It's not strictly true that gluons and quarks cannot be separated, but it requires very extreme conditions that have not existed in nature since the first tiny fractions of a second after the Big Bang.

During that time, before any hadrons had even formed, the infant universe was filled with a soup of free quarks and gluons known as the quark-gluon plasma (plus leptons such as neutrinos and electrons).

The universe very quickly cooled as it expanded, and by the first millionth of a second, the temperature had dropped sufficiently, to 2 trillion degrees(opens in new tab), to allow the strong force to bind quarks and gluons together to form the first hadrons.

(Image credit: CERN)(opens in new tab)The atomic nuclei of heavy elements such as gold or lead are smashed together at almost the speed of light, resulting in a miniature fireball that for a brief moment is hot enough to dissolve hadrons into a quark-gluon plasma.

By learning about properties such as these, recreating the quark-gluon plasma in particle accelerators can give scientists a window back in time to the very birth of the universe and the immediate aftermath of the Big Bang when matter first came into being.

Read the story of the discovery of gluons in 1979, as told by DESY physicists Ilka Flegel and Paul Söding in the CERN Courier(opens in new tab).