Britannica Sources used in this answer

How do classical physics and modern physics differ?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A diagram of a cylindrical vessel containing gas, with a movable piston held down by a weight. Countless small dots beneath the piston represent gas particles in rapid, random motion, illustrating Bernoulli's model of gas pressure. — physics As conceived by Daniel Bernoulli in *Hydrodynamica* (1738), gases consist of numerous particles in rapid random motion. He assumed that the pressure of a gas is produced by the direct impact of the particles on the walls of the container.

Classical physics and modern physics differ primarily in their scope of application and their fundamental assumptions regarding the nature of reality.12

Classical Physics

Developed largely before the 20th century, classical physics describes the motion of macroscopic objects moving at speeds much slower than the speed of light. It is built upon Newton’s laws of motion and assumes an absolute, independent framework of space and time. In this view, physical phenomena are deterministic, and objects possess definite properties regardless of observation.13456

Modern Physics

Modern physics, emerging in the early 20th century, addresses phenomena at extreme scales—such as high speeds, massive objects, and subatomic particles—where classical laws fail. Its foundations rest on two pillars:12
* Relativity: Replaces absolute space and time with a four-dimensional space-time and shows that measurements depend on the observer's motion.24
* Quantum Mechanics: Replaces determinism with probability and uncertainty, revealing that energy and matter exhibit both wave and particle characteristics.26

While modern physics provides a more profound understanding of the universe, classical physics remains a highly accurate approximation for everyday macroscopic experiences.37