Introduction to LIGO & Gravitational Waves

"Ripples on Space-time"

Artist's impression of gravitational waves from two orbiting black holes. [Image: T. Carnahan (NASA GSFC)]
Waves on Space-time
Physicists sometimes call these gravitational waves "ripples on space-time," where space-time includes time as well as the 3 spatial dimensions we are used to. Relativity very accurately describes gravitation in this 4-dimensional 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 space-time in 3 dimensions. Einstein stated that gravity is the result of the curvature of space-time (the surface of our trampoline in our visualization).  If there is no mass on this surface to make depressions on it, then space-time 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 light-years 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.

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