Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [192]
Cloud chamber
A device invented by C.T.R. Wilson around 1911 that enables the detection of particles by observing their tracks through a chamber containing saturated vapour.
Collapse of the wave function
According to the Copenhagen interpretation, until it is observed or measured, a microphysical object like an electron does not exist anywhere. Between one measurement and the next it has no existence outside the abstract possibilities of the wave function. It is only when an observation or measurement is made that one of the 'possible' states of the electron becomes its 'actual' state and the probabilities of all the other possibilities become zero. This sudden, discontinuous change in the wave function due to an act of measurement is called the 'collapse of the wave function'.
Commutativity
Two variables A and B are said to commute if A×B=B×A. For example, if A and B are the numbers 5 and 4, then 5×4=4×5. Multiplcation of numbers is commutative, since the order in which they are multiplied makes no difference. If A and B are matrices, then A×B does not necessarily equal B×A. When this happens, A and B are said to be non-commutative.
Complementarity
A principle advocated by Niels Bohr that the wave and particle aspects of light and matter are complementary but exclusive. This dual nature of light and matter is like the two sides of the same coin that can display either face, but not both simultaneously. For example, an experiment can be devised to reveal either the wave properties of light or its particle nature, but not both at the same time.
Complex number
A number written in the form a+ib, where a and b are ordinary real numbers familiar from arithmetic. i is the square root of -1, so that (-1)2=-1, and b is called the 'imaginary' part of the complex number.
Compton effect
The scattering of photons by atomic electrons discovered by the American physicist Arthur H. Compton in 1923.
Conjugate variables
A pair of dynamical variables such as position and momentum, or energy and time, that are related to one another through the uncertainty principle, are called conjugate variables or conjugate pairs.
Conservation law
A law which states that some physical quantity, such as momentum or energy, is conserved in all physical processes.
Conservation of energy
The principle that energy cannot be created or destroyed, but can only be converted from one form to another. For example, when an apple falls from a tree, its potential energy is converted into kinetic energy.
Copenhagen interpretation
An interpretation of quantum mechanics, whose principal architect Niels Bohr was based in Copenhagen. Over the years there were differences of opinion between Bohr and other leading advocates of the Copenhagen interpretation such as Werner Heisenberg. However, all agreed on its central tenets: Bohr's correspondence principle, Heisenberg's uncertainty principle, Born's probability interpretation of the wave function, Bohr's principle of complementarity, and the collapse of the wave function. There is no quantum reality beyond what is revealed by an act of measurement or observation. Hence it is meaningless to say, for example, that an electron exists somewhere independent of an actual observation. Bohr and his supporters maintained that quantum mechanics was a complete theory, a claim challenged by Einstein.
Correspondence principle
A guiding principle advocated by Niels Bohr in which the laws and equations of quantum physics reduce to those of classical physics under conditions where the impact of Planck's constant is negligible.
De Broglie wavelength
The wavelength of a particle is related to the momentum p of the particle by the relationship =h/p, where h is Planck's constant.
Degrees of freedom
A system is said to have n degrees of freedom if it requires n coordinates to specify each state of the system. Each degree of freedom represents an independent way in which a body can move or a system can change. An object in the everyday world has three degrees of freedom corresponding