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22h
awarded  Nice Answer
Jul
11
awarded  thermodynamics
Jul
2
awarded  Curious
Jun
25
awarded  Nice Answer
May
26
comment Why do we study the scalar field in QFT when there is no such thing in nature?
@user22180: They are massive, hence not gauge bosons! They are described by fields described, e.g. in Chapter 5.7 of Weinberg's QFT I book.
May
25
comment Why do we study the scalar field in QFT when there is no such thing in nature?
@user22180: There are lots of composite particles with spin>2, both bosons and fermions; see the tables from the Particle Data Group. They are all massive. It is almost generally believed that there are no massless particles of spin >2.
May
23
comment microcausality and locality
see physicsoverflow.org/16870/microcausality-and-locality for an answer
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.