17 particles that explain the world: The Standard Model of Physics describes the structure of matter and the fundamental forces interacting between matter particles. The forces are mediated by force carriers (gauge bosons): photons transfer the electromagnetic interaction, gluons the strong, and W and Z bosons the weak interaction. No carrier boson has yet been demonstrated for the fourth fundamental interaction, gravitation. Physicists have coined the term "graviton" for this as yet undemonstrated particle.

In his general theory of relativity, Albert Einstein defined gravitation as a geometrical property of spacetime, curved by mass or energy. His theory describes the graviton as a massless, spin-2 particle – in contrast to the already known, spin-1 gauge bosons.