In earlier chapters we considered neutrons and protons as elementary particles that could not be subdivided. Recent research with high-energy accelerators has led to the development of a theory describing neutrons, protons, and other "elementary" particles as being made up of particles called quarks. Quarks have a charge of one-third or two-thirds that of the electron. No one has ever isolated an individual quark, but the results of scattering experiments conducted at high-energy accelerators provide evidence that quarks exist. There must be a strong force between nucleons in order to have protons exist in such a small volume, because the nucleus the positive charge on the protons causes them to be repelled from each other due to the electrostatic force of repulsion between like charges. This strong attractive force between nucleons is called the strong nuclear interaction.

There is another force involved in nuclear processes called the weak nuclear force. This force was discovered in investigations of b decay. Physicists now refer to this weak nuclear force and the electromagnetic force that we studied earlier as different manifestations of one fundamental force. This force is called the electroweak interaction. Thus there are three fundamental forces in nature: the strong nuclear interaction, the electroweak force, and the gravitational force.

Astronomers know that the universe is expanding. They also know that other galaxies are expanding away from our galaxy, the Milky Way. One explanation of this expansion is provided by the "Big Bang" theory, which holds that the universe came into existence as an explosion followed by an expansion. This expansion of the universe continues.

Semiconductors are made from materials that are not normally good conductors of electricity but can be made to be good conductors by "doping" them with very small amounts of the appropriate materials. Semiconductors are used to produce diodes, transistors, and integrated circuits that are used in the modern electronic devices. Semiconductors are important in the development and manufacture of digital computers.

Superconductors are materials that have no electrical resistance when they are cooled below a certain critical temperature. Until 1986 no material was known to be a superconductor at a temperature above 28K, but recent discoveries have identified superconductors with critical temperatures as high as 100 K. It is much easier and cheaper to maintain a temperatue of 100 K than to maintain a temperature of 28 K. In general these high temperature superconductors are brittle, so researchers are investigating methods of fabricating them into useful devices.

For many years the neutron and proton were thought to be elementary particles that could not be subdivided. Experiments conducted by bombarding targets with high-energy particles produce evidence in the form of tracks in photographic emulsions, cloud chambers, bubble chambers, and other particle detectors that neutrons and protons are composed of quarks. Quarks do not exist as individual entities but only in combination with each other. They have charges that are one-third or two-third the size of the charge of the electron. The strong nuclear interaction force holds them together. In the "Big Bang" theory for the origin of the universe all the matter of the universe is viewed as having been just a sea of quarks. After the "Big Bang" the universe expanded, and it continues to expand.

"Doping" materials such as silicon and germanium with phosphorus, arsenic, antimony, boron, gallium, or indium produces semiconductors. Semiconductor materials are used to produce diodes, transistors, and integrated circuits, which are good electrical conductors under certain conditions but not under other conditions. Thus semiconductors are useful in producing devices such as voltage-controlled switches. Digital computers use semiconductor devices to implement digital logic that is based upon the binary or "base two" number system.