# All Questions

136,649 questions
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### Entanglement breaking quantum channels

An entanglement breaking quantum channel is defined as one where $\sigma_{AB}=(\Phi_A\otimes I_B)(\rho_{AB})$ is separable, even for entangled inputs $\rho_{AB}$. Of course, if the input $\rho_{AB}$ ...
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### Bernoulli's equation: distributing energy per unit volume into static pressure vs. dynamic pressure

In any fluids problem solvable using Bernoulli's equation, the question always seems to assume initial values of static pressure ($P$) vs. dynamic pressure ($1/2\rho v^2$). In the initial setup of a ...
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### Calculating refractionindex of water given permittivity

I'm a bit confused. The refraction index is calculated by $$n=\frac{c}{v},\tag1$$ where $c$ is the lightspeed and $v$ is the phasevelocity of light in the medium of interest, water in this case. In ...
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### Somehow unexisting book even though volume 2 exists

I am desperately trying to find a book Electromagnetic Interactions of Hadrons, Vol.1, A Donnachie; G Shaw; New York 1978 Somehow, the second volume is easily ...
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### Why Aren't Hadron Masses More Useful In Determining Standard Model Constant Values?

Some of the most exquisitely precise experimental measurements in all of physics are the masses of the various hadrons. Consider these examples: the proton mass is known to eleven significant digits. ...
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### If an electric field passing through a dielectric medium, back into the original medium, is it “back to normal”?

Suppose there is an electric point charge causing an electric field E in a medium with a dielectric constant $\epsilon_1$. You can calculate the scalar potential $\phi$ at a given distance $r$, as ...
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### Show two Lagrangians are equivalent

I need to show that these two Lagrangians are equivalent: \begin{align} L(\dot{x},\dot{y},x,y)&=\dot x^2+\dot y + x^2-y ,\\ \tilde{L}(\dot x, \dot y, x, y)&=\dot x^2+\dot y -2y^3. \end{align} ...
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### Does it make sense to speak in a total derivative of a functional? Part III

In this third part of the series, I will continue the deduction of Noether's theorem initiated in the previous post - Does it make sense to speak in a total derivative of a functional? Part II. ...
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### Insufficiency of Newton's third law to solve constrained motion problems

In The Variational Principles of Mechanics Lanczos describes what he calls 'vectorial mechanics': the process of solving mechanical problems by recourse to the immediate consequences of Newton's laws, ...
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### Why there are 3 significant figures in 86400 but 5 in 86400 s ( because of Unit)

My book states one rule for determining the number of significant figures as under: All zeroes to the right of a non zero digit but to the left of an understood decimal point are not significant . ...
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### Spaceship A and Spaceship B move in opposite directions at half the speed of light, and A fires a laser at B, does the laser light reach Spaceship B? [on hold]

This question has been bugging me for a week now! I believe it would, but logically, I feel like I'm missing something. Please give a full explanation.
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### Reciprocal lattice points and diffraction peaks

I am having a issue with the concept of what this question is asking. Question For a FCC crystal describe all the reciprocal lattice points corresponding to the two diffraction peaks. Here is my ...
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### Conventions in the FRW metric

So the FRW metric is $$ds^2=-c^2dt^2+a(t)^2\left(\frac{dr^2}{1-kr^2}+r^2(d\theta^2+\sin^2\theta d\phi^2)\right).$$ I have seen books mention that $k=1,0,-1$ - depending one of the values listed ...
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### How can compact fluoscent lamps light up?

My understanding of fluorescent tubes is that filament on one side gets very hot, undergoes thermoionisation and shoots electrons into the gas. The collision between a mercury gas atom and a fast ...
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### Why does friction occur? [duplicate]

I'm pretty sure this is a rather stupid question but I just cannot wrap my mind around the reasoning behind friction. Now, an obvious explanation would be that since all surfaces are inherently rough,...
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### Inertial frames with unknown past

Let's suppose two ships that cross in space at constant speed. We do not know their previous acceleration or velocity history. When they cross, they synchronize watches. From that moment, if they ...
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### Muon capture cross-section

I would like to understand how the cross-section of muon capture depends on the nuclear charge. My attempts are based on the following thinking. Muon is captured to "high" orbital and then muon goes ...
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### Why are rockets with High specific impulse not used for transferring satellites?

I believe one reason could be that high Specific impulse rockets would take longer time to reach the orbit. Is higher time the only reason for using rockets with low specific impulses like Falcon 9 ...
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