According to physics, there are four fundamental forces in the Universe: gravity, electromagnetism, the strong force, and the weak force. Each of these can be visualized as a field surrounding the source of the force, i.e. gravity field, magnetic field, electric field, etc. According to quantum physics, everything is both a wave and a particle at the same time, so according to particle physics, these fields all have associated particles that mediate them. Force-mediating particles are called gauge bosons, and there's a different kind for each force:
- Graviton - mediates gravity
- Photon - mediates electromagnetism
- Gluons (8 kinds) - mediate the strong force
- W+, W-, and Z bosons - mediate the weak force

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According to
the Standard Model of particle physics which currently explains the latter three forces, all of the gauge bosons should have zero mass. This is true for the photon and gluons. The W
+, W
-, and Z bosons not only have mass, however, they have lots of it (subatomically speaking). This called for an explanation. The current prevailing hypothesis, proposed in 1964, is called the Higgs mechanism.