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Physics deals with the behaviour and composition of matter and its interactions at the most fundamental level. Its domain stretches from inside the tiny nucleus of an atom to the vast expanses of the universe. Geology, chemistry, engineering, and astronomy all require an understanding of the principles of physics. Physics also finds many applications in biology, physiology, and medicine.

hevelius.jpg (14713 bytes)Between 1600 and 1900, three broad areas were developed in what is called classical physics:

  • Classical mechanics, the study of the motion of particles and fluids;
  • Thermodynamics, the study of heat transfer, temperature, and the behaviour of large numbers of particles;
  • Electromagnetism, which involves electricity, magnetism, and electromagnetic waves and optics.

These three areas encompass virtually all the physical phenomena with which we are familiar. Galileo Galilei (1564 1642) made significant contributions to classical mechanics through his work on the laws of motion with constant acceleration. In the same era, Johannes Kepler (1571 1630) used astronomical observations to develop empirical laws for the motion of planetary bodies.

The most important contributions to classical mechanics were provided by Isaac Newton (1642 1727), who developed classical mechanics as a systematic theory and was one of the originators of the calculus as a mathematical tool. Although major developments in classical physics continued in the 18th century, thermodynamics was not developed until the latter part of the 19th century, principally because the apparatus for controlled experiments was either too crude or not available. Although many electric and magnetic phenomena had been studied earlier, it was the work of James Clerk Maxwell (1831 - 1879) that provided the unified theory of electromagnetism.

wheel.jpg (14134 bytes)When a discrepancy between theory and experimentation arises, the theories must be modified and experiments performed to test the predictions of the modified theories. Many times a theory is satisfactory under limited conditions; a more general theory might be satisfactory without such limitations.

A classic example of this kind of modification of theories occurred around 1905 when Albert Einstein (1879 1955) was working on what would happen to objects moving at speeds comparable to the speed of light. He found that Newtonian mechanics did not apply at these great speeds; it was necessary to develop a more general theory of motion - his special theory of relativity - which successfully predicts the motions of objects at speeds approaching the speed of light. This new theory led to a radical revision of our ideas of space, time and energy and the birth of modern physics.

Three important theories in modern physics are:

  • Special relativity, which is a theory describing particles moving at any speed, even those approaching the speed of light.
  • Quantum mechanics, a theory dealing with the behaviour of sub-microscopic particles as well as the macroscopic world. It also required a profound upheaval in our vision of how nature operates.
  • General relativity, a theory that relates the force of gravity to the geometrical properties of space.

The goal of the physicist is to explain physical phenomena in the simplest and most economical terms. According to our present state of knowledge, ordinary matter is constructed from atoms, the atoms from nuclei and electrons, the nuclei from neutrons and protons, the neutrons and protons from quarks. Indeed all elementary particles (of which there are hundreds) can be constructed from just two types of particles: quarks and leptons.

slac.jpg (22009 bytes)We encounter great variety of forces in nature: forces exerted by ropes, springs, fluids, electric charges, magnets, the Earth and the Sun, chemical forces, nuclear forces, and so on. But despite this variety, physicists can explain all these kinds of forces in terms of just four basic interactions: the strong, weak, electromagnetic, and gravitational forces.

The gravitational interaction produces an attractive force between all particles. It is responsible for our weight, causes apples to fall, and holds the planets in their orbits around the Sun. The electromagnetic interaction between charges is evident in chemical reactions, light, radio and TV signals, X-rays, friction, and all other forces we experience every day. It also governs the transmission of signals along nerve fibres. The strong interaction between quarks and most other sub-nuclear particles holds the particles within the nucleus. The weak interaction between quarks and leptons is associated with radioactivity.

The dream of physicists is to discover a single fundamental interaction from which all forces can be derived. In 1983 it was confirmed that the electromagnetic and weak interactions are different forms of a more basic electroweak interaction. Progress is also being made in attempts to combine the strong and electroweak interactions into a Grand Unified Theory.

See also What Do Physicists Do?      and  Are there jobs in Physics?

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