# All Questions

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### How fast can fire spread?

You can build a home-made flamethrower by using a spray and a lighter: (from this YouTube video) But this can be dangerous if the fire gets in the can and the can explodes. So I guess the speed at ...
608 views

### What are the main algorithms the LHC particle detectors use to reconstruct decay pathways?

I am just starting to look into how we understand the data from particle collisions. My question is, what are the algorithms or ways that these detectors interpret the data? Are there standard ...
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### When I move my arm forward in vacuum, will my body move backward?

Let's say I stay at point $x=0$ in vacuum. When I move my arm forward such that it will have a positive $x$ position (say $x=5$) will the rest of my body move backward such that it will have a ...
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### Small sphere rolling off the top of a large sphere [closed]

A heavy sphere of radius r = 1.00 meter is fixed with respect to the ground. A small uniform solid sphere is placed at the top of the larger sphere. After a slight disturbance, the smaller sphere ...
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### Rotating a complex number

Let us begin in a two-dimensional Euclidean plane. The vector is e.g. $\vec{V}(x,y)$ It is often useful – but in this case, it's just a mathematical trick that doesn't make the complex numbers ...
411 views

### Crystal Momentum in a Periodic Potential

I'm working through some basic theory on periodic potentials, and I would appreciate help in understanding the crystal momentum. Suppose we have a Bravais lattice with lattice vectors $\textbf{R}$. ...
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### Coordinates of the extra dimensions

If we live in more than three spatial dimensions, is it not right to conclude that all matter observable to us shares almost the same coordinates of extra dimensions. Or is it just that ordinary ...
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### Deriving conserved currents by promoting parameter

I currently reading Tong's text on String Theory. In Chapter 4.1.1 he alludes to a technique to derive conserved currents Recall that we can usually derive conserved currents by promoting the ...
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### How muscle force work? [closed]

If muscle works with force on joint and joint works with exact force on muscle(newton III law) how can muscle drag much heavier weight than itself?
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### Kinematics problem involving rocket

A rocket is fired vertically and ascends with a constant vertical acceleration of 20 m/s^2 for 1 minute. It's fuel is then all used and it continues as a free particle. Find the Maximum height ...
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As far as my knowledge is concerned, a vector quantity should possess magnitude and direction & more over it should also obey the laws of vector addition. As we all know that the vector sum of 3 ...
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### Rectifying incomplete popular notions in cosmology

In looking at the answers to this question regarding light from distant galaxies ever being visible to us: Expansion of the Universe, will light from some galaxies never reach us? I came across a ...
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### Why does the Sun's (or other stars') nuclear reaction not use up all its “fuel” immediately?

The temperature and pressure everywhere inside the Sun reach the critical point to start nuclear reactions - there is no reason for it to take such a long time to complete the reaction process. Just ...
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### The equivalence of two worlds related by T-duality

T-duality in string theory relates a world containing open and closed strings with a D$p$-brane with a compact dimension with radius $R$ with a dual world with a D$(p-1)$-brane with a radius ...
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The Gell-Mann matrices $\lambda^\alpha$ are the generators of $SU(3)$. Applying an SU(3) - transformation on the triple $q = ( u , d, s )$ of 4-spinors looks like this: $$q \rightarrow q' = e^{i ... 3answers 457 views ### The Gluon - Does It Exhibit Wave Properties? Do Gluons have frequencies and wavelengths? I assume that they do, but have been unable to find anything on point in SE or Wikipedia. Just beginning to study university-level physics here. 1answer 345 views ### Quantum Entanglement and BB84 protocol Can Quantum entanglement can be used to strengthen the security or increase the efficiency of communication via the BB84 protocol? If so, how? 5answers 471 views ### Can “vacuum be brought” from outer space? Instead of creating vacuum in chambers on earth, why can't vacuum be brought from outer space in chambers? Outer space pressure ranges from 10^{-6}torr to 10^{-17}torr very very low. Is it ... 2answers 2k views ### How can it be that the sun emits more than a black body? As far as I know, a black body is an ideal emitter. So how can it be that a non-ideal emitter emits more radiation than a black body? This happens only in a very limited area at around 500nm, but it ... 0answers 39 views ### Are signals exchanged between a rotating frame and a central frame assymetric? This question pertains to EM signals exchanged between an inertial central clock and an orbiting clock i.e. non-inertial frame. (I edited out the variables connected with gravity and separation ... 0answers 37 views ### Requesting some research study problems on Classical Mechanics [duplicate] Someone please tell (advice) me some research study problems on Classical Mechanics that can be tackled with Undergraduate level knowledge. 1answer 185 views ### Rational ratio of frequencies leads to isolating integral of motion Padmanabhan's discussion of dynamics mentions that in general the two dimensional harmonic oscillator fills the surface of a two torus. He further notes that there will be an extra isolating integral ... 0answers 40 views ### Is spin an observable quantity for fundamental particles? [duplicate] How we know that the spin of fundamental particles? For example spin-0, 1/2, 1,2.. What is the experimental facts about spin? 3answers 116 views ### Finding the mass of pure matter [closed] How would I find the mass of "pure" matter, that is, non-quantized matter, for a given volume?Let's say I have a volume equal to 1 meter squared, and I completely filled it up with matter - that is, ... 0answers 68 views ### In KK theory, is proper time defined using the 5 dimensional or the 4 dimensional line element? Let's consider five dimensional KK theory. This is Klein's metric \hat{g}_{AB}= \begin{pmatrix} g_{00}+A_{0}A_{0}&g_{01}+A_{0}A_{1}&g_{02}+A_{0}A_{2}&g_{03}+A_{0}A_{3}&A_ 0\\ ... 0answers 68 views ### Young elasticity modulus anisotropic media Im studying anisotropic system composed by a elastic matrix (Young modulus E_m) filled with oriented rods. Given this filler orientation, the material is elastic-anisotropic, with Young elastic ... 1answer 218 views ### Lattice geometry and dispersion relation Is there a general theorem which gives some information about which influence have the lattice geometry (for example sub-lattice structure, square lattice, honeycomb lattice, lattice symmetries, ...) ... 0answers 31 views ### Static Friction on a Circular Road [duplicate] I was recently brushing up on my dynamics, when I saw the interpretation of how a car moves in a circular road. Apparently, the car can stay in a circle due to static friction acting inwards. But I ... 0answers 59 views ### Why did the Aharonov-Bohm effect mystify people? [duplicate] Of course it is intriguing. But I think the Schroedinger equation for a charged particle in a magnetic field was known at the very beginning of wave mechanics. Therefore, the A-B effect should not be ... 1answer 230 views ### Maximum speed higher than the speed of light I wanted to ask if higher speed than the speed of light will be discovered, will scientists be able to adjust the special relativity to the new situation? I read that informations transmitted faster ... 2answers 1k views ### Gravitational Length Contraction General Relativity predicts that a clock at rest in a gravitational field will run slower than a clock in free fall. Similarly, will a vertical ruler on the earth's surface be shorter than a ruler in ... 1answer 143 views ### Why is it more convenient to consider space or time as a continuum? I often find that phisicists and cosmologists make use of Planck's units. I have read propositions that sound like "...at the level of Planck's units many law of physics break down" ... 1answer 75 views ### Mellin-Barnes (MB) integrals and hypergeometic functions I'm trying to understand a step in arXiv:1104.2661. Equation 3.4 reads, ... 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 170 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 208 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 436 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} $$. If I add the spatial part of the wavefunction as two ... 0answers 207 views ### Radial Null Geodesics in Static Maximally Symmetric DeSitter Space Given a DeSitter-space metric from the line element:$$ 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 321 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 446 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 455 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 343 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 223 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 84 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 48 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 103 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 131 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}^{*} \\ ...
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 ...