  We develop a general energy method for proving the optimal time decay rates
of the solutions to the dissipative equations in the whole space. Our method is
applied to classical examples such as the heat equation, the compressible
Navier-Stokes equations and the Boltzmann equation. In particular, the optimal
decay rates of the higher-order spatial derivatives of solutions are obtained.
The negative Sobolev norms are shown to be preserved along time evolution and
enhance the decay rates. We use a family of scaled energy estimates with
minimum derivative counts and interpolations among them without linear decay
analysis.
