Unifications in Physics
Many of the greatest discoveries in physics have been the result of unification of concepts that were previously thought to be distinct. For instance, James Clerk Maxwell unified electricity and magnetism with his electromagnetic theory and Einstein unified space, time, and gravity as well as matter and energy with his theory of relativity. The ultimate unification would be a Theory of Everything - a single theory which would in principle be capable of predicting all natural phenomena. Many physicists are hopeful of finding such a theory.
Galileo was responsible for one of the first great unifications, namely the unification of systems at rest with moving systems. The unification was Galilean relativity, the idea of which is that there is no way to distinguish between being at rest and being in motion at constant velocity. This is why we do not feel the Earth rotating or moving through space.
Isaac Newton came up with another great unification, the unification of planetary motion with terrestrial gravity. His law of universal gravitation did this by asserting that the same force which makes an apple fall is responsible for keeping the moon in orbit about the earth as well as the planets orbiting the sun.
James Clerk Maxwell completed great unification, namely the unification of electricity and magnetism, two forces previously thought to be distinct. This unification was begun by others including Ampere and Faraday. Maxwell's unification was very powerful in that it made new predictions, namely the existence of electromagnetic waves and that light is such a wave.
Albert Einstein was arguably the greatest unifier of phenomena previously believed to be distinct. His special theory of relativity unified space and time as well as matter and energy. His general theory of relativity further unified space, time, and accelerated motion with gravity.
The ultimate dream of physicists is the search for a Theory of Everything. Part of this program would be to find a unified field theory, which would unify each of the four known forces, namely electromagnetism and gravity as well as the strong and weak nuclear forces. Glashow, Weinberg, and Salam already succeeded in part of this unification with their electroweak theory, which unified electromagnetism and the weak force. Grand Unified Theories (GUTs) attempt to further unify the strong force. String theory offers hope of unifying all the forces, though after over 20 years, it has been unable to offer any testable predictions. Physicists are also trying to unify general relativity with quantum mechanics, which is turning out to be an equally difficult challenge.