auxsvr
Reputation
1,262
Next privilege 2,000 Rep.
 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 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 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 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. 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. Aug 27 comment Euclidean derivation of the black hole temperature; conical singularities G. F. R. Ellis and B. G. Schmidt. Singular space-times. General Relativity and Gravitation, 8(11):915, 1977 has more details about conical (conelike) singularities. Jul 16 comment Why does locking the rear tires on a vehicle cause it to spin? When the tyres lock, static friction decreases significantly. This is the reason cars have ABS. Jul 12 comment Generalised velocities enough to be deterministic in Lagrangian mechanics? Where does the dependence of $\ddot{q}$ on $(q, \dot{q})$ come from? This is not valid in the general case. Jul 8 comment Classical models with unbounded particle number The Boltzmann equation is about the distribution of particles in phase space. It allows for different kinds of particles that change from one form into another, which implies particle creation and destruction. It has applications in plasma physics. Jun 20 comment How do you take the derivative with respect to a rank two tensor? Every index combination indicates a different function. The derivative of a function w.r.t another functionally independent function, which is the case for functions with different indices, is zero and the only non-zero case is when the function to be derived and the one to be derived with are the same. Jun 11 comment Is there a scientific term for the right-hand-(grip-)rule? This is known as orientation. There are two orientations depending on the order of the basis vectors and only one of them can be used to define the volume element with positive sign. May 10 comment If the electrostatic potential is zero, why doesn't the electric field have to be zero? Do you mean $V$ is zero identically or that there's a value of $\vec{x}$, so that $V=0$?