Reality
Reality is the sum or aggregate of all that is real or existent within the universe, as opposed to that which is only imaginary, nonexistent or nonactual. The term is also used to refer to the ontological status of things, indicating their existence.[1] In physical terms, reality is the totality of a system, known and unknown.[2]
Not to be confused with Realty.Philosophical questions about the nature of reality or existence or being are considered under the rubric of ontology, which is a major branch of metaphysics in the Western philosophical tradition. Ontological questions also feature in diverse branches of philosophy, including the philosophy of science, of religion, of mathematics, and philosophical logic. These include questions about whether only physical objects are real (i.e., physicalism), whether reality is fundamentally immaterial (e.g. idealism), whether hypothetical unobservable entities posited by scientific theories exist, whether a god or gods exist, whether numbers and other abstract objects exist, and whether possible worlds exist. Epistemology is concerned with what can be known or inferred as likely and how, whereby in the modern world emphasis is put on reason, empirical evidence and science as sources and methods to determine or investigate reality.
Physical sciences
Scientific realism
Scientific realism is, at the most general level, the view that the world (the universe) described by science (perhaps ideal science) is the real world, as it is, independent of what we might take it to be. Within philosophy of science, it is often framed as an answer to the question "how is the success of science to be explained?" The debate over what the success of science involves centers primarily on the status of entities that are not directly observable discussed by scientific theories. Generally, those who are scientific realists state that one can make reliable claims about these entities (viz., that they have the same ontological status) as directly observable entities, as opposed to instrumentalism. The most used and studied scientific theories today state more or less the truth.
Realism and locality in physics
Realism in the sense used by physicists does not equate to realism in metaphysics.[42]
The latter is the claim that the world is mind-independent: that even if the results of a measurement do not pre-exist the act of measurement, that does not require that they are the creation of the observer. Furthermore, a mind-independent property does not have to be the value of some physical variable such as position or momentum. A property can be dispositional (or potential), i.e. it can be a tendency: in the way that glass objects tend to break, or are disposed to break, even if they do not actually break. Likewise, the mind-independent properties of quantum systems could consist of a tendency to respond to particular measurements with particular values with ascertainable probability.[43] Such an ontology would be metaphysically realistic, without being realistic in the physicist's sense of "local realism" (which would require that a single value be produced with certainty).
A closely related term is counterfactual definiteness (CFD), used to refer to the claim that one can meaningfully speak of the definiteness of results of measurements that have not been performed (i.e. the ability to assume the existence of objects, and properties of objects, even when they have not been measured).
Local realism is a significant feature of classical mechanics, of general relativity, and of electrodynamics; but quantum mechanics has shown that quantum entanglement is possible. This was rejected by Einstein, who proposed the EPR paradox, but it was subsequently quantified by Bell's inequalities.[44] If Bell's inequalities are violated, either local realism or counterfactual definiteness must be incorrect; but some physicists dispute that experiments have demonstrated Bell's violations, on the grounds that the sub-class of inhomogeneous Bell inequalities has not been tested or due to experimental limitations in the tests. Different interpretations of quantum mechanics violate different parts of local realism and/or counterfactual definiteness.
The transition from "possible" to "actual" is a major topic of quantum physics, with related theories including quantum darwinism.