| EPR paper | |
|---|---|
| Name | "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" |
| Author | Albert Einstein, Boris Podolsky, Nathan Rosen |
| Language | English |
| Published | 1935 |
| Publisher | Physical Review |
| Pages | 775–780 |
| Genre | Scientific paper |
EPR paper
The EPR paper is the 1935 paper "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" by Albert Einstein, Boris Podolsky and Nathan Rosen that challenged the philosophical and technical foundations of quantum mechanics. It presented a thought experiment intended to show that quantum theory, as formulated in the Copenhagen interpretation, yields results that imply either incompleteness of the theory or the existence of what the authors called "elements of reality" not described by the theory. The paper catalyzed decades of conceptual and experimental work on entanglement, locality, and the interpretation of quantum theory.
The EPR paper arose in the context of debates between proponents of the Copenhagen interpretation such as Niels Bohr and critics like Albert Einstein who sought a deterministic and locally causal account of physical phenomena. In the early 20th century, developments by Max Planck, Einstein himself (on the photoelectric effect), Erwin Schrödinger (wave mechanics), and Paul Dirac (quantum algebra) established a formal framework for microscopic systems. By the 1930s, the formalism of wave functions and the Born rule had become central, but foundational puzzles—most notably the measurement problem and nonlocal correlations—prompted scrutiny. The EPR paper targeted the question of whether the quantum state provides a complete description of physical reality and invoked notions of locality and reality that were integral to classical relativity and statistical mechanics traditions.
The paper was authored by Albert Einstein, then at the Institute for Advanced Study (though Einstein's institutional moves are complex), Boris Podolsky, a young theoretical physicist, and Nathan Rosen, who had collaborated with Einstein on gravitational and quantum problems. It was submitted to and published in the journal Physical Review in 1935. The paper formed part of a broader exchange between Einstein and Bohr; Bohr subsequently published a response defending the Copenhagen viewpoint. The EPR paper became a canonical reference in discussions involving foundations of quantum mechanics, philosophy of science, and later in operational and information-theoretic studies by groups at institutions such as Copenhagen University and Harvard University.
EPR constructed a thought experiment using a pair of particles prepared in a correlated quantum state now recognized as an entangled state. They considered measurements of two noncommuting observables (e.g., position and momentum) on spatially separated subsystems. By assuming locality—that operations performed on one system cannot instantaneously influence a distant system—and a criterion for "elements of reality" (if one can predict with certainty the value of a physical quantity without disturbing the system, then there exists an element of reality corresponding to that quantity), the authors argued that both position and momentum for a distant particle would be elements of reality. Since quantum mechanics forbids simultaneous eigenstates of noncommuting observables, they concluded that the quantum-mechanical description must be incomplete: there should exist additional variables (often referred to later as hidden variables) that supplement the wave function to restore completeness and possibly determinism.
The EPR argument forced clarification of terms like completeness, realism, and locality within quantum theory. It prompted defenders of the Copenhagen interpretation, especially Niels Bohr, to respond that the EPR criterion of reality misapplied the quantum formalism by ignoring the holistic role of experimental arrangement and complementarity. The paper stimulated exploration of alternative frameworks including hidden variable theories and reforms of the measurement postulates. It also deepened interest in entanglement as a physically significant resource rather than a mere mathematical curiosity, influencing later formalizations in quantum information theory and foundational analyses by figures such as Erwin Schrödinger (who coined "entanglement") and John von Neumann.
In subsequent decades, David Bohm reformulated the EPR argument in terms of spin variables and developed the de Broglie–Bohm theory (pilot-wave theory), a deterministic nonlocal hidden-variable model reproducing quantum predictions. Bohm's version made the puzzle more accessible and connected it to concrete models. The critical breakthrough came with John Stewart Bell's 1964 theorem, which derived quantitative inequalities (now called Bell's theorem and Bell inequalities) that any local hidden-variable theory must satisfy. Bell showed that predictions of quantum mechanics—and therefore certain experimental statistics—can violate these inequalities, demonstrating that no local realistic completion of quantum mechanics can reproduce all quantum predictions. Bell's work linked the EPR philosophical challenge to experimentally testable criteria and motivated analysis by researchers at CERN, Bell Labs, and universities worldwide.
Starting in the 1970s and accelerating in the 1980s, experimental tests by Alain Aspect and collaborators, and later loophole-closing experiments by groups including Anton Zeilinger's and John Clauser's teams, measured correlations in entangled photon and atomic systems and observed violations of Bell inequalities consistent with quantum mechanics. Experiments addressed locality, detection-efficiency, and freedom-of-choice loopholes; contemporary tests are increasingly sophisticated, using techniques from quantum optics, atomic physics, and superconducting qubits. The EPR paper's legacy extends into quantum information science—notably quantum teleportation, quantum cryptography, and entanglement theory—and into ongoing foundational debates over realism, locality, and the nature of quantum states. The EPR thought experiment remains a central historical and conceptual touchstone for discussions about the meaning and scope of quantum theory.
Category:Quantum mechanics Category:1935 works Category:Philosophy of physics