© Blayne Heckel.
A gravitational wave traveling into the page stretches and compresses spacetime along perpendicular axes. Upper diagram: For free masses initially arranged in a circle, a gravitational wave with a period, T, stretches and then compresses space along the vertical axis and vice versa along the horizontal axis. This is called the "+" polarization state of the wave. Lower diagram: The "X" polarization state stretches and compresses space along axes tilted 45 degrees from vertical. (Unit: 3)