  Starting with a Nambu-Goto action, a Dirac-like equation can be constructed
by taking the square-root of the momentum constraint. The eigenvalues of the
resulting Hamiltonian are real and correspond to masses of the excited string.
In particular there are no tachyons. A special case of radial oscillations of a
closed string in Minkowski space-time admits exact solutions in terms of wave
functions of the harmonic oscillator.
