The magnetic monopole predicted by Dirac nearly a century ago(1) was found in spin ice as a quasi-particle(2). My question is: why doesn't the magnetic monopole found in spin ice modify Maxwell's Equations? (I know they are not elementary particles but quasi-particles.)
(1) Dirac, P. A. M. Quantised singularities in the electromagnetic field. Proc. R. Soc. A 133, 60–72 (1931). DOI: 10.1098/rspa.1931.0130.
(2) Castelnovo, C., Moessner, R. & Sondhi, S. Magnetic monopoles in spin ice. Nature 451, 42–45 (2008). arXiv: 0710.5515 [cond-mat.str-el].