Unification of theories in physics

Unification of theories about observable fundamental phenomena of nature is one of the primary goals of physics.^[1][2][3] The two great unifications to date are Isaac Newton’s unification of gravity and astronomy, and James Clerk Maxwell’s unification of electromagnetism; the latter has been further unified with the concept of electroweak interaction. This process of "unifying" forces continues today, with the ultimate goal of finding a theory of everything.

The "first great unification" was Isaac Newton's 17th century unification of gravity, which brought together the understandings of the observable phenomena of gravity on Earth with the observable behaviour of celestial bodies in space.^[2][4][5]

His work is credited with laying the foundations of future endeavors for a grand unified theory. For example, it has been stated that "If we have to take any single individual as the originator of the quest for a unified theory of physics, and, by implication, the whole of knowledge, it has to be Newton."^[6] Physicist Steven Weinberg stated that "It is with Isaac Newton that the modern dream of a final theory really begins".^[7]

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.^[8] 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 (electricity and magnetism). 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 (and more broadly, the spectrum of electromagnetic radiation).^[9] This was followed in the 20th century by Albert Einstein's unification of space and time, and of mass and energy through his theory of special relativity.^[9] Later, Paul Dirac developed quantum field theory, unifying quantum mechanics and special relativity.^[10]

A relatively recent unification of electromagnetism and the weak nuclear force now consider them to be two aspects of the electroweak interaction.

This process of "unifying" forces continues today, with the ultimate goal of finding a theory of everything – it remains perhaps the most prominent of the unsolved problems in physics. There remain four fundamental forces which have not been decisively unified: the gravitational and electromagnetic interactions, which produce significant long-range forces whose effects can be seen directly in everyday life, and the strong and weak interactions, which produce forces at minuscule, subatomic distances and govern nuclear interactions. Electromagnetism and the weak interactions are widely considered to be two aspects of the electroweak interaction. Attempts to unify quantum mechanics and general relativity into a single theory of quantum gravity, a program ongoing for over half a century, have not yet been decisively resolved; current leading candidates are M-theory, superstring theory and loop quantum gravity.^[2]