# All Questions

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### How to compute scattering amplitude $\gamma\pi^+\to\pi^+\pi^0$

I wish to find the amplitude for process $\gamma\pi^+\to\pi^+\pi^0$ at low energies. I am familiar with the basic concepts and techniques of QFT but have never dealt with the scattering processes ...
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### Definition of 1 calorie

Why on defining 1 calorie its been taken as heating of water from 14.5 to 15.5 degree centigrade of 1gram of water at standard pressure? Any specific reason for taking the temperature from 14.5 to ...
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### Why is a beam reach the fastest point of sail on modern sailboats?

I've heard that a beam reach (perpendicular to the wind) is the fastest point of sail on modern sailboats, but I haven't heard a satisfying explanation of the physics behind the claim. Triangular ...
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### If a second is defined as a duration of 9192631770 periods of radiation - what is time?

Given the wiki definition of 1 second as: "the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 ...
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### Why friction increases with increase in normal force?

Why is friction dependent on normal force and not the actual weight? I have also read that friction is horizontal component of contact force. Is this true?
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### What does a scale accelerating on an incline read?

I was watching an online video lecture about dynamics, and then I came across this brain teaser, and I've been thinking it over for a couple of hours but can't seem to find the solution. I hope ...
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### About the property of the mirror [duplicate]

It is actually said that left appears to be right and right appears to be left in a mirror. But when I stood in front of the mirror and raised my right hand, then according to this property my mirror ...
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### Lifting capacity of superconductors

How does the strength of the magnetic field the superconductor is exposed to affect its ability to levitate/lock heavier objects? Or is there no relationship (for superconductors) between magnetic ...
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### Sum of powers of complex exponentials

I encountered the identity $\sum_{j=1}^{N} {e^{ikja}} = N\delta_{k,0}$ when reading the section on phonon modes (2.4) in Lancaster & Blundell's "Quantum Field Theory for the Gifted Amateur". I ...
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### What is antimatter and its role in the universe?

As we know everything has its opposite. Like matter its opposite is antimatter. So why the universe is not made by antimatter instead of matter. And what is the main role of antimatter in the ...
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### First fundamental form in the Gibbons-Hawking-York boundary term

Let me expose my problem, I am trying to perform the explicit variation of the Gibbons-Hawking-York boundary term, $$S_{GH}=\int_{\partial M} d^{n-1}x\sqrt{\left|h\right|}K$$ The problem I have is ...
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### Energy levels in molecules

I apologize in advance if this turns out to be a duplicate question. As far as I can understand, if you bring two or more atoms together their wave functions begin to interfere and, since there ...
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### Calculating the probability of a given energy

Given a normalised wavefunction say $\psi(x) = A\sin(n\pi x)$, (where $A$ is a normalisation constant) I can calculate the probability of finding the particle being between a position $x$ and $x + dx$ ...
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### With the LHC about to restart as max energy, are there absolutely no hints or tantalizing signs of Supersymmetry in previous data?

Over the last couple of years I've seen several articles talk about hints or bumps in the data that might point to Supersymmetry. An article in NewScientist from Summer 2012 discussed the discovery of ...
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### Are photon energies conserved in general relativity?

As I understand it, both Maxwell's wave equation and the null geodesics of general relativity are scale invariant. Thus an electromagnetic wave can be shifted along a null geodesic without changing ...
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### Is there a word for all the particles in an atom?

That is to say, is there a word that picks out protons, neutrons, AND electrons, rather than just saying "nucleons plus electrons"?
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### What is the energy levels of 1d hydrogen by using WKB?

I need to find the energy levels of 1d hydrogen using WKB and $V(x)=-1/x$? can you help me please?
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### Notation of polarization of light

Upon my research of polarization of light I stumbled upon the following formula: What do the three dots mean? I have never seen them before. Edit: Found this notation in the book "Nonlinear fiber ...
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### Polchinski equation 11.2.7

In Polchinski's string theory volume 2, when discussing the GSO projection for the heterotic string he says: In the IIA and IIB superstrings the GSO projection acted separately on the left- and ...
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### Central Force - Trajectory [on hold]

There is a central force $\vec{F}(\vec{r})=f(r)\vec{e_r}$. 1.1.: The trajectory/flight path is defined by $r=r(\phi)$, compute $f(r)$. 1.2.: The path is defined by $u=u(\phi)$ with ...
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### Is the light from the stars within our galaxy redshifted?

I know that metric expansion does not happen within a galaxy. But is the light from stars in our galaxy somehow redshifted? Also, everyone says that our galaxy isn't expanding. But is there a slight ...
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### Minimizing the Lagrangian action of an impossible problem

I'm working my way through Structure and Interpretation of Classical Mechanics (SICM), and am stuck on an exercise in Section 1.4: Exercise 1.6. Minimizing action: Suppose we try to obtain a ...
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### Time dilation: reference frame

So I'm calculating how long it will take for half of some muons to decay for in a stationary observers frame of reference. They have a half life of 2.2 * 10^-6 and are moving at a speed of .98c ...
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### How can we be sure that a new frequency standard is better than the old one?

Lets assume for this question that at one point in scientific history everyone was running along well with their cesium frequency standards, and someone has a brilliant idea and builds an active ...
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### What voltage is induced between the endpoints of bar, if we swing it by an amplitude [on hold]

After a long horizontal conductor carrying current of 10 A is vertically suspended 2m long insulated conductive rod. We swing it by an amplitude of 10 cm. (a) in the direction of the wire; (b) in ...
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### Rigorous definition of superselection sector/quasiparticle type in anyon systems

The systems I have in mind are for example Kitaev's toric code model (arXiv:quant-ph/9707021) and Kitaev's honeycomb model (arXiv:cond-mat/0506438). What I'm looking for is a mathematically rigorous ...
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### Explaining a quote by Weinberg about the signifcance of symmetry groups in physics

When skimming through a book, I found this quote: The universe is an enormous direct product of representations of symmetry groups. —Steven Weinberg I am a mathematician (so I know only basic ...
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### With what frequency triangle swings if it shuns from equilibrium? [on hold]

Guide V-shaped is with no friction in the swivel bearings A and B (see Fig.) Of the magnet with a vertical magnetic field of 0.8 T. The weight of the guide is 50 g, the length AB 4 cm, and the length ...
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### How do vinyl records work?

I know that vinyl records save the sound as ups and downs on the record plate surface. Let's say two people with different voice timbers say "I like vinyl records" with the same level of voice. It ...
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If I send two rockets from the Earth in opposite directions, at, say, 60% of the speed of light relative to the Earth, then relative to each other they are travelling at 120% of the speed of light. ...
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### Calculating curvature of spacetime when energy is present

I am only about half-way of studying SR and GR and I am not yet familiar with a formula to calculate the curvature of spacetime when energy is present. To be more specific, I want to calculate ...
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### What is the value (order of magnitude) of electric charge density of the atom and the nucleus?

The mass density of the nucleus is constant. But what is the electric charge density of the nucleus and the atom (any order of magnitude)?
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### How to count the symmetric wavefunctions made by 6 (u, d, s, $\bar{u},\bar{d},\bar{s}$) quarks to form Baryons (i.e., 3 particle state)?

Peskin said 6 (particles + Anti-Particles) guys make (6($\Rightarrow$particles) x7x8)/(3x2)= 56 total symmetric state. How?
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### does the internal energy and enthalpy of the GAS changes in a free expansion process?

when a balloon kept in an evacuated room is punctured and the ideal gas in the balloon goes on to fill the entire room,does the gas perform any work in this expansion, and is there in any change in ...
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### 'Doppler redshift' Vs 'Cosmological redshift'

It is not difficult to understand the differences between 'Doppler redshift' and 'Cosmological redshift' conceptually. But how do astronomers distinguish them when doing observations?
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### How much is if my internet provider says “we have increased your WiFi connection by 2 decibels”?

I understand the definition of decibel as a logarithmic unit for intensity of anything. I just cannot imagine its "actual size" when comes to the wireless connection. Can you provide something ...
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### Why wouldn't any Emission Theory work?

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/origins_pathway/#Emission Here, at the Emission theories of light, I loved the discussed theory. There seems to be a contradiction right ...