# All Questions

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### Uncertainty of permittivity of vacuum

Question: The value of permittivity of vacuum, $\epsilon_0$, is given with absolutely no uncertainty in NIST Why is this the case? More details: The permeability of vacuum can be given by ...
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### Thermal propagator for a scalar field (KMS condition)

I'm having some troubles following the derivation of the scalar field thermal propagator. I'm following the article "Finite Temperature Quantum Field Theory in Minkwoski space" by Niemi and Semenoff ...
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### How to solve this torque problem

The Answer to the Question is 0.15Kg Is there anything wrong with the question or is it just me. Please Help. I tried working backward but didn't work.
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### Proton for fission

I am just wondering why is proton not used for nuclear fission? Is it because it is charged, so we will have to input more unnecessary energy to overcome the Coulomb force?
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### The origin of the colour of the sun

I have learned so many concepts under astrophysics and unfortunately, I have muddled everything together... Let me try to illustrate my problem: When a star is in main sequence, it fuses hydrogen to ...
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### How much of Minkowski spacetime structure can be recovered from its causal structure?

A beginner's question: I have always understood that (four-dimensional) Minkowski spacetime can be recovered up to a constant factor—i.e. 'up to a dilation' or 'up to global scale'—from its causal ...
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### Charge by Induction

For example, Initially, there are two metal sphere touching each other and the ball on the left has a charge of +5q, while the other is neutral. Then a rod with charge -3q is brought near the left ...
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### How do we know that that the universe expansion is accelerating?

The universe is expanding in all directions. We can detect this by the redshift of electromagnetic radiation from other galaxies. We detect a higher redshift in galaxies which are further away, and ...
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### Three-plate capacitor [on hold]

What is the total capacitance of this capacitor? Is it analogous to the case of inserting a plate into a capacitor (essentially creating two capacitors in series)?
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### Calculate speed of proton at infinity

I have this problem to solve: Two protons are on the x axis at x=-1 and x=0. An alpha particle is placed at x=2, and the proton on the left is released, find its speed at infinity. I know that this ...
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### Does the mass of a star change as it collapses into a black hole?

I know (I think!) that when a really big star collapses on itself it creates a black hole. My question: When a star collapses, is the mass equal to the mass of the star when it's not a black hole? Or ...
Why do we need to introduce the spin connection coefficients $\omega_{\mu \space \space b}^{\space \space a}$ in General Relativity? To me, they just look (mathematically) like the Christoffel ...