow">Big Bang Theory</a>. I know about parallel universes (and I wonder if one codingtales in one of the paral... | g9563 | [
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<p>Related: <a href="http://physics.stackexchange.com/questions/18117/is-a-white-object-always-white">Is a white object always white?</a></p>
<p>If you are standing in front of a glass window during the day, you can see your dim white t-shirt’s reflection in the window. The reflection is dim because only 4% of the lig... | g9564 | [
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<p>Suppose we have a container divided into equal halves. Right half is fixed at temperature $T$, volume $\frac{V}{2}$. </p>
<p>Initially it has pressure $P_0$, a hole of area $A$ is opened between them. I have to show that the pressure in the right half is:</p>
<p>$$ p = \frac{p_0}{2} \left[ 1 + exp \left(-\langle v... | g9565 | [
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<p>OK, so $\ce{Fe}$ is the most 'stable element'. As such, why do all elements above it not decay into $\ce{Fe}$? In all cases, would it not lead to an increase in binding energy and therefore energy been released, meaning it is energetically feasible, and should happen spontaneously (given enough time)? </p> | g9566 | [
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<p><strong>Disclaimer</strong>: I am not asking whether or not the events that occur in Anathem are possible. I understand that this book is a work of speculative science fiction and this site has no interest in it as such. I'm asking about the physics of a <strong>particular</strong> plot device.</p>
<p>In Anathem, N... | g9567 | [
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<p>I'm revisiting the elementary algorithms of renormalization that are taught in a classroom setting and find that the procedure taught to students is as follows:</p>
<ol>
<li>Write down the bare Lagrangian: $\mathcal{L}_0$</li>
<li>Renormalize the field strengths: $\phi_0\rightarrow Z^{1/2}\phi_r$</li>
<li>Renormali... | g9568 | [
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<p>Can some one please explain to me why electricity flowing though a copper coil generates a magnetic field or where I could possibly find that information? Are there other materials that produce a magnetic field when a current is run through them in a different shape? Thanks!</p> | g9569 | [
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<p>Consider the typical problem, "You have a conducting sphere of charge $Q$ and a point charge q a certain distance away, what is the force on the point charge?". The solution is a simple application of the method of images, but in Griffiths part of the argument relies on saying
$$Q_{conductor} = \sum_i q_{image}$$
Or... | g9570 | [
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<p>Hydrogen is the lightest element, so it's cable of lifting the most weight in out atmosphere (probably not the best terminology there, but you get the picture)</p>
<p>Would hot hydrogen (in the same sense as hot air) be able to lift even more mass?
Would a higher or lower density of hydrogen in a ballon lift more?
... | g978 | [
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-0.02570066973567009,
0.0050882906652987,
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-0.0134... |
<p>The change of entropy is defined $$\Delta S = \int \frac{dQ_\mathrm{rev}}{T}.$$ If a system is isolated the heat transfer between the system and the surroundings is zero ($dQ = 0$), thus $\Delta S = 0$.</p>
<p>However, it is commonly stated that the entropy of an isolated system can increase. How is this possible... | g9571 | [
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... |
<p>I was in Solid State Physics lecture yesterday and we BRIEFLY went over what causes phonons to collide with one another. Things such as crystal imperfections, boundaries, Temperature, but I was wondering:</p>
<p>Do Optical mode phonons interact differently than acoustic ones? Also, is there a way I can quantify thi... | g9572 | [
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0... |
<p>As I see it, as light comes through approximately same angular area of the lens, it should have less aberrations, and at the same time, it should still have high θ -> diffraction limit should stay as without this diaphragm.</p>
<p>What am I missing?</p> | g9573 | [
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<p>I'm looking for a list of immiscible (and miscible) liquids.</p>
<p>I currently developing a game that will use the property of differents liquid, some of them will mix, other will not (positioning in the recipient is important so density is).</p>
<p>I'm searching but not found anything very relevant.</p>
<p>Of c... | g9574 | [
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0... |
<p>This may sound as a mathematical question, but it should be very familiar to physicists. I am trying to perform an expansion of the function $$f(x) = \sum_{n=1}^{\infty} \frac{K_2(nx)}{n^2 x^2},$$ for $x \ll 1$. Here, $K_2(x)$ is the modified Bessel function of the second kind. This series is a result of solving the... | g9575 | [
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<p>The dynamic viscosity of air is $1.78\cdot 10^{-5}Pas = 1.78\cdot 10^{-5}\frac{kg}{ms}$. How do I convert this to the minute scale? I see a reason for multiplying the value with 60 (because of the $\frac{kg}{ms}$) or dividing it by 60 (because of the $Pa s$).</p>
<p>Background: I need the value in a numerical CFD s... | g9576 | [
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<p>What do the supercharges in <a href="http://en.wikipedia.org/wiki/Supersymmetry_algebra">extended supersymmetry</a> do?</p>
<p>In $N=1$ supersymmetry there are a certain number of fermions and and equal number of bosons. You can transform all fermions to the bosons (and vice versa) in a 1 to 1 fashion using a singl... | g9577 | [
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<p>The other day we derived Kepler's third law.</p>
<p>$$
\left( \frac{T_1}{T_2} \right)^2 = \left( \frac{r_1}{r_2} \right) ^3
$$</p>
<p>In order to derive this, you can look at a given planet that revolves around the sun. If you assume that the sun is way heavier than the planet and that the planet moves on a circ... | g9578 | [
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<p>So this appears in a problem which looks simple enough in its context; It's something like this:</p>
<blockquote>
<p><em>Two discs, <strong>A</strong> and <strong>B</strong>, are mounted coaxially on a vertical axle. The discs have moments of inertia $I$ and $2I$ respectively about the common axis. Disc A is impa... | g9579 | [
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<p>The Hamiltonian for an electron of mass $m$ and charge $e$ in an exterior electromagnetic field is
$$H=\frac{1}{2m}(p-(e/c)A)^2+e\varphi.$$
The corresponding (via canonical quantization) quantum mechanical Hamiltonian is not invariant under Gauge transformations (for $A$ and $\varphi$). What's the physical meaning ... | g9580 | [
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<p>In <a href="http://www.itp.uzh.ch/lectures/jetzer/GR2013/GR.pdf" rel="nofollow">these lecture notes</a> the static isotropic metric is treated as follows (p. 71):</p>
<p>Take a spherically symmetric, bounded, static distribution of matter, then we will have a spherically symmetric metric which is asymptotically the... | g9581 | [
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<p>For an harmonic oscillator we can write the Hamiltonian eigenvalues in the basis of the amplitude eigenvalues : for example the ground state is a gaussian : $⟨x|0⟩=a.e^{-b.x^{2}}$. </p>
<p>I was wondering if we can do that for the energy states of a field : can we calculate $⟨a|n⟩$ with |a⟩ an eigenvalue of the fie... | g9582 | [
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<p>Two inertial frames K and k’ are considered. They are in relative uniform motion along the x-x’ direction with relative speed =v.
In the frame K’ we have a cuboidal piece of dielectric [at rest wrt K’]with a flat face perpendicular to the x-x’ direction, that is, this particular face is parallel to the y-z direction... | g9583 | [
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<p>So the wave moves like. a wave, it moves up and down, up and down. But how do photons move? Do they follow the same path or do they just go straight forward without oscillating?</p> | g9584 | [
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<p>Let's say I have a cylindrical piston filled with air and fitted with an electrical resistor. The resistor has a current going across it and a voltage for a certain amount of time. My system would be the air inside the piston.</p>
<p>In the first law of thermodynamics: Q - W = E,</p>
<p>Would the power from the re... | g9585 | [
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<p>Not sure if this is perfectly a physics question:</p>
<p>A rope is wrapped around a cylinder of diameter D. The cylinder is slightly eccentric so that the distance between the axis of the cylinder and the axis of rotation is e. As the cylinder rotates, it pulls the rope. At t=0, the far end of the rope is at positi... | g9586 | [
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<p>For example; is $$\partial_{\rho}\partial_{\sigma}h_{\mu\nu} - \partial_{\sigma}\partial_{\rho}h_{\mu\nu}=0$$ correct?</p> | g9587 | [
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<p>First I must let you know that I don't have much understanding of neither GR nor quantum mechanics, and therefore this question.</p>
<p>I've mentally pictured Newtonian physics, GR and quantum mechanics all somewhat competing (in terms of use case) physics models. For one situation Newtonian physics model may be su... | g9588 | [
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<p>A free particle moves along geodesics, one form being</p>
<p>\begin{split}
\ddot x^\mu &= -\Gamma^{\mu}_{\sigma \rho} \dot x^\sigma \dot x^\rho \\
&= -\frac{1}{2}g^{\mu \nu}(\partial_\sigma g_{\rho \nu} + \partial_\rho g_{\sigma \nu} - \partial_\nu g_{\rho \sigma}) \dot x^\sigma \dot x^\rho
\end{split}</p>
... | g9589 | [
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<p>Is there a software (open source preferred) where I would input something like "Ingoing: a fermion $(p1, s1)$ and a photon $(p2, s2)$. Output: A fermion $(k1, r1)$ and a photon $(k2, r2)$" and I would could then get each diagram (that is, each term in the Dyson series) up to whichever order I choose, also showing te... | g9590 | [
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<p>I can not do the following problem:</p>
<p>Prove the identity:</p>
<p>$$\left( \frac{\partial x}{\partial y} \right)_{z}\left( \frac{\partial y}{\partial z} \right)_{x}\left( \frac{\partial z}{\partial x} \right)_{y}=-1$$</p>
<p>State the properties that must be $x=x(y,z)$, $y=y(x,z)$, $z=(x,y)$.</p>
<p>The trut... | g9591 | [
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<p>Say I have a laser. If I spin the laser so that the beam sweeps in an arc along a very distant object, could that dot travel faster than the speed of light?</p>
<p>In Diagram form:
<img src="http://i.stack.imgur.com/tqv8C.jpg" alt="laser beam thought experiment"></p> | g312 | [
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<p>A candle is floating in a liquid placed in a container. The container is a cylinder of diameter $D$, and the candle is of width $d$. ($D>d$) The height of the liquid from the bottom of the container is $p$, and the height of the candle flame from the bottom is $h$. The density of the candle material is $p_c$ and ... | g9592 | [
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<p>So I am wondering what kind of two dimensional Fermi surface is called quasi one dimensional, what is its character? Also, when there are orbital hybridization taking place in lattice site, what are characters of hybridized Fermi surface?</p> | g9593 | [
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<p>I'm studying the introduction of flavour degrees of freedom in the AdS/CFT correspondence and now I'm supposed to calculate the mass spectrum of mesons in the <a href="http://www.google.com/search?as_q=coulomb+branch" rel="nofollow">Coulomb branch</a>. I have searched the concept but I always find very long and comp... | g9594 | [
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<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/8659/invariant-spacetime-distance-circular-motion">Invariant spacetime - distance - Circular Motion</a> </p>
</blockquote>
<p>This is a question that I thought up a few years ago when I was taking mechan... | g306 | [
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<p>I have difficulty understanding the following vector calculus example. Text can be found <a href="http://www.nhn.ou.edu/~shaferry/41832005_files/l9.pdf" rel="nofollow">here</a>. It is the 5th Q&A -- starting with equation (31.1035).It concerns finding the vector potential of a current loop.</p>
<p>I get the arg... | g9595 | [
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<p>In a nutshell, Noether's theorem states that for every continuous symmetry a corresponding conserved quantity exists.</p>
<p>Now, the Hamiltonian equations of motion (let's talk about a classical system here) are invarinat under addition of any constant to the Hamiltonian function</p>
<p>$$H\rightarrow H+ \text{co... | g9596 | [
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<p>The action</p>
<p>$$S=\int L \;\mathrm{d}t$$</p>
<p>is an important physical quantity. But can it be understood more intuitively? The Hamiltonian corresponds to the energy, whereas the action has dimension of energy × time, the same as angular momentum.</p>
<p>I've heard the action being described as a measure of... | g9597 | [
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<p>I've had this conceptual problem with Faraday's law and inductance for a while now.</p>
<p>Take the example of a simple current loop with increasing area in a constant field (as in <a href="http://physics.stackexchange.com/a/1790/12255">this answer</a>). So Faraday's law states that the increasing flux (due to the ... | g9598 | [
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0.036018... |
<p>I've had a wild idea which I can not discuss at length in this forum, but it comes down to the following problem:</p>
<p>A sphere of radius R=~10μm and mass m=~10-16Kgr is travelling towards the earth at v = ~10^8m/sec. The sphere carries a charge Q and intersects the earth's magnetic field perpendicularly, at a di... | g9599 | [
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<p>Wigner crystal and FQH effect are both due to strong electron-electron interaction under magnetic field. As we know, Landau's symmetry-breaking cannot be used to describe FQH state. But can it be used to describe Wigner crystal state and which symmetries are broken in Wigner crystal?</p> | g9600 | [
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<p><em>Note:</em> This is a rewrite of the original question, which was titled <a href="http://physics.stackexchange.com/revisions/20511/2">What would time be for 2D beings?</a></p>
<hr>
<p>In my current, non-physicist's understanding, every instant of our three‑dimensional world is just another 'slice' of a fo... | g9601 | [
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<p>How do we estimate the energy released from knocking on a wooden table or a door?</p> | g9602 | [
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<p>I am learning about circular motion and not quite sure how to approach this particular problem. Any help would be greatly appreciated!</p>
<blockquote>
<p>A particle moves along a circular path over a horizontal $x$-$y$ coordinate system, at constant speed. At time $t_1 = 5$ s, it is at point ($4.00$ m, $8.00$ m)... | g9603 | [
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<p>When you boil a liquid the KE stays the same (as temp is the same), so where does the extra energy go? I know the extra energy put in breaks bonds between the molecules/ atoms, but once it has broken the bonds where does the energy go?</p>
<p>And where does the potential energy in a liquid go- as (ideal) gases don'... | g9604 | [
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<p>Let's say we have two systems A and B. Each system is described by a density matrix $\rho_A$ and $\rho_B$. I'm wondering about the formal notation to write down the expectation value of an operator that takes us from system A to system B or vice versa.</p>
<p>Normally the expectation value of an operator M acting o... | g9605 | [
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<p>I have to find the Lagrangian for a system.
In the point of interest I have come up with the following position coordinates:</p>
<p>$$x = Rcos(\omega t)+\ell sin(\phi)$$
and
$$y = Rsin(\omega t)-\ell cos(\phi)$$
Now, since I want to find the Lagrangian I need to take the derivative with respect to $t$ of both of th... | g9606 | [
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<p>I posted this in the mathematical forums. Maybe you will help me. I found an hard article <a href="http://prola.aps.org/abstract/PR/v105/i3/p776_1" rel="nofollow">http://prola.aps.org/abstract/PR/v105/i3/p776_1</a> of yang huang and luttinger. The authors begins with the sum: $$\frac{{E_2 }}{{E_0 }} = \frac{{16\pi ^... | g9607 | [
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<p>I have action as shown below
$$S=\int \mathrm{d}t \int \mathrm{d}x^3 \bar\psi\left(i\partial_t\psi +\frac1{2m}\bar\nabla^2\psi-V(x)\psi\right)$$ </p>
<p>How do I manipulate it to obtain the Lagrangian density in Standard form
$$\mathcal{L}\left(\{\varphi_r\},\{\partial_\mu\varphi_r\} \right)$$ </p> | g9608 | [
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<p>how electronic ink display retains the particles on top of micro capsule when
charge on electrode is released or when device is powered off?</p>
<p>Doesnt that make particles loose on the dielectric fluid on capsule after volatge on electrode is removed?</p>
<p>How e ink maintains image even when device is powere... | g9609 | [
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<p>I have a question on the reformulation of the Klein Gordon equation in terms of Killing fields.
Suppose we have a static spacetime with timelike Killingfield $\xi^{\mu}$
(e.g. Schwarzschild).
Then the wave eq. ($\nabla_{\mu}\nabla^{\mu}-m^{2})\phi=0$
can be rewritten in the following form
$$
\partial_{t}^{2}\ph... | g9610 | [
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0.04... |
<p>Inspired by <a href="http://arxiv.org/abs/1401.5761" rel="nofollow">Stephen Hawking</a> I recently tripped upon an idea of what is really inside a black hole.</p>
<p>I thought if NASA (or any other space agency) could send a super protected camera into a black hole, then we could see what's inside black hole.</p>
... | g9611 | [
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<p>A recent paper by the CDMS collaboration (<a href="http://prl.aps.org/abstract/PRL/v111/i25/e251301">PRL here</a> and <a href="http://cdms.berkeley.edu/CDMSII_Si_DM_Results.pdf">free text here</a>) makes this statement in the abstract:</p>
<blockquote>
<p>This blind analysis of 140.2 kg day of data taken between ... | g9612 | [
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... |
<p><img src="http://i.stack.imgur.com/MEo7X.jpg" alt="enter image description here">
<img src="http://i.stack.imgur.com/U1dyL.png" alt="enter image description here"></p>
<p>My interpretation: When we have no angular momentum, the potential well looks like this, my question is: How do you find the point where the wave... | g9613 | [
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... |
<p>Suppose you accelerate a body to very near the speed of light $c$ where $v = c - \epsilon$. Although this would take an enormous energy, is it possible the last arbitrarily small velocity needed -- $\epsilon$ -- could be overcome with a minor bump in velocity due to the <a href="http://en.wikipedia.org/wiki/Uncerta... | g9614 | [
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<p>It appears that velocity is a quantity of motion meaning that all objects can have assigned to them a particular velocity. Through the application of forces (ex: gravity, E&m) we measure changes in velocity, which is called acceleration but here is the crux:</p>
<p>Once the force ceases to be applied the body w... | g9615 | [
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<p>What's the physical difference between the quantities $\left\langle v_{i}v_{j}\right\rangle $ and $\left\langle v_{i}\right\rangle \left\langle v_{j}\right\rangle $? </p>
<p>Where $\left\langle v_{i}\right\rangle$ is the mean value of the $v_{i}$ velocity component with $i=1,2,3$. </p>
<p>(Same story with $v_{j}$.... | g9616 | [
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<p>A pion is made out of a pair of up and/or down quarks. A neutron or proton is three up or down quarks. So naively I'd expect a pion to be about 2/3 the mass of a nucleon.</p>
<p>In fact it's less than 1/6 the mass. Why is it so much lighter? Is there some reason why a lot less energy is contained in the gluon field... | g9617 | [
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<p>We tried to measure background radiation using a geiger counter for a experiment at school. The meter showed $0.12$-$0.21$ microSv/h during the day averaging at about $0.14$ mcSv/h.</p>
<p>As we tried to see ways how to shield incoming radiotion nothing seemed to work. Taking a cue from nuclear power plant we put ... | g9618 | [
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<p>So I am told that pressure = Force per Area --> F/A..</p>
<p>When considering the units of Force I find that force = kg * m/s^2</p>
<p>When considering the units of Area I find that area = m^2</p>
<p>Thus the units of Pressure = kg/s^2 *1/m = d^2(mass)/dt^2 * 1/m </p>
<p>If I interpret this correctly that means ... | g9619 | [
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<p>Consider the following problem taken from a problem booklet. My questions are:</p>
<ol>
<li>What is displacement vector?</li>
<li>And how to determine the direction of displacement vector at a certain point?</li>
<li>Where is the position with zero displacement vector?</li>
</ol>
<p><img src="http://i.stack.imgur.... | g9620 | [
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<p>I'm trying to write down the bath Hamiltonian for a system of dimers and trimers. Imagine each of the monomers in the excited state can interact with several phonons with given frequencies. The bath Hamiltonian has the following form
$\sum\limits_{n = 1}^N {\sum\limits_{j = 1}^M {{\omega _{nj}}} } a_{nj}^\dagger {a... | g9621 | [
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<p>Why <a href="http://en.wikipedia.org/wiki/Perpetual_motion" rel="nofollow">perpetual motion</a> wouldn't be possible if we are so technological advanced?</p>
<p>It is just a thing that I was wondering for too long. I mean, we are able to create so powerful permanent magnets, like neodymium magnets which can store a... | g9622 | [
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<p>I was going through <a href="http://arxiv.org/abs/quant-ph/0001106" rel="nofollow">arXiv:quant-ph/0001106v1</a>, the first paper by Farhi on adiabatic quantum computation.
Equation 2.24 says, $$\tilde{H}(s) = (1-s)H_B + sH_P$$ which means the adiabatic evolution starts from the ground state of $H_B$ and slowly evol... | g9623 | [
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<p>I am given the following equations.
$$\frac{\partial ^2\tilde \psi(x,\omega)}{\partial x^2}+k^2(x,\omega)\tilde\psi(x,\omega)=0 $$
Where
$$k^2 =\frac{2m\omega}{\hbar}$$
And then
$$\frac{\partial ^2\tilde E(x,\omega)}{\partial x^2}+\frac{n^2(x)\omega^2}{c^2}\tilde E(x,\omega)=0$$
We have the boundary conditions:
$$\... | g9624 | [
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0.... |
<p>My questions are:</p>
<ol>
<li>How does a black body absorb photons? </li>
<li>Why does a black body absorb the most photons of all objects (e.g. those with another color)? </li>
<li>Are there any relationship between the electron and a black body?</li>
</ol> | g9625 | [
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0.05... |
<p>Does the inverse square law have anything to do with conservative behavior of the central forces? </p> | g9626 | [
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<p>The book I am reading takes the unjustified step $$e^{-\frac{i}{\hbar}\vec{p}\cdot\vec{r}}f(\vec{p}) = f(\frac{\hbar}{i}\vec{\nabla})e^{-\frac{i}{\hbar}\vec{p}\cdot\vec{r}}$$</p>
<p>and similarly, he uses elsewhere $$e^{-\frac{i}{\hbar}\vec{p}\cdot\vec{r}}g(\vec{r}) = g(i\hbar\vec{\nabla}_p)e^{-\frac{i}{\hbar}\vec{... | g9627 | [
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<p>I know that by Heisenberg's Uncertainty Principle that it is not possible to know the exact values of position and momentum of a particle simultaneously, but can we know the exact values of momentum and velocity of a particle simultaneously? I would think the answer would be no because even if we were 100% certain o... | g9628 | [
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<p>I'm a little confused.. When I write the equation of the circuit, do I have to keep the signs of the induced emf and self-inductance opposite?</p>
<p>I'll try to explain better my doubt:
Consider a coil that has resistance R and inductance L. Running a current will give rise to both an induced emf and self-inductan... | g9629 | [
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<p>Dynamic structure factor is the spatial and temporal Fourier transform of Van Hoves time dependent pair correlation function. It is written as</p>
<p>$$ S(k,\omega)= \frac{1}{2\pi}\int F(k,t)\exp(i\omega t)dt $$</p>
<p>$F(k,t)$ is intermediate scattering function.
My question is how did we use spatial and tempora... | g9630 | [
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<p>Connect the positive terminal of a battery to a piece of p-doped semiconductor, say, silicon doped with boron. Will the terminal pull electrons out of the doped silicon, or equivalently, inject holes into it?</p>
<p>The atomic structure of p-doped semiconductors certainly <em>accepts</em> electrons freely, but I fe... | g9631 | [
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<p>In inflationary theory, many papers start off by making the slow-roll approximation, on which many things depend. This approximation is usually presented by requiring that two 'slow-roll parameters' are small:
$$\epsilon_V\equiv\frac{1}{16\pi G}\left(\frac{V'}{V}\right)^2 \ll 1$$
$$|\eta_V|\equiv \frac{1}{8\pi G}\le... | g9632 | [
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<p>I'm new to physics and am just going through some of the free online classes at World Science U, and after watching this video on the nature of the speed of light and its constancy, a question came to mind about photons. (Video: <a href="http://www.youtube.com/watch?v=fDm1PQgIq7U">YouTube Video</a>, <a href="http:... | g9633 | [
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<p>given a Hamiltonian and the semiclassical WKB partition function in units $ \hbar =2m=1 $</p>
<p>$ \Theta (t) = \frac{1}{2\pi} \iint dx dp exp(-tp^{2}-tV(x)) $</p>
<p>can i use this Theta function to evaluate the Funntional determinant ??</p>
<p>$ \zeta (s,z^{2}) \Gamma (s)= \int_{0}^{\infty}dt\Theta (t) exp(-tz^... | g9634 | [
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<p>Put the classical electron-emitting double-slit apparatus in a sealed box. At each slit there's a counter to check whether an electron has passed it or not, so had the apparatus been left in open, we would have observed dots pile after the two slits on the screen with no interference pattern. But now the apparatus i... | g9635 | [
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<p>I'm supposed to calculate the capacitance of an unknown capacitor in series, but I'm not sure exactly which equation to use.</p>
<p>I know the voltage across the resistor (Vr), voltage across the capacitor (Vc), the total voltage (V), the resistance value of the resistor (R), the frequency (f), and since I was usin... | g9636 | [
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<p>What's the difference between particle's helicity, spinor's helicity and chirality ?</p>
<p>I read a saying that right-handed spinor correspond to the left-handed positron, right-handed spinor correspond to the right-handed positron. </p>
<p>Why the particle's helicity is different from the spinor's? </p> | g9637 | [
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<p>In the late nineties Bender has started a research program on what is called PT symmetric QM, or non hermitian QM, in which he has shown that if the hamiltonian enjoys a PT symmetry then the spectrum is real even if the hamiltonian was not hermitian. Now a search for "PT symmetric" on the arxiv will return over 400 ... | g9638 | [
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... |
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