  The ratio between the proton and electron masses was shown to be close to the
ratio between the shortest lifetimes of particles, decaying by the
electromagnetic and strong interactions. The inherent property of each
fundamental interaction is defined, namely the Minimal lifetime of the
interaction (MLTI). The rest mass of the Lightest free massive stable particle
(LFMSP), acted upon by a particular interaction, is shown to be inversely
proportional to MLTI. The found mass relation unifies the masses of four stable
particles of completely different kinds (proton, electron, electron neutrino
and graviton) and covers an extremely wide range of values, exceeding 40 orders
of magnitude. On the basis of this mass relation, the electron neutrino and
graviton masses have been approximately estimated to 6.5x10^(-4) eV/c^2 and
H*h_bar/c^2 = 1.5x10^(-33) eV/c^2, respectively. Besides, the last value has
been obtained independently by dimensional analysis by means of the fundamental
parameters speed of light (c), reduced Planck constant (h_bar) and Hubble
distance (H). It was shown that the equivalent energy of LFMSP, acted upon by a
particular interaction, is close to Breit-Wigner's energy width of the shortest
living state, decaying by the respective interaction.
