A proof that new massive gravity - the massive 3D gravity model proposed by
Bergshoeff, Hohm and Townsend (BHT) - is the only unitary system at the tree
level that can be constructed by augmenting planar gravity through the
curvature-squared terms, is presented. Two interesting gravitational properties
of the BHT model, namely, time dilation and time delay, which have no
counterpart in the usual Einstein 3D gravity, are analyzed as well.Comment: Submitted to Classical and Quantum Gravit
