


Timespeak
You and I know it as a time machine. Physicists, on the other hand, call it a
"closed timelike curve." Below, feast on the concepts and conjectures, the
dialects and definitions that physicists rely on when musing about the
possibility of time travel. If this list only whets your appetite for more, we
recommend you have a gander at the book from which we excerpted this glossary:
Black Holes and Time Warps: Einstein's Outrageous Legacy, by Kip S.
Thorne (Norton, 1994).
absolute space: Newton's conception of the threedimensional 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
19thcentury thinking about the Universe.
perturbation: A small distortion (from its normal shape) of an object or
of the spacetime curvature around an object.
PlanckWheeler length: The PlanckWheeler 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 foamlike structure of space that probably
makes up the cores of singularities, and that probably occurs in ordinary space
on scales of the PlanckWheeler 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 fourdimensional "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."
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.
From Black Holes and Time Warps: Einstein's Outrageous Legacy by Kip S. Thorne.
Copyright © 1994 by Kip S. Thorne. Reprinted by permission of
W. W. Norton & Company, Inc. and Macmillan.
Photo: Photodisc Imaging
Sagan on Time Travel 
Traveling Through Time
Timespeak 
Think Like Einstein 
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