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bio website motls.blogspot.com
location Czech Republic
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visits member for 4 years, 3 months
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Hi, I am a string theorist and a publicist.


Jan
28
revised Where does the energy come from when a current heats a wire (resistor)?
added 183 characters in body
Jan
28
answered Where does the energy come from when a current heats a wire (resistor)?
Jan
28
answered Does a charging capacitor emit an electromagnetic wave?
Jan
28
revised why is it hard to push the empty upside down mug inside the water
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Jan
28
comment Do Maxwell's Equations overdetermine the electric and magnetic fields?
Dear @Vladimir, at one moment, e.g. in the initial state, $\vec E,\vec B$ at each point are independent - so 6 components per point - but they're constrained by ${\rm div}\,\,\vec D=\rho$ and ${\rm div}\,\,\vec B=0$, respectively. So it's effectively four independent components per point. Again, static fields are not the same problem as the dynamical ones, the counting is different for generic points away from the initial state.
Jan
28
answered What is the mass of individual components in a gravitationally bound system?
Jan
28
answered why is it hard to push the empty upside down mug inside the water
Jan
28
comment Purple doesn't occur in rainbow - or does it?
Oh, I see, this could be the right argument. I think that you're right that if you mix the red and blue kind of equally, so that it's more red than violet, you won't find a single frequency in the spectrum because they're either too blue, too red, or excite green in the middle as well. Now, I understand where the statement of my undergraduate classmate that the "CRT monitor can't manage purple" came from. I mixed a beautiful purple for him, kind of #FF00FF, or something like that, and he was beaten, but I still didn't know where it came from. Now I probably now what's the right statement.
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?
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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
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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!