Basic concepts of quantum mechanics

This article is a non-technical introduction to the subject. For the main encyclopedia article, see Quantum mechanics.

We now know how completely the quantum permeates all of existence. With the physicist it has become almost an obsession, haunting his every equation, dictating his every experiment, and leading him into long and not always fruitful argument with philosopher and priest on God and free will.

— Banesh Hoffman, The Strange Story of the Quantum

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Conception of a wave packet, an electron circling the nucleus of an atom, illustrating the uncertainty principle.

Quantum mechanics, also called wave mechanics, explains the behaviour of matter and energy on the tiny scale of atoms and subatomic particles. The basic concepts of quantum mechanics are summarized in this article. Clicking on any word or phrase in blue will take you to a more in-depth article. At the bottom and below right are collapsible boxes with links that also lead to related articles.

Physicist Stephen Hawking wrote in 2001 that quantum mechanics is:

the basis of modern developments in chemistry, molecular biology, and electronics, and the foundation for the technology that has transformed the world in the last fifty years.

Many experiments have been carried out to prove the concept, which has been repeatedly translated into practical use.

Practical use

Main article: Quantum_mechanics § Applications

Quantum mechanics is a physical theory that has practical application. Examples include the laser, the transistor, the electron microscope, and magnetic resonance imaging. The study of semiconductors led to the invention of the diode and the transistor, which are indispensable for modern electronics.

In even the simple light switch, quantum tunneling is vital, as otherwise the electrons in the electric current could not penetrate the potential barrier made up of a layer of oxide. Flash memory chips found in USB drives also use quantum tunneling to erase their memory cells.

Derivation of the term

Quantum is a form of the Latin word for "how much." It refers to "any of the very small increments or parcels into which many forms of energy are subdivided." It was first used (in a different sense) in English in 1567. Its plural form is quanta. Mechanics is, in this sense, the "branch of physical science that deals with energy and forces and their effect on bodies." First used in 1612, it is plural in form but takes either a plural or singular verb.

Particles and waves

Main article: Wave-particle duality

The beams streaming through these windows at Chicago's Union Station in 1943 were both waves and particles.

Quantum mechanics reveals how subatomic particles can have wave-like properties and waves can have particle-like properties. This phenomenon is known as wave–particle duality. The explanation stems from a theory proposed by French physicist Louis de Broglie in 1924 that subatomic particles like electrons are associated with waves. Experiments later found he was correct: Electrons can bend around objects and can display wave shapes.

Neither wave nor particle is an entirely satisfactory model to use in understanding light. Indeed, astrophysicist A.S. Eddington proposed in 1927 that "We can scarcely describe such an entity as a wave or as a particle; perhaps as a compromise we had better call it a 'wavicle' ". This term was later popularized by mathematician Banesh Hoffmann.

Classical physics

Main article: Classical physics

Sir Isaac Newton

Classical mechanics, or classical physics, is a general term for the principles developed before the rise of general relativity and quantum mechanics. It is still in use to measure the action of forces on material bodies, but it cannot be used accurately on the micro-scale of atoms and molecules. Still, it remains the key to the measurement for much of modern science and technology.

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Christiaan Huygens, on a Dutch 25 guilder banknote from the 1950s

Beginning with Sir Isaac Newton (1643–1727), theorists believed that light consisted of a stream of particles; later, Christian Huygens and his followers favored the theory that light consisted of waves flowing through some medium.

Later experiments identified phenomena, such as the photoelectric effect, that were consistent only with a packet or "quantum" model of light. When light strikes an electrical conductor, electrons seem to move away from their original positions. These observations could be explained only by assuming that light delivered energy in definite packets.

In 1874, George Johnstone Stoney was the first to propose that a physical quantity, namely an electric charge, could not vary by less than some irreducible amount, or, in later terminology by a quantum. Thus the electric charge was the first physical quantity to be theoretically quantized. Stoney termed this unit as an electrine.

Modern physics

Main article: History of quantum mechanics

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An artist's vision of the speed of light

In the late 19th century, classical physics appeared to explain just about everything. But during the 20th century observers found that not to be the case — in both the large (macro) and the small (micro) worlds.

Macro: Relativity theory postulated that time does not pass at the same rate for all observers, that matter can convert to energy and vice versa, that two objects each moving at more than half the speed of light cannot approach each other at a speed exceeding the speed of light, that time progresses more slowly near massive objects, and so on.

Micro: Scientists found they could not determine the exact location of a photon or an electron, nor could they trace a path between the point where one was emitted and the point where it was detected. The points where such a particle might be detected were not where one would expect them to be, based on everyday experience.

The remainder of this section is developed in roughly chronological order. A complete timeline is available here.

Planck constant

Main article: Planck constant

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Max Planck on a German postage stamp

Quantum mechanics formally began with a paper that German physicist Max Planck delivered in 1900 on black body radiation—that is, a surface that completely absorbs all radiant energy falling upon it. Planck said that neither the wave nor the particle model could explain electromagnetic radiation.

His paper marked him as the originator of the quantum theory and won him the Nobel Prize for Physics in 1918.

In that speech, the researcher revealed his mathematical precept later called the Planck constant. The significance of Planck’s constant in the context of quantum mechanics is that radiation, such as light, is emitted, transmitted, and absorbed in discrete energy packets, or quanta, determined by the frequency of the radiation and the value of the constant.[5]

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Einstein's portrait by Harm Kamerlingh Onnes at the University of Leiden in 1920

In 1905, Albert Einstein used Planck's constant to explain the photoelectric effect — "the way certain metals give off electrons when light falls on them" — by postulating that the energy in a beam of light occurs in packets he called light quanta, and that later came to be called photons.According to Einstein's account, a single photon of a given frequency delivers an invariant amount of energy.

The particle analogy became favored, as it helped understand how light delivers energy in multiples of certain set values, called quanta of energy. Nevertheless, the wave analogy remained indispensable for helping to understand other light phenomena, such as diffraction.

Bohr model of the atom

Main article: Bohr model

Sketch by Christian Gori from a photograph of Niels Bohr

One great question of early 20th century physics was: "How do electrons normally remain in stable orbits around the nucleus of an atom?"

In 1913, Niels Bohr solved this problem by applying the notion of discrete (non-continuous) quanta to electron orbits. This solution became known as the Bohr model of the atom. He theorized that electrons could inhabit only certain orbits around the atom. These orbits could be derived by looking at the spectral lines, or colors, produced by pure elements.

Bohr proposed that when an atom emitted or absorbed energy, its electron or electrons did not move in a continuous trajectory from one orbit around the nucleus to another. Instead, the electrons suddenly disappeared from their original orbits and instantly reappeared in another one — a quantum leap.

Bohr's model of the atom was essentially two-dimensional — an electron orbiting in a plane around its nuclear "sun." In modern theory, orbital has replaced the earlier word orbit concerning the position of an electron in relation to the nucleus of an atom. It is often depicted as a three-dimensional region within which there is a 95 percent probability of finding the electron.

Quantum leap

Main article: Quantum leap

A quantum leap or quantum jump is a change of an electron from one quantum state to another within an atom. It is discontinuous; the electron jumps from one energy level to another instantaneously. The phenomenon contradicts classical theories, which expected energy levels to be continuous. Quantum leaps cause the emission of electromagnetic radiation, including that of light, which occurs in the form of quantized units called photons.

Uncertainty principle

Main article: Uncertainty principle

Werner Heisenberg at the age of 26

In 1927, German physicist Werner Heisenberg discovered a discrepancy between the measured position of a particle and its measured momentum. His conclusion as to the reason for this variance came to be called Heisenberg's Uncertainty Principle.

The principle states that an electron cannot be viewed as having an exact location at any given time. The concepts of exact position and exact velocity really have NO meaning in nature. Rather, the observer has to evaluate every possible location for the electron. This conclusion gave rise to a depiction of the electron orbital as a spherically shaped "cloud of points," with the cloud having a maximum density at a certain distance from the nucleus and growing less dense at greater and lesser distances. Mathematicians refer to such a cloud of points as a probability distribution. (See animated illustration at top of page.)

In quantum theory orbital has replaced the earlier word orbit concerning the position of the electron in relation to the nucleus of the atom. The orbital is often depicted as a three-dimensional region within which there is a 95 percent probability of finding the electron.

Schrödinger wave equation

Main article: Schrödinger equation

Erwin Schrödinger, about 1933, age 46

Building on De Broglie's theoretical model of particles as waves, Austrian physicist Erwin Schrödinger brought forth in 1926 what has been called "the fundamental equation" of quantum mechanics.

The equation describes the form of the probability waves that govern the motion of small particles, "and it specifies how these waves are altered by external influences. Schrödinger established the correctness of the equation by applying it to the hydrogen atom, predicting many of its properties with remarkable accuracy. The equation is used extensively in atomic, nuclear, and solid-state physics."

Although Werner Heisenberg saw no problem in the existence of discontinuous quantum jumps, Schrödinger hoped that a theory based on continuous wave-like properties could avoid what he called (in the reported words of Wilhelm Wien) "this nonsense about quantum jumps."

Quantum field theory

This sculpture in Bristol, England — a series of clustering cones — presents the idea of small worlds which Paul Dirac studied to reach his discovery of anti-matter.

Main article: Quantum field theory

The idea of quantum field theory began in the late 1920s with British physicist Paul Dirac, when he attempted to quantize the electromagnetic field — a procedure for constructing a quantum theory starting from a classical theory.

A field in physics is "a region or space in which a given effect (as magnetism) exists." Other effects resulting from fields are gravitation and electricity In 2008, physicist Richard Hamilton wrote that

Sometimes we distinguish between quantum mechanics (QM) and quantum field theory (QFT). QM refers to a system in which the number of particles is fixed, and the fields (such as the electromechanical field) are continuous classical entities. QFT . . . goes a step further and allows for the creation and annihilation of particles . . . .

He added, however, that quantum mechanics is often used to refer to "the entire notion of quantum view."

In 1931, Dirac proposed the existence of particles that later became known as anti-matter. Dirac shared the Nobel Prize in physics for 1933 with Schrödinger, "for the discovery of new productive forms of atomic theory."

Further reading

• Richard Feynman, 1985. QED: The Strange Theory of Light and Matter, Princeton University Press. ISBN 0-691-08388-6

External links

• Quantum mechanics on Youtube
• Quantum mechanics on Google video