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1 Causality

The property of causalityAlthough causality the relationship between causes and effects, is often examined in the fields of philosophy, computer science, and statistics, it has a place in the study of physics as well. In classical physics, it was assumed that all events are cause, a fundamental principle of theoreticalTheoretical physics attempts to understand the world by making a model of reality, used for rationalizing, explaining, predicting physical phenomena through a physical theory . There are three types of theories in physics; mainstream theories, proposed th particle physicsQuantum field theory (QFT) is the application of quantum mechanics to fields. It provides a theoretical framework widely used in particle physics and condensed matter physics. In particular, the quantum theory of the electromagnetic field, known as quantu, poses a problem for the physical existence of tachyons. If a tachyon were to exist and were allowed to interact with ordinary ( time-likeIn the context of special relativity, time-like separated points (or events) in spacetime have a spacetime interval greater than 0 (see sign convention). This means that the events can be causally connected (i. one can be a cause or effect of the other) b) matter, causality could be violated: roughly, there would no longer be a way to tell the difference between the future and the past along the worldline of a given piece of ordinary matter. A particle could send energy or information into its own past, forming a so-called causal loop. This would lead to logical paradoxFor other meanings of "Paradox", see Paradox (disambiguation). Robert Boyle's self-flowing flask fills itself in this diagram, but perpetual motion machines don't exist. A paradox is an apparently true statement or group of statements that seems to lead tes such as the grandfather paradoxThis article refers to the time travel paradox. In novels based on the television series Doctor Who, "Grandfather Paradox" is the semi-mythical founder of Faction Paradox. The grandfather paradox is a paradox of time travel, supposedly first conceived by, unless the theory was set up in such a way as to prevent them. At present such a fix is not known: for example, the Novikov self-consistency principle has not been obtained within a quantum field theory, but has to be imposed by hand. At the very least the principle of special relativity would have to be discarded.

In the theory of general relativity, it is possible to construct spacetimes in which particles travel faster than the speed of light, relative to a distant observer. One example is the Alcubierre metric. However, these are not tachyons in the above sense, as they do not exceed the speed of light locally.