Theories of the Universe: The Forces You Already Know

The Forces You Already Know

There are only four forces that are known to operate between elementary particles. Two of these we just covered, the strong force and the weak force. The other two, gravity and electromagnetism, are familiar to us because, not only have we covered them, but they also operate in the everyday world. Gravity is the weakest of the four forces and was the first to be discovered. But even though it is the weakest of the four, its range is infinite, while the strong and weak forces are limited in range to the nucleus of the atom. Electromagnetism is much stronger than gravity, but both electricity and magnetism come in two varieties—positive and negative charge, north and south poles. These varieties tend to cancel each other out, reducing their overall influence. But as you know, Maxwell combined these two separate forces into one: electromagnetism. This force holds atoms together (electrons in their probable positions around the nucleus) as well as molecules, but is overcome by the strong force that binds the nucleus together. This all might be explained a little better by the following chart. You can compare the four forces and their relative strengths.

The Four Forces
Name	Strength Compared to Strong Force	Effective Range in Centimeters	What It Does
Gravity	6 × 10-39	Infinite	Holds planets, stars, and galaxies together
Weak	1 × 10-5	1 × 10-15	Causes particles to decay
Electromagnetism	7 × 10-3	Infinite	Holds atoms together
Strong	1 × 100 = 1	1 × 10-13	Holds atomic nucleus together

Cosmic Rays, Greek Particles, and Accelerators

In the last sections, we'll be taking a look at the plethora of particles that go to make up what is known as the standard model of particle physics. Contained within this model are the particles responsible for carrying out the work of the four fundamental forces we've already discussed. Since Einstein's famous equation showed that matter and energy are equivalent, physicists describe particles in terms of their energy content rather than just their mass. This unit of measure is called the electron volt (eV). This is an extremely small amount of energy, less than the amount used by a bug to flap its wings.

Before accelerators were built to study the elementary particles that were detected in their subatomic collision experiments, nature had provided a natural laboratory in which the interaction of elementary particles could be studied. Cosmic rays are elementary particles that travel to earth from our sun and other stars in our galaxy. They are mostly protons and they interact with other particles in our atmosphere as they make their journey to the surface of the Earth. The collisions that are created by these particles cause a cascading effect. As one particle hits another, more particles are created and these in turn hit other particles and so on.

Seeing an elementary particle is not an easy thing to do, so an apparatus was built to observe these cosmic ray collisions. In the 1930s, the Wilson cloud chamber was invented to fill this need.

When a particle travels through the gas in the chamber, it leaves a trail of vapor, similar to the trail from jets in the sky. Out of these experiments came the discovery of the first particle of antimatter, the positive electron, or positron. The cloud chamber proved to be a very useful observation device, because more and more particles were discovered.

Mu and Pi

Three of the more significant particles that were discovered with the use of a cloud chamber all were assigned Greek letters. (They were all members of a new classification of particles based on nationality … not really, physicists just love to use the Greek alphabet when assigning new names.) In 1936, two particles were discovered that were called mu-mesons. They both had equal masses but opposite electrical charges. They were called mesons because their mass was 210 times that of the electron and this just happened to be about midway between the lightest and heaviest particles known at the time. (Meson means intermediate one.) The name mu-meson was later shortened to muon and was assigned the Greek letter µ, mu.

Not long after, a slightly more massive meson was discovered, the pi-meson. And yes, as you may suspect, this name was shortened as well to pion, represented by the Greek letter for pi, π. Neither muons or pions last for very long. The muon has a half-life of 2.19 microseconds. (A microsecond is a millionth of a second.) A pion was also discovered to be a messenger particle. As you will soon see, the four forces we discussed earlier all depend on what are called messenger particles that “carry” the force between interactions.

From Natural to Man-Made

The cosmic ray experiments had revealed that there were many previously unsuspected particles in nature, but this natural supply of particles had its limitations. For one thing, you would have to sit around and wait until the particles you wanted happened along. All of these new particles were unstable, so if you wanted to see what these particles were like, you needed a way to produce them in large enough quantities to study. This is the reason that particle accelerators were developed. To take a normal particle and give it high energy requires that the particle be accelerated. And this is exactly what accelerators do.

There are two general classes of accelerators. In the one kind, particles are accelerated while they travel down a long straight tube. This is a linear accelerator. In the other type, the particle is made to move in a circular path by applying a magnetic field and then boosting its energy each time it comes past a given point on the circle. This sort of machine is called a cyclotron or a synchrotron, depending on how the magnetic field is applied. The first accelerators were linear and were built back in the late 1920s and '30s. They were capable of producing energies between 400,000 and 750,000 eV. Today the largest circular accelerator, which is four miles in circumference, is located at Fermilab outside of Chicago. This machine is capable of producing 1 TeV, one trillion electron volts.