History of ultimate models [of physics]
From the earliest days of Greek science until well into the 1900s, it seems to have often been believed that an ultimate model of the universe was not far away. In antiquity there were vague ideas about everything being made of elements like fire and water. In the 1700s, following the success of Newtonian mechanics, a common assumption seems to have been that everything (with the possible exception of light) must consist of tiny corpuscles with gravity-like forces between them. In the 1800s the notion of fields—and the ether—began to develop, and in the 1880s it was suggested that atoms might be knotted vortices in the ether (see page 1044). When the electron was discovered in 1897 it was briefly thought that it might be the fundamental constituent of everything. And later it was imagined that perhaps electromagnetic fields could underlie everything. Then after the introduction of general relativity for the gravitational field in 1915, there were efforts, especially in the 1930s, to introduce extensions that would yield unified field theories of everything (see page 1028). By the 1950s, however, an increasing number of subatomic particles were being found, and most efforts at unification became considerably more modest. In the 1960s the quark model began to explain many of the particles that were seen. Then in the 1970s work in quantum field theory encouraged the use of gauge theories and by the late 1970s the so-called Standard Model had emerged, with the Weinberg–Salam SU(2) × U(1) gauge theory for weak interactions and electromagnetism, and the QCD SU(3) gauge theory for strong interactions. The discoveries of the c quark, τ lepton and b quark were largely unexpected, but by the late 1970s there was widespread enthusiasm for the idea of a single "grand unified" gauge theory, based say on SU(5), that would explain all forces except gravity. By the mid-1980s failure to observe expected proton decay cast doubts on simple versions of such models, and various possibilities based on supersymmetry and groups like SO(10) were considered. Occasional attempts to construct quantum theories of gravity had been made since the 1930s, and in the late 1980s these began to be pursued more vigorously. In the mid-1980s the discovery that string theory could be given various mathematical features that were considered desirable made it emerge as the main hope for an ultimate "theory of everything". But despite all sorts of elegant mathematical work, the theory remains rather distant from observed features of our universe. In some parts of particle physics, it is still sometimes claimed that an ultimate theory is not far away, but outside it generally seems to be assumed that physics is somehow instead an endless frontier—that will continue to yield a stream of surprising and increasingly complex discoveries forever—with no ultimate theory ever being found.