# All Questions

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### What is the fate of a hyper sphere universe?

This PBS video postulates a positively curved universe which is in the shape of something the narrator calls a "hypersphere". https://youtu.be/AwwIFcdUFrE?t=6m33s Such a universe would have no edge, ...
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### Eigenkets in matrix representation

We use base kets in matrix representation of an operator $X$. Could the base kets be possibly be eigenkets also, of operator X? (Here, I'm taking X as a general operator , not only observable,i.e. ...
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### Physics Culture and the Attitude Towards Pure Math [on hold]

To preface my question, I think it's important that I explain my experience studying both physics and pure math at the university level. I am American (as I'm sure education in different countries ...
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### “Big Crunch” Ending of Universe?

So, I am reading Introduction to Modern Cosmology by Andrew Liddle and have just learned that the universe is cooling as it is expanding. Now, I am mathematician knowing very little about physics, but ...
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### Why does galilean invariance imply that particles that start rest stay on the same line?

I'm reading Arnol'd for self study. I'm struggling with this question: "Show that any system of two particles will remain on the same line that connected them at the initial moment, if they started at ...
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### Torque on electric dipole placed in non uniform electric field

When electric dipole placed in non uniform electric field, what is the approach to calculate torque acting on it? Can it be zero?
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### How do flavoured mesons decay?

I've seen (Cf. Griffiths' book) many diagrams of weak interaction of the type $q_1 \rightarrow q_2 + W^{\pm}$ (which are justified ultimately in terms of CKM transition values). But I don't understand ...
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### Why does AC Conductivty in Drude Theory have an Imaginary and a Real Part?

In the Drude Model the direct current (DC) conductivity is given by the following formula: $$\sigma_0=\frac{ne^2 \tau}{m}$$ where $\tau$ is the relaxation time. Furthermore, the AC conductivity in ...
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### Third Brillouin zone for a quadratic 2D lattice

As far as I understand, the construction of Brillouin zones stems from the relation $$2 \vec{k}\cdot \vec{G} +G^2 = 0$$, where k is the wave vector and and G is the reciprocal lattice vector. This ...
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### How extremely low temperatures (near absolute zero) are actually measured [on hold]

How do the industrial or laboratory thermometers for this purpose work like: what effects are based on, what are other alternatives how accurate they all are
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### When water turns to steam at 100 degrees Celsius why does both the internal and potential energy increase?

I thought during this transition only the potential energy increases as energy is used to weaken the van Der Waal forces between the molecules. And that the internal energy is only dependent on the ...
I'm trying to find the friction coefficient that makes the body roll without slipping but I just can't reach a value. The force is applied on a small central disk of radius $r=0,03\, m$ and mass \$m=0,...