Quantum Mechanics

Quantum mechanics, pronounced KWAHN tuhm, is a field of physics that describes the structure of the atom and the motion of atomic particles. It also explains how atoms absorb and give off energy as light, and it clarifies the nature of light.

Quantum mechanics goes beyond the limits of classical physics, which is based on the laws formulated by the English scientist Sir Isaac Newton. It ranks as one of the major scientific achievements of the 1900's. Quantum mechanics has contributed greatly to the development of such important devices as lasers and transistors. It also has enabled scientists to gain a better understanding of chemical bonds and chemical reactions.

Understanding quantum mechanics. In an atom, tiny particles of negative electric charge called electrons move in orbits around a nucleus of positive charge. Quantum mechanics shows that the electrons can move only in certain orbits. Each orbit, called a quantized orbit, has a particular value of energy. When an electron is in a given orbit, it exists at a specific energy level and does not release or absorb energy. An electron remains in this normal state as long as its atom is not disturbed. But if outside forces act on the atom, the electron can change to another quantized orbit.

When an electron jumps from an orbit of higher energy to one of lower energy, it gives off energy as light. This light is released in the form of a tiny bundle of energy called a quantum or photon. The energy of a photon corresponds to the difference in energy of the two orbits between which the jump occurs. An electron also can absorb a photon and jump from an orbit of lower energy to one of higher energy.

Scientists once believed light was a wave emitted as a continuous flow. But quantum mechanics explains that light is a stream of separate photons, which have characteristics of both particles and waves. A photon behaves like a particle because it occupies a fixed amount of space. A photon also behaves like a wave because it has a definite frequency (number of vibrations per second), which is proportional to its energy.

A photon's frequency forms a single spectrum line that represents a particular wavelength or color. The atoms of a chemical element give off photons of a wide range of frequencies to produce many different lines. This series of lines makes up the chemical element's spectrum, which differs from that of any other element.

Quantum mechanics shows that electrons and other atomic particles of matter are also associated with waves. These waves, called matter waves, have a specific wavelength. The wavelength is inversely proportional to the particle's momentum. The particle's momentum is calculated by multiplying the mass of the particle by its velocity.

Matter waves provide an explanation for the arrangement of electrons in separate orbits. When an electron is undisturbed, its wave fits around the atom's nucleus at a distance such that the wave can join smoothly onto itself. The electrons of a single atom have waves of different wavelengths. These electrons form orbits at varying distances from the nucleus.

Another fundamental idea of quantum mechanics is the uncertainty principle. According to this principle, the position and velocity of a particle cannot simultaneously be measured with exactness. The principle is valid because a particle has certain wave properties.

History. In 1900, the German physicist Max Planck introduced the idea of quanta to explain the spectrum of light emitted by certain heated objects. In 1905, the German-born physicist Albert Einstein broadened Planck's idea to explain a phenomenon called the photoelectric effect. In doing so, Einstein firmly established that light consists of particles of energy that have wave properties. Niels Bohr, a Danish physicist, proposed the theory of the atom's electron structure in 1913. He also showed how atoms radiate light. Scientists call Bohr's work quantum theory to distinguish it from the broader system of quantum mechanics.

Louis de Broglie, a French physicist, introduced the idea of matter waves in 1924. The physicists Erwin Schrodinger of Austria and Werner Heisenberg of Germany independently developed forms of quantum mechanics in the mid-1920's. Since that year, these forms have been unified into a system and applied to several scientific fields, including chemistry, molecular biology, and solid-state physics.

Contributor: Francis T. Cole, Ph.D., Former Physicist, Fermi National Accelerator Laboratory.

Quantum Mechanics

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