# All Questions

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### What is the critical diameter ratio of a cone shaped reducer before air overflows

Let's assume we have a cone shaped cylindrical reducer with $D_1$ inlet and $D_2$ outlet diameter (funnel). Air flows through it with velocity $v_1$ at inlet and $v_2$ at outlet. With the velocity ...
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### What does the ratio (x-ct)/(x'-ct') signify?

$(x,t)$ being coordinates of any event in an inertial frame $A$ and $(x',t')$ being the coordinates of the same event in another frame moving with uniform velocity $v$ along the same direction in ...
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### Tidal effects in Global Positioning System?

Are tidal effects from the Sun and/or Moon taken into account in GPS systems? More broadly, is the heliocentric system used somehow in GPS?
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### Why is Fermi-Dirac type of distribution used in semiconductors?

We assume that distribution of electrons follows Fermi-Dirac distribution / statistics in semiconductor model which will help to find the concentrations of electron and holes and the relationship ...
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### Superstring NS tachyon vertex operator

After reading some confusing chapter of various string theory book I'm trying to construct the Tachyon vertex operator for superstring theory. I know that this is removed after GSO projection, but for ...
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### What causes this IR emission in UV 365nm LED?

I've just got access to spectrometer and measuring spectrum of everything. But when I took a look at 365nm UV led from chinese UV flashlight I got something really strange - it had some IR at exactly ...
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### Intitutive meaning the unit of Force $N$

I infer 1 Newton of force = 1 kg.m/ sec squared means a force if acting continuosly on a body at rest would make it gain an acceleration of 1 m/sec squared each second.So the change in velocity is 1-0 ...
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### Probability density function as number of particles per unit volume?

In this book Quantum Mechanics by P.J.E. Peebles pg 365 it hints at the idea of the wave function been the probability of finding $n$ particles per unit volume. I have looked in other books and on ...
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### Solving the Klein-Gordon equation via Fourier transform

I have been writing a personal set of notes on QFT and I'm currently writing up a section on solving the Klein-Gordon (K-G) equation. I many texts that I've read, the author starts by expressing the ...
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### how tough is research work? [on hold]

I am going to be a university graduate with good C GPA but still I am worried that whether I can do M Phil or not? I want to ask is it difficult to do research and get your work published in a ...
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### Is the mass of the Z due to mixing with the photon “precursor” B or to interaction with the Higgs?

I want to get something clear that I do not seem to understand. I used to read that the photon A and the Z boson are (different) linear combinations between the W^0 (neutral weak boson before SU(2) ...
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### Can the uncertainty principle be redefined for different standard deviations?

$$\sigma_x \cdot \sigma_p \ge {{\hbar} \over {2}}$$ Where the $\sigma$ is the standard deviation. What happens to the inequality if you use a different definition of $\sigma$. For instance what ...
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### Are measurements of time and distance independent or always coupled by the speed of light

In texts about special relativity the light clock is usually described as a tool to measure time $t$. It counts how often light travels along a given distance $d$, i.e. $t = d/c$, where $c$ is the ...
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### Is causality a necessary and sufficient condition for special relativity?

Is there a way to "build" special relativity by using causality as one of the axioms? Or is it just a consequence of a fact that the speed of light is same in all reference frames?
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### Why can the probability function for a particle in an infinite square well be larger than 1?

For a particle in a one dimensional infinite potential well of width $L$ the probability function is: $$P_n(x)=\left(\frac{2}{L}\right)\sin^2\left(\frac{n\pi x}{L}\right)$$ for $0\leq x\leq L$. The ...
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### Square root of matrix appears in massive gravity. How to calculate the square root of matrix $A+B$ perturbatively?

$A=\text{diag}\{\lambda_1,...,\lambda_n\}$, where $\lambda_i$ can be any number and not necessarily a small number, $\lambda_i>0$, $B$ is a positive definite symmetric matrix, and ...
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### What are mesons and leptons?

What are mesons and leptons? I get decay processes where the decay products are leptons (electrons, positrons, neutrinos, mesons etc.). What are these?
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### What makes electrons behave like particles or waves at different times? [duplicate]

I am quite puzzled about the theory that electrons or light often behave as particles and sometimes as waves. So, I wanted to know more about this phenomena and what happens when and why.
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### Vertical throwing exercise [on hold]

I'm struggling with this question Assume an object is resting on the ground, in $t=0$ there is a force that starting to work on it, which grants him vertical acceleration towards up, $a=3m/s^2$, ...
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### Why is hydrogen, nitrogen, carbon and oxygen the most common constituents of interstellar molecules?

Could someone elaborate more on the use of the word "constituent"? Is the question basically asking why these four elements are the most prevalent in interstellar molecules? If so, why are they the ...
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### Derivative of effective action with respect to classical fields

I have some trouble with the effective action involving fermions. Consider the following effective action (QED): \varGamma = \int \mathrm{d}^4 x \left( j_\mu \mathcal{A}_\mu + \bar{\varPsi} \eta + ...
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### would a clock at Absolute zero have no mass

since Absolute zero corresponds to the theoretical state in which particles have no energy at all. and m = E/c^2 Then would an unwound clock have zero mass if it was cooled ,theoretically , to ...
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### Estimate Uncertainties in Rietveld Refinement

I have the output of an x-ray diffraction experiment made on a thin film. Here I report the output of the experiment (I analyzed those data using powdercell and coloured regions have been excluded ...
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### Is harmonic oscillator continuous variable system?

In the literature I have seen that the notions "our system is continuous variable system", "Hilbert space of our system is infinite" were used as if they were equivalent. For example for harmonic ...
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### Nucleus validation

Inside the nucleus of an atom is the Coulomb's law valid? I mean that between proton, neutron and other elementary particles, i.e., meson etc. What will be the limit or validity of charges between ...