# All Questions

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### is it possible to extract spectrum (color in general) from an animal [on hold]

if i may ask....can this be ASAP? :3 my problem: okay this is going to sound lame, but i was wondering if it is possible to extract color (specifically spectrum) from an animal. this question ...
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### Is the decay $B\rightarrow K^* \gamma$ decay allowed in the Standard Model?

This is my idea of the Feynman diagram of the $B^0$ to a $K^0$ decay: The photon is radiated off by one of the particles, and by $up$ quarks I just mean ($u$,$c$ and $t$) and their antiquarks. how ...
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### Quantum Mechanics: Eigenstates for Quantum operators [on hold]

1. The problem statement, all variables and given/known data Suppose that a state |Ψ> is an eigenstate of operator B, with eigenvalue bi. 2. Relevant equations i. What is the expectation value of ...
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### Minimum information necessary to represent a pure quantum state

I was thinking about how quantum states are represented for various types of systems, and how the amount of classical information (bits) required to represent a state depends on its basis. Let's take ...
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### The needs of photonic senseor for biomedical applications

I am doing a literature review on the topic mentioned in the title. When I write the review, I found myself a contradiction. Just want to listen to opinions from others :P Nowadays, the development ...
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### Question on calculating the number density of free electrons in the sun's photosphere

I am writing a paper on the effect H$^-$ (a hydrogen atom with an additional electron) has on the opacity in the sun's photosphere. As such, I need to calculate its abundance. Doing so is ...
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### The classical hydrogen atom

Suppose we want to analyze a hydrogen atom using purely classical mechanics. This obviously is not exactly how things work - quantum mechanics plays a huge role and probability distributions are ...
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### Does wind cause more wind?

(Eulerian) wind is air movement from an area of higher pressure to an area of lower pressure. As the air moves its hydrostatic pressure sinks relative to the environment and so the "wind region" ...
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### Realisation of arbitrary controlled quantum gate

I am interested if there's some known approach when constructing quantum gates out of universal set. I was trying to construct Controlled Z gate (gate that will apply pauli-Z gate on the target qbit ...
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### How to compute the normal ordered angular momentum of a Klein-Gordon real scalar in terms of ladder operators?

I'm trying to compute the angular momentum $$Q_i=-2\epsilon_{ijk}\int{d^3x}\,x^kT^{0j}\tag{1}$$ where ...
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### Heavy beam suspended by nylon and steel cables, angle problem [on hold]

I'm having trouble with the following simple physics problem: A heavy uniform beam of mass 8000kg and length 2,0m is suspended at one end by a nylon rope of diameter 2,5cm and at the other end by a ...
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### How can randomness exist? [duplicate]

Apparently radioactive decay cannot is entirely random (I'm just picking something that's currently accepted as random). However, since it's caused by something, if you had the means to do so, surely ...
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### Weight of a material

I'm dealing with a problem here. Does a material lose weight when it transforms from solid to liquid? I believe it does. It loses weight because some of it becomes gas. Can anyone help me? Thank ...
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### Does a tire need to slip to generate force?

Recently, I have been doing some research on racing and tire modelling. While I was doing this, I encountered many curves like those shown below. While I understand the need of slip angles to ...
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### Can anyone provide a good exposition on acoustic impedance?

Can anyone provide a good exposition on acoustic impedance? Does anyone know of a good paper or textbook that does so? It seems like most sources kind of just start using it without deriving any ...
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### Explanation for negative specific heat capacities in stars?

I've just found out that a negative specific heat capacity is possible. But I have been trying to find an explanation for this with no success. Negative heat capacity would mean that when a system ...
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### Is it possible for two holograms to be on the same holographic film?

I was thinking if you could use a laser of a specific frequency to write a hologram on a film, and then to try to record over the same film but with a laser of another frequency the the first. Is it ...
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### Why does gravity enact force? [on hold]

Gravity would cause two objects in a vacuum to move toward each other. I understand that gravity is a force that exists as a product of energy's original conversion into mass and the continuing change ...
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### How can nuclear decay both annihilate and create?

This'll be back to basics for many of you, but here's something I still don't get. How can nuclear decay of an unstable atom both create and annihilate positron-electron pairs? You have an unstable ...
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### Does entropy increases due to gravity? [on hold]

Imagine travelling to the surface of a neutron star where the very intense gravity would make you weigh 140 trillion times more than we do on earth. It's enough to significantly raise the entropy of ...
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### Convergence of light by light scattering amplitude

Perhaps I'm too exhausted to see the answer of why the photon-photon scattering should contain no divergences. In Peskin and Schroeder page 320 we find that because of the Ward identity the ...
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### how can we apply the formula p=VI in power transmission lines?

i mean we don't know the value of voltage across the power lines...we just know the potential at which it is generated which is not the potential difference across its ends..say electricity is ...
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### Density distribution of black hole while cycling energy? [on hold]

I have not yet taken a course on general relativity, but from what I understand: -Black holes condense their entire collective mass into a central point (or ring if the black hole is spinning), and ...
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### Surviving collapsing buildings: Which floor maximizes your chances of survival? [on hold]

Although this is not a purely physical question, I think this is the best StackExchange place to ask it. When a building collapses, which floor should you go in order to maximize your chance of ...
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### Transmission lines…which one is high? potential or potential difference?

The electric potential at generating station is 11,000 V...why such high potential is generated there? From generating station it is transferred to local station...is there any electric potential at ...
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### When a planet is heated through gravitational pull, where is the energy taken from?

Jupiters moon Io is heated through the gravitational pull of Jupiter, but when Io is heated because of this, where does that energy come from? How does conservation of energy work for this effect, ...
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### Proportionality and units

This might be very easy, but I'm not 100% sure how it's done. Lets say I have this equation: $$R = R_{0} \cdot \left[1 - \frac{P_{0}R_{0}}{GM_{0}\rho_{0}}\right]^{-1},$$ where I know $P_{0}$, ...
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### The reciprocal lattice of HCP lattice

There is a very similar question here Reciprocal Lattice of a non-bravais lattice, but I don't fully understand the answer, and the question is now obsolete so I feel that I should ask it again. How ...
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### What is the actual energy content of the Sun?

Beside the energy released by nuclear fusion in the core, the Sun is a hot plasma of hydrogen and helium ranging from thousands to million of degrees. So how does that translate into energy ? I have ...
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### Position operator in QFT

My Professor in QFT did a move which I cannot follow: Given the state $$\hat\phi|0\rangle = \int \frac{d^3p}{(2\pi)^3 2 E_p} a^\dagger_p e^{- i p_\mu x^\mu}|0\rangle,$$ he wanted to show that this ...
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### What's so special about velocity dispersion relations in astrophysics (M-sigma relation, Tully-Fisher, etc)?

I'm confused. Velocity dispersion of stars is measured by Doppler broadening, and through virial theorem that provides information about enclosed masses of galaxy bulges, galaxies, etc, as explained, ...
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### Electric field due to continuous volume charge distribution [duplicate]

Since the Electric field at point (0,0,z) is $$dE = \frac{\rho vdv}{4\pi\epsilon_0R^2}\hat{R}$$ $$\hat{R} = \cos\alpha \hat{z} + \sin\alpha \hat{\rho}$$ where $\hat{R}$ means unit vector $R$. ...
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### Inconsistency in the delta potential

I encountered an inconsistency in the one-dimensional delta potential. Suppose we have a one-dimensional infinitely deep square well from $-L$ to $+L$. We know the eigenstates are sine and cosine ...
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### How can we derive the gauge field Lagrangian?

I learned the gauge field Lagrangian is given in this form: $$\mathcal{L} = -\frac{1}{4} \mathrm{Tr}(F_{\mu \nu}F^{\mu \nu}).$$ But how one can derive this equation starting from defining the ...
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### how refraction takes place at atomic level? [duplicate]

when light beam is incidented at some angle on the surface of a medium some part of the light is reflected and some refracted. I want to know about refraction. As the photons interact with the atom ...
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### Is it possible to improve the efficacy of light bulbs with a semipermeable mirror? (The greenhouse bulb)

Let us imagine that there exists a material that reflects infrared radiation, but is transparent for visible light. Could we take an incandescent bulb and add this material to the inner surface of the ...
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### Can a liquid with angle of contact as 90 degrees with a solid surface definitely wet it?

Can we conclusively say something about wetting conditions at this boundary value of angle of contact ($90^o$)?
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### Kinetic energy of fluids in Venturi Injector

Since the cross sectional area is smaller in the middle the water flows faster and experiences a smaller hydrostatic pressure. Additional mass is sucked in from the middle tube and gets accelerated ...
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### Dalitz plot for $\pi \to \gamma (1) + e^+ (2) + e^- (3)$

I am studying the decay above and I need an exact form of Dalitz plot in coordinates $m^2_{12}, m^2_{23}$ to integrate over three-particle phase-volume. Please, tell where to read about it? I tried ...
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### Pattern on mesh office chair emulates thin film interference for curved surface. Possible to form an equation?

I saw a chair that looked like this at the dentists office yesterday, and the pattern caught my interest, as the light patterns emulated thin film interference fringes formed by an object that has a ...
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### Why can we say that $\bar{d}Q=TdS$?

When we introduce entropy we do this by saying that: $$\bar{d}Q=TdS.$$ Now I was wondering why this should be true? I know that by looking at a Carnot cycle, we do get this relation for reversible ...
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### Help needed to understand the dispersion curve of a 1D lattice with diatomic basis

I am trying to understand the dispersion curve (as shown below) of a 1D lattice with diatomic basis. Here are my questions: Can both optical and acoustic branch of phonon simultaneously exist in ...
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### What is the Quantum Transition Time for Photon Emission?

When an electron in an atom changes energy states to emit a photon, how long does the process take? Is this question even meaningful?
Suppose I have 3 fermions($\left|\psi_1\right\rangle$, $\left|\psi_2\right\rangle$, $\left|\psi_3\right\rangle$) and a system with 3 states ( $\left|1\right\rangle$, $\left|2\right\rangle$, ...
I am trying to work out the length contraction using the Lorentz transformations. Here is how I stated the problem: Suppose a bar (whose proper length is $L$) is moving at speed $u$ (to the right) ...