Quantum Mechanics

Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.

Overview

Quantum mechanics introduces a major shift in the framework of classical mechanics. Classical mechanics describes the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets, stars, and galaxies. However, classical mechanics fails to describe phenomena at very small scales, such as the subatomic level. Quantum mechanics differs from classical mechanics primarily at the quantum realm of atoms and smaller particles.

History

The development of quantum mechanics was initially motivated by two main observations which demonstrated the inadequacy of classical physics. The first was the problem of black-body radiation proposed by Max Planck in 1900, and the second was the explanation of the photoelectric effect by Albert Einstein in 1905. Further development was made in the 1920s by Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and others. The formulation of the theory was completed in the mid-1920s with the work of Heisenberg on matrix mechanics and Schrödinger on wave mechanics.

Principles

Quantum mechanics is based on several key principles that distinguish it from classical mechanics:

• Quantum Superposition: The principle that any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum state.
• Quantum Entanglement: A physical phenomenon that occurs when pairs or groups of particles are generated, interact, or share spatial proximity in ways such that the quantum state of each particle cannot be described independently of the state of the others.
• Wave-Particle Duality: The concept that every particle or quantum entity can exhibit both particle-like and wave-like behavior.
• Heisenberg Uncertainty Principle: Formulated by Werner Heisenberg, this principle states that the more precisely the position of some particle is determined, the less precisely its momentum can be known, and vice versa.

Mathematical Formulation

The mathematical formulations of quantum mechanics are abstract. A system is described by a "state", which is represented by a wave function, symbolized as ψ (psi). This wave function is an element of a complex vector space—variously called the "state space" or "Hilbert space". The evolution of a quantum state is described by the Schrödinger equation, which predicts how a quantum system will change with time.

Applications

Quantum mechanics has radically altered our understanding of many fundamental aspects of the universe and led to the development of many technologies including:

• Quantum computing
• Quantum cryptography
• Quantum teleportation
• Semiconductors and transistors
• MRI scanners in the field of medicine

Challenges and Implications

Quantum mechanics also poses profound challenges to several philosophical questions about the nature of reality, including the role of the observer and measurements, the nature of reality, and the place of determinism in the universe.

See Also

• Quantum field theory
• Quantum gravity
• String theory