1
vote
0answers
45 views

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 ...
1
vote
1answer
39 views

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 ...
2
votes
1answer
47 views

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 ...
0
votes
0answers
20 views

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.
2
votes
1answer
26 views

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 ...
2
votes
1answer
48 views

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 ...
2
votes
1answer
25 views

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 ...
0
votes
0answers
38 views

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.
0
votes
1answer
28 views

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 ...
-3
votes
0answers
27 views

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.
2
votes
1answer
44 views

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?
0
votes
2answers
24 views

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 ...
-1
votes
1answer
53 views

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 ...
2
votes
2answers
39 views

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 ...
0
votes
1answer
30 views

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 ...
3
votes
1answer
43 views

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 ...
23
votes
3answers
3k views

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 ...
1
vote
1answer
106 views

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, $\psi$ ...
0
votes
1answer
49 views

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?
0
votes
2answers
25 views

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 ...
1
vote
0answers
26 views

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 ...
4
votes
1answer
144 views

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 ...
3
votes
2answers
75 views

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 ...
1
vote
1answer
38 views

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.... ...
1
vote
1answer
35 views

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 ...
0
votes
0answers
23 views

Boundary Element Method or Boundary Integral Method Computational Aspects

I have to solve a Helmholtz equation inside a simply connected domain. I know that in general the boundary integral can be written as, $$\phi(x)=\int_V G(x,x') \rho(x')\ d^3x'+\int_S ...
1
vote
1answer
50 views

Finding particles in the classically forbidden regions [duplicate]

Have particles ever been found in the classically forbidden regions of potentials? For example, in a square well: has an experiment been able to find an electron outside the rectangular well (i.e. in ...
0
votes
1answer
26 views

Entangled Electrons

What attribute would help determine in a spaghetti of tangled-entangled electrons, which two are the respective entangled electron pairs, since obviously there is an attribute other than charge or ...
12
votes
2answers
861 views

Evidence of why the Standard Model is a successful theory of particle physics

When discussing physics with laypersons, I'm often in the situation where I have to explain what the Standard Model is, and why it's a successful theory of particle physics. To help in such ...
1
vote
1answer
41 views

Calculating equipotential lines and current density in a rectangular conductor

(This isn't homework, I'm trying to make an illustration for an article I'm writing.) Let's say that I have a thin rectangular bar of uniform conductivity, and I have point probes at various places: ...
0
votes
1answer
34 views

What indicates if object will be reflected - certain example

If I throw a small rock(1kg) at a big rock(100kg) the small rock is reflected; Let's say my weight is 80kg - if I would jump into a big rock instead of being reflected I would move in the same ...
3
votes
0answers
37 views

Output of a beamsplitter with photon number (Fock) state inputs

Given a beamsplitter drawn below, where $\hat{a}$ and $\hat{b}$ are input modal annihilation operators, transmissivity is $\tau\in[0,1]$, and output modal annihilation operators are ...
2
votes
1answer
30 views

Effect of aircraft speed on barometer

Many aircrafts depend on several types of static pressure sensors (barometers) to find the altitude. I assume this sensor need to be located outside the aircraft main body, so that it has a contact ...
1
vote
0answers
26 views

Point of rotation appears as an ellipse when calculated from gps coordinates

First question here. Feel free to write if I posted in the wrong place. While rotating a ship as accurately as we can around some point within the ship, we can calculate the point of rotation from ...
1
vote
2answers
50 views

Moving coil contradiction of Faraday Induction

In Faraday's Induction Experiment, the e.m.f. induced in the induction coil becomes zero when the relative velocity of the coil and the magnet becomes zero. But one can also argue from a stationary ...
4
votes
2answers
137 views

Gravitational slingshot maximum

I have recently read an article about gravitation slingshot assist used by Voyagers 1-2, and was thinking on why this hasn't been used for travel between solar and other systems. I mean sligshot can ...
3
votes
1answer
65 views

Has the possibility of Higgs boson being a composite particle excluded already?

I heard some theory, such as technicolor, predicts the Higgs-like particle discovered at LHC should be a composite particle (correct me if I am wrong). Has this possible been completely excluded ...
2
votes
0answers
37 views

Is Gravity a part of the Strong Nuclear Force? [duplicate]

In episode 7 season 5 of Through the Wormhole, they say that gravity could just be a manifestation of the strong nuclear force and that the force carrier are a pair of Gluons. Is this valid in anyway? ...
1
vote
1answer
34 views

From where does the sound come from when two charged objects meet in real life?

I am sure all of us have played with rubbing things and producing static electricity and when I was charging my comb by rubbing it on my hair and watching it attracting a small piece of paper, I heard ...
1
vote
0answers
23 views

where does the energy for capillary action come from? [duplicate]

I think capillary action occurs because water molecules have a little magnetic field with poles so they can stack up on each other somewhat. When it happens though, where is the energy derived from to ...
0
votes
0answers
29 views

What makes photons travel at the speed of light? [duplicate]

Photons are the only elementary particles which can travel at speed of light. What are the qualities of the photon that enable it to travel with the speed of light?
14
votes
3answers
1k views

Is the frequency of light restricted?

What are the factors that limit the frequency of light? Can it have wavelengths ranging between zero and infinity?
0
votes
1answer
21 views

At what gap width between two plates does convection not occur?

Does the Grashof number lead to the answer? The Wikipedia article (https://en.wikipedia.org/wiki/Grashof_number) yields an equation for vertical plates $$Gr_L = \frac{g\beta(T_s-T_\infty ...
0
votes
0answers
18 views

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 ...
1
vote
2answers
51 views

Phase space derivation of quantum harmonic oscillator partition function

I would like to derive the partition function for the quantum Harmonic oscillator from scratch: $$\tag{1} Z = \int dp \, dx\, e^{-\beta H}.$$ The free particle appears in many textbooks. $H = ...
2
votes
1answer
22 views

Color in Different media

When a light wave passes from one medium to other, the wavelength of the light wave changes and the frequency remains constant. If then color of the light waves depends on changing wavelength or ...
0
votes
0answers
14 views

Quantum observation in the double slit experiment? [duplicate]

As far as quantum observation, when a human observes electrons going through the double whole experiment, have scientist tried having a blind person observe the experiment to see if the electrons ...
1
vote
1answer
15 views

Critical size and Radioactive Nuclei

Nuclear fission requires the mass of the fissile material above the critical mass. So that the explosion takes place at least in the case of a nuclear bomb. But once a single nucleus got involved in ...
3
votes
1answer
75 views

What is physical meaning of $|\phi|^2$ in quantum field theory and pedagogical spontaneous symmetry breaking?

If wavefunction is $\phi$, we know that $|\phi (x)|^2$ represents probability of finding a particle at $x$. Now let us talk about some pedagogical example in spontaneous symmetry breaking in QFT, ...
2
votes
0answers
15 views

Equation describing the motion of an object attracted to magnet [on hold]

Imagine there is a strong magnet at (0, 0). There is a horizontal rail on y=1 on which a ferromagnetic object rides. The object ...

15 30 50 per page