# All Questions

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### What new insights are gained in quantum information by doing tasks (e.g. creating BECs) in space?

Recently, I have seen a lot of discussion surrounding NASA and quantum technologies. Specifically, there is a paper that came out in Nature titled "Quantum gas mixtures and dual-species atom ...
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### Interpretation of density wave observables

Given some Hamiltonian quantum system, one can ask questions about (expectation values of) observables. While some have an intuitive meaning, e.g. energy or magnetisation, I find it hard to grasp the ...
14 views

### Limiting Summation to the First Brillouin Zone

I am stuck on validating whether the procedure I am doing is correct. If we have an 'infinite' sum which looks as follows: $$\sum_\vec{q} a(\vec{q})$$ where $\vec{q}$ is a momentum transfer. I aim to ...
• 21
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### What is topological about topologically ordered states?

My naive understanding based off section 4.8 of Nakahara's Geometry Topology and Physics is that topologically ordered phases are ground states that are homotopically inequivalent to other possible ...
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### Clarifying the properties and meaning of scaling dimensions of lattice operators

I'm trying to understand scaling dimensions of lattice operators in translationally invariant critical spin chains. The main example I have in mind is the transverse field Ising model. There's a nice ...
• 1,644
22 views

### Help finding mass of an object using circular motion [closed]

I am currently doing a lab where I need to find the mass of an object using uniform circular motion. The lab goes as follows: you attach a rubber duck to a string, feed that string through a small ...
27 views

### Potential energy of a moving mass-spring system

I want to figure out the Lagrangian $\mathcal{L}(q_j, \dot{q}_j^{})$ of the arrangement given in the figure with $x$ and $u$ as generalized coordinates, respectively, the horizontal position of the ...
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1 vote
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### How to rotate a 3D matrix about a line by a certain degree? [closed]

Physics Ch 67.1 Advanced E&M: Review Vectors (15 of 55) Coordinate Transformation in 3-D: Ex. 2 I was watching the following video where the goal was to find the Coordinate Transformation matrix ...
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1 vote
38 views

### Mechanics vs thermodynamics? [closed]

Many systems of classical interacting particles resonate predominantly at a single well-defined scale although they actually should theoretically allow for broad range of behaviours. For example, a ...
25 views

### Piecewise electromagnetic quantities. How limits of piecewise intervals transform in boosted frame?

Context I am studying electromagnetic quantities in the framework of special relativity. I understand that I can write the contravariant components of the four potential $A^\mu$, and that they ...
• 392
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### Why is mechanical advantage for a wheel and axle not consistent for all wheels and axles? [closed]

This question plagues my existence. Why is it that sometimes the ratio for wheel and axles are the radius of the wheel / radius of the axle, but other times its reversed? I researched and found that ...
18 views

### Electric Circuits [closed]

I got the first part of the question right but once i short circuit the v1 source I can't seem to get it right.
• 1
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### QED feyman graph Coordinate space doubt

So I know that there are the Feyman rules to transform mathematical equation into graphs but to me it's not too much clear when I should draw the graph vertically or horizontally i.e. how I determine ...
• 1
15 views

### Is the ray model appropriate only for spherical/planar waves?

In ray optics, light propagation is modeled using rays, i.e., curves that are perpendicular to the wavefront. In general, rays can be curved; however, if the medium is homogeneous, it follows that the ...
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### Hydraulic press

Let's say I have an asymmetrical U-shaped tube, one branch is wider than the other. If I now pour some liquid into this tube, the liquid reach the same high in both branches. Then, I will cover both ...
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### Why is the general equation for interplanar spacing not applicable here [closed]

https://imgur.com/a/w2vXbPd So I just got back the solutions for one of my midterms, and I'm confused by this answer. I thought the interplanar spacing was, $$d_{hkl}=a/\sqrt{h^2+k^2+l^2}$$ for ...
13 views

### Help with question from Isaac Physics (Path and Phase Difference 11.5) [closed]

This question was taken from Isaac Physics Path and Phase Difference 11.5 The question states: 20 GHz microwaves pass through a pair of narrow slits 10cm apart. Calculate the fringe spacing (y when n=...
1 vote
59 views

### How do they set the clocks on spacecraft visiting several other planets

Scientists have sent spacecraft into distant space passing nearby other planets on the voyage. How do they set the clocks to adjust for kinetic time dilation (special relativity) and gravitational ...
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1 vote
34 views

### Why does hysteresis even exist in ferromagnet or Ising model?

I understand the math where mean field theory gives 2 (+ and -) self-consistent magnetization values for n-dimensional Ising model when the temperature is below the critical temperature. How does this ...
• 544
85 views

### How Quantum Mechanics reconciles with Classical Mechanics?

Imagine we have to charged particles. The kinetic energy of the system is: $$T = \frac{1}{2}(m_1 + m_2) \mathbf{\dot{R}}_{cm}^2 + \frac{1}{2} \mu \dot{R}^2 + \frac{L^2}{2 \mu R^2}$$ and its ...
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### Simple atmospheric air pressure and temperature model with velocity sinks and sources [closed]

I'm seeking to understand how to make a simplistic model for pressure, temperature and velocity in an earth-like atmosphere, given the following features: Initially static atmosphere (zero air ...
1 vote
78 views

### Angular momentum operator in second quantization

I wanted to know what the angular momentum operator in the second quantization would look like in terms of the annihilation and creation operators.