The inability of scientists to create a theory of quantum gravity arises from long-standing tensions between general relativity and quantum mechanics. There have been few approaches with any success. In this video, Fermilab’s Dr. Don Lincoln explains one of the few promising ideas, called loop quantum gravity.

Further reading:
http://www.einstein-online.info/spotlights/spin_networks.1.html

Full Quantum series: https://www.youtube.com/playlist?list=PLCfRa7MXBEsq5JoO2gU3yWhxTr06SY2De