Causality (physics)
Although causality, the relationship between causes and effects, is often examined in the fields of philosophy, computer science, statistics, it has a place in the study of physics as well.
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 by extending 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.
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.
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.
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