question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>I understand that the black-and-white images you see looking through a household telescope are only like that due to the intensity of the light that reaches us, and that most of the astronomy images we find online have some color modification. But if we were to eventually travel between galaxies, how would those gal... | 1,217 |
<p>I understand the mathematical derivation of the Tully Fisher relation from basic physics formulas, as shown on <a href="http://www.jca.umbc.edu/~george/html/courses/glossary/tully_fisher.html" rel="nofollow">this site</a>. However, after using the physics equations, it seems that several assumptions are made from th... | 1,218 |
<p>How can I prove Thevenin's and Norton's theorem? <a href="http://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem" rel="nofollow">Thévenin's theorem</a> can be used to convert any circuit's sources and impedances to a Thévenin equivalent.</p> | 1,219 |
<p>I'm doing simulations of copper, where the temperature can reach up to ~1300 K.</p>
<p>Some calculations depend on the mean free path (MFP) of copper. The only value I've found for it is 39nm and it's always accompanied with the clause of "at room temperature".</p>
<p>Can anybody tell me (or provide some reference... | 1,220 |
<ol>
<li><p>What does a <a href="http://en.wikipedia.org/wiki/Quantum_state" rel="nofollow">"STATE"</a> exactly mean in <a href="http://en.wikipedia.org/wiki/Quantum_mechanics" rel="nofollow">quantum mechanics</a>? </p></li>
<li><p>What is the equivalence of "STATE" in classical mechanics?</p></li>
<li><p>If we have a ... | 1,221 |
<p>If I drop a ball in a train moving at a constant speed, will it land on the spot I aimed it at or a little away as the train has moved while it was in air? If it lands away, will the observer not know that he is in a moving frame of reference? If it lands on the intended spot, how did the ball know it is inside a tr... | 1,222 |
<p>I'm struggling with the notion of an inertial frame of reference. I suspect my difficulty lies with the difference between Newtonian and relativistic inertial frames, but I can't see it. </p>
<p>I've read that Newton's laws apply in any non accelerating frame of reference, which are called inertial frames. So, if I... | 285 |
<p>Example of pendulum is inertial frame of reference or non inertial frame of reference?
because if pendulum starts moving its continuously moves without changing there period of time but is changes its velocity with there motion... </p> | 1,223 |
<p>Let $\rho = \begin{bmatrix}\ 1&0 \\ 0&0 \end{bmatrix}$, $\rho' = \begin{bmatrix}\ 0&0 \\ 0&1 \end{bmatrix}$, $\rho'' = \dfrac{1}{2}\begin{bmatrix}\ 1&1 \\ 1&1 \end{bmatrix}$ (all density operators).</p>
<p>Consider a physical operation $\phi$ such that $\phi(\rho) = \rho$, $\phi(\rho') = \rh... | 1,224 |
<p>I remember considering in class in college, the case of a photon gas trapped in a d-dimensional box as a subject of interest, whose energy distribution, heat capacity, etc. should be calculated. </p>
<p>This momentum/energy distribution was then related to that emitted by a black body - that up to a constant factor... | 1,225 |
<p>There are some definitions and properties for <a href="http://en.wikipedia.org/wiki/Pauli_matrices" rel="nofollow">Pauli matrices</a> and their combinations:</p>
<p>$$
\varepsilon^{\alpha \beta } = \varepsilon^{\dot {\alpha} \dot {\beta} } = \begin{pmatrix} 0 & 1 \\ -1 & 0 \end{pmatrix}_{\alpha \beta }, \qu... | 1,226 |
<p>I'm curious if you have a radiator or say a block of metal (lets say it's copper since it has the highest thermal conductivity) and on one side is a processor producing heat.</p>
<p>At idle the processor heats the block to 50°C, and the block is 120mm x 120mm. The ambient room temp is 22.2°C. You put a fan on the b... | 1,227 |
<p>If I have two balls with masses and charges $m_1, q_1^{+}$, $m_2, q_2^{+}$, initially held at distance $d$, and then released, how can I know the kinetic energies of each of the balls at infinite distance between them? I'm quite stuck on that, because they both have the same potential energy at the beginning, and it... | 1,228 |
<p>Or: can it be proved that anti-matter definitely is nót matter going backwards in time?</p>
<p><a href="http://en.wikipedia.org/wiki/Antimatter" rel="nofollow">From wikipedia</a>:</p>
<blockquote>
<p><em>There is considerable speculation as to why the observable universe is apparently almost entirely matter [...... | 420 |
<p><strong>Short question</strong></p>
<p>Given any Lagrangian density of fields one could possibly conceive, is it the case that after one has performed a Legendre transformation, if the Hamiltonian is then expressed in terms of the original fields, will it contain all of the terms originally in the Lagrangian but wi... | 1,229 |
<p>I have a set of trajectories of three particles and their instantaneous velocities. I would like to compute the 3 components of the angular velocity pseudovector of the fictive triangle formed by these three particles. How can I proceed for that?</p> | 1,230 |
<p>Several different sources online state that the average temperature of interstellar space (or the universe in general) is around 2-3K.</p>
<p>I learned that temperature is basically the wiggling of matter, and I find it somewhat counterintuitive that the wiggling of so few particles can cause a temperature of 2-3K.... | 1,231 |
<p>Is it possible to write down a partition function for a <a href="http://en.wikipedia.org/wiki/Microcanonical_ensemble" rel="nofollow">microcanonical ensemble</a>?</p> | 1,232 |
<p>I am very curious if an easy calculable formula for the bremstrahlung radiation of deeply relativistic, charged particles exists, if they are moving on circular orbit:</p>
<p>$P(E,m_0,Q,r)=?$</p>
<p>...where</p>
<ul>
<li>$P$ is the power of the Bremstrahlung radiation;</li>
<li>$E$ is the total kinetic energy of ... | 1,233 |
<p>I came across the notion of complex energy while studying instanton method to study the unstable state. Unstable states are those which have energy with an imaginary part. But as we know Hamiltonian is a Hermitian operator. So how can the energy be imaginary?</p> | 1,234 |
<p>Where do the terms <a href="http://en.wikipedia.org/wiki/Microcanonical_ensemble" rel="nofollow">microcanonical</a>, <a href="http://en.wikipedia.org/wiki/Canonical_ensemble" rel="nofollow">canonical</a> and <a href="http://en.wikipedia.org/wiki/Grand_canonical_ensemble" rel="nofollow">grand canonical</a> (ensemble)... | 1,235 |
<p>Trying to understand <a href="https://en.wikipedia.org/wiki/Limits_to_computation" rel="nofollow">the physical limits to computation</a>, I notice that among these we have two types of limits that constrain the minimum allowable energy for a computation.</p>
<ul>
<li>Limits that constrain <strong>the product of ene... | 1,236 |
<p>By reading <a href="http://www.sciencedirect.com/science/article/pii/0550321388904555" rel="nofollow">an article</a>, I found a partition function that, according to the author, describes an interacting with random variables as coupling constant.</p>
<p>$$Z =\int \mathrm{d} \lambda_i e^{i(K^{ij}\lambda_i\lambda_j +... | 1,237 |
<p>I'm studying optics at the moment and I am wondering how the refractive index changes when a liquid is in super critical state?</p>
<p>Thank you.</p> | 1,238 |
<p>I recently learnt that the conservation laws are a consequence of the symmetries of space and time (the Lagrangian in Newton mechanics). Since space-time change in a black hole wouldn't quantum mechanics also change?</p>
<p>Let me give an example by what I mean: <br>
In classical space: <br>
$$x' = x + e \\
L(x') =... | 1,239 |
<p>I'm trying to calculate the spin and color averaged gg->gg cross section, and I am stumbling upon gauge invariance:</p>
<p>Must the amplitude not be invariant under replacements $\epsilon_i \to \epsilon_i + \kappa_i p_i$ where $p_i\cdot\epsilon_i=0$ and $\kappa_i$ is arbitrary? For me it is not :/</p>
<p>I have su... | 1,240 |
<p>Squeezed electromagnetic vacuum does have a renormalized energy density smaller than the vacuum. So it makes it in my opinion a inconspicuous candidate for a dark energy carrier.</p>
<p>Are there observatories at the moment attempting to detect squeezed radiation from astrophysical and cosmic background sources? If... | 1,241 |
<p>If the atmosphere is filled with electromagnetic waves all oscillating at different wavelengths and speeds how is it that they don’t all interfere with each other? For example turning on your light seems to have no effect on the sound coming from your radio. </p> | 1,242 |
<p>Does time exist in space or does space exist in time?</p>
<p>Alternatively, perhaps both are independent existences?</p>
<p>Another possibility is that this question is beyond the capacities of the human mind.</p> | 11 |
<p>So here is an interesting question about inductors or coils in general. </p>
<p>Suppose you had a inductor which was 12 inch's in diameter and say 12 inch's in length and has 100 turns of wire. The total length of that wire would be 12(diam) x 3.14(pie) x 100(turns) = 3,768 inches/314 ft in length. </p>
<p>So if ... | 1,243 |
<p>For projector $p$, in finite dimension say, some unitaries $u, v$ does $upu^\dagger = vpv^\dagger$ implies $u = v$ ?
Intuitively, can we not say that a unitary is matrix permuting the basis and since $p$ is diagonal then obviously $u$ is $v$ ?
But for an exact proof ?</p>
<p>what if further, $p = upu^\dagger = vpv^... | 1,244 |
<p>I know the main problem with a quantum computer is that you can't read and write data from it until after it has run though all of its calculations. But would reading and writing quantum data to particles <i>without collapsing their probability fields</i> make this possible?</p> | 1,245 |
<p>Interacting fermionic SPT phases in 1d and 3d with $\mathbb{Z}_2^T$ symmetry are classified by $\mathbb{Z}_8$ and $\mathbb{Z}_{16}$ respectively, as shown in the paper by Fidkowski and Kitaev <a href="http://arxiv.org/abs/1008.4138" rel="nofollow">http://arxiv.org/abs/1008.4138</a>, and Wang and Senthil <a href="htt... | 1,246 |
<p>This is a problem that has been periodically bugging me, so I finally decided to work on it. I haven't done any physics since high school, so I'm a bit out of practice:</p>
<p>Consider a doorway with two hinged doors, one being the mirror of the other. Someone opens one of the doors (which open outward) and lets go... | 1,247 |
<p>I find the profound asymmetry in the sensitivity of left and right chiral particles to be one of the most remarkable analytical observations captured in the Standard Model. Yet for some, I've not found much in the way of discussions that worry about <em>why</em> of such as truly remarkable fact is true. I can't help... | 1,248 |
<p>Since the Higgs mechanism is so intimately tied to binding together massless chiral fermions, does it happen to have anything to say about the spin statistics issue?</p>
<p>I'm actually assuming the answer is no, but perhaps there is a deeper tie there that I'm missing completely. So, just curious...</p> | 1,249 |
<p>According to equation (6) on the first page of <a href="http://www.nhn.ou.edu/~gut/notes/cm/lect_14.pdf" rel="nofollow">some lecture notes online</a>, the above equation is used to prove the virial theorem. For rectangular coordinates, the relation</p>
<p>$$
2T~=~\sum_i p_{i}\dot{q}^{i}
$$</p>
<p>is obvious. How w... | 1,250 |
<p>My question is about <code>photo electric</code> but it could be applied to other daily routine phenomenon. As we know rest mass of photon is zero. When a photon strikes the metal surface it transfers its energy to the electrons. Whether electron will be emitted or not, it depends upon work function. But my question... | 1,251 |
<p>Suppose, in whatever dimension and theory, the action $S$ is invariant for a global symmetry with a continuous parameter $\epsilon$.</p>
<p>The trick to get the Noether current consists in making the variation local: the standard argument, which does't convince me and for which I'd like a more formal explanation, i... | 1,252 |
<p>This is a theoretical problem. Suppose we have two ropes of the same material and diameter, but differing in length. We excert force on both ropes until they break. Let rope A be the longer one. What can be said about the stress in rope A compared to the stress in rope B at the moment of breaking, taking in account ... | 1,253 |
<p>All particles exhibit wave-particle duality. And I have a strange question. </p>
<p>Why does a larger system, liken an atom that is just a set of smaller systems, itself exhibit wave-particle duality?</p>
<p>In principle all large systems can be defined as a set of smaller systems. An atom is a set of nucleus (a s... | 1,254 |
<p>I saw a diagram of the photosensitivity (Current per Power) of a photodiode.
<img src="http://i.stack.imgur.com/44MQw.png" alt="Photodiode sensitivity"></p>
<p>So there is this diagonal stating the 100% quantum efficiency.
I wondered why the sensitivity for bluer light lower than that of redder light. </p>
<p>Is... | 1,255 |
<p>What is a practical way in an engineering or physics laboratory to create bubbles of a specified size in water within plastic tubing? The tubing is a few mm inner diameter. We'd like to make bubbles smaller in size than the diameter, for example 1/2 the diameter. We'd like to have control over the size, especiall... | 1,256 |
<p>Why is it that in stars undergoing gravitational collapse <a href="http://en.wikipedia.org/wiki/Electron_degeneracy_pressure" rel="nofollow">electron degeneracy</a> kicks in? Why couldn't the electrons form <a href="http://en.wikipedia.org/wiki/Energy_bands" rel="nofollow">energy bands</a> like in semiconductors? </... | 1,257 |
<p>For my bachelor's thesis, I am investigating <a href="http://en.wikipedia.org/wiki/Divergent_series" rel="nofollow">Divergent Series</a>. Apart from the mathematical theory behind them (which I find fascinating), I am also interested in their applications in physics. </p>
<p>Currently, I am studying the divergent s... | 1,258 |
<p>Let's say I am flowing a fluid through a pipe that has a constant surface temperature. Is it possible to determine the heat flux on the inner surface without knowing the pipe's length when the only two temperatures I know are the pipe surface temperature and the inlet flow temperature?</p>
<p>I am able to assume th... | 1,259 |
<p>The $\mathrm{U(1)}$ QED case has two physical degrees of freedom, which is easy to understand because the free electromagnetic field must be transverse to the direction of propagation. But what are the physical degrees of freedom the $\mathrm{SU(N)}$ Yang-Mills theory? </p>
<p>I believe that the $\mathrm{SU(N)}$ Y.... | 192 |
<p>I'm currently studying at the spectra of some supersymmetric models, and would like to know whether the parameter points I'm looking at are ruled out due to excessive CP violation.</p>
<p>I am using <a href="http://www.physik.uni-wuerzburg.de/~porod/SPheno.html" rel="nofollow">SPheno</a>, which allows me to test my... | 1,260 |
<p>Is it true:From equation $C=Q/V$</p>
<p>1)If Potential difference is kept
<strong>constant</strong>
$C \propto Q$</p>
<p>2)If charge is kept <strong>constant</strong>
$C \propto 1/V$
Then if 2nd is true I do have a question:
Don't we say that capacitance is the ability of a body to store charge,so my question goe... | 1,261 |
<p>I know QFT at graduate level but I'll like to master the skill of working with Feynman diagrams. I'm looking for a book of solved exercises on this topic. </p>
<p>Specifically, I'm looking for the kind of repetitive exercises, just like in elementary school you have to do e.g. 10 multiplications, 10 divisions, etc.... | 12 |
<p>These are some further important queries regarding the question here <a href="http://physics.stackexchange.com/questions/102910/why-would-spacetime-curvature-cause-gravity">Why would spacetime curvature cause gravity?</a></p>
<p>Q1. Explain the statement “Everything in spacetime moves at the speed of light”. Is thi... | 1,262 |
<p>This is just a quick question. I would figure this out myself if I wouldn't have an exam about this tomorrow.</p>
<p>I am working on the non-relativistic approximation of the Dirac equation for an electron in an EM field. On one point, I need the following relation:</p>
<p>$$
\epsilon^{klm} \sigma^{m} = \sigma^m ... | 1,263 |
<p>I'm slightly befuddled by is what it means when I'm asked to </p>
<blockquote>
<p>Draw the Feynman diagram in momentum space for the two point function of $\frac{\lambda}{3!}\phi^3$ theory for order $O(\lambda^2).$</p>
</blockquote>
<p>I can draw Feynman diagrams, and I thought two-point function meant</p>
<p>... | 1,264 |
<p>I try to understand why Algodoo software (freeware) give an efficiency greater than unity. I tested several scenes and at each time I can look the software give energy, I don't say it's all the time the case, a lot of scene have energy constant or decrease but with particular forces Algodoo give energy. </p>
<p>The... | 1,265 |
<p>I'm fine with $U(1)$ symmetry and <a href="http://en.wikipedia.org/wiki/Noether%27s_theorem" rel="nofollow">Noether's Theorem</a>, but struggling with the translations of the field; namely</p>
<p>$$\phi'(x^{\mu})=\phi(x^{\mu}-a^{\mu}),$$ where $a^{\mu}$ constant four-vector</p>
<p>$$x^{\mu}=x^{\mu}+a^{\mu},$$</p>... | 1,266 |
<p>As I understand it, <a href="http://en.wikipedia.org/wiki/Paramagnetism" rel="nofollow">paramagnetism</a> is similar in its short-term effect to <a href="http://en.wikipedia.org/wiki/Ferromagnetism" rel="nofollow">ferromagnetism</a> (spins of the electrons line up with the magnetic field, etc.), though apparently th... | 1,267 |
<p>I can conceive of a particle existing in empty spacetime, but not a wave. A wave appears to me at least, to insist upon a medium for its very definition. </p>
<p>I understand that the 19C physicists postulated the lumineferous aether for this very reason, but abandoned it in favour of the electromagnetic field.</p>... | 1,268 |
<p>As a fun project, I would like to roughly simulate the suspension operation of a full-suspension mountain bike. This is <em>not</em> another one of those "How does a bicycle stay upright?" questions.</p>
<p>Follow some metrics of interest.</p>
<ul>
<li>Pedal bob - the amount of movement in the rear triangle upon p... | 1,269 |
<p>In order to determine the relative motion between the Earth and the ether (the medium through which light supposedly propagated. It has zero density and complete transparency), scientists used the concept of the <a href="http://en.wikipedia.org/wiki/Aberration_of_light" rel="nofollow">aberration of starlight</a>. If... | 1,270 |
<p>Context: My room is being painted, and i sit and study in a corner of the room, surrounded by walls on 2 sides, such that i am facing the wall. A tube light is at my 4-5 o'clock, and a door(transparent) at my 7 o'clock. These are the only two sources of light. There is a TV mounted just in front of my desk. </p>
<p... | 1,271 |
<p>When writing a method for an experiment, does it always have to be set out in orderly numbered steps? Can it not also be a paragraph of text that outlines the method?</p>
<p><strong>A mundane example</strong>:</p>
<ol>
<li>Place a tripod on a table, and the wire gause mesh ontop </li>
<li>Following that, place a b... | 1,272 |
<p>Does the curvature of space-time cause objects to look smaller than they really are? What is the relationship between the optical distortion and the mass of the objects?</p> | 1,273 |
<p>Are virtual particles only popping in and out of existence where the local energy density is below a certain point? What I wonder is, does any kind of matter prevent the pairs from appearing? Is there a shell surrounding an atom or maybe I should call it a boundary beyond which particle pair production occurs, and... | 1,274 |
<p>In a wave guide, graphics of propagation of Transversal Magnetic modes show closed field lines for the electric field. </p>
<p>For example, for a rectangular guide:</p>
<p>$E_x (x,y,z) = \frac {-j\beta m \pi}{a k^2_c} B_{mn}\cos\frac{m\pi x}{a}\sin\frac{n\pi y}{b}e^{-j(\beta z + \omega t)}$</p>
<p>$E_y (x,y,z) = ... | 1,275 |
<p>Recently I have started to study the classical theory of gravity. In Landau, Classical Theory of Field, paragraph 84 ("Distances and time intervals") , it is written</p>
<p>We also state that the determinanats $g$ and $\gamma$, formed respectively from the quantities $g_{ik}$ and $\gamma_{\alpha\beta}$ are related ... | 1,276 |
<p>Suppose I am trying to determine the value of the acceleration due to gravity by simply dropping objects from a ledge. If I did not account for air resistance, would the acceleration due to gravity measured be less than if I did account for air resistance?</p>
<p>This is what a thought: Suppose I had an object of m... | 1,277 |
<p>The question goes along the lines:</p>
<blockquote>
<p>Uranium-$236$ fissions when it absorbs a slow-moving neutron. The two fission fragments can be almost any two nuclei whose charges $Q_1$ and $Q_2$ add up to $92e$ ($e$ is the charge of a proton, $1.6 \times 10^{-19}C$), and whose nucleons add up to $236$ prot... | 1,278 |
<p>I realize I'm trying to get a simple answer on a complicated subject, but here goes anyway. I've done some research and I understand (well, mostly) about how Vc is the amount of energy required to get two nucleons past the electric repulsive force so the strong force can take over. I know about the Maxwell-Boltzmann... | 1,279 |
<p>There is a very fundamental flaw in the common explanation given of the space-time curvature due to massive objects. It is said that a massive object curves space time just like a bowling ball on a rubber sheet, and another object near the massive object simply rolls down-hill on the rubber sheet, that is the reason... | 13 |
<p>When I was young I read one book in which is written that you get more tan on the morning than on evening even light angle from Sun to Earth is the same. Don't remember exact reason, I think because ultraviolet is more absorbed on the evening because air is more humid or something like this. Is it true?</p> | 1,280 |
<p>Condensed matter physicists have shown using quantum information that in many condensed matter systems, entanglement entropy only scales as the area of the boundary, and not the volume. This is the basis for the density matrix renormalization group and Projected Entangled Pair States (PEPS). Does this also explain t... | 1,281 |
<p>I just thought of an interesting question.Is the sun actually moving?
I have learnt that the way the lunar landers and the space ships of the Mercury and Apallo missions moved and controlled their positions through the use of thrusters.NoW apply that same concept to the sun are the constant explosions on the surface... | 1,282 |
<p>Our calculus book, Stewart, has a problem where they claim that for a metal cable (inner radius $r$) encased in insulation (outer radius $R$), the speed of an electrical impulse is given by</p>
<p>$$v = - k \left(\frac{r}{R}\right)^2 \ln \left(\frac{r}{R}\right)$$</p>
<p>where $k$ is a positive constant.</p>
<p>... | 1,283 |
<p>I'm a maths undergrad taking a course on electromagnetism, I've drawn a diagram to represent this following question, but I'm having a bit of trouble approaching it:</p>
<p>"Two tiny balls of mass m = 0:1 g and charge q are suspended on silk threads of
length l = 30 cm (see picture below). Their centers are separat... | 14 |
<p>If an astronomer moves at relativistic speed, the stars and constellations are distorted. He sees the stars towards which he is moving blue shifted, while the ones he's moving away from are red shifted. In addition, the apparent direction of distant stars is modified. I think that the coolest way of representing thi... | 1,284 |
<p>It would be nice to have a cute method that uses Lorentz transformations of basis vectors by exponential transformation using gamma matrices. To avoid confusion, let's assume -+++ signature. Given $\gamma_\mu$ as gamma matrices that satisfy ${\rm tr}(\gamma_\mu\gamma_\nu) = 4\eta_{\mu\nu}$, then we have ${\rm tr}(\g... | 1,285 |
<p>You may have noticed over the last few years that Moore's law is no longer applying to the real world.
This observation states that over the history of computing hardware, the number of transistors on integrated circuits doubles approximately every two years.</p>
<p>However, as microprocessors in computers have con... | 1,286 |
<ol>
<li><p>Where the gravitational pull of Earth exist up to? </p></li>
<li><p>What distance from Earth it will be zero?</p></li>
<li><p>How do the skydivers fly at a same altitude? </p></li>
<li><p>Won't they feel gravitational pull? </p></li>
<li><p>What is the Earth's gravitational pull?</p></li>
</ol> | 1,287 |
<p>What are the units of the damping constant from the following equation by dimensional analysis?</p>
<p>$$\zeta = \frac{c}{2\sqrt{mk}}$$</p>
<p>I'm assuming the units have to be s^-1, as the damping constant is present in the exponential equation which plots damping of y=Ae^kt (which plots amplitude vs time). Is th... | 1,288 |
<p>Let us say a block of mass is placed on the surface of earth. Then while drawing the forces on that body, we say:</p>
<ol>
<li>Force $F = mg$ acting towards the center of Earth.</li>
<li>Normal reaction $N$ offered by the surface of Earth.</li>
</ol>
<p>If no other forces are acting, we say $F=N$. But what about t... | 18 |
<p>I am working on a variational problem involving elastic stability of a beam.</p>
<p>The deformation of the beam is given by six functions of the material coordinate along the beams longitudinal axis. The governing functional looks like
$$
\int_{s_1}^{s_2} F \left(
u(s),v(s),w(s),\phi(s),\chi(s),\psi(s) \right) ds.
... | 1,289 |
<p>Consider a thick wire that narrows for some part of its length, connected to the terminals of a battery, hence carrying some current.</p>
<p>Presumably the electrons have to speed up as the wire gets thinner - if the current is constant everywhere, then if the wire cross-section is less, they have to go through fas... | 1,290 |
<p>I tried to calculate earth's orbital period using <a href="https://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion#Third_law" rel="nofollow">Kepler's third law</a>, but I found 365.2075 days for the orbital period instead of 365.256363004 which is the correct value. I checked everything, and I couldn't fin... | 1,291 |
<p>for any real and positive 's' and in the sense of semiclassical approximation </p>
<p>is this valid ??</p>
<p>$ \sum_{n}exp(-sE_{n})\sim \iint_{C}dxdpe^{-s(p^{2}+V(x))}$ valid for every 's'</p>
<p>here simply both the sum and integral are convergent, so in this case the approximation can be seen as the replacemen... | 1,292 |
<p>Can someone answer and explain it please.
Here are the options:</p>
<ul>
<li>Moving downwards and decelerating.</li>
<li>Moving downwards with a constant velocity.</li>
<li>Moving upwards and decelerating.</li>
<li>Moving upwards with a constant velocity.</li>
</ul> | 1,293 |
<p>I suggest the following thought experiment that describes a machine which makes everybody happy.</p>
<p>Suppose a lottery is conducted. The winner is awarded a billion dollars plus the title of eternal Earth dictator.</p>
<p>The winner is determined in the following way.</p>
<ul>
<li><p>There is a thorough isolat... | 1,294 |
<p>I have the following reaction: $^{10}_5\mathrm{Be} +\space ^2_1\mathrm{H} \rightarrow \space^{11}_5\mathrm{B} + \space ^1_1\mathrm{H}$ </p>
<p>And I know that I have to use the formula: $E = \Delta m\cdot c^2 = \Delta m \cdot \frac{931,5MeV}{u}$.</p>
<p>So I just need $\Delta m$ which is equal to:
$\Delta m = m_b... | 1,295 |
<p>Consider a $\varphi^3$ theory:
$$
Z_1(J) \propto \exp\left[\frac{i}{6} Z_g g\int \mathrm{d}^4 x \left(\frac{1}{i}\frac{\delta}{\delta J}\right)^3\right] Z_0(J),
$$
where
$$
Z_0(J) = \exp\left[\frac{i}{2} \int \mathrm{d}^4 x \mathrm{d}^4 x' J(x)\Delta(x-x')J(x')\right].
$$
That is
$$
Z_1(J) \propto \sum_{V=0}^\inft... | 1,296 |
<p>I have the following equation:</p>
<p>$$\frac{\partial U}{\partial t}=k\frac{\partial^2 U}{\partial x^2}-v_{0}\frac{\partial U}{\partial x}, x>0$$</p>
<p>with initial conditions:</p>
<p>$$U(0,t)=0$$</p>
<p>$$U(x,0)=f(x)$$</p>
<p>In the problem is requested to give an interpretation of each of the terms in th... | 1,297 |
<p>$$y(x,t)=2A\sin(Kx)\cos(\omega t)$$
$A$ and $x$ are in metre, $\omega$ is angular frequency.
Then find dimensions of $A$ and $K$.</p>
<p>In this equation how can I find the dimension of $K$?</p> | 1,298 |
<p>I have been taught that space-time should be viewed as a fabric and that objects with a large gravitational influence indent that fabric. My question is, if the singularity of a black-hole punctures space-time, how is this accomplished if the universe is 3D? Can an object move completely around the black-hole in all... | 13 |
<p>We know that the CMB is isotropic when viewed outside of the spinning and revolving earth.</p>
<p>Is it homogeneous?</p>
<p>Can we relate the CMB to an inertial frame in the Newtonian sense (in which space and time are homogeneous and isotropic)? Or can it just provide an idea to build upon a new theory in which g... | 1,299 |
<blockquote>
<p>Bodies $A$ and $B$ are moving in the same direction in a straight line with a constant velocities on a frictionless surface. The mass and the velocity of $A$ are $2 \text{kg}$ and $10 \text{m/s}$. The mass and the velocity of $B$ are $6 \text{kg}$ and $4 \text{m/s}$. A spring is connected to the back ... | 1,300 |
<p>This question is trying to see if anyone has some simple notation (or tricks) for dealing with operators acting on coherent states in a Fock space. I use bosons for concreteness; what I'm interested in might not be applicable to fermions.</p>
<p>If I have a multiparticle state defined by</p>
<p>$$|\phi\rangle=|n_1... | 1,301 |
<p>See Griffiths Quantum Mechanics, eq. 11.21. Evidently,
$$\psi(r,\theta,\phi)=Ae^{ikz}+A\sum\limits_{l,m}^{\infty}C_{l,m}h_{l}(kr)Y_{l}^{m}(\theta,\phi).$$
But I don't see why the $l$th Hankel function necessarily needs to multiply the $l$th spherical harmonic. Seeing as we arrived at this equation through separation... | 1,302 |
<ul>
<li><p>How does unitarity require that every scalar operator in a $2+1$ SCFT will have to have a scaling dimension $\geq \frac{1}{2}$ ? </p></li>
<li><p>Why is an operator with scaling dimension exactly equal to $\frac{1}{2}$ said to be "free (i.e decoupled from the rest of the theory)" ? </p></li>
<li><p>Let us ... | 1,303 |
<p>I have read an answer on this site regarding the change of laws over time . However a physisct told me that the laws did evolve at planck era and then stopped evolving after it , is that true even in case of a TOE ( namingly as the best candidate string theory )</p> | 1,304 |
<p>A clock near the surface of the earth will run slower than one on the top of the mountain. If the equivalence principal tells us that being at rest in a gravitational field is equivalent to being in an accelerated frame of reference in free space, shouldn't the clock near the earth run <em>increasingly</em> slower ... | 1,305 |
<p>I have heard that area is a vector quantity in 3 dimensions, e.g. <a href="http://physics.stackexchange.com/q/14165/">this</a> Phys.SE post, what about the length/distance? Since area is the product of two lengths, does this mean that length is also a vector quantity, and why?</p> | 1,306 |
<p>One usually hears about graphene as a good thermal conductor, and good light absorber due to its tunable bandgap properties. But i haven't heard about its aplicability as an optical mirror. In fact, mostly the opposite is true: the optical transmissivity of monolayer graphene is very high ($\approx$ 98%)</p>
<p>Sin... | 1,307 |
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