What is a Gravitational Wave?

Gravitational waves are somewhat analogous to the waves of electromagnetic energy that we call light. Light waves are created by electric charges in motion; similarly, gravitational waves are created by masses in motion. Gravity waves are easiest to conceptualize when they are regarded in a similar sense as electromagnetic waves: they carry information about a change in a gravitational field with time.

Imagine a point source of any sort of field, moving with respect to an observer. As the field source moves away from the observer, one expects the intensity of the field around the observer to decrease. However, this change in potential is limited to the speed of light; as the source of the field moves, a "ripple" of change spreads outward from the source at the speed of light, communicating the change in potential to all objects it reaches. The ripple eventually reaches the observer, who feels the change in potential. This effect can be applied to both electromagnetic and gravitational fields.

These ripples, however, are not true gravitational waves; they simply help to visualize the process by which waves are formed from changing fields. True gravitational waves are produced by masses which move in such a way that the change in their quadrupole moment with respect to time is non-zero; a definition and examples of this are given in the next section.

The final piece of the picture is the addition of the work of Einstein to the concept of the gravitational wave. Einstein’s development of the theory of relativity brought about a new definition of gravity: the force of gravity is actually an effect created by the curvature of space-time around massive bodies. This definition implies that gravitational waves are not waves in a gravitational field analogous to electromagnetic waves, as was imagined previously. In reality, gravitational waves are oscillations in the fabric of space-time itself, creating changing curvatures in the space through which they pass. (In fact, gravitational waves were predicted as an effect of general relativity, and are not part of Newtonian physics at all as classical gravitational fields carry no energy of their own.)

Consider a test point mass caught in a gravitational wave: how will the mass respond to the changing curvature around it? In reality, if the mass is alone in space, it will not notice any change. The mass will be freely falling, which means that it will experience no forces. If we consider a pair of point test masses, however, the effect of the wave becomes clear. If the two masses are arranged in a line perpendicular to the direction of the gravitational wave, then the relative distance between the masses will alternatively stretch and shrink as the wave passes through. The wave will affect the masses in such a way that the center of mass of the system is not displaced.

If a third point test mass is added to the system and arrayed perpendicular both to the line between the first two masses and to the direction of travel of the gravitational wave, the distance between the first two masses will be observed to shrink as the distance between the third mass and the first two masses stretches, and vice versa. As with the two-mass system, the wave will affect the masses such that the center of mass of the entire system remains constant. This effect is put to use in the construction of gravitational wave detectors.

Counterintuitively, if the masses are arranged in a line parallel to the direction of the gravitational wave, the masses will experience no change in relative distance. This is due to the fact that there is no change in the metric in the component along the axis of the wave, and is discussed in more detail in the section on general relativity.