Editing Theory of everything (section)

===Late 20th century and the nuclear interactions===
In the 20th century, the search for a unifying theory was interrupted by the discovery of the [[strong force|strong]] and [[weak force|weak]] nuclear forces, which differ both from gravity and from electromagnetism. A further hurdle was the acceptance that in a theory of everything, quantum mechanics had to be incorporated from the outset, rather than emerging as a consequence of a deterministic unified theory, as Einstein had hoped.

Gravity and electromagnetism are able to coexist as entries in a list of classical forces, but for many years it seemed that gravity could not be incorporated into the quantum framework, let alone unified with the other fundamental forces. For this reason, work on unification, for much of the 20th century, focused on understanding the three forces described by quantum mechanics: electromagnetism and the weak and strong forces. The first two were [[electroweak interaction|combined]] in 1967–1968 by [[Sheldon Glashow]], [[Steven Weinberg]], and [[Abdus Salam]] into the electroweak force.<ref>Weinberg (1993), Ch. 5</ref>
Electroweak unification is a [[broken symmetry]]: the electromagnetic and weak forces appear distinct at low energies because the particles carrying the weak force, the [[W and Z bosons]], have non-zero masses ({{val|80.4|u=GeV/c2}} and {{val|91.2|u=GeV/c2}}, respectively), whereas the [[photon]], which carries the electromagnetic force, is massless. At higher energies W bosons and Z bosons can be [[matter creation|created]] easily and the unified nature of the force becomes apparent.

While the strong and electroweak forces coexist under the [[Standard Model]] of particle physics, they remain distinct. Thus, the pursuit of a theory of everything remained unsuccessful: neither a unification of the strong and electroweak forces&nbsp;– which Laplace would have called 'contact forces'&nbsp;– nor a unification of these forces with gravitation had been achieved.