8h
answered Origins of many-particle interactions
Apr
15
comment Lie algebra and Lie group about quantum harmonic oscillator
It is a group of upper triangular 3 by 3 matrices; see physicsoverflow.org/14831
Apr
14
revised Crash course in classical thermodynamics
corrected description of reference [2]
Apr
13
comment What exactly is regularization in QFT?
@joshphysics: I hope that the recently started Physics Overflow at physicsoverflow.org , which tries to continue the tradition of Theoretical Physics SE, will become in due time the place where more advanced questions can be asked and will be answered.
Mar
24
comment Wave function of a photon?
In any case, the Fourier transform of f(ν) can never be interpreted as something in space. For a spatial interpretation you'd need to Fourier transform the direction-dependent density f(nu,p) with respect to momentum, but because of the transversal nature of photons, this gives something easily interpretable only along planes perpendicular to the momentum p.
Mar
24
comment Wave function of a photon?
@thyme: Independent of the application, the Fourier transform of a function in frequency space is always a function in time. For photons, the fourier transform of $f(\nu)$ is meaningless, as the time oscillations are extremely rapid, while the observation process is slow. E.g., our eyes observe the frequency distribution $f(\nu)$ itself, not its Fourier transform.
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?