These interactions govern how particles and macroscopic objects interact and how certain particles decay. Generally, they can be classified into one of four fundamental forces:
- Gravitational force. Gravity was the first force to be investigated scientifically. The gravitational force was described systematically by Isaac Newton in the 17th century. Newton stated that the gravitational force acts between all objects having mass (including objects ranging from atoms and photons to planets and stars) and is directly proportional to the masses of the bodies and inversely proportional to the square of the distance between the bodies. Since energy and mass are equivalent, all forms of energy (including light) cause gravitation and are under its influence. Gravity is the weakest of the four fundamental forces of physics, approximately 1038 times weaker than the strong force. On the other hand, gravity is additive. Every speck of matter that you put into a lump contributes to the overall gravity of the lump. Since it is also a very long-range force, it is the dominant force at the macroscopic scale and is the cause of the formation, shape, and trajectory (orbit) of astronomical bodies.
- Electromagnetic force. The electromagnetic force is responsible for all electromagnetic processes, and it acts between electrically charged particles. It is an infinite-ranged force, much stronger than the gravitational force, and obeys the inverse square law. Still, neither electricity nor magnetism adds up in the way that gravitational force does. Since there are positive and negative charges (poles), these charges tend to cancel each other out.
- Weak force. The weak interaction or weak force is one of the four fundamental forces and involves the exchange of the intermediate vector bosons, the W and the Z. Since these bosons are very massive (on the order of 80 GeV, the uncertainty principle dictates a range of about 10-18 meters which is less than the diameter of a proton. As a result, the weak interaction occurs only at very small, sub-atomic distances.
- Strong force. The strong interaction or strong force is one of the four fundamental forces and involves the exchange of the vector gauge bosons known as gluons. In general, the strong interaction is complicated because it varies significantly with distance. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as the proton and neutron. Moreover, the strong force is the force that can hold a nucleus together against the enormous forces of repulsion of the protons is strong indeed.
These fundamental interactions are characterized based on the following four criteria:
- the types of particles that experience the force
- the range over which the force is effective
- the relative strength of the force
- the nature of the particles that mediate the force