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Why are quantum mechanics and relativity needed for very small objects or objects moving near the speed of light?

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Title page of Isaac Newton's Philosophiae Naturalis Principia Mathematica, 1687, featuring large Latin text and publication details in a bordered layout. — Newton’s laws of motion The title page of Isaac Newton's *Philosophiae Naturalis Principia Mathematica* (1687; *Mathematical Principles of Natural Philosophy*), the work in which the physicist introduced his three laws of motion.

Newtonian mechanics is highly effective for everyday, large-scale objects, but it fails to accurately describe the physical world at the extremes of size and speed.

High-Speed Motion and Relativity

Newtonian mechanics breaks down when objects move at speeds approaching the speed of light. Classical physics assumes that velocities are additive and that time and space are absolute. However, special relativity demonstrates that the speed of light is constant for all observers, leading to phenomena such as time dilation and length contraction. At these high speeds, mass and energy are also shown to be equivalent, as expressed by the equation $E = m c^{2}$ .12

Subatomic Scales and Quantum Mechanics

For very small objects like atoms and subatomic particles, classical physics is replaced by quantum mechanics. In this regime, particles exhibit wave-particle duality, and their properties are often restricted to discrete, quantized values. Furthermore, the uncertainty principle dictates that one cannot simultaneously know a particle's position and momentum with perfect precision, precluding the deterministic predictions characteristic of Newtonian physics.34