  We discuss the excitation spectrum of a disordered, isotropic and
translationally invariant spin state in the 2D Heisenberg antiferromagnet. The
starting point is the nearest-neighbor RVB state which plays the role of the
vacuum of the theory, in a similar sense as the Neel state is the vacuum for
antiferromagnetic spin wave theory. We discuss the elementary excitations of
this state and show that these are not Fermionic spin-1/2 `spinons' but spin-1
excited dimers which must be modeled by bond Bosons. We derive an effective
Hamiltonian describing the excited dimers which is formally analogous to spin
wave theory. Condensation of the bond-Bosons at zero temperature into the state
with momentum (pi,pi) is shown to be equivalent to antiferromagnetic ordering.
The latter is a key ingredient for a microscopic interpretation of Zhang's
SO(5) theory of cuprate superconductivity
