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.[1] It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.
Classical physics, the collection of theories that existed before the advent of quantum mechanics, describes many aspects of nature at an ordinary (macroscopic) scale, but is not sufficient for describing them at small scales and low energies of individual atoms and subatomic particles.[2]
"If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet." — Niels Bohr
All quantum theories make use of mathematical tools. In quantum mechanics, the wave function of a physical system provides a probability distribution of the outcome of each possible measurement made on the system.[3]
History
Quantum mechanics grew out of several late-19th and early-20th-century problems in physics. At this time, physics was dominated by the classical physics theories of classical mechanics, classical electromagnetism, and classical thermodynamics.
The foundation of quantum mechanics was laid by Max Planck in 1900 when he introduced the concept of energy quantization to explain black-body radiation. This was followed by Albert Einstein's explanation of the photoelectric effect in 1905, which established the photon concept.[4]
Mathematical Formulation
The mathematical formulation of quantum mechanics was developed by several physicists in the 1920s. The two primary formulations are the Schrödinger equation (wave mechanics) and Heisenberg's matrix mechanics. These were later shown to be mathematically equivalent.
The above equation is the time-dependent Schrödinger equation, where \(i\) is the imaginary unit, \(\hbar\) is the reduced Planck constant, \(\Psi\) is the wave function, and \(\hat{H}\) is the Hamiltonian operator.[5]
Key Principles
Wave-Particle Duality
One of the central concepts of quantum mechanics is wave-particle duality. This principle states that every particle or quantum entity may be described as either a particle or a wave. It expresses the unified nature of all quantum entities: they are neither what we conventionally call particles nor waves, but exhibit qualities of both concepts depending on how they are observed.[6]
Uncertainty Principle
The Heisenberg uncertainty principle states that there is a fundamental limit to the precision with which certain pairs of physical properties, such as position and momentum, can be known simultaneously. The more precisely the position is determined, the less precisely the momentum is known in this time, and vice versa.[7]
Applications
Quantum mechanics has practical applications that have revolutionized modern technology:
- Semiconductors: The basis of all modern electronics, including computers and smartphones.
- Lasers: Used in telecommunications, medical surgery, and industrial cutting.
- MRI: Magnetic resonance imaging relies on quantum mechanical properties of nuclear spin.
- Quantum Computing: Emerging technology leveraging superposition and entanglement for computation.
Interpretations
There is no consensus on the correct interpretation of quantum mechanics. Major interpretations include the Copenhagen interpretation, the Many-worlds interpretation, and the De Broglie–Bohm theory. These interpretations differ primarily in their ontological claims about the nature of reality and the role of the observer.[8]