question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>I read that thermodynamic entropy is a measure of the number of microenergy states. What is the derivation for $S=k\log N$, where $k$ is Boltzmann constant, $N$ number of microenergy states.</p>
<p>How is the logarithmic measure justified?</p>
<p>Does thermodynamic entropy have anything to do with information entr... | 3,722 |
<p>I am currently studying for an exam in Quantum Mechanics and came across a solution to a problem I have trouble with understanding.</p>
<p>The Problem:</p>
<p>A Particle sits in an infinite potential well described by</p>
<p>\begin{align}
V(x) &= 0, & 0 \leq x \leq L \\
V(x) &= \infty, & \text{oth... | 3,723 |
<p>I was recently reading Atkins' Physical Chemistry, the topic of rotational energies of molecules. It states the degeneracies of spherical top, symmetric and linear molecules as being $(2J+1)^2$, $2(2J+1)$ and $(2J+1)$, respectively ($J$ being the rotational quantum number). </p>
<p>Atkins however does not discuss ... | 3,724 |
<p>Suppose I have two charged capacitor plates that both are isolated and carry a charge density $D = \frac QA$. According to textbook physics the electric field between them is given by $E=\frac D {\epsilon\epsilon_0}$ and the voltage by $U = Ed = \frac {Dd}{\epsilon\epsilon_0}$ with $d$ the distance between the plate... | 3,725 |
<p>Suppose there are two objects in the universe. Earth, with a gravitational acceleration of g = 9.8m/s/s, and a typical electron. </p>
<p>The electron is dropped from a certain height, say 1000m above the Earth's surface.</p>
<p>The initial energy of the electron is only the potential energy, $mgh = m_eg\times100... | 3,726 |
<p>The experment would involve a small NIB magnet levitating between or on the diamagnetic material pyrolytic graphite, unlike other forms of levitation this doesn't require power to run such as electricity or that the levitating item be moving to maintain the effect. This does seem to require occasional adjustments, p... | 3,727 |
<p>Whether it is necessary to search still for variants of an explanation of spontaneously breaking gauge symmetry, giving masses for a W, Z-bosons?</p>
<p>Goldstone bosons are bosons that appear necessarily in models exhibiting spontaneous breakdown of continuous symmetries, thus it is clear that the Higgs bogon - is... | 3,728 |
<p>Magnets have a magnetic north and south pole. Solenoids too have north and south pole from which magnetic fields comes out and goes in respectively. But is it that every magnetic configuration have a north and south pole? Electrons have magnetic moment and they can be regarded as very tiny magnets. So, where is its ... | 3,729 |
<p>As we know the eigenfunctions for a particle of mass m in an infinite square well defined by $V(x) = 0$ if $0 \leq x \leq a$ and $V(x) = \infty$ otherwise are:</p>
<p>$\psi_n (x) = \sqrt{(2/a)} sin(n \pi x/a)$</p>
<p>The question now is: How does the ground state function look like in momentum space? As far as i r... | 3,730 |
<p>I am having trouble with a velocity-versus-time graph. I recently took a Physics test that asked this question: The graph shows the velocity versus time for a particle moving along the $x$ axis. The $x$ position of the particle at $t$=0 seconds was 8 meters. What was the $x$ position of the particle at $t$=2 seconds... | 3,731 |
<p>Sometimes I need to look up a certain cross section, say the inclusive Z production cross section at $\sqrt{s}$ = 7 TeV. Is there a place where 'all the' cross sections are tabulated (experimental and theoretical), like the branching ratios are in the <a href="http://en.wikipedia.org/wiki/Particle_Dat... | 3,732 |
<p>A fairly common technique in experimental particle physics is event-by-event reweighting. The idea is that you have a sample of background-model events, either from a Monte Carlo simulation, or from a data-driven method. However, your background model doesn't describe the data well enough, so you go into a sideband ... | 3,733 |
<p>This is a past exam question from one of our lectures, and we have an issue with (i), I believe I need to use the equation $\rho=\frac{RA}{l}$, but I am not sure - could someone enlighten me on the issue?</p>
<blockquote>
<p>A mild steel ring of magnetic permeability 380, having a cross sectional area of $500mm^2... | 3,734 |
<p>What does the following statement mean and why is it true?</p>
<blockquote>
<p><em>The Weak Equivalence Principle (WEP) implies that in general curved space-time there is no privileged coordinate system.</em></p>
</blockquote>
<p>I have looked up the <a href="http://en.wikipedia.org/wiki/Equivalence_principle" r... | 3,735 |
<p>I am reading a paper, and I came across the Green-Kubo formulation, where the conductivity $\sigma$ of charged particles is related to the time correlation function of the $z$-component of the collective ionic current $J_z(t)$:</p>
<p>$$\sigma_{GK} = \frac{1}{V k_B T} \int_0^{\infty} dt \; C_{JJ}(t)$$</p>
<p>where... | 3,736 |
<p>This is not a joke. It is something I have wondered about for a long time. Nearly every picture of Werner Heisenberg shows him smiling. </p>
<p><img src="http://i.stack.imgur.com/1kvEV.jpg" alt="Heisenberg is happy">
<img src="http://i.stack.imgur.com/amoRb.jpg" alt="Heisenberg is still happy">
<img src="http://i.... | 3,737 |
<p>Does the state of whether an object if moving or stationary affect the likelihood of it being struck by lightning?</p>
<hr>
<p>I suppose some things that could be considered would be:</p>
<p>Whether the movement means the object is not continually earthed, for example, a horse galloping across an open plain, duri... | 3,738 |
<p>I need some ideas on a problem. </p>
<p>The first part says: Whats the posible rise in the temperature of the water falling 49.4 m in the Niagara Falls? That one was easy, with answer 0.112 Kelvin. ($\Delta T = \frac{g*h}{c_{H_2 O}}$)</p>
<p>The second part asks what factors tend to prevent that rise in temperatur... | 3,739 |
<p>I was going over my notes for an introductory course to electricity and magnetism and was intrigued by something I don't have an answer to. I remember my professor mentioning, to the best I can remember, that electric current is actually not the flow of electrons but the propagation of the electric field. My questio... | 3,740 |
<p>When beginning a study of the special theory of relativity, one discovers that the theory of special relativity has as an axiom that the laws of physics are invariant with respect to transformations between inertial frames. The theory then states that Maxwell's equations are laws of physics and thus invariant betwee... | 3,741 |
<p>If you have some random object at rest and you apply a <a href="http://en.wikipedia.org/wiki/Couple_%28mechanics%29" rel="nofollow">couple</a> to it, the net force acting on it is zero. However because a moment acts on it, it starts to rotate.</p>
<p>So you had an object at rest, a net force of zero was applied to ... | 3,742 |
<p>The Boltzmann equation in absence of external force reads:</p>
<p>$\frac{\partial f}{\partial t} + \vec{v} \cdot \frac{\partial f}{\partial \vec{r}} = \left( \frac{\partial f}{\partial t}\right)_{coll}$</p>
<p>Where the r.h.s. stands for the change in the distribution function of the velocities owing to collisions... | 3,743 |
<p>In class my prof said that when showing a system is at equilibrium it suffices to show that the moment at one point is zero. Why? Why does showing the moment at a point is zero imply the moment of the whole system is zero?</p>
<p>If I misunderstood him and the statement above isn't true, how do you show the system ... | 3,744 |
<p>To the extent that I know:</p>
<p>There are symmetry groups like the rotation groups SO(3), the Groups of Poincare Transformations,... If the physics of a system has a symmetry group G, then it can be described by a representation of G and the vector space acted on.</p>
<p>Correct me if I'm wrong.</p>
<p>If I'm n... | 3,745 |
<p>Say you have a sphere, and you have several torque vectors acting on it, all at different points. Say you have the vector (6i + 3j + 5k) originating from point A, and the vector (3i + 1j + 9k) originating at point B, and (7i + 2j + 9k) acting on point C.</p>
<p>Summing the vectors gives you (16i + 6j + 23k) which i... | 3,746 |
<p>Where I can find photos of nuclear explosions just after detonation (before 5-10 ms, the shorter the better)?</p> | 3,747 |
<p>I am having trouble using the <a href="http://en.wikipedia.org/wiki/Right-hand_rule" rel="nofollow">right hand rule</a> properly and often find myself putting my hand in awkward orientations. I know you point your hand in the direction of $r$ and then point your fingers in the direction of $F$ but that doesn’t reall... | 3,748 |
<p>Bohr-Sommerfeld quantization provides an approximate recipe for recovering the spectrum of a quantum integrable system. Is there a mathematically rigorous explanation why this recipe works? In particular, I suppose it gives an exact description of the large quantum number asymptotics, which should be a theorem.</p>... | 3,749 |
<p>In a type Ia Supernova, the carbon accumulated in the earlier stages of a stars death fissues to create even heavier elements. Could this be used by humans aswell? Is it theoretically possible to build a Carbon Bomb?</p>
<p><a href="http://physics.stackexchange.com/questions/30755/pressures-necessary-for-carbon-det... | 3,750 |
<p>This may be a very basic question but I am not seeing how it works.
Consider the standard example of an ice skate rotating about his/her center of mass and pulling in his/her arms. The torque is zero so we have conservation of angular momentum. This implies that $\omega$ increases to keep $I\omega$ constant, but the... | 594 |
<p>Suppose we are given a mechanical frame consisting of two points. How can we prove that assuming any initial conditions there is an inertial frame of reference in which these points will be in a static plane?</p> | 3,751 |
<p>I have heard numerous times when getting x-rays, MRIs, CAT Scans, etc. that each one is equivalent to a cross country airplane trip. Disregarding the different types of radiation as asked in <a href="http://physics.stackexchange.com/questions/20665/can-x-ray-radiation-be-compared-to-background-radiation">this quest... | 3,752 |
<p>The quantum mechanics of Coloumb-force bound states of atomic nuclei and electrons lead to the extremely rich theory of molecules. In particular, I think the richness of the theory is related to the large mass ratio between the nucleon and the electron. This mass ratio leads to the Born-Oppenheimer approximation whi... | 3,753 |
<p>I realised, reading another Phys.SE question about <a href="http://physics.stackexchange.com/q/86774/">balloons moving forwards in an accelerating car</a> that I don't really understand how <a href="http://en.wikipedia.org/wiki/Buoyancy" rel="nofollow">buoyancy</a> works. Particularly concerning, for a SCUBA diver.<... | 3,754 |
<p>Starting from a homework problem:</p>
<blockquote>
<p>An aluminum cup of $100 cm^3$ capacity is completely filled with glycerin at $22°C$. How much glycerin, if any, will spill out of the cup if the temperature of both the cup and the glycerin is increased to $28°C$? (The coefficient of volume expansion... | 3,755 |
<p>I had a question on one of the details of the derivation of the second law of thermodynamics starting from the phase space volume. I'll type out what I understand so far:</p>
<p>Letting the Hamiltonian depend on some external parameter $a$ so that $H=H(a)$. The phase space volume can be written as </p>
<p>$$\bar{\... | 3,756 |
<p>Consider a particle in a box system.Assume its state to be a superposition of the ground and the first excited energy states.Consider two observers A and B (rest of the world).A made the measurement of the energy of the system and got energy corresponding to one of the states.
Consider two scenarios from now.</p>
1.... | 3,757 |
<p>As you know the active noise cancellation technology used in many application such as protection of aircraft cabins and car interiors to reduce engine noise also some headphones use this feature to reduce unwanted ambient sounds.</p>
<p>Actually I don't know the details of how this feature works, but I do know that... | 107 |
<p>Hi Guys generally when you evaluate the 3 open string tachyon tree level amplitude in CFT, you do a conformal transformation mapping the worldsheet to the upper half of the complex plane and the incoming and outgoing strings become points on the real axis. However in evaluating the string field theory tree level amp... | 3,758 |
<p>Suppose I was designing an apparatus which needed to lift 250kg 5cm high, hold it there for a few seconds, and then lower the object back to the original height. Such a process would need to be repeated every certain time interval.</p>
<p>What kind of mechanism (pneumatic, hydraulic, electric, etc) is likely to hav... | 3,759 |
<p>Does Coulomb's law apply to Plasma?</p> | 3,760 |
<p>I have been exploring for some time both the Special and General Relativity, hoping to glean at least a conceptual grasp of their basic tenets.
In reading the book "Gravitation" by Misner, Thorne and Wheeler, the authors stress that Riemann came very close to make a decisive connection between gravitation and curva... | 3,761 |
<p>I am trying to do a homework problem where we re-write the mass, momentum and energy conservation formulas for downward flow in a vertical pipe and it says "where all hydrodynamic entrance effects have disappeared". What exactly does this mean? Also I am using an incompressible substance if that helps.</p>
<p>Someo... | 3,762 |
<p>My question essentially revolves around multi-electron atoms and spectroscopic terms. I understand the idea that the total wavefunction for Fermions should be antisymmetric. Consider as an example, the $2p^2$ electrons in a partially filled p shell; that is, the outer shell of Carbon. The two electrons both have ... | 3,763 |
<p>In order to find the magnetic field generate by an infinitely long straight wire of radius R, I have to use local Ampère's Law :</p>
<p>$\boxed{ \oint_L B\cdot dl = \mu_0 \sum_j I_j}$</p>
<p>If $r > R$ then $ \oint_L B\cdot dl = B(r)\cdot 2\pi r = \mu_0 I \Rightarrow B(r) = \dfrac{\mu_0 I}{2\pi r} $</p>
<p>(I... | 3,764 |
<p>The <a href="http://i.stack.imgur.com/Qzc6I.png">following question</a> was on a quiz in physics class:</p>
<blockquote>
<p>If the net work done on a particle is zero, which of the following statements must be true?</p>
<p>a) The velocity is zero<br>
b) The velocity is decreased<br>
c) The velocity is un... | 3,765 |
<p>I was wondering how <a href="http://www.wickedlasers.com/phosforce" rel="nofollow">this lens</a> works. It converts blue laser light into white light and effectively turns a portable laser into a flashlight. The info mentions phosphor coating. I used my Google Fu and found this: <a href="http://www.photonics.com/Art... | 3,766 |
<p><img src="http://i.stack.imgur.com/KaLim.jpg" alt="enter image description here"></p>
<p>Ans:
Applying Gauss’s law the net flux can be calculated. And for option (B), I guess the flux will be 0. But not sure. Can anyone explain all the 3 options?</p>
<p>For left and rignt face, EA = 300*(0.05)^2 = 0.75 Nm^2/c , bu... | 3,767 |
<p>As we all know that anticommutator of one set of supercharges in massive extended supersymmetry is something like $$\{b_\alpha, b_\beta^\dagger \} = \delta_{\alpha \beta} (M-\sqrt{2} Z).$$
My question is that everyone says that it is obvious that <a href="http://en.wikipedia.org/wiki/BPS_state" rel="nofollow">BPS st... | 3,768 |
<p>Currently in my last year of high school, and I have always been told that <a href="http://en.wikipedia.org/wiki/Centrifugal_force">centrifugal force</a> does not exist by my physics teachers. Today my girlfriend in the year below asked me what centrifugal force was, I told her it didn't exist, and then she told me ... | 3,769 |
<p>I am curious due to the course of the idle research that I am doing (my hobby), and I am curious for various reasons as to the answer of this question. Please forgive me in advance for possible convoluted language, as I occasionally have trouble explaining my questions. How could one go about calculating a <a href="... | 3,770 |
<p>On page 1 of this recent <a href="http://arxiv.org/abs/1105.3796" rel="nofollow">paper</a> by Bousso and Susskind we read.</p>
<blockquote>
<p>This question is not about philosophy.
Without a precise form of decoherence,
one cannot claim that anything really
"happened", including the specific
outcomes of ... | 3,771 |
<p>Is there software available that can analyse a 5MHz RF pulse to give a plot of frequency spectrum.
The signal data is visible on a LCD screen or a print out could be obtained.</p> | 3,772 |
<p>Spin echo experiments have been able to reverse the motions of all the molecules in a gas in statistical mechanics in the manner of Loschmidt. The Fermi-Ulam-Pasta model has solutions with a single mode dispersing, only to recohere after quite some time has elapsed. Can the same thing happen for decoherence? What ar... | 3,773 |
<p><strong>Question</strong>: are they mathematically possible at all? physically?</p>
<p>with finite mass systems, usually the binding energy contributes to the rest-mass of the system. It would seem that even if you could bind two massless fields/particles, the coupled system would still have a finite rest mass beca... | 3,774 |
<p>Using the renormalization group approach, coupling constants are "running". If we apply this to the fine structure (coupling) constant, we do know that, e.g., at energies around the Z mass, $$\alpha \approx 1/128$$ instead of 1/137. We know that $$\alpha =e^2/ \hbar c$$. Therefore, if alpha is running with energy, a... | 3,775 |
<p>I know that $\frac {dv}{dt}=a$ is acceleration,
but:</p>
<ol>
<li><p>what is convective acceleration of a flow velocity?</p></li>
<li><p>what is difference between $(v\cdot \nabla) v$ and $v\cdot (\nabla v)$, ?</p></li>
</ol> | 3,776 |
<p>I'm quite sure that similar questions like this have been asked for more than thousands of times on here but <strong><em>since each person's background and interests are unique</em></strong> I believe questions like this should not be considered duplicates.</p>
<p>I'm a first year pure math undergradudate student. ... | 108 |
<p>I have a question... Do you round with significant digits during each subcalculation of a problem or only when the entire problem is complete?</p>
<p>Example:</p>
<p>multiply the following number:</p>
<p>$$1.8 \times 2.01 \times 1.542$$</p>
<p>saving rounding until the end:</p>
<p>$$(1.8 \times 2.10) \times (1.... | 3,777 |
<p>In Classical Mechanics, both Goldstein and Taylor (authors of different books with the same title) talk about the centrifugal force term when solving the Euler-Lagrange equation for the two body problem, and I'm a little confused about what it exactly means - is it a real centrifugal force or a mathematical conseque... | 3,778 |
<p>I want to know if my solution to a textbook problem has any major problems with it. Here is the problem:</p>
<blockquote>
<p>Ethanol has a given density of 0.789 g/mL at 20 degrees Celsius and isopropanol has a given density of 0.785 g/mL at 20 degrees Celsius. A chemist analyzes a substance using a pipet that is... | 3,779 |
<p>A particle moves with force</p>
<p>$$F(x) = -kx +\frac{kx^3}{A^2}$$</p>
<p>Where k and A are positive constants.</p>
<p>if $KE_o$ at x = 0 is $T_0$ what is the total energy of the system?</p>
<p>$$ \Delta\ KE(x) + \Delta\ U(x) = 0$$</p>
<p>$$F(x) = -\frac{dU}{dx} = m\frac{dv}{dt} = m v\frac{dv}{dx}$$</p>
<p>I... | 3,780 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/11398/double-light-speed">Double light speed</a><br>
<a href="http://physics.stackexchange.com/questions/30505/someting-almost-faster-than-light-traveling-on-something-else-almost-faster-than">Someting alm... | 109 |
<p>$$\langle \hat A \rangle \langle \hat B \rangle=\langle \hat A\hat B \rangle,$$</p>
<p>$$\langle \hat A \rangle + \langle \hat B \rangle=\langle \hat A + \hat B \rangle,$$</p>
<p>$$\langle \hat A^2 \rangle \langle \hat B^2 \rangle=\langle \hat A^2 \hat B^2 \rangle,$$</p>
<p>$$\langle \hat A^2 \rangle + \langle \... | 3,781 |
<p>Even if many interesting similarities between the classical and the quantum mechanical framework have been worked out, e.g. in the subject of deformation quantization, in general, there are some mathematical problems. And in the conventional formulation, you don't want to make things like $\hbar\rightarrow 0$ for th... | 3,782 |
<p>Is black hole entropy, computed by means of quantum field theory on curved spacetime, the entropy of matter degrees of freedom i.e. non-gravitational dofs? What is one actually counting?</p> | 3,783 |
<p>What is the physical meaning/significance of the <a href="http://en.wikipedia.org/wiki/Classical_electron_radius" rel="nofollow">classical radius of the electron</a> if we know from experiments that the electron is point like?</p>
<p>Is there similarly a classical radius of the photon? The W and Z bosons?</p> | 3,784 |
<p>I'm working through <a href="http://natureofcode.com/" rel="nofollow">The Nature of Code</a>, which is an awesome book, lots of fun.</p>
<p>I've come across Exercise 3.13 and I'm not sure how to solve it. I'm assuming that the force of gravity and the force of friction are provided. I'm not sure in which directions... | 3,785 |
<p>I wanted to make sure I understand induction well enough.</p>
<p>Assume we have two wires running parallel to each other. Wire A has a signal of $f(t)$, wire B has a signal of $\hat{f}(t)$.</p>
<p>Let's connect a signal generator to wire A, therefore putting
$$f(t) = A \cdot sin(2\pi f_{c} t)$$
where $A$ is the a... | 3,786 |
<p>I am reading on magnetic monopoles from a variety of sources, eg. <a href="http://arxiv.org/abs/hep-th/9603086/" rel="nofollow">the Jeff Harvey lectures.</a>. It talks about something called the winding $N$, which is used to calculate the magnetic flux. I searched the internet but am not being able to understand the... | 3,787 |
<p>Is there any scientific evidence that demonstrates why time passes?
Or is it just an opened question?</p> | 3,788 |
<p>From what I gather the interstellar medium has about about 1 atom per cubic centimeter. But on the other hand, as they say, "Space is big, really really big" So if it is known (or at least theorized about) what percent of the universes matter is in the following: black holes, stars, and interstellar medium?</p> | 3,789 |
<p>I'm doing a paper for school about space navigation. It's an interesting subject, but upon searching google, all I could find were non-detailed explanations of astrophysics that I think are meant for children, and a few somewhat-more-detailed articles about other subjects.</p>
<p>I remember learning about the subje... | 3,790 |
<p>Since we see the <a href="http://en.wikipedia.org/wiki/New_moon" rel="nofollow">new moon</a> at least once in a month when the Moon gets in between of the Sun and the Moon at the night and as far as I know if this happens during the day, you'll get to see a <a href="http://en.wikipedia.org/wiki/Solar_eclipse" rel="n... | 110 |
<p>Suppose a string element oscillating transversely in <em>SHM</em> having both kinetic energy and elastic potential energy travels with velocity $u$. Let $y(m)$ be the amplitude. When it reaches $y(0)$, it has max. $K.E.$ and elastic $P.E.$ together at the same time. But in <em>SHM</em>, when the kinetic energy is ma... | 3,791 |
<p>An often quoted figure is that the LHC magnets take a month to completely cool and a month to warm. There is never an explanation as to why that is. I can conjure any number of reasons (slow changes to prevent stress, very low temperature deltas, gremlins can hold their breath for 29 days, etc) but I can't find any ... | 3,792 |
<p>Diamond is one of the best thermal conductors you can get. If the diamond is crushed into dust and spread out over a flat surface, but still held fairly compact (for instance in a small petri dish), would it still conduct as well as before?</p> | 3,793 |
<p>Let's say we have a meter stick with a single rope attached to it. One end of the rope is attached to one end of the meter stick, the other end of the rope is attached to the opposite end of the meter stick. By hanging the rope from a beam in the center of the rope, the meter stick (which is attached to the rope) ba... | 3,794 |
<p>When I boil water in the kettle on my electric stove, sometimes it rocks back and forth making an annoying sound at a frequency of about 6Hz. When that happens, I move the kettle slightly to make it stop rocking. It occurred to me that although my motivation is to stop the annoying sound, it might also make the wat... | 3,795 |
<p>Say I have a pot of water that boils in 20 minutes, at whatever temperature.</p>
<p>If I leave the fire on, take the pot off, pour the hot water into a container, refill the pot with tap water and put it back on the fire.</p>
<p>How long will the 2nd pot take to boil and why?</p> | 3,796 |
<p>Single photons: Is there a 90° offset of the electric to the magnetic component in the direction of propagation?</p>
<p><img src="http://i.stack.imgur.com/ksHYn.png" alt="Electromagnetic wave"></p> | 3,797 |
<p>I've taken a class on elementary number theory (for fun), but now I wonder: was it at all useful to learn number theory for my future career in physics?</p>
<p>More to the point, are there any applications of elementary number theory (the kind that would be taught in a first or second undergrad-level course) in the... | 111 |
<p>The universe is expanding at an ever increasing rate. Virtual particles are being created and then destroyed everywhere at every moment. As space expands even faster, will a time come when these virtual particles will be ripped apart before they can destroy each other, much like what happens at gravitational event h... | 3,798 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/1801/why-space-expansion-affects-matter">Why space expansion affects matter?</a> </p>
</blockquote>
<p>Imagine two tiny spacecrafts that are moving with the Hubble flow and so are moving away from each o... | 56 |
<p>In the paper, <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.52.997" rel="nofollow">"Density-functional theory for time-dependent systems" Physical Review Letters 52 (12): 997</a> the authors mentioned that the action
$$ A= \int_{t_0}^{t_1} dt \langle \Phi(t) | i \hbar\partial / \partial t - \hat... | 3,799 |
<p>A positron is odd under charge conjugation and parity reversal but nevertheless even with respect to time reversal. Is a theoretical positron which would be odd under all three symmetries (C, P, T) physical?</p> | 3,800 |
<p>I thought light like singularity is where the geodesics end on a lightlike hypersurface and can't be extended anymore. I guess its different than light cone singularity. Lot's of places have mention of it, but I wanted to know its definition for sure.</p> | 3,801 |
<p>Suppose a simple circuit with a <strong>DC voltage</strong> source and a <strong>resistor</strong>. The voltage of the source will be situated over the resistor.
So the <strong>electric field</strong> (which is the gradient of the potential) will be constant in the resistor (if you assume a linear potential functio... | 3,802 |
<p>In particle physics, when you read $J^P$, does it mean Spin parity or total angular momentum parity?</p>
<p>I know that the letter $J$ is used for TOTAL angular momentum but I think I read somewhere that sometimes it is meant spin when used in this $J^P$ notation.</p> | 3,803 |
<p>Sorry if this is rather simple, but I've only just started learning about using logarithms in experimental physics. </p>
<p>I did an experiment to test the amount of time it would take for an amount of water to leave a burette. I used the starting volume of water in the burette as a control variable, $50cm^3$. I re... | 3,804 |
<p>I've recently heard about this new material called Quantum Stealth which is suppost to be a replicate of Harry Potter's 'Invisibilty cloak' . Apparently the material, bends light waves around a target which allows complete invisibility. Is all this true?</p> | 3,805 |
<p>I'm dealing with a problem here. Today my professor asked us a question :</p>
<p>How should a metallic wire move in Earth's magnetic field such that she gets maximum potential difference in it's ends:</p>
<p>a) Horizontally
b) Vertically
c) Normal in the magnetic field lines </p>
<p>Can anyone help me with this q... | 3,806 |
<p>Fermi surfaces are surfaces of constant energy in reciprocal space. They provide information about the properties of a material in solid state physics.</p>
<p>Constant mean curvature surfaces are a superset of minimal surfaces, which minimize area and have zero mean curvature. Soap bubbles are surfaces of constant ... | 3,807 |
<p>In $\phi^3$ theory, the generating functional for interacting field theory is given by:
$$ Z_1(J) = \sum_{V=0}^{\infty} \frac{1}{V!} \Big[ \frac{iZ_g g}{6} \int \Big( \frac{1}{i}\frac{\delta}{\delta J}\Big)^3 d^4 x \Big]^V \times
\sum_{P=0}^{\infty} \frac{1}{P!} \Big[ \frac{i}{2} \int J(y) \Delta(y-z) J(z) \, d^4 y... | 3,808 |
<p><a href="http://en.wikipedia.org/wiki/Energy" rel="nofollow">Energy</a> seems to me to be a very abstract thing, and while it clearly works out very nicely, I don't understand how anyone would have thought to come up with it. Where does the concept of energy find it's roots, and how was it settled down on as a 'usef... | 3,809 |
<p>I've noticed something curious about the rotation of a rectangular prism. If I take a box with height $\neq$ width $\neq$ depth and flip it into the air around different axes of rotation, some motions seem more stable than others. The 3 axes which best illustrate what I mean are:</p>
<p>(1) Through the centre of ma... | 3,810 |
<p>I've read sci-fi stories in which a spaceship crew, moving at some significant fraction of 'c', observes stars ahead as bluer, stars behind as redder, stars to port and starboard as...well, you get the idea.</p>
<p>This seems wrong, though I'm not sure if it is. Seems to me the red/blue shifts would only be apparen... | 3,811 |
<p>I may be wrong, but it seems that only logarithmic divergences need to be retained when using the Callan-Symanzik equation, finding running couplings, etc. Why is this the case? Is there some simple intuitive understanding for why the logarithmic divergences are most important for these applications?</p> | 3,812 |
<p>I was thinking about the Google XPrize for Space Travel the other day.</p>
<p>In order to claim the prize of building a robot that goes to the moon, travels 500m, and relays data, I had the idea of building a tiny vessel the size of a marble and shooting it at the moon with a railgun.</p>
<p>The theory is that sin... | 3,813 |
<ul>
<li><p>In this paper, <a href="http://arxiv.org/abs/1301.4504" rel="nofollow">http://arxiv.org/abs/1301.4504</a> in equation 4.1 in what sense are the two states a "9-qubit state"? I did not understand this counting. </p></li>
<li><p>Can someone explain what are the different $X_i$ and $Z_i$ in 4.2? How is say $X_... | 3,814 |
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