Modern physics brings all existing forces down to four fundamental forces. All of these fundamental forces – physicists prefer to say: 'interactions' – are transmitted by special force particles. The electromagnetic interaction acts between electrically charged particles and is transmitted by a force particle called the photon. This force has effects in technical applications and in the human metabolism. Light is the interaction of photons with the retina in our eyes. Chemical bonds, but also physical phenomena like friction, elasticity or surface tension, are ultimately consequences of the electromagnetic interaction between the exterior electrons of atoms. The strong interaction acts inside the atomic nucleus and is transmitted by gluons. They 'glue' together the quarks into protons and neutrons and assure the solidity of the atomic nucleus. The weak interaction permits the transmutation of quarks into other quarks and of leptons into other leptons. It is transmitted by the so-called W and Z bosons. Without the weak interaction, there would be no radioactivity and the sun could not shine. The fourth interaction is gravity, which we know from the terrestrial attraction. It is presumably transmitted by the graviton. All force particles have been observed experimentally, with the exception of the graviton.