17,984 reputation
2957
bio website mat.univie.ac.at/~neum
location Vienna, Austria
age
visits member for 2 years, 6 months
seen 9 hours ago

Mar
24
comment Wave function of a photon?
@thyme: The Fourier transform is a function of time and describes the oscillations in time.
Mar
23
revised Hilbert Space of (quantum) Gauge theory
Added the cohomological construction of the physical Hilbert space
Mar
23
awarded  Necromancer
Mar
23
comment Galilean, SE(3), Poincare groups - Central Extension
@user35952: Look the terms up in Wikipedia; they are defined rigorously, there is no cycle. - Wave fucntions $\psi$ are ambiguous; only the associated density matrix $\rho=\psi\psi^*$ contains physical information. Thus projective representations are the natural objects in QM. Expressed in terms of the symmetry group represented, it leads automatically to central extensions. In case these are nontrivial, they cannot be avoided.
Mar
22
comment Quantum Wave Mechanics
@iota: Single particles are idealized asymptotic objects. They occur in nature only approximately, after careful preparation. In QFT, almost all states are a superposition of an indefinite number of particles.
Mar
22
comment Galilean, SE(3), Poincare groups - Central Extension
@user35952: A projective representation of a group is the same thing as an ordinary representation of a corresponding central extension.
Mar
21
revised Where do our 4 macroscopic spacetime dimensions reside in multidimensional models of the universe?
corrected misprints
Mar
21
answered Any simple reason why spin 2 polarization tensor should be symmetric in $\mu\nu$?
Mar
21
answered A question to clarify the use of divergent series in calculating the casimir effect
Mar
21
comment What exactly is regularization in QFT?
@joshphysics: Unfortunately, it lost this potential during the revolution. They preferred to sacrifice experts rather than nicety rules. As a commercial organization, they preferred mass visibility to quality of content.
Mar
21
comment What exactly is regularization in QFT?
@joshphysics: Since the 2012 revolution here, I consider myself here only a guest. I check and answer reasonably regularly only the responses to my own postings and the featured questions as far as I can answer them with little effort. Very occasionally (like today, where I am too tired to do something really productive), I look at some contributions that are either very recent and my specialty, that were written by people I learnt from when still active here, or that were linked to these and sounded interesting. Just for recreation, no longer as dedicated effort.
Mar
21
revised Where do our 4 macroscopic spacetime dimensions reside in multidimensional models of the universe?
added comment on boundaries
Mar
21
comment Where do our 4 macroscopic spacetime dimensions reside in multidimensional models of the universe?
@PlaysDice: I would visualize it as they would be ''around'' us. But words are not really telling as our imagination of higher dimensions is poor. Thus it is up to the imagination of the author to choose a word such as ''below'', ''above'', or ''around''. Note that flatlanders have no way of distinguishing between ''below'' and ''above'', as these dimensions are not perceptible by them - what they perceive is symmetric in the third dimension.
Mar
21
answered Where do our 4 macroscopic spacetime dimensions reside in multidimensional models of the universe?
Mar
21
comment What is the Reduced Density Matrix?
Your $\rho$ is a pure state, not mixed, hence the trace of $\rho^2$ is 1.
Mar
21
answered Wave function of a photon?
Mar
21
answered In what sense is a scalar field observable in QFT?
Mar
21
revised What exactly is regularization in QFT?
added reference to two useful books
Mar
21
answered Galilean, SE(3), Poincare groups - Central Extension
Mar
21
comment Lie group of Schrodinger Wave equation
@user35952: In some cases, every central extension is trivial. Thiis can be found out by calculating the corresponding cohomology group.