What are the fundamental forces of Physics?

Sometimes there are references to the "four fundamental forces of physics." What are the fundamental forces? Why are they fundamental?

Answer: The fundamental forces (or fundamental interactions) of physics are the ways that individual particles interact with each other. It turns out that for every single interaction that we've observed take place in the universe, they can be broken down to be described by only four (well, generally four - more on that later) types of interactions:

Gravity

Electromagnetism

Weak Interaction (or Weak Nuclear Force)

Strong Interaction (or Strong Nuclear Force)

Gravity

Of these forces, gravity has the furthest reach but it's the weakest in actual magnitude.

It is a purely attractive force which reaches through even the "empty" void of space to draw two masses toward each other. It keeps the planets in orbit around the sun and the moon in orbit around the Earth.

Gravitation is described under the theory of general relativity, which defines it as the curvature of spacetime around an object of mass. This curvature, in turn, creates a situation where the path of least energy is toward the other object of mass.

Electromagnetism

Electromagnetism is the interaction of particles with an electrical charge. Charged particles at rest interact through electrostatic forces, while in motion they interact through both electrical and magnetic forces.

For a long time, the electric and magnetic forces were considered to be different forces, but they were finally unified by James Clerk Maxwell in 1864, under Maxwell's equations. In the 1940s, quantum electrodynamics consolidated electromagnetism with quantum physics.

Electromagnetism is perhaps the most obviously prevalent force in our world, as it can affect things at a reasonable distance and with a fair amount of force.

Weak Interaction

The weak interaction is a very powerful force that acts on the scale of the atomic nucleus. It causes phenomena such as beta decay. It has been consolidated with electromagnetism as a single interaction called the "electroweak interaction."

Strong Interaction

The strongest of the forces is the aptly-named strong interaction, which is the force that, among other things, keeps nucleons (protons & neutrons) bound together. In the helium atom, for example, it is strong enough to bind two protons together despite the fact that their positive electrical charges cause them to repulse each other.

In essence, the strong interaction allows particles called gluons to bind together quarks to create the nucleons in the first place. Gluons can also interact with other gluons, which gives the strong interaction a theoretically infinite distance, although it's major manifestations are all at the subatomic level.

Unifying the Fundamental Forces

Many physicists believe that all four of the fundamental forces are, in fact, the manifestations of a single underlying (or unified) force which has yet to be discovered. Just as electricity, magnetism, and the weak force were unified into the electroweak interaction, they work to unify all of the fundamental forces.

The current quantum mechanical interpretation of these forces is that the particles do not interact directly, but rather manifest virtual particles that mediate the actual interactions. All of the forces except for gravity have been consolidated into this "Standard Model" of interaction.

The effort to unify gravity with the other three fundamental forces is called quantum gravity. It postulates the existence of a virtual particle called the graviton, which would be the mediating element in gravity interactions. To date, gravitons have not been detected and no theories of quantum gravity have been successful or universally adopted.