<human>:  Who was the first person to propose the existence of gravitational waves?
<bot>: Albert Einstein was the first to propose the existence of gravitational waves. He wrote a paper called "On Gravitational Waves" in 1916, and he predicted that gravity could travel through space as ripples in the fabric of spacetime. He also predicted that these waves should be produced by massive objects such as stars, black holes, and neutron stars.