# All Questions

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### Are there any experimental tests of non-locality / Bell inequalities that do not rely on spin?

All the experiments I know, which have been performed to test Bell inequalities, are somehow based on measuring the spin degree-of-freedom of some particle (usually photons, sometimes electrons). I ...
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### meteorite-like impact of a drop of yogurt

When I throw a drop of Yogurt (through spoon) into a dish of Yogurt, it caused something like a meteor hitting the earth and something occurs similar to the place a meteor hitting the earth(Barringer ...
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### Accelerated wedge [on hold]

Consider the following figure, where a mass $m$ is placed on a wedge which is accelerated to the right ($\vec a$) which could be negative. The coefficient of friction between the mass and the wedge ...
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### Green function for simple harmonic oscillator

I'm interested in examples on how to use Green function (GF)for simple harmonic oscillator (SHO)? I am from undergrad physics, so I need a fundamental math and quantum mechanical application of GF ...
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### A problem regarding equivalent resistance between two points [on hold]

I came across this problem in a book which has solutions for all questions except this one. I can't get how to do this partly due my doubt whether the R and 4R resistors will get short-circuited or ...
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### Could a trial wavefunction providing exact eigenenergy differ from the exact eigenfunction by a zero measure function?

Given the eigenequation of a Hamiltonian $$H |n \rangle = E_n |n \rangle \tag{1}$$ We write it in the position representation $$\langle x | H | n \rangle = E_n \langle x | n \rangle \tag{2}$$ ...
A metallic wire is being rotated in a vertical circle with a mass $m$ fastened to its end. My understanding says that the net force experienced by this mass should be the centripetal force: $T - mg ... 1answer 27 views ### How to get the direction of bending light? When light passes from a more dense to a less dense substance, (for example passing from water into air), the light is refracted (or bent) away from the normal. My question, is there any ... 1answer 55 views ### Why does the magnetic force exerted by two charged particles on each other do no work? An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system during motion, one ignores the magnetic force of one on another. ... 1answer 26 views ### Working of fusion reactor The present type fusion reactors use the magnetic confinement technique for the production of energy. Moreover only 48% for the energy is only absorbed and converted into useful energy . Why we are ... 1answer 19 views ### Does no variables change in an adiabatic + isothermal system? Adiabatic + isothermal = no heat and temperature change. Does that mean the system will remain as it is? If it is adiabatic + isovolumetric or adiabatic + isobaric , is it as well no change in any ... 0answers 27 views ### Do all atoms get accelerated in a synchrotron? [on hold] How does an electron accelerator work? I have been going on the accelerators and I have lots of doubts on how to minimise the cost and size of the accelerators. 1answer 28 views ### What is the difference between reversible and irreversible adiabatic expansion? What is the difference between reversible and irreversible adiabatic expansion? Is it true that the work done by the gas is the same but the pressure applied externally differ between two process? ... 1answer 22 views ### question about ICRF/J2000 equinox orientation The DE406 ephemeris data and the NASA Horizons website all report the Earth's current coordinates on 2014 Sep 27, a few days after the Fall equinox, as approximately [1,0,0] AU. However every ... 1answer 18 views ### Direction of friction force between two boxes There are two boxes on top of each other. The bottom box is dragged right by a constant force. What is the direction of friction of the top box against the bottom box? 2answers 137 views ### Confusion between magnetic field and magnetic flux I've been learning about electromagnetism and Maxwell's equations (in integral form), and I'm slightly confused. The Ampere-Maxwell law (as I know it): $$... 1answer 165 views ### Why is wet paper transparent? [duplicate] When paper get wet, it becomes transparent. How does this happen? Is it because of refraction, or bcause water extends the space between molecules that consist of the paper? I have no clue. 1answer 79 views ### What is the meaning of a state in QFT? I guess this may be more of a mathematical than a physics question, but it comes down to physical interpretations, so I'm posting it here. In classical Quantum Mechanics, we can define a state ... 2answers 61 views ### Where does the \partial \vec{E}/\partial t term from Maxwell's equation go in Ampere's Law? One of Maxwell's Equations (ME) is:$$\nabla\times\vec B = \mu_0\vec J+\epsilon_0\mu_0 \frac{\partial \vec E}{\partial t}.$$While Ampere's Law (AL) is:$$\nabla\times\vec B = \mu_0\vec J.$$... 1answer 26 views ### Diffraction and Shape in the grating Suppose, I use a laser with a known wavelength of 650 nm through a "opaque grating" that has a pattern of holes of same shape and size. We can see the pattern on the screen. We know the distance ... 0answers 36 views ### Show that any rotation can be represented by two successive reflections [on hold] Show that any rotation can be represented by two successive reflections in two planes, both of which passing through the axis of rotation with the planner angle \phi/2 between them 0answers 31 views ### Will there be an induced emf in this situation? Here is a (horrible) illustration: The first ring has a voltage V_1 applied to it through the voltage source on the left. Ring 1 has resistance R_1 whereas ring 2 has resistance R_2. Will ... 2answers 92 views ### When does a singularity start to exist during a black hole formation? Excuse me if the question is naïve. I am not a physicist. Let's imagine formation of a black hole from a homogeneous collapsing star. At certain moment when enough of matter is inside of a small ... 0answers 18 views ### Find the angle and force constituent [on hold] See picture for the problem description. I know of one way to solve this problem: Use the law of cosines to obtain:$$ 500^2=F^2+750^2-2 \cdot 750 \cdot F \cdot \cos(30°) $$Rearranging and ... 1answer 23 views ### Is it only possible when 600J of heat is exhausted and 400J of heat is converted into work? [on hold] Is it only possible when 600J of heat is exhausted and 400J of heat is converted into work? Is the 600J heat will enter the cold reservoir and available to do work again? q1/T hot + q3 / T cold = 0 ... 1answer 53 views ### What's the definition of incompleteness of a coordinate system and a spacetime? I always see in GR textbooks that some coordinates or some spacetime is incomplete, such as Rindler spacetime and spacetially flat FRW universe with only positive cosmological constant. This ... 0answers 34 views ### Does Hawking radiation need an apparent horizon and when does it switch on during stellar collapse? I've read that Hawking radiation is implicitly linked with the existence of an apparent horizon (1). This seems a slightly less onerous than linking Hawking radiation with a genuine bona fide event ... 1answer 59 views ### Do Photons Accelerate Upon Creation? [duplicate] Of course the speed of light is constant relative to everything. Its frequency can change, but its speed is constant. Photons may be absorbed by atoms or release by atoms as energy, but as soon as it ... 0answers 23 views ### What happens to a particle in the exact center of a Kerr black hole? Kerr black holes (and Kerr-Newman black holes), instead of the "point" singularity theorized in spherically symmetric black holes, instead have a "ring" singularity, spread along the equatorial plane ... 0answers 36 views ### Potential due to a Uniform Ring [on hold] Consider a uniform ring of mass M and radius a (?). I would like to prove that for r > a:$$\Phi(r) = \frac{GM}{r} \sum_0^\infty[P_n(0)]^2 \left(\frac{a}{r}\right)^n $$Where \Phi(r) is ... 2answers 41 views ### Why won't the efficiency of a heat pump increase when T_\mathrm{cold} decreases? Why won't the efficiency of a heat pump increase when T_\mathrm{cold} decreases? In the carnot cycle, efficiency increases when T_\mathrm{cold} gets colder and T_\mathrm{hot} gets hotter. Are ... 1answer 33 views ### Nails and Screws without Friction In my physics class we're just starting the chapter on friction, and our book began the chapter by describing what a world without friction would be like. Among other examples, it said "nails and ... 2answers 257 views ### If conservation of energy was wrong, how would we know about it? Suppose you just started learning physics and you've been introduced to conservation of energy and kinetic energy. Apart from those concepts you know next to nothing. Then you observe an inelastic ... 2answers 102 views ### What is the expectation value of the 3D delta function for the Hydrogen atom ground state? I'm trying to evaluate the expectation value of some perturbation Hamiltonian$$H=\alpha \delta^3(\vec{r}),$$where \alpha is a positive constant, for the ground state wavefunction of the hydrogen ... 0answers 28 views ### Problem in Solving an Equation in Quantum Mechanics [duplicate] I am trying to reproduce this paper : http://www.ias.ac.in/pramana/v73/p573/fulltext.pdf But, somehow I am stuck at equation (7). The equation that I am trying to solve for particle outside the well ... 0answers 18 views ### Comparison between isobaric, isothermal and adiabatic expansion Please check if my understandings of these terms are right because they are a bit confusing to me. For all three types of expansions, a change in volume means work is done by the system to the ... 0answers 18 views ### Relative humidity of an air parcel with decreasing temperature I'm a bit confused while doing relative humidity calculations. To make the case simple: let's say I have my room temperature at 25 degrees with a 50% relative humidity (RH). If I take an air parcel ... 0answers 86 views ### Forced Quantum Harmonic Oscillator I'm just starting my journey to QFT and Particles physics and I have a question about the problem of QHO witch we hit with a force F(t) for t< t' , for which the force is zero for t>t'. ... 0answers 19 views ### Property of mass moment about center of mass [on hold] Can any one give the analytical (mathematical) proof of the following statement: "The summation of mass moment of all the component of the system about its centre of mass is always equal to zero." 0answers 40 views ### Conservation Laws and relativity [on hold] A 50 kg meteorite moving at 1000 m/s strikes Earth. Assume the velocity is along the line joining Earth's center of mass and the meteor's center of mass. What is the gain in Earth's kinetic energy? 2answers 45 views ### What “exactly” happens when a ray is incident at the critical angle? The question arose when my physics teacher taught me geometrical optics and told that the phenomenon of both refraction and reflection occur by change in medium. First of all, I'm not sure about the ... 0answers 49 views ### Fermions in Schwarzschild spacetime To my understanding Geroch proved that on 4-dimensional non-compact manifold a necessary and sufficient condition for a manifold to have a notion of spinors is to be parallelizabe .1 (General ... 0answers 6 views ### Are there any potential pitfalls of deferring admission to a PhD program? [migrated] I'm currently in my final year of undergraduate study, and will be applying to physics PhD programs (for condensed matter theory) soon. Without going into details, I am convinced that deferring my ... 1answer 53 views ### Deriving photon propagator In Peskin & Schroeder's book on page 297 in deriving the photon propagator the authors say that$$\left(-k^2g_{\mu\nu}+(1-\frac{1}{\xi})k_\mu k_\nu\right)D^{\nu\rho}_F(k)=i\delta^\rho_\mu ... 1answer 53 views ### Help with equations for deflecting an asteroid This is a homework assignment, but not the physics part. I have to write a python script to simulate the size of a warhead required to deflect an asteroid (parameters inputted by user) that is on a ... 0answers 14 views ### How big can be gradient of magnetic induction (in lab) I'm thinking about magnetic levitation and how big force can push a diamagnetic when we put it in inhomogenious magnetic field. As far as I know, force exerted on a diamagnetic is proportionald to ... 1answer 35 views ### Symmetry transformation on Quantum Field I stumbled upon this point several times, the latest beeing this question: Connection between conserved charge and the generator of a symmetry I want to understand, why Quantum fields transform under ... 2answers 32 views ### For change in entropy dS = dq/T, is T the temperature of system or surrounding or both? For change in entropy dS = dqrev/T, is T the temperature of system or surrounding or both? I am confused about Thot, Tcold, Tsys and Tsurr. If qrev, are we talking about the reversible cycle such as ... 1answer 26 views ### Is there a simple explanation for the fact that the Lorentz transformation must be linear? [duplicate] In my physics class, for deriving the Lorentz transformatuons my professor assumed that there is a linearity relatioin between the coordinates of the two systems. But why is that?? Can anyone please ... 0answers 21 views ### Commutator of$SU(2)\$ angular momentum operators [on hold]
Prove the following statement. $$[J_i,J_j] = i\hbar\epsilon_{ijk}J_{k}$$ using rotation operators.