0
votes
3answers
2k views

The meaning of scale invariance in power law distribution

A function $f(ax)$ that satisfies $$ f(ax)=a^\Delta f(x)\,\,\, (\Delta \in R) $$ is said to be scale invariant. The most general function $f(x)$ that satisfies the previous condition is of the form ...
3
votes
2answers
2k views

Conservation of angular momentum in helicopter

I have a small RC-controlled toy helicopter with removable tail rotor. Suppose I remove the tail rotor, hold the tail with my hand, start the rotor until it moves with constant angular velocity and ...
5
votes
2answers
342 views

Did anyone claim that quantum theory meant lasers would never work

I've been reading 'How the Hippies saved Physics', which describes a design for a superluminal communication device, of which the crucial part was a laser which duplicated an incoming photon many ...
3
votes
1answer
52k views

Solving for initial velocity required to launch a projectile to a given destination at a different height

I need to calculate the initial velocity required to launch a projectile at a given angle from point A to point B. The only force acting on the projectile after launch will be gravity – zero air ...
1
vote
1answer
354 views

How to determine n equidistant vectors from point P in three dimensions

As an assignment for uni I need to figure out an algorithm that explodes a particle of mass $m$, velocity $v$, into $n$ pieces. For the first part of the assignment, the particle has mass $m$, ...
6
votes
4answers
1k views

What's the distinctions between Yang-Mills theory and QCD?

So Yang-Mills theory is a non-abelian gauge theory, and we used a lot in QCD calculation. But what are the distinctions between Yang-Mills theory and QCD? And distinctions between supersymmetric ...
1
vote
1answer
651 views

speed of sound relative to density of medium through which sound travels

I know that sound travels faster in water compared to air and say faster in steel than in what're so What would the density have to be to cause sound to approach the speed of light
0
votes
1answer
49 views

observation and implied time since creation

I read on a post Big Bang and Cosmic microwave background radiation? We detect light from another 13 billion years ago does this mean that one billion years ago we could only detect light from about ...
-1
votes
1answer
281 views

Will a black hole increase the speed of sound above the speed of light in this medium

For the sake of this question we are inside the EH and a sound wave enters from our perspective as the sound moves closer to us at the EH would it speed up. Specifically how would the extreme ...
2
votes
2answers
589 views

how does heat energy start to speed up a gas molecule?

If it was possible to place a single gas molecule in a cell and freeze it to near absolute zero. What would the molecule do as it thawed out? Would it translate the heat energy into it's electrons ...
1
vote
0answers
379 views

Newton's Law of Gravitation, Gauss Law and GR

From One of My Unpublished Papers $$\frac{d^2 x^{\alpha}}{d\tau^2}=-\Gamma^{\alpha}_{\beta \gamma}\frac{dx^{\beta}}{d\tau}\frac{dx^{\gamma}}{d\tau} \tag{1}$$ For radial motion in Schwarzschild’s ...
5
votes
4answers
521 views

Examples of exact many-body ground state wavefunction

Is there any non-trivial many-body system for which the exact solution to Schrödinger's equation is known? (By non-trivial, I mean a system with particle-particle interactions.) Perhaps something like ...
2
votes
1answer
372 views

How is it possible that we see light from shortly after the big bang?

How can astronomers see light from shortly after the big bang? How did we get "here" before the light that emanated from our "creation"?
2
votes
2answers
259 views

Interaction speed between electric charges and magnetic materials

Einstein said that the speed of a matter in universe cannot exceed the speed of light. Is it correct for electric force transmission speed from one electric charge to other one? What is ...
1
vote
2answers
2k views

What is the difference between an accelerometer and an inclinometer?

When looking through papers for a controls project I found one that seemed to be referencing the fusing of sensor data from an inclinometer and an accelerometer. I always had thought that it was ...
15
votes
1answer
396 views

How is the logarithmic correction to the entropy of a non extremal black hole derived?

I`ve just read, that for non extremal black holes, there exists a logarithmic (and other) correction(s) to the well known term proportional to the area of the horizon such that $S = \frac{A}{4G} + K ...
6
votes
0answers
346 views

Why is Werner Heisenberg smiling? [closed]

This is not a joke. It is something I have wondered about for a long time. Nearly every picture of Werner Heisenberg shows him smiling. This is unusual. The difference between Heisenberg and ...
1
vote
1answer
1k views

Magnetic Permeability & Reluctance on old exam question

This is a past exam question from one of our lectures, and we have an issue with (i), I believe I need to use the equation $\rho=\frac{RA}{l}$, but I am not sure - could someone enlighten me on the ...
3
votes
1answer
522 views

What is the time correlation function in the Green-Kubo formulation of ionic current?

I am reading a paper, and I came across the Green-Kubo formulation, where the conductivity $\sigma$ of charged particles is related to the time correlation function of the $z$-component of the ...
32
votes
2answers
8k views

Before a once-warm lake starts to freeze, must its temperature be 4°C throughout at some point?

This is a problem I just started puzzling over, and I felt this would be a good forum to check my reasoning. So here are the relevant observations followed by my question: Water achieves its maximum ...
4
votes
2answers
994 views

Meaning of the phrase “dipole moment of the combination”

Here is a question I came across in a book: Three point charges $-q$,$-q$ and $2q$ are placed on the vertices of an equilateral triangle of side length $d$ units.What is the dipole moment of the ...
5
votes
3answers
1k views

Can the spin of a photon change during its “life”?

Or is the spin set in one of two possible states at its moment of creation and does not change for the rest of the duration of its "life"?
3
votes
1answer
476 views

How much more effective is it to stir in both directions?

I have been told that industrial mixing machines (say, for cake batter) switch directions periodically, first stirring in one direction, then the other, because this mixes the material more ...
18
votes
5answers
689 views

Particles for all forces: how do they know where to go, and what to avoid?

Here's an intuitive problem which I can't get around, can someone please explain it? Consider a proton P and an electron E moving through the electromagnetic field (or other particles for other ...
13
votes
2answers
543 views

black hole no-hair theorems vs. entropy and surface area

I was revisiting some old popular science books a while ago and two statements struck me as incompatible. No-hair theorems: a black hole is fully-described by just a few numbers (mass, spin etc) ...
2
votes
1answer
117 views

Black Holes, New Branes and Stars

Could a dying star in one brane ( assuming that we live in a world of multiple branes { not a Randal-Sumdrum model of a single brane}) be the start of the birth of the star ( more precisely Super ...
13
votes
4answers
1k views

Why is exhaling more forceful than inhaling?

By blowing at pencil, a piece of paper, or another object up to fifty centimeters away, I can cause it to move away from me significantly. But I can't move an object toward myself by inhaling sharply ...
1
vote
1answer
150 views

Pseudo-Superluminal Motion and the Synchronization of Clocks

Let's consider two points A an B separated by a finite distance in curved space time. A light ray flashes across an infinitesimally small spatial interval at B. Metric: ...
5
votes
1answer
936 views

Angular momentum operator in terms of ladder operators

I wanted to show that the angular momentum of the particle state with zero momentum $| \vec{0} \rangle$ is $0$, that is to say the intrinsic spin of a scalar field is $0$ using a mode expansion. ...
2
votes
1answer
225 views

A certain $\cal{N}=2$ superconformal theory (or is it?)

I want to look at the following theory in $1+1$ dimensions with $\Phi$ being the chiral superfield, $L = \int d^2x d^4\theta \bar{\Phi}\Phi - \int d^2x d^2\theta \frac{\Phi^{k+2}}{k+2} - \int d^2x ...
0
votes
2answers
391 views

Dynamics - Beginner question

I am trying to understand this question, but I keep coming to a dead end: Two identical rectangular masses rest on top of one another, the top is tied to a wall on the left, while the one on the ...
4
votes
1answer
210 views

Eigenvalue of $L_z$

In section 4.3 of Griffths' "Introduction to Quantum Mechanics", just below Figure 4.6, the sentence begins Let $\hbar \ell$ be the eigenvalue of $L_z$ at this top rung... Why is this valid? ...
1
vote
1answer
901 views

Curvature of spacetime in only required to explain tidal forces?

I'm a bit confused about the equivalence principle in GR. I'm quoting from Wikipedia: An observer in an accelerated reference frame must introduce what physicists call fictitious forces to ...
0
votes
3answers
19k views

Difference b/w Kinetics & Kinematics w/concrete example

(I know whether I understand this or not doesn't matter much to my work & study but am just curious.) I still can't differentiate in my head kinetics and kinematics (similar thread is found but ...
7
votes
2answers
1k views

Can a force in an explicitly time dependent classical system be conservative?

If I consider equations of motion derived from the pinciple of least action for an explicilty time dependend Lagrangian $$\delta S[L[q(\text{t}),q'(\text{t}),{\bf t}]]=0,$$ under what ...
13
votes
1answer
581 views

Backward causality: A question/extension to Ma et al.'s “Experimental delayed-choice entanglement swapping”

In a philosophically rather interesting experiment, Ma et al. show that backward causality exists in quantum physics. An Ars Technnica-article gives a less technical account. From Ars Technica: ...
0
votes
4answers
14k views

Why is force described as rate of change of momentum? [closed]

momentum = mass * velocity Differentiating both sides leads to force = mass * acceleration since the mass doesn't participate in the differentiation as it is constant. Is this a sound ...
2
votes
1answer
2k views

How does a voltage independent current source work?

So I'm having some trouble understanding what a voltage independent current source is. How can you have a current without a voltage. as I understand it voltage or EMF is the force that drives the ...
2
votes
2answers
927 views

is the voltage drop of a battery that is being short circuited the same as the battery's advertised voltige

So I know this is a pretty fundamental question, but I'll ask it any way. lets say you have a 12V battery, and the positive is connected to the negative directly by a wire with negligible ...
0
votes
1answer
396 views

Equipotential Contour mapping and making it 3D [closed]

I have seen Contour diagrams for Equipotentials . That are drawn like so: I also saw One image for these contours that was in 3D (Negative Point Charge) : I was Wondering If there's any ...
2
votes
1answer
384 views

How much water is destroyed in photosynthesis, relative to the world's supply?

Water is involved in the photosynthesis. How much water are we talking about compared with the total amount on water on Earth? Is it enough to have an effect on the average age of water molecules?
11
votes
1answer
414 views

Fermi statistics and Berry phase

When the positions of two fermions are exchanged adiabatically in three-dimensional space, we know that the wave function gains a factor of $-1$. Is this related to Berry's phase?
3
votes
0answers
125 views

Polyakov action as broken symmetry effective action

I would like to ask if it is possible to regard the Polyakov action as an effective action that describes the broken symmetric phase of a more general model. Could someone draw an analogy with O(N) ...
1
vote
1answer
124 views

How to deterministically distinguish the following quantum states?

(1) How to deterministically distinguish the following quantum states: $$\frac{1}{\sqrt{2}}[|+0\rangle|0\rangle+|-1\rangle|1\rangle$$, $$\frac{1}{\sqrt{2}}|-0\rangle|0\rangle+|+1\rangle|1\rangle$$, ...
5
votes
5answers
3k views

Producing photons with same frequency, different amplitude wave [duplicate]

I don't understand how two photons of the same frequency can have different amplitudes, neither how to produce them. I know that classically the square of the amplitude is proportional to the energy, ...
6
votes
2answers
496 views

Branch-point twist fields and operator insertions on a Riemann manifold

I am having trouble understanding how Eq (2.6) in this paper (PDF) $$Z[\mathcal{L},\mathcal{M}_{n}]\propto\langle\Phi(u,0)\tilde{\Phi}(v,0)\rangle_{\mathcal{L}^{(n)},\mathbb{R}^{2}}$$ generalizes to ...
12
votes
6answers
3k views

Why is there a $\frac 1 2$ in $\frac 1 2 mv^2$?

For elastic collisions of n particles, we know that momentum in the three orthogonal directions are independently conserved:$$ \frac{d}{dt}\sum\limits_i^n m_iv_{ij} =0,\quad j=1,2,3$$ From this, it ...
0
votes
3answers
253 views

Apparent non-aberration of gravity waves

Since GR assumes that gravity waves travel at speed c, we expect we would be able to some day detect an aberration effect similar the that of light. Of course, gravity waves are so tiny in magnitude, ...
3
votes
1answer
327 views

Brachistochrone problem for 3 points

I wonder how I can solve the Brachistochrone problem for 3 points? The matter starts from point A that is the highest point and it must pass from B and must finish with point C. (No any friction in ...
2
votes
0answers
99 views

Temperature of the CMB when the Earth formed and the faint young Sun paradox

The cosmic microwave background (CMB) has a modern temperature of about 2.7 K. At the time of the origin of the CMB, about 13.6 billion years ago, it had a temperature of about 3000 K. ...

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