19,588 reputation
3771
bio website mat.univie.ac.at/~neum
location Vienna, Austria
age
visits member for 3 years, 4 months
seen 6 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


  • Jun
    28
    comment Choice of framing in Gravitational Chern-Simons
    There is an answer at physicsoverflow.org/32208
    Jun
    20
    comment Anomalous magnetic moment of electron
    @MarkWayne: Of course; g=1 also violates experimental results directly. I was just pointing out that without invoking the Dirac equation one has a 1-parameter family of Hamiltonians parameterized by g, so that choosing one needs experimental input, while the Dirac equation fixes g=2, close to the the experimental value of g.
    Jun
    18
    comment Anomalous magnetic moment of electron
    @MarkWayne: H_0=(p^2+(σ⋅p)(σ⋅p))/(4m) should give g=1.
    Jun
    17
    comment Anomalous magnetic moment of electron
    @MarkWayne: interesting. By writing H_0 in other quadratic patterns before doing the minimal substitution one can probably get an arbitrary value of g.
    Jun
    16
    comment If a symmetry operator S in a QFT annihilates the vacuum, why does S preserve the space of 1-particle states?
    More answers (among them mine) together with an extensive discussion can be found at physicsoverflow.org/30822
    Jun
    11
    awarded  Nice Answer
    Jun
    10
    comment Question regarding moduli space of a Calabi-Yau manifold
    another answer is at physicsoverflow.org/31683
    Jun
    9
    awarded  Enthusiast
    Jun
    7
    comment Symplectic leaves, tori and Poisson manifolds
    Qmechanic's suggestion to remove the nambu part is sensible - you can pose the removed part as a new question. It is never good to ask too much and too disparate things in one question.
    Jun
    5
    revised Symplectic leaves, tori and Poisson manifolds
    grammar corrected
    Jun
    5
    comment Symplectic leaves, tori and Poisson manifolds
    @JanetthePhysicist: The relation to the Dirac bracket is complicated - it amounts to giving a more explicit description of the centralizer.
    Jun
    4
    revised Symplectic leaves, tori and Poisson manifolds
    added detail
    Jun
    4
    answered Symplectic leaves, tori and Poisson manifolds
    Jun
    2
    comment What is the physical interpretation of second quantization?
    @DanielSank: To write a general $3$-particle antisymmetric wave function $\psi(x_1,x_2,x_3)$ for the analysis of a 3-atom molecule in terms of creation and annihilation operators is already awkward. People use creation and annihilation operators for fixed particle number, e.g., in coupled cluster quantum chemistry computations, but there the meaning is quite different, describing excitations of a few quasiparticles although the total number of particles is fixed.
    Jun
    1
    comment What is the physical interpretation of second quantization?
    My statement was just that it is necessary when particle number is indefinite. In ordinary QM, the only relevant situation of indefinite particle number that I know of is the multicanonical ensemble, and one relevant case is enough to motivate the concept (which was the question of the OP). But I agree that it may be useful in other circumstances as well, though it is overkill in case $N$ is fixed.
    Jun
    1
    comment What is the physical interpretation of second quantization?
    @yjc: Together with my answer given, your comment explains why one says that in QFT, elementary particles are the elementary excitations of the quantum field. Indeed, this is a much better notion of particles than the semiclassical picture generally considered.
    Jun
    1
    comment Uniqueness of expression of a Lie group element
    But you say in the sentence ending with (1): ''defined by unique $\tau_j\in R$''. It is this statement that is incorrect as stated. The neighborhood is introduced only later in the proof ''Indeed...'', but it should be in the claim!
    Jun
    1
    comment Uniqueness of expression of a Lie group element
    If $X_j$ is a compact generator, the exponent cannot be unique, as the exponential is periodic. Your local uniqueness result only holds if you assume that the coefficients in the exponents are modulo the period, or ar continuous function of the group element.
    Jun
    1
    answered What is the physical interpretation of second quantization?
    May
    31
    answered Uniqueness of expression of a Lie group element