Quantum mechanics
| Scientific Theory | |
|---|---|
| Name | Quantum mechanics |
| Type | Physical theory and its disputed interpretation |
| Author(s) | Formalism: Heisenberg, Schrödinger, Dirac and others; interpretation disputed — see David Bohm, Louis de Broglie, Jean Pierre Vigier |
| Keywords | quantum mechanics, Copenhagen interpretation, measurement problem, hidden variables, pilot wave, non-locality, Bell's theorem |
| Year | 1925 onward |
Quantum mechanics is the physical theory governing matter and radiation at atomic and subatomic scales. Its mathematical formalism is among the most accurately tested in science, and that success is not in dispute. What is disputed — and what the critical tradition documented on this wiki concerns itself with — is what the formalism means.
The orthodox account, the Copenhagen interpretation, holds that nature is irreducibly probabilistic, that a system has no definite properties until it is measured, and that asking what happens between measurements is meaningless. A long line of physicists has objected that this is an interpretive choice presented as an experimental result: that the same predictions follow from realist, causal theories in which particles have definite properties at all times, and that the alternatives were dismissed on the authority of a flawed proof rather than on evidence.
The formalism and its success
Developed from 1925 by Heisenberg, Schrödinger, Dirac and others, quantum mechanics describes a system by a wave function evolving deterministically under the Schrödinger equation, from which probabilities of measurement outcomes are computed via the Born rule. It accounts for atomic spectra, chemical bonding, semiconductors, lasers and much else, often to extraordinary precision. Critics of the orthodox interpretation generally accept all of this: the dispute is not about whether the equations work.
The Copenhagen interpretation
The interpretation associated with Bohr and Heisenberg holds that:
- the wave function is a complete description of an individual system, containing all that can be said about it;
- measurement outcomes are irreducibly random, not merely unknown;
- the wave function "collapses" on measurement, discontinuously and outside the Schrödinger dynamics;
- properties such as position and momentum are not possessed by a system prior to observation.
This became the standard teaching account, and for much of the twentieth century questioning it was treated as a mark of misunderstanding rather than a legitimate research programme.
Why the interpretation is disputed
The measurement problem
The theory contains two incompatible rules of evolution: smooth, deterministic Schrödinger evolution, and abrupt collapse on measurement. Nothing in the formalism specifies what constitutes a "measurement" or when collapse occurs — leaving an ill-defined boundary between quantum system and classical apparatus, and inviting the observer, or even consciousness, into fundamental physics. Critics regard this as an unresolved incoherence at the theory's foundation rather than a philosophical footnote.
The failure of the impossibility proof
For two decades, the possibility of a realist alternative was widely held to have been ruled out by John von Neumann's 1932 "impossibility proof" against hidden variables. That proof rested on an assumption stronger than physics requires. The flaw was identified by the philosopher Grete Hermann in 1935 and ignored; it was rediscovered and made widely known only by John Stewart Bell in the 1960s. In the interim the proof served to foreclose an entire line of inquiry.
Realist and causal alternatives
In 1952 David Bohm produced an explicit counterexample: a fully worked causal interpretation in which particles have definite trajectories guided by a real field through a quantum potential, reproducing every standard prediction without collapse, observers, or irreducible randomness. It built on the pilot-wave proposal Louis de Broglie had advanced and abandoned in 1927, and de Broglie returned to the programme after Bohm's work. Jean Pierre Vigier, de Broglie's assistant, developed a stochastic version in which quantum behaviour arises from an underlying subquantum medium.
That such theories exist and work is the central point: indeterminism and observer-dependence are not forced on us by the evidence.
Non-locality and Bell's theorem
Engaging with Bohm's theory led John Stewart Bell to his 1964 theorem, which showed that no local theory assigning definite pre-existing values can reproduce all quantum predictions. Experiment has since favoured the quantum predictions. This is often presented as a defeat for hidden variables; proponents note that it is more precisely a result about locality — Bohm's theory is non-local and untouched by it — and that Bell's own sympathies lay with the realist programme he had done so much to clarify.
Alternative interpretations documented here
Besides the causal and stochastic programmes, the literature catalogued on this wiki includes classical and semi-classical reconstructions of quantum phenomena, probabilistic and ensemble readings of the wave function, multivalued-logic treatments of entanglement, and proposals to rebuild quantum theory on electromagnetic or subquantum foundations. Papers include Classical Interpretation of Quantum Mechanics, On the Interpretation of Quantum Mechanics, Copenhagen's Interpretation in the Balance, Quantum Reprogramming - A Long Overdue and Least Intrusive Reality Adaptation of the Copenhagen Interpretation, The Need For a Probabilistic Interpretation of Quantum Mechanics: Cause and Results, Interpretation of Quantum Mechanics and Entanglement with Multivalued Logic and Jean-Pierre Vigier and the Stochastic Interpretation of Quantum Mechanics.
On this wiki
Researchers with pages here whose work bears on quantum foundations include David Bohm, Louis de Broglie, Jean Pierre Vigier, Robert E French and James Keene. Further material is indexed under Category:Quantum Theory.