At very high energy levels, the electromagnetic and weak forces were one and the same

 "There are four known forces in the universe: gravity; electromagnetism; the strong force, which binds the nuclei of atoms together; and the weak force, which causes radioactive decay. The first two forces have been known for centuries, but the other two were discovered only in the first two decades of the 20th century.
Over the next decades, physicists struggled to find a theory that would account for all the forces, or what Einstein called a theory of everything. Though there were significant discoveries, particularly of new particles with exotic names like quarks (the components of protons and neutrons in the nucleus) and leptons (which include electrons but also more esoteric particles called muons and taus), a unified theory or model remained elusive.

In 1967, Dr. Weinberg began using something called gauge theory to study the interactions in weak forces, which had not been successfully explained up to that point.
Gauge theory had been developed in the 19th century by James Clerk Maxwell, a British physicist, in his seminal work to explain electromagnetism. In the 1950s, it was used by Robert Mills and Chen Ning Yang, a Chinese American physicist, who later won the Nobel Prize, to understand strong-force interactions.
But Dr. Weinberg’s application of gauge theory to the weak force soon ran into a problem.
Electromagnetism is a force that acts at large distances, but the weak force acts only at very short distances — smaller than the nucleus of an atom. In electromagnetism, when two particles — say, electrons — collide, they exchange a massless neutral particle called a photon, which is also known as a gauge boson. If two particles collide because of the weak force, gauge theory requires — because of the short distances of the interaction — that the gauge bosons that are exchanged be massive and possibly electrically charged.
Fortunately, several years earlier, physicists had come up with a way to generate mass for gauge bosons called the Higgs Mechanism. It was named for Peter Higgs, a British physicist, and it predicted the existence of a previously unknown particle that is responsible for giving other particles their mass. The particle was given the name the Higgs boson, and its discovery, in 2012, brought Dr. Higgs and his colleague François Englert the 2013 Nobel Prize.

Using this new idea, Dr. Weinberg was able to create a model in which weak interactions produced massive, at least by atomic standards, gauge boson particles. He called them W and Z bosons.
His theory also predicted that in some collisions — for example, between two electrically neutral particles like a neutron and a neutrino — a neutral current, as opposed to a charged one, would be created, indicating that there had been an exchange of a Z boson.
Dr. Weinberg theorized that there was a link between the photon and the W and Z bosons, suggesting that they were created by the same force. The conclusion was that, at very high energy levels, the electromagnetic and weak forces were one and the same. It was a step on the path to the unified theory that physicists had been searching for."