## Time Travel: Terms and Definitions

absolute space: Newton's
conception of the three-dimensional space in which we live as
having a notion of absolute rest, and as having the property
that the lengths of objects are independent of the motion of the
reference frame in which they are measured.absolute time: Newton's conception of time as
being universal, with a unique, universally agreed upon notion
of simultaneity of events and a unique, universally agreed upon
time interval between any two events.chronology protection conjecture: Stephen
Hawking's conjecture that the laws of physics do not allow time
machines.curvature of space or spacetime: The property
of space or spacetime that makes it violate Euclid's or
Minkowski's notions of geometry; that is, the property that
enables straight lines that are initially parallel to cross.event: A point in spacetime; that is, a
location in space at a specific moment of time. Alternatively,
something that happens at a point in spacetime, for example, the
explosion of a firecracker.exotic material: Material that has a negative
average energy density, as measured by someone moving through it
at nearly the speed of light.field: Something that is distributed
continuously and smoothly in space. Examples are the electric
field, the magnetic field, the curvature of spacetime, and a
gravitational wave.freely falling object: An object on which no
forces act except gravity.general relativity: Einstein's laws of physics
in which gravity is described by a curvature of spacetime.geodesic: A straight line in a curved space or
curved spacetime. On the Earth's surface the geodesics are the
great circles.gravitational time dilation: The slowing of the
flow of time near a gravitating body.gravitational wave: A ripple of spacetime
curvature that travels with the speed of light.hyperspace: A fictitious flat space in which
one imagines pieces of our Universe's curved space as embedded.mouth: An entrance to a wormhole. There is a
mouth at each of the two ends of the wormhole.Newtonian laws of physics: The laws of physics,
built on Newton's conception of space and time as absolute,
which were the centerpiece of 19th-century thinking about the
Universe.perturbation: A small distortion (from its
normal shape) of an object or of the spacetime curvature around
an object.Planck-Wheeler length: The Planck-Wheeler
length (1.62 x 10-33 centimeters) is the length scale below
which space as we know it ceases to exist and becomes quantum
foam.principle of absoluteness of the speed of light:
Einstein's principle that the speed of light is a universal
constant, the same in all directions and the same in every
inertial reference frame in the absence of gravity.quantum foam: A probabilistic foam-like
structure of space that probably makes up the cores of
singularities, and that probably occurs in ordinary space on
scales of the Planck-Wheeler length and less.quantum gravity: The laws of physics that are
obtained by merging ("marrying") general relativity with quantum
mechanics.quantum mechanics: The laws of physics that
govern the realm of the small (atoms, molecules, electrons,
protons), and that also underlie the realm of the large, but
rarely show themselves there.reference frame: A (possibly imaginary)
laboratory for making physical measurements, which moves through
the Universe in some particular manner.relative: Dependent on one's reference frame;
different, as measured in one frame which moves through the
Universe in one manner, than as measured in another frame which
moves in another manner.simultaneity breakdown: The fact that events
which are simultaneous as measured in one reference frame are
not simultaneous as measured in another frame that moves
relative to the first.singularity: A region of spacetime where
spacetime curvature becomes so strong that the general
relativistic laws break down and the laws of quantum gravity
take over. If one tries to describe a singularity using general
relativity alone, one finds (incorrectly) that tidal gravity and
spacetime curvature are infinitely strong there. Quantum gravity
probably replaces these infinities by quantum foam.spacetime: The four-dimensional "fabric" that
results when space and time are unified.spacetime curvature: The property of spacetime
that causes freely falling particles that are initially moving
along parallel world lines to subsequently move together or
apart. Spacetime curvature and tidal gravity are different names
for the same thing.spacetime diagram: A diagram with time plotted
upward and space plotted horizontally.special relativity: Einstein's laws of physics
in the absence of gravity.tidal gravity: Gravitational accelerations that
squeeze objects along some directions and stretch them along
others. Tidal gravity produced by the moon and sun is
responsible for the tides on the Earth's oceans.time machine: A device for traveling backward
in time. In physicists' jargon, a "closed timelike curve."time reactor: Any of several devices in which
the extraction of energy from curved spacetime is initiated and
controlled, with the resulting reaction creating concentrated
fields of closed time-link curves for time rate control or power
generation.time-warped field: A self-contained filed in
which time rates within the field may be accelerated or
decelerated relative to time rates outside of the field.warpage of spacetime: Same as curvature of
spacetime.world line: The path of an object through
spacetime or through a spacetime diagram.wormhole: A "handle" in the topology of space,
connecting two widely separated locations in our Universe. |