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visits member for 2 years, 9 months
seen Dec 9 at 9:05

Aug
11
comment Is mass an observable in Quantum Mechanics?
I'd just drop the corresponding paragraph. What you write about elementary systems in the sense of Wigner is technically correct, but misleading since elementary particles are not elementary systems in this sense. In today's theories, elementary particles are irreducible representations of the full symmetry group, which is not just the Poincare group. If the mass transforms nontrivially under the internal symmetry group, it is a matrix rather than a number.
Aug
11
comment Is mass an observable in Quantum Mechanics?
You made a sharp distinction but you still write ''Therefore, in relativistic quantum mechanics, the elementary systems must have trivial mass operator, which as before, can be considered as a given non-quantum parameter.'', which is wrong.
Aug
11
comment Is mass an observable in Quantum Mechanics?
-1. In the relativistic case, mass is no longer a superselection rule, hence the mass operator is often nontrivial. Otherwise one couldn't have a nontrivial mass matrices of quarks and neutrinos, which are experimentally verified to show mass mixing.
Aug
10
awarded  Necromancer
Aug
8
answered Is mass an observable in Quantum Mechanics?
Aug
7
comment Interesting Hamiltonian System
There is an answer on physicsoverflow.org/21606/interesting-hamiltonian-system
Jul
22
awarded  Nice Answer
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?