# All Questions

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### Why is the majorana particle a fermion?

My knowledge of quantum mechanics is rather limited, but what I always understood was that Bosons have integer spins and Fermions have half-integer spins. My question is very simple: the Majorana ...
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### three-particle quantum entanglement

So I know that two particles can be entangled in a quantum way, but is it possible that more than two particles be entangled in a quantum way? Most descriptions provide with two-particles cases, so I ...
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### Sound Wave Propagation: Why HF are more specular while LF are more omni

The propagation of high frequencies sound waves is more directional (specular), and they don't diffract as much as low frequencies. Low-frequencies diffract and thus propagate in a more omni-spherical ...
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I am reading "Supercollision cooling in undoped graphene." There the authors write: Above $T_{BG}$ (the Bloch-Gruneisen temperature), only a fraction of acoustic phonons with wave vector $q\le ... 2answers 256 views ### How does locality decouple the UV and IR behaviour of a QFT? I came a comment in this paper: Scattering Amplitudes and the positive grassmannian in the last paragraph of page 104 which says: "One of the most fundamental consequences of space-time locality is ... 1answer 181 views ### Influence of air resistance in space Consider the following situation: You are locked inside a cylindric container allowing you to move around freely without being in contact with any of the items or surfaces aboard. The container is ... 2answers 91 views ### What symmetries does a lattice calculation need to preserve? I've heard that it is impossible to have a properly Lorentz-invariant lattice QFT simulation, as the Lorentz invariance is spoiled by the nonzero lattice distance$a$. I've also heard that there are ... 0answers 180 views ### Does the Standard Model have a Landau pole? I have seen the statement that the Standard Model has a Landau pole, or at least it its believed that it does at$\sim 10^{34}$GeV. Has this actually been proven (at least in perturbation theory, as ... 1answer 755 views ### Finding the Electric Field (and other information, besides) The problem I am working on is: Two parallel plates having charges of equal magnitude but opposite sign are separated by 29.0 cm. Each plate has a surface charge density of 33.0 nC/m2. A proton is ... 1answer 845 views ### Historic derivation of Wien's law Every book I've read, including a lot of websites, Wikipedia, etc, say that Wien derived this: $$\rho_\nu(T)=\rho(\nu,T)=\nu^3f\left(\frac{\nu}{T}\right)$$ Being$\rho_v(T)$the spectral enegy ... 1answer 112 views ### Partial derivative potential energy of 'free' vibration I have this rather mathematical question about the calculation of the partial derivative of a potential energy function given by: $$U(x_i)=\frac{1}{2}\sum_{i,j}\frac{\partial^2U(0)}{\partial ... 3answers 1k views ### Why isn't average speed equal to displacement over time? I'm in an introductory Physics course and I need help! During a one-hour trip, a small boat travels 80.0km north and then travels 60.0km east. What is the boat's average speed during the one-hour ... 1answer 58 views ### What are the units of the Bekenstein bound? Working with the Wikipedia definition of the Bekenstein bound: S \leq \frac{2 \pi R k_bE}{\hbar c} 2\pi R \ is m^2 k_b is \frac{J}{K} E is J \hbar is J*s c is \frac{m}{s} ... 4answers 1k views ### Collision of two photons Could someone explain me how will be look like collision of two photons? Will they behave like: Electromagnetic waves, they will interpher with each other and keep they wave nature Particles and ... 4answers 211 views ### Another question about energy(Improved) I read this thread about energy What Is Energy? Where did it come from? EDIT: after some sleep I tried to read the answers again and found better questions to ask. so here you go As most guys have ... 1answer 420 views ### In what limit does string theory reproduce general relativity? [duplicate] In quantum mechanical systems which have classical counterparts, we can typically recover classical mechanics by letting \hbar \rightarrow 0. Is recovering Einstein's field equations (conceptually) ... 2answers 1k views ### Root Mean Square Speed of Gas The RMS speed of particles in a gas is v_{rms} = \sqrt{\frac{3RT}{M}} where M = molar mass; according to this Wiki entry: http://en.wikipedia.org/wiki/Root-mean-square_speed The gas laws ... 0answers 122 views ### How to make DIY flight detector for double slit experiment? I want to reproduce double slit experiment. So, is it possible to build flight detector (situated near one slit) at home? Is it possible to buy it somewhere? 3answers 293 views ### Does Unruh Radiation replace the cosmic horizon radiation? A recent paper, titled Inertia from an Asymmetric Casimir Effect, discusses the universal horizons relative to an accelerating observer (Rindler space). A figure it used to demonstrate its point ... 1answer 278 views ### What dark matter can AMS currently find (or exclude)? The rumor mill is running again, this time it's about the AMS experiment (Alpha Magnetic Spectrometer) that's going to make a major announcement soon. I suppose they are looking for peaks in gamma ... 1answer 80 views ### Oscillations of a fluid particle in a stably stratified fluid Could anyone help me work through the following set of equations: I found these online at http://marian.fsik.cvut.cz/~bodnar/PragueSum_2012/Staquet_1-2.pdf and I'm trying to work through them. I ... 1answer 1k views ### Calculation of number density from material density Material density is given by \rho =m/V, where m is mass and V is volume. Again number density given by n=N/V, where N is the total number of particle. How can I calculate number density n ... 1answer 405 views ### How to calculate pressure field in potential flow I haven't ever studied fluid dynamics before and may mix something here, so please, be patient :). Given flow potential of the form (homogeneus flow over a dipole):$$ \phi = u_\infty x ... 2answers 260 views ### Electric field lines If electric field lines cannot terminate in empty space, in the simple example of two equal charges what happens to the line starting from one of the charges toward the other in the middle? 1answer 278 views ### Semi-conductor band-gap and deformation potential Submitting a semi-conductor to stress leads to a deformation in the energy-bands, roughly described by:$$H_{ij} = {\cal{D}}_{ij}^{\alpha\beta}\;\epsilon_{\alpha\beta}$$$\epsilon$being the strain ... 0answers 59 views ### How to calculate the correct coordinates from a distorted video of a projectile? I am working on a high school project that is related to projectile motion. I am exploring how exactly the position of the center of mass affects the trajectory of a long but thin, javelin-like ... 1answer 181 views ### Equation of state of cosmic strings and branes I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ... 3answers 2k views ### Why are AC quantities represented by sine waves always? Usually we use a sinusoidal wave form to represent a alternating quantity. Why not a cosinusoidal wave or a ramp wave form? In sine wave forms we can indicate the maximum and minimum amplitude and ... 1answer 128 views ### When moving something by entrainment in a fluid stream, where does the energy come from? The particular Something I had in mind here would be air bubbles that are pulled downwards against their buoyancy by a stream of water falling down a shaft. The work required to push those bubbles ... 1answer 190 views ### In a column of rising hot air, is the velocity higher at the top? Since the movement of the air is induced by buoyancy, i. e. there's a constant force acting on the air, so I would expect the velocity to increase during ascent, much like an object falling down due ... 1answer 163 views ### A classically trivial quantum field theory of electromagnetism Presumably there is a field theory of electromagnetism that classically gives trivial equations of motion, but when quantized shows interesting topological phenomena. I am talking about the Lagrangian ... 3answers 205 views ### Effective operator in four-fermion interaction In one book, I have got the following lines which I found myself unable to understand what is effective operator? The paragraph is given below: The weak interaction describes nuclear beta decay, ... 1answer 394 views ### Spontaneous breaking of Lorentz invariance in gauge theories I was browsing through the hep-th arXiv and came across this article: Spontaneous Lorentz Violation in Gauge Theories. A. P. Balachandran, S. Vaidya. arXiv:1302.3406 [hep-th]. (Submitted on 14 ... 1answer 137 views ### Sideways motion between a vertical launch from a planet and landing [duplicate] I saw a video some days ago (Hello Kitty in Space) of a schoolgirl successfully launching a balloon into space which later popped and landed ~47 km from launch site. If I vertically launch an object ... 1answer 160 views ### Thermionic emission and delayed emission I want to understand the concepts behind the thermionic emission. In thermionic emission, the energy randomization occurs and the energy may be split to electronic or roto-vibrational states. If this ... 1answer 82 views ### In the expansion of the scattered wave function, why do these two functions have the same index? See Griffiths Quantum Mechanics, eq. 11.21. Evidently, $$\psi(r,\theta,\phi)=Ae^{ikz}+A\sum\limits_{l,m}^{\infty}C_{l,m}h_{l}(kr)Y_{l}^{m}(\theta,\phi).$$ But I don't see why the$l$th Hankel function ... 1answer 341 views ### Puzzled by magnetized aluminum! For a school project I bought a set of magnets and an aluminum strip. The magnets are places along both sides of the aluminum strip (all in the same direction). Now the aluminum strip is exhibiting ... 2answers 300 views ### Water-cooled fast neutron reactors Can anyone explain why fast neutron reactor designs use sodium/lead/salt cooling, instead of water (heavy/light)? Is that because neutron absorption by water would not allow to break even in fuel ... 0answers 49 views ### A question related to tractrix I'm a novice to physics, so maybe it's rather stupid. According to wiki, the tractrix could be considered a trajectory: Suppose$AB$is a stick on a smooth plane$\pi$, and the initial position of ... 1answer 166 views ### What is the momentum of these emitted photons? 5 x 10^7 kg of radioactive material emits energy in the form of photons of red light (700 nm). (Note: photons have no mass.) What is the momentum of each photon? ... 1answer 170 views ### acceleration of the universe Moments after the Big Bang, the universe was expanding at an incredible rate, (I've heard) faster than the speed of light. Due to dark energy, scientists predict the rate of expansion will pick up ... 0answers 85 views ### Why Does Germanium Spontaneously Extrude Long Screw Dislocations? Pure germanium is known to spontaneously extrude very long screw dislocations. They are one of the primary reasons for the failure of older diodes and transistors made from germanium; depending ... 3answers 356 views ### Could a universe with purely Newtonian mechanics exist? I've heard about the fine-tuning principle and how if the fundamental constants of the Universe were changed even a slight bit, life could not exist as we know it. However, study on this subject ... 2answers 121 views ### What does it mean to gauge a group? I'm starting to learn about gauge theories and Goldstone bosons. What does it mean for a group to be gauged? 3answers 302 views ### Could quantum mechanics work without the Born rule? Slightly inspired by this question about the historical origins of the Born rule, I wondered whether quantum mechanics could still work without the Born rule. I realize it's one of the most ... 3answers 474 views ### Normalisation factor$\psi_0$for wave function$\psi = \psi_0 \sin(kx-\omega t)$I know that if I integrate probabilitlity$|\psi|^2$over a whole volume$V$I am supposed to get 1. This equation describes this. $$\int \limits^{}_{V} \left|\psi \right|^2 \, \textrm{d} V = 1\\$$ ... 1answer 68 views ### Erratic light spot in optical projection I once had an old microscope, that included a projection screen that could be mounted instead the eyepiece. It showed a quite decent palm-sized image. Now I have a new microscope, and removed the ... 0answers 201 views ### Quantum Entanglement Versus Inflation in the Early Universe? Quantum entanglement is one of the most fascinating and mysterious phenomena in nature. It needs no interactions, or any sort of exchange for it to take place. It is possible, not against any rules of ... 3answers 461 views ### Why is$\langle \partial_{\mu} f(x) \rangle=0\$?

I'm reading page 488 of Hobson, Efstathiou & Lasenby, and I don't understand something they write... so I came here. The concept they describe is in linearised general relativity. In particular, ...