# All Questions

134 views

### Why do we do partial and not covariant differentiation with $x^{\nu}$?

Why when taking the velocity vector we make $$u^{\nu}=\frac{d}{d\tau}x^{\nu}$$ and not $$u^{\nu}=\frac{\nabla}{d\tau}x^{\nu}$$ where in the last equation I meant the covariant derivative. Why?
83 views

### How should semiconductors be understood? [duplicate]

In the chemistry setting, a doped semiconductor is a crystal lattice with holes/extra electrons in it. In the band/quantum mechanical picture, these holes/extra electrons can be seen as the amount ...
270 views

### Huygen's principle in curved spacetimes

Does Huygen's principle hold in even dimensional (2m+1,1) curved spacetimes, or are there certain necessary conditions for it to hold? In other words, if I have Cauchy data for a field satisfying the ...
191 views

Though the mathematical concepts underlying quadratic air drag are quite straightforward (a single variable differential, just like the linear drag equation), my text book (and online text books) ...
173 views

### Turning a car on a perfect surface

Say you have a car with perfect wheels and a perfect surface with so much friction that it never skids. When the car is traveling forward, the wheels INSTANTANEOUSLY turn 90 degrees. Does the car ...
229 views

### What are the implications for the AdS/Cft program if AdS is unstable?

To my understanding recent progress in the study of the non linear stability of AdS spacetime suggest that $AdS$ might be unstable. If this is true, what are the physical and mathematical ...
175 views

### GR exercise: falling particles on earth's surface

I'm having some trouble with Exercise 5.1 in Shapiro's BH,WD&NS book, which goes as follows: Consider two particles of mass $m$ at distance $r$ and $r+h$, such that $h\ll r$, on the same ...
110 views

### target category of extended field theory

For a topological field theory to be a true “extension” of an Atiyah-Segal theory, the top two levels of its target (ie its $(n-1)^{\text{st}}$ loop space) must look like $\text{Vect}$. What other ...
62 views

### How obtain conserved quantities in integrable models in accordance with Liouville's theorem, via Sklyanin Poisson algebra?

In classical integrable models, in the discrete case we have the Sklyanin algebra, $$\lbrace T_{a}(u),T_{b}(v)\rbrace =[r_{ab}(u,v),T_{a}(u)T_{b}(v)].$$ How to prove that the conserved quantities are ...
33 views

### Why do physical bodies in the universe follow the law of physics ( or any rule/pattern ) ? [duplicate]

Why do physical bodies in universe are bound to follow a rule ( eg. laws of physics ) ? Is their any imaginary condition possible, where they stop following any rule. And everything becomes random ? ( ...
88 views

### Adding matter to make galaxies bigger

What are the actual factors that play a role in the accretion of matter into galaxies? I read about Accretion Disks but I don't quite understand how they work yet.
2k views

### Forces other than the fundamental interactions, e.g. friction

Forgive me for the silly question, but I just don't get it. I just completed an elementary course in mechanics, and I am curious to know what I am about to ask. We have, all year, dealt with many ...
297 views

### Diffusion and Drift currents in a PN junction

In a forward-biased PN junction, the potential barrier decreases, allowing more majority carriers from one side to diffuse to the other side where they are minority carriers. After they cross the ...
322 views

### Can Minkowski spacetime be redefined as a non-flat riemannian manifold?

Minkowski space time is defined in terms of a flat pseudo-Riemannian manifold. I have wondered if it can be redefined as Riamannian manifold and in the case what type of curvature would there appear. ...
82 views

1k views

### Do we breathe air by creating a vacuum?

How do we suck air into our lungs, are we generating a vacuum?
2k views

### Electron shells in atoms: What causes them to exist as they do?

I have seen similar posts, but I haven't seen what seems to be a clear and direct answer. Why do only a certain number of electrons occupy each shell? Why are the shells arranged in certain distances ...
145 views

### I need to know how many pounds of force a 3 million pound object has moving at 1 mph [closed]

How many pounds of force does a 3 million pound object have moving at 0.1, 0.5, and 1 mph?
827 views

### Amplitude decrease during geometrical spreading of a seismic pulse

I'm looking at amplitude decrease of a seismic pulse as a result of geometrical spreading. Starting with the energy contained in a unit area, $I = E / (4 \pi r^2)$, where $E$ is the original energy ...
94 views

### Spaghettification while still in orbit

THIS POST does a pretty good job of giving an idea as to what the differences in forces becomes as you approach a black hole. For a small enough black hole, you would reach the dangerous tidal force ...
443 views

### What does the $y$-axis represent in the atomic spectra and what is its significance?

The picture is an emission spectrum of Helium. The spectrum has sharp lines (peaks) at certain wave lengths characterizing it as helium. Agreed that it characterizes Helium as atomic spectral line ...
304 views

### Can the compactified dimensions of M-Theory/String Theory become uncurled?

Is it possible for the curled dimensions described in superstring theories to become uncurled and open up. I have read that the big bang could have been the uncurling over 3 dimensions through ...
61 views

### Metric convention conversions between expressions

I'm sure this has caused many people headaches. First, is there a metric $(-+++)\leftrightarrow(+---)$ convention conversion chart where many common expressions are listed? Thank you in advance for ...
80 views

496 views

### Has anyone developed tunable notch filters in the visible RGB wavelengths which have very fast response times?

I am interested in whether the combination of a spectrometer and a tunable notch filter with notch frequencies at the visible red, green and blue wavelengths which have very fast response times could ...
279 views

### What algorithms can be used to compute the binodal in a ternary Flory-Huggins theory?

What are the most popular algorithms used to obtain a binodal curve for the ternary mixture (starting from Flory-Huggins theory)? I would like to obtain a plot similar to the one calculated here ...
110 views

### Electromagnetic waves and boundaries

I am reading about dielectric boundaries and electromagnetic plane waves following griffiths ch7. When considering a boundary at z=0 with electric fields perpendicular to the plane of incidence it ...