  For a compact subset $K$ of a closed symplectic manifold $(M, \omega)$, we
prove that $K$ is heavy if and only if its relative symplectic cohomology over
the Novikov field is non-zero. As an application we show that if two compact
sets are not heavy and Poisson commuting, then their union is also not heavy. A
discussion on superheaviness together with some partial results are also
included.
