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bio website mat.univie.ac.at/~neum
location Vienna, Austria
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visits member for 2 years, 8 months
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May
5
comment Origins of many-particle interactions
@garyp: all potentials are effective potentials. For example, the Coulomb interactions are obtained by neglecting higher order terms that appear in a more exact QED solution. These corrections are important for larger atoms, e.g., to explain the color of gold or the liquidity of mercury.
May
5
comment Does the vacuum energy problem of quantum field theory only occur in the Hamiltonian approach, or also in the path integral approach and in AQFT?
@fqq: I had explained how. Which constants one gets is found out only after successful renormalization, as only then the formal infinities are converted into finite quantities.
Apr
25
comment Origins of many-particle interactions
On the level of nuclei and eelectrons, the pairwise Coulomb interaction is an excellent approximation (though not perfect). On the level of atoms, triple interactions of Axilrod-Teller type are already necessary for high quality models. Both levels are microscopic.
Apr
24
revised Origins of many-particle interactions
added explanation of approximations
Apr
24
comment Origins of many-particle interactions
Approximations of what? Of the true multiparticle potential, which is just an arbitrary translation, rotation, and permutation invariant potential. A real system has some N-particle potential, but which one must be decided by experiment (or deduced from a more detailed model). Pair potentials are simply the simplest class of approximations.
Apr
24
comment Origins of many-particle interactions
This is because the physics tradition proceeds (unlike the rtradition of mathematicians, which I prefer) through learning by example. So textbooks just consider the simplest case that is enough to illustrate the typical methods and its difficulties. The complexities come early enough when one is treating real applications rather than teaching examples, since then one often cannot ignore more complex terms.
Apr
20
comment One question about Weinberg's derivation of arbitrary spin fields expressions
I couldn't find them there. Please give equation numbers.
Apr
20
answered How could there be a truly “pure” state?
Apr
19
comment One question about Weinberg's derivation of arbitrary spin fields expressions
Please give the page numbers where the two formulas appear.
Apr
18
answered Origins of many-particle interactions
Apr
14
revised Crash course in classical thermodynamics
corrected description of reference [2]
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