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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).
In modern physics, this notion has largely been abandoned. The discoveries leading to the theory of special relativity challenged the notion of an absolute measure of time, making it more difficult (and sometimes impossible) to state that some event A happened "before" another event B. The principle of locality is essentially an attempt to reclaim the sense of time ordering lost in the process, but this concept has in turn been challenged by developments in quantum physics. In particular, Bell's Theorem complicates the physical notion of causality. If the currently-accepted interpretation of the experimental tests is correct (the various loopholes being inoperative) it extends the set of events which can affect a physical measurement to a practically unmeasureable universe. Furthermore, all statements of quantum mechanics about observable events are probabilistic in nature, so that an absolute connection between a cause and an effect can never exist. See also Quantum entanglement.
Despite these difficulties, causality remains an important 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 conjectureThe chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel (" closed timelike curves") on all but sub-microscopic scales. In a 1992 paper, Hawking uses the me.
Within special relativity, causality can be preserved by forbidding information from travelling faster than the speed of light. It is strongly suspected that general relativity also preserves causality and forbids agents from changing the past, although this has not been rigorously demonstrated.
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 theoristA string theory is a physical model whose fundamental building blocks are one-dimensional extended objects (strings) rather than the zero-dimensional points (particles) that were the basis of most earlier physics. For this reason, string theories are able Edward WittenEdward Witten (born August 26, 1951) is a professor at the Institute for Advanced Study, near Princeton University. He was born in Baltimore, Maryland, son of a physicist specializing in gravitation and general relativity. Edward Witten is a mathematical are in causal contact as you both could have potentially been affected by Wikipedia's article on the Antarctic Treaty SystemThe Antarctic Treaty and related agreements, collectively called the Antarctic Treaty System or ATS regulate the international relations with respect to Antarctica, Earth's only uninhabited continent. For the purposes of the treaty system, Antarctica is d.
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 seeing as an event occurring 13 billion light 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).
See particle horizonThe particle horizon in cosmology is the distance from which particles (of positive mass or of zero mass) can have travelled to the observer in the age of the Universe. It is different from the event horizon. The particle horizon is defined as the largest.