Editing Tachyon (section)

=== Superluminal information ===
[[File:Faster than light implies time travel diagram.svg|thumb|upright=1|[[Spacetime diagram]] showing that moving faster than light implies time travel in the context of special relativity. A spaceship departs from Earth from A to C slower than light. At B, Earth emits a tachyon, which travels faster than light but forward in time in Earth's reference frame. It reaches the spaceship at C. The spaceship then sends another tachyon back to Earth from C to D. This tachyon also travels forward in time in the spaceship's reference frame. This effectively allows Earth to send a signal from B to D, back in time.]]

If tachyons can transmit information faster than light, then, according to relativity, they violate causality, leading to logical paradoxes of the [[Grandfather paradox|"kill your own grandfather"]] type. This is often illustrated with thought experiments such as the "tachyon telephone paradox"<ref name="Benford" /> or "logically pernicious self-inhibitor."<ref name="Fitzgerald" />

The problem can be understood in terms of the [[relativity of simultaneity]] in special relativity, which says that different [[Inertial frame of reference|inertial reference frames]] will disagree on whether two events at different locations happened "at the same time" or not, and they can also disagree on the order of the two events. (Technically, these disagreements occur when the [[Spacetime#Spacetime intervals|spacetime interval]] between the events is 'space-like', meaning that neither event lies in the future [[light cone]] of the other.)<ref name="Jarrell" />

If one of the two events represents the sending of a signal from one location and the second event represents the reception of the same signal at another location, then, as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity ensures that all reference frames agree that the transmission-event happened before the reception-event.<ref name="Jarrell" /> However, in the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received before it was sent, so that the signal could be said to have moved backward in time. Because one of the two fundamental [[postulates of special relativity]] says that the laws of physics should work the same way in every inertial frame, if it is possible for signals to move backward in time in any one frame, it must be possible in all frames. This means that if observer A sends a signal to observer B which moves faster than light in A's frame but backwards in time in B's frame, and then B sends a reply which moves faster than light in B's frame but backwards in time in A's frame, it could work out that A receives the reply before sending the original signal, challenging causality in ''every'' frame and opening the door to severe logical paradoxes.<ref name="Gron" /> This is known as the [[tachyonic antitelephone]].

==== Reinterpretation principle ====
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The '''reinterpretation principle'''<ref name="Feinberg 1967-1969" /><ref name="sudarshan62" /><ref name="Gron" /> asserts that a tachyon sent ''back'' in time can always be ''reinterpreted'' as a tachyon traveling ''forward'' in time, because observers cannot distinguish between the emission and absorption of tachyons. The attempt to ''detect'' a tachyon ''from'' the future (and violate causality) would actually ''create'' the same tachyon and send it ''forward'' in time (which is causal).

However, this principle is not widely accepted as resolving the paradoxes.<ref name="Benford" /><ref name="Gron" /><ref name="Recami" /> Instead, what would be required to avoid paradoxes is that, unlike any known particle, tachyons do not interact in any way and can never be detected or observed, because otherwise a tachyon beam could be modulated and used to create an anti-telephone<ref name="Benford" /> or a "logically pernicious self-inhibitor".<ref name="Fitzgerald" /> All forms of energy are positted to interact at least gravitationally, and many authors state that superluminal propagation in Lorentz invariant theories always leads to causal paradoxes.<ref name="Barcelo" /><ref name="Arkani" />