All Questions

1
vote
1answer
475 views

Covariance of the Dirac equation and Infinitesimal Lorentz transformation

Okay, I've got really desperate now. I've spend 10 hours of work (for a few days) trying to prove 'trivial' equation in Quantum Electrodynamics. To anybody who want to write an answer for my questions ...
3
votes
1answer
123 views

Can the Solar System be assumed a single body concentrated in the Sun?

This question springs from a comment against my question posted on the Space SE My questions may seem inane, or obvious to most of you real physics people too ... Any number of sources put the peg ...
0
votes
1answer
126 views

Explicit solutions to 2-d Dirac Equation

The 2-d Dirac equation without any constants is represented usually as $$i*dt (\phi) = D (\phi)$$ where $D = m\sigma_2-i\sigma_1dx-i\sigma_3dy$. Where can I find explicit closed form solutions to ...
6
votes
1answer
605 views

Wavefront RMS errors, mirror surface roughness and Gaussian beam scattering

What I'm interested is in the scattered power of Gaussian beams reflected from mirrors with a given surface RMS. Usually the surface RMS $\sigma_{s}$ of a mirror translates in an error for the ...
1
vote
1answer
222 views

Classical Rutherford scattering (partial) derivation

I am having trouble answering the following question, please could you help! Thank you in advance for any assistance you can give. Consider classical Rutherford scattering of a particle with mass $m$ ...
0
votes
3answers
105 views

Please explain me this relativistic occurrence [duplicate]

So, for example, we have the planet, a spaceship (cylinder shaped) and a ball in it (spaceship is like transparent so we can see the ball). We are on the planet looking at the ship. The spaceship ...
8
votes
0answers
155 views

Gauge fields in Polyakov's treatment of renormalization for nonlinear sigma model

I am deriving the results of renormalization for nonlinear sigma model using Polyakov approach. I am closely following chapter 2 of Polyakov's book--- ``Gauge fields and strings''. Action for the ...
0
votes
1answer
174 views

Virial Theorem and the Energy in a Gas

I clearly am interpreting the Virial Theorem incorrectly, but I don't know how. In dipole gases, the molecules can exhibit five kinetic modes, while they can only experience 2 potential modes. Doesn't ...
3
votes
2answers
317 views

Water level difference by pumping [closed]

Open tank is partially filled with water and is divided into two equal parts by the wall. The wall has pipe that connects both sections. Obviously, water levels are equal from both sides (Fig.1). ...
2
votes
1answer
104 views

Key difference between 3D and 4D solutions

In many papers in theoretical physics especially in the more advanced theories, I notice that physicists solve problems in 3 dimensions (2 spatial+ 1 temporal). In some specific papers (I can't seem ...
2
votes
1answer
121 views

Proper acceleration asymetry in twin paradox

In the Wikipedia article on the twin pararadox, there is an interesting chapter which calculates the difference of age for the twins, with steps of accelerated movement, and steps with constant speed, ...
17
votes
1answer
229 views

What are the equations of motion of a hole in a soap bubble?

Imagine the following situation: I have a thin stationary water film, like a soap bubble, suspended inside a large ring. I throw a small loop of string onto the film and punch a hole inside it. How ...
8
votes
1answer
164 views

Which way do black hole jets spin?

The centers of black holes and quasars often have jets coming out the two poles of an accretion disk, say north and south. Is it known if the two jets spin in the same direction or opposite ...
2
votes
1answer
284 views

What is the geometry behind special relativity?

Many books on special relativity eventually mention that the geometry of spacetime is special because the metric has a signature $(-,+,+,+)$ which is non-Euclidean. I have encountered many ways this ...
3
votes
1answer
89 views

Nonlinear waves and shock formation

In the cases of nonlinear acoustics, why is shock formation unlikely when the dispersion is strong when compared to the nonlinearity of the wave?
6
votes
2answers
942 views

Galaxy Spectra: Emission and Absorption Lines

Spectra from galaxies include both absorption and emission lines. I do understand how both types of spectral lines are produced but I am not quite sure where each type is coming from when we observe a ...
1
vote
1answer
83 views

While finding the equivalent resistance why isn't the 8 ohm in series with the 5 ohm?

Is there any way to tell if the circuit below is in series or parallel quickly and accurately?
4
votes
2answers
364 views

The statistical nature of the 2nd Law of Thermodynamics

Ok, so entropy increases... This is supposed to be an absolute statement about entropy. But then someone imagines a box with a 10 particle gas, and finds that every now and then all particles are in ...
4
votes
1answer
388 views

How many oxygen molecules touch you in your lifetime?

Following up on a comment by BlueRaja to this beautiful answer of Ilmari Karonen, I would like to phrase this follow up question: How many air molecules hit your average human's skin during their ...
3
votes
3answers
385 views

Is a spinning object heavier than a non-spinning object?

Does spinning an object make it heavier? A real-world example: I was mowing the lawn in front of my house, a lawn that tends to have some steep inclines. I realized that the lawn-mower was easier ...
0
votes
1answer
1k views

How does (kg/m^3) * (m/s^2) * (m) come out to be units of N/m^2? [closed]

To me it seems to come out to be kg/(m*s^2). Is this somehow equivalent to N/m^2?
0
votes
1answer
91 views

Cylindrical charge distribution [closed]

I'll just list the problem first and then explain what I need help with. Consider a cylindrical charge distribution extending from $r = 0$ to $r = 8.2 $cm of charge density $\rho = r/a_0$, where ...
3
votes
2answers
150 views

Proper Time with a non-zero Initial Velocity

I have a question regarding how to find the proper time for a body with an initial velocity to slow down to 0. For example, the equation I have been working with looks like: $$\int^\tau_0 ...
0
votes
0answers
130 views

Please clarify how freezer size and load affects rate of freezing

I was discussing how to make a freezer slow freeze food with somebody and asked them a question, they then gave me the below reply however I don't understand it properly. Can somebody please explain ...
-1
votes
1answer
123 views

Investigation of a pendulum's period, problem creating equation to sum the dynamic velocity

I am investigating the period of a pendulum swing. This is a simple harmonic pendulum and I am already aware of the common, but slightly inaccurate, $2\pi \sqrt{\frac{L}{G}}$ formula. My problem is ...
2
votes
2answers
119 views

How do we deal with irrationals in Physics? [duplicate]

This question is really very basic: how do we deal with irrationals in Physics? If for instance, in some meaningful calculation we get a length of $\pi$ meters, or a force of $\pi$ Newtons, how should ...
0
votes
2answers
845 views

Pressure and height of water in a pipe [closed]

In a building, at what height does water rise from the ground-floor in a pipe, if the pressure there measures P=3atm? The correct answer is 31m. What formula should I apply?
1
vote
1answer
69 views

Couple Masses - Change in Basis

I'm having trouble with the linear algebra used to solved a coupled mass problem. $\ddot{x}_1 = -(2k/m)x_1 + (k/m)x_2$ and $\ddot{x}_2 = (k/m)x_1 - (2k/m)x_2$ Shankar then sets the equation up in ...
1
vote
1answer
205 views

Spinor formalism in QFT

We can describe fields by two formalisms: vector and spinor. This is the result of possibility of representation of the Lorentz's group irreducible rep as straight cross product of two $SU(2)$ or two ...
2
votes
1answer
174 views

Precision and accuracy calculations

Precision is usually understood as the number or significant figures in some experiment. Accuracy is the difference between the best measurement and the real value. How are precision and accuracy ...
9
votes
2answers
326 views

Mixing of quarks, neutrinos… and leptons?

This is a quite simple question: quarks do mix (through the CKM matrix), neutrinos do mix (through the PMNS matrix). Then why do charged leptons not mix?
3
votes
1answer
366 views

Using Wien's Law to show spectral distruibution function of one temperature represents all temperatures

This is a exercise question from Quantum Mechanic textbook by Bransden: Using Wien's Law to show that if the spectral distribution function of black body radiation, $\rho(\lambda,T)$ is known at ...
1
vote
2answers
85 views

How are propositions concerning spacetime curvature constructed explicitly in terms of coincidences?

Is Einstein's insight [1] that All our well-substantiated space-time propositions amount to the determination of space-time coincidences [such as] encounters between two or more [...] material ...
5
votes
2answers
294 views

Is a total solar eclipse a common phenomenon?

Is our moon "special" for creating a total solar eclipse? Is it common or rather rare for a moon to create a total solar eclipse the way our moon does, with corona and such?
4
votes
2answers
390 views

What is the definition of a quantum integrable model?

What is the definition of a quantum integrable model? To be specific: given a quantum Hamiltonian, what makes it integrable?
1
vote
0answers
62 views

Maximise magnetic force at one point while minimizing at another

Is it possible to have a magnet that attracts one object strongly, but not another object behind that first object? Given an (electro-) magnet M above two thin parallel steel sheets P1 and P2 in the ...
2
votes
4answers
200 views

Expansion of the universe and strain

From cosmological models that involve expansion of the universe, can we not say that there are ever increasing tidal forces felt by solid bodies? If so, the material in solid bodies like metal ...
2
votes
1answer
191 views

Why is the value of thrust for a perfectly spherical event equal to ${\frac{1}{2}}$?

In a particle collision the thrust is defined by: $$ T = max_{\hat{n}} \frac {\sum_i \left| \hat{p}_i . \hat{n}_i \right|}{\sum_i \left| \hat{p}_i \right|} $$ where $\hat{n}$ is the unit vector that ...
2
votes
3answers
122 views

Question about the Dirac notation for partial trace

I saw the following definition for the partial trace operator: $\rho_A=\sum_k \langle e_k|\rho_{AB}|e_k\rangle$, where $e_k$ is basis for the state space of system $B$. From what I know, in the ...
6
votes
2answers
281 views

Holographic principle “inside-out view”

From the perspective inside a black hole: Is information about everything outside a black hole - the rest of the cosmos - represented on the inside of the (event) horizon too? NB. I realize it is the ...
1
vote
1answer
255 views

Aerofoil Theory Project

I'm doing a project (dissertation) on the mathematics of Aerofoil Theory. I wonder if I could get some advice on a possible structure. I'm new to fluid dynamics, so it's quite hard to know where to ...
2
votes
1answer
507 views

An objective question from electromagnetic induction

A bar magnet is released from rest along the axis of a very long, vertical copper tube. After some the the magnet : (a) will stop in the tube. (b) will move with almost constant speed. ...
6
votes
2answers
474 views

Vector Potential for Magnetic field when the field is not in simply-connected region

According to Poincare's Lemma, if $U\subset \mathbb{R}^n$ is a star-shaped set and if $\omega$ is a $k$-form defined in $U$ that is closed, then $\omega$ is exact, meaning that there's some ...
2
votes
0answers
45 views

Impact of friction on a fallen body [closed]

If a body travelling at speed falls to the ground through an arc from a certain height (someone walking who trips, for example), how does friction of the ground surface affect the forces impacting ...
2
votes
2answers
189 views

How would one show that a nonabelian field strength tensor transforms in a certain way under a local gauge transformation?

How would one show that the nonabelian ${F_{\mu\nu}}$ field strength tensor transforms as ${F_{\mu\nu}\to F_{\mu\nu}^{\prime}=UF_{\mu\nu}U^{-1}}$ under a local gauge transformation? Rather than going ...
1
vote
1answer
287 views

How widespread is the meme “QM is counterintuitive” in academic physics? [closed]

I have recently entered university — studying CS — and I have spoken to many physics students on campus. Most of these — when propmted — will gladly proclaim that QM is ...
2
votes
1answer
73 views

Increasing the maximum angle of laser scanners with optics?

With laser scanning devices, very roughly speaking, the maximum deflection angle seems to be inversly proportional to speed. So for example, while acousto-optic deflectors can steer the beam very ...
20
votes
3answers
821 views

Are the air particles in today's wind on earth (more or less) the same as the air 2/3 billion years ago?

So the air pressure on earth remains relatively constant, right? Is there air gained or lost through transitions of any sort? e.g. plumes going out into space, earth gaining new air particles with ...
3
votes
1answer
146 views

Index raising and lowering - how does it work?

In the context of four-dimensional spacetime, how does the metric turn a tangent vector into a gradient, and vice versa? By this I mean that I know the metric can be used to raise and lower indices: ...
0
votes
1answer
162 views

Symmetry groups [closed]

I am quite new to this subject. I am just repeating in a few words, what I have learned so far: There are 4 fundamental forces of nature: strong, weak, electromagnetism and gravity. Physicists are ...

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