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seen Mar 18 at 22:46

Diego Mazón


Mar
7
comment Why am I wrong about how to view gauge theory?
In this context a redundant transformations means that it does not change the physical state of the system but only the field related to it (and that creates spurious d.o.f).
Mar
7
comment Why am I wrong about how to view gauge theory?
1) Are you saying that non-large gauge transformations (those that are continuously connected with the identity) change the physical state of a system? 2) The gauge transformations you say that they are not redundancies, do they tend to the identity in the space-time boundary? Are they the so-called "large gauge transformations"? Thanks.
Feb
20
awarded  Nice Answer
Feb
17
awarded  quantum-field-theory
Feb
10
comment When is quasiparticle same as elementary excitation, and when is not?
@huotuichang The Poincare algebra has two quadratic Casimir invariant: the square of the cuadri-momentum (the particle's mass) and the square of the Pauli- Lubanski (related to the spin or helicity of the particle), which self-commute. Therefore, the eigenvalues of these operators are good label. This classification is originally due to Wigner. Elementary particles are also classified according to the way they transform under the gauge group of the standard model $U_y(1)\times SU_l(2)\times SU_c(3)$
Feb
7
comment When is quasiparticle same as elementary excitation, and when is not?
Hello @IsidoreSeville Then, can some of you provide the definition of "elementary excitation" in condensed matter physics? I only see one sense in which an "elementary excitation" can be elementary. If this answer is correct, then, in my opinion, this condensed matter terminology should be abandoned because it is confusing and useless.
Feb
7
revised When is quasiparticle same as elementary excitation, and when is not?
dictionary and short answer added
Feb
7
comment When is quasiparticle same as elementary excitation, and when is not?
Hello, you read correctly, but excitation=particle, while elementary$\neq$collective . Thanks.
Feb
6
revised When is quasiparticle same as elementary excitation, and when is not?
added 232 characters in body
Feb
6
awarded  Custodian
Feb
6
reviewed Approve suggested edit on What determines color — wavelength or frequency?
Feb
6
reviewed Approve suggested edit on What is the geometrical interpretation of Ricci tensor?
Feb
6
comment When is quasiparticle same as elementary excitation, and when is not?
The question asks about the difference between quasiparticles and elementary particles.
Feb
6
revised When is quasiparticle same as elementary excitation, and when is not?
deleted 191 characters in body
Feb
6
answered When is quasiparticle same as elementary excitation, and when is not?
Feb
5
comment What exactly is regularization in QFT?
(cont) require us to do analytic continuation to imaginary time. Regarding the naturalness problem, in theories where Lorentz invariance is just an emergent or approximate low-energy symmetry, the renormalization group flow makes the coefficient of $(\nabla \phi )^2$ (or the equivalent in spinorial or tensorial theories) run with energy, implying stronger constraints than the naive (tree-level) ones. We can move to a chat room in the case you want to discuss.
Feb
5
comment What exactly is regularization in QFT?
(cont.) When one talks about a physical regulator, one means a regulator such that the intermediate steps in the calculation make sense, and therefore do not break any gauge symmetry or quantum unitarity. Of course, the existence of such a regulator is not a strict requirement of a theory (what matters are final results), but the search of such physical regulator may perhaps help find a more fundamental theory. In condensed matter QFT, one DOES has a physical regulator making this case completely different in this regard. For example, this regulator does not break unitarity or does not (cont)
Feb
5
comment What exactly is regularization in QFT?
@Adam One can define a 4d euclidean lattice for QFT (there are issues with chiral theories though) and the cubic discrete of the lattice makes the theory O(4) rotational invariant in the continuum limit. There is nothing analogous in Minkowski. One may be happy with analytical continuation to imaginary times as a calculation device or trick, likewise one may be happy with a cut-off in momentum space in spite of the fact it breaks explicit gauge invariance in intermediate steps. The final results agree each other and with experiments, and from a positivistic viewpoint this is enough. (cont.)
Feb
5
comment What exactly is regularization in QFT?
(cont.) than the Planck length. Having said this, there are claims I don't understand that say that string theory and LQG provide the SM with physical regulators.
Feb
5
comment What exactly is regularization in QFT?
@Adam In order for the eucl. lattice to be physical, i.e., that spacetime is euclidean and discrete at short enough distances, you don't have to imagine anything, you have to provide a mechanism to make it compatible with the well-known physics. For example, you have to explain how spacetime seems to be Lorentzian (Minkowsian) rather than Riemannian (Euclidean), that is, how the signature changes. You also have to explain why we do see Lorentz inv., so that either you have to address the "speed of light naturalness problem" or the lattice spacing is many orders of magnitude lower (cont.)