next up previous
Next: Energy Conservation Up: Classical Lattice Gauge Previous: Classical Lattice Gauge

Gauss' Law and its Rate of Change

The Noether charge, corresponding to lattice gauge theory is similar to the one derived for the motion of a point particle on the unit circle, with the however important difference that this time a rotation, i.e. a multiplication with a unitary matrix G has to be done either at the beginning or at the end of the link influencing naturally some further links attached to that site. This kind of local unitary rotation is a gauge transformation.

The total change of a link variable U by such infinitesimal rotations at its beginning and end independently can be disantangled to a sum of transformation generators located at the lattice sites. Since such transformations leave the Hamiltonian ( 50 ) due to its construction using traces of group elements multiplied over oriented closed link doubles --- like <P,P> --- or elementary squares (plaquettes) --- like --- invariant, the above mentioned infinitesimal generators of the transformation define a Noether charge. In the continous time limit it is a quaternion (for SU(2) ) defined on each lattice site summing up contributions from outgoing () and subtarcting reverse contributions from incoming () links,

Its rate of change, upon using the continuum time equation of motion and the correct value of the Lagrange multiplier obtained from the orthogonality condition becomes

The corrections steming from the use of the Lagrange multiplier manage to make each incoming and outgoing link contribution to the change rate of the Noether charge --- to the non commutivity of the Gauss' law with the equations of motion --- traceless. It is now relatively easy to convince ourselves that this total change is zero if using a little bit of geometrical imagination. Denoting each group element multiplication by oriented lines sweeping through the corresponding links --- in positive x- y- or z- direction if encountering a U and in the reverse direction if encountering a --- the complement variable can be viewed as oriented pathes matching to the link of U. The products and consist then of complete plaquette pathes oriented correspondingly. (These pathes are not closed, because in calculating the Gauss law we do not take the trace of these products.)

Summing up now outgoing and subtracting incoming plaquette contributions one immediately realizes that half of these plaquette pathes (in two dimensions the right upper and the left lower plaquettes attached to the site) directly cancel while the other half (the left upper and right lower corner plaquettes) consists of pairwise oppositely oriented sums. An oppositely oriented sum for any path --- i.e. for any product of link variables --- means, however, an expression like , a result equal to the unit element multiplied by . But exactly this has been substracted by the corresponding <U,V>1 terms steming from the Lagrange multiplier (in other words the Noether charge change rate is traceless if orthogonality is conserved)!

Finally the respective definitions of the finite time step recursion equations and the Noether charge, norm and orthogonality at half integer time points ensure that the same proof holds for the numerical recursion for a general Type III algorithm.



next up previous
Next: Energy Conservation Up: Classical Lattice Gauge Previous: Classical Lattice Gauge