Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [196]
Quantum number
Numbers that specify quantised physical quantities such as energy, quantum spin or angular momentum. For example, the quantised energy levels of a hydrogen atom are denoted by a set of numbers beginning with n=1 for the ground state, where n is the principal quantum number.
Quantum spin
A fundamental property of particles with no direct counter-part in classical physics. Any picturesque comparison of a 'spinning' electron to a spinning top is merely a poor aid that fails to capture the essence of this quantum concept. The quantum spin of a particle cannot be explained in terms of classical rotation since it can only have certain values that are equal to either a whole number or half a whole number multiplied by Planck's constant h divided by 2 (h, a quantity called h-bar). Quantum spin is said to be either up (clockwise) or down (anti-clockwise) with respect to the direction of measurement.
Radiation
The emission of energy or particles. Examples include electromagnetic radiation, thermal radiation and radioactivity.
Radioactivity
When an unstable atomic nucleus spontaneously disintegrates to acquire a more stable configuration by emitting alpha, beta or gamma radiation, the process is called radioactivity or radioactive decay.
Realism
The philosophical worldview which maintains that there exists a reality 'out there' independent of an observer. For a realist, the moon exists when no one is looking at it.
Relativity, general
Einstein's theory of gravitation in which the gravitational force is explained as a distortion of space-time.
Relativity, special
Einstein's 1905 theory of space-time in which the speed of light remains the same for all observers however fast they are moving. It is called 'special' because it does not describe objects that are accelerating, or gravity.
Scattering
The deflection of one particle by another.
Schrödinger's cat
A thought experiment devised by Erwin Schrödinger in which, according to the rules of quantum mechanics, a cat exists in a superposition of alive and dead states until it is observed.
Schrödinger's equation
The fundamental equation of the wave mechanics version of quantum mechanics that governs the behaviour of a particle or the evolution of a physical system by encoding how its wave function varies with time.
where m is the mass of the particle, is a mathematical entity called the 'del-squared operator' which is responsible for tracking how the wave function changes from place to place, V captures the forces acting on the particle, i is the square root of –1, /t describes how the wave function changes in time, and h is Planck's constant h divided by 2 and is pronounced 'h-bar'. There is another form of the equation that gives a snapshot in time and is called the time-independent Schrödinger equation.
Spectral energy distribution of blackbody radiation
At any given temperature, it is the intensity of electromagnetic radiation emitted by a blackbody at each wavelength (or frequency). Also known simply as the blackbody spectrum.
Spectral lines
The pattern of coloured lines of light on a black background is called an emission spectrum. A series of black lines on a coloured background is called an absorption spectrum. Each element has a unique set of both emission and absorption spectral lines produced respectively by the emission and absorption of photons as electrons within the atoms of the element jump between different energy levels.
Spectroscopy
The area of physics concerned with analysing and studying absorption and emission spectra.
Spontaneous emission
The spontaneous emission of a photon as an atom makes the transition from an excited state to a lower energy state.
Stark effect
The splitting of spectral lines when atoms are placed in an electric field.
Stimulated emission
When an incident photon is not absorbed by an excited atom,