1
vote
1answer
75 views

Unheated intermediate room - positive or negative effect on flats temp. insulation?

This is a question I have heard quite some contrary opinions, so I want to ask it here, as it deals with physics in principle:) The question is basically that, if having a unheated intermediate (in ...
4
votes
0answers
87 views

Degrees of freedom in m(atrix) theory

The Hamiltonian for m(atrix) theory is given by $$H=\frac{1}{2\lambda}\text{Tr}\left(P^{a}P_{a}+\frac{1}{2}\left[X^{a},X^{b}\right]^{2}+\theta^{T}\gamma_{a}\left[X^{a},\theta\right]\right).$$ Where ...
26
votes
4answers
5k views

Why is a new moon not the same as a solar eclipse?

Forgive the elementary nature of this question: Because a new moon occurs when the moon is positioned between the earth and sun, doesn't this also mean that somewhere on the Earth, a solar eclipse ...
4
votes
0answers
1k views

Difference between Eulerian and Lagrangian formulation of Fluid Dynamics

Difference between Eulerian and Lagrangian formulation of Fluid Dynamics. I am completely new to fluid mechanics. Until now definition $F = ma$ was sufficient for me to solve any rigid body problems ...
2
votes
1answer
63 views

Expanding Universe now and $\sim14$Gyr ago

All know that Inflation may solve some of the problems of the Standard Model of Cosmology and today we suppose that the field responsible for that is the inflaton. The properties of the potential ...
0
votes
1answer
87 views

K-factor in special relativity

A sends out a series of flashes of light to B, where the interval between flashes is denoted by T according to A's clock. Then it is plausible to assume that the intervals of reception by B's clock ...
0
votes
1answer
117 views

Can Planck's temperature ever be reached?

Planck's temperature is theorized to be the highest possible temperature in the universe. But, has it been reached? Or, will any object or phenomenon ever reach this absolute temperature?
4
votes
1answer
88 views

What happens when the frequency of A.C becomes infinite?

In an alternating current, the flow of electric charge periodically reverses direction, and the number of times it does that is called the frequency of the current. However, if the frequency of an AC ...
2
votes
1answer
68 views

Any difference between “Mueller matrix” and “Scattering matrix”?

I find in some references 4x4 Mueller matrix and in other references 4x4 Scattering matrix. Are they different or identical? If they are different, I would like to know the book or any research ...
1
vote
1answer
155 views

Tricky operator identity: $[L^2,[L^2,\vec{r}]]=2 \hbar ^2 \{ L^2, \vec{r}\}$?

This operator identity showed up in a course I was taking, and it was given without proof. $$[L^2,[L^2,\vec{r}]]=2 \hbar ^2 \{ L^2, \vec{r}\}$$ The curly brackets denote the anticommutator, $AB+BA$. ...
0
votes
1answer
69 views

Hawking radiation and entropy

What is your opinion of the hawking radiation mechanism, does that actually lower the entropy of the black hole?
0
votes
0answers
120 views

Field inside a conductor?

If the above image is a cross section of a conductor, the field at the point shown is not zero. So the field inside a conductor is not zero at all points. You could argue that the electrons would ...
7
votes
2answers
2k views

The error of the natural logarithm

Can anyone explain why the error for $\ln (x)$ (where for $x$ we have $x\pm\Delta x$) is simply said to be $\frac{\Delta x}{x}$? I would very much appreciate a somewhat rigorous rationalization of ...
0
votes
1answer
30 views

Does KI have to be a salt of stable I-127 isotope?

I was recently watching a TV show where some people exposed to radiation were asked to take KI. I googled for the information and discovered this and also the Wikipedia article. From what I read, I ...
9
votes
1answer
244 views

How deep is the region near an event horizon where Hawking radiation is generated?

In other words, how strong does gravity have to be to cause Hawking radiation to occur?
11
votes
3answers
1k views

Reconciling topological insulators and topological order

We make an important distinction between the topological insulators (which are essentially uncorrelated band insulators, "with a twist") and topological order (which covers a variety of exotic ...
6
votes
2answers
249 views

Quantum Anomalies in Non-Gauge Theories?

I'm reading about quantum anomalies in QFT and all the examples seem to arise in gauge theories. Is it true that theories without a local gauge invariance don't have quantum anomalies? I can't think ...
2
votes
0answers
64 views

Angles in cosmology

We have several definitions of distances in cosmology. Take angular diameter distance. $$d_A = a(t)\int \frac{dt}{a(t)}\;.$$ If an object has proper length $L$ then the angle $\theta$ it is seen at ...
0
votes
0answers
57 views

Different units in the equations of electrostatics in macroscopic media

I have the usual equations of electrostatics in macroscopic media expressed in Gaussian units: $$\nabla\cdot\vec{D}=4\pi\rho$$ $$\nabla\times\vec{E}=0$$ $$\vec{D}=\vec{E}+4\pi\vec{P} \tag1$$ My ...
6
votes
3answers
266 views

Maximal kinetic energy due to gravitational attraction

Two related questions: Small object of mass $m$ is falling into the supermassive black hole of mass $M$. What is the maximal kinetic energy can be acquired by the small object, from the point of ...
3
votes
1answer
91 views

Differential Realizations of certain algebras

I'm a first year graduate student in Mathematical Physics, and I am trying to generalise a certain method involving the so-called "Differential realizations" of certain algebras. The problem I'm ...
0
votes
1answer
81 views

Fixed Angular Momentum

Say I'm given the following Schrodinger equation $$\frac{d^2u}{dx^2}+ \left[E - V(x)+ \frac{a}{x^2}\right]u(x) =0$$ Where $a \in \mathbb{R}$. What are the physical interpretations of this equation? ...
0
votes
1answer
186 views

Toppling of a cylinder on a block

A uniform cylinder rests on a cart.The height and diameter is given.coefficient of static friction is given.How can i find the minimum acceleration of block such that the block topples? Morever what ...
2
votes
1answer
71 views

Triangular lattice arrangement of vortices in a superfluid

In a simply connected container containing a superfluid and rotating, there is a net circulation of superfluid. This is found due to the vortices formed, around which the superfluid rotates. These ...
1
vote
1answer
83 views

Question on the preservation of information via mapping to free field states

In Hawking's paper, "Breakdown of predictability in gravitational collapse", the crux of Hawking's argument is as follows: ...,one can extend the principle to treatments in which the gravitational ...
0
votes
0answers
67 views

How to measure distances to stars by means of spectroscopic parallaxes?

How to measure distances to stars by means of spectroscopic parallaxes on practice? What is the accuracy of measuring distances using this method compared with distances based on HIPPARCOS ...
5
votes
3answers
227 views

Countable Matrix Representation

In my quantum mechanics class, my professor explained that the Hamiltonian along with position and momentum operators can be represented by matrices of countable dimension. This is especially usefull ...
6
votes
2answers
265 views

Mass gap for photons

I am puzzled by the answers to the question: What is a mass gap? There, Ron Maimon's answer gives a clear-cut definition, which I suppose applies to any quantum field theory with Hamiltonian $H$, ...
3
votes
2answers
77 views

Energy spectrum, mass spectrum, and mass gap

In Arnold Neumaier's nice answer to this question: The energy spectrum in quantum field theory it is remarked that 'If there is a mass gap (i.e., if no representation of tiny positive mass exists), ...
-8
votes
1answer
84 views

There can be nothing with zero mass [closed]

Zero mass has no meaning and is an outcome of 2nd postulate of STR .The STR can be developed without this postulate and thus the postulate is redundant besides being contradictory to 1st postulate.The ...
1
vote
3answers
326 views

Difference between $SU(2)$ and $SU(2)$ gauge transformations?

I hear this jargon all the time, so what is the difference? (Of course this is nothing special to $SU(2)$, but rather I just took it as an example)
0
votes
1answer
172 views

Anticlock wise nature of motion of planets and electrons.

From my bookish knowledge all planet revolve around the sun in anticlockwise direction and electron also moves in the anti clock around the nucleus. Is there any specific reason behind the stability ...
2
votes
1answer
58 views

Wave equation in varying mediums

I recently stumbled upon the above image describing partial transmittance, and was wondering what sort of equation would model such a wave propagating through varying mediums. Is there also an ...
2
votes
1answer
67 views

HOMEWORK: Minimum angle to start the mop moving [closed]

Situation: I'm a janitor who wants to clean the floor with a mop. I need to find a minimum angle theta so that the mop starts moving. First, I constructed the Newton's equations: $$ma_x = ...
-1
votes
1answer
30 views

Could each non-dependent physical contant represent dimentions, and our universe be a point on this n-dimentional structure?

For example say the gravitation constant instead of equaling G, was actually a range bounded between 0 and infinity. Our Universe would be at a point on this range (equal to our G value) where ...
16
votes
3answers
1k views

Can space expand with unlimited speed?

At the beginning, right after the Big Bang, the universe was the size of a coin. One millionth of a second after the universe was the size of the Solar System (acording to ...
1
vote
3answers
2k views

Magnitude of magnetic field at the center of circular wire

I'm preparing for an exam by solving the sample questions , here is the one I'm having difficulty with : Following is the given circuit. Which contains two resistance $R_1$ and $R_2$ in form of ...
0
votes
1answer
117 views

Parametric equations in ballistics

while studying some material on ballistic trajectories (the basic gravity-only parabola), I've tried to come up with closed-form expressions for another case, where in addition to gravity we add a ...
16
votes
4answers
847 views

How would we see a near-lightspeed object emitting light?

Consider an object travelling near the speed of light relative to us (let it be a spaceship or a star), which is emitting light (consider it monochromatic resulting from a two level electronic ...
0
votes
4answers
678 views

How do electrons get the energy to jump from one orbital to the next when in stationary orbits the electron does not radiate energy

My Question is how do electrons get energy/lose energy to jump up or down an orbital as in a stationary orbit they do not gain energy and their energy is finite?
1
vote
0answers
112 views

Confused by Many-Body Formalism: Creation/Annihilation to Field Operators

I'm going through an introduction to many-body theory and I am getting tripped up on the formalism. I understand quantities such as $\hat {N} = ...
3
votes
1answer
54 views

Cepheid variable light curves

Basically, given a set of noisy observations for the apparent magnitude of a Cepheid variable, how is this fit to a curve which allows the period, and therefore distance, to be found? Cepheids' ...
27
votes
1answer
3k views

What conservation law corresponds to Lorentz boosts?

Noether's Theorem is used to related the invariance under certain continuous transformations to conserved currents. A common example is that translations in spacetime correspond to the conservation of ...
2
votes
2answers
135 views

what is the 'resolution' of the reality in pixel terms

1.What resolution should a TV screen have so that its image were so faithful as reality as if the TV were a window ? 2.Also what would happen if Physics could reproduce a 'pixel' of the size $ ...
1
vote
0answers
114 views

Relativistic beaming - which power of the Doppler factor

The context is that I'm building a simulation of a starfield, as seen from a relativistic spaceship. (EDIT: the simulation can now be found here.) One reference that I'm using is this paper by John ...
0
votes
4answers
258 views

Can we create invisible things?

We are able to see this beautiful world because of light. When light gets reflected from the object, the reflected light enters our eye retina and thus we are able see that particular object from ...
-1
votes
1answer
63 views

Is Tube light polychromatic?

Is tube light/fluorescent lamp monochromatic or polychromatic? Please justify.
1
vote
0answers
55 views

Contracting Indices in General relativity [duplicate]

I was reading a book about general relativity and I came across these two equations $$ \begin{align} \mathrm{g}^{\mu\nu}_{,\rho}+ \mathrm{g}^{\sigma\nu}{\Gamma}^{\mu}_{\sigma\rho}+ ...
1
vote
0answers
68 views

What is a Nicolai map?

I couldn't find the definition of a Nicolai map. What is it and what is a simple example which helps understanding it?
4
votes
2answers
192 views

What is a simple mathematical model of a star?

I had a discussion at work regarding a recent fusion experiment in China that resulted in temperatures five times hotter than the Sun. Someone mentioned that no one can know the temperature of the ...

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