What are the four fundamental forces of nature?

­The first force that you ever became aware of was probably gravity. As a toddler, you had to learn to rise up against it and walk. When you stumbled, you immediately felt gravity bring you back down to the floor. Besides giving toddlers trouble, gravity holds the moon, planets, sun, stars and galaxies together in the universe in their respective orbits. It can work over immense distances and has an infinite range.

Isaac Newton envisioned gravity as a pull between any two objects that was directly related to their masses and inversely related to the square of the distance separating them. His law of gravitation enabled mankind to send astronauts to the moon and robotic probes to the outer reaches of our solar system. From 1687 until the early 20th century, Newton's idea of gravity as a "tug-of-war" between any two objects dominated physics.

But one phenomenon that Newton's theories couldn't explain was the peculiar orbit of Mercury. The orbit itself appeared to rotate (also known as precession). This observation frustrated astronomers since the mid-1800s. In 1915, Albert Einstein realized that Newton's laws of motion and gravity didn't apply to objects in high gravity or at high speeds, like the speed of light.

In his general theory of relativity, Albert Einstein envisioned gravity as a distortion of space caused by mass. Imagine that you place a bowling ball in the middle of a rubber sheet. The ball makes a depression in the sheet (a gravity well or gravity field). If you roll a marble toward the ball, it will fall into the depression (be attracted to the ball) and may even circle the ball (orbit) before it hits. Depending upon the speed of the marble, it may escape the depression and pass the ball, but the depression might alter the marble's path. Gravity fields around massive objects like the sun do the same. Einstein derived Newton's law of gravity from his own theory of relativity and showed that Newton's ideas were a special case of relativity, specifically one applying to weak gravity and low speeds.

When considering massive objects (Earth, stars, galaxies), gravity appears to be the most powerful force. However, when you apply gravity to the atomic level, it has little effect because the masses of subatomic particles are so small. On this level, it's actually downgraded to the weakest force.

Let's look at electromagnetism, the next fundamental force.

­If you brush your hair several times, your hair may stand on end and be attracted to the brush. Why? The movement of the brush imparts electrical charges to each hair and the identically charged individual hairs repel each other. Similarly, if you place identical poles of two bar magnets together, they will repel each other. But set the opposite poles of the magnets near one another, and the magnets will attract each other. These are familiar examples of electromagnetic force; opposite charges attract, while like charges repel.

Scientists have studied electromagnetism since the 18th century, with several making notable contributions.

When scientists worked out the structure of the atom in the early 20th century, they learned that subatomic particles exerted electromagnetic forces on each other. For example, positively charged protons could hold negatively charged electrons in orbit around the nucleus. Furthermore, electrons of one atom attracted protons of neighboring atoms to form a residual electromagnetic force, which prevents you from falling through your chair.

But how does electromagnetism work at an infinite range in the large world and a short range at the atomic level? Physicists thought that photons transmitted electromagnetic force over large distances. But they had to devise theories to reconcile electromagnetism at the atomic level, and this led to the field of quantum electrodynamics (QED). According to QED, photons transmit electromagnetic force both macroscopically and microscopically; however, subatomic particles constantly exchange virtual photons during their electromagnetic interactions.

­But electromagnetism can't explain how the nucleus holds together. That's where nuclear forces come into play.

­The nucleus of any atom is made of positively charged protons and neutral neutrons. Electromagnetism tells us that protons should repel each other and the nucleus should fly apart. We also know that gravity doesn't play a role on a subatomic scale, so some other force must exist within the nucleus that is stronger than gravity and electromagnetism. In addition, since we don't perceive this force every day as we do with gravity and electromagnetism, then it must operate over very short distances, say, on the scale of the atom.

The force holding the nucleus together is called the strong force, alternately called the strong nuclear force or strong nuclear interaction. In 1935, Hideki Yukawa modeled this force and proposed that protons interacting with each other and with neutrons exchanged a particle called a meson -- later called a pion -- to transmit the strong force.

In the 1950s, physicists built particle accelerators to explore the structure of the nucleus. When they crashed atoms together at high speeds, they found the pions predicted by Yukawa. They also found that protons and neutrons were made of smaller particles called quarks. So, the strong force held the quarks together, which in turn held the nucleus together.

One other nuclear phenomenon had to be explained: radioactive decay. In beta emission, a neutron decays into a proton, anti-neutrino and electron (beta particle). The electron and anti-neutrino are ejected from the nucleus. The force responsible for this decay and emission must be different and weaker than the strong force, thus it's unfortunate name -- the weak force or the weak nuclear force or weak nuclear interaction.

With the discovery of quarks, the weak force was shown to be responsible for changing one type of quark into another through the exchange of particles called W and Z bosons, which were discovered in 1983. Ultimately, the weak force makes nuclear fusion in the sun and stars possible because it allows the hydrogen isotope deuterium to form and fuse.

­Now that you can name the four forces -- gravity, electromagnetism, the weak force and the strong force -- we'll see how they compare and interact with one another.

From the fields of QED and quantum chromodynamics, or QCD, the field of physics that describes the interactions between subatomic particles and nuclear forces, we see that many of the forces are transmitted by objects exchanging particles called gauge particles or gauge bosons. These objects can be quarks, protons, electrons, atoms, magnets or even planets. So, how does exchanging particles transmit a force? Consider two ice skaters standing at some distance apart. If one skater throws a ball to the other, the skaters will move farther away from each other. Forces work in a similar way.

Physicists have isolated the gauge particles for most of the forces. The strong force uses pions and another particle called a gluon. The weak force uses W and Z bosons. The electromagnetic force uses photons. Gravity is thought to be conveyed by a particle called a graviton; however, gravitons haven't been found yet. Some of the gauge particles associated with the nuclear forces have mass, while others don't (electromagnetism, gravity). Because electromagnetic force and gravity can operate over huge distances like light-years, their gauge particles must be able to travel at the speed of light, perhaps even faster for gravitons. Physicists don't know how gravity is transmitted. But according to Einstein's theory of special relativity, no object with mass can travel at the speed of light, so it makes sense that photons and gravitons are mass-less gauge particles. In fact, physicists have firmly established that photons have no mass.

Which force is the mightiest of them all? That would be the strong nuclear force. However, it acts only over a short range, approximately the size of a nucleus. The weak nuclear force is one-millionth as strong as the strong nuclear force and has an even shorter range, less than a proton's diameter. The electromagnetic force is about 0.7 percent as strong as the strong nuclear force, but has an infinite range because photons carrying the electromagnetic force travel at the speed of light. Finally, gravity is the weakest force at about 6 x 10^-29 times that of the strong nuclear force. Gravity, however, has an infinite range.

­Physicists are currently pursuing the ideas that the four fundamental forces may be related and that they sprang from one force early in the universe. The idea isn't unprecedented. We once thought of electricity and magnetism as separate entities, but the work of Oersted, Faraday, Maxwell and others showed that they were related. Theories that relate the fundamental forces and subatomic particles are called fittingly grand unified theories. More on them next.

­Science never rests, so the work on fundamental forces is far from finished. The next challenge is to construct one grand unified theory of the four forces, an especially difficult task since scientists have struggled to reconcile theories of gravity with those of quantum mechanics.

That's where particle accelerators, which can induce collisions at higher energies, come in handy. In 1963, physicists Sheldon Glashow, Abdul Salam and Steve Weinberg suggested that the weak nuclear force and electromagnetic force might combine at higher energies in what would be called the electroweak force. They predicted that this would occur at an energy of about 100 giga-electron volts (100GeV) or a temperature of 10¹⁵ K, which occurred shortly after the Big Bang. In 1983, physicists reached these temperatures in a particle accelerator and showed that the electromagnetic force and weak nuclear force were related.

Theories predict that the strong force will unite with the electroweak force at energies above 10¹⁵ GeV and that all the forces may unite at energies above 10¹⁹ GeV. These energies approach the temperature at the earliest portion of the Big Bang. Physicists are striving to build particle accelerators that might reach these temperatures. The largest particle accelerator is the Large Hadron Collider at CERN in Geneva, Switzerland. When it comes online, it will be capable of accelerating protons to 99.99 percent the speed of light and reaching collision energies of 14 tera-electron volts or 14 TeV, which is equal to 14,000 GeV or 1.4 x 10⁴ GeV.

­If physicists can show that the four fundamental forces indeed came from one unified force when the universe cooled from the Big Bang, will that change your daily life? Probably not. However, it will advance our understanding of the nature of forces, as well as the origins and fate of the universe.