What is the Difference Between Classical and Quantum Mechanics

The main difference between classical and quantum mechanics is that classical mechanics describes the motion of objects on a macroscopic scale using Newton’s laws of motion, while quantum mechanics deals with the behavior of particles on a microscopic scale, incorporating principles like wave-particle duality and Heisenberg’s uncertainty principle.

Classical mechanics and quantum mechanics are two foundational pillars in the realm of physics. They offer distinct frameworks to understand the behavior of matter and energy at different scales.

Key Areas Covered

1. What is Classical Mechanics
      – Definition, Features, Applications
2. What is Quantum Mechanics
      – Definition, Features, Applications
3. Similarities Between Classical and Quantum Mechanics
      – Outline of Common Features
4. Difference Between Classical and Quantum Mechanics
      – Comparison of Key Differences
5. FAQ: Classical and Quantum Mechanics
      – Frequently Asked Questions

Key Terms

Classical Mechanics, Quantum Mechanics

Compare Classical and Quantum Mechanics

What is Classical Mechanics

Classical mechanics, formulated by Sir Isaac Newton in the 17th century, stands as the cornerstone of classical physics, describing the motion of objects at everyday scales. At its core, classical mechanics relies on three fundamental laws: Newton’s laws of motion. The first law states that an object at rest remains at rest, and an object in motion continues in a straight line at a constant speed unless acted upon by a net external force. The second law relates the force applied to an object with its mass and acceleration, expressed mathematically as F = ma. Finally, the third law asserts that for every action, there is an equal and opposite reaction.

These principles provide a strong framework for understanding the motion of objects, guiding the analysis of diverse phenomena from the graceful arc of a thrown ball to the orbital dynamics of celestial bodies. Classical mechanics, however, faced challenges with the advent of quantum mechanics and relativistic physics in the early 20th century. These unveiled the limitations of Newtonian principles at extreme scales and speeds.

Compare Classical and Quantum Mechanics

The elegance of classical mechanics lies in its simplicity and applicability to everyday scenarios. It has enabled the development of technologies ranging from classical engineering marvels to space exploration missions. Moreover, classical mechanics has found its place in theoretical physics, serving as a stepping stone for more advanced theories. Despite its widespread success, classical mechanics does not provide a complete picture of the physical world, prompting the development of quantum mechanics and relativity to address the behavior of matter at the smallest scales and speeds approaching the speed of light.

What is Quantum Mechanics

Quantum mechanics is a fundamental theory in physics that describes the behavior of matter and energy at the smallest scales, typically involving particles such as atoms and subatomic particles. Developed in the early 20th century, it revolutionized our understanding of the physical world, challenging classical Newtonian physics.

At the heart of quantum mechanics is the concept of wave-particle duality, suggesting that particles exhibit both wave-like and particle-like properties. This duality is encapsulated in the famous Schrödinger equation, which describes the evolution of quantum states over time. The wave function, a mathematical expression derived from the equation, represents the probability amplitude of finding a particle in a particular state.

Classical vs Quantum Mechanics

One of the most perplexing aspects of quantum mechanics is superposition, where particles can exist in multiple states simultaneously until observed. This challenges our classical intuition, as particles seem to exist in a combination of possibilities until a measurement is made, collapsing the wave function to a definite state.

Entanglement is another intriguing phenomenon in quantum mechanics. When particles become entangled, the state of one particle becomes directly correlated with the state of another, regardless of the distance between them. This phenomenon, famously referred to as “spooky action at a distance” by Einstein, challenges our classical notions of locality and separability.

The uncertainty principle, formulated by Werner Heisenberg, states that certain pairs of properties, such as position and momentum, cannot be precisely known simultaneously. This inherent uncertainty is a fundamental characteristic of the quantum world and has profound implications for our understanding of determinism.

Quantum mechanics plays a crucial role in various technological advancements, including the development of quantum computers and quantum cryptography.

Similarities Between Classical and Quantum Mechanics

  • Both classical mechanics and quantum mechanics describe the behavior of physical systems.
  • They use mathematical equations to describe physical phenomena.

Difference Between Classical and Quantum Mechanics


Classical mechanics operates on deterministic principles, meaning that the state of a system can be precisely predicted given initial conditions. In contrast, quantum mechanics introduces inherent probabilistic aspects, making it impossible to precisely predict the outcome of certain measurements.


Classical mechanics deals with continuous variables, such as the position and momentum of a particle. Quantum mechanics, on the other hand, involves discrete quantities, such as quantized energy levels and the concept of particles having distinct quantum states.


While classical mechanics treats particles strictly as particles, quantum mechanics introduces the wave-particle duality concept.

FAQ: Classical and Quantum Mechanics

Which physics is the hardest?

Most people find quantum mechanics the hardest because it introduces concepts like probability-based behavior, the dual nature of particles, and non-local connections between particles. Additionally, the complex math involved in quantum physics can be challenging for many to understand.

Who is the father of quantum physics?

Niels Bohr and Max Planck are considered the founding fathers of quantum physics.


Classical mechanics describes the motion of objects on a macroscopic scale using Newton’s laws of motion, while quantum mechanics deals with the behavior of particles on a microscopic scale, incorporating principles like wave-particle duality and Heisenberg’s uncertainty principle. Thus, this is the main difference between classical and quantum mechanics.


1. “Classical Mechanics.” Encyclopedia Britannica. 
2. “Quantum Mechanics.” Encyclopedia Britannica. 

Image Courtesy:

1. “Orbital motion” By Own work – (Public Domain) via Commons Wikimedia
2. “Hydrogen Density Plots” By PoorLeno (talk) – Forinash, Kyle. Hydrogen W Simulation. Indiana University Southeast. Retrieved on 2008-12-18.Tokita, Sumio; Sugiyama, Takao; Noguchi, Fumio; Fujii, Hidehiko; Kobayashi, Hidehiko (2006). “An Attempt to Construct an Isosurface Having Symmetry Elements”. Journal of Computer Chemistry, Japan 5 (3): 159–164. DOI:10.2477/jccj.5.159. (Public Domain) via Commons Wikimedia

About the Author: Hasini A

Hasini is a graduate of Applied Science with a strong background in forestry, environmental science, chemistry, and management science. She is an amateur photographer with a keen interest in exploring the wonders of nature and science.

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