Unphysical

In the philosophy of physics, the term unphysical means a physical prediction or entity that is in contradiction to rational understanding of the physical world. In his Oxford Handbook of Philosophy of Physics the philosopher Robert W. Batterman uses the term "defying our antecedent expectations as to what ought to happen".^[1]

Common examples of this include equations with singularities,^[2] physical quantities with infinite values, or events with negative probabilities or infinite probabilities.^[3]

Norton's taxonomy

In a 2006 paper, the philosopher of physics John D. Norton has extensively examined the concept of what it means for something to be unphysical.^[4] Norton states that the term means "cannot obtain in the real world", listing the following possibilities:^[4]

• Unphysical as gauge (over description), in which "a theory admits more structures than are in the world for descriptive convenience"

• Unphysical as false, in which "a theory makes a prediction that turns out to be false and quite far from approximations to the actual"

• Unphysical as pathological, in which "a physical theory is used to generate conclusions that turn out to contradict the original theory"
• Unphysical through under description, in which "a theory may under describe or under constrain a system’s properties, with the outcome that the theory admits solutions that do not apply to the system"

See also

• Ghost (physics)
• Naked singularity
• Non-physical entity
• Unobservable