  The ac driven sine-Gordon equation is studied analytically and numerically,
with the aim of providing a full description of how soliton solutions behave.
To date, there is much controversy about when ac driven dc motion is possible.
Our work shows that kink solitons exhibit dc or oscillatory motion depending on
the relation between their initial velocity and the force parameters. Such
motion is proven to be impossible in the presence of damping terms. For
breathers, the force amplitude range for which they exist when dissipation is
absent is found. All the analytical results are compared with numerical
simulations, which in addition exhibit no dc motion at all for breathers, and
an excellent agreement is found. In the conclusion, the generality of our
results and connections to others systems for which a similar phenomenology may
arise are discussed.
