There is at least one more fundamental force, which is responsible for fundamental particles masses, and which completes the scheme of the five interaction types needed to explain all physically observed phenomena.

In 1967, Abdus Salam, of London’s Imperial College, and Steven Weinberg, of Harvard University, proposed theories that would unify the electromagnetic with the weak interaction (achievement comparable to Maxwell’s one). The predictions of the theories coincided so snugly with the experiments that, in 1979, Salam and Weinberg (together with Sheldon Glascow of Harvard University, who proposed similar theories) were awarded with the Nobel Prize for physics.

All these unifications have lead to the creation of the Standard Model of Fundamental Particles and Interactions, which includes the electroweak (unified electromagnetic and weak forces) and the strong interaction.

In quantum mechanics, it is believed that all interactions between particles are due to the exchange of force-carrying particles (in the Standard Model of Fundamental Particles and Interactions, these particles are called bosons) in the following way:

A particle transmits one force-carrying particle and (just like canons move backwards after the shot) it changes its velocity. Afterwards, the force-carrier particle jams with another particle and, due to the jam, the speed of the last particle changes, too. The final outcome is equal to a force acting between the first and the last particle.

Each interaction is transmitted with its own characteristic bosons: the gravitational with the graviton (not yet directly observed since, at the subatomic level, the gravitational force is many orders of magnitude weaker than the other three fundamental interactions), the electromagnetic with the photon (which is also the carrier particle of light), the weak with the W (W+ and W-) and Z0 bosons (the different bosons of the weak force introduce different kinds of decays) and the strong with the gluon (which glues the strongly-interacting particles together).

That is why gravity is not easy to be unified with the other forces; the theories describing it (the general relativity, which suggests that gravity is not even a force, and the quantum mechanics) do not ‘agree’ with each other.

The next steps in the unification of physics is the so-called Grand Unified Theory (GUT), which would unify the electroweak with the strong interaction, as well as the unification of the general relativity with quantum mechanics, thus, the development of a theory for gravity that would be in accordance with all the other theories describing it and would make the gravitational force easy to be unified with the other interactions.

Finally, all the problems might perhaps be solved with the so-called Strings Theory, but this can be the topic of another article.