The Standard Model of particle physics is a theory which describes the strong, weak, and electromagnetic fundamental forces, as well as the fundamental particles that make up all matter. Developed between 1970 and 1973, it is a quantum field theory, and consistent with both quantum mechanics and special relativity. To date, almost all experimental tests of the three forces described by the Standard Model have agreed with its predictions. However, the Standard Model is not a complete theory of fundamental interactions, primarily because it does not describe the gravitational force.

The Standard Model

In the standard model of particle physics, particles are considered to be points moving through space, tracing out a line called the World Line. To take into account the different interactions observed in Nature we need to provide particles with more degrees of freedom than only their position and velocity, such as mass, electric charge, colour (which is the "charge" associated with the strong interaction) or spin. The standard model was designed within a framework known as “Quantum Field Theory” (QFT), which gives us the tools to build theories consistent both with quantum mechanics and the special theory of relativity.

Using QFT theories were built which describe with great success three, of the four known interactions in Nature: Electromagnetism and the Strong and Weak nuclear forces. Furthermore, a very successful unification between Electromagnetism and the Weak force was achieved (Electroweak Theory), and promising ideas put forward to try to include the Strong force. But unfortunately the fourth interaction, gravity, beautifully described by Einstein's General Relativity (GR), does not seem to fit into this scheme. Whenever an attempt to apply the rules of QFT to GR is applied, results are obtained which make no sense. For instance, the force between two gravitons (the particles that mediate gravitational interactions), becomes infinite as yet it is unknown how to get rid of these infinities to get physically sensible results.