question stringlengths 37 38.8k | group_id stringlengths 2 6 | sentence_embeddings listlengths 768 768 |
|---|---|---|
<p>Does the <a href="http://en.wikipedia.org/wiki/Born-Oppenheimer_Approximation" rel="nofollow">Born-Oppenheimer approximation</a> fail for muonic molecules (i.e. molecules where one or more electrons are replaced with muons)?</p> | g18486 | [
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<p>In quantum mechanics, we have the word second quantization for identical particles. However, when dealing with localized states, first quantization version of quantum mechanics is also very convenient. I wonder if there is a first quantization version of QFT which is handy in dealing with, for example, two particles... | g18487 | [
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<p>I am not sure if this is more of a Chemistry or a Physics question, but in my Organic Chem class we discussed that chiral molecules will rotate plane polarized light. However, my professor did not discuss the mechanism at all. She also said that the only way to determine the angle of rotation was experimentally. ... | g18488 | [
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<p>Calculate the electric potential energy stored in a capacitor that stores 3.40 x 10^-10 C of charge at 20.0v</p>
<p>I am lost.
I am scared. </p> | g18489 | [
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<p>I always wondered how much time means 1 second? </p>
<p>Technically, the SI second is defined as</p>
<blockquote>
<p>the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. </p>
</blockquote>
<p>(Source:... | g18490 | [
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<p>As you all know, journal subscriptions are extremely expensive, and many colleges and libraries don't subscribe to them. Many important articles aren't available on the preprint server arxiv. My question is, where can I find pirated copies of journals? Your help means so much to me.</p> | g18491 | [
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<p>I've encountered in some books (and even completed an exercise from the Goldstein by using it), a strange notation that seems to work exactly like a gradient, I have tried to look for an explanation but found none yet and I think I can reduce the time spent looking for my answer by posting this question, even if you... | g18492 | [
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<p>Consider a theory for a finite number of real scalar fields $\phi _i$ with interaction terms of the form
$$
-\lambda _{ijk}\phi _i\partial _\mu \phi _j\partial ^\mu \phi _k,
$$
with the sum over $i,j,k$ being implicit. Without loss of generality, assume that $\lambda _{ijk}$ is symmetric in $j$ and $k$.</p>
<p>Con... | g18493 | [
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<p>The plain old double slit experiment displays interference when we don't measure which slit the photon passed through, and no interference when it is measured. Let's turn our attention to the case with no detectors at the slits. If instead of a back screen, we replace that with a focusing lens with a screen further ... | g18494 | [
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<p>$$S_x= \frac{\hbar}{2}\quad\begin{pmatrix}0&1\\1&0\end{pmatrix}\quad$$
$$S_x{X_+}^{(x)}=\frac{\hbar}{2}{X_+}^{(x)}$$
How can I find the eigenvalue of $S_x$?
My book says
$$
\left| \begin{array}{cc}
-\lambda & \frac{\hbar}{2} \\
\frac{\hbar}{2} & -\lambda\\\end{array} \right|=0 $$
So $\lambda=\frac... | g18495 | [
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<p>I tried to sum up two weight ranges of the <a href="http://en.wikipedia.org/wiki/Initial_mass_function" rel="nofollow">IMF</a> which wouldn't not work so my question is, if I'm doing something wrong.</p>
<p>Let's say my weight ranges are $\left[X M_{\mbox{sun}}, Y M_{\mbox{sun}}\right)$ and $\left[Y M_{\mbox{sun}},... | g18496 | [
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<p>I was wondering, in a windy weather on the beach is it better to play volleyball with a fully blown ball or a ball filled with less air?</p>
<ul>
<li>What are the important factors that make a ball float easier in the air?</li>
<li>Please illustrate the effectiveness of the mass of the ball Vs. it's volume in the a... | g18497 | [
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<p>Why don't we chose a random point in the void present between galaxy clusters and define it as the absolute origin? </p>
<p>I know it is not at absolute rest and space expands, but we can easily keep that point at origin and explain the movement of all other bodies in both space and time, if it is acceptable to dea... | g18498 | [
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<p>In Weinberg's textbook on QFT(<a href="http://books.google.com.sg/books?id=NcHmZ0DfecUC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false" rel="nofollow">google book preview</a>), he discussed the phase acquired after interchanging particle labels in the last paragraph of page... | g18499 | [
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<p>A police inspecter P is at point $ (\acute{x},\acute{y})$ and a thief X is at point $(x, y)$. X has a constant velocity $V_x\hat{i} + V_y\hat{j} $, where $\hat{i}$ and $\hat{j}$ are unit vectors in $X$ and $Y$ direction respectively. Maximum speed of P is S.
What is the minimum time in which P will catch X? (assumi... | g18500 | [
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<p>Are there any materials for which the refractive index decreases with density? It seems that for most materials we would expect there to be a positive correlation between density and refractive index, but perhaps there are exceptions. I am particularly interested in any gases that might not obey this rule.</p> | g18501 | [
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<p>Jaynes In his paper "Information theory and Statistical mechanics" says </p>
<p>"Previously, one constructed a theory based on equations of motion, supplemented by additional hypothesis of ergodicity, Metric transitivity, or equal a priori probabilities, and the identification of entropy was made only at the end, b... | g18502 | [
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<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/12175/book-recommendations">Book recommendations</a><br>
<a href="http://physics.stackexchange.com/questions/33215/what-is-a-good-introductory-book-on-quantum-mechanics">What is a good introductory book on... | g98 | [
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<p>Is there is any difference between Electric Flux ($\Psi$) and Total number of Electric Field lines ($E\times$Area)?</p>
<p>$\psi = \Sigma Q$, and $\phi$ = Electric field intensity $\times$ Area</p>
<p>where Area = Total surface integral over Gaussian surface</p> | g18503 | [
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<p>I have a couple of related questions that have been bothering me for a while. They might sound unscientific, but here is goes:</p>
<ol>
<li><p>What are the building blocks of energy? What does energy consist of? Is there 1 fundamental, theoretical particle or something similar that causes energy, just like higgs bo... | g18504 | [
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<h1>Semiconductor Optoelectronics:</h1>
<p>What are the differences between multiple Quantum Wells and supperlattices.</p> | g18505 | [
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<p>My textbook mainly deals with laminar flows. The book derives the equation of continuity, which states that the cross-sectional area times the velocity of a flow is always constant. But nowhere in the derivation does the textbook explicitly assumes that the flow is laminar. So, does the equation hold for turbulent f... | g18506 | [
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<p>For the sake of simplicity, I’d like to believe that there is one master non-linear partial differential equation
governing physics. In particular, consider a Klein-Gordon form:</p>
<p>$$
\frac{\partial^2 f}{\partial t^2}=v^2 \left (\frac{\partial^2 f}{\partial x^2}+\frac{\partial^2 f}{\partial y^2}+\frac{\partial... | g18507 | [
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<p>I recently stumbled across a small peculiarity I don't understand:</p>
<p>According to de Broglie, the frequency of a matterwave can be written as $f=\frac{E}{h}$, and its wavelength as $\lambda = \frac{h}{p}$.</p>
<p>However, relativistically for a particle with matter the following formulae are valid:
$E = \gamm... | g18508 | [
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<p>According to my naive understanding of topological insulators, an inverted band strucure in the bulk (inverted with respect to the vaccum/trivial insulator surrounding it) implies the existence of a gapless state at the surface (interface with the trivial insulator). Is this a sufficient condition as well? What role... | g18509 | [
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<p><img src="http://i.stack.imgur.com/jYZxe.png" alt="enter image description here"></p>
<p>In the above image taken from wikipedia, at the string level the strings have been shown as some loops, the article in wikipedia says that in <a href="http://en.wikipedia.org/wiki/String_theory" rel="nofollow">string theory</a>... | g18510 | [
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<p>Does the <a href="http://en.wikipedia.org/wiki/Logistic_map" rel="nofollow">logistic map</a> have a <a href="http://en.wikipedia.org/wiki/Attractor" rel="nofollow">strange attractor</a> for some "chaotic" values of the parameter?</p> | g18511 | [
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<p><s>Like these <a href="http://en.wikipedia.org/wiki/Impossible_colors" rel="nofollow">impossible colors</a> (yellow/blue and green/red) or these <a href="http://en.wikipedia.org/wiki/Imaginary_color" rel="nofollow">imaginary colors</a>, for example saturated red more intense than pure spectral red can be viewed in o... | g18512 | [
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<p>I know that at very large scales, solids behave like liquids, which is why planets are round, rather than knobbly. I also understand that you can pour some gases, and, if I have this right, that the air around the Earth is like a like an additional layer of liquid, but very much lighter. You can still be crushed by ... | g18513 | [
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<p>Using currently available fuels and technology and an unlimited budget. How big would a rocket without stages have to be to make it to a stable orbit?
and how big would a rocket without staged have to be to escape Earth's gravity? "edited" - or for the easier question how big would the rocket have to be if it was ma... | g18514 | [
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<p>I know that for an inductor having self inductance $L$ energy stored in its steady state when a current $I$ has been established is given by $U = \frac{LI^2}{2}$.</p>
<p>But after this current has been established, if we suddenly cut the wires attaching the inductor to the potential source or short the circuit, wha... | g18515 | [
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-0... |
<p>The minimum mass for the spontaneous collapse of a gas cloud is the <a href="http://en.wikipedia.org/wiki/Jeans_instability" rel="nofollow">Jeans mass</a>. For low temperatures and high densities, this jeans mass is greater than the mass of a typical star. What are the implications of the fact that the Jeans masses ... | g18516 | [
-0.01554923877120018,
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... |
<p>I already know that a charged polyethylene plastic (done by rubbing it with paper) can be used to attract a cardboard. Now, can I 'charge' the cardboard by touching it with the plastic? Supposedly, the cardboard gets the same charge as the plastic, it would repel the plastic after I charge the plastic again and try ... | g18517 | [
0.019970843568444252,
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0.02012118138372898,
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0.0064... |
<p>When solving Schrodinger's equation, we end up with the following differential equation:</p>
<p>$$\frac{{d}^{2}\psi}{dx^2} = -\frac{2m(E - V)}{\hbar}\psi$$</p>
<p>As I understand it, the next step is to guess the wave function, so let $\psi = {e}^{i\kappa x}$ or let $\psi = \sin(\kappa x)$, both of which I underst... | g18518 | [
0.03549155592918396,
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<p>From what I understand photons only move in straight lines unless reflected or refracted (other than influences from gravitational fields and their usual wavelike movement). And since they are a fundamental force carrier I am having difficulty understanding how they are "pulled" into a pole of a magnet after leaving... | g18519 | [
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<p>Tycho proposed a model of the solar system where all planets but Earth move around the sun while the sun and the moon move around the earth.</p>
<p>I wonder how this model could explain the retrograde motion.
Thanks!</p> | g18520 | [
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<p>The effective mass is proportional to the second derivative of the dispersion relation d2k/dE2.
Do we say that phonon have effective mass through it ? Spin wave have.</p> | g18521 | [
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<p>In the wiki it is article only about shear modulus and it is written that it's unit is Pascal.
At the same time in all researches about Red Blood Cells shear stresses, authors write "in-plane shear stress" and measure it in N/m. For instance, here - <a href="http://web.mit.edu/spectroscopy/research/biomedresearch/B... | g18522 | [
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<p>Now that we know the mass of the Higgs boson, which system would be better for the production of Higgs bosons, the LEP ramped up or the LHC?</p> | g18523 | [
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<p>The standard scene follows: The good guys have finally captured the enemies and have them in an airlock chamber on a spaceship. A button is pressed and out they go, violently propelled by the blast of air moving from a pressurized zone to a zone of much less pressure (space).</p>
<p>Here's my question: If people <a... | g18524 | [
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<blockquote>
<p>There is a spacecraft with mass $M_1$ moving in a certain trajectory with a constant speed $V_r$. To modify its trajectory, a engine is turned on and starts ejecting gas at a speed $V_g$, which is constant relative to the spacecraft and perpendicular to the spacecraft's trajectory. The engine is turne... | g18525 | [
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<p>In Minkowski spacetime time is subjective [or more precisely: time is different for every particle/ reference frame]. It is the coordinate time of an observer whose reference frame travels up the $ct$-axis. Before Minkowski, we thought that time was the same for everybody, but now we know that the slightest movement... | g18526 | [
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<p>Following Gross-Manes paper (which came right after Gross-Mende), the planar one-loop amplitude (the annulus) is dominated by the region in moduli space where the radius of the annulus shrinks to a point, i.e. it looks like the tree amplitude. Does anyone know if this generalizes for to all-loops? </p> | g18527 | [
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<p>I was reading <a href="http://www.guardian.co.uk/science/blog/2011/dec/06/is-higgs-boson-real" rel="nofollow">an article</a> and these paragraphs got me wondering...</p>
<blockquote>
<p>Before I list the replies, here is some background. The Higgs mechanism describes an invisible field that, it is argued, split o... | g18528 | [
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<p>i need to derive a formula for the photon gas correlation function
$\left\langle\partial n_i\partial n_j\right\rangle $
where $$\partial n_i=n_i -\left \langle n_i \right \rangle.$$</p>
<p>whilst solving I saw that
$$ \left \langle n_in_j \right \rangle = \frac{1}E. \frac{\partial ^2E}{\partial \beta \epsilon _i... | g18529 | [
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<p>I can't understand the relation between Schrödinger's Cat and this amazing experiment of the double slit. </p>
<p>It seems like in the double slit we know if we observe the particle the wave function collapse and the electron acts like a normal particle, so doesn't that solve the problem in Schrödinger's Cat?</p>
... | g18530 | [
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<p><img src="http://i.stack.imgur.com/eixuu.jpg" alt="enter image description here"></p>
<p>The lemma 4 is given in the above picture. My question is, how to verify linear dependence (20.15) for diagram (a)? I tried to extend the matrix for the simple root in <a href="http://en.wikipedia.org/wiki/E6_%28mathematics%29"... | g18531 | [
0.03618649020791054,
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0.02221740409731865,
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0.0... |
<p>There was an exam question that read approximatly:</p>
<blockquote>
<p><em>Let $\vec j = \vec l + \vec s$. Given eigenvalues of $\vec l^2$ and $\vec s^2$, calculate the eigenvalue for $\vec j^2$.</em></p>
</blockquote>
<p>We came up with
$$\vec j^2 = (\vec l + \vec s)^2 = \vec l^2 + \vec s^2 + 2 \vec l \cdot\ve... | g18532 | [
0.0398838147521019,
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0.0938... |
<p>I will begin my research on AdS/CMT, however I find AdS/CMT is only a phenomelogical method, so I want to know can AdS/CMT give some results the condensed matter physicists can not give, or even predict some unseen exotic matter states? </p> | g18533 | [
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<p>Consider the <a href="http://en.wikipedia.org/wiki/Quantum_teleportation" rel="nofollow">quantum state teleportation protocol</a> of Bennett et. al.
How does one prove that this protocol would never violate the conservation of energy? At the face of it, it doesn't seem to be something obvious, as the various measure... | g18534 | [
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<p>I have been watching some slow motion video and I was intrigued by the slow motion underwater gunshot. The first moments of the video go as expected. The gun fires and a cloud forms in front of the gun. Then the bullet rips through the water, leaving an open space (which I assume is a cavitation vacuum.) Soon af... | g18535 | [
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<p>As the question in the title states I am wondering what material can be effectively used to shield gamma rays apart from lead? I believe concrete is often used, but it is nowhere near as effective as lead (6 cm to match 1 cm of lead as I understand it). I also hear significant bodies of water helps, as does tightly ... | g18536 | [
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<p>I am a student at the 2nd year of a B.Sc. in Biotechnology. I started reading some papers about complex systems and nonlinear dynamics applied to economy, biology of course, climate model etc...
I would like to study theme better so i need a textbook from which i can start. I toke a calculus course and i am going to... | g572 | [
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<p><em>Neutron matter</em> is matter comprised entirely of neutrons, as it exists in <a href="http://en.wikipedia.org/wiki/Neutron_star" rel="nofollow">neutron stars</a>.</p>
<p>Most optical phenomena encountered in everyday life, such as light reflection and spectral absorption (i.e. color appearance) are the result ... | g18537 | [
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<p>Say we have an observable that describes a Hilbert space and that observable acts on state kets. Lets take the position observable for example. Then $\langle y|x\rangle = \delta(y - x)$. But can the eigenstates of the position observable be individually thought of as delta functions?
$$ A |x\rangle = x'|x\rangle $$... | g18538 | [
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... |
<p>I'm trying to understand the units in:</p>
<p>$mx''+kx=0$</p>
<p>And the general solution is $x(t)=A \cos(\omega_0 t)+B \sin(\omega_0 t)$</p>
<p>Let $\omega_0 =\sqrt{\frac{k}{m}}$ - the unit for the spring constant $k$ is $kgms^{-2}$
or $Nm^{-1}$, where $m$ is in $kg$, so that the units of $\omega_0$ seem to be "... | g210 | [
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<p>So I was applying some mathematical techniques I learned to physics, and one thing that captured my interest, is the power or luminosity flux of a star. So modeling the situation, taking the scalar field of Luminosity($L$), and the vector surface(stellar surface), $S$, I began by setting up a Surface (Flux, in this ... | g18539 | [
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<p>I am going through the paper <a href="http://arxiv.org/abs/quant-ph/0604077" rel="nofollow">Quantum adiabatic evolutions that can't be used to design efficient algorithms</a> by Zhaohui Wei and Mingsheng Ying. On the second page they prove a lemma. The statement goes as follows.</p>
<blockquote>
<p><strong>Lemma ... | g18540 | [
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0.055... |
<p>What is the minimum number of electrons necessary to propagate a plasmon? Is it possible for just 1 electron to oscillate in a plasmon. </p> | g18541 | [
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<p>In general, when one performs spin-wave theory, he has to assume some kind of long-range ordered state and perform spin-wave theory (using for instance Holstein-Primakoff bosons) around that state. Recall that in 1D there is no long-range order. Then it seems that spin-wave theory in 1D would be meaningless. <strong... | g18542 | [
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<p>How can I derive the Dirac equation from the Lagrangian density for the Dirac field?</p> | g18543 | [
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<p>By chance we received for free some monolayer graphene sheets (20 cm x 20 cm) and mixed coper-graphene wires at our University. I would like to prepare some very easy to do experiments for the chemistry students that show the awesome properties of graphene, and which could be carried out with very simple lab equipme... | g18544 | [
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... |
<p>if a simple pendulum is dropped in a elevator with a acceleration greater than acceleration due to gravity then what will be its frequency ? We know time period depends on frequency.</p> | g18545 | [
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0.08552013337612152,
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0.023827310651540756,
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0.00717221898958087,
0.03534328... |
<p>I have an exercise here with a solution.</p>
<p>The basic outline of the problem is:</p>
<p>Calculate the mass of air inside a hot air balloon, given the mass of the air balloon at rest is $m_b = 500~\mathrm{kg}$.</p>
<hr>
<p>The solution goes like this:</p>
<p>The buoyant force of the air and the gravitational... | g18546 | [
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-0.029866816475987434,
... |
<p><img src="http://i.stack.imgur.com/tRPyd.jpg" alt="The picture shows the situation described"></p>
<p>I have the above question and I have though about it every way and can't seem to find out. So what I have concluded (ignore the answers on the img) Is that the force is on the z axis because the cross product of th... | g18547 | [
0.04391541704535484,
0.044071536511182785,
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0.05253662168979645,
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-0.011... |
<p>I recently came to know that any type of force is exerted by a particle on another with the help of gauge bosons/virtual particles which are in reality a mathematical aide and do not have actual existence.</p>
<p>Since, <a href="http://en.m.wikipedia.org/wiki/Force_field_%28physics%29" rel="nofollow">force fields</... | g18548 | [
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0.0... |
<p>As we know that light photon cannot escape the gravity of <a href="http://en.wikipedia.org/wiki/Black_hole" rel="nofollow">black hole</a> so I was thinking that if that is the surface of the black hole would be bright as all the photons would be there only. Am I right or wrong?</p> | g18549 | [
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<p>Clay Mathematics Institute writes about the Yang-Mills and mass gap problem on this page <a href="http://www.claymath.org/millennium/Yang-Mills_Theory/" rel="nofollow">http://www.claymath.org/millennium/Yang-Mills_Theory/</a>:</p>
<blockquote>
<p>The successful use of Yang-Mills theory to describe the strong
in... | g18550 | [
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0.06516212224960327,
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-0.002... |
<p>I have got so far the 2D density of states as $g(\epsilon)=\frac{Am}{\pi\hbar^2}$ where $A$ is the area of the "square" and $m$ is the the electron mass. Then I have found an expression for the the chemical potential for the gas using:
$N={\int_0^\infty}g(\epsilon)n_Fd\epsilon$,
where $n_F$ is the fermi distributio... | g18551 | [
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<p>I'm trying to follow the derivation of the Boltzmann equation in my Theory of Heat script, but have a little trouble understanding the following:</p>
<p>The cross-section $d\sigma$ is defined as: <strong>The amount of particles which get scattered into the solid angle $d\Omega$ per time over the incoming flux inten... | g18552 | [
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<p>The derivation of drift velocity in case of electrons is equivalent to the case of an charged ionic gas and therefore all the arguments also apply there. Now for an ideal "ionic" gas which interacts with each others only as perfect spheres would (for arguments sake), when we apply an external electric field, they wo... | g18553 | [
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<p>I have recently encountered <a href="http://en.wikipedia.org/wiki/Haag%27s_theorem" rel="nofollow">Haag's theorem</a> and according to Wikipedia: </p>
<blockquote>
<p><em>Rudolf Haag postulated [1] that the interaction picture does not exist in an interacting, relativistic quantum field theory (QFT), something no... | g573 | [
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<p>What is the <em>effective</em> physical difference between a large region of curved space-time and an equally large region of a polarized vacuum? Consider the fact that vacuum polarization amounts to an effective deviation in speed of light in a local region due to the deviation in electric permittivity of free spa... | g18554 | [
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<p>I'm trying to solve the massive Klein-Gordon equation in good old Minkowski space-time: $$(\square + m^2) \phi = \rho(t,\mathbf{x})$$ where $\square = \partial_{\mu} \partial^{\mu} = \partial_{t}^2 - \nabla^2$. So one can use a Green's function approach to find the fundamental solution of the form $$(\square + m^2) ... | g18555 | [
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0.020271940156817436,
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<p>I'm an electrical engineer, and I understand wave propagation, interference patterns, and so on. But I'm missing something basic, so perhaps my understanding isn't as good as I believe. I'll show my thinking; please tell me where I am mistaken.</p>
<p>Say that I have two guitars. Each one is precisely tuned. Each g... | g18556 | [
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<p>I have learnt that upon measurement, the state of the system changes to the eigenstate corresponding to the observed eigenvalue. Specifically, we obtain the eigenstate that is the normalized projection of the original state onto the eigenspace. However, if you multiply a state by a scalar of absolute value 1, it wil... | g18557 | [
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<p>I am currently coding something with a moving object and can't figure out the physics with my school knowledge only. I hope this question is not below the standards of this forum.</p>
<p>I have a moving object at ($x_0, y_0, \theta_0, v_0, \omega_0$), where $x$ and $y$ are the 2D-coordinates, $\theta$ the angle (to... | g18558 | [
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<p>How does an increase/decrease in temperature affect:</p>
<ul>
<li>Strength of the magnet</li>
<li>Polarity</li>
<li>Other changes?</li>
</ul>
<p>I have made some research and I have found that increasing the temperature turns ferromagnetic and antiferromagnetic substances in to paramagnetic. As far as I can unders... | g18559 | [
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<p>I wondered if someone could help me understand <a href="https://en.wikipedia.org/wiki/Spontaneous_symmetry_breaking">spontaneous symmetry breaking</a> (SSB) in classical mechanics, quantum mechanics and quantum field theory. Consider a Higgs-like potential, with a local maximum surrounded by a degenerate ground stat... | g18560 | [
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<p>In reading an article about theories as to how the slow retrograde rotation of Venus may have come into being, the article <a href="http://www.scientificamerican.com/article.cfm?id=why-venus-spins-the-wrong" rel="nofollow">Why Venus Spins the Wrong Way</a> (Franzen, 2001) stated that</p>
<blockquote>
<p>it is bou... | g18561 | [
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<p>We all know that gravitation force between two small (not heavenly) bodies is negligible. We give a reason that their mass is VERY small. But according to inverse square law, as $r\to 0$, then $F\to \infty$. But in real life we observe that even if we bring two objects very close, no such force is seen.</p>
<p>Why ... | g18562 | [
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0.005809450056403875,
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<p>I know $K(a,b,t)$ is the probability amplitude of find a particle that starts at point a in b in a time t later. There is also an expression that sometimes is called green function:<br>
$$G(a,b,E)=(i/\hbar)\int_{-\infty}^\infty\;\exp(iEt/\hbar)\;K(a,b,t)\;dt$$</p>
<p>or Fourier transform of Feynman propagator<br>
... | g18563 | [
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0.... |
<p>The age of the universe is 13.798±0.037 billion years, yet the age of HD 140283 is 14.46±0.8 billion years, how this can be the case?</p> | g18564 | [
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<p>I was wondering if anyone could help guide me in finding the Feynman rules for a real pseudoscalar field ($\phi$) interacting with the electromagnetic field $(F^{\mu\nu})$.</p>
<p>The (effective) interaction Lagrangian is:
$$ \phi \epsilon^{\alpha\mu\beta\nu} F_{\mu\alpha} F_{\nu\beta} $$</p>
<p>In my textbooks, ... | g18565 | [
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<p>Could anyone point out what went wrong in this argument?</p>
<p>Setup:</p>
<p>We have a system with 2 energy levels say with energies $0,e$ respectively.
We consider the grand canonical ensemble for the system occupied by identical spinless fermions. Then there are clearly 4 feasible microstates -- </p>
<ol>
<li>... | g18566 | [
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<p>The reason for the long life time of $B$-hadrons is that the CKM element $|V_{tb}| > 0.999$, meaning that the preferred decay of the $b$-quark is to a $t$-quark (and vice versa). However because $m_{top} >> m_{bottom}$ this `isn't allowed' for $b$-quarks.</p>
<p>My feeling is that at very high energies, i.... | g18567 | [
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<p>Through my research, I learned that;</p>
<blockquote>
<p>According to thermophysics, heat always moves from and area of high
heat to an area of low heat. Space has no heat at all. It is extremely
cold</p>
</blockquote>
<p>However, if I am not mistaken (I am not a physics guy), based on the <code>Inflation Th... | g18568 | [
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<p>Hello physicists/engineers,</p>
<p>I have a somehow very specific question I hope some expert can answer me:
I'm working at university and try to build an experiment which requires comparable high magnetic field (2000Gauss or 0.2T if you prefer) at room temperature. This field is actually no problem using Neodymium... | g18569 | [
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<p>Other day, I bumped my bookshelf and a coin fell down. This gave me an idea. Is it possible to compute the mass of a coin, based on the sound emitted when it falls? </p>
<p>I think that there should be a way to do it. But how?</p> | g18570 | [
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<p>When discussing higher dimensional operators in a theory with fermions, why do I never see anyone ever talk about the dimension five operator $\partial_\mu\bar\psi\partial^\mu\psi$?</p>
<p>How does the Fermion field behave when such a strange operator is its Lagrangian?</p>
<p>$$\mathcal{L}=\bar\psi(i\partial_\mu\... | g18571 | [
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<p>A photon of frequency f, and another of frequency f' (take f' as given) collide to create an electron-positron pair. The frequency f is such that when the collision is head on, there is exactly enough energy to produce the electron-positron pair at rest in the center-of-mass frame. What is f)</p>
<p>So in the rest ... | g18572 | [
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0.03310105577111244,
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-... |
<p>In explaining the Maxwell distribution of molecular speeds, my pchem textbook uses the following figure: <img src="http://i.stack.imgur.com/tjEtG.jpg" alt="enter image description here"></p>
<p>We are basically trying to find the probability of having a particle with a speed $u$ between $u$ and $u + du$. The axes a... | g18573 | [
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0.03108... |
<p>On page 166 of Wald's General Relativity book, he claims that the equation (7.1.20),
$$ 0 = R^t{}_t + R^\phi{}_\phi = (\nabla_a t) R^a{}_b \xi^b + (\nabla_a \phi) R^a{}_b \psi^b, $$
yields (7.1.21),
$$ D^a D_a \rho = 0. $$
He is working in an axisymmetric spacetime, and $D_a$ is the covariant derivative in the $\rho... | g18574 | [
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0.0... |
<p>I am a mathematician. I am studying and working on <a href="http://en.wikipedia.org/wiki/Hecke_algebra" rel="nofollow">Hecke pairs</a> which I am going to give the related definitions in the following. But first let me explain what I am looking for to learn by asking this question. </p>
<p>I have observed that the ... | g18575 | [
-0.01455844845622778,
0.003577557858079672,
-0.010480335913598537,
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... |
<p>I'm working on global seismology and I'm currently facing troubles understanding how an equation is obtained. The equation concerned is the following one :
$$
\rho^{E1} = -\nabla \cdot (\rho^0\mathbf{s})
$$
(From the book Theoretical Global Seismology, Princeton Press : <a href="http://books.google.fr/books?id=GWnuB... | g18576 | [
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0.01440801564604044,
0.002101... |
<p>This is a very layman level question. I watch a lot of documentaries, that's my only qualification. I have seen in documentaries that gamma rays have an extraordinary penetrating power. So if we could have a gamma ray gun on one end of planet Jupiter and something analogous to an X-Ray film of the other end, would i... | g18577 | [
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<p>Is it physically possible to have one device, that will scan one object atom by atom and record it to some computer file and then send it to some other machine that could use this blueprint to rebuild that object? What are limitations of this?</p> | g574 | [
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0.01646331511437893,
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<p>Holes are treated as particles in solid-state physics, so I've had some trouble with reasoning through this properly.</p> | g18578 | [
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-0.0021424125880002975,
... |
<p>According to the table at the bottom of the <a href="https://en.wikipedia.org/wiki/Candela" rel="nofollow">Wikipedia page for the candela</a>, the dimension for candelas is J (joules). Why is this not W (watts)? </p>
<blockquote>
<p>The luminous intensity for light of a particular wavelength λ is given
by</p>
... | g18579 | [
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<p>Scenario:</p>
<p>Imagine a collimated beam of white light falling on one refracting face of a prism. Let the light emerging from the second face be focused by a lens onto a screen. Suppose there is linear dispersion at the screen. Now by introducing a narrow "exit slit" (on the order of 0.01cm) in the screen, one h... | g18580 | [
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0... |
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