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 Yearling
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Dec
14
answered Ernst potential from Kaluza-Klein reduction of axisymmetric space-time
Nov
23
awarded  Yearling
Nov
15
comment Why do we obtain classical physics by taking the limit of Planck's constant to zero?
Well, one obvious reason is that $h$ does not appear in the equations of classical physics. If $[x,p]=0$, every state is defined simultaneously by position and momentum, which differentiates between the classical and the quantum theory. This answer does not explain certain details, which appear in chapter VI, §1 of Quantum mechanics by Messiah.
Nov
13
comment Conservation of energy and Killing-field
Also, it is not necessary that the Killing vector is timelike, it is necessary that it is asymptotically timelike, and this is for the reason described in my answer below, namely that the scalar defined using it has the appropriate asymptotic behaviour. In Kerr space-time, $K^a$ may be space-like!
Nov
13
comment Conservation of energy and Killing-field
"Physically, asymptotically flat space-times represent isolated systems", cf. Robert Geroch and Jeffrey Winicour. Linkages in general relativity. Journal of Mathematical Physics, 22(4):803-812, 1981.
Nov
13
revised Conservation of energy and Killing-field
Probably used the wrong word "for" instead of "in". The former might confuse someone in that the answer refers to the energy of the spacetime.
Nov
12
answered Curvature of Conical spacetime
Nov
12
comment Conservation of energy and Killing-field
What do you mean by "isolated system"?
Nov
12
comment Conservation of energy and Killing-field
Isn't the reason asymptotically flat space-times are preferable to model physical systems that they can be considered as isolated?
Nov
12
answered Conservation of energy and Killing-field
Nov
12
comment Does gravity acting on a resting object produce any heat?
Yes, you're right, I missed that line.
Nov
11
comment Does gravity acting on a resting object produce any heat?
If the object is not a point particle, as you imply, then tidal forces can produce heat.
Nov
11
answered Is it possible to calculate the energy of a spark?
Nov
6
comment How is hydrostatic pressure overcome when a star is formed?
By gravity you mean the acceleration of gravity, right? How did you derive $P \propto 1/r^3$? By using $PV = NkT$ and keeping $T$ constant? But the temperature changes, too. You need to assume a realistic equation of state for the cloud to collapse.
Nov
6
comment How is hydrostatic pressure overcome when a star is formed?
@user2800708 What do you mean by these numbers?
Nov
5
comment How is hydrostatic pressure overcome when a star is formed?
The OP assumed certain equation of state in his question and wonders whether the cloud will collapse or not, therefore the virial theorem must be expressed in terms of the thermodynamic variables of the cloud. I'm not going further than this, because I'm not certain what the OP is claiming.
Nov
5
comment How is hydrostatic pressure overcome when a star is formed?
Yes, I noticed that, and that's why I added the last sentence.
Nov
5
answered How is hydrostatic pressure overcome when a star is formed?
Sep
24
comment Determining eccentricity of satellite orbit from velocity vectors and altitude
Also, the easiest way to find the eccentricity is to calculate the Runge-Lenz vector.
Sep
24
comment Determining eccentricity of satellite orbit from velocity vectors and altitude
Newton's equation is of second order, therefore knowledge of the initial position and velocity are enough to find the orbit.