Three Roads To Quantum Gravity

GLOSSARY
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Newton's gravitational constant
The fundamental constant that measures the strength of the gravitational
force.
Newtonian physics
All physical theories formulated on the pattern of Newton's laws of
motion. See classical physics, which is a synonymous term.
non-commutative geometry
A description of a space in which it is impossible to determine
enough information to locate a point, but which can have many other
properties of space including the fact that it can support a description
of particles and ®elds evolving in time.
past or causal past
For a particular event, all other events that could have in¯uenced it by
sending energy or information to it.
past light cone
The past light cone of an event consists of all those events that could
have sent a light signal to it.
Pauli exclusion principle
The principle that no two fermions can be put into exactly the same
quantum state; named after Wolfgang Pauli.
perturbation theory
An approach to making calculations in physics in which some phenomena
are represented in terms of small deviations from or oscillations
of some stable state, or the interactions among such oscillations.
Planck scale
The scale of distance, time and energy on which quantum gravity
effects are important. It is de®ned roughly by the Planck units ±
processes on the Planck scale take around a Planck time, which is
10 743 of a second. To observe on the Planck scale, distances of around
the Planck length must be probed. This is about 10 733 of a centimetre.
Planck's constant
A fundamental constant that sets the scale of quantum effects;
normally denoted by h.
Planck units
The basic units of measure in a quantum theory of gravity. Each is
given by a unique combination of three basic constants: Planck's
constant, Newton's gravitational constant and the speed of light.
Planck units include the Planck length, Planck energy, Planck mass,
Planck time and Planck temperature.
quantum chromodynamics (QCD)
The theory of the forces between quarks.
quantum electrodynamics (QED)
The marriage of quantum theory with electrodynamics. It describes
light and the electric and magnetic forces in quantum terms.