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Physics, Quantum

Physics, Quantum


Quantum theory is one of the most successful theories in the history of physics. The accuracy of its predictions is astounding. The breath of its application is impressive. Quantum theory is used to explain how atoms behave, how elements can combine to form molecules, how light behaves, and even how black holes behave. There can be no doubt that there is something very right about quantum theory.

But at the same time, it is difficult to understand what quantum theory is really saying about the world. In fact, it is not clear that quantum theory gives any consistent picture of what the physical world is like. Quantum theory seems to say that light is both wavelike and particlelike. It seems to say that objects can be in two places at once, or even that cats can be both alive and dead, or neither alive nor dead, orwhat? There can be no doubt that there is something troubling about quantum theory.


Early research

Quantum theory, more or less as it is known at the beginning of the twenty-first century, was developed during the first quarter of the twentieth century in response to several problems that had arisen with classical mechanics. The first is the problem of blackbody radiation. A blackbody is any physical body that absorbs all incident radiation. As the blackbody continues to absorb radiation, its internal energy increases until, like a bucket full of water, it can hold no more and must re-emit radiation equal in energy to any additional incident radiation. The problem is, most simply, that the classical prediction for the energy of the emitted radiation as a function of its frequency is wrong. The problem was well known but unsolved until the German physicist Max Planck (18581947) proposed in 1900 the hypothesis that the energy absorbed and emitted by the blackbody could come only in discrete amounts, multiples of some constant, finite, amount of energy. While Planck himself never felt satisfied with this hypothesis as more than a localized, phenomenological description of the behavior of blackbodies, others eventually accepted Planck's hypothesis as a revolution, a claim that energy itself can come in only discrete amounts, the quanta of quantum theory.

A second problem with classical mechanics was the challenge of describing the spectrum of hydrogen, and eventually, other elements. Atomic spectra are most easily understood in light of a fundamental formula linking the energy of light with its frequency: E = h ν, where E is the energy of light, h is a constant (Planck's constant, as it turns out), and ν is the frequency of the light (which determines the color of the visible light).

Suppose, now, that the energy of some atom (for example, an atom of hydrogen) is increased. If the atom is subsequently allowed to relax, it releases the added energy in the form of (electromagnetic) radiation. The relationship E = h ν reveals that the frequency of the light depends on the amount of energy that the atom emits as it relaxes. Prior to the development of quantum theory, the best classical theory of the atom was Ernest Rutherford's (18711937), according to which negatively charged electrons orbit a central positively charged nucleus. The energy of a hydrogen atom (which has only one electron) corresponds to the distance of the electron from the nucleus. (The further the electron is, the higher its energy is.) Rutherford's model predicts that the radiation emitted by a hydrogen atom could have any of a continuous set of possible energies, depending on the distance of its electron from the nucleus. Hence a large number of hydrogen atoms with energies randomly distributed among them will emit light of many frequencies. However, in the nineteenth century it was well known that hydrogen emits only a few frequencies of visible light.

In 1913, Niels Bohr (18851962) introduced the hypothesis that the electrons in an atom can be only certain distances from the nucleus; that is, they can exist in only certain "orbits" around the nucleus. The differences in the energies of these orbits correspond to the possible energies of the radiation emitted by the atom. When an electron with high energy "falls" to a lower orbit, it releases just the amount of energy that is the difference between the energies of the higher and lower orbits. Because only certain orbits are possible, the atom can emit only certain frequencies of light.

The crucial part of Bohr's proposal is that electrons cannot occupy the space between the orbits, so that when the electron passes from one orbit to another, it "jumps" between them without passing through the space in between. Thus, Bohr's model violates the principle of classical mechanics that particles always follow continuous trajectories. In other words, Bohr's model left little doubt that classical mechanics had to be abandoned.

Over the next twelve years, the search was on for a replacement. By 1926, as the result of considerable experimental and theoretical work on the part of numerous physicists, two theoriesexperimentally equivalentwere introduced, namely, Werner Heisenberg's (19011976) matrix mechanics and Erwin Schrödinger's (18871961) wave mechanics.


Matrix mechanics. Heisenberg's matrix mechanics arose out of a general approach to quantum theory advocated already by Bohr and Wolfgang Pauli (19001958), among others. In Heisenberg's hands, this approach became a commitment to remove from the theory any quantities that cannot be observed. Heisenberg took as his "observable" such things as the transition probabilities of the hydrogen atom (the probability that an electron would make a transition from a higher to a lower orbit). Heisenberg introduced operators that, in essence, represented such observable quantities mathematically. Soon thereafter, Max Born (18821970) recognized Heisenberg's operators as matrices, which were already well understood mathematically.

Heisenberg's operators can be used in place of the continuous variables of Newtonian physics. Indeed, one can replace Newtonian position and momentum with their matrix "equivalents" and obtain the equations of motion of quantum theory, commonly called (in this form) Heisenberg's equations. The procedure of replacing classical (Newtonian) quantities with the analogous operators is known as quantization. A complete understanding of quantization remains elusive, due primarily to the fact that quantum-mechanical operators can be incompatible, which means in particular that they cannot be comeasured.

Wave mechanics. Schrödinger's wave mechanics arose from a different line of reasoning, primarily due to Louis de Broglie (18921987) and Albert Einstein (18791955). Einstein had for some time expressed a commitment to a physical world that can be adequately described causally, which meant that it could be described in terms of quantities that evolve continuously in time. Einstein, who was primarily responsible for showing that light has both particlelike and wavelike properties, hoped early on for a theory that somehow "fused" these two aspects of light into a single consistent theory.

In 1923, de Broglie instituted the program of wave mechanics. He was impressed by the Hamilton-Jacobi approach to classical physics, in which the fundamental equations are wave equations, but the fundamental objects of the theory are still particles, whose trajectories are determined by the waves. Recalling this formalism, de Broglie suggested that the particlelike and wavelike properties of light might be reconcilable in similar fashion. Einstein's enthusiasm for de Broglie's ideasboth because de Broglie's waves evolved continuously and because the theory fused the wavelike and particlelike properties of light and matterstimulated Schrödinger to work on the problem from that point of view, and in 1926 Schrödinger published his wave mechanics.

It was quickly realized that matrix mechanics and wave mechanics are experimentally equivalent. Shortly thereafter, in 1932, John von Neumann (19031957) showed their equivalence rigorously by introducing the Hilbert space formalism of quantum theory. The Uncertainty Principle serves to illustrate the equivalence. The Uncertainty Principle follows immediately from Heisenberg's matrix mechanics. Indeed, in only a few lines of argument, one can arrive at the mathematical statement of the Uncertainty Principle for any operators (physical quantities) A and B : ΔA ΔB Kh, where K is a constant that depends on A and B, and h is Planck's constant. The symbol ΔA means "root mean square deviation of A " and is a measure of the statistical dispersion (uncertainty) in a set of values of A. So the Uncertainty Principle says that the statistical dispersion in values of A times the statistical dispersion in values of B are always greater than or equal to some constant. If (and only if) A and B are incompatible (see above) then this constant is greater than zero, so that it is impossible to measure a both A and B on an ensemble of physical systems in such a way as to have no dispersion in the results.

Schrödinger's wave mechanics gives rise to the same result. It is easiest to see how it does so in the context of the classic example involving position and momentum, which are incompatible quantities. In the context of Schrödinger's wave mechanics, the probability of finding a particle at a given location is determined by the amplitude (height) of the wave at that location. Hence, a particle with a definite position is represented by a "wave" that is zero everywhere except at the location of the particle. On the other hand, a particle with definite momentum is represented by a wave that is flat (i.e., has the same amplitude at all points), and, conversely to position, momentum becomes more and more "spread" as the wave becomes more sharply peaked. Hence the more precisely one can predict the location of a particle, the less precisely one can predict its momentum. A more quantitative version of these considerations leads, again, to the Uncertainty Principle.

Quantum field theory. Perhaps the major development after the original formulation of quantum theory by Heisenberg and Schrödinger (with further articulation by many others) was the extension of quantum mechanics to fields, resulting in quantum field theory. Paul Dirac (19021984) and others extended the work to relativistic field theories. The central idea is the same: The quantities of classical field theory are quanticized in an appropriate way. Work on quantum field theory is ongoing, a central unresolved issue being how one can incorporate the force of gravity, and specifically Einstein's relativistic field theory of gravity, into the framework of relativistic quantum field theory. A related, though even more speculative, area of research is quantum cosmology, which is, more or less, the attempt to discern how Big Bang theory (itself derived from Einstein's Theory of Gravity) will have to be modified in the light of quantum gravity.


Contemporary research

Contemporary research in the interpretation of quantum theory focuses on two key issues: the "measurement problem" and locality (Bell's Theorem).

Schrödinger's cat. Although the essence of the measurement problem was clear to several researchers even before 1925, it was perhaps first clearly stated in 1935 by Schrödinger. In his famous example, Schrödinger imagines a cat in the following unfortunate situation. A box, containing the cat, also contains a sample of some radioactive substance that has a probability of 1/2 to decay within one hour. Any decay is detected by a Geiger counter, which releases poison into the box if it detects a decay. At the end of an hour, the state of the cat is indeterminate between "alive" and "dead," in much the same way that a state of definite position is indeterminate with regard to momentum.

The cat is said to be in a superposition of the alive state and the dead state. In standard quantum theory, such a superposition is interpreted to mean that the cat is neither determinately alive, nor determinately dead. But, says Schrödinger, while one might be able to accept that particles such as electrons are somehow indeterminate with respect to position or momentum, one can hardly accept indeterminacy in the state of a cat.

More generally, Schrödinger's point is that indeterminacy at the level of the usual objects of quantum theory (electrons, protons, and so on) can easily be transformed into indeterminacy at the level of everyday objects (such as cats, pointers on measuring apparatuses, and so on) simply by coupling the state of the everyday object to the state of the quantum object. Such couplings are exactly the source of our ability to measure the quantum objects in the first place. Hence, the problem that Schrödinger originally raised with respect to the cat is now called the measurement problem : Everyday objects such as cats and pointers can, according to standard quantum theory, be indeterminate in state. For example, a cat might be indeterminate with respect to whether it is alive. A pointer might be indeterminate with respect to its location (i.e., it is pointing in no particular direction).

Approaches to the measurement problem. Thus, the interpretation of quantum theory faces a serious problem, the measurement problem, to which there have been many approaches. One approach, apparently advocated by Einstein, is to search for a hidden-variables theory to underwrite the probabilities of standard quantum theory. The central idea here is that the indeterminate description of physical systems provided by quantum theory is incomplete. Hidden variables (so-called because they are "hidden" from standard quantum theory) complete the quantum-mechanical description in a way that renders the state of the system determinate in the relevant sense. The most famous example of a successful hidden-variables theory is the 1952 theory of David Bohm (19171992), itself an extension of a theory proposed by Louis de Broglie in the 1920s. In the Broglie-Bohm theory, particles always have determinate positions, and those positions evolve deterministically as a function of their own initial position and the initial positions of all the other particles in the universe. The probabilities of standard quantum theory are obtained by averaging over the possible initial positions of the particles, so that the probabilities of standard quantum theory are due to ignorance of the initial conditions, just as in classical mechanics. According to some, the problematic feature of this theory is its nonlocality the velocity of a given particle can depend instantaneously on the positions of particles arbitrarily far away.

Other hidden-variables theories exist, both deterministic and indeterministic. They have some basic features in common with the de Broglie-Bohm theory, although they do not all take position to be "preferred"some choose other preferred quantities. In the de Broglie-Bohm theory, position is said to be "preferred" because all particles always have a definite position, by stipulation.

There are other approaches to solving the measurement problem. One set of approaches involves so-called Many-worlds interpretations, according to which each of the possibilities inherent in a superposition is in fact actual, though each in its own distinct and independent "world." There is a variant, the Many-minds theory, according to which each observer observes each possibility, though with distinct and independent "minds." These interpretations have a notoriously difficult time reproducing the probabilities of quantum theory in a convincing way. A slightly more technical, but perhaps even more troubling, issue arises from the fact that any superposition can be "decomposed" into possibilities in an infinity of ways. So, for example, a superposition of "alive" and "dead" can also be decomposed into other pairs of possibilities. It is unclear how Many-worlds interpretations determine which decomposition is used to define the "worlds," though there are various proposals.

Yet another set of approaches to the measurement problem is loosely connected to the Copenhagen Interpretation of quantum theory. According to these approaches, physical quantities have meaning only in the context of an experimental arrangement designed to measure them. These approaches insist that the standard quantum-mechanical state is considered to describe our ignorance about which properties a system has in cases where the possible properties are determined by the experimental context. Only those properties that could be revealed in this experimental context are considered "possible." In this way, these interpretations sidestep the issue of which decomposition of a superposition one should take to describe the possibilities over which the probabilities are defined. Once a measurement is made, the superposition is "collapsed" to the possibility that was in fact realized by the measurement. In this context, the collapse is a natural thing to do, because the quantum mechanical state represents our ignorance about which experimental possibility would turn up. The major problem facing these approaches is to define measurement and experimental context in a sufficiently rigorous way.

Another set of approaches are the realistic collapse proposals. Like the Copenhagen approaches, they take the quantum-mechanical state of a system to be its complete description, but unlike them, these approaches allow the meaningfulness of physical properties even outside of the appropriate experimental contexts. The issue of how to specify when collapse will occur is thus somewhat more pressing for these approaches because the collapse represents not a change in our knowledge, but a physical change in the world. There are several attempts to provide an account of when collapse will occur, perhaps the two most famous being observer-induced collapse and spontaneous localization theories. According to the former, notably advocated by Eugene Wigner (19021995), the act of observation by a conscious being has a real effect on the physical state of the world, causing it to change from a superposition to a state representing the world as perceived by the conscious observer. This approach faces the very significant problem of explaining why there should be any connection between the act of conscious observation and the state of, for example, some electron in a hydrogen atom.

The spontaneous-localization theories define an observer-independent mechanism for collapse that depends, for example, on the number of particles in a physical system. For low numbers of particles the rate of collapse is very slow, whereas for higher values, the rate of collapse is very high. The collapse itself occurs continuously, by means of a randomly distributed infinitesimal deformation of the quantum state. The dynamics of the collapse are designed to reproduce the probabilities of quantum theory to a very high degree of accuracy.


The problem of nonlocality. The other major issue facing the interpretation of quantum theory is nonlocality. In 1964, John Bell (19281990) proved that, under natural conditions, any interpretation of quantum theory must be nonlocal. More precisely, in certain experimental situations, the states of well-separated pairs of particles are correlated in a way that cannot be explained in terms of a common cause. One can think, here, of everyday cases to illustrate the point. Suppose you write the same word on two pieces of paper and send them to two people, who open the envelopes simultaneously and discover the word. There is a correlation between these two events (they both see the same word), but the correlation is easily explained in terms of a common cause, you.

Under certain experimental circumstances, particles exhibit similar correlations in their states, and yet those correlations cannot be explained in terms of a common cause. It seems, instead, that one must invoke nonlocal explanations, explanations that resort to the idea that something in the vicinity of one of the particles instantaneously influences the state of the other particle, even though the particles are far apart.

On the face of it, nonlocality contradicts special relativity. According to standard interpretations of the theory of relativity, causal influences cannot travel faster than light, and in particular, events in one region of space cannot influence events in other regions of space if the influence would have to travel faster than light to get from one region to the other in time to influence the event.

However, the matter is not so simple as a direct contradiction between quantum theory and relativity. The best arguments for the absence of faster-than-light influences in relativity are based on the fact that faster-than-light communicationmore specifically, transfer of informationcan lead to causal paradoxes. But in the situations to which Bell's theorem applies, the purported faster-than-light influences cannot be exploited to enable faster-than-light communication. This result is attributable to the indeterministic nature of standard quantum theory. In de Broglie and Bohm's deterministic hidden-variable theory, one could exploit knowledge of the values of the hidden variables to send faster-than-light signals; however, such knowledge is, in Bohm's theory, physically impossible in principle.

Other areas of research. There are of course many other areas of research in the interpretation of quantum theory. These include traditional areas of concern, such as the classical limit of quantum theory. How do the nonclassical predictions of quantum theory become (roughly) equivalent to the (roughly accurate) predictions of classical mechanics in some appropriate limit? How is this limit defined? In general, what is the relationship between classical and quantum theory? Other areas of research arise from work in quantum theory itself, perhaps the most notable being the work in quantum computation. It appears that a quantum computer could perform computations in qualitatively faster time than a classical computer. Apart from obvious practical considerations, the possibility of quantum computers raises questions about traditional conceptions of computation, and possibly, thereby, about traditional philosophical uses of those conceptions, especially concerning the analogies often drawn between human thought and computation.


Applications to religious thought

Quantum theory was the concern of numerous religious thinkers during the twentieth century. Given the obviously provisional status of the theory, not to mention the extremely uncertain state of its interpretation, one must proceed with great caution here, but we can at least note some areas of religious thought to which quantum theory, or its interpretation, has often been taken to be relevant.

Perhaps the most obvious is the issue of whether the world is ultimately deterministic or not. Several thinkers, including such scientists as Isaac Newton (16421727) and Pierre-Simon Laplace (17491827), have seen important ties to religious thought. In the case of classical mechanics, Newton had good reason to believe that his theory did not completely determine the phenomena, whereas Laplace (who played a key role in patching up the areas where Newton saw the theory to fail) had good reason to think that the theory did completely and deterministically describe the world. Newton thus saw room for God's action in the world; Laplace did not.

In the case of quantum theory the situation is considerably more difficult because there exist both indeterministic and deterministic interpretations of the theory, each of which is empirically adequate. Indeed, they are empirically equivalent. Those who, for various reasons, have adopted one or the other interpretation, though, have gone on to investigate the consequences for religious thought. Some, for example, see in quantum indeterminism an explanation of the possibility of human free will. Others have suggested that quantum indeterminism leaves an important role for God in the universe, namely, as the source of the agreement between actual relative frequencies and the probabilistic predictions of quantum theory.

Other thinkers have seen similarities between aspects of quantum theory and Eastern religions, notably various strains of Buddhism and Daoism. Fritjof Capra (1939), who is perhaps most famous in this regard, has drawn analogies between issues that arise from the measurement problem and quantum nonlocality and what he takes to be Eastern commitments to the "connectedness" of all things. Other thinkers have seen in the interpretive problems of quantum theory evidence of a limitation in science's ability to provide a comprehensive understanding of the world, thus making room for other, perhaps religious, modes of understanding. Still others, drawing on views such as Wigner's (according to which conscious observation plays a crucial role in making the world determinate), see in quantum theory a justification of what they take to be traditional religious views about the role of conscious beings in the world. Others, including Capra, see affinities between wave-particle duality, or more generally, the duality implicit in the Uncertainty Principle, and various purportedly Eastern views about duality (for example, the Taoist doctrine of yin and yang, or the Buddhist use of koans).

Finally, quantum cosmology has provided some with material for speculation. One must be extraordinarily careful here because there is, at present, no satisfactory theory of quantum gravity, much less of quantum cosmology. Nonetheless, a couple of (largely negative) points can be made. First, it is clear that the standard Big Bang theory will have to be modified, somehow or other, in light of quantum theory. Hence, the considerable discussion to date of the religious consequences of the Big Bang theory will also need to be reevaluated. Second, due to considerations that arise from the time-energy Uncertainty Principle, even a satisfactory quantum cosmology is unlikely to address what happened in the early universe prior to the Planck time (approximately 1043 seconds) because quantum theory itself holds that units of time less than the Planck time are (perhaps) meaningless. Some have seen here a fundamental limit in scientific analysis, a limit that is implied by the science itself. Of course, others see an opportunity for a successor theory.

This situation is, in fact, indicative of the state of quantum theory as a whole. While it is an empirically successful theory, its interpretations, and hence any consequences it might have for religious thought, remain matters of speculation.


See also Copenhagen Interpretation; EPR Paradox; Heisenberg's Uncertainty Principle; Indeterminism; Locality; Many-worlds Hypothesis; Phase Space; Planck Time; Quantum Cosmologies; Quantum Field Theory; SchrÖdinger's Cat; Wave-particle Duality


Bibliography

bohm, david. quantum theory. new york: dover, 1989.

gribbin, john. in search of schrödinger's cat: quantum physics and reality. new york: bantam, 1984.


heisenberg, werner. physical principles of the quantum theory. new york: dover, 1930.

shankar, ramamurti. principles of quantum mechanics. new york: plenum, 1994.


w. michael dickson

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Quantum Mechanics

Quantum mechanics

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 lightsuch as reflection and refractioncan 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.

Macroscopic and submicroscopic properties

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 (15641642) and English physicist Isaac Newton (16421727).

But the methods of classical physics do not work nearly as welland sometimes they don't work at allwhen 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.

Words to Know

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.

Uncertainty principle

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 physicsthe uncertainty principle. In 1927, German physicist Werner Heisenberg (19011976) 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.

A new physics

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.

[See also Light; Subatomic particles ]

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quantum mechanics

quantum mechanics Branch of physics that uses the quantum theory to explain the behaviour of elementary particles. According to quantum theory, all radiant energy emits and absorbs in multiples of tiny ‘packets’ or quanta. Atomic particles have wavelike properties and thereby exhibit a wave-particle duality. Sometimes the wave properties dominate, and other times the particle aspects dominate. The quantum theory uses four quantum numbers to classify electrons and their atomic states: energy level, angular momentum, energy in a magnetic field and spin. The exclusion principle says any two electrons in an atom cannot have the same energy and spin. A change in an electron, atom or molecule from one quantum state to another, called a quantum jump, is accompanied by the absorption or emission of a quantum. The quantum field theory seeks to explain this exchange. The strong interactions between quarks and between protons and neutrons are described by quantum chronodynamics. The idea that energy radiates and absorbs in packets was first proposed by German theoretical physicist Max Planck in 1900 to explain black body radiation. Using Planck's work, German-born US physicist Albert Einstein quantized light radiation, and in 1905 explained the photoelectric effect. He chose the name of photon for a quantum of light energy. In 1913, Danish physicist Niels Bohr used quantum theory to explain atomic structure and atomic spectra, showing the relationship between the energy levels of an atom's electrons and the frequencies of radiation emitted or absorbed by the atom. In 1924, French physicist Louis de Broglie suggested that particles have wave properties, the converse having been postulated in 1905 by Albert Einstein. In 1926, Austrian physicist Erwin Schrödinger used this hypothesis of wave mechanics to predict particle behaviour on the basis of wave properties, but a year earlier German physicist Werner Heisenberg had produced a mathematical equivalent to Schrödinger's theory without using wave concepts at all. In 1928, English physicist Paul Dirac unified these approaches while incorporating relativity into quantum mechanics (especially when large speeds are involved). This predicted the existence of antimatter and helped develop the quantum electrodynamics theory of how charged subatomic particles interact within electric and magnetic fields. The superstring theory provides a possible answer to gravitational interaction. The complete, modern theory of quantum mechanics is the quantum field theory of quantum electrodynamics, also known as the quantum theory of light. It was derived by US theoretical physicist Richard Feynman in the 1940s. The theory predicts that a collision between an electron and a proton should result in the production of a photon of electromagnetic radiation, which is exchanged between the colliding particles. Quantum mechanics remains a difficult system because the uncertainty principle, formulated in 1927 by Heisenberg, states that nothing on the atomic scale can be measured or observed without disturbing it. This makes it impossible to know the position and momentum of a particle at the same time.

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quantum mechanics

quan·tum me·chan·ics • pl. n. [treated as sing.] Physics the branch of mechanics that deals with the mathematical description of the motion and interaction of subatomic particles, incorporating the concepts of quantization of energy, wave-particle duality, the uncertainty principle, and the correspondence principle. DERIVATIVES: quan·tum-me·chan·i·cal adj.

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Quantum Mechanics

Quantum Mechanics

See Physics, Quantum

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Quantum Physics

Quantum Physics

See Physics, Quantum

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quantum mechanics

quantum mechanics: see quantum theory.

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"quantum mechanics." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 24 Jun. 2017 <http://www.encyclopedia.com>.

"quantum mechanics." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (June 24, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/quantum-mechanics

"quantum mechanics." The Columbia Encyclopedia, 6th ed.. . Retrieved June 24, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/quantum-mechanics