Quantum mechanics is a method of studying the natural world based on the concept that waves of energy also have certain properties normally associated with matter, and that matter sometimes has properties that we usually associate with energy. For example, physicists normally talk about light as if it were some form of wave traveling through space. Many properties of light—such as reflection and refraction—can be understood if we think of light as waves bouncing off an object or passing through the object.
But some optical (light) phenomena cannot be explained by thinking of light as if it traveled in waves. One can only understand these phenomena by imagining tiny discrete particles of light somewhat similar to atoms. These tiny particles of light are known as photons. Photons are often described as quanta (the plural of quantum) of light. The term quantum comes from the Latin word for "how much." A quantum, or photon, of light, then, tells how much light energy there is in a "package" or "atom" of light.
The fact that waves sometimes act like matter and waves sometimes acts like waves is now known as the principle of duality. The term duality means that many phenomena have two different faces, depending on the circumstances in which they are being studied.
Until the 1920s, physicists thought they understood the macroscopic properties of nature rather well. The term macroscopic refers to properties that can be observed with the five human senses, aided or unaided. For example, the path followed by a bullet as it travels through the air can be described very accurately using only the laws of classical physics, the kind of physics originally developed by Italian scientist Galileo Galilei (1564–1642) and English physicist Isaac Newton (1642–1727).
But the methods of classical physics do not work nearly as well—and sometimes they don't work at all—when problems at the submicroscopic level are studied. The submicroscopic level involves objects and events that are too small to be seen even with the very best microscopes. The movement of an electron in an atom is an example of a submicroscopic phenomenon.
Classical mechanics: A collection of theories and laws that was developed early in the history of physics and that can be used to describe the motion of most macroscopic objects.
Macroscopic: A term describing objects and events that can be observed with the five human senses, aided or unaided.
Photon: A unit of energy.
Quantum: A discrete amount of any form of energy.
Wave: A disturbance in a medium that carries energy from one place to another.
In the first two decades of the twentieth century, physicists found that the old, familiar tools of classical physics produced peculiar answers or no answers at all in dealing with submicroscopic phenomena. As a result, they developed an entirely new way of thinking about and dealing with problems on the atomic level.
Some of the concepts involved in quantum mechanics are very surprising, and they often run counter to our common sense. One of these is another revolutionary concept in physics—the uncertainty principle. In 1927, German physicist Werner Heisenberg (1901–1976) made a remarkable discovery about the path taken by an electron in an atom. In the macroscopic world, we always see objects by shining light on them. Why not shine light on the electron so that its movement could be seen?
But the submicroscopic world presents new problems, Heisenberg said. The electron is so small that the simple act of shining light on it will knock it out of its normal path. What a scientist would see, then, is not the electron as it really exists in an atom but as it exists when moved by a light shining on it. In general, Heisenberg went on, the very act of measuring very small objects changes the objects. What we see is not what they are but what they have become as a result of looking at them. Heisenberg called his theory the uncertainty principle. The term means that one can never be sure as to the state of affairs for any object or event at the submicroscopic level.
Both the principle of duality and the uncertainty principle shook the foundations of physics. Concepts such as Newton's laws of motion still held true for events at the macroscopic level, but they were essentially worthless in dealing with submicroscopic phenomena. As a result, physicists essentially had to start over in thinking about the ways they studied nature. Many new techniques and methods were developed to deal with the problems of the submicroscopic world. Those techniques and methods are what we think of today as quantum physics or quantum mechanics.