Hidden VariablesEdit
Hidden variables are a set of theoretical ideas in quantum physics that aim to explain the apparent randomness of quantum events by positing underlying states that determine outcomes. The notion challenges the standard view that quantum phenomena are inherently probabilistic and that the best description of nature is through statistical laws rather than a deeper, determinate reality. The term is most closely associated with attempts to restore a classical sense of reality to the microworld, and it remains a focal point in debates about the foundations of quantum mechanics and the nature of causality itself.
From a traditional, results-first perspective, the appeal of hidden variables lies in offering a coherent picture of an orderly world where effects track their causes, even if those causes are not directly observable in every experimental situation. Proponents have drawn on the historical discomfort with quantum indeterminacy to argue that a more complete theory could exist—one that describes a set of underlying conditions that fully determine measurement outcomes. The controversy is not merely academic: it touches how scientists conceive what counts as an explanation, how much of reality can be described independently of observation, and what kinds of assumptions we are willing to accept about causality and locality. hidden variables quantum mechanics Einstein Niels Bohr
Foundations
The EPR debate and local realism
The debate over hidden variables was sharpened by the 1935 paper of EPR paradox by Einstein, Podolsky, and Rosen, which argued that quantum mechanics might be incomplete if it could not be supplemented by local hidden variables. They insisted that a complete theory should assign definite properties to physical systems prior to measurement, and that information cannot travel faster than light. The EPR challenge was a catalyst for a long-running discussion about locality, realism, and the possibility of an underlying deterministic description that preserves a classical intuition about the world. EPR paradox local realism Copenhagen interpretation
Determinism, locality, and the orthodox view
The mainstream, long-dominant view in physics is that quantum mechanics does not require hidden variables to be a valid description of nature, but rather that probabilities reflect the limits of what we can know about a system given its quantum state. This perspective is associated with interpretations that emphasize the role of measurement and the non-deterministic character of quantum phenomena. Yet determinist intuitions remain influential, and the question of whether there could be a deeper, determinate theory—perhaps with features that make determinism compatible with observed randomness—is far from settled. Copenhagen interpretation local realism hidden variables
Theories of hidden variables
Bohmian mechanics and the price of determinism
One of the best-known recipients of the hidden-variable label is the de Broglie-Bohm theory, commonly called Bohmian mechanics or the de Broglie-Bohm theory. In this view, particles follow precise trajectories guided by a wave function, and the apparent randomness arises from our uncertainty about the exact configuration of hidden variables. Notably, Bohmian mechanics preserves determinism but at the cost of embracing nonlocal connections—instant influences that transcend classical locality. For many, this trade-off is acceptable as a logically coherent alternative to the standard probabilistic picture, while others view nonlocality as a controversial concession. de Broglie-Bohm theory Bohmian mechanics Bell's theorem
Contextual hidden variables and the Kochen-Specker result
Another line of thought considers contextual hidden variables, where the value of a measurement depends on the experimental context rather than being a pre-existing property of the system alone. The Kochen-Specker theorem shows that certain kinds of noncontextual hidden-variable models cannot reproduce the predictions of quantum mechanics in systems with enough dimensionality. This has led some researchers to pursue contextual schemes that might align with observed phenomena while preserving a form of determinism, albeit within a more nuanced framework. Kochen-Specker theorem contextuality hidden variables
Other proposals and interpretations
There have been additional proposals aimed at reconciling determinism with quantum predictions, including ideas that attempt to reconstruct quantum behavior from more classical substrates, such as certain cellular-automaton-inspired notions. While these remain on the fringe relative to mainstream quantum theory, they illustrate the persistent appeal of a deterministic underpinning to the physical world. cellular automaton interpretation t'Hooft quantum mechanics
Experimental status and controversies
Bell’s theorem and the tests of local realism
A central focus of the modern debate is Bell’s theorem, which shows that no local hidden-variable theory can reproduce all the predictions of quantum mechanics. A sequence of experiments testing Bell inequalities has provided strong evidence against local realism in its simplest form, though experimentalists continually scrutinize assumptions and loopholes. Prominent generations of tests have pushed toward closing major loopholes, including those related to detection efficiency and the independence of measurement settings. Bell's theorem Bell test experiments local realism
Loopholes, nonlocality, and the meaning of results
While many experiments have become increasingly robust, some critics have pointed to loopholes that could, in principle, preserve a local-hidden-variable explanation. Scientists argue that as loopholes close, the plausibility of local realism diminishes, making deterministic, local theories harder to defend. Others emphasize that accepting nonlocal determinism (as in Bohmian mechanics) may be scientifically acceptable if it yields a coherent, predictive framework consistent with observed data. loophole nonlocality Bohmian mechanics
Superdeterminism and the limits of experimental testability
A particularly provocative angle is superdeterminism, which posits that the choices of measurement settings are not free but correlated with hidden variables in the system being measured. If true, many standard tests of Bell inequalities would be undermined. Proponents argue it is a logically possible—but empirically challenging—route to reconcile determinism with quantum correlations, while critics worry it risks rendering experimental science untestable. superdeterminism Bell test experiments EPR paradox
Philosophical and practical implications
From a practical, results-driven standpoint, the hidden-variables conversation underscores a core point: theories are judged by their explanatory power and empirical success, not by how neatly they align with intuitive notions of how the world ought to be. For some scientists and thinkers, the appeal of a determinate foundation remains strong because it preserves a sense of objective causality and avoids reliance on probabilistic narratives as the ultimate story of physical reality. For others, the nonlocal and contextual features necessary to make such theories work—when they are workable at all—represent a price worth paying for a theory that aligns with the experimental fabric of quantum phenomena. quantum mechanics Bell's theorem Copenhagen interpretation nonlocality
In discussions about science and society more broadly, proponents of a straightforward, empirical approach argue that the best path forward is to let the data lead, rather than allowing narrative incentives to reshape interpretation. Critics who push a broader political or social agenda into scientific discourse are often charged with conflating epistemology with advocacy, a stance that some see as confusing scientific debates with identity-driven narratives. Advocates of the traditional scientific method contend that objective inquiry—rooted in careful experimentation, transparent reasoning, and reproducible results—should trump attempts to recast physics to fit contemporary social theories. This view holds that the strength of physics lies in its disciplined, evidence-based pursuit of truth, independent of fashionable ideas about which worldview is most palatable. EPR paradox Copenhagen interpretation local realism hidden variables