  The frequency dependent specific heat has been measured under pressure for
the molecular glass forming liquid 5-polyphenyl-4-ether in the viscous regime
close to the glass transition. The temperature and pressure dependence of the
characteristic timescale associated with the specific heat is compared to the
equivalent timescale from dielectric spectroscopy performed under identical
conditions. It is shown that the ratio between the two timescales is
independent of both temperature and pressure. This observation is non-trivial
and demonstrates the existence of specially simple molecular liquids in which
different physical relaxation processes are both as function of temperature and
pressure/density governed by the same underlying "inner clock". Furthermore,
the results are discussed in terms of the recent conjecture that van der Waals
liquids, like the measured liquid, comply to the isomorph theory.
