Causality (physics)

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.

Causal contact

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.

Related Topics:
String theorist - Edward Witten - Antarctic Treaty System

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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).

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