question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>I was told today that the Polyakov action for a $p$-brane is (superficially) re-normalizable iff $p\leq 1$. Of course, when I went to check for myself, I screwed up my power-counting, and I'm having trouble seeing why.</p>
<p>We work in units with $c=1=\hbar$, so that $L=T=M^{-1}$. In these units, any action must... | 2,153 |
<p>As defined by Wikipedia:</p>
<blockquote>
<p>The heat death of the universe is a suggested ultimate fate of the
universe in which the universe has diminished to a state of no
thermodynamic free energy and therefore can no longer sustain
processes that consume energy (including computation and life). Heat
... | 2,154 |
<p>I have a question regarding Jackson's Classical Electrodynamics. Consider the equation</p>
<p>$$\varphi \left ( x\right )=\tfrac{1}{4\pi\epsilon _{0}} \int_V \frac{\varrho ( x )}{R}d^{3}x+\tfrac{1}{4\pi} \oint_{\partial V} \left(\frac{1}{R}\frac{\partial }{\partial n}\varphi -\varphi \frac{\partial }{\partial n}\f... | 2,155 |
<p>I'm attempting to simulate the Fraunhofer diffraction pattern due to a single slit.
We know that the intensity at an angle $\theta$ is $I(\theta)=I_0 \text{sinc} ^2(\beta)$ where $\beta=k\,b\,\sin(\theta) / 2.$</p>
<p>Considering the setup shown in the image where O is the origin (the precise center of the single ... | 2,156 |
<p>I am trying to relate the surface-area-to-volume-ratio of a sphere to the <a href="http://en.wikipedia.org/wiki/Bekenstein_bound" rel="nofollow">Bekenstein bound</a>. Since the surface-area-to-volume-ratio decreases with increasing volume, one would surmise that, per unit of volume, a small space is richer in inform... | 2,157 |
<p>I would appreciate it if someone tells me how a cft on a compactified manifold (e.g. by means of periodic boundary conditions) can be meaningful? The global conformal invariance is broken due to the scale over which the manifold is compactified (e.g. the period). The local conformal invariance of course still classi... | 2,158 |
<p>According to this review</p>
<blockquote>
<p>Photon wave function. Iwo Bialynicki-Birula. <a href="http://dx.doi.org/10.1016/S0079-6638%2808%2970316-0" rel="nofollow"><em>Progress in Optics</em> <strong>36</strong> V (1996), pp. 245-294</a>. <a href="http://www.arxiv.org/abs/quant-ph/0508202" rel="nofollow">arXiv... | 2,159 |
<p>In the context of the AdS/CFT correspondence I was trying to understand how the symmetry group of the underlying space $AdS_5 X S^5$ comes out to be the supergroup $SU(2,2|4)$. I can see how the bosonic subrgoup $SU(2,2)XSU(4)_R$ crops up as the group of isometries of $AdS_5XS^5$, since $SU(2,2)$ is a double cover ... | 2,160 |
<p>Is there a physical method to prove for example when the zeta regularization of a series</p>
<p>$$ 1+2^{k}+3^{k}+............= \zeta (-k) $$</p>
<p>gives the correct result: Casimir effect, vacuum energy and when does it 'fail'?</p>
<p>For example we plug the zeta regulator inside the renormalization group equati... | 2,161 |
<p>I'm having a party.</p>
<p>Suppose I'd like to have a fridge <em>full</em> of cold ($6~^\circ\text{C}$ or below) beer bottles, <em>in as short a time frame as possible</em>. The fridge indicates that it is targeting (and presumably currently at) $4~^\circ\text{C}$. All the bottles are currently at $30~^\circ\text{C... | 2,162 |
<p>Just a small question regarding collisions.
Imagine a head-on collision between a photon and a particle with mass that moves with a non-relativistic speed, the particle was on its ground state, completely absorbs the photon, and moves to its next energy level. Is it always the case that the particle ends up with a n... | 2,163 |
<p>I know that <em>WolframAlpha</em> questions are off topic but as I cannot find a Q&A site for WA I decided to ask here. It should be a piece of cake for you guys as I think it is a fairly simple question. </p>
<p>I have been trying to calculate my derivative in WolframAlpha by inputing this: </p>
<pre><code>A=... | 2,164 |
<p>I'm writing a quantum computer simulator (about 8 qubits) and I know most of the basics (i.e. how to calculate the effect of a quantum gate on a qubit). But I have hit a wall.</p>
<p>Is it possible, with two qubits of $ a |0\rangle + b|1\rangle $ and $c|0\rangle + d|1\rangle$
to calculate the superposition state of... | 2,165 |
<p>Say we are in 3 dimensions and use $(-++)$. If we have the metric
$$ds^2=-dt^2+dr^2+r^2df^2(t),$$
then what is the third coordinate if the first two were $t$ and $r$?
$$X^iX_i=-t^2+r^2+?$$</p> | 2,166 |
<p>Due to my task of writing orbit prediction routines I am trying to understand the reference frames better and how to use them ( particularly for Earth orbits ).</p>
<p>I think I get the idea of what <a href="https://en.wikipedia.org/wiki/Earth-centered_inertial" rel="nofollow">ECI (Earth centered inertial)</a> is a... | 2,167 |
<p>Imagine there's a little girl/boy sitting on one of the seats of a Ferris wheel. She/he throws out a stone and sees that it's experiencing the velocity <strong>v</strong>. Meanwhile, another observer standing on the ground sees that the stone is thrown with a horizontal velocity, <strong>u</strong>. What's the conne... | 2,168 |
<p>Solar neutrino deficit was first observed in the late 1960's. And theory of neutrino oscillation was developed in 1967. But,in 2001, the first convincing evidence of solar neutrino oscillation came in SNO. Why did it take nearly 35 years to verify the neutrino oscillation?</p>
<p>reference: <a href="http://en.wikip... | 2,169 |
<p>Suppose we set up an experiment where we have an inclined ramp, and a spherical basketball. If we were to assume the ball to be perfectly round, and rolls down in a vertical manner and the situation friction less. The simplified equation that would be used would be $\frac{2}{3} G x \sin\theta$. I'm wondering why is ... | 2,170 |
<p>During analysis the constraint from a theory,
suppose my canonical Hamiltonian is $$H_c=P^A\dot{A}+P^B\dot{B}-L$$
where $P^A=\frac{\partial L}{\partial \dot A}$ and $P^B=\frac{\partial L}{\partial \dot B}$
In this case does the commutation of $[P^A,P^B]=0$?</p> | 2,171 |
<p>The claim that the young universe was in a low-entropy state seems at odds with</p>
<ul>
<li>maximal entropy being thermal equilibrium, and</li>
<li>the young universe being in thermal equilibrium.</li>
</ul>
<p>I've looked at <a href="http://physics.stackexchange.com/questions/18702/why-was-the-universe-in-a-extr... | 2,172 |
<p>Well, Is it really possible to maintain such low temperatures required for super-conductors (taking <a href="http://en.wikipedia.org/wiki/High-temperature_superconductivity" rel="nofollow">High-temperature superconductivity</a> into account) over <em>large distances</em>?</p>
<p>What I say is - Even if we were able... | 2,173 |
<p>Following with a series of questions regarding quantum squeezing, let me add another one: quantum squeezing of vacuum is a real propagating state of the field, it can be switched on and off, squeezing can be altered and information can be sent modulating it. So, there must be real degrees of freedom in the QED field... | 2,174 |
<p>I will say right away that I don't mean standard-model sphalerons, I mean the sphalerons of some extension of the standard model. </p>
<p>The reason to even think about this is last year's paper by <a href="http://arxiv.org/abs/1310.3904" rel="nofollow">Frampton and Hung</a> <a href="http://www.physicsforums.com/sh... | 2,175 |
<p>A weather satellite ($m_s = 4350$ kg) is in a stable circular orbit around the Earth ($m_E = 5.97 \cdot 10^{24}$ kg). It completes an orbit once every 2 and a half hours.</p>
<p>(I'm sure about these 2 answers)
At what distance from the center of the Earth does the satellite orbit?
$r_i = 9.35 \cdot 10^6$ m</p>
<p... | 2,176 |
<p>Why is that when you scuff with your shoes on, charges move (since electrometer moves back and forth), but if you don't have your shoes on, the electrometer doesn't move.</p>
<p>Here's the corresponding video detailing the event.
<a href="http://www.youtube.com/watch?v=cJSp8v0YJrA" rel="nofollow">http://www.youtub... | 2,177 |
<p>I have a question concerning the Wald's book: General Relativity.
In the appendix E, he derived the Einstein equation by considering the surface term (GHY).
I do not understand what he said after the equation (E.1.38).</p>
<p>Actually he considers that $h^{bc}\nabla_c(\delta g_{ab})=0$, because we fix $\delta g_{a... | 2,178 |
<p>We know that electron trapped by nuclear, like the hydrogen system, is described by quantum state,and never fall to the nuclear. So is there any similar situation in the case of electron near the black hole but not fall into it? And what is "falling " in gravitation mean when considering quantum mechanics? What does... | 2,179 |
<p>Given the following potential:
$$V(\theta,\phi)=\frac{Q}{a}\left(\sin\theta \cos\phi+\frac{1}{2}\cos^2\theta\right)$$ on the surface of a sphere of radius $a$
I am trying to solve Laplace's Equation outside the sphere (where there aren't any charges).
I know the general solution to Laplace's Equation outside the sph... | 2,180 |
<p>I work in the Lorentzian manifolds, more generally in pseudo Riemannian manifolds and applications to general relativity.
I know the definitions of <a href="http://en.wikipedia.org/wiki/Conformal_vector_field" rel="nofollow">conformal</a>, <a href="http://en.wikipedia.org/wiki/Killing_vector_field" rel="nofollow">Ki... | 2,181 |
<p>When I am studying the total reflection phenomenon, I calculated the Poynting vector of the transmitted wave, which can be written as $S_t=A(k_{x}\hat{x}+i\alpha\hat{z})$
A is some constant.
I choosed $z=0$ as the interface, light incident from the region $z>0$,
If total reflection occurs,
the z-component becom... | 2,182 |
<p>It is well known that complex polarizability of uniform dielectric sphere with radius $r$ and complex permittivity $\hat\epsilon_{in}(\omega)$ placed in a medium with complex permittivity $\hat\epsilon_{out}(\omega)$ under homogeneous electric field with circular frequency $\omega$ is defined by (in the <a href="htt... | 2,183 |
<p>There is <a href="https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality" rel="nofollow">Wave–particle duality</a>. According to this theory, light is a wave and a particle at once.</p>
<p>What about magnetic field? Can it be so, that it is also a wave and particle, but this particle has not yet been discovere... | 2,184 |
<p>How is it that lead can block radiation, and things are lead lined. In the Indiana Jones 4 movie he climbs inside a lead-lined fridge and he somehow survives the blast and radiation?</p> | 2,185 |
<p>I am not very good with physics terms, so please treat me as an ignorant.</p>
<p>I am trying to calculate a damping coefficient dynamically for a hydraulic-controlled door that opens and closes due to hydraulic pressure (opening/closing). The formula where I need my damping coefficient is: </p>
<p>viscous_fric_mom... | 2,186 |
<p>Is it possible to focus white light into a single mode optical fiber? I thought no because in order to focus it should be a solution to the Helmholtz equation but I am not too sure about it. Also since the equation features the wave number, shouldn't it be impossible? </p> | 2,187 |
<p>If $ use $(+,-,-,-)$ sign convention then four position, four velocity become positive but four acceleration becomes negative! </p>
<p>$x_{\mu}x^{\mu}=\tau^2c^2,$</p>
<p>$U_{\mu}U^{\mu}=c^2,$</p>
<p>$a_{\mu}a^{\mu}=-(a\gamma^3)^2,$</p>
<p>in other hands If I use $(-,+,+,+)$ sign convention then four position, f... | 2,188 |
<p>Sun is the major source of electromagnetic radiation. Then if the source is the same, how are different EM waves with differing wavelengths produced?</p> | 2,189 |
<p>Well, this question has been puzzling me for kinda long time, many people believe that orbiting astronauts feel weightless because they are "beyond the pull of Earth's gravity"...How far from the Earth would a spacecraft have to travel to be truly beyond the Earth's gravitational influence? If a spacecraft were real... | 33 |
<p>I regularly find that I'll understand where the field content in a particular physics paper comes from, but then a Lagrangian or action or superpotential is stated and I don't know how it's derived. Is there a set of general rules for building a Lagrangian/action/superpotential if you already know the field content ... | 2,190 |
<p>$\angle I +\angle E=\angle A+\angle D$</p>
<p>Angle of incidence + angle of emergence = angle of prism (Normally $60^\circ$) + angle of deviation.</p>
<p>If their sum is not equal,we made personal error in doing an experiment with prism. Please make sense of this equation. </p> | 2,191 |
<p>Blackholes may be really strong but they act in a very short range. For example if the sun was a black having the same mass, it will be dark but we will still be revolving around it. It wont engulf us.</p>
<p>Also I hear that the outer stars in a galaxy rotates around the galaxy with same speed as the inside stars?... | 2,192 |
<p>Why is $SU(3)$ chosen as the gauge group. Why not $U(3)$? Why does it even have to be unitary?</p> | 2,193 |
<p>I want to understand why we do not build space stations in a similar way that we build a home, piece by piece. Instead we construct modules on the ground and fly them up. Are there some technical limitations other than money that make this impossible?</p>
<p>Why don't we fly up raw materials, prefabbed metal panels... | 2,194 |
<p>A <a href="http://www.nasa.gov/mission_pages/WISE/news/wise20110823.html" rel="nofollow">recent article from NASA</a> said they found some stars with temperatures "as cool as the human body." How is this possible? Does fusion still occur in these stars?</p> | 2,195 |
<p>For the infinite well:
$$U(x)=\quad\infty : x \leq 0\quad
0 : 0 < x < L\quad
\infty : x \geq L$$</p>
<p>$\psi_n=$$\sqrt{\frac{2}{L}}\sin{\frac{n\pi x}{L}}$</p>
<p>Find $\Delta x_n$, the uncertainty in position for some arbitrary eigenstate psi.n</p>
<p>So the attempt I made at doing this was to find using $... | 2,196 |
<p>Several years ago, I was laying on my bed and had a CD shaped transparent plastic disk (which was covering a 100 CD stack), basically a transparent CD. I don't know why but I took my phone and took a picture of the light bulb in my room through the hole of that plastic disk. Here is the result:</p>
<p><img src="htt... | 2,197 |
<p>Suppose we have a circular loop of wire, and we put a long perfect solenoid inside it which is connected to an AC voltage source so that the magnetic field inside it starts to vary by time, does this varying magnetic field induce an emf in our loop according to Faraday's law of induction? (suppose that the radius of... | 2,198 |
<p>So in QFT, quantum field operator $\psi$ is there. $\psi$ seems to take the role of wavefunction in QM, which now acts upon vacuum state. Then, in lagrangian of various quantum field theories, $\psi$ appears, but now it is called quantum field. So is quantum field operator no different from quantum field here? If it... | 2,199 |
<p>Could anyone tell me what equations can I obtain from the Lagrangian density</p>
<p>$${\cal L}(\phi,\,\,\phi_{,i},\,\,A_i, \dot A_i,\,\,A_{i,j})~=~\frac{1}{2}|\dot A+\nabla\phi|^2-\frac{1}{2}|\nabla \times A|^2-\rho\phi+J\cdot A$$
by the Euler-Lagrange equations?</p> | 2,200 |
<p>Interesting behavior of strong correlation between electrons occur in metals with partially filled d or f orbitals (transition metals). Why these strong correlations do not appear with elements with incomplete p or s orbital for example ?</p> | 2,201 |
<p>Shouldn't the absorption rate of a beam of particles strongly depend on the angle between the beam and the target material's crystal-axis (if the target material is a monocrystal)? At certain angles, all the nuclei will be stacked behind each other, offering a very small cross section, whereas other orientations wi... | 2,202 |
<p>This question comes out of my other question "<a href="http://physics.stackexchange.com/questions/71987/time-ordering-and-time-derivative-in-path-integral-formalism-and-operator-formal">Time ordering and time derivative in path integral formalism and operator formalism</a>", especally from the discussion with <a hre... | 2,203 |
<p>With $Y_{lm}(\vartheta,\varphi)$ being the Spherical Harmonics and $z_l^{(j)}(r)$ being the Spherical Bessel functions ($j=1$), Neumann functions ($j=2$) or Hankel functions ($j=3,4$) defining $$\psi_{lm}^{(j)}(r,\vartheta,\varphi)=z_l^{(j)}(r)Y_{lm}(\vartheta,\varphi),$$ what are representations of the Poincaré tra... | 2,204 |
<p>I'm struggling to find a solution to a project for college. I have looked through my textbook countless times and googled for days, but I just can't seem to figure it out. </p>
<p>The question is that we need to prove the angular speed of a hanging rotating weight is given by the expression:</p>
<p>W = root(g/Lcos... | 2,205 |
<p>I just finished learning M(atrix) theory and the basics of the <a href="http://en.wikipedia.org/wiki/Compactification_%28physics%29" rel="nofollow">compactification</a> of extra dimensions.</p>
<p>The extra 6 dimensions of superstring theory can be compactified on 3 Calabi-Yau manifolds (because 6 real dimensions m... | 2,206 |
<p>What energy changes take place when you operate a <a href="http://en.wikipedia.org/wiki/Jet_ski" rel="nofollow">jet-ski</a>?</p> | 2,207 |
<p>I'm considering a metric of the following form (signature $(+,-,-,-)$):
$$ds^2 = (F(r,t)-G(r,t))dt^2 - (F(r,t)+G(r,t))dr^2 - r^2(d\Omega)^2$$</p>
<p>where $F(r,t)$ and $G(r,t)$ are arbitrary scalar functions.</p>
<p>I am trying to find a coordinate and/or conformal transformation to one of the 'standard' Robertson... | 2,208 |
<p>I have a really simple doubt about finding the potential difference in electrostatics. Well, first of all, the definition of potential difference is very clear to me: we take a path between the points of interest and we sum the tangential components of the electrict field $E$ along the path. In equation:</p>
<p>$$\... | 2,209 |
<p>This question developed out of conversation between myself and Joe Fitzsimons. Is there a succinct stabilizer representation for symmetric states, on systems of <em>n</em> spin-1/2 or (more generally) <em>n</em> higher spin particles?</p>
<p>By a "stabilizer representation", I mean that:</p>
<ul>
<li><p>every symm... | 2,210 |
<ul>
<li><p>If $\Phi$ is a multi-component scalar field which is transforming in some representation of a gauge group say $G$ then how general a proof can one give to argue that the potential can only be a function of the G-invariant function, $\Phi^\dagger \Phi$? </p>
<p>This issue gets especially more confusing when... | 2,211 |
<p>I enjoy thinking about theoretical astrophysics because I want to understand black holes. Given that no one understands black holes, I like to ponder the nearest thing to a black hole: a neutron star! I have searched around the web for pedagogical discussions of the structure of neutron stars such as this link from ... | 2,212 |
<p>I have always been interested in space and astronomy (in my youth - I wanted to be an astronaut).</p>
<p>However for various reasons, I never quite got started. I now want to get started - small but steadily. I live in a city, ergo: light pollution is a problem - however, I would like to get a telescope (maybe a se... | 2,213 |
<p><a href="http://hpiers.obspm.fr/eop-pc/models/constants.html" rel="nofollow">http://hpiers.obspm.fr/eop-pc/models/constants.html</a> states the
tropical year is $365.242190402$ days.</p>
<p>The Gregorian calendar's average year is $365.2425$ days (every 4th year
a leap year, except every 100 years, except every 4... | 2,214 |
<p>I have a particle system of seven protons and seven (or sometimes eight) neutrons (each formed by their appropriate quarks, etc.) bound together in a state that can be macroscopically described as a nucleus. If relevant, there are also about seven electrons that are bound to this arrangement. These particle system... | 944 |
<p>I hope this is the right word to use.</p>
<p>To me, these forces seem kind of fanciful (except for General Relativity and Gravity, which have a geometric interpretation).</p>
<p>For example, how do two charged particles <strong>know</strong> that they are to move apart from each other?</p>
<p>Do they communicate ... | 2,215 |
<p>I've always heard people saying, "Faster than light information transfer can't happen because it would violate causality! Effects can precede their causes!"</p>
<p>However, I'm trying to think of a situation where this would happen. I'm sure it has something to do with relativity or something like that. I sort o... | 2,216 |
<p>Why does a magnetic core saturate? What is its actual mechanism?</p> | 2,217 |
<p>What are the best textbooks to read for the mathematical background you need for modern physics, such as, string theory?</p>
<p>Some subjects off the top of my head that probably need covering:</p>
<ul>
<li>Differential geometry, Manifolds, etc.</li>
<li>Lie groups, Lie algebras and their representation theory.</l... | 185 |
<p>Suppose you put wheels under a compressed air tank</p>
<p><img src="http://i.stack.imgur.com/3KYwa.jpg" alt="compressed air tank"></p>
<p>so that it can move horizontally to the right and to the left. Suppose there is a nozzle on the right hand side of the tank (in the picture on the backside, if you like).</p>
<... | 2,218 |
<p>Suppose we have a voltic cell. In one half cell, we have a zinc electrode (or any other) and in the other we have a hydrogen electrode (or any other). </p>
<p>Now, if I say that the electrode potential of zinc is 5 volts, would it mean that it (5 volts) is actually the potential difference between the two electrode... | 2,219 |
<p>In the book <em>Arthur Beiser - Concepts of modern physics [page 213]</em> author separates the variables in the polar Schrödinger equation assuming: </p>
<p>$$\psi_{nlm}=R(r)\Phi(\phi)\Theta(\theta)$$</p>
<p>then there a statement that the differential od space in the polar coordinate system is: </p>
<p>$$dV=(dr... | 2,220 |
<p>I was reading up about <a href="http://en.wikipedia.org/wiki/Depth_of_field" rel="nofollow">Depth of Focus</a> and wondered if glasses affect depth of focus. If yes, is it noticeable to the user?</p> | 2,221 |
<p>In an electrostatic case it is clear that that in a space enclosed with a conductor (without charge in it) the electric field is zero. </p>
<p>This is often demonstrated in physics shows like on the following image:</p>
<p><img src="http://i.stack.imgur.com/mKkdX.jpg" alt="enter image description here"></p>
<p>Ho... | 2,222 |
<h2>The problem statement, all variables and given/known data</h2>
<p>Consider the following arrangement:</p>
<p><img src="http://i.stack.imgur.com/Jge7p.jpg" alt="enter image description here"></p>
<p>Calculate the work done by tension on 2kg block during its motion on circular track from point $A$ to point $B$.</p... | 2,223 |
<p>How big is an <a href="http://en.wikipedia.org/wiki/Inertial_frame_of_reference" rel="nofollow">inertial frame</a>?</p>
<p>Consider a huge rod which is rotating about a fixed point in a plane, its length is 1 light year.</p>
<p>Thus light from its end closer to the fixed point to the end farther from the fixed poi... | 2,224 |
<p>Can we call <a href="http://en.wikipedia.org/wiki/Speed" rel="nofollow">speed</a> the <em>change in distance per time</em>?</p>
<p>Further, would you phrase <a href="http://en.wikipedia.org/wiki/Heat_capacity" rel="nofollow">heat capacity</a> as <em>change in heat per temperature</em> or <em>change in temperature p... | 2,225 |
<p>The GPS is a very handy example in explaining to a broad audience why it is useful for humanity to know the laws of general relativity. It nicely bridges the abstract theory with daily life technologies! I'd like to know an analogous example of a technology which could not have been developed by engineers who didn... | 868 |
<p>Suppose that we have a known set of unitaries $U_1,...,U_n$ randomly selected from the Haar measure and suppose that each unitary is applied with probability $\frac{1}{n}$ to some input state $\rho$ which is pure and lives in a $d$-dimensional Hilbert space. </p>
<p>For $n\rightarrow\infty$ it is clear that:
\begin... | 2,226 |
<p>All stars are revolving around the center of galaxy. And so I wonder how long it takes before we can see any noticeable change in the shape of any constellation? What are the factors one will use to find this besides the speed of revolution of the concerned stars? Let us say a noticeable change corresponds to about ... | 2,227 |
<p>I want to see a some physical process (experimental) that could explain the many transformations of coordinates into this mathematical procedure.
(really two transformations, but i think that is a big number here)</p> | 2,228 |
<p>How can it be shown that the Majorana mass violates the fermion number by two units? Can even a Noether charge be defined in presence of Majorana mass term?</p> | 2,229 |
<p>I was thinking about Hydrogen balloons and that large ones which are used for weather balloons which sometimes go up to 100,000 ft (approx 30km). Then I was wondering, how much potential energy has the balloon gained with the balloon and the weight it carries to get up to 100,000 ft. It seems the object would have a... | 2,230 |
<p>You can read everywhere about water's extraordinary property of <em>expanding</em> when frozen, thus the reason ice floats on liquid water. </p>
<p>What other substances do this? There are claims of mercury, silica, germanium, bismuth, and antimony, but I've had trouble tracking down the data to back these up.</p> | 2,231 |
<p>I am currently doing coursework for my A2 physics course. I am dropping a charged oil drop in a horizontal electric field and I was wondering how to calculate how much the droplet would deviate given that the voltage across the plates is 5500 volts.</p>
<p>The height I am dropping the drop from is 22 cm, the distan... | 2,232 |
<p>I'm starting university, but I've not made up my mind yet: I like both physics and maths. I pretty much know what mathematical research is about, but I've no clue about physics. Can anyone suggest some reading (easy stuff) to get an idea of what theoretical physics is all about when it comes to research? </p> | 2,233 |
<p>The dominant channels in the <a href="http://en.wikipedia.org/wiki/Greisen%E2%80%93Zatsepin%E2%80%93Kuzmin_limit">GZK</a> process are</p>
<p>$$p+\gamma_{\rm CMB}\to\Delta^+\to p+\pi^0,$$
$$p+\gamma_{\rm CMB}\to\Delta^+\to n+\pi^+.$$</p>
<p>According to the <a href="http://pdg.lbl.gov/">pdg</a>, $\Delta\to N+\pi$ m... | 2,234 |
<p>In the <a href="https://en.wikipedia.org/wiki/Proper_acceleration#Acceleration_in_.281.2B1.29D" rel="nofollow">Wikipedia page for proper acceleration</a>, an equation for proper acceleration in terms of rapidity is given as $\alpha = \frac{\Delta \eta}{\Delta \tau}$, where $\eta$ is the rapidity of the moving frame,... | 2,235 |
<p>If incident electromagnetic wave is given as:</p>
<p>$$\begin{align*}E_i&=A_e \cos(\omega t + bz)\\
H_i&=A_h \cos(\omega t + bz)\end{align*}$$</p>
<p>What would be relation for REFLECTED wave? </p>
<p>Does it go like this?
$$\begin{align*}E_r&=A_e \cos(\omega t - bz)\\
H_r&=A_h \cos(\omega t - b... | 2,236 |
<p>Recently I am studying the projective symmetry group (PSG) and the associated concept of quantum order first <a href="http://prb.aps.org/abstract/PRB/v65/i16/e165113" rel="nofollow">proposed by prof.Wen</a>.</p>
<p>In Wen's <a href="http://www.sciencedirect.com/science/article/pii/S0375960102008083" rel="nofollow">... | 2,237 |
<p>If one takes a bundle of wood up high to the mountains so its potential energy increases, would there be obtained more heat by burning it? </p> | 2,238 |
<p>I been reading some physics articles (related to the recent discovery of the particle that could be a Higgs boson) posted online and it was talking about electron <a href="http://en.wikipedia.org/wiki/Spin_%28physics%29" rel="nofollow">spin</a> and how it can only have values of either up or down and that always con... | 2,239 |
<p><a href="http://en.wikipedia.org/wiki/T-symmetry" rel="nofollow">Time reversal symmetry</a> seems to be a very useful concept and is mentioned in a good number of papers I recently came across. Most of the time people claim that a certain system or Hamiltonian is time reversal invariant and deduce certain properties... | 2,240 |
<p>If I have two disc-shaped magnets (radius r=0.05m, width w=0.03m, Remanance Br=1.06 T) separated by a distance d, how can I calculate the magnetic flux density somewhere between them?</p>
<p>I found <a href="http://www.supermagnete.de/eng/faq/How-do-you-calculate-the-magnetic-flux-density#cylinder-magnet" rel="nofo... | 2,241 |
<p>I am talking about thermal ionization, Is to possible to make any material or is there any element that can be ionized at let say 20 degree centigrade Temperature? </p>
<p>I am a computer engineer and into science that much but it is just curiosity, Please correct me If I am talking nonsense. </p>
<p><strong>ED... | 2,242 |
<p>I am working on the transport properties of two dimensional electron gas in semiconductor heterostructures and am interested in the characteristic length and time scales of the system like elastic scattering time and length, phase coherence length, thermal length, etc. Kindly suggest a good review paper on explainin... | 2,243 |
<p>My question is short and simple. If a damped, travelling wave (say on a string) could be described as </p>
<p>$y\left( x,t\right) =Ae^{-\gamma x}\sin \left( kx-\omega t\right)$</p>
<p>how could/would one determine the decay/dampening constant $\gamma$ mathematically?</p> | 2,244 |
<p>I am looking to pump water from a pool up to a roof for solar heating (black plastic tubing) and then back into the pool with the original source water. Does the gravitational force of the water flowing back down the pipe into the pool assist the pump and therefore decrease the pumps required strength?</p>
<p>I hav... | 2,245 |
<p>This is of course a purely theoretical question and it would take energy to get the magnet moving in the first place but once it's moving in the vacuum of space, with no gravity or magnetic field nearby, could it spin nearly forever (as in billions of years) producing a magnetic field from which you could get electr... | 2,246 |
<p>The question is: if </p>
<ul>
<li>A bowling ball and ping pong ball </li>
<li>are moving at <strong>same momentum</strong></li>
<li>and you exert <strong>same force</strong> to stop each one</li>
<li>which will take a longer time? or some? </li>
<li>which will have a longer stopping distance? </li>
</ul>
<hr>
<p>... | 2,247 |
<p>Look up <a href="http://en.wikipedia.org/wiki/Linearized_gravity" rel="nofollow"><em>linearized Einstein field equations</em></a> anywhere and the first thing you'll see will be a discussion of gravitational waves. Using the linearized EFE's is pretty handy when studying gravitational waves, but it doesn't seem like... | 2,248 |
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