0
votes
0answers
21 views

Angular and luminosity distance in general?

Consider a non-Friedmannian Universe in which we know the trajectories of photons, ie in which we know null geodesics $\left(\eta, x^{1}, x^{2}, x^{3}, a, z\right)$ where : $\eta$ is the conformal ...
0
votes
1answer
103 views

Photoelectric Effect Problem

Say in a photoelectric experiment, we find a stopping potential of 1.85V for $\lambda=3000\overset{\circ}{A}$ and of 0.82V for $\lambda = 4000 \overset{\circ}{A}$. How can I get the Planck's ...
4
votes
1answer
175 views

Clebsch-Gordan Coefficients for two spin-1 particles - Why is there a ∣0⟩∣0⟩ ket?

I have used the rules for addition of angular momenta to work out the Clebsch-Gordan coefficients, which all seem right except for state $\lvert0,0\rangle$: For n = 1 \begin{align} \lvert1,1\rangle ...
4
votes
2answers
1k views

What is the symbol Å?

I saw this symbol like: $$\lambda=3000\overset{\circ}{\text{A}}$$ and I don't know what this means. Is it a frequency? (since $\lambda$ is usually used for frequency)
1
vote
0answers
39 views

Optical Spectra and the Diffraction Grating

I'm doing a lab where we are identifying different gases by their diffraction spectra. This what my lab book says: Verify and calibrate your spectrometer, by measuring Hg and Ne. Measure ...
3
votes
1answer
205 views

Why are there gapless excitations in the anti-ferromagnetic Heisenberg model while the true ground state is a singlet?

The true ground state of the anti ferromagnetic quantum Heisenberg Model (nearest neighbor only)is known to be a singlet (I think this is Liebs theorem.) Since a singlet is invariant under rotations, ...
0
votes
2answers
138 views

Why does the alcubierre drive get roasted by hawking radiation?

I read that an alcubierre drive would get destroyed by hawking radiation inside the warp bubble. Where does this hawking radiation come from? And why does it only happen to objects inside the bubble? ...
1
vote
0answers
35 views

What is energy? [duplicate]

What exactly is energy (in terms a non-physicist/non-mathematician can understand)? I've been researching this topic, and it seems that energy is not a real physical state, i.e. it is only a scalar ...
1
vote
2answers
165 views

How to determine a strong or weak interaction (Strange particles)?

As you know $\pi^-$ meson + proton ---> $K^+$ meson + $\Sigma^-$ particle. (AntiUp,down) + (up up down) --> (up antistrange) + (down down strange) I know that the quark number has be conserved in ...
1
vote
1answer
98 views

Is linear momentum conserved in a system with open boundary conditions?

In a one dimensional lattice system with periodic boundary conditions, in which the last and the first site of the lattice are the same site, momentum is conserved modulo a vector of the reciprocal ...
3
votes
2answers
121 views

Electromagnetic current for interaction with Dirac spinors

The covariant form of the Dirac equation is given by $$(i\gamma^{\mu}\partial_{\mu} - M) \Psi(x) = 0 $$ Einstein's summation is implied here, $x=(x^0,x^1,x^2,x^3)^T$. I am simply looking for the ...
1
vote
1answer
101 views

Fluid mechanics of aircraft at 10000 m

I have come up with the answers to parts a and b as shown in the image above. However in part c, I can't seem to find the answer, even though it only involves fitting numbers into Bernoulli's ...
5
votes
2answers
482 views

What are the technological challenges to building a muon collider?

There is a strong physics case for building and running a muon collider (primarily as a Higgs factory). Currently, what is the principal technological hold-up in making the muon collider possible?
1
vote
0answers
41 views

Any photon colliders in the past?

People have been thinking about a photon-photon collider (see this and this) as an add-on or to supplement the ILC, the next generation linear collider. My question is, have there been any ...
0
votes
1answer
63 views

Electric field energy

Is it true that $\int_V \rho \phi $ is the equation for the energy of a charge distribution in an external potential and $\frac{1}{2} \int_V \rho \phi$ is the equation for the energy of a charge ...
1
vote
2answers
80 views

Does the ice melt faster when breaks free from larger sheet and drops into Antarctic Ocean waters?

My question is, when Ice is intact with the large sheet and exposed only to air, it melts at some pace. But does it melt faster or slower when the big piece breaks free from the sheet and drops into ...
0
votes
1answer
299 views

Model the gyroscopic effect for 3axes stabilisation

Context I am trying to stabilise a platform on a guimbal using 3 axes gyroscopes. I have made tons of research (same problem for weeks even months now) without being able to properly model the 3 ...
4
votes
2answers
226 views

Ten-ping bowling: Can a ping pong ball knock over a bowling pin?

In this video we see a rather unorthodox bowling technique on display. The gentleman appears to knock over all of the pins using only a ping pong ball. It's a fake, of course: you could never knock ...
2
votes
1answer
259 views

Why the orbital angular momentum equal zero for electron in s state? does it mean that the electron doesn't orbiting in s [duplicate]

Why is the orbital angular momentum $l$ equal to zero for electrons in the $s$ state? Does it mean that the electrons aren't actually orbiting?
0
votes
3answers
94 views

Why does a $1/2$ appear in this simple capacitor equation?

The maximum energy stored by a capacitor is when it's (theoretically) fully charged or just before it reaches dielectric breakdown, if you will. It seems, then, that the maximum energy ($E$) $=QV$ ...
0
votes
2answers
71 views

Why only vertical component of the stress tensor on vertically suspended bar?

EDIT: I am gonna rephrase the question entirely. Imagine we have a bar which we will analyze in the linear elastic regime. The shape of the cross section is irrelevant. The bar is suspended from a ...
1
vote
1answer
132 views

I'm getting weird autocorrelations when simulating an Ising model below the critical temperature

So I'm simulating an Ising model using Monte Carlo and the Metropolis algorithm. After letting it reach equilibrium, I try to calculate the autocorrelation of the magnetization. As long as the system ...
4
votes
0answers
61 views

Dark Matter and Modified Gravity

Could someone please explain briefly or refer me to an article or manuscript that shows how f(R) modified gravity theories can be used to explain the problem of Dark Matter, particularly Galaxy ...
0
votes
1answer
67 views

relation between photon number and energy

Suppose there are two light beams. One is red while the other is violet. The energy of both is the same. Which one of these beams has a larger number of photons, or is the number of photons relevant? ...
0
votes
1answer
30 views

Voyager Golden Record

Has there been any subsequent researches or any respond from deep outer space in any type of form with these images and sounds?
1
vote
1answer
112 views

A formula in Sung-Sik Lee's paper

I want to ask if anyone has gone through the derivation of the second equality in the following formula which comes from http://journals.aps.org/prb/abstract/10.1103/PhysRevB.80.165102.
8
votes
1answer
70 views

Why are no CMB experiments able to measure the whole range of multipole moments?

Have a look at the following diagram: This shows measurements of the CMB by various experiments, with multipole moment $l$ on the x-axis and the temperature of the corresponding moment on the ...
1
vote
1answer
70 views

Projectile trajectory with linear air resistance [closed]

I have the following equations $$\ x′′(t)=−\frac km x′(t)$$ $$\ y′′(t)=−\frac km y′(t) - g$$ where $k$ is the drag, $m$ is the mass of the object and $t$ is the time. $g$ is the gravity constant. ...
1
vote
0answers
80 views

Big Bang, Heat Death, and cause and effect

If the Universe has two 'end points', one being the Big Bang, and the other being heat death, is there anything in the laws of physics which forbid a random fluctuation in the heat death state from ...
1
vote
2answers
168 views

Why neglect friction when computing torque?

There is a block of wood on a plank, with friction, and the block is moving down with uniform speed. They are asking what is the torque acting through the center of the block. My doubt is that ...
12
votes
2answers
435 views

Noether's theorem and gauge symmetry

I'm confused about Noether's theorem applied to gauge symmetry. Say we have $$\mathcal L=-\frac14F_{ab}F^{ab}.$$ Then it's invariant under $A_a\rightarrow A_a+\partial_a\Lambda.$ But can I say that ...
1
vote
0answers
40 views

What happens when a polarized light get reflected by a rough dielectric material?

I have a polarized light (lets say p polarized) which happens to undergo scattering when it is obstructed by a rough opaque dielectric material. What will happen to polarization? Will some part of it ...
0
votes
0answers
127 views

Magnetic field or EM field around wire with electric current?

My question is based on the experiment where iron filings scattered around a piece of paper with a wire carrying an electric current poking through shows magnetic field lines. If you start to vary ...
3
votes
1answer
119 views

Taylor expansion of the metric

Consider a coordinate change $$ x^a\mapsto \tilde x^a=x^a+\epsilon y^a $$ In the note I am reading, the author calculate the change of metric by $$ g_{ab}(x) = \tilde g_{ab}(\tilde x)=\tilde ...
3
votes
2answers
171 views

What is the first non-vanishing multipole moment of this configuration?

Imagine that you have a triangle where each side has the length $a$ and a charge $q$ sitting at every vertex. Additionally, we have a charge $-3q$ sitting in the center of the triangle. What is the ...
3
votes
0answers
41 views

Recommendation: Advanced topics in quantum field theory [duplicate]

I have read Srednicki's Quantum Field Theory book. I want to learn more about advanced topics in field theory, such as geometry and topology in field theory, topology defect, anomaly, soliton, ...
5
votes
2answers
347 views

Confusion about Lie derivative on metric

According to this site, the Lie derivative of a $(0,2)$-tensor is $$ \mathcal{L}_XT_{ab}=\partial_XT_{ab}+T_{cb}\partial_aX^c+T_{ac}\partial_bX^c $$ However, according the same website, the Lie ...
2
votes
1answer
74 views

Synchronized rotation of the moon

Does the moon rotates? Yes. The rotation matches exactly the orbit of the Earth. Which means in 28 days the moon makes one rotation. Shouldn't this be also happening with the Earth rotation around the ...
24
votes
5answers
4k views

Artificial gravity on rotating spaceship?

One of the possible ways to simulate gravity in outer space is to have a rotating spaceship, so that the centrifugal force experienced provides a gravity-like force. My question is: shouldn't this ...
3
votes
2answers
487 views

Flight time Toronto to Moscow the same [duplicate]

I have a question. How come that the flight from Toronto to Moscow takes the same time as the flight from Moscow to Toronto. Should it take much longer because of the earth rotation? One direction we ...
3
votes
2answers
253 views

Domain walls intersection

I was reading this article(On domain shapes and processes in supersymmetric theories). In the paragraph about domain walls intersection (paragraph $4$, page $7$) the authors say: In a one-field ...
0
votes
3answers
282 views

Charge On parallel plate capacitor?

Suppose if we supply two plates of parallel plate capacitor with charges $Q_1$ and $Q_2$.What would be the charge of the capacitor? As the field inside plate should be zero and hence using gauss law I ...
6
votes
3answers
212 views

$E=mc^2$ resembles kinetic energy formula?

The simplest equation expressing mass–energy equivalence is the famous $E=mc^2$ where $c$ represents the speed of light. Compare this with $E_K = \frac{1}{2}mv^2$. Since $E=mc^2$ can be applied to ...
1
vote
0answers
69 views

How to make a base tranformation for a linear operator in QM? [closed]

I have 2 bases A and B with the following kets: Base A: $|a_1\rangle$ and $|a_2\rangle$ Base B: $|b_1\rangle = \frac{1}{\sqrt2} \cdot(|a_1\rangle + i\cdot|a_2\rangle)$ $|b_2\rangle = ...
18
votes
3answers
3k views

How quickly was the Earth rotating 250 million years ago?

The Earth is slowing at a rate of $4.7\times10^{-4}$ miles per second every 100 years due to tidal forces of the moon. See: http://en.wikipedia.org/wiki/Earth%27s_rotation ...
5
votes
2answers
408 views

Forced harmonic oscillator with two springs

Consider a vertical system of two springs in series, with a mass(50 g) between them. From below the system is driven by a vibration generator. The setup is shown here, but the picture is taken while ...
0
votes
1answer
32 views

Lattice with snowflakes

Trying to understand these better. I'm trying to find a conventional primitive unit mesh and find the planes of symmetry. Now my attempt was simply finding the next points where it repeated itself. ...
3
votes
1answer
129 views

Conservation of total angular momentum in $\Phi$-meson decay

I am looking into the decay of a $\Phi$-meson decaying into $K^+$, $K^-$. My problem is, the $\Phi$-meson has a total angular momentum of 1 and the two Kaons have a total angular momentum of 0. On the ...
6
votes
0answers
106 views

Is the Higgs bare mass larger than the physical mass?

The Higgs boson propagator can be written $$\frac{1}{p^2-m^2+\Sigma(p^2)}$$. If we take $p^2=m_P^2$ the physical mass, we get $m_P^2=m^2-\Sigma(m_P^2)$. Now, if $\Sigma\sim \Lambda^2$, we get ...
2
votes
0answers
85 views

Entangled event horizons

Assuming it is possible in principle to entangle the degrees of freedom of the event horizons of two black holes, and that this is something that can be done, either after the black hole is formed, or ...

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