Causality describes the relationship between causes and effects, and is fundamental to all natural science, especially physics. It is also studied from the perspectives of philosophy, computer science, and statistics.
In classical physics, it was assumed that all events are caused by earlier ones according to the known laws of nature, culminating in Pierre-Simon Laplace's claim that if the current state of the world would be known with precision, it could be computed for any time in the future. This is known as determinism (see Causal determinism).
According to classical physics, the cause simply had to precede its effect. In modern physics, the notion of causality had to be clarified.
The insights of the theory of special relativity confirmed the assumption of causality, but they made the meaning of the word "precede" observer-dependent. Consequently, the relativistic principle of causality says that the cause must precede its effect according to all inertial observers. This is equivalent to the statement that the cause and its effect are separated by a timelike interval, and the effect belongs to the future light cone of its cause. Equivalently, special relativity has shown that it is not only impossible to influence the past; it is also impossible to influence distant objects by signals that are superluminal (faster than light).
In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the spacetime is curved. New subtleties must be taken into account when we investigate causality in quantum mechanics and relativistic quantum field theory in particular. In quantum field theory, causality is closely related to the principle of locality. A careful analysis of the phenomena is needed, and the outcome slightly depends on the chosen interpretation of quantum mechanics: this is especially the case of the experiments involving quantum entanglement that require Bell's Theorem for their implications to be fully understood.
Despite these subtleties, causality remains an important and valid concept in physical theories. For example, the notion that events can be ordered into causes and effects is necessary to prevent paradoxes such as the grandfather paradox, which asks what happens if a time-traveller kills his own grandfather before he ever meets his grandmother. See also Chronology protection conjecture.
In physics, two entities are said to be in causal contact if there may be an event that has affected both in a causal way. Most things we deal with on a daily basis are in causal contact. For example, the reader and string theorist Edward Witten are in causal contact as you both could have potentially been affected by Wikipedia's article on the Antarctic Treaty System.
The only objects that are not in causal contact (according to accepted physics) are those for which there is no event in the history of the universe that could have sent a beam of light to both. For example, if the universe were not expanding and had existed for 13 billion years, anything more than 26 billion light-years away from the earth would not be in causal contact with it. Anything less than 26 billion light-years away would because an event occurring 13 billion years in the past that was 13 billion light-years away from both the earth and the object under question could have affected both (perhaps an alien shining a flashlight in both directions).
The basis of physical causality is energetic processes governed by the conservation of energy.
See particle horizon.