Quantum physics deals with phenomena at the atomic and subatomic scale. It studies how matter behaves at this small scale through space and time, and how it relates to energy and forces. Some of the fundamental theories that defined quantum physics include Planck's discovery that energy and frequency of light are proportional, and Einstein's theories of the photoelectric effect and relativity. Quantum physics seeks to explain phenomena that classical physics cannot, such as at the subatomic level, and may help explain many unknown aspects of the universe.