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revised Why does gravitational collapse occur suddenly in a supernova progenitor?
edited body
2d
comment Why does gravitational collapse occur suddenly in a supernova progenitor?
I'm not arguing whether electron degeneracy is essential for stellar equilibrium or not, I'm just claiming that one needs to decide at what depth to extend an answer and assume certain knowledge by the OP, otherwise answering becomes a very time-consuming process. The core consists of Fe mainly (not Ni), because it has the maximum binding energy per nucleon. The model with a second shock wave is known as delayed hydrodynamic explosion. You're right about the free-fall time-scale; what I wrote is correct about the outer layers of the core.
Oct
18
comment Why does gravitational collapse occur suddenly in a supernova progenitor?
I didn't write about electron degeneracy pressure, because the question is about the process after the hydrostatic equilibrium condition ceases to be valid. I had assumed that the OP is aware of the process that keeps the stars in equilibrium.
Oct
18
comment Why does gravitational collapse occur suddenly in a supernova progenitor?
Since the time-scale of free-fall collapse is decreasing as the density increases, the outer layers fall faster than the inner ones and separate completely from the rest of the star as they reach supersonic speed. Splitting Fe reduces Fe's mass (the core is primarily Fe; products of its reactions eventually escape). The free-fall time-scale is essential here, because we have loss of hydrostatic equilibrium. The explosion does not always occur in a single stage; it is possible for part of the iron core to remain after the bounce, which triggers a second shock wave.
Aug
7
comment Unknown Function in the Tolman-Bondi-de Sitter Metric
$e^{-2\Psi}$ is a function that allows you to express a spherically symmetric metric in its most general form. It takes specific values depending on extra assumptions you make, e.g. static Einstein, Friedmann etc. The reprinted article has more examples.
Aug
7
comment Does a square wave “smooth out” in the air?
Try plotting it: a square wave is formed by the infinite sum of waves with integral multiples of the fundamental frequency. The more frequencies you add, the more the sine shape resembles a square. Since air is dispersive, the square will be deformed, especially for the higher frequencies. Higher frequencies affect the edges of the waveform, therefore the edges will soften as you lose supersonic frequencies.
Jul
29
comment Why does Pluto's orbit cross Neptune's orbit?
The solar system is not stable, as it demonstrates chaotic behaviour on time-scale of order of ten million years, as indicated by numerical simulations. In particular, the probability of collision is ~1% before the Sun dies. See this for details.
Jul
29
comment Is it possible that black holes are also neutron stars, but so dark that we cannot see them?
@RobJeffries Yes, sorry. I wrote that the Schwarzschild radius is of order 1 km, meaning that in general it is smaller than the radius of a neutron star, which is one order higher. I was giving rough estimates, because the equation of state is not known and exact values may change in the future. The range you give is realistic.
Jul
29
comment Is it possible that black holes are also neutron stars, but so dark that we cannot see them?
@RobJeffries According to p.12,15, of this, it is 7-11 km with the most conservative estimate being 9 km. This estimate hasn't changed much since Shapiro & Teukolsky was published.
Jul
25
comment Calculate loudness of sound: why am I getting contradictory answers?
Also, the temperature gradient in the vertical direction is known to affect the propagation of sound waves significantly.
Jul
23
revised Why do bubbles make a sound?
deleted 21 characters in body
Jul
23
answered Why do bubbles make a sound?
Jul
22
comment Counting Problems in Physics
Non-trivial counting is often done in group theory.
Jul
21
comment Show that getting parallel transported does not change angle between them- Tensors
If you use google, there are lots of lecture notes on GR, but they require understanding of calculus, linear algebra, mechanics, differential geometry, differential equations etc.
Jul
21
comment Show that getting parallel transported does not change angle between them- Tensors
I'm using $\nabla_a$ instead of ${}_{;a}$ and $s$ is the affine parameter of $X^a$.
Jul
18
answered Show that getting parallel transported does not change angle between them- Tensors
Jul
17
comment How to understand “accelerating charge radiate” using intuition?
There is an intuitive way to understand this as Compton scattering: a flux of photons is scattered by an electron, hence the electron is accelerated and the radiation comprises the scattered photons.
Jul
16
answered Poincare Generators in terms of Position and Momentum
Jul
15
comment Are there any hamiltonian systems without a periodic orbit?
A free particle does not have periodic orbit.
Jul
15
comment Why doesn't matter pass through other matter if atoms are 99.999% empty space?
Also of relevance here is the cross-section for the scattering, which indicates that in most cases the effective size of the particle for the interaction is not related to the physical size, if one calculates it classically as the OP suggests.