Quantum Mechanical indeterminacy, or often just "quantum indeterminacy" refers to the same fundamental physics phenomenon as does the more frequently used Heisenberg uncertainty principle.

Quantum uncertainty is usually described in terms of a particle with a definitely measured momentum for which there must be a fundamental limit to how precisely its location can be specified. This quantum uncertainty principle can be expressed in terms of other variables, for example, a particle with a definitely measured energy has a fundamental limit to how precisely one can specify how long it will have that energy. The units involved in quantum uncertainty are on the order of Planck's constant (found experimentally to be 6.6 x 10^-34 J·s).

Quantum indeterminacy is usually mentioned when one is concerned with the predictability of events. For example, predictability explicitly arises in the context of the type of determinism that has been called Scientific determinism. Some philosophers have tried to identify the basic types of indeterminacy that underly the inability of humans to predict the future. Four types of indeterminacy are:

• quantum indeterminacy
• indeterminacy due to chaos as described in chaos theory
• indeterminacy caused by limited powers of observation
• limitations due to the nature of human memory and thought processes

Within most interpretations of quantum mechanics, it is fundamentally unavoidable. The existence of quantum indeterminacy was deeply troubling to Albert Einstein who proposed hidden variable theory to address what he considered to be a defect. These theories have since proven problematic.

Some physicists and philosophers have suggested that the human experience of free will may involve quantum indeterminacy. In his book Elemental Mind , Nick Herbert explores the idea that our minds exercise free will by modulating which quantum mechanical possibilities actually occur. Herbert traces the development of this idea through the work of John von Neumann, Eugene Wigner, Henry Stapp, Roger Penrose, and John Eccles. Other philosophers such as Daniel Dennett have argued against this view, suggesting that quantum indeterminacy cannot constructively contribute to the sense of one's control of human behavior, an aspect that seems fundamental to our experience of free will. Dennett's position was given in his book Elbow Room.

However one major difficulty in arguments that free will arises from quantum indeterminacy has been the lack of an agreed physical mechanism by which quantum effects can affect cognition. The temperature, time, and length scales of the human brain are far above the temperatures and scales at which quantum decoherence occur making it questionable whether quantum mechanics plays any functional role in the brain at all. To deal with this problem, some physicists most notably Roger Penrose argue that microtubules rather than neuron and synapses are important in cognition, a position that most neurologists find absurd.

See also

• Mind-body problem
• Quantum mind