Gravitational Waves - What Are They?

Gravitationa waves are the source of those gravitational waves. They can be, for example, the collision of two neutron stars. It's incredible wecan see them here on earth. We have these lazers underground in mile long tunnels, bouncing lazer beams off of mirrors. And we can tell when a gravitational wave has passed because it makes the path of that lazer a tiny bit longer or a tiny bit shurer.