 | Quantum mechanics: Encyclopedia II - Quantum mechanics - Introduction
Quantum mechanics - Introduction
The term quantum (Latin, "how much") refers to the discrete units that the theory assigns to certain physical quantities, such as the energy of an atom at rest (see Figure 1, at right). The discovery that waves could be measured in particle-like small packets of energy called quanta led to the branch of physics that deals with atomic and subatomic systems which we today call Quantum Mechanics. The foundations of quantum mechanics were established during the first half of the 20th century by Max Planck, Albert Einstein, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Wolfgang Pauli and others. Some fundamental aspects of the theory are still actively studied.
Quantum mechanics is a more fundamental theory than Newtonian mechanics and classical electromagnetism, in the sense that it provides accurate and precise descriptions for many phenomena that these "classical" theories simply cannot explain on the atomic and subatomic level. It is necessary to use quantum mechanics to understand the behavior of systems at atomic length scales and smaller. For example, Newtonian mechanics say unlike charges attract so that an electron in a hydrogen atom which has a negative charge will be attracted to the nucleus which has a positive charge at great speed and collapse making it highly unstable. However, in the natural world the electron normally remains in a stable orbit around a nucleus seeming to defy classical electromagnetism.
Quantum mechanics was initially developed to explain the atom and especially the orbit and position of the electron. Therefore, quantum theory developed an explanation for the electron staying in its orbital that couldn't be explained by Newton's laws of gravity nor could be explained by classical electromagnetism. Quantum mechanics uses probability distributions to explain such effects. Probability in the context of quantum mechanics has to do with the likelihood of finding a particle, such as an electron, in a particular region around the nucleus at a particular time. Therefore, electrons cannot be pictured as localized particles in space but rather should be thought of as clouds of negative charge spread out over the entire orbit. These clouds represent the regions around the nucleus where the probability of finding an electron is the largest. This probability cloud arises from a quantum mechanical principle called Heisenberg's Uncertainty Principle which states that there is an uncertainty in position of any subatomic particle including the electron so to describe where an electron or other particle is, the entire range of possible values is used describing a probability distribution. So in normal atoms with electrons in a stable orbit, the probability of the electron being at the nucleus is nearly zero according to the Uncertainty Principle (it is nearly zero as the nucleus has a volume and is not a point). Therefore, the laws of quantum mechanics, unlike Newton's deterministic laws, lead to a probabilistic description of nature.
The other great phenomenon that led to quantum mechanics was the study of electromagnetic waves such as light. When it was found in 1900 by Max Planck that the energy of waves could be described as consisting of small packets or quanta, Albert Einstein exploited this idea to show that an electromagnetic wave such as light could be described by a particle called the photon. This led to a theory of unity between subatomic particles and electromagnetic waves called wave-particle duality in which a particle and a wave were neither one nor the other, but had the properties of both simultaneously. It was from this view of an electron's wave-particle duality that the uncertainty principle arose and indeed it is the foundation of quantum mechanics.
Therefore, quantum mechanics describes the micro world, the world of the very small, such as the atom and subatomic particles, but quantum mechanics is also used to describe certain "macroscopic quantum systems" such as superconductors and superfluids.
Broadly speaking, quantum mechanics incorporates four classes of phenomena that classical physics cannot account for: (i) the quantization (discretization) of certain physical quantities, (ii) wave-particle duality, (iii) the uncertainty principle, and (iv) quantum entanglement. Each of these phenomena will be described in greater detail in subsequent sections.
The predictions of quantum mechanics have never been disproved after a century of experiments. Most physicists believe that quantum mechanics provides a correct description for the physical world under almost all circumstances. However, the effects of quantum mechanics are generally not significant when considering the observable Universe as a whole. This is because although atoms and subatomic particles are the building blocks of matter and the universe is made of matter, when analyzing the universe on large scales one finds that the dominant force becomes gravity governed by Einstein's theory of general relativity. In some cases, both general relativity and quantum mechanics converge. As an example, general relativity is unable to explain what will happen if a subatomic particle hits the singularity of a black hole which is a phenomenon predicted by general relativity and involves gravity in the macro world. Only quantum mechanics can provide the answer stating that the particle will have an uncertainty in position according to the Heisenberg Uncertainty Principle already discussed, such that it might not really reach the singularity and thus escape the possible collapse to infinite density.
It is believed that the theories of general relativity and quantum mechanics, the two great achievements of physics in the 20th century, contradict one another for two main reasons. One is that the former is an essentially deterministic theory and the latter is essentially indeterministic. And two, general relativity relies mainly on the force of gravity while quantum mechanics relies mainly on the other three fundamental forces being the strong, the weak, and the electromagnetic. The question of how to resolve this contradiction remains an area of active research (see, for example, the article on quantum gravity).
In certain situations, the laws of classical physics approximate the laws of quantum mechanics to a high degree of precision. This is often expressed by saying that quantum mechanics "reduces" to classical mechanics and classical electromagnetism. The situation in which this reduction occurs is called the correspondence, or classical limit.
Quantum mechanics can be formulated in either a relativistic or non-relativistic manner. Relativistic quantum mechanics (quantum field theory) provides the framework for some of the most accurate physical theories known, though non-relativistic quantum mechanics is also frequently used for reasons of convenience. We will use the terms quantum mechanics, quantum physics, and quantum theory synonymously, to refer to both relativistic and non-relativistic quantum mechanics. It should be noted, however, that certain authors refer to "quantum mechanics" in the more restricted sense of non-relativistic quantum mechanics. Also, in quantum mechanics, the use of the term particle refers to an elementary or subatomic particle.
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