  Nonrelativistic Fermi liquids in d+1 dimensions exhibit generalized Fermi
surfaces: (d-p)-dimensional submanifolds in the momentum-frequency space
supporting gapless excitations. We show that the universality classes of stable
Fermi surfaces are classified by K-theory, with the pattern of stability
determined by Bott periodicity. The Atiyah-Bott-Shapiro construction implies
that the low-energy modes near a Fermi surface exhibit relativistic invariance
in the transverse p+1 dimensions. This suggests an intriguing parallel between
norelativistic Fermi liquids and D-branes of string theory.
