The term modern physics refers to the post-Newtonian concept of physics, i.e. the end of the dominance of the classical description of physical phenomena, and therefore the accurate "modern" description of reality requires theories that include elements of quantum mechanics or Einstein's relativity or both, and this term is usually used to refer to any branch of physics It was developed at or after the beginning of the twentieth century or was greatly influenced by the physics of the beginning of the twentieth century. Modern physics often includes extreme cases such as quantum mechanics guaranteeing distances close to the atomic dimensions (10−9 meters), and relativity guaranteeing speeds close to the speed of light (108 m/s). , since large distances and small speeds are involved in classical mechanics.