Recent Questions - Physics Stack Exchange most recent 30 from physics.stackexchange.com 2022-05-28T11:08:00Z https://physics.stackexchange.com/feeds https://creativecommons.org/licenses/by-sa/4.0/rdf https://physics.stackexchange.com/q/710978 0 Derivation of pseudo forces in non inertial rotating frames Dhrxv https://physics.stackexchange.com/users/257194 2022-05-28T10:51:10Z 2022-05-28T10:56:25Z <p>Considering a rotating frame with angular velocity <span class="math-container">$\mathbf{\omega}$</span>. Using the transport theorem , ('R' representing rotating frame and 'I' inertial frame) <span class="math-container">$$\left.\frac{\mathrm d\mathbf{r}}{\mathrm dt}\right|_I - \mathbf{\omega} \times \mathbf{r} = \left.\frac{\mathrm d\mathbf{r}}{\mathbf dt}\right|_R ,$$</span> So, <span class="math-container">\begin{align}\left.\frac{\mathrm d^2\mathbf{r}}{\mathrm dt^2}\right|_R &amp;= \left( \left.\frac{\mathrm d}{\mathrm dt}\right|_I - \mathrm{\omega} \times \right)^2 \mathbf{r} \\&amp;= \left.(\frac{\mathrm d^2\mathbf{r}}{\mathrm dt^2}\right|_I) - (2\mathbf{\omega} \times \left.\frac{\mathrm d\mathbf{r}}{\mathrm dt}\right|_I )- (\left.\frac{\mathrm d\mathbf{\omega}}{\mathrm dt}\right. \times \mathbf{r}) +( \mathbf{\omega} \times (\mathbf{\omega} \times \mathbf{r}) ).\end{align}</span></p> <p>The sign in front of the <span class="math-container">$\mathbf{\omega} \times (\mathbf{\omega} \times \mathbf{r}) \$</span> term should be reversed to obtain the correct expression of centrifugal force . What is wrong in my method?</p> https://physics.stackexchange.com/q/710975 1 How do we extract the classical description of a particle from the QFT description? Ryder Rude https://physics.stackexchange.com/users/156987 2022-05-28T10:35:19Z 2022-05-28T10:38:28Z <p>I'm coming off <a href="https://physics.stackexchange.com/q/710826/">this</a> post.</p> <p>The free field QFT of an elementary particle moving at a relativistic speed can be approximated by the model of a classical particle.</p> <p>The QFT description of a free particle is <span class="math-container">$a_{p}^{\dagger } |0 \rangle$</span>. The time evolution is <span class="math-container">$a_{p}^{\dagger } e^{-iE_p t}|0\rangle$</span></p> <p>The classical description of a free particle is two numbers : <span class="math-container">$(x,p')$</span> position and momentum. The time evolution is a straight line.</p> <p>I'm using <span class="math-container">$p'$</span> instead of <span class="math-container">$p$</span> to not confuse it with the quantum field momentum eigenvalues <span class="math-container">$p$</span>.</p> <p>There's supposed to be some correspondence between these two descriptions. So I'm looking for some procedure to extract the approximate classical description from the QFT description.</p> <p>How do we get the numbers <span class="math-container">$(x,p')$</span> of the classical model? Is <span class="math-container">$p'$</span> somehow equal to <span class="math-container">$p$</span>? What about <span class="math-container">$x$</span> then?</p> https://physics.stackexchange.com/q/710974 0 Vector form of centrepetal force madness https://physics.stackexchange.com/users/311011 2022-05-28T10:31:43Z 2022-05-28T10:39:15Z <p>We know the centripetal force <span class="math-container">$F_c$</span> had magnitude <span class="math-container">$m\omega^2r$</span>. But let's try to write it in vector form.</p> <p>First of all,since it is directed along the radius,the unit vector in radial direction in this case is <span class="math-container">$-\hat{\boldsymbol{r}}$</span>. And since the magnitude is <span class="math-container">$m\omega^2r$</span>,we finally get <span class="math-container">$$\vec{\mathrm{F_c}}=-\mathrm{m}\mathrm{\omega}^2\mathrm{r}\hat{\boldsymbol{r}}=-\mathrm{m}\frac{v^2}{r^2}\vec{\mathrm{r}}.$$</span></p> <p>But this is not how it us done in the books. According to them,<span class="math-container">$$\vec{\mathrm{F_c}}=-\mathrm{m}\frac{v^2}{r^3}\vec{\mathrm{r}}.$$</span></p> <p>I don't understand how they got it,the one i did seems to be completely fine to me. Could anyone tell me the mistake i made?</p> https://physics.stackexchange.com/q/710971 0 Can you combine photons to ionise electrons? John Hon https://physics.stackexchange.com/users/115337 2022-05-28T10:14:56Z 2022-05-28T10:37:30Z <p>I was talking with a friend about the photoelectric effect. I know that only light of a certain energy will eject an electron from a metal plate.</p> <p>But consider this.</p> <p>A photon (red) had the exact energy such that it could be excite an electron to a higher energy state, <em>but</em> not enough for ionisation.</p> <p>Then another photon (blue) tag teams with it, whilst the electron is in the higher energy state. Now with the new energy, it could be ionised.</p> <p><a href="https://i.stack.imgur.com/nHJqa.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nHJqa.png" alt="enter image description here" /></a></p> <p>Is this possible? If not (which is my suspicion), why not?</p> https://physics.stackexchange.com/q/710970 0 Stuck on a rotational dynamics related question Random https://physics.stackexchange.com/users/334963 2022-05-28T09:58:22Z 2022-05-28T10:29:17Z <blockquote> <p>A ball of mass <span class="math-container">$\frac M 2$</span> strikes the bottom point <span class="math-container">$P$</span> of a rod of mass <span class="math-container">$M$</span> and length <span class="math-container">$L$</span> hinged at the top point O with velocity <span class="math-container">$u$</span>. What is the angular velocity of the rod-ball system just after collision?</p> </blockquote> <p>I went about solving the question through conservation of angular momentum since external torque is zero and got the correct answer by conserving it about the hinge with <span class="math-container">$w=\frac{3u}{5l}$</span>. Then I thought about conserving angular momentum about the point <span class="math-container">$P$</span> but couldn't since the <span class="math-container">$L_i$</span> equals zero since <span class="math-container">$L_i=\frac M 2 ur_⊥=0$</span> due to <span class="math-container">$r_⊥$</span> being zero while <span class="math-container">$L_f = Iw + \frac M 2 L^2w$</span> where <span class="math-container">$Iw$</span> can be written as <span class="math-container">$ML^2/3 w$</span> Moreover I can't get the same value of <span class="math-container">$w$</span> via conservation of translational and rotational kinetic energy where I took into account rotational kinetic energy of the rod-ball system about the hinge and the ball's kinetic energy at point <span class="math-container">$P$</span>. I'm stumped for now.</p> https://physics.stackexchange.com/q/710969 0 How do we calculate the effective mass? Atom https://physics.stackexchange.com/users/332123 2022-05-28T09:57:40Z 2022-05-28T10:52:59Z <p>In a band structure calculation, the dispersion relation for electrons is found to be <span class="math-container">$\epsilon_k = \beta (cosk_xa+ cosk_ya+ cosk_za)$</span>, where <span class="math-container">$β$</span> is a constant and a is the lattice constant. Prove that the effective mass at the boundary of the first Brillouin zone is <span class="math-container">$\frac{\hbar^2}{3 \beta a^2}$</span>.</p> <p>Effective Mass, <span class="math-container">$m^* = \frac{\hbar^2}{\frac{d^2\epsilon_k}{dk^2}}$</span></p> https://physics.stackexchange.com/q/710966 0 Responsivity of a photo detector abhishek bhat https://physics.stackexchange.com/users/127358 2022-05-28T09:46:44Z 2022-05-28T09:46:44Z <p>I have a pn junction diode of which responsivity (R = Photocurrent / optical power) is to be calculated. For this purpose i have 5 LEDs of various wavelengths (UV, RED, BLUE etc).</p> <ol> <li><p>In this case, i have kept the optical power of all the LEDs the same by ensuring electrical power supplied to the LED sources are the same. Note that photon density is different among various LEDs</p> </li> <li><p>Here, i keep the photon density a constant, then optical power of the LEDs would be different.</p> </li> </ol> <p>Question is, if i calculate R with above two constraints,i.e. keeping optical power a constant in the first case, and photon density in the other case. Does R value vary depending upon the constraint applied ? Or, is it independent ? Also, is the assumption that electrical power supplied ~ optical power a reasonable assumption ?</p> https://physics.stackexchange.com/q/710964 0 Absence of topology in semi-dirac materials Feynnman pilows https://physics.stackexchange.com/users/336263 2022-05-28T09:28:37Z 2022-05-28T09:28:37Z <p>Good morning everybody, I am facing a problem when calculating the topological invariant in a semi-dirac system, whose Hamiltonian is: <span class="math-container">$$H=k_x^2\sigma_x+k_y\sigma_y$$</span> My question is that this Hamiltonian has time-reversal symmetry, of the form <span class="math-container">$H(k)=-H(k)^*$</span> and therefore should not have topological invariant. Instead I have calculated the edge states and they exist. Does anyone know why it has edge states, but in theory it does not have topological invariance. Is it because the model is developed to too low order? Can anyone think of a way to prove the topology?</p> https://physics.stackexchange.com/q/710960 0 Displacement relation of a progresive wave Cover Spot https://physics.stackexchange.com/users/336655 2022-05-28T08:47:26Z 2022-05-28T10:44:42Z <p>I know that the displacement relation of a body in SHM is given by <span class="math-container">$$x(t) = Acos(\omega t+\phi)$$</span> Displacement relation of a progressive wave is a similar one: <span class="math-container">$$y(x,t) = Acos(kx-\omega t+\phi)$$</span> Is there a relation between the two? Can The progressive wave relation be derived from the SHM relation? I know that a progressive wave is not the same as the sinusodial wave produced by a rigid body in SHM because i tried to use the same concept to derive the displacement relation for progressive wave but it did not work.</p> https://physics.stackexchange.com/q/710958 -1 Has anyone made a objection to fluid sink model? [closed] H JeF https://physics.stackexchange.com/users/336657 2022-05-28T08:44:35Z 2022-05-28T09:32:42Z <p>Poincares objection is <a href="https://physics.stackexchange.com/q/707843/">here</a> (<a href="https://web.archive.org/web/20220510043531/https://physics.stackexchange.com/questions/707843/bad-explanation-by-poincare" rel="nofollow noreferrer">WaybackMachine</a>), the Wikipedia objections are terrible, has anyone made any actual objection to <a href="https://en.wikipedia.org/wiki/Le_Sage%27s_theory_of_gravitation" rel="nofollow noreferrer">Le Sage theory</a>? Not defending it or anything just wondering why Wikipedia is so bad here.</p> https://physics.stackexchange.com/q/710956 0 How ample is a wave amplitude? user157860 https://physics.stackexchange.com/users/157860 2022-05-28T08:37:36Z 2022-05-28T09:45:47Z <p>Is it possible to actually measure/deduce the amplitude of a wave? We know that the length of am infrared wave is a fraction of a millimetre, do you have any idea what is the range of the physical length of a less/more intense wave?</p> <p>Or it makes no sense to talk about the width of an em wave as we can do about a water wave?</p> https://physics.stackexchange.com/q/710953 0 Is there no net voltage over an ideal inductor/coil? No voltage drop? bananenheld https://physics.stackexchange.com/users/318923 2022-05-28T08:30:06Z 2022-05-28T09:52:44Z <p>If you have a coil with self inductance: <span class="math-container">$$\varepsilon= - L \frac{dI}{dt}$$</span></p> <p>Then the current is lagging behind the voltage.</p> <p>If you attach a battery on the coil/inductor and have an AC power source, then at the highest current there is no internal resisting voltage produced bij the self inductor.</p> <p><span class="math-container">$$V_{\text{produced by battery}} - \varepsilon =0 (?)$$</span></p> <p><a href="https://i.stack.imgur.com/p4bQ3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/p4bQ3.png" alt="enter image description here" /></a></p> <p>My question. Is there no net voltage over the inductor at any moment? I.e. the inductor will always perfectly cancel the voltage produced by the battery? How do you calculate this?</p> https://physics.stackexchange.com/q/710945 0 Can we really prove that $E_0 = mc^2$ for a photon? [closed] Emile Couzin https://physics.stackexchange.com/users/333176 2022-05-28T07:16:27Z 2022-05-28T10:42:45Z <p>Let <span class="math-container">$E$</span> be the totale energy of a particle, <span class="math-container">$E_0$</span> the energy when its speed is nil, <span class="math-container">$m$</span> its rest mass, <span class="math-container">$\vec{p}$</span> the momentum vector, <span class="math-container">$\vec{v}$</span> the velocity vector, <span class="math-container">$c$</span> the speed of light, <span class="math-container">$\gamma$</span> a photon and <span class="math-container">$P$</span> the quadrivector energy-momentum.</p> <p>For any particle, we prove <span class="math-container">$E_0 = mc^2$</span> by the invariance of the quadrivector energy-impulsion: <span class="math-container">$$P = \left(\frac{E}{c}, \vec{p}\right)$$</span> <span class="math-container">$$\Longleftrightarrow P^2 = \frac{E^2}{c^2} - \vec{p} \cdot \vec{p}$$</span> Indeed, if we choose a referential where the velocity (and thus the momentum) of the particle equals <span class="math-container">$0$</span>: <span class="math-container">$$P^2 = \frac{E^2}{c^2}$$</span> and as: <span class="math-container">$$\vec{v} = \frac{c^2}{E}\vec{p}$$</span> My book claims that <span class="math-container">$E_0 = mc^2$</span>.</p> <ol> <li><p>I don't understand this demonstration: it seems to me that we use <span class="math-container">$E_0 = mc^2$</span> to prove <span class="math-container">$E=mc^2$</span>.</p> </li> <li><p>What if you have a photon? You can't say that there exists a referential where the velocity equals <span class="math-container">$0$</span>, so how do you prove <span class="math-container">$E_{0, \gamma} = mc^2 = 0$</span> ?</p> </li> </ol> https://physics.stackexchange.com/q/710943 0 What is the displacement time graph for this object which is in x-y plane going in sine wave like path from A to B.Also can velocity constant inpath? Pankaj Shukla https://physics.stackexchange.com/users/336647 2022-05-28T06:52:56Z 2022-05-28T10:46:14Z <p><a href="https://i.stack.imgur.com/fYtTY.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fYtTY.jpg" alt="enter image description here" /></a></p> <p>The path is from A to B in sine wave curve while the displacement is straight line.So how displacement is calculated for graph purpose here</p> https://physics.stackexchange.com/q/710934 2 How does the heat energy from burning a gallon of gasoline compare to the solar energy the resulting $\rm CO_2$ absorbs in the atmosphere? sbirch https://physics.stackexchange.com/users/336638 2022-05-28T04:28:50Z 2022-05-28T09:38:29Z <p>I think essentially all of the energy from a gallon of gasoline burned in, say, a car is eventually converted into heat in the atmosphere. But I don't <em>think</em> the heat from burning fossil fuels contributes in any material way to global warming -- the idea of heating the earth directly, even at the scale we currently burn fossil fuels, seems unlikely. But the earth is nonetheless warming up.</p> <p>That suggests, a little counterintuitively to me, that the solar heat adsorbed by the <span class="math-container">$\rm CO_2$</span> generated from burning a gallon of gas must exceed the heat from burning that gallon of gas (at least over the timescale that that <span class="math-container">$\rm CO_2$</span> remains in the atmosphere -- Google gives me a half-life of ~120 years.) Perhaps even by a few orders of magnitude.</p> <p>To put a specific point on it: what fraction of the total heat from burning a gallon of gasoline actually comes from burning the gasoline? (Over the whole lifetime of the <span class="math-container">$\rm CO_2$</span> in the atmosphere.)</p> https://physics.stackexchange.com/q/710925 1 Evaluating conjugate momentum from a given Lagrangian density kowalski https://physics.stackexchange.com/users/180244 2022-05-28T00:56:17Z 2022-05-28T08:47:05Z <p>I have the following Lagrangian density <span class="math-container">$\mathcal{L}$</span> where</p> <p><span class="math-container">$$\mathcal{L}=\frac{1}{2}\left(c[\partial_{t}\phi(x,t)]^{2}-\frac{1}{l}[\partial_{x}\phi(x,t)]^{2}+\frac{1}{\omega_{J}^{2}l}[\partial_{x}\partial_{t}\phi(x,t)]^{2}+\gamma[\partial_{x}\phi(x,t)]^{4}\right)$$</span></p> <p>where <span class="math-container">$c,l,\omega_{J},\gamma$</span> are constants. Defining the usual conjugate momenta <span class="math-container">$\pi$</span> such that <span class="math-container">$$\pi=\frac{\partial\mathcal{L}}{\partial(\partial_{t}\phi(x,t))}$$</span></p> <p>How should I evaluate the third term <span class="math-container">$[\partial_{x}\partial_{t}\phi(x,t)]^{2}$</span> where there is also an <span class="math-container">$x$</span>-derivative?</p> <p><strong>Edit:</strong> I found a solution to this. It seems that I cannot use the regular convention for defining conjugate momenta. Rather I have to define it such that <span class="math-container">$$\pi=\frac{\delta\mathcal{L}}{\delta[\partial_{t}\phi]}=\frac{\partial\mathcal{L}}{\partial[\partial_{t}\phi]}-\partial_{x}\frac{\partial\mathcal{L}}{\partial[\partial_{x}\partial_{t}\phi]} = c\partial_{t}\phi-\frac{1}{\omega_{J}^{2}l}\partial_{x}^{2}\partial_{t}\phi$$</span></p> <p>I do not understand this definition of conjugate momenta. Can someone explain why is it defined like so?</p> https://physics.stackexchange.com/q/710906 2 If the escape velocity at the event horizon is the speed of light does it mean that slower bodies won't move away at all? Krešimir Bradvica https://physics.stackexchange.com/users/253499 2022-05-27T20:33:59Z 2022-05-28T10:36:11Z <p>If we say that the escape velocity from a planet is say 10 km/s we think that a slower body will move away from that planet but will be eventually forced to fall back on the planet. In simple words we don't say the body won't move at all but it couldn't leave for ever the planet. What is confusing for me is the escape velocity at the black hole event horizon. If it is the speed of light does it mean that a slower body would leave the horizon but fall down again or that is impossible for that body to make a path at all even 1mm away from the event horizon?</p> https://physics.stackexchange.com/q/710820 0 Extended Conformal Thin-sandwich Method Kabouter9 https://physics.stackexchange.com/users/315313 2022-05-27T09:03:20Z 2022-05-28T08:33:09Z <p>I got a question about the following <a href="https://arxiv.org/abs/gr-qc/0703035" rel="nofollow noreferrer">lecture notes</a>.</p> <p>My question concerns equation (8.95), page 141: <span class="math-container">$$\tilde{D}_i \tilde{D}^i N + 2\tilde{D}_i \log\Psi \tilde{D}^i N = \Psi^{-1}\left(\tilde{D}_i\tilde{D}^i\left(N\Psi\right) + N \tilde{D}_i\tilde{D}^i\Psi \right) \, .$$</span> I'm not able to prove this identity. By working both sides out, I reduced the problem to <span class="math-container">$$\tilde{D}_i \Psi \tilde{D}^i N = N \tilde{D}_i \tilde{D}^i \Psi \, ,$$</span> but I don't see why this is true. Intuitively, I can see why this is true when there is a minus sign (I see it then when integrated over, as partial integration where the boundary terms are zero). Moreover, if there is a minus sign, the next equation (8.96) should also work out which, right now does not work out when I work it out.</p> <p>Thanks in advance!</p> https://physics.stackexchange.com/q/710804 1 Is there any clear and concrete proof that says the Earth is rotating and orbiting? [duplicate] an4s911 https://physics.stackexchange.com/users/335464 2022-05-27T07:33:39Z 2022-05-28T10:28:19Z <p>As the title states, I am wondering if there exists a clear and concrete proof that proves that the earth is revolving around the sun on an orbit, and proves that the sun is not revolving around the earth.</p> <p>I've heard of the Foucault pendulum, but that doesn't prove that the sun isn't revolving around the earth. And stellar parallax, I was able to find a lot of material explaining about what it is and why it works, and all the theory behind it is explained in detail on several articles and videos, but I couldn't find any solid experiments or evidences to prove it in any of them.</p> https://physics.stackexchange.com/q/710668 12 Why is it impossible for the reactor of the nuclear power plant to turn into an explosive nuclear bomb? Lionheart https://physics.stackexchange.com/users/336488 2022-05-26T12:30:21Z 2022-05-28T09:43:55Z <p>Is it true that both work on the same principle of operation: the interactive fission chain reaction U 235/Pu239 and the bombardment of Uranium-235 by fast neutrons produce a fission chain reaction accompanied by a enormous thermal energy in addition to beta and gamma radiation? The most probable fission products are known to be Sr 94 amu and Xe 140 amu plus two fast neutrons.If both reactions are a chain reaction and almost instantaneous, then why not?.</p> https://physics.stackexchange.com/q/710571 1 Is the identity $\rho=\sum_m M_m\rho M^t_m$ possible for measurement operators with $\sum_m M_m^t M_m=I$? facenian https://physics.stackexchange.com/users/80627 2022-05-25T20:25:32Z 2022-05-28T09:16:34Z <p>Nielsen and Chuang quantum information book has the following identity <span class="math-container">$$\rho=\sum_m M_m\rho M^t_m$$</span> Where <span class="math-container">$M_m$</span> are measurement operators and <span class="math-container">$\sum M^t_m M_m=I$</span>.</p> <p>I suspect it must be a typo. I think that <span class="math-container">$\rho$</span> on the left hand side cannot be the same on the right hand side. Can anyone confirm this?</p> https://physics.stackexchange.com/q/698035 1 Relation between divergence of unit normal and radius of curvature Apoorv Mishra https://physics.stackexchange.com/users/290932 2022-03-08T07:37:00Z 2022-05-28T09:17:28Z <p>I don't understand how does the divergence of a unit normal vector to a curve at a point gives the local radius of curvature. For simplicity consider a 2-D curve. <span class="math-container">$$\nabla.n=\frac{1}{R}$$</span> I want to understand the mathematical proof for the expression and also some physical intuition to understand why is this true.</p> https://physics.stackexchange.com/q/665903 1 Would an extremely slowly forming star ignite? blademan9999 https://physics.stackexchange.com/users/263465 2021-09-14T10:18:27Z 2022-05-28T10:49:19Z <p>Nuclear fusion requires extremely high temperatures and pressures, both of which are crated by the collapse of protostars.</p> <p>But, what if the accretion of matter happened slowly enough that the core never got very hot.</p> <p>For our example, we can take Jupiter, then very slowly (e.g. at a rate of say one Earth mass every few trillin years) would we be able to get it to 100 Jupiter masses without igniting fusion?</p> https://physics.stackexchange.com/q/587727 0 In Franck-Hertz experiment, why mercury emits UV light while neon emits visible light despite neon having greater excitation energy? ThePratama190 https://physics.stackexchange.com/users/277408 2020-10-17T10:16:58Z 2022-05-28T09:00:40Z <p>The reference that I checked shows that Neon has an excitation energy of 18.2 eV, while mercury has 4.9 eV. However, the reference also shows that the wavelength emitted by the mercury is at the ultraviolet range of the electromagnetic spectrum (254 nm precisely). How could this be, despite Neon having greater excitation energy than mercury?</p> https://physics.stackexchange.com/q/514265 0 How is the voltage in this parallel circuit different across each component? Ceecee https://physics.stackexchange.com/users/247472 2019-11-16T19:30:07Z 2022-05-28T11:01:50Z <p>I'm studying up on circuit calculations and came across this circuit: <a href="https://i.stack.imgur.com/ysBcr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ysBcr.png" alt="enter image description here"></a></p> <p>I was taught that voltage in a parallel circuit is the same across all components, which would be the voltage supplied by the cells. So why is the reading across the resistor 4V? Shouldn't it be 9V, or is it treated differently because the components within the branch are in series with each other? So could I say the voltage is the same across each branch instead, with different components having different voltages across them, due to their differing resistances?</p> <p>If so, am I right in saying V1= 9V and V2= 5V?</p> https://physics.stackexchange.com/q/409380 3 What portion of the total power emitted by the Sun comes from photons? yalis https://physics.stackexchange.com/users/1937 2018-05-31T17:35:11Z 2022-05-28T09:09:50Z <p>We usually talk about luminosity, which is the total power from emitted photons. The other sources are neutrinos and the solar wind (which includes protons and other particles). </p> <p>I assume that most of the emitted energy comes from photons. What portion comes from the other stuff? </p> https://physics.stackexchange.com/q/394955 4 Do stellar model luminosities include neutrino losses? ProfRob https://physics.stackexchange.com/users/43351 2018-03-22T13:59:23Z 2022-05-28T09:11:55Z <p>I have had a sudden crisis in my understanding of the published outputs from stellar evolutionary model calculations.</p> <p>Usually these models output a &quot;luminosity&quot; that one can then use, along with the temperature and an atmosphere model to work out how bright the star would be (in terms of magnitudes) through various filters (e.g. U, B, V etc.)</p> <p>But do the quoted luminosities <em>only</em> include electromagnetic radiation or does it also include the neutrino losses (which then obviously make no contribution to the electromagnetic flux)? This would be a 2.3% effect in the case of the Sun, where the usually quoted value of <span class="math-container">$L_{\odot} = 3.83\times 10^{26}$</span> W does <em>not</em> include neutrinos.</p> <p>And whether the models do or don't include the neutrino losses, where can I find those separately?</p> https://physics.stackexchange.com/q/389891 1 Isothermal and adiabatic compression in the Carnot cycle Gokulakrishnan Shankar https://physics.stackexchange.com/users/185977 2018-03-03T16:30:14Z 2022-05-28T10:06:55Z <p>I am reading about the Carnot Engine and I understood the first 2 stages (Isothermal and Adiabatic Expansion) well. In the 3rd stage where isothermal compression takes place, the surroundings do work on the system. The following is an image from wikipedia:</p> <p><a href="https://i.stack.imgur.com/R21wO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/R21wO.png" alt="enter image description here" /></a></p> <ol> <li><p>My question is that do we <em>physically</em> have to compress it or does the gas get compressed on its own?</p> </li> <li><p>What is not quite intuitive to me is how is it that after 4 stages, the temperature goes back to being the original temperature?</p> </li> </ol> https://physics.stackexchange.com/q/301324 0 Relation between Casimir Effect and Quantum Tunnelling Dhruv Saxena https://physics.stackexchange.com/users/139130 2016-12-27T19:15:30Z 2022-05-28T09:23:08Z <p>Are Casimir Effect and Quantum Tunnelling dependent on each other? Also, is it reasonable to conclude that if Casimir Effect is already observed in a system, then Quantum Tunnelling has also taken place (or vice versa)? Alternatively, can both the phenomena be observed simultaneously in a single system? </p> https://physics.stackexchange.com/q/27849 7 Branch-point twist fields and operator insertions on a Riemann manifold dbrane https://physics.stackexchange.com/users/1228 2012-05-05T23:38:54Z 2022-05-28T08:36:17Z <p>I am having trouble understanding how Eq (2.6) in <a href="https://arxiv.org/abs/0706.3384" rel="nofollow noreferrer">this</a> paper <a href="https://arxiv.org/pdf/0706.3384v2.pdf" rel="nofollow noreferrer">(PDF)</a> <span class="math-container">$$Z[\mathcal{L},\mathcal{M}_{n}]\propto\langle\Phi(u,0)\tilde{\Phi}(v,0)\rangle_{\mathcal{L}^{(n)},\mathbb{R}^{2}}$$</span></p> <p>generalizes to Eqn (2.7) <span class="math-container">$$\langle\mathcal{O}(x,y;\mbox{ sheet i })...\rangle_{\mathcal{L},\mathcal{M}_{n}}=\frac{\langle\Phi(u,0)\tilde{\Phi}(v,0)\mathcal{O}_{i}(x,y)...\rangle_{\mathcal{L}^{(n)},\mathbb{R}^{2}}}{\langle\Phi(u,0)\tilde{\Phi}(v,0)\rangle_{\mathcal{L}^{(n)},\mathbb{R}^{2}}}$$</span></p> <p>It's quite possible that some of you with more experience in CFTs will be able to immediately answer this for me without the context, but here it is anyway: we want to evaluate the partition function on the Riemann manifold <span class="math-container">$\mathcal{M}_{n}$</span> which consists of <span class="math-container">$n$</span> flat 2D sheets joined together at the branch cut between <span class="math-container">$u$</span> and <span class="math-container">$v$</span> in the manner shown in Fig 1 in the paper. We do this by modeling the manifold as <span class="math-container">$n$</span> disconnected flat sheets with twist fields inserted at the branch points. It turns out that the original partition function is proportional to the correlation function of two twist fields as in Eq. 2.6.</p> <p>Later on, the paper makes use of the correlation function with insertions of the stress-energy tensor, so that generalization (with the equality sign) is crucial. Your help is appreciated!</p> <p>In particular, how does this NOT mean that the partition function in Eq 2.6 is not actually proportional to the two-point function but is simply equal to one? (I am replacing the <span class="math-container">$\mathcal{O}$</span>'s in (2.7) with one to make this claim)</p>