  The article presents a review of studying unstable states in the relativistic
dissociation of nuclei $^{10}$B, $^{11,12}$C, $^{16}$O, $^{22}$Ne, $^{28}$Si,
$^{84}$Kr and $^{197}$Au in the energy range from hundreds of MeV/nucleon to
several tens of GeV/nucleon using the nuclear track emulsion method. A
systematic study of the fragmentation of incident nuclei with multiple
formation of the lightest fragments of He and H. This made it possible to study
the dynamics of unstable nuclear states of $^8$Be, the Hoyle state and the
4$\alpha$-particle state of the $^{16}$O nucleus above the threshold in the
relativistic dissociation of nuclei due to precision measurements of fragment
emission angles. It is shown that to reconstruct the relativistic decays of
unstable nuclei in a nuclear track emulsion, it is sufficient to determine the
invariant mass of the system of He and H fragments in the approximation of
conservation of momentum per nucleon of the parent nucleus. This approach
allows one to search for more complex nuclear states. An indication has been
obtained of growing probability to detect $^8$Be with increasing the number of
relativistic $\alpha$ particles in the event.
