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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.
Jul
12
comment A rigorous treatment of distributions in quantum mechanics
Messiah's book covers the formalism with rigour, but is a bit old and verbose.
Jul
12
comment Is the apparent lack of (Ricci) curvature in the Schwarzschild metric due to a choice of coordinates?
There are many ways to measure curvature (and more than one type of curvature). The Ricci scalar is one of them, but the Riemann tensor is the one we use to say whether a spacetime is curved or not, and it is nonzero for the Schwarzschild metric, therefore it is curved.
Jul
12
comment Temperature of Bose-Einstein-Condensate in space
The temperature in space in vacuum in absence of radiation etc is $2.7 \text{K}$ due to the CMB, while on Earth it is $300 \text{K}$ on average. This makes it easier to reach lower temperatures in space, but I cannot argue about specifics in this case.
Jul
5
answered Seeing one's back on the event horizon