Question

Explain why there are four fundamental interactions in nature; strong, weak, gravitational, and electromagnetic.

Answer 1

Well first off, let me just say that this is definitely a physics question. Second off, I'm not exactly sure what you mean by your question. I mean, the only real answer to WHY there are the four fundamental interactions of the universe is because they were, for a very short period after the Big Bang (if you will), one universal force, but as the universe expanded and cooled exponentially (keep in mind that all of this is taking place in inconceivable fractions of a second) the forces began to be independent of one another.

But perhaps you are looking for a description of the forces? If so, I will oblige. If not, I'll let you decide if you want to continue reading or not. Well start with gravity.

Gravity:
Gravity is the single weakest (known) force in the universe and serves as the attractive force between objects, rather large or small (though we have an EXTREMELY small, if virtually no, understanding of how gravity works on the small scale of Quantum Mechanics). That's the general gist of gravity. If you want to know more about it's mechanics in respect to General Relativity (large scale stuff, like planets and stars), pick up an astronomy book and flip it around for awhile. Gravity is theorized to be mediated by the (theoretical) boson known as the graviton.

Electromagnetism:
Electromagnetism is essentially the force that causes electrically charged particles to interact. It is involved in almost every phenomena we encounter, with the exception of gravity. In general, all the forces involving some kind of atomic interactions are caused by the electromagnetic force acting on the electrically charged protons and electrons inside the atoms. Electromagnetism is mediated by the boson known as the photon.

Weak Nuclear Force:
The WNF is probably the easiest to understand when it is described in it's simplest terms. It is the cause of radiation. Simple as that. The WNF is mediated by the W and Z bosons.

Strong Nuclear Force:
The SNF is by far the most complicated of the forces, so I'll try to keep it simple. On a (relatively) large scale, it is the force that holds together atomic nuclei; the protons and neutrons of any particular atom. On an even smaller scale, it is the force that binds quarks together to form protons, neutrons, and other hadrons. The SNF is generally believed to be mediated by gluons.