  Starting from a Poincar\'e invariant field theory of a real scalar field with
interactions governed by a double-well potential in 2+1 dimensions, the Lorentz
representation induced on the collective coordinates describing low-energy
excitations about an effective string background is derived. In this
representation, Lorentz transformations are given in terms of an infinite
series, in powers of derivatives along the worldsheet. Transformations that act
on the direction transverse to the string worldsheet involve a universal
dimension $-1$ term. As a consequence, Lorentz invariance holds in this theory
of long effective strings due to cancellations in the action between irrelevant
terms and the dimension two term that describes free massless scalar fields in
two dimensions. (in plain tex, no macropackages necessary)
