Overview
In the conceptual model of fundamental interactions, matter consists of fermions, which carry properties called charges and spin ±1⁄2 (intrinsic angular momentum ±ħ⁄2, where ħ is the reduced Planck constant). They attract or repel each other by exchanging bosons.
The interaction of any pair of fermions in perturbation theory can then be modeled thus:
- Two fermions go in → interaction by boson exchange → Two changed fermions go out.
The exchange of bosons always carries energy and momentum between the fermions, thereby changing their speed and direction. The exchange may also transport a charge between the fermions, changing the charges of the fermions in the process (e.g., turn them from one type of fermion to another). Since bosons carry one unit of angular momentum, the fermion's spin direction will flip from +1⁄2 to −1⁄2 (or vice versa) during such an exchange (in units of the reduced Planck's constant).
Because an interaction results in fermions attracting and repelling each other, an older term for "interaction" is force.
According to the present understanding, there are four fundamental interactions or forces: gravitation, electromagnetism, the weak interaction, and the strong interaction. Their magnitude and behavior vary greatly, as described in the table below. Modern physics attempts to explain every observed physical phenomenon by these fundamental interactions. Moreover, reducing the number of different interaction types is seen as desirable. Two cases in point are the unification of:
- Electric and magnetic force into electromagnetism;
- The electromagnetic interaction and the weak interaction into the electroweak interaction; see below.
Both magnitude ("relative strength") and "range", as given in the table, are meaningful only within a rather complex theoretical framework. It should also be noted that the table below lists properties of a conceptual scheme that is still the subject of ongoing research.
| Interaction | Current theory | Mediators | Relative strength | Long-distance behavior | Range (m) |
|---|---|---|---|---|---|
| Strong | Quantum chromodynamics (QCD) |
gluons | 1038 | (see discussion below) |
10−15 |
| Electromagnetic | Quantum electrodynamics (QED) |
photons | 1036 | ∞ | |
| Weak | Electroweak Theory | W and Z bosons | 1025 | 10−18 | |
| Gravitation | General Relativity (GR) |
gravitons (hypothetical) | 1 | ∞ |
The modern (perturbative) quantum mechanical view of the fundamental forces other than gravity is that particles of matter (fermions) do not directly interact with each other, but rather carry a charge, and exchange virtual particles (gauge bosons), which are the interaction carriers or force mediators. For example, photons mediate the interaction of electric charges, and gluons mediate the interaction of color charges.
Read more about this topic: Fundamental Interaction