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Fermi showed that electrons, as a result of their quantum nature, form a gasof particles where the temperature and density follow the so called Fermidistribution. In a metal, as shown by Landau, that despite their strong Coulombinteraction with each other and the positive background ions, the electronscontinue to act like free quantum mechanical particles but with enhancedmasses. This state of matter, the Landau-Fermi liquid, is recognizedexperimentally by an electrical resistivity that is proportional to the squareof the absolute temperature plus a term proportional to the square of thefrequency of the applied field. Calculations show that, if electron-electronscattering dominates the resistivity in a Landau Fermi liquid, the ratio of thetwo terms, $b$ has the universal value of {\em b} = 4. We find that in thenormal state of the heavy Fermion metal URu$_2$Si$_2$, instead of the Fermiliquid value of 4 the coefficient $b$ =1 $\pm$ 0.1. This unexpected resultimplies that the electrons in this material are experiencing a uniquescattering process. This scattering is intrinsic and we suggest that, theuranium $f$ electrons do not hybridize to form a coherent Fermi liquid butinstead act like a dense array of elastic impurities, interacting incoherentlywith the charge carriers. This behavior is not restricted to URu$_2$Si$_2$.Fermi liquid like states with $b \neq$ 4 have been observed in a number ofdisparate systems but the significance of this result has not been recognized.

Optical spectroscopy shows that the normal state of URu$_2$Si$_2$ is an anomalous Fermi liquid