  The tachyonic instability of the open bosonic string is analyzed using the
level truncation approach to string field theory. We have calculated all terms
in the cubic action of the string field theory describing zero-momentum
interactions of up to level 20 between scalars of level 10 or less. These
results are used to study the tachyon effective potential and the
nonperturbative stable vacuum. We find that the energy gap between the unstable
and stable vacua converges much more quickly than the coefficients of the
effective tachyon potential. By including fields up to level 10, 99.91% of the
energy from the bosonic D-brane tension is cancelled in the nonperturbative
stable vacuum. It appears that the perturbative expansion of the effective
tachyon potential around the unstable vacuum has a small but finite radius of
convergence. We find evidence for a critical point in the tachyon effective
potential at a small negative value of the tachyon field corresponding to this
radius of convergence. We study the branch structure of the effective potential
in the vicinity of this point and speculate that the tachyon effective
potential is globally nonnegative.
