The theory that would need to be adapted or replaced in order to unify the theories of relativity and quantum mechanics is classical mechanics, also known as Newtonian mechanics. Classical mechanics, formulated by Sir Isaac Newton, provides a description of motion and the behavior of objects at macroscopic scales. It works well for everyday objects but breaks down when dealing with extremely small particles, high speeds, or strong gravitational fields.
Both the theory of relativity and quantum mechanics have been incredibly successful in their respective domains, but they are fundamentally different and incompatible with each other within their current frameworks. General relativity, which describes gravity and the behavior of spacetime on large scales, is a classical theory based on continuous fields. On the other hand, quantum mechanics deals with the behavior of particles and fields at the microscopic scale, involving discrete energy levels, probabilities, and wave-particle duality.
The challenge lies in reconciling these two theories into a unified framework known as "quantum gravity" or a "theory of everything." Several approaches have been proposed to achieve this goal, such as string theory, loop quantum gravity, and other quantum field theories. These approaches attempt to provide a consistent mathematical framework that combines both quantum mechanics and general relativity.
However, it's important to note that a definitive theory of quantum gravity that successfully unifies quantum mechanics and general relativity has not been established. The quest for a unified theory remains an active area of research in theoretical physics, and scientists continue to explore different avenues to bridge the gap between these two fundamental theories.