# All Questions

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### Au, Ag nanoparticles plasmon peak position?

Is the interaction between metallic nanoparticles (~ 20 nm) and light in the UV-Vis-NIR range governed by Mie theory or by Rayleigh scattering theory? Where are the Au and Ag plasmonic peaks located? ...
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### To what fraction of the speed-of-light has any multi-atom molecule (or polyatomic ion) been accelerated?

Of course photons go 'c' in a vacuum, and seems there's data on electrons getting to 0.999999999976c and protons to 0.99999912c. For the twins paradox to be plausible, one of the twins must reach ...
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### Would our universe seemed static from outside?

As the time we experience does not exist outside our universe and all the change we experience are measured with respect to the time, how our universe would seem from outside? Will be like an static ...
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### Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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### How can one reasonably theoretically model polycrystalline materials?

Many techniques are taught in advanced solid state courses but they are almost all derived for perfectly crystalline materials. For example, band structure really only appears theoretically when you ...
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### Can energy be extracted indefinitely from a mass dipole/mass negative mass pair in perpetual motion?

Some days ago I have read this short piece of article by Hammond which caused me to revisit the theoretical properties of negative mass. It is known that a pair consists of ordinary mass and negative ...
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### Does Graphene have a honeycomb lattice?

In my grand ignorance I would state that graphene has a honeycomb lattice. Some tend to agree with me and some others do not. I'm curious to know what members of the SE community think is the right ...
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### Is it possible to tell time within a steady state?

(Inspired from this question (A) which unfortunately the link in the selected answer is now 404 broken (which might have contained my answer), thus cannot read any deeper than that) From the above ...
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### Notion of distance in a Conformal Field Theory

I'm confused about the how the notion of distance is used in Conformal Field Theory. Let's take for example the Operator Product Expansion (OPE). In a conformal field theory, due to the scale ...
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### Simplest example of spontaneous breaking of time reversal symmetry

Consider a two-dimensional fluid flow, confined to a square, where the bottom is held at a higher temperature than the top. With appropriate choices of the parameters, this will form a single ...
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### How did special relativity change physicists views on the two prominent inverse square laws (ie Newton grav and Coulomb's law)?

On page 107 in Hartle's Gravity -- An introduction to Einstein's General Relativity, he says the following With the success of special relativity it became apparent that the Newtonian theory of ...
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### Superconducting electromagnets?

Is it true that superconducting electromagnets don't need any power? So... energy to created a magnetic field via a superconductor would be zero? Since resistance is zero, does that also mean that ...
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### Is there an exact formal definition of the Universe?

I've read several articles about observable Universe, Universe and Hubble volume, including Wikipedia article and references on it, and I wondered: Is there a formal, rigorous definition in physics of ...
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### Capacitance and voltage drops

Let's say that Switch $S$ is closed. I have a conceptual question: If a stream of particles acquire a potential difference upon exiting the battery, why will they lose some of that potential ...
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### Uncertainty of energy for harmonic oscillator at ground state and first excited state

How does one calculate the energy uncertainty of the harmonic oscillator in the ground state and first excited state?
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### How does one normalize this wavefunction? [on hold]

Here is the question: So I could write $N = \dfrac{1}{{\sqrt{<Ψ|Ψ>}}}$, right? Considering the parentheses in the exponential term, it looks like a good idea to switch to spherical polar ...
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### How can I replace the Euler method by RK4 to determine the free fall time? [on hold]

How can I replace the Euler method by Runge-Kutta 4th order to determine the free fall motion in not constant gravitional magnitude (eg. free fall from 10 000 km above ground)? So far I wrote simple ...
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### Using a charged capacitor to charge two others

Here is the homework question in question: The figure (below) displays a 13.1 V battery and three uncharged capacitors of capacitances C1 = 4.08 μF,C2 = 6.19 μF and C3 = 3.30 μF. The switch is thrown ...
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### How does electrostatic force affect electronic devices?

How does electrostatic force generated by two seperate plates having opposite charges affect electronic devices? I know that magnetic fields have some harmful effects to electronic devices but I am ...
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### Why is the scalar product of four-velocity with itself -1?

My GR book Hartle says the scalar product of four-velocity with itself $-1$? Consider the definition of four velocity $\mathbf{u} = \frac{dx^{\alpha}}{d\tau}$. Suppose I take the scalar product of ...
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### How holes conduct current?

I had questions about holes in semiconductor, when i checked the net it is said holes are said to be equivalent positive charge and they say because the hole moves from one place to another when it is ...
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### Non-degenerate or degenerate perturbation theory for a non-degenerate level of a system with other levels degenerate?

To decide whether I have to use non-degenerate or degenerate perturbation theory, I have to look only on whether the energy level I am calculating corrections to is degenerate, the degree of ...
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### Upper limit to electric charge confinement?

The following questions are not intended for conventional capacitors (which stores excess charge on the outside surface), rather a type of hypothetical capacitor which is designed to confine charge by ...
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### How to accurately mark with arrows an electric circuit

As I understood, we choose to mark the current with arrows going from the plus pole to the minus pole (even though we know that in reality it is the contrary). As I'm looking at electric circuits, I ...
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### SU(3) irreducible representations with tensor method

I am dealing with the tensor product representation of SU(3) and I have some problems in understanding some decomposition. 1) Let's find the irreducible representation of $3\otimes\bar{3}$ we have ...
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### Can observations of cosmic super voids teach us anything about time, dark energy, or dark matter?

Can observations of cosmic super voids teach us anything about... Time - If gravity affects time is there a chance we could gain any further information from studying a region with lower than ...
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### Mode groups in an optical fiber

I know what modes in an optical fiber mean but what are exactly mode groups in an optical fiber? From what I read until now, I have the impression that modes that have close propagation constants ...
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### What justification is necessary for convolutional variational principles to be considered legitimate?

I recently asked a related question and was interested in why/or why we cannot use convolutional variational principles in practice or in theory. Summarizing the points I made in the earlier post: ...
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### Are empty black holes possible

When just considering GR without evaporation nor QM, is an empty (containing no matter or anything) black hole possible ? Let's say that there is only GR and nothing else (no matter or boson fields), ...
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### What arguments and examples did Newton give in support of Absolute time and space?

As everyone knows, Sir Newton advocated the Absolute Space & Time theory. But what examples and arguments did he confer to establish his theory? I read the wiki page where it was written that he ...
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### Why does this derivation of the dependence of free energy on pressure not work?

It is well known that the Gibbs Free Energy of a gas depends on the pressure via the following formula: $$G_m(p) = G^\circ_m + RT\ln{\frac{p}{p^{\circ}}}$$ Where $G_m$ is the molar gibbs free energy ...
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### Working out the penetration of radioactive decay products

From my understanding of the products of radioactive decay (alpha particles, beta particles, and gamma are all I know of), the particles (or energy I guess?) are stopped by a medium according to it's ...
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### Ambiguity in Asymptotic Perturbative Series and Instantons

I know there are a number of questions about the asymptoticity of perturbative series and about instantons on StackExchange (e.g. Instantons and Non Perturbative Amplitudes in Gravity from user566, ...
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### How do you accelerate atoms/particles?

So I'm in 11th grade, and I'm writing a theory for my Physics professor and I need help. I need to know how you can make atoms/particles travel faster. For example, making sound travel a little bit ...
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### Balancing a pencil

I came across this equation for balancing a pencil while solving some problems: $$ml\ddot { \theta } =mg\theta$$ Where $l=$the length of the pencil, and $\theta$ is the angle it makes with vertical. ...
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### What frequencies will get lost/diminished when you listen to a colloidal speaker without the backside

I was checking the connections of my colloidal speakers, so I had to listen to music without the backside of the speakers. There was clearly a difference in the quality of the music, but I couldn't ...
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### Are there any equations describing solid, liquid, or plasma containing the gas constant $R$? [on hold]

The gas constant $R$ relates so much to Boltzmann constant $k_B$. Boltzmann constant= gas constant divided by the Avogadro constant. For example, $R$ can be used in Nernst equation. So, $R$ is not ...
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### Can we prove that zero point energy exists everywhere and not just locally around individual particles or our universe as a whole?

If zero point energy only exists locally around the particles that we are using to observe it and not globally then the zero point energy would have to rise in value when we put two particles close ...
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### Laser cutting - ignition

How is that sometimes laser cutting by burning material or "flame cutting" causes ignition of the whole material like here where the whole paper starts to burn and sometimes doesn't allowing precise ...
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### Calculate the mass of a Schwarzchild black hole with Komar integral

In Wald's GR, Komar integral is Eq. (11.2.9): $$M=-\frac{1}{8\pi}\int_S\epsilon_{abcd}\nabla^c\xi^d$$ $S$ can be chosen as a 2-sphere, the boundary of a spacelike hypersurface $\Sigma$ such that the ...
### Compute $Z_2$ Invariant of 2D Topological Insulators without Computing the Eigenstates
For 2D Time-Reversal Invariant systems ($T H(\vec{k}) T^{-1} = H(-\vec{k})$), there is a formula by Fu-Kane-Mele in order to determine whether the system belongs to either one of distinct topological ...