# All Questions

1answer
57 views

### “All energy gets taken from the particles and put into the fields”. What does that mean?

In the The Emperor's New Mind book. While author explains why classical physics fails to explain our universe, says: The root cause of these is that two kinds of physical object must coexist: ...
1answer
178 views

### At what speed will objects hit a singularity? [duplicate]

I have read in numerous places like here that an object in free fall into a black hole will be travelling the speed of light when it passes the event horizon. How is it possible to go this fast? ...
0answers
216 views

### How to prove the Hubble law is the unique expansion law compatible with homogeneity and isotropy?

In the book physical foundations of cosmology, it saids that Hubble law is unique and a problem seems to be a hint of proving that. In order for a general expansion law,v=f(r,t), to be the same ...
1answer
445 views

### Is entangled state after measurement still an entangled state?

Suppose there is an entangled state of two electrons, the spin part is $$| \downarrow \uparrow \rangle - | \uparrow \downarrow \rangle \tag{1}$$. If I add the spatial part of the wavefunction as two ...
0answers
211 views

Given a DeSitter-space metric from the line element: $$ds^2=\left(1-\frac{r^2}{R^2}\right)dt^2-\left(1-\frac{r^2}{R^2}\right)^{-1}dr^2-r^2d\Omega^2$$ Where $R=\sqrt{\frac{3}{\Lambda}}$, and $\... 1answer 348 views ### A question about the asymptotic series in perturbative expansion in QFT Related post I heard about the argument that the perturbative expansion in QFT must be asymptotic, such as http://ncatlab.org/nlab/show/perturbation+theory#DivergenceConvergence Roughly this can ... 1answer 465 views ### Understanding the four fundamental forces of the standard model - are they magic [closed] Don't misunderstand the question, my purpose is exploration and understanding of what defines "mainstream physics". It is not asked idly, or with ill purpose.... My understanding of current ... 4answers 491 views ### Is relative velocity invariant under special relativity? If a metre stick passes an observer at speed$v$, would all observers in any inertial frame of reference say the speed of the meter stick relative to the observer is exactly$v$? If so what is it ... 1answer 374 views ### Is a vacuum needed in photoelectric effect? This question was asked to me. My first thought was that electrons may ionise the air and potential difference that was applied may increase or decrease the current which should have been observed. ... 1answer 233 views ### Magnetic multipoles in spherical harmonic Does someone know explain me how to identify the multipoles magnetic terms of the multipolar expansion (Dipole, quadrupole, etc) in spherical harmonics? 2answers 87 views ### About time measurements What we regard as TIME is just a way of measuring duration for various phenomena. Like a ruler is a measuring device for measuring length ( or breadth or width). Saying Time is an illusion is like ... 1answer 106 views ### Conversion of energy to matter or antimatter? We all know that matter converts into energy, but will energy convert into matter? Does it form antimatter by converting? Illustrate me with example. 0answers 49 views ### Why did we see intereference from a single point source? In class we were doing an experiment where an agitator was used to create spherical waves on a ripple tank. I noticed that when using only a single paddle (so a single point source) that you could see ... 1answer 111 views ### Is it possible that matter/antimatter collisions emit resonance frequencies of H/He? Is it possible that matter/antimatter collisions emit resonance frequencies of H/He? I am researching a high energy phenomenon that occurs between the frequencies of 1200-1580 MHz. This come from a ... 1answer 134 views ### Seesaw type-1 and integrating out heavy fields Let's assume seesaw 1 type of generation of left neutrino Majorana mass: $$L_{m} = -G_{ij}\begin{pmatrix} \bar{\nu}_{L}& \bar{l}_{L}\end{pmatrix}^{i}i\sigma_{2}\begin{pmatrix}\varphi_{1}^{*} \\ \... 1answer 156 views ### Diffusion in the standard map Consider the standard map (also known as Chirikov map):$$ p_{n+1} = p_n + K \sin(\theta_n) \\ \theta_{n+1} = \theta_n + p_{n+1} $$I know that the diffusion coefficient according to the ... 1answer 3k views ### Bondi-Metzner-Sachs (BMS) symmetry of asymptotically flat space-times I started studying the BMS symmetry in connection with the paper: http://arxiv.org/abs/1312.2229 and there are a few strange things I noticed. First of all, from reading the original papers by Bondi, ... 3answers 1k views ### How do photons know they can or can't excite electrons in atoms? This might be a stupid question, but nonetheless, it has been bothering me. If you take a photon, make it go through some atoms in a solid, liquid or whatever, then you have the chance of this photon ... 1answer 132 views ### Mainstream view on the measurement problem What is the modern mainstream way to deal with the measurement problem? For this question, the measurement problem is the problem of formalizing wave function collapse when the observer is part of the ... 1answer 213 views ### Lagrangian and grassmann numbers Why sometimes we remember that "classical" lagrangians of fermions are constructed from grassmann numbers, while sometimes don't? For example, for Majorana's field in terms of 2-component spinors (... 0answers 82 views ### Why don't we experience dilatations in Minkowski spacetime? The question This question follows on from the use of projective coords for spacetime in Notation for Translation Group Generators . Under Felix Klein's Erlangen Program, Minkowski spacetime starts ... 1answer 48 views ### Index of refraction appearing in the radiative transfer equation In this publication the Radiative Transfer Equation (RTE) (eq. (7)) contains the following emission term:$$n_i^2\kappa_{d,i}L_{b,i}(\mathbf{r})$$where n_i is the refractive index of component i... 2answers 483 views ### Applying a force on a rigid body which is used to create torque In the above picture, I have a rigid body, in turquoise, which is connected to a point, the red circle. The dotted lines are used to divide objects or lengths, they are not part of the rigid body. ... 2answers 95 views ### How can one calculate the central term of the conformal field theory algebra (and show it's really the virasoro algebra)? So I'm following Szabo's book "An Introduction to String Theory and D-brane Dynamics (2nd ed, 2011); still on the canonical treatment in chapter 3. After doing a mode expansion, we get (up to a ... 1answer 429 views ### Angular momentum of the electric field of a point-like electric charge and the magnetic field of a monopole I am currently reading "Magnetic Monopoles" of Ya. Shnir. My problem is I can not retrieve a result the author provides in the first chapter of the first part. In this chapter, he studies the non-... 5answers 2k views ### When is it useful to distinguish between vectors and pseudovectors in experimental & theoretical physics? My understanding of pseudovectors vs vectors is pretty basic. Both transform in the same way under a rotation, but differently upon reflection. I might even be able to summarize that using an equation,... 1answer 87 views ### What is the power of the most powerful quasar found? Trying to find an answer to this question, I came across many sources that are in complete in contradiction. For example Wikipedia states that a typical quasar has a power of 10^{40} watts while ... 1answer 155 views ### Magnetic flux of a loop due to another loop The scenario is the following, I am given 2 loops with the same radius, r, a distance of d, and same current of I. In the left loop the current goes counter clockwise, in the right loop the current is ... 2answers 230 views ### Has a theory of a small Big Bang in a much larger universe been proposed? I have a theory I think might explain why the universe has accelerated in a way that doesn't require dark energy. I'm wondering if someone has proposed this theory before (did some research and couldn'... 1answer 134 views ### Considering the theory of special relativity: Is torque still a vector? Considering the theory of special relativity: Is torque still a vector? In classical mechanics it is easy: You have 3 axes and thus 3 planes. Every plane has its own torque so torque has 3 ... 0answers 201 views ### Max speed as a function of engine power My question started out as finding the maximum speed of a go kart, taking into account only the drag forces as force opposing the motor. I've done some investigation to find:$$ F_{drag}=\; \frac{\... 1answer 2k views ### Distance between Newton's Rings fringes is does not seem linear On the outer edges of Newton Ring patterns, the fringes are really close together, and much more uneven. Also, the spacing does not seem to decrease linearly at all as you move from the center. Why is ... 2answers 2k views ### Relations between pressure and temperature I have several questions concerning thermodynamics and I order them in 4 points that may be related: What's the difference between heat and work at the atomic level? Isn't heat simply work between ... 0answers 112 views ### Electrodynamics using exterior calculus Can anyone suggest resource for full course of electrodynamics using exterior calculus? 2answers 162 views ### What materials are used in non thermal plasma? While reading about non-thermal plasmas, I came across their ionization potentials(~1%), and other capabilities, such as their non Maxwellian energy distributions. At what temperatures, and pressures ... 1answer 2k views ### A two-dimensional event horizon? Today, I came across an article which says that the Universe began with the collapse of a 4-dimensional [spatial] Universe. Well, I don't know if there's any way to confirm these like it is with all ... 1answer 96 views ### Understanding EM Fields I am an electronics engineering major and some questions arise when studying communications technology that utilizes wireless technology. In particular, I am more of a complete picture kind of person, ... 2answers 275 views ### Looking for lists of student difficulties with electromagnetism, circuits (DC and AC) [closed] I'm trying to help with an Instructor's Guide for an introductory calc-based physics text, and it would really help if I could locate some lists of typical student difficulties with topics like ... ... 0answers 89 views ### Symmetry and Algebra I'm trying to get a more concrete idea how symmetry is understood in quantum theories, as broad as possible. Consider a infinitesimal transformation of states in quantum physics of the form: $$|\... 3answers 193 views ### Understanding ket notation and the unitary matrix, what does a/the unitary matrix represent? I am reading over this paper http://www.scottaaronson.com/thesis.pdf, specifically page 24 under "Quantum computing cheat sheet". The author is explaining ket notation and how it relates to ... 1answer 215 views ### Torque definition and right hand rule not arbitrary I have read the following: http://www.feynmanlectures.caltech.edu/I_20.html#Ch20-S1 The formula for \tau_{xy} is derived in this chapter: http://www.feynmanlectures.caltech.edu/I_18.html#Ch18-S2. ... 2answers 199 views ### Spin of anti-particles Pretty simple question (I think), probably stemming from my lack of formal background in physics. I've been reading questions like this one related to attraction with respect to spin (a term I'm not ... 0answers 80 views ### U(1) local gauge transformation for Dirac spinor field How can we define U(1) local gauge transformation for Dirac spinor field?, like scalar fields? 2answers 469 views ### Can we “fill” an atom with alpha particles? The following passage has been extracted from the book "Modern's abc of Chemistry": ..Heisenberg in 1927, put forward a principle known as Heisenberg's uncertainty principle. It states that, it ... 0answers 45 views ### Dispersing ultrasound in a tube - patters I am looking for patterns (Surface texture, inserted material, grooves, spirals, elasticity of materials etc) to efficiently disperse reflected ultrasound in the range of wavelengths 1mm to 4mm within ... 0answers 115 views ### Trebuchet launch into space I was reading some questions on your site about using a Trebuchet to launch a payload into space. Would it be possible to use the same sort of principle, but to have, say a 100 mile steel cable ... 2answers 263 views ### Does a 4-current J determine a unique maxwell-faraday F tensor up to isometry? Maxwell's equations on a pseudo-Riemannian manifold (M,g_{ab}) say,$$d_a F_{bc} = \nabla_{[a}F_{bc]} = 0,\nabla_a F^{ab} = J^b,$$where$d_a$is the exterior derivative,$\nabla_a$is the ... 1answer 176 views ### The inverse square law of sound through solids? We all know about the inverse square law of sound. In short the power of the wave will get evenly spread on an ever increasing spherical expansion and this will dissipate the power of the wave at a ... 5answers 15k views ### Potential difference (PD) and electromotive force (EMF) in terms of electrons? I am trying to understand potential difference and electromotive force in terms of electrons. In the above 3 circuit diagrams the red shows (what I think to be) the distribution of electrons around ... 1answer 159 views ### Force and torque on rigid body in space Assume that we have two disks in space. If we act same force F on each disk like above figure, how they react? I first thought that since$F_{ext}=\frac{dP_{cm}}{dt}\$, the CM(center of mass) ...

15 30 50 per page