In addition, these theories were being developed just as other physicists were beginning to discover quantum mechanics, which would ultimately prove successful in describing known forces such as electromagnetism, as well as new nuclear forces that were being discovered throughout the middle part of the century.
Dodatkowo teorie te rozwijały się, gdy inni fizycy zaczynali odkrywać mechanikę kwantową, która ostatecznie przyniosła sukcesy w opisie znanych oddziaływań, jak elektromagnetyzm, jak też nowych sił jądrowych, które odkryto w połowie XX wieku.