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Oct
8
comment Spacetime Torsion, the Spin tensor, and intrinsic spin in Einstein-Cartan theory
Sorry, the first sentence in my comments should be: The scalar and Dirac field examples were not given for the purpose of showing how to couple torsion to classical fields, they were given as examples of matter.
Oct
8
comment Spacetime Torsion, the Spin tensor, and intrinsic spin in Einstein-Cartan theory
The answer to your second comment is that nothing was added by hand, the coupling term is obtained according to the rules of the minimal coupling. The only manipulation done is to separate the symmetric part of the affine connection and include it in the spin connection, and writing the antisymmetrical components separately giving the interaction term. In summary, the coupling of the matter fields to Einstein-Cartan theory can be viewed as a stage in the determination of its spin tensor.
Oct
8
comment Spacetime Torsion, the Spin tensor, and intrinsic spin in Einstein-Cartan theory
This is given in equation (2.20) in Shapiro. The spin tensor is also analogous to the energy momentum tensor being a Noether current of the Lorentz symmetry as the energy momentum tensor is the Noether current of the translation symmetry. (Together they generate the Poincare symmetry).
Oct
8
comment Spacetime Torsion, the Spin tensor, and intrinsic spin in Einstein-Cartan theory
This is exactly analogous to the energy momentum tensor: In order to obtain the energy momentum tensor, first one minimally couples the matter theory to gravity, and then varies the Lagrangian with respect to the metric. The analogy for the spin tensor: first one couples the matter theory to gravity with torsion (i.e., with a nonsymmetrical affine connection), then varies the Lagrangian with respect to the contorsion.
Oct
8
comment Spacetime Torsion, the Spin tensor, and intrinsic spin in Einstein-Cartan theory
The scalar and Dirac field examples were given for the purpose of showing how to couple torsion to classical fields, they were given as examples of matter, please see that these examples are given in Shapiro's review in section 2.3 titled as "Interaction of torsion with matter". The interaction appears as a term of the form: Spin tensor $\times$ contorsion tensor as can be seen in the Dirac example.
Aug
4
comment What does it mean to renormalize an effective field theory?
@ New_new_newbie It may happen that some process (like photon-photon scattering in QED) does not exist at the tree level approximation, then you can say that radiative corrections predict new physics. But apart from that, the control of the quantum corrections is mainly required to ensure that these corrections will not spoil the leading order.
Jul
3
comment Expressing an adjoint representation Wilson line in terms of the fundamental representation
Sorry, $E^{ij}$ is the matrix with all elements zero except a unit at the place $i,j$
Jun
5
comment The $U(1)$ charge of a representation
@user41746, cont.Now, Slansky insists to identify the highest weight of the $SU(5) $ representation with the highest weight of the quark representation after the symmetry breaking. For that he needs to act on the weights with a “projection matrix” given in equation 6.7. After the action of the projection matrix the fourth component of the weights is lost, but nevertheless, the dual U(1) vector can be obtained by insisting that it has a constant scalar product with each subrepresentation weights. As you can see all the exercises lead to the same result for the hypercharges.
Jun
5
comment The $U(1)$ charge of a representation
@user41746, Sorry I noticed that I used $a_1, a_2$ for the $SU(3)$ labels and $a_4$ for the $SU(2)$ and in your example you used $a_3, a_4$ for $SU(3)$ labels and $a_1$ for SU(2). Thus you should use the second row in $G$ for your example. You will get the same result with the normalization factor $\frac{1}{3}$.
Jun
5
comment The $U(1)$ charge of a representation
@user41746 $SU(5)$ is $A_4$, so the metric tensor should be $4\times 4$ matrix, please see the first matrix in table 7 page 82. Its third row for n=4 is just the given vector. I can’t see this exercise on page 84 (This page belongs to the appendix and it contains tables 10 and 11a. However, on page 16 equation (3.3), Slansky obtained the same hypercharges as in the question.
May
22
comment The derivation of the Belinfante-Rosenfeld tensor
@ramanujan_dirac: The energy momentum tensor constitutes of the source term in Einstein equations: $R_{\mu\nu}-g_{\mu\nu}R = \frac{8\pi G}{c^4}T_{\mu\nu}$, and both the Ricci curvature and the metric tensor are symmetric.
May
1
comment The curvature of the space of commuting hermitian matrices
@vesofilev: Sorry for not understanding you. The following reference arxiv.org/abs/0801.2913 contains a parameterization of the matrix $U$ for the flag manifold $\frac{SU(3)}{U(1)\times U(1)}$ on page 9, the third paragraph. (The authors call it the dressing matrix).
Apr
30
comment The curvature of the space of commuting hermitian matrices
@vesofilev: The $SU(2)$ case is a special case. Take for example the space of one commuting matrix in $SU(3)$, the spaces obtained from the adjoint action on $diag(0,0,1)$ and $diag(0,1,1)$ are disconnected.
Apr
30
comment The curvature of the space of commuting hermitian matrices
The space of $p$ commuting $n\times n$ matrices is not connected, since a unitary transformation does not change the rank of a matrix. Each connected component corresponds to a specific choice of the ranks of the commuting matrices. The connected components are generalized flag manifolds. Since they are homogeneous spaces, they are of constant curvature and there are many ways to calculate their curvatures.The following lecture notes may be of help to you uregina.ca/~mareal/flag-coh.pdf
Mar
25
comment Soliton Moduli Spaces and Homotopy Theory
@jamalS cont. There are more applications besides anomalies such as spinc quantization, charge quantization and fractionalization, Brane charges, supersymmetric quantum mechanics, Berezin-Toeplitz quantization and may be more. I don’t know of a single review comprising all these applications.
Mar
25
comment Soliton Moduli Spaces and Homotopy Theory
@jamalS There are analogous computations for the monopole moduli space dimension in the following review arxiv.org/abs/hep-th/0609055v2 by Weinberg and Yi, and for nonAbelian vortices in arxiv.org/abs/hep-th/0306150v1 by Hannay and Tong. There is a more mathematical review of the instanton and Seiberg-Witten moduli spaces dimension computation by David Bleecker milne.ruc.dk/~Booss/A-S-Index-Book/BlckBss_2012_04_25.pdf.
Mar
20
comment Lie group of Schrodinger Wave equation
@user35952 I'll try to answer your new question soon, I'll try to clarify the previous "Poincare group vs Galilean group" answer
Mar
13
comment Which symmetric pure qudit states can be reached within local operations?
@Piotr Migdal Thank you very much
Mar
9
comment Lie algebra of axial charges
@gian_25 The matrix representation is just the 4-dimensional fundamental representation of $SO(4) = SU(2) \times SU(2)$, denoting the indices of the 4-space corresponding to $(\sigma, \pi_1, \pi_2, \pi_3)$ by $0,1,2,3$. Then$ (T^{V}_i)_{jk} = i \epsilon_{ijk}, (T^{(A)}_i)_{jk} = i (\delta_{ji}\delta_{k0}+ \delta_{ki}\delta_{i0})$. Please observe that the commutator of two axial generators is a vector generator.
Mar
6
comment How to prove the covariant derivative cannot be written as an eigendecomposition of the partial derivative?
@MBN there are partial results, for example Ellis meghnad.iucaa.ernet.in/~tarun/pprnt/topology/… mentions a result by Clarke that every 3+1 noncompact space-time can be isometrically embedded into a flat $\mathbb{R}^{87,3}$.