Introduction

At the temperatures common to our world, four discrete forces govern the interactions of matter - gravity, electromagnetism, the weak nuclear force, and the strong nuclear force. Each force is carried by a separate "messenger particle" unique to it and still being researched. The strong force is by far the strongest of the forces, followed by the electromagnetic force, the weak force, and finally the extremely feeble gravitational force. Though these four forces govern every matter interaction, a theory that unites them all is still being sought. The most recent candidate is string theory, further discussed in Introduction to String Theory.

Messenger Particles

There is a unique messenger particle associated with each of the four fundamental forces. These particles can be considered the "smallest amount" of each force than can exist in nature. Experiments have confirmed the existence of three of the four particles, but the graviton has yet to be discovered. Calculations show that it should be massless. The weak gauge bosons come in two separate varieties with different masses.

Gravity

Gravity, the weakest of the four forces, is about 10^-36 times the strength of the strong force. This weakness is easily demonstrable - on a dry day, rub a comb across your shirt to give it static electricity, then hold it over a piece of paper on a desk. If you were successful, the piece of paper lifts off the desk. It takes an entire planet to keep the paper on the desk, but this force is easily overcome with everyday materials employing the electromagnetic force.

However, the range of gravity is unlimited - every object in the universe exerts a gravitational force on everything else. The effects of gravity depend on two things: the mass of two bodies and the distance between them. In more precise terms, the attractive force between any two bodies is directly proportional to the product of the masses and inversely proportional to the square of the distance between the bodies. The dominance of gravity on macroscopic scales is due not to any intrinsic strength but instead to its enormous range and constant attractive nature, especially as compared to the other forces. These properties of gravity have made it extremely difficult to incorporate gravity into modern theoretical frameworks.

The messenger particle of gravity is the graviton. It has not been experimentally verified, mainly because it is extremely hard to find the smallest denomination of the weakest force. Recent calculations show that it will likely be massless. Interestingly, all versions of modern string theory incorporate gravity (unlike previous quantum theories) and not only allow but require a particle with the properties of the graviton. Its discovery will likely represent a major victory for string theory, since previous quantum theories based on the model of point particles give illogical, infinite answers when gravity is incorporated.

Electromagnetism

The electromagnetic force is actually second in effective strength only to the strong force, but it is listed out of order here because it, like gravity, is more familiar to most people. Its strength is less than 1% of that of the strong force, but it, like gravity, has infinite range. However, unlike gravity, the electromagnetism has both attractive and repulsive properties that can combine or cancel each other out. Whereas gravity is always attractive, electromagnetism comes in two charges: positive and negative. Two positive or two negative things will repel each other, but one positive and one negative attract each other. This can be neatly illustrated by magnets: two of the same "pole" will repel each other, but two opposite poles attract each other.

This is the principle that keeps atoms together: the positively charged nucleus and the negatively charged electrons attract each other. This is also the principle of atomic size: more electrons have greater repulsive force, so atoms with more electrons are larger because of the electrons' mutual repulsion. Similarly, atoms with larger nuclei and the same number of electrons are smaller overall because they exert a greater attractive force on the electrons.

The messenger particle of electromagnetism is the photon, a massless particle that logically (since light is a manifestation of electromagnetism) travels at the speed of light (299 792 458 m/s or 299 972 km/s).

The Weak Nuclear Force

The weak nuclear force is one of the less familiar fundamental forces. It operates only on the extremely short distance scales found in an atomic nucleus. The weak force is responsible for radioactive decay. In actuality, it is stronger than electromagnetism, but its messenger particles (W and Z bosons) are so massive and sluggish that they do not faithfully transmit its intrinsic strength.

The Strong Nuclear Force

The strong nuclear force is the other unfamiliar fundamental force. Like the weak force, its range is limited to subatomic distances. Its "duties" are keeping quarks together inside protons and neutrons, and keeping protons and neutrons inside atomic nuclei. Its messenger particle is the massless gluon, so named because it "glues" elementary particles together.

If the Forces Were Different . . .

Just as questions arise about the reasons behind the properties of elementary particles, scientists wonder about the reasons behind the properties of the forces and their particles as well. For example, why are three of the messenger particles massless, and the fourth is one of the heaviest known particles? Similarly, why do the ranges and strengths of the forces differ so drastically? Yet it cannot be disputed that the universe would be strikingly different if the force properties were otherwise. The formation of stable nuclei depends on the ratio of the strong and electromagnetic forces - the protons in a nucleus repel each other, but the strong force overcomes this repulsion. A small change in their relative strengths could allow the electromagnetic force to overcome the strong force, and atoms could not exist. If electrons were any more massive, then electrons and protons would be disposed to bond and form neutrons, thus disrupting the formation of heavy elements. The strength of gravity is also important: if it were any stronger, stellar matter would bind more strongly and stars would use their nuclear fuel much faster, thus negating the possibility of the evolution of life. If gravity were any weaker, matter might not "clump together" to form larger structures, thereby preventing the formation of stars in the first place. The answer to the question of why the forces are the way they are may yet come with string theory.