3
votes
1answer
606 views

Non-relativistic limit of complex scalar field

In page 42 of David Tong's lectures on Quantum Field Theory, he says that one can also derive the Schrödinger Lagrangian by taking the non-relativistic limit of the (complex?) scalar field Lagrangian. ...
4
votes
1answer
292 views

Identifying the position of the moon

I encountered the following question in a previous year paper of a graduation-based test. On a certain night the moon in its waning phase was a half-moon. At midnight the moon will be (choose one ...
0
votes
1answer
460 views

Gauss's / Divergence theorem in Classical electrodynamics for the Electric field [duplicate]

Can somebody explain the proof of Gauss's theorem / divergence theorem taking the vector as electric field $$\iiint(\nabla\cdot\vec E)\mbox{ d} V=\iint \vec E \cdot\hat{n} \mbox{ d} A?$...
0
votes
2answers
831 views

Gauss's (Divergence) theorem in Classical Electrodynamics

How does divergence theorem holds good for electric field. How does this hold true- $$\iiint\limits_{\mathcal{V}} (\vec{\nabla}\cdot\vec{E})\ \mbox{d}V=\mathop{{\int\!\!\!\!\!\int}\mkern-21mu \...
2
votes
1answer
286 views

Conformal dimensions of the energy-momentum tensor

Currently I am reading the Di Francesco-Mathieu-Sénéchal textbook on conformal field theory. Above the equation 5.52, the author argues that the EM tensor should have scaling dimension 2 and spin 2. ...
-1
votes
1answer
173 views

Physical interpretation of $\iiint (∇\cdot\vec E)\mbox{d} V$ [duplicate]

Can anybody explain the physical interpretation of Gauss's law $$\iiint (\nabla\cdot \vec E)~\mbox{d}V~=~\frac{Q}{\epsilon_0}? $$ Also, how is the differential form of Gauss's law obtained from the ...
15
votes
3answers
5k views

Why do we use operators in quantum mechanics?

In classical mechanics, physical quantities, such as, e.g. the coordinates of position, velocity, momentum, energy, etc, are real numbers, but in quantum mechanics they become operators. Why is this ...
-1
votes
1answer
189 views

Explanation for the minus sign in $\Omega_3$ in the Kappa symmetry of the Green - Schwarz formalism for F1 strings

Just so that there can be more higher - level physics questions here, let me post this question + answer. Also because I'm a bit sad that there are almost no questions on the Green-Schwarz formalism....
6
votes
1answer
179 views

Renormalizability of the Polyakov Action

I was told today that the Polyakov action for a $p$-brane is (superficially) re-normalizable iff $p\leq 1$. Of course, when I went to check for myself, I screwed up my power-counting, and I'm having ...
0
votes
2answers
199 views

Is it possible to deduce the Archimedes' law of the lever using only the laws of conservation of the physics?

Is it possible to deduce the Archimedes' law of the lever using only the laws of conservation of the classical mechanics? I never saw (which is strange), but I think that it's possible.
0
votes
1answer
306 views

Drawing the circuit from a differential equation

Can you please help me in modelling a circuit using the differential equation? In the following equation, $u(t)$ is the input voltage and $y(t)$ is the output voltage. $$y(t)=2u(t)+3\frac{du(t)}{dt}+...
0
votes
1answer
331 views

How possible is liberal human flight using a flying superconductor bed?

I saw a video by Michio Kaku, showcasing levitation of a small object with a superconductor cooled by liquid nitrogen. It's the same technology used by some trains. Hypothetically, how possible is it ...
4
votes
2answers
138 views

What happens to gravitational waves after arbitrarily long propagation?

Given that some systems may radiate energy in the form of gravity waves, and that gravitational waves weaken proportionally to the distance travelled, what would happen to the waves that never hit ...
0
votes
0answers
145 views

Where to find information about rocket launches (speed, height, trajectory)?

I want to find experimental data from real launches, with some description of a rocket like full mass, fuel mass, thrust and so on. Just trying to model it. Where to read about precise methods of ...
3
votes
1answer
164 views

Hypervelocity black holes

Is it possible for a black hole to be ejected from a galaxy after an encounter with the central supermassive black hole? What would be the impact of the passage of the hyperV-BH through the galaxy? ...
8
votes
1answer
582 views

How does entanglement work independent of time?

A recent experiment shows that it is possible to entangle two particles that never co-existed in time. Time line diagram. (I) Birth of photons 1 and 2, (II) detection of photon 1, (III) birth of ...
0
votes
2answers
129 views

What would a person *unmoving* look like on a Minkowski diagram?

If you were to draw a person on a Minkowski diagram, that was unmoving (though time was still passing) what would that look like? (As a light cone and ignore the observer.) Also what would it look ...
3
votes
2answers
335 views

Thermalisation - Open quantum systems

I would like to understand better a phenomenon of a quantum heat bath. Below I present one example, which seems quite clear to me. It would be great to see some less-discrete models, and more ...
8
votes
2answers
3k views

Why can't I just think the spin as rotating?

I'm going mad about the problem. I really don't understand why do electron have 1/2 spin number, why they are not actually spinning. I can accept that the electrons have their own magnetic field, ...
0
votes
0answers
375 views

Resistance of circle of wire

Today I was wondering about this situation. Let's imagine I have a circle of wire with a homogeneous section. I connect the wire with 2 terminals from ohmmeter. What minimum and maximum resistance can ...
7
votes
3answers
2k views

Speed of light in a gravitational field?

How do I solve the speed of light in gravitational field? Should I just add gravitational acceleration in speed of light? $$c'=c_0+g(r)t~?$$
3
votes
1answer
712 views

Time reversal and the solutions to Maxwell equations

The standard Lienard-Wiechert potentials describe the electromagnetic field at a point $P$ at time $t$ due to an arbitrarily moving charge $q$ at the retarded time $t-r/c$. An electromagnetic ...
2
votes
1answer
145 views

Can micro black holes form in the Sun?

Wikipedia writes to say ...
6
votes
1answer
372 views

How do non-transverse photon polarizations cancel in Euclidean QED?

First, recall how to write scattering amplitudes in covariant fashion in Minkowskian QED. One starts by considering some process with an external photon whose momentum is chosen to be $k^\mu=(k,0,0,k)...
4
votes
1answer
322 views

Some questions about the paper, “AdS description of induced higher spin gauge theory”

I am referring to this paper. I guess that in this paper one is trying to relate the massless spin $s$ gauge fields in $AdS_4$ to conformal spin $s$ theory on $S^3$. So am I right that the ...
2
votes
0answers
132 views

Is time ordering defined for a single operator depending of two time variables?

The time ordering for the purpose of quantum mechanics is e.g. given by $${\mathcal T} \left[A(x) B(y)\right] := \begin{matrix} A(x) B(y) & \textrm{ if } & x_0 > y_0 \\ \pm B(y)A(x) & \...
5
votes
1answer
131 views

Visualizing irreps of SU(N)

What physical system can one use as an example while considering irreps of SU(N)? What is the correspondence between the system and the irreps?
2
votes
1answer
561 views

Path integral & Gaussian integration

The following is from Ref. 1. Given the (Euclidean) action for a particle ($q$) coupled to a bath of harmonic oscillators $q_\alpha$. Goal is to find an effective action for the particle, e.g ...
1
vote
3answers
646 views

Free neutrons pass through most materials but how do they damage materials?

Neutrons has no charge so they are hard to interact with materials. That's what I'm thinking. But they collide with atoms and it causes nuclear fission. And they can also be a dangerous radioactive ...
2
votes
2answers
294 views

Does the total mass of an isolated object include the mass stored in its gravitational field?

Since neither the object nor its field could exist without the other, it would seem strange not to include the field energy as part of the object. But how exactly does the accounting go? How is the ...
3
votes
0answers
124 views

Optimality of product input state in quantum channel

Let $\mathcal N^{A_1\rightarrow B_1}_1,..,\mathcal N^{A_1\rightarrow B_1}_k$ be a set of valid quantum evolutions with equal input and output dimensions. And let the effect of a channel on a system $\...
1
vote
1answer
124 views

Relativity question: frequency shift under constant acceleration

Okay, my buddy tells me this: Let there be a starship, ovoid, and me and my buddy stand each at the extremities of the ship, him below the roof, me on the floor. We start a journey and the ship ...
7
votes
2answers
595 views

“Correlation energy” using the pair correlation function

In this paper on the Quantum Hall effect the authors refer to something called the correlation energy of electrons. It is defined at the top of page 5 as $E=\frac{n}{2}\int (g(r)-1)V(r)dA\ ,$ where $...
3
votes
2answers
165 views

Questions about gravitational and inertial mass [duplicate]

What differences between gravitational mass and inertial mass? I cannot tell the differences between them. In history, which concept was put forward firstly? Are there some experiments to prove they ...
4
votes
2answers
155 views

Strong response to UV radiation despite severe attenuation

Recently, I performed experiments to characterise the ultraviolet-A response in a smartphone camera (with the lens still attached). This question focuses on Figure 2 of my paper "Characterization of a ...
12
votes
2answers
5k views

Weak force: attractive or repulsive?

We are always told that there are the four fundamental forces or interactions of nature: gravitation, electromagnetism, and the weak and strong forces. We know that gravitation is attractive, that ...
0
votes
1answer
841 views

Engineering Mechanics (Dynamics) Problem [closed]

Cylinder A has a constant downward speed of 1 m/s. compute the velocity of cylinder B for θ= 30 and θ= 45. The spring is in tension throughout the motion range of interest, and the pulleys are ...
6
votes
4answers
2k views

Explaining Newton's Laws of motion to a 6 year old

An old professor of mine once said that an effective means to get people interested in Physics is to get them started early. What would be an effective and meaningful (and fun) means to explain ...
-1
votes
1answer
278 views

What is weak interaction? I need easy and short answer

What is weak interaction? I need easy and short answer. I can't understand the definition of weak force. Why is it so difficult? Bosons, Mesons, Fermions, parity etc. Why the fundamental force is ...
3
votes
3answers
865 views

How does the gravity of a massive non-spherical object act on things around it?

Firstly, not sure if this question ought to be in the space SE site. Please let me know if it should. (Posted in both for now) Secondly, I don't know a whole lot about physics (I'm just inquisitive). ...
2
votes
2answers
97 views

Can contemporary technology simulate the spectrum of a star?

The ability to manipulate light spectra (as in mercury lamps, sodium lamps, infra red) is within the ambit of contemporary technology. Stars are said to be unique in the spectrum they emit ; http://...
3
votes
2answers
1k views

Example of inviscid flow with drag

This was my engineering viva question and I couldn't think of any example, for I am totally convinced that there cannot be drag in a inviscid flow. It also bothers me whether it is possible, a ...
1
vote
1answer
197 views

Extremizing a Hamilton-Jacobi Equation

How can one make sense of the idea of extremizing a Hamilton-Jacobi equation? In Schrödinger's paper "Quantisation as a Problem of Proper Values I" (Annalen der Physik (4), vol. 79, 1926, p. 1, ...
4
votes
1answer
191 views

LSZ theorem for photons

Is there any discussion on what the LSZ theorem for photons looks like? I would rather like a discussion of this in axial gauge $A_0 = 0$ (instead of the usual Lorenz gauge), but anything would do.
18
votes
2answers
2k views

What is the difference between the Balmer series of hydrogen and deuterium?

In my quantum mechanics textbook, it claims that the Balmer series between hydrogen and deuterium is different. However, I was under the impression that the Balmer series $$H_\alpha, H_\beta, H_\...
3
votes
2answers
265 views

“Complex Variables Method” in Diff. Eq. - Justification and physical meaning?

A common method of simplifying calculations that involve differential equations - particularly involving oscillation - is to replace $\cos(\theta)$ with $e^{i \omega t}$, evaluate, and then take the ...
2
votes
2answers
663 views

Showing Wien's Displacement Law from Wien's Law

Does anyone know how I would show that $\lambda * T$ is constant, using only Wien's Law? That $\rho(\lambda,T) = 1/\lambda^5*f(\lambda T)$ I differentiated, but all I could get was $\lambda T = 5f(\...
0
votes
1answer
470 views

Spectral Distribution Function Related to Energy Density

I'm a little confused as to why the Energy Density of a black body is related, in particular by $4/C$, to the spectral distribution function. One is per area, while the other is per volume, I'm lost. ...
2
votes
3answers
329 views

Proper Time for Deceleration in Special Relativity

As stated here: http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html A rocket which accelerates to some point, then decelerates back to 0 over the same distance will measure a proper time ...
2
votes
1answer
739 views

What is the speed of a neutral pion and how is it measured?

Alvager et al 1964 purportedly disproved Ritz's emitter theory in an experiment that generated neutral pions with a vellocity of $v =B.c$ where B is slightly smaller than one. How is the velocity of ...

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