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Apr
19
answered Understanding Well Defined States
Apr
19
reviewed Approve suggested edit on Understanding Well Defined States
Apr
18
reviewed Approve suggested edit on Universal gravity at small distance
Apr
17
reviewed Approve suggested edit on $U(1){\times}U(1)$ local gauge invariance derivative
Apr
14
answered Entanglement in single particle state
Apr
14
reviewed Approve suggested edit on What is the notion of a spatial angle in general relativity?
Apr
12
comment Can dimensional regularization solve the fine-tuning problem?
Every regulator carries an opinion about the UV physics. Intuitively, I think dimensional regularization assumes that there is no new UV physics. Further, one consistent way to compute divergent integrals is to just drop the power-law divergences, which is what dim-reg does. So it is unphysical inasmuch as you expect to see new UV physics.
Apr
12
answered why do the electroweak vacuum have to be charge and color neutral?
Apr
12
comment why cannot fermions have non-zero vacuum expectation value?
@Paul: VEV = Vacuum expectation value i.e. a property of the vacuum. So if any object in a non-trivial representation of the Poincare algebra picks up a VEV, then some of the spacetime symmetries will be spontaneously broken by the vacuum (state). One can expect the same to apply to fluctuations around the vacuum. That would mean that the corresponding conserved quantities are not really conserved. And as far as we can see, conservation of energy-momentum and angular momentum apply quite perfectly to our universe.
Apr
12
comment why cannot fermions have non-zero vacuum expectation value?
@innisfree: In any "derivatively-coupled" theory, one cannot have a vev, right? Since there is no special point in field space.
Apr
11
reviewed Approve suggested edit on quadrupole moment and higher for simple current loop
Apr
10
comment Is $\langle k \vert k_1k_2\rangle=0$
In such a case, you'd have a 1-particle state in the initial Fock space and a 2-particle space in the final Fock space and an insertion of the time evolution operator in between them. In the case of an interacing theory with a corresponding trivalent vertex, the evolution operator can indeed cause one particle to decay into two.
Apr
10
comment Is $\langle k \vert k_1k_2\rangle=0$
In other words, a two particle state is orthogonal to a one particle state.
Apr
4
comment Dirac, Weyl and Majorana Spinors
Whoops! That was a bad slip on my part. Thanks for the correction @RobinEkman. It is indeed the direct sum.
Mar
30
comment How to conclude that an interaction is attractive from its Fourier transform (momentum space representation)?
Take a look at Anthony Zee's book: QFT in a nutshell. He explains why interactions mediated by spin 1 particles are repulsive for like charges and attractive for unlike charges, while they're the other way round for spin 0 or spin 2 mediators.
Mar
27
reviewed Approve suggested edit on Experimentally Verifying a Clock's Accuracy
Mar
27
comment Free particle propagator - Evaluating Integral
I don't think this answer is helpful. For starters, to get a feel for what is happening, just expand out each of the quadratic expressions in the exponent and then do the Gaussian integral for $x_1$ ($x_0$ and $x_2$ will be constants).
Mar
27
answered Free particle propagator - Evaluating Integral
Mar
26
comment What transformation is the metric of general relativity invariant under?
@Prahar: Depends on whether you're a mathematician or a physicist :P Physicists talk about objects with indices as if they transform (which they do). Mathematicians prefer to contract all indices with the corresponding basis and talk about "invariants".
Mar
26
answered There are two definitions of S operator (or S matrix) in quantum field theory. Are they equivalent?