unphysical

Unphysical is an adjective used in science and philosophy to describe ideas, models, quantities, or results that do not correspond to entities or phenomena that can exist, be observed, or be measured in the real world. In common usage, something unphysical fails to satisfy the empirical or theoretical constraints that govern physical reality, such as conservation laws, causality, or gauge invariance. The term often signals that a construct is intended as a formal tool, a limiting case, or a mathematical artifact rather than a physically realizable object.

In physics, unphysical elements frequently arise from mathematical formalisms that include redundancies or idealizations. For example, gauge theories introduce degrees of freedom that do not correspond to observable states; certain polarization components of gauge bosons can be gauge artifacts and are classified as unphysical because they cannot be measured. Ghost fields in quantization are likewise unphysical but are needed for consistency. When performing calculations, physicists typically apply constraints or gauge fixing to isolate the physical subspace and ensure that unphysical contributions cancel in observable quantities.

In mathematics and philosophy of science, an unphysical solution is a mathematical solution to a problem that violates the problem’s constraints or would require nonrealizable conditions. Distinguishing physical from unphysical results helps prevent misinterpretation and guides the formulation of theories toward empirically testable predictions.

Overall, unphysical describes not the abstract truth of a model but its relation to the real, observable world, and it often guides the refinement of theories to exclude nonrealizable aspects.