# All Questions

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### Lagrangian for FRW metric

For the metric $$ds^2=-dt^2+a^2(t)(dx^2+dy^2+dz^2),$$ $$L= \sqrt{-g_{\alpha\beta}\frac{dx^\alpha}{dt}\frac{dx^\beta}{dt}}$$ How does this become $$L= \sqrt{1-a^2 (\frac{dx}{dt})^2}~?$$ I guess ...
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### Number of Stars vs Value of Omega (Crtitical Density of the Universe)

I may be badly mixing things up here. If I am, please kindly correct me. As I understand it, if the universe was too dense at the start of the big bang, it would have collapsed back in on itself. Too ...
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### Negative energy/mass bounds on de-Sitter spacetime

There exists a Positive Energy theorem for General Relativity in Anti-de Sitter and asymptotically flat spacetimes, but there is no equivalent theorem for de Sitter spacetimes Question: Is there ...
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### how is “ electrostatic discharge of metals” done? Say for example of a sattelite [on hold]

how is " electrostatic discharge of metals" done? Say , for example of a sattelite.
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### Lifting frictional force and relative values for kinetic vs static friction

My physics textbook says that friction mainly arises due to intermolecular attraction between atoms of the objects in contact and clashes between peaks are only significant for rough surfaces. I was ...
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### Importance of the Higgs field being formulated in quantum gauge theory

As far as my knowledge goes, Higgs field is only currently formulated in term of classical gauge theory. What is the importance of Higgs field being formulated in term of quantum gauge theory? In ...
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### Why is the metric linearized to determine the mass spectrum of five dimensional Kaluza-Klein?

In this review about Kaluza-Klein theories, (page 1115) in order to determine the mass spectrum of the 5 dimensional theory the metric is expanded to first order. Why this? Why not retain the full ...
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### How long does it take to read and truly understand the Feynman Lectures on Physics? [on hold]

I plan on reading the Feynman lectures and was wondering how long it would take to learn the whole thing so that I can incorporate that into my daily schedule.
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### How do you calculate the power needed to keep something stationary? [duplicate]

1) If I place an object on a table, it will sit there forever. There's no change in potential or kinetic energy. If I hold the object at the end of my outstretched arm, eventually I can't do it ...
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### How to calculate integral? [migrated]

$\int_0^\infty\mathrm{d}l/(r^2+l^2)^{3/2}$ the solution is supposed to look like this, unfortunately I can't derrive it.
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### Why does nuclear matter tend to maximize pressure?

I'm reading a text about equations of state of dense nuclear matter. It is often stated that the phase with maximum pressure is preferred. Why is that?
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### Finding Interatomic Spacing

Here is the problem I am working on For what temperatures are the atoms in an ideal gas at pressure P quantum mechanical ? Hint: use the ideal gas law P V = N kT to deduce the interatomic spacing ...
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### Please explain these equations relating to Newtonian gravity [on hold]

Here is an extract from some badly-written slides: A planet in an orbit with radius $R$ having a period $T$ falls through a distance of $R$ in a time $T/4$. A planet has a speed of $R/T$ and an ...
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### Are all methods of propulsion based on the same principle?

I am struggling with coming up with a way to differentiate between propulsion on the most basic level. Are all methods of propulsion based on throwing stuff in the opposite direction? Can the be no ...
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### Single slit diffraction pattern in 2D

I was looking at previous exams and I saw a question with single slit diffraction. Please look at picture on the website: http://www.physicsforums.com/showthread.php?p=4807732#post4807732 So, this ...
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### Is that a result of thin-film interference?

Several years ago, I was laying on my bed and had a CD shaped transparent plastic disk (which was covering a 100 CD stack), basically a transparent CD. I don't know why but I took my phone and took a ...
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### Is this really a golden ratio spiral?

In this blog post, I found this picture: Does the water really form golden ratio spiral in such cases? Or is the photo just a provocative example, without physics grounds for claims about ...
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### Is quantum field operator $\psi$ same as quantum field $\psi$?

So in QFT, quantum field operator $\psi$ is there. $\psi$ seems to take the role of wavefunction in QM, which now acts upon vacuum state. Then, in lagrangian of various quantum field theories, $\psi$ ...
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### Electricity supplication [on hold]

Is their any reason I can search up as to why electricity is generally harder to inaugurate in regions of vast inhabitability?
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### Work done by a frictional force on a cylinder

A cylinder of mass $m$ and radius $R$, is rolled on surface with coefficient of kinetic friction $\mu_{k}$ about the axis passing through the center and parallel to the surface, with initial ...
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### Is the plasma state always visible?

All the examples of plasma I have come across are visible. Is there any plasma which is not visible? For example, during a dark lightening we don't see the radiation because its gamma radiation. Is ...
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### The Aharonov-Bohm effect is purely classical, right?

Every discussion I've ever seen of the Aharonov-Bohm effect makes a big deal of its being a quantum effect with no classical analogue. But as far as I can tell it is present already at the classical ...
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### Physical reason why (hot) objects glow? [duplicate]

Every object at a non-zero temperature radiates light, i.e. it glows. (Is that called blackbody radiation?) What is the physical reason to this? Is it because more heat implies that the atoms ...
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### What happens when Dark Matter comes in contact with the event horizon of a large Black Hole

Josh Hill, 9, Oakdale Elementary has always talked Theory of Relativity and Astrophysics etc., I can answer most but lately he has stumped me and has been begging me to ask a pro, so here it is.... ...
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### Who proposed the bulk-edge correspondence principle?

Who proposed the bulk-edge correspondence principle? The principle is often quoted in counting the number of zero energy states localized on the interface between two insulators with distinct band ...
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### Electron transition creating Photons

If the transition of the electrons result in production of photons, Is photon creation is instantaneous? If Yes,At what moment rightly the photon gets created? Else, Will the photon be stationary ...