The International Solvay Institutes for Physics and Chemistry, located in Brussels, was founded by the Belgian industrialist Ernest Solvay in 1912, following the historic invitation-only 1911 conference, considered a turning point in the world of physics.
Hendrik Lorentz was chairman of the first Solvay Conference held in Brussels from 30 October to 3 November 1911. The subject was Radiation and Quanta. This conference looked at the problems of having two approaches, namely classical physics and quantum theory. See that fine-looking young gentleman, standing, second from right, with the dark moustache. That's Albert Einstein. Other members of the Solvay Congress included such luminaries as Marie Curie and Henri Poincare.
Einstein, though better known for his theories of relativity, also happens to be one of the founding fathers of quantum physics. His 1905 paper on the photoelectric effect, which upended Maxwell's theory by suggesting that the electromagnetic field itself ought to be quantized, was so revolutionary, even a decade later some of his friends were convinced that the paper was wrong and that even great scientists, like Einstein, sometimes make mistakes.
Well, this 'mistake' earned Einstein his Nobel prize.
What Einstein didn't accept was that quantum mechanics, or more specifically, its probabilistic interpretation is known as the Copenhagen interpretation, was the final word on the subject. He believed that quantum mechanics, though works well in practice, must be an "effective theory", an approximation of a deeper, more elegant theory that does away with probabilities.
This led him on a quest towards a classical unified theory. Meanwhile, the world of physics marched ahead in a different direction, developing quantum field theory, achieving successes that made Einstein's later efforts look unnecessary. This fruitless quest consumed the last few decades of his life. Perhaps if Einstein had lived longer and seen the development of group theoretical methods, non-Abelian quantum field theory, and ultimately, the standard model of particle physics, his views would have mellowed and he would have sought a resolution to the question of how we interpret the quantum world in a different direction. But his life ended in 1955 when all this was still in its infancy.