“Albert Einstein may be most famous for his mass-energy equivalence formula E = mc2, but his work also laid down the foundation for modern quantum mechanics.
His analysis of the “spookiness” of quantum mechanics opened up a whole range of applications including quantum teleportation and quantum cryptography, but he wasn’t completely convinced by the theory of quantum mechanics – and that story is as fascinating as the theory he attempted to nail down...”
Read more at: https://phys.org/news/2014-06-einstein-quantum-mechanics-hed-today.html#jCp
“, but he wasn’t completely convinced by the theory of quantum mechanics”
Not of gravity waves.
“Together with a young collaborator, I arrived at the interesting result that gravitational waves do not exist, though they had been assumed a certainty to the first approximation,” he wrote in a letter to his friend Max Born.
Einstein submitted his change of heart in a paper to the Physical Review Letters titled “Do gravitational waves exist?”