  The Bethe-Salpeter equation is related to a generalized quantum-mechanical
Hamiltonian. Instability of the presumed vacuum, indicated by a tachyon, is
related to a negative energy eigenstate of this Hamiltonian. The variational
method shows that an arbitrarily weak long-range attraction leads to chiral
symmetry breaking, except in the scale-invariant case when the instability
occurs at a critical value of the coupling. In the case of short-range
attraction, an upper bound for the critical coupling is obtained.
