1
vote
1answer
5 views

Uncertainty of permittivity of vacuum

Question: The value of permittivity of vacuum, $\epsilon_0$, is given with absolutely no uncertainty in NIST Why is this the case? More details: The permeability of vacuum can be given by ...
1
vote
0answers
10 views

Thermal propagator for a scalar field (KMS condition)

I'm having some troubles following the derivation of the scalar field thermal propagator. I'm following the article "Finite Temperature Quantum Field Theory in Minkwoski space" by Niemi and Semenoff ...
-1
votes
0answers
9 views

How to solve this torque problem

The Answer to the Question is 0.15Kg Is there anything wrong with the question or is it just me. Please Help. I tried working backward but didn't work.
1
vote
1answer
11 views

Proton for fission

I am just wondering why is proton not used for nuclear fission? Is it because it is charged, so we will have to input more unnecessary energy to overcome the Coulomb force?
3
votes
1answer
13 views

The origin of the colour of the sun

I have learned so many concepts under astrophysics and unfortunately, I have muddled everything together... Let me try to illustrate my problem: When a star is in main sequence, it fuses hydrogen to ...
2
votes
0answers
14 views

How much of Minkowski spacetime structure can be recovered from its causal structure?

A beginner's question: I have always understood that (four-dimensional) Minkowski spacetime can be recovered up to a constant factor—i.e. 'up to a dilation' or 'up to global scale'—from its causal ...
0
votes
0answers
18 views

Charge by Induction

For example, Initially, there are two metal sphere touching each other and the ball on the left has a charge of +5q, while the other is neutral. Then a rod with charge -3q is brought near the left ...
0
votes
0answers
9 views

How do we know that that the universe expansion is accelerating?

The universe is expanding in all directions. We can detect this by the redshift of electromagnetic radiation from other galaxies. We detect a higher redshift in galaxies which are further away, and ...
0
votes
0answers
10 views

Parity transformation of polarization vectors

In quantum field theory, for vector fields, we can write the following expansion: $$V^\mu = \sum_{p,\lambda} \left[ \epsilon^{\lambda, \mu}\left(p\right) a^\lambda\left(p\right)e^{-ip \cdot x} + ...
1
vote
0answers
28 views

QFT in curved space

Can someone exactly tell me what one gains from doing QFT in curved space, and how reliable these new results are. I want to know if it is worth while putting some man hours towards this. Please ...
1
vote
1answer
23 views

Keep an object at a certain hight, does it require energy? [duplicate]

If i have an object with a defined mass, like a stone, and put it on the shelf it will sit there forever (lets forget all external influences). But if I have to hold up the stone I will eventually ...
2
votes
1answer
38 views

The momentum of a hole

I'm currently working through "A Guide to Feynman Diagrams in the Many-Body Problem" by R.D. Mattuck (self study, not a homework problem) and am stumped by the following problem: "In a system of free ...
0
votes
1answer
17 views

Electric Magnetic potential and Lorentz transform

I have heard that the scalar potential and the magnetic vector potential in the electromagnetic four potential become the four vector by the Lorentz transform. Thereafter, the Lorentz transform leads ...
2
votes
1answer
260 views

Why we see the laser beam?

The laser produces the coherent beam of photons with particular $\mathbf{k}$ and $\mathbf{\omega}$. So, if there is no particular $\mathbf{k}_0$ which directed to our eyes, why we see the laser ...
1
vote
1answer
13 views

Questions on beta-decay

According to my textbook, for all the beta decays, it is required that the mass of the original atom to be heavier than the mass of the final atom. Is this due to the fact that all the beta decays ...
3
votes
0answers
37 views

Why is the Ricci tensor diagonal for isotropic spacetime?

I'm reading Zee's Einstein Gravity in a Nutshell and while calculating the Ricci tensor for FRW spacetime he claims that because the spacelike slices of constant $t$ are rotationally invariant, the ...
1
vote
1answer
14 views

Plane Polar coordinates for simple pendulum in moving lift(elevator)

Can any one help me with the following. A simple pendulum is suspended from the ceiling of a lift. It is moving upward with acceleration $a$. The string of the effective length of the pendulum makes ...
1
vote
1answer
44 views

About quantum mechanics

Can quantum mechanics be partly explained in terms of mathematical filters? Is there a way to explain some of it with matrices on an amateur level?
1
vote
0answers
25 views

Conflict between Lippmann–Schwinger equation and Gell-Mann and Low theorem about energy

Lippmann–Schwinger equation states that scattering state will have the same energy as free state, while Gell-Mann Low theorem says that they have different enery. Lippmann–Schwinger equation says: ...
0
votes
0answers
12 views

Wattage of a motor need to move a load?

How can I calculate the wattage needed for a motor at 48 volts that is directly attached to a wheel with a diameter of 17" to move a 200 pound load that is above the single axle/wheel? Assuming the ...
3
votes
1answer
23 views

woven appearance of water-streams by the side of the street

It's pretty rainy in Boston now, and there are streams of water by the sides of the street. Why do they have a 'woven' appearance? What's the formal name for this kind of fluid flow? With lots of ...
1
vote
2answers
50 views

Question about $E=mc^2$

Is $E=mc^2$ not just $E=m$. What does the speed of light have to do with this other than to give it a really big number so it looks cool? What spectrum of light is used? How can we test the speed of ...
1
vote
0answers
17 views

Imaginary number for extinction coefficient in complex refractive index

In complex refractive index on a material, $n=n'+ ik$, the imaginary part $k$ is physical meaning, as it shows absorption in the material but it is an imaginary. How we measure an imaginary values in ...
0
votes
0answers
9 views

Sphere between two parallel plates [on hold]

I'm stuck on the following physics question; I'm not sure how much, if any, of what I'm doing is right: A small sphere of mass 1.0 × 10−6 kg carries a total charge of 2.0 × 10−8 C. The sphere hangs ...
0
votes
1answer
37 views

Speed=mass=time? [on hold]

I was wondering why objects gain mass with speed? How can we measure speed if there is no truly stationary point to measure from? And if an object comes to a stand still will it loose its mass? ...
1
vote
0answers
14 views

About accelerating particles

Can a particle moving below the speed of light be accelerated more and more until it is travelling at c? IF so does it behave like electro-magnetic radiation?
0
votes
2answers
28 views

Would an Object Near a Pre-Blackhole Star Experience the Same Gravity as Post-Blackhole?

My question was inspired by this question, which got me thinking. According to Newton's Law of Gravitation,$$F = \frac{m_1m_2}{r}$$the gravity of an object is inversely proportional to the distance ...
2
votes
0answers
29 views

Photon Angular Momentum

Essentially I am wanting to evaluate $$\langle j\, m \mid a^\dagger(\mathbf{k}, \lambda) \mid 0 \rangle \,,$$ where $\lambda$ indicates the circular polarization (about $\mathbf{k}$). We have that ...
2
votes
1answer
19 views

Does the unit of Inertia include radians?

The unit for angular acceleration $\alpha$ is: $$\mathrm{rad/s^2}$$ The unit for torque is $\mathrm{Nm}$: $$\mathrm{kg\ m^2/s^2}$$ And their relationship with Inertia is: $$I = \tau/\alpha$$ So ...
1
vote
1answer
37 views

Our universe the surface of a 4-dimensional sphere?

The cosmic microwave background that we observe uniformly around us is usually explained by assuming that our universe is the surface of a four dimensional sphere. That way the uniformity makes sense ...
0
votes
1answer
8 views

Probability distribution of two particle types system

Suppose that particles of two different species, A and B, can be chosen with probability $p_A$ and $p_B$, respectively. What would be the probability (and distribution) $p(N_A;N)$ that $N_A$ out of ...
4
votes
1answer
39 views

Calculating the electrostatic energy per unit length of a cylindrical shell surrounded by a coaxial cable

Suppose an infinitely long cylindrical shell of radius $a$ carries a surface charge density $\sigma_0$ and is surrounded by a coaxial cable of inner radius $b$ and outer radius $c$ with uniform charge ...
1
vote
4answers
36 views

Acceleration is zero, for non-zero net force

I understand a problem like this has already been asked, but I have not found an answer that makes it clear. A force is applied to a box on a table(lets ignore friction), and the box moves with some ...
0
votes
0answers
22 views

Energy in an electromagnetic wave

A radio antenna creates EM waves through switching the polarization in the antenna at a certain frequency. I assume the the energy of the photons produced in this process amount to E=hf for each ...
2
votes
2answers
101 views

2D Gauss law vs residue theorem

I used to have a vague feeling that the residue theorem is a close analogy to 2D electrostatics in which the residues themselves play a role of point charges. However, the equations don't seem to add ...
-4
votes
0answers
19 views

I need help with angular and linear kinematics ASAP! [on hold]

I am writing a paper for kinesiology about the biomechanics of cutting a pizza and one of the sections has to deal with measuring angles and angular and linear kinematics. Is there anyone who can help ...
1
vote
0answers
36 views

Green Function for Proca Equation

I have tried to find a retarded and advanced Green function for Proca field equation. $(\Box - \mu^2)A^{\mu}=J^{\mu}$ where $\mu$ is the mass term. How I did it: first: I made Fourier ...
0
votes
0answers
9 views

Waveguide in TE mode. Power guided [on hold]

I have the following problem and don't know where to begin. Could someone give me a hand with it? A waveguide is formed by two conducting parallel plates spaced apart a distance $d=3cm$, and width ...
6
votes
1answer
69 views

Why doesn't gravity ruin satellites?

Recently I watched a documentary about Io, a moon around Jupiter. Io has volcanic activity even though it is small and can't retain heat well because gravity from Jupiter and a nearby moon cause it to ...
4
votes
0answers
37 views

Intuitively what's the relationship between forces and connections?

In Einstein's General Relativity we relate the effects of gravity with the curvature of the Levi-Civita connection on the spacetime manifold. Also, when we get the electromagnetic tensor $F = dA$ ...
3
votes
2answers
242 views

Where does this term “shell” with prefix “on-/off-” come from?

Is there some historical reasons or is there a specific reason behind it? This question is connected to: Why on-shell vs. off-shell matters?
1
vote
0answers
12 views

Entropy of Reeh-Schlieder correlations

Any state analytic in energy (which includes most physical states since they have bounded energy) contains non-local correlations described by the Reeh-Schlieder theorem in AQFT. It is further shown ...
1
vote
2answers
25 views

water pressure in the oceans or great lakes

Is there a way to create energy from water depth pressure? Convert the pressure to any type of energy? Does there need to be movement? Can someone comment on this process? Thanks
1
vote
0answers
50 views

Rewriting $\langle {\bf k} \vert E,l,m \rangle$ as $\langle {\bf k} \vert ~k,l,m \rangle$ Spherical Harmonics

From Sakurai eq. 6.4.21a we have that $$\langle {\bf k} \vert E,l,m \rangle=\frac{\hbar}{\sqrt{M k}}\delta\left(E-\frac{\hbar^2 k^2 }{2M}\right) Y_l^m({\bf\hat k}),$$ where $M$ is the mass of the ...
0
votes
0answers
30 views

How to derive the schwarzchild metric?

I'm having trouble differentiating the following when making a change of co-ordinates to determine the Schwarzchild metric. $$r'^{2}=r^{2}C(r)$$ Then taking the total derivative of both sides, the ...
0
votes
1answer
28 views

Three-plate capacitor [on hold]

What is the total capacitance of this capacitor? Is it analogous to the case of inserting a plate into a capacitor (essentially creating two capacitors in series)?
1
vote
1answer
40 views

Calculate speed of proton at infinity

I have this problem to solve: Two protons are on the x axis at x=-1 and x=0. An alpha particle is placed at x=2, and the proton on the left is released, find its speed at infinity. I know that this ...
4
votes
2answers
161 views

Does the mass of a star change as it collapses into a black hole?

I know (I think!) that when a really big star collapses on itself it creates a black hole. My question: When a star collapses, is the mass equal to the mass of the star when it's not a black hole? Or ...
0
votes
1answer
49 views

The Spin Connection

Why do we need to introduce the spin connection coefficients $\omega_{\mu \space \space b}^{\space \space a} $ in General Relativity? To me, they just look (mathematically) like the Christoffel ...
0
votes
0answers
25 views

What is the motion of the sun? [on hold]

Well it seems that the we are in a big hurry to get somewhere, But here is a ? for you really bright folks. Describe the motion of the sun along the midpoint of the plane of the galaxy? I suspect it ...

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