In 1924, Louis de Broglie proposed that not only do light waves sometimes exhibit particle-like properties, but particles may also exhibit wave-like properties. Two different formulations of quantum mechanics were presented following de Broglie's suggestion. The wave mechanics of Erwin Schrödinger (1926) involves the use of a mathematical entity, the wave function, which is related to the probability of finding a particle at a given point in space. The matrix mechanics of Werner Heisenberg (1925) makes no mention of wave functions or similar concepts but was shown to be mathematically equivalent to Schrödinger's theory. A particularly important discovery of the quantum theory is the uncertainty principle, enunciated by Heisenberg in 1927, which places an absolute theoretical limit on the accuracy of certain measurements; as a result, the assumption by earlier scientists that the physical state of a system could be measured exactly and used to predict future states had to be abandoned. Quantum mechanics was combined with the theory of relativity in the formulation of Paul Dirac. Other developments include quantum statistics, quantum electrodynamics, concerned with interactions between charged particles and electromagnetic fields; and its generalization, quantum field theory. String Theory A possible candidate for the theory of everything, this theory combines the theory of general relativity and quantum mechanics to make a single theory. This theory can predict about properties of both small and big objects. This theory is currently under the developmental stage. Optics and acoustics Optics is the study of light motions including reflection, refraction, diffraction, and interference. Acoustics is the branch of physics involving the study of mechanical waves in different mediums. Condensed matter physics The study of the physical properties of matter in a condensed phase. High-energy particle physics and nuclear physics Particle physics studies the nature of particles, while nuclear physics studies the atomic nuclei. Cosmology Cosmology studies how the universe came to be, and its eventual fate. It is studied by physicists and astrophysicists. Interdisciplinary fields To the interdisciplinary fields, which define partially sciences of their own, belong e.g. the agrophysics is a branch of science bordering on agronomy and physics astrophysics, the physics in the universe, including the properties and interactions of celestial bodies in astronomy. space physics is the study of plasmas as they occur naturally in the Earth's upper atmosphere (aeronomy) and within the Solar System. biophysics, studying the physical interactions of biological processes. chemical physics, the science of physical