2
votes
1answer
454 views

What prevents a star from collapsing after stellar death?

When the star stops burning because heavier elements like Iron are formed in its core. Then the gas pressure stops and as you know the gas pressure helps keep a star in equilibrium because it provides ...
0
votes
1answer
106 views

Newton's Third Law Opposite Angles

I am asking a related question here : Newton's Third Law Angular Bouncing Why, given newton's third law would a ball bounce at an opposite angle to it's original, if an equal and opposite force ...
0
votes
1answer
195 views

Newton's Third Law Angular Bouncing [duplicate]

I am asking a related question here: Newton's Third Law Opposite Angles If a ball is accelerating at a surface at a non-straight angle, it will bounce off with the opposite trajectory, like if ...
5
votes
1answer
113 views

Baryon in terms of quark fields – spinor index structure

What is the most general way to write down a current describing a baryon made from quarks $\psi_i^\alpha$? Let's say we suppress flavour indices but want to write colour $(i,j)$ as well as spinor $(\...
6
votes
0answers
104 views

Is there a null incomplete spacetime which is spacelike and timelike complete?

Geodesic completeness, the fact we can make the domain of the geodesic parametrized with respect an affine parameter the whole real line, is an important concept in GR. Especially, because the lack of ...
3
votes
3answers
2k views

What is the physical meaning of electric potential, potential difference, and voltage?

When resembling the electricity flow through a wire to people walking through a street: electrons are people, current is the number of people, resistance is the barriers on the way. But what is the ...
1
vote
0answers
93 views

How to construct singlet and other multiplets from two triplets

Let an $SU(2)$ isotriplet operator is given by\begin{equation}\bar{l^c}i\tau_2\vec \tau l=l^T Ci\tau_2\vec \tau l\sim 3\end{equation} and an isotriplet Higgs field \begin{equation}\vec \Delta\sim 3\...
1
vote
1answer
49 views

Why does an external laser drive only couples certain levels?

I was always wondering how is it that all the quantum optics levels schemes are depicted as if the laser couples only two certain levels with some frequency. For exmaple the standard lambda system ...
4
votes
2answers
2k views

Linear vs. quadratic dispersion relation

In wave mechanics the dispersion relation between frequency $\omega$ and wave number $k$ is linear: $$\omega_n=c k_n$$ But in quantum mechanics, based on Schrödinger's equation, one can show that we ...
0
votes
1answer
54 views

Flip of polarisation of light

Consider an optical experiment with photons or light pulses. Is there an optical element that acts in the polarisation degree of freedom like the unitary $$ U = \frac 1 {\sqrt 2} \begin{pmatrix} 1 &...
2
votes
1answer
67 views

Does velocity determine a geodesic?

If the gravity effect we witness is due to objects travelling along geodesics, why is the geodesic different for objects with different velocities if there is no gravitational force as such? For ...
2
votes
0answers
109 views

A driven quantum harmonic oscillator (DQHO) [closed]

I'm trying to find the dispersion relation for the DQHO with Lagrangian $$ L(q,\dot q,t)=\frac{1}{2}\dot{q}^2-\frac{1}{2}\omega q^2+F(t)q $$ with $F(t)$ being non-zero for $0<t<T$. The ...
2
votes
0answers
246 views

Feynman propagator with general $\xi$ parameter

Hey from my notes in my PS book it seems I have solved this some time in the past, but I cannot seem to get the indices straight this time around. So in deriving the Feynman photon-propagator which ...
1
vote
1answer
3k views

Relationship between polarization density and electric field

Imagine I have an electric field $E$ created due to some free charges. Then I introduce a dielectric material somewhere. What confuses me is the polarization field $P$, which is now proportional to ...
7
votes
3answers
786 views

Mathematical understanding of Quantum Mechanics

Assuming that $\phi(r) = F (\psi(r))$ for some operator $F$ in Quantum Mechanics. Then, in our lecture today, we said that $$\phi(r) = \langle r|F |\psi\rangle = \int_{\mathbb{R}} \langle r |F| r' \...
1
vote
0answers
80 views

Geometrised Units

I am curious as to how to express a area units in geometrised ones. I was reading on wikipedia and saw that when angular momentum is converted into geometrised units, it is expressed as a dimension of ...
2
votes
1answer
123 views

Mirror that flips polarisation?

Is it possible to build a mirror which not just reflects a photon but also flips its polarisation from horizontal tho vertical (or vice versa)? The reason why I ask is the following: If I put an ...
0
votes
1answer
75 views

Can a current source provide Voltage to a resistor?

We know that the tension between a perfect current source is 0 We have Va-Vb= 0 but we also have by ohm's law Va-Vb=RI=5V so 5 = 0?
0
votes
0answers
41 views

A confusion about the proof of $\hat{p}|x\rangle=i\hbar\frac{\partial}{\partial x}|x\rangle,$ using $[\hat{x},\hat{p}]=i\hbar~?$ [duplicate]

How to prove $$\hat{p}|x\rangle=i\hbar\frac{\partial}{\partial x}|x\rangle,$$ using $$[\hat{x},\hat{p}]=i\hbar~?$$ The question seems to be uncomplete because for any $f(x)$ $$[\hat{x},\hat{p}+f(x)]=i\...
1
vote
2answers
160 views

Is the temperature rise caused by reflection of sunlight linear?

I have saw a YouTube video showing large Fresnel lens can focus sunlight into a spot which can melt iron. I am very interesting of that, but large Fresnel lens are not that popular, so I was ...
1
vote
5answers
1k views

What is divergence?

What is divergence? I was learning about Maxwells equations and don't understand the divergence part of it. Can someone give an intuition of what divergence is in relation to maxwells equation. To ...
1
vote
0answers
104 views

What is DX technology and what are the effects on digital cameras?

I am photographing in a high power voltage field in Industry producing aluminium, called DX technology. How do I know if my digital camera will not be affected? At 350kA current my film cameras were ...
1
vote
0answers
28 views

Two conductor's in a magnetic field, canceling each other's EMF?

If two conductor's are in a $\Delta$B, oriented the same in the field, and they induce equal $\epsilon$, can their wiring be oriented in a way that they can cancel each other's induced $\epsilon$? So ...
54
votes
3answers
5k views

Why do the big nuts always remain at top? The Brazil-nut Effect

Most of the time when I mixed different nuts in a bowl, I observed that the big Brazil nuts were always on top while the small peanuts settled near the base. No matter how you take it, if the big nuts ...
1
vote
1answer
289 views

Constructing matrix for spin in Stern-Gerlach experiment for arbitrary angle

This is a conceptual question about a problem in Sakurai. I understand how to solve the problem, but there's something about it that irks me, and it feels like I'm missing something. In the problem, ...
6
votes
2answers
234 views

Classical logic in concern with QM Mathematics

In no way am I a physicist, so please excuse improperly used terms. It is in my understanding that Quantum Physics does not obey Classical Logic, hence the existence of Quantum Logic. My questions ...
1
vote
1answer
296 views

How to derive the critical temperature for Bose-Einstein condensation of photon?

I found in Nature magazine that photon can have Bose-Einstein condensation. But I have a question how to derive the critical temperature for photon? Because the chemical potential of photon is zero ...
4
votes
2answers
774 views

Diffraction by small holes

This is a follow-up to this question: What happens to waves when they hit smaller apertures than their wavelenghts? Hans Bethe wrote a paper in 1944, "Theory of Diffraction by Small Holes," Phys. Rev....
4
votes
6answers
1k views

What happens to waves when they hit smaller apertures than their wavelenghts?

I was wondering this for quite a long time now. Let's say you have a water wave (like ripples, not the ones you see during tsunamis) with wavelength 10 m. Imagine you put a boundary with an opening of ...
3
votes
0answers
58 views

How could we find preons?

If I understand correctly, scientists were able to find strong evidence for the existence of quarks by using deep inelastic scattering. If quarks (hypothetically) are composed of preons, could we use ...
12
votes
1answer
733 views

Why is it so coincident that Palatini variation of Einstein-Hilbert action will obtain an equation that connection is Levi-Civita connection?

There are two ways to do the variation of Einstein-Hilbert action. First one is Einstein formalism which takes only metric independent. After variation of action, we get the Einstein field equation. ...
2
votes
2answers
339 views

Photoelectric effect stopping potential

$q_eV_s = hf - \phi$ My question is... suppose we are testing the photoelectric effect. One plate is illuminated. We have applied the stopping potential. Suppose an electron leaves one plate with $KE ...
3
votes
3answers
293 views

What is the significance of clamping the center of the spring?

7. A block is hung on a spring, and the frequency $f$ of the oscillation of the system is measured. The block, a second identical block, and the spring are carried in the Space Shuttle to space. The ...
0
votes
0answers
173 views

Collision force calculation with no future knowledge

For a personal project, I am attempting to write a fairly realistic collision simulator (for relatively large objects, not quantum stuff). As I was consulting my physics textbook and various online ...
1
vote
1answer
103 views

Does this type of equation for an airfoil exist?

In a common airfoil, such as a plane's wing, diminished pressure over the top of the wing and increased pressure under the wing, generally speaking, will cause the wing to lift. Now, a man standing ...
3
votes
1answer
178 views

Is the Wikipedia article on the Kramers-Kronig relations correct?

Reading the "Physical interpretation and alternate form" section of the Wikipedia article on the Kramers-Kronig relations, it says: The imaginary part of a response function describes how a system ...
7
votes
2answers
254 views

Is manufacturing roughness really the only reason we don't see optical interference in thick dielectrics like windows?

I had always kind of wondered why we didn't see interference in things like windows -- we were taught that the condition is that the thickness of the film/slab/medium just has to be an integral number ...
7
votes
2answers
1k views

Can the High beta fusion reactor work?

Are the claims made about the http://en.wikipedia.org/wiki/High_beta_fusion_reactor realistic? Can such a small fusion reactor really work?
0
votes
3answers
1k views

Can Thermal Energy be converted into usable energy?

I'm going to go out on a limb and ask a question that will probably be a bit humiliating: Can Thermal Energy be converted into usable energy? I've been reading about conservation of energy, and I know ...
3
votes
2answers
186 views

Why does nuclear waste have to be stored until the constituent elements decay naturally?

Fair warning, I have a bachelors in CS and have chemistry 211/212 under my belt. My understanding of the atom consists of a proton, neutron, and electron quasi-orbiting it in some sort of strange ...
-1
votes
1answer
63 views

Angular displacement after full rotation

I was wondering is why angular displacement isn't $0$ after $n$ full rotations?
0
votes
1answer
134 views

Freezing or melting Brine

In the freezing of brine, is it possible to do it at a rater or in such a way that the water only freezes or the ice is somehow isolated? Or conversely is this possible in the melting of brine-ice?
0
votes
1answer
208 views

Are the Pauli matrices closed under commutation?

I tried to make a group multiplication table for the Pauli matrices, but I keep getting multiples in front of the elements. What am I doing wrong? I thought the Pauli matrices formed a group that was ...
0
votes
1answer
729 views

Sound Wave interference Experiment

I was wondering, can we use two sound sources so as to create a destructive interference at the position of a recorder at home? If possible, what is the easiest way?
10
votes
3answers
1k views

Planets and Pluto? Neptune?

If one of the rules to be a planet is that it needs to clear ALL objects from their orbit, does this also make Neptune a non-planet? Since it has thus far failed to clear Pluto from it's orbit. Or ...
1
vote
5answers
1k views

Why is kinetic energy stored as potential energy in a body during its motion against gravity?

Everyone knows kinetic energy is converted to potential energy in the body when it moves up against the earth's opposing gravitational force. But I am facing some problem with this. What I learnt ...
3
votes
1answer
168 views

Yang-Mills constraints and Poisson brackets

Let's have constraints for Yang-Mills theory: $$ \varphi_{a} = \partial_{i}\pi^{i}_{a} - f_{abc}\pi^{b}_{i}A^{c}_{i}. $$ I have read the statement that $$ \tag 1 [\varphi_{a}(\mathbf x), \varphi_{b}(\...
1
vote
1answer
1k views

Energy Dispersion in Young's Double Slit Experiment

In Young's double slit experiment, when you see the diffraction pattern, why does the intensity of the light fade out as you move from the central maximum? I think it has something to do with the wave-...
3
votes
1answer
164 views

How do door-less refrigerators keep cool air inside?

I got curious when I saw a door-less refrigerator for the first time. It was in a shop and I couldn't control myself so I began to look around to see how they work but I couldn't figure it out. They ...
7
votes
1answer
4k views

Poincaré maps and interpretation

What are Poincaré maps and how to understand them? Wikipedia says: In mathematics, particularly in dynamical systems, a first recurrence map or Poincaré map, named after Henri Poincaré, is ...

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