19,275 reputation
3670
bio website mat.univie.ac.at/~neum
location Vienna, Austria
age
visits member for 3 years, 2 months
seen 9 hours ago

I am currently primarily active on Physics Overflow. Here I mainly respond to comments on my old postings, sometimes upon request by email, and sometimes because I happen to see an interesting question for which I know a quick but useful answer. Occasionally I also post purely mathematical questions.

Some physics material I wrote:

  • A theoretical physics FAQ
  • Classical and Quantum Mechanics via Lie algebras
  • Renormalization without infinities - an elementary tutorial
  • Classical and quantum field aspects of light
  • Optical models for quantum mechanics
  • Molecular modeling of proteins and mathematical prediction of protein structure


  • Mar
    30
    comment Is it theoretically possible to reach 0 kelvin?
    1. How do you deduce $p\ne 1$ from quantum physics? 2. ''reach'' requires a subject, an experimenter.
    Mar
    30
    comment Is it theoretically possible to reach 0 kelvin?
    @MarcelKöpke: Temperature is mathematically well-defined defined only in the thermodynamic limit, i.e., for an infinite system. It becomes an approximate concept for finite systems, but being approximate, the question of reaching exactly T=0 becomes meaningless. Also note that the slightest interaction of a subsystem with its envirenment turns its state into a mixed state, whereas in the absence of degeneracy, T=0 means pure. We cannot shield a subsystem of any significant size from its environment.
    Mar
    30
    comment Is it theoretically possible to reach 0 kelvin?
    @MarcelKöpke: It is very improbable to happen (probability=0) but impossible to achieve, since we cannot (even in principle) control a macroscopic or mesoscopic system (only for these is temperature defined) to the extent needed to turn it into a pure state.
    Mar
    30
    comment 2 + 1 dimensional gravity as an exactly soluble system
    For the corrected formulas and another answer see physicsoverflow.org/29140
    Mar
    27
    awarded  Enlightened
    Mar
    27
    awarded  Nice Answer
    Mar
    22
    comment Intuition for S-duality
    an answer is at physicsoverflow.org/28238
    Mar
    17
    comment Fermion counting operator for open superstring
    an answer is at physicsoverflow.org/28072
    Mar
    10
    awarded  Notable Question
    Mar
    10
    comment What exactly does it mean to wrap a D-brane or a M-brane in a Riemann surface $\Sigma_g$?
    There is an answer at physicsoverflow.org/27416
    Mar
    10
    comment Supergravity prerequisites for branes in string theory
    There is an answer at physicsoverflow.org/27209
    Mar
    10
    comment Green function for single impurity
    There is an answer at physicsoverflow.org/25400
    Mar
    10
    comment Scattering states of Hydrogen atom in non-relativistic perturbation theory
    @Jiang-minZhang: You may understand it by reverting to the usual inner product by a transformation of the wave functions to $\bar\psi(x):=|x|^{-1/2}\psi(x)$. Then you get the standard inner product on the transformed wave functions. Instead you have now to worry about the right boundary conditions at $x=0$.
    Mar
    8
    comment Does the existence of instantons imply non-trivial cohomology of spacetime?
    There is an answer at physicsoverflow.org/27926
    Mar
    8
    comment Reference request: Relation between $Sp(N)$, $Spin(N) $, $SU(N)$ groups and physics
    There is a specific answer at physicsoverflow.org/25980
    Mar
    8
    comment Questions regarding $D=4 $ ${\cal N}=4$ supersymmetric Yang-Mills
    There is an answer at physicsoverflow.org/26677
    Mar
    8
    comment Why is there an Euler density in SCFT $T_{\mu}^{\nu}$?
    There is an answer at physicsoverflow.org/25564
    Mar
    7
    comment Forming massive $\mathcal{N} = 2$ supermultiplets
    There is an answer at physicsoverflow.org/25692
    Mar
    7
    comment Why the bosonic part of the superconformal group $SU(2,2|1)$ is $SO(4,1) \times U(1)_R$?
    There is an answer at physicsoverflow.org/25607
    Mar
    7
    comment How to prove explicitly that by including Dirac fermions into the Einstein-Hilbert action we make torsion to be non-zero?
    There is another answer at physicsoverflow.org/25818