Popper believed that there was a contrast between the theories of Sigmund Freud and Alfred Adler, which he considered non-scientific, and Albert Einstein's theory of relativity which set off the revolution in physics in the early 20th century. Popper thought that Einstein's theory, as a theory properly grounded in scientific thought and method, was highly "risky", in the sense that it was possible to deduce consequences from it which differed considerably from those of the then-dominant Newtonian physics; one such prediction, that gravity could deflect light, was verified by Eddington's experiments in 1919. In contrast he thought that nothing could, even in principle, falsify psychoanalytic theories. He thus came to the conclusion that they had more in common with primitive myths than with genuine science.