Unification of theories in physics
Unification of theories about observable fundamental phenomena of nature is one of the primary goals of physics. Isaac Newton’s unification of terrestrial and celestial mechanics into one theory of gravity in the 17th century
and James Clerk Maxwell’s unification of electricity with magnetism into electromagnetism in the 19th were the first two unifications. In the 20th century the focus shifted to the unification of fundamental interactions, beginning with the unification of electromagnetic forces with the weak force to create electroweak interaction. This process of "unifying" forces continues today, with the ultimate goal of finding a theory of everything.
Past instances
Unification of gravity on Earth with astronomical behaviors
The "first great unification" was Isaac Newton's 17th century unification of gravity, in which he brought together the understandings of the observable phenomena of gravity on Earth with the observable laws of behaviour of celestial bodies in space, formulating a fundamentally new, universal mathematical framework that applied to every particle in the universe. This new law accounted for both terrestrial and celestial mechanics, superseding the local, approximate laws of Galileo Galilei and Johannes Kepler with a single, abstract principle that governed the entire cosmos.Newton's unification is considered a foundational step in the quest for a unified theory of physics. Physicist Steven Weinberg stated that "It is with Isaac Newton that the modern dream of a final theory really begins".
Unification of magnetism, electricity, light and related radiation
The ancient Chinese people observed that certain rocks such as lodestone and magnetite were attracted to one another by an invisible force. This effect was later called magnetism, which was first rigorously studied in the 17th century. However, prior to ancient Chinese observations of magnetism, the ancient Greeks knew of other objects such as amber, that when rubbed with fur would cause a similar invisible attraction between the two. This was also studied rigorously in the 17th century and came to be called electricity. Thus, physics had come to understand two observations of nature in terms of some root cause. However, work in the 19th century revealed that these two forces were just two different aspects of one force – electromagnetism.The "second great unification" was James Clerk Maxwell's 19th century unification of electromagnetism. It brought together the understandings of the observable phenomena of magnetism, electricity and light.
This was followed in the 20th century by Albert Einstein's unification of the description of space and time into an inseparable continuum, and of mass and energy through his theory of special relativity.