This book highlights selected contributions presented at the 15th annual international symposium Frontiers of Fundamental Physics (FFP15), with the aim of informing readers about the most important recent advances in fundamental physics and physics education research. The FFP series offers a platform for physicists from around the world to present their latest theories and findings. The latest symposium was held in Orihuela, Spain and covered diverse fields of research, including gravitation, astronomy and astrophysics, physics of complex systems, high-energy physics, and mathematical physics. Considerable attention was also paid to physics education research, teacher education in physics, and the popularization of physics. In a knowledge-based society, research into fundamental physics plays a vital role in both the advancement of human knowledge and the development of new technologies. Presenting valuable new peer-reviewed contributions submitted from 15 countries, this book will appeal to a broad audience of scholars and researchers.