Gravitational waves do not compress or expand space in the same way that gravitational forces do. Gravity is indeed a force that attracts masses toward each other, leading to compression of space in the vicinity of massive objects. However, gravitational waves operate differently.
Gravitational waves are ripples in the fabric of spacetime itself. They are generated by accelerating masses, such as binary systems of compact objects (like black holes or neutron stars) orbiting each other or supernova explosions. When these masses undergo acceleration or asymmetrical motion, they create disturbances in spacetime that propagate outward as waves.
Gravitational waves can be visualized as stretching and squeezing of spacetime in mutually perpendicular directions, forming an oscillating pattern. This stretching and squeezing is often referred to as "compression" and "expansion," but it's important to note that it is not compression and expansion of space in the same sense as the gravitational force acting on matter.
As a gravitational wave passes through space, it alternately stretches and compresses spacetime along the direction of wave propagation while simultaneously squeezing and expanding it perpendicularly. This distortion of spacetime causes objects located in the vicinity of the wave's path to undergo slight changes in distance between them.
It's important to emphasize that these changes in distance caused by gravitational waves are extremely tiny—on the order of the fraction of the size of an atomic nucleus—making them challenging to detect. However, sensitive instruments such as interferometers, like LIGO (Laser Interferometer Gravitational-Wave Observatory), have been able to detect and measure these minuscule distortions in spacetime caused by gravitational waves.
In summary, gravitational waves do not compress or expand space in the same manner as gravitational forces. Instead, they cause oscillatory stretching and squeezing of spacetime as they propagate through it.