Introduction to LIGO & Gravitational Waves
"Ripples on Spacetime"
Artist's impression of gravitational waves from two orbiting black holes.
[Image: T. Carnahan (NASA GSFC)]

Physicists sometimes call these gravitational waves "ripples on
spacetime," where spacetime includes time as well as the 3 spatial
dimensions we are used to.
Relativity very accurately describes gravitation in
this 4dimensional universe. Since it is very difficult to
visualize 4 dimensions, we can use a flexible surface such as a
trampoline as a simplified model for spacetime in 3 dimensions.
Einstein stated that gravity is the result of the curvature of
spacetime (the surface of our trampoline in our visualization).
If there is no mass on this surface to make depressions on it, then
spacetime is flat and a rolling ball on the surface will move in a
straight line. But if there is a large mass that makes a
depression on this surface, the rolling ball will be deflected toward
the mass by the curvature of the surface, just as if there were a
gravitational attraction between the two masses. Any change in
position of the masses will make ripples on this surface representing
our changing gravitational field  or gravitational waves.
Gravitational waves are created by moving masses, much as
electromagnetic waves are created by moving charges. But because
gravity is the weakest of the four fundamental forces (the others being
the electromagnetic, weak nuclear, and strong nuclear), gravitational
waves are exceedingly small. For physicists, a strong
gravitational wave will produce displacements on the order of 10
^{18}
meters  this is 1000 times smaller than the diameter of a
proton.
Waves of this strength will be produced by very massive systems
undergoing large accelerations, like two orbiting black holes that are
about to merge into one. Since systems like these are rare, these
sources will be lightyears away. Therefore, the search for
gravitational waves is seeking the minute effects of some of the most
energetic astrophysical systems from the depths of the universe.