Newtonian physics, also known as classical physics, was replaced by relativity and quantum theory because they provided more accurate and comprehensive explanations of the physical world, especially in extreme conditions or at very small scales. Here are the main reasons for their replacement:
Inadequacy in Explaining High-Speed and Massive Objects: Newtonian physics is based on the concept of absolute space and time, and it assumes that measurements of time and space are the same for all observers. However, as objects approach the speed of light or become very massive, Newtonian physics fails to accurately describe their behavior. Albert Einstein's theory of relativity, particularly special relativity and later general relativity, provided a more accurate and consistent framework for describing the dynamics of objects in these regimes.
Experimental Evidence: Several experimental observations challenged the predictions of Newtonian physics. For example, the Michelson-Morley experiment in 1887 failed to detect the existence of the "ether," a medium proposed by classical physics to carry light waves. This experiment supported the idea that the speed of light is constant in all inertial reference frames, a principle central to special relativity. Other experimental confirmations, such as the bending of starlight around massive objects (confirmed during a solar eclipse) and the redshift of light in gravitational fields, further supported the validity of general relativity.
Quantum Behavior at Small Scales: Classical physics treated particles as point-like objects with well-defined trajectories and properties. However, observations at the atomic and subatomic scales revealed phenomena that could not be explained by classical physics. Quantum theory, developed in the early 20th century, introduced the idea of wave-particle duality and probabilistic behavior of particles. It provided a framework to understand phenomena like quantization of energy levels, the uncertainty principle, and wave interference, which classical physics could not account for.
Explanation of Atomic and Molecular Structure: Quantum theory successfully explained the behavior and stability of atoms and molecules. It provided insights into electron energy levels, chemical bonding, and the spectra of elements. These explanations were crucial for advancing our understanding of chemistry, materials science, and atomic physics.
Consistency with Experimental Observations: Both relativity and quantum theory have been extensively tested and have shown remarkable agreement with experimental observations across a wide range of phenomena. These theories have made precise predictions that have been confirmed by experiments, such as the time dilation of moving clocks (special relativity), the gravitational redshift (general relativity), and the behavior of particles in accelerators (quantum theory).
Overall, the replacement of Newtonian physics by relativity and quantum theory was driven by the need for more accurate descriptions of physical phenomena, as well as the ability to explain experimental observations that could not be accounted for by classical physics. These theories have fundamentally reshaped our understanding of the universe and play a crucial role in modern physics and technology.