Theories of the Universe

The Forces You Already Know


It's interesting that the fundamental forces of the universe happen to manifest as the number 4. If you remember back to the first section when I discussed the role of certain numbers, such as 4 and 7, which were used by ancient cultures to define the cosmological structure of their universe, the tribal Native American people believed that “the Great Spirit caused everything to be in fours.” Maybe they already knew something that has taken science centuries to find out.

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
NameStrength Compared to Strong ForceEffective Range in CentimetersWhat It Does
Gravity6 × 10-39InfiniteHolds planets, stars, and galaxies together
Weak1 × 10-51 × 10-15Causes particles to decay
Electromagnetism7 × 10-3InfiniteHolds atoms together
Strong1 × 100 = 11 × 10-13Holds atomic nucleus together
Universal Constants

Cosmic rays are energetic particles from space, including electrons and protons, some of which interact with the nuclei of atoms in the atmosphere of the Earth to produce showers of secondary particles. Before the development of particle accelerators, cosmic rays provided physicists with their only source of high-energy particles to study.

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.

Universal Constants

An electron volt (eV) is a measure of energy introduced in 1912. It's equal to the energy gained by one electron when it is accelerated across an electric potential difference of 1 volt. (For you budding scientists out there, this would read 1eV = 1.602 × 10-19 joules.) Because this unit is so small, it is more commonly encountered as keV (thousand electron volts), MeV (million electron volts) or CeV (billion electron volts). If you dropped a book from a height of about 3 inches, it would be accelerated by gravity to kinetic energy of about 1 billion electron volts. A 100-watt light bulb burns energy at the rate of 6.24 × 1020 per second. But it takes only 13.6eV to knock an electron right out of an atom of hydrogen and the energy of particles produced in radioactive decay are typically several MeV. This gives you an idea of the different energies associated with chemical and nuclear processes.

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.

Universal Constants

Antimatter is a form of matter in which each particle has the opposite set of quantum properties (such as electric charge) to its counterpart in the everyday world. When an antiparticle meets its particle counterpart they annihilate each other, converting their mass into energy. There are antimatter counterparts for all matter of particles—antiprotons, antineutrons, antineutrinos, and so on.

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.


Between the 1930s and late 1950s, many particles were discovered that left physicists with no idea as to their role or purpose in particle interaction. They knew that they existed for some purpose, but it was like trying to complete a jigsaw puzzle with some pieces missing. The role that many of these particles ended up having was to predict the theoretical existence of other particles that were needed to complete the picture. As more and more particles were discovered using accelerators, the particle universe began to get quite con-fusing. Rather than a beautiful, simple, elegant system of interactions, hundreds of particles made a rather strange picture. Could the universe really be this complicated? Fortunately things did calm down, at least a little with the theory of the existence of quarks. You'll read about those guys very soon.

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.

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