  For a general thermodynamic system described as a Markov process, we prove a
general lower bound for dissipation in terms of the square of the heat current,
thus establishing that nonvanishing current inevitably implies dissipation.
This leads to a universal trade-off relation between efficiency and power, with
which we rigorously prove that a heat engine with nonvanishing power never
attains the Carnot efficiency. Our theory applies to systems arbitrarily far
from equilibrium, and does not assume any specific symmetry of the model.
