Einstein's theory of relativity, both the special theory and the general theory, has been remarkably successful in describing and predicting the behavior of objects in a wide range of situations. However, like any scientific theory, it is not without its limitations or flaws. Here are a few limitations and challenges associated with Einstein's theory of relativity:
Quantum Mechanics: Einstein's theory of relativity does not incorporate the principles of quantum mechanics, which describe the behavior of particles on a very small scale. Quantum mechanics and relativity are both highly successful theories in their respective domains, but they are not yet fully reconciled into a single unified theory. Attempts to unify the two theories, such as quantum field theory and string theory, are ongoing but remain areas of active research.
Gravitational Singularities: Einstein's general theory of relativity predicts the existence of gravitational singularities, points of infinite density and curvature, such as those found at the center of black holes. These singularities pose theoretical challenges as they imply breakdowns in our understanding of physics. The behavior of matter and energy in these extreme conditions is not fully understood, and a complete theory of quantum gravity is needed to address these issues.
Dark Matter and Dark Energy: Einstein's theory of relativity does not account for the presence of dark matter and dark energy, which are thought to make up a significant portion of the universe. These components are invoked to explain the observed gravitational effects and the accelerated expansion of the universe. Understanding the nature of dark matter and dark energy remains an open question in cosmology.
Regarding your second question, Einstein did not simply use his theory of relativity as an approximation but rather as a fundamental framework for understanding the nature of space, time, and gravity. Einstein's theory of relativity emerged as a result of his dissatisfaction with the limitations of Newtonian mechanics, which did not adequately explain certain phenomena, such as the behavior of light, the perihelion shift of Mercury, and the nature of gravity.
Einstein's theory of relativity introduced profound conceptual shifts in our understanding of the physical world. It provided a more accurate and comprehensive framework for describing the behavior of objects at high speeds, in strong gravitational fields, and in the context of the overall structure of the universe. The theory's success in explaining and predicting a wide range of phenomena, confirmed through experimental observations and empirical evidence, contributed to its widespread acceptance and application.
While Newton's laws of motion continue to be used in many everyday situations due to their simplicity and accuracy within certain limits, Einstein's theory of relativity is necessary to explain phenomena that go beyond the scope of classical mechanics, such as the behavior of objects near the speed of light or in the presence of strong gravitational fields. As such, Einstein's theory is not merely an approximation but a more fundamental and comprehensive description of the universe.