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bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 3 months
seen 4 hours ago

Hi, I am a string theorist and a publicist.


Jan
27
comment Do Maxwell's Equations overdetermine the electric and magnetic fields?
Dear Vladimir, I have answered your question in detail. Again. There's a 1-parameter ambiguity in the 4 potentials - the U(1) gauge invariance - because locally in spacetime, the 4 potentials are only constrained by 3 equations, curl H = $j+\partial D / \partial t$. The fourth equation with currents, ${\rm div}\,\, D=\rho$, isn't independent: its time derivative follows from the previous three. The remaining 3+1 equations for $B,E$ are satisfied automatically if $B,E$ are expressed in terms of the 4 potentials, they're Bianchi identities.
Jan
27
reviewed Approve Numerical simulation of mechanics problem
Jan
27
reviewed Approve What practical issues remain for the adoption of Thorium reactors?
Jan
27
comment Nonabelian gauge theories and range of the corresponding force
QCD forces only look "short range" because they're so powerful at long distances that they're confining. That's why only color-neutral objects may exist in isolation and the residual forces acting on such color-neutral objects (like nucleons) decreases quickly with the distance.
Jan
27
revised Do we need a quantum deformation of the diffeomorphism group in string theory?
deleted 18 characters in body
Jan
27
answered Do Maxwell's Equations overdetermine the electric and magnetic fields?
Jan
26
answered Reduced density matrices for free fermions are thermal
Jan
26
revised Conformal transformation equation
added 16 characters in body
Jan
26
comment Do we need a quantum deformation of the diffeomorphism group in string theory?
Dear Ron, it's the upper limit on cross section as a function of energy for very high energies and some particular scaling of the impact angle. The inequality may be derived from locality and analyticity in QFT but no ordinary QFT saturates it. Open strings saturate it. But I wouldn't be sure about the inequality on the top of my head, sorry.
Jan
26
comment Is the stability of the nucleus due to pions or gluons?
This is apparently a popular question with at least two duplicates here: physics.stackexchange.com/questions/9663/… (exactly the same question) and physics.stackexchange.com/questions/9661/… (related)
Jan
26
answered Conformal transformation equation
Jan
26
comment Why is there no theta-angle (topological term) for the weak interactions?
Thanks a lot, David!
Jan
26
comment Why is there no theta-angle (topological term) for the weak interactions?
Nice, would you add one or two formulae? What is the parameter of the transformation (and which one) to remove the $\theta\cdot F\wedge F$ term? And a related question: is there some simple way to add some chiral couplings of new fermions to $SU(3)_{color}$ to solve the strong CP-problem?
Jan
26
comment How to choose the right units to compute the phase space volume in classical statistical mechanics?
Dear @Nick, nope, the entropy of the vacuum is zero. And indeed, as you guessed, in the previous comment, "energy difference" should have been "entropy difference". The vacuum's entropy is zero because it is a unique (1) state and the logarithm 1 is zero. Every argument (linked to energy and virtual particles?) that the vacuum should or could have a different entropy than 0 is sloppy and the conclusion would be flawed.
Jan
26
comment Why is there no theta-angle (topological term) for the weak interactions?
Good question, and looking forward to the answers if any. ;-)
Jan
26
comment Intuitive sketch of the correspondence of a string theory to its limiting quantum field theory
If you only want an intuitive sketch, this 600-character comment is more than enough. Histories in string theory look like Riemann surfaces. The long-distance limit inevitably makes all the tubs in the diagram much thinner than they're long - because one pays with energy for the spatial circumference of the cross section (i.e. length of the string). So then one has Feynman rules involving the lowest vibration states of strings - and they look like pointlike particles and have discrete spectrum - and they interact by some vertices (given by the tube junctions), so we get Feynman rules of QFTs.
Jan
25
revised When do I know if energy stored in an object is 0 or nonzero? (Heat transfer)
rplaced "x" with "\times"
Jan
25
comment Why don't electrons crash into the nuclei they “orbit”?
Dear Anna, you authoritatively sensed some age of the OP, hopefully correctly, and acquired the role of a wise old-generation teacher and you got +1. ;-)
Jan
25
comment Definition of Fluctuations and Perturbations
OK answer. One could also discuss oscillations and corrections in the same thread. There are various differences. Oscillations are small numbers (a bit bigger than fluctuations) around the "normal level" which are time-dependent. The time dependence sort of applies to fluctuations, too: fluctuations should be time-dependent. On the other hand, perturbations are changes made to the laws of physics, "before" we "kickstart" time evolution. Corrections are like perturbations except that perturbations should be corrections to a solvable/understood zeroth approximation, not a generic one. And so on.
Jan
25
comment Vacuum Wavefunctional
You're welcome. The stumbling block you mentioned may be atypical but the fact that there are often atypical stumbling blocks is pretty typical. ;-)