The Elegant Universe - Brian Greene [225]
Newton's laws of motion. Laws describing the motion of bodies based on the conception of an absolute and immutable space and time; these laws held sway until Einstein's discovery of special relativity.
Newton's universal theory of gravity. Theory of gravity declaring that the force of attraction between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them. Subsequently supplanted by Einstein's general relativity.
Nonperturbative. Feature of a theory whose validity is not dependent on approximate, perturbative calculations; an exact feature of a theory.
Nucleus. The core of an atom, consisting of protons and neutrons.
Observer. Idealized person or piece of equipment, often hypothetical, that measures relevant properties of a physical system.
One-loop process. Contribution to a calculation in perturbation theory in which one virtual pair of strings (or particles in a point-particle theory) is involved.
Open string. A type of string with two free ends.
Oscillatory pattern. See vibrational pattern.
Particle accelerator. Machine for boosting particles to nearly light speed and slamming them together in order to probe the structure of matter.
Perturbation theory. Framework for simplifying a difficult problem by finding an approximate solution that is subsequently refined as more details, initially ignored, are systematically included.
Perturbative approach, Perturbative method. See perturbation theory.
Phase. When used in reference to matter, describes its possible states: solid phase, liquid phase, gas phase. More generally, refers to the possible descriptions of a physical system as features on which it depends (temperature, string coupling constant values, form of spacetime, etc.) are varied.
Phase transition. Evolution of a physical system from one phase to another.
Photoelectric effect. Phenomenon in which electrons are ejected from a metallic surface when light is shone upon it.
Photon. Smallest packet of the electromagnetic force field; messenger particle of the electromagnetic force; smallest bundle of light.
Planck energy. About 1,000 kilowatt hours. The energy necessary to probe to distances as small as the Planck length. The typical energy of a vibrating string in string theory.
Planck length. About 10-33 centimeters. The scale below which quantum fluctuations in the fabric of spacetime would become enormous. The size of a typical string in string theory.
Planck mass. About ten billion billion times the mass of a proton; about one-hundredth of a thousandth of a gram; about the mass of a small grain of dust. The typical mass equivalent of a vibrating string in string theory.
Planck's constant. Denoted by the symbol h, Planck's constant is a fundamental parameter in quantum mechanics. It determines the size of the discrete units of energy, mass, spin, etc. into which the microscopic world is partitioned. Its value is 1.05 × 10-27 grams-cm/sec.
Planck tension. About 1039 tons. The tension on a typical string in string theory.
Planck time. About 10-43 seconds. Time at which the size of the universe was roughly the Planck length; more precisely, time it takes light to travel the Planck length.
Primordial nucleosynthesis. Production of atomic nuclei occurring during the first three minutes after the big bang.
Principle of equivalence. Core principle of general relativity declaring the indistinguishability of accelerated motion and immersion in a gravitational field (over small enough regions of observation). Generalizes the principle of relativity by showing that all observers, regardless of their state of motion, can claim to be at rest, so long as they acknowledge the presence of a suitable gravitational field.
Principle of relativity. Core principle of special relativity declaring that all constant-velocity observers are subject to an identical set of physical laws and that, therefore, every constant-velocity observer is justified in claiming that he or she