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bio website motls.blogspot.com
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Hi, I am a string theorist and a publicist.


Jan
22
revised What is the meaning of speed of light $c$ in $E=mc^2$?
added 276 characters in body
Jan
22
comment Calculating gravity when taking into account the change of gravitational force
Let me say Steven's lesson in different words. The first equation of yours, using $gt^2$, is only OK if the acceleration $g$ is constant, i.e. in the "uniform" gravitational field. For example, on the surface of Earth, the field and acceleration are approximately uniform. The accuracy is so high that we don't need to talk about $g$ being height-dependent in any everyday applications. However, $g$ does depend on $r$ and therefore on $t$, so $y=gt^2/2$ for a fixed $g$ isn't the right description anymore. Instead, it must be replaced by the solution of the differential equation, $g\to\ddot y$.
Jan
22
comment Calculating gravity when taking into account the change of gravitational force
Dear @Frxstrem, differential equations are equations for whole functions $r(t)$ which relate the so-called "derivatives". The equation with $\ddot{r}$ is actually the right way to write your equation for the acceleration. I think that if you don't know at all what differential equations are, this thread on Physics SE isn't the right arena to learn this rather extensive subdiscipline of maths. Steven: I disagree that the analytic solution to the Kepler problem etc. is useless and infinitely contrived; after all, Newton mastered it to explain Kepler's laws, the first big success of his theory.
Jan
22
revised What is the meaning of speed of light $c$ in $E=mc^2$?
added 1323 characters in body
Jan
22
answered What is the meaning of speed of light $c$ in $E=mc^2$?
Jan
22
comment Source term of the Einstein field equation
Otherwise what the mass "including the pressure" is depends on what we exactly calculate; Feynman's was an estimate. But in various situations, the mass may be modified by $\pm C \int p\,\, {\rm d}V/c^2$ where $C$ is a numerical constant. In other contexts, it's important that the influence of the pressure is nonlinear, and therefore only in higher orders, and so on.
Jan
22
comment Source term of the Einstein field equation
Dear @Kernel, it seems to me that you are repeating the same pre-existing opinions of yours and you didn't really pay much attention to Kyle who explains what the actual core of Penrose's complaint was, and in my opinion, correctly so. When you have a star, it has not only mass density inside; it also have a pressure because the particles are moving somewhat quickly. In GR, the very presence of pressure - inside the matter (so "pressure on what" is a totally irrelevant question here) - is affecting the properties of the gravitational field because the whole $T_{\mu\nu}$ including $p$ is RHS.
Jan
21
awarded  Enlightened
Jan
21
comment What is an isoscalar factor?
I see, so they're for the maximally symmetrized representations only.
Jan
21
comment What is an isoscalar factor?
This whole question looks strange because while $SO(3)$ irreducible representations are uniquely specified by a value of $L$, it is not the case for $SO(n)$ for $n>3$. For more complicated groups, one must use Young diagrams, not just a simple value of $L$, and the Racah coefficients - another name for the isoscalar factor - also depends on the whole diagram. Where did you see the comments and formulae you reproduced?
Jan
21
revised Quantum Field Theory Variants
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Jan
21
revised Quantum Field Theory Variants
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Jan
21
answered How does Newton's 2nd law correspond to GR in the weak field limit?
Jan
21
awarded  Nice Answer
Jan
21
comment Feynman rule 4-point vertex WW -> ZZ
There isn't any helicity-dependence in the coupling of gauge bosons; only fermions are chiral in the electroweak theory. Up to different coupling constants and structure constants, the WWZZ vertex is identical to that of 4 gluons and may be found in any textbook such as Peskin and Schroeder.
Jan
21
answered Where does the photon's energy come from in X-ray bremsstrahlung?
Jan
21
comment relativity and aberration of light
Good luck, Kostis, but maybe you're taking some details too seriously. The signs (plus or minus) either follow from the text in a way you may understand, or you should ignore them if you just "roughly read" the text. In a broad variety of situations, the changed signs are just changes in conventions and both options may be equally valid assuming that other signs are adjusted as well. One must be careful which of the signs are really variable and which of them would be mistake if one changed them. Much of the structure beyond the signs is more important and characteristic about the equations.
Jan
21
revised Quantum Field Theory Variants
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Jan
21
answered Quantum Field Theory Variants
Jan
21
comment Some questions regarding $n+m$-dimensional spacetime
I have a big problem with the "unstable" and other labels. Whether a spacetime is stable depends on the dynamics in it. And be sure that supersymmetric $n+1$ dimensional spacetimes are always stable, and there are lots of them in string theory and outside string theory, too. The label "too simple" may have a point but one would have to discuss what it exactly means, much like the "ultrahyperbolic/unpredictive" label. Those things are for a long discussion with many aspects and it's not clear which of them you're really interested in, especially if you add fractional and negative dims (WTF?).