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


Jan
21
revised Quantum Field Theory Variants
added 385 characters in body
Jan
21
revised Quantum Field Theory Variants
added 385 characters in body
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
added 528 characters in body
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?).
Jan
20
comment Is there a 1-1 correspondence between symmetry and group theory?
@Revo, translations represent an additive group of real numbers because if you compose a translation by distance $L_1$ with a translation by $L_2$, you get a translation by $L_1+L_2$ which is the sum of the two real numbers.
Jan
20
revised A dimensional regularization identity
added 79 characters in body
Jan
20
comment A dimensional regularization identity
Dear user, you should make the steps in the order I indicated. You first bring the denominator to the standard form $1/(p^2)^a$ by completing the squares. Then, with this definition of the variable $p$, you get something in the numerator whatever it is (it's a new polynomial, different from the orig. one, written in terms of $p$), and this is then treated by the second step I described which is relevant for the numerator. You seem eager to randomly permute the steps or otherwise damage the procedure I carefully sketched and then you seem to be surprised that yours doesn't work. But mine does.
Jan
20
answered At the molecular level, how is the pressure at the bottom of a lake higher than at the top?
Jan
20
answered What is a dynamical variable
Jan
19
comment relativity and aberration of light
Apologies, I probably don't understand this question of yours. If the two equations - regardless of the physical interpretations - differ by a sign, it either means that $x$ in one language means $-x$ in the other, or $c$ means $-c$ in the other, or $a$ means $\pi\pm a$ in the other, or all these three things combined. Depending on the context, some of the 4 options may be impossible.
Jan
19
comment Modification of de Donder gauge
I am pretty sure that the term $\nabla_a (\nabla_b v^b)$ which is a gradient of the divergence $\nabla\cdot v$ that you don't cancel for $n\neq 1/2$ is bad for the solvability. When the term cancels, the equations for individual components $v_a$ are pretty much independent, but they get mixed up if the term is there which probably damages the existence or uniqueness of solutions. There is a good reason why only the de Donder gauge with the right coefficient is being used but I don't have the answer in my head clearly enough to post it as a full answer.
Jan
19
answered relativity and aberration of light
Jan
19
revised relativity and aberration of light
deleted 3 characters in body
Jan
19
comment How many colors exist?
Dear @Ron, I agree you may be right: the Hubble-scale issues were sketched in the part of my answer about the lower limit on frequencies. For a universe with boundaries, one could indeed get a quantization of frequencies, like in a box, but with an insanely low spacing.