Einstein referred to his theory of relativity as "incomplete" primarily because it had not yet been successfully reconciled with quantum mechanics, which is the branch of physics that describes the behavior of particles on a very small scale.
Einstein's theory of relativity, both special and general relativity, provided a revolutionary framework for understanding the nature of space, time, and gravity. However, it did not incorporate the principles of quantum mechanics, which had emerged as a separate branch of physics in the early 20th century.
Quantum mechanics introduced a probabilistic and wave-like description of particles, challenging the deterministic worldview of classical physics. Einstein was known for his dissatisfaction with the probabilistic nature of quantum mechanics and his preference for a deterministic understanding of reality. He famously remarked, "God does not play dice with the universe," expressing his skepticism about the indeterministic nature of quantum mechanics.
Einstein spent much of his later years searching for a unified theory that could reconcile the principles of general relativity and quantum mechanics. He sought a theory that would provide a complete and consistent description of the fundamental laws governing the universe.
Unfortunately, Einstein was not able to achieve this goal before his passing, and the challenge of unifying general relativity and quantum mechanics remains one of the outstanding problems in theoretical physics. Despite its remarkable success in explaining the macroscopic world, general relativity's lack of integration with quantum mechanics is often cited as one of its limitations and the reason why Einstein considered it "incomplete."
It is important to note that Einstein's theory of relativity itself is highly accurate and successful within its domain of applicability, especially in describing the behavior of objects on large scales and in the presence of strong gravitational fields. However, the quest for a theory that unifies all fundamental forces and provides a complete description of the universe continues to be a topic of ongoing research.