Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [191]
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A quantum theory of gravity? A Theory of Everything? A theory beyond the quantum?
GLOSSARY
Terms in italics have an entry in the glossary.
Alkali elements
Elements such as lithium, sodium and potassium in group one of the periodic table that share the same chemical properties.
Alpha decay
A process of radioactive decay in which the nucleus of an atom emits an alpha particle.
Alpha particle
A subatomic particle consisting of two protons and two neutrons bound together. Emitted during alpha decay, it is identical to the nucleus of a helium atom.
Amplitude
The maximum displacement of a wave or an oscillation that is equal to half the distance from the top of the wave (or oscillation) to the bottom. In quantum mechanics, the amplitude of a process is a number that is linked to the probability of that process occurring.
Angular momentum
A property of a rotating object akin to the momentum of an object moving in a straight line. The angular momentum of an object depends on its mass, its size, and the speed with which it is spinning. One object orbiting another also possesses angular momentum that depends on its mass, the radius of its orbit, and its velocity. In the atomic realm, angular momentum is quantised. It can change only by amounts that are whole-number multiples of Planck's constant divided by 2.
Atom
The smallest component of an element consisting of a positively-charged nucleus surrounded by a bound system of negatively-charged electrons. Since an atom is neutral, the number of positively-charged protons in the nucleus is equal to the number of electrons.
Atomic number (Z)
The number of protons in the nucleus of an atom. Every element has a unique atomic number. Hydrogen, with a single proton making up its nucleus and one electron orbiting it, has an atomic number of 1. Uranium, with 92 protons and 92 electrons, has an atomic number of 92.
Balmer series
The set of emission or absorption lines in the spectrum of hydrogen caused by the transitions of its electron between the second and higher energy levels.
Bell's inequality
A mathematical condition derived by John Bell in 1964 concerning the degree of correlation of the quantum spins of entangled pairs of particles that has to be satisfied by any local hidden variables theory.
Bell's theorem
A mathematical proof discovered by John Bell in 1964 that any hidden variables theory whose predictions agree with those of quantum mechanics must be non-local. See non-locality.
Beta particle
A fast moving electron ejected from the nucleus of a radioactive element due to the interconversion of protons and neutrons. Faster and more penetrating than alpha particles, it can be stopped by a thin sheet of metal.
Blackbody
A hypothetical, idealised body that absorbs and emits all electromagnetic radiation that strikes it. It can be approximated in the laboratory as a heated box with a pinhole in one of its walls.
Blackbody radiation
Electromagnetic radiation emitted by a blackbody.
Brownian motion
The erratic motion of pollen grains suspended in a fluid first observed, in 1827, by Robert Brown. In 1905 Einstein explained that Brownian motion was due to the random buffeting of the pollen grains by the molecules of the fluid.
Causality
Every cause has an effect.
Classical mechanics
The name given to the physics that originates from Newton's three laws of motion. Also called Newtonian mechanics, in which the properties of particles such as position and momentum are, in principle, simultaneously measurable with unlimited accuracy.
Classical physics
The description applied to all non-quantum physics such as electromagnetism and thermodynamics. Although Einstein's general theory of relativity is regarded by physicists as 'modern' twentieth-century physics, it is nevertheless