The elusive tachyon

Tachyon (from the greek ταχύς, tachýs, "fast") is a hypothetical particle having imaginary mass and faster than the speed of light.
The first theoretical -conceptual description is attributed to Arnold Sommerfeld, while attempts at interpretation within the relativity were made by George Sudarshan in 1962. The term " tachyon " was used for the first time in 1964 by Gerald Feinberg. Research in physics modern concept appears in various contexts, in particular in string theory.
The hypothetical existence of Tachyon is compatible with the special theory of relativity, according to which he could never slow down the speed of light or inferior to it.
If its energy and momentum are real , the rest mass is imaginary, or if the rest mass and momentum were real , the energy would be imaginary. It is difficult to interpret the physical meaning of a mass of complex value. A curious effect is that, unlike the ordinary particles, the speed of a tachyon increases with a decrease of its energy. This is a consequence of relativity as tachyon, in theory, has a mass which squared is negative.
If tachyons are particles faster than light, the common particles are called " bradions " or " tardions ." Similarly where for bradions is impossible to overcome the barrier of the speed of light, the same applies to tachyons, who may not have speeds lower than that of light.
The existence of such particles pose interesting problems of modern physics. For example, consider the formulas of electromagnetic radiation and assume that a tachyon had an electric charge (we can not say a priori whether a tachyon is neutral or equipped with charge); then a tachyon in acceleration should generate electromagnetic waves, as any particle with charge. But, as we have seen, decreasing the energy of a tachyon increases the speed and hence in such a situation a small acceleration would produce a greater, leading to a chain reaction similar to the ultraviolet catastrophe.
In 1973, Philip Crough and Roger Clay announced a particle faster than light apparently due to a wave of cosmic rays (observation has been neither confirmed nor repeated).
The property of causality, a fundamental principle of particle physics, poses a problem for the physical existence of tachyons. If a tachyon existed and could interact with ordinary matter, causality could be violated: in broad terms, there would be no way to tell the difference between the future and the past along the line of the events of a given amount of ordinary matter. A particle could send energy or information in his past, forming a so-called causal loop. This would lead to logical paradoxes such as the grandfather paradox, unless the theory is not set up to prevent them. Currently, such a solution is not known. As a minimum the principle of special relativity should be abandoned. According to the theory of general relativity, however, it is possible to construct models of spacetime in which some particles traveling faster than light relative to a distant observer. However, these are not tachyons as the previous, as locally do not exceed the speed of light.
Another possibility to solve the causal paradoxes, proposed at the time by David Bohm, is to require the existence of a privileged frame of reference in which you do not ever observe signals that move back in his time on (at this point a kind of universal time in the broad sense). In this way it is impossible to create time loops, in any reference system. The apparent motion back in time some signals would in that case only a kind of optical effect. In the system of reference, there is no motion back in time.
In quantum field theory, a tachyon is a quantum of a field, usually a scalar field, whose mass squared is negative (ie is expressed by an imaginary number). The existence of such a particle involves the instability of space-time vacuum, since the energy of the vacuum exhibits a maximum rather than a minimum (at least with respect to the direction of tachyon). A pulse very small (even a normal quantum fluctuations) will lead the field to collapse, with an exponential growth of heights, causing the tachyon condensation. The Higgs mechanism is an elementary example, but important to understand that once the tachyon field has reached the minimum potential, that has undergone the process of condensation, its not how many more are tachyons but Higgs bosons, which have mass positive.
Tachyons appear in many versions of string theory. In general, string theory says that what we see as "particles" (electrons, photons, gravitons, and so on) are actually different vibration modes of the same basic structures, strings. The mass of a particle can be deduced from the vibrations of the string as saying that the mass depends on the "notes" played by the string. Tachyons frequently appear in the spectrum of possible states of the strings in the sense that some states have imaginary mass; an example is the ground state of the bosonic string.
The tachyon is often cited in the literature of science fiction, although usually with properties corresponding to those not entirely scientific.
Because of their mysterious nature - particularly of the feature to move faster than the speed of light - tachyons have often excited the imagination of science fiction writers. In many science fiction stories tachyons are used as a means of making communication faster than light, with or without reference to the results of randomness; sometimes they are part of some technology for time travel. Quotes of the tachyon are in fact in many science fiction series Star Trek, in the comics or novels, like Timescape, doors or Anubis The hem of the Foundation of I. Asimov.

Graphical representation of a tachyon.
Since a tachyon moves faster than ligh, we can not see it coming. After we passed, we should see two images appear, that separate and move away in opposite directions. The black line is the front of the shock wave of Cherenkov radiation, shown only in an instant. This double image effect is more obvious to an observer that is along the path of an object faster than light. The right image is formed by light Doppler shifted toward the blue that reaches the observer placed at the top of the wave of Cerenkov radiation; the left image is formed by the light shifted due to Doppler effect towards the red that leaves the object after the latter has passed the observer.