Superluminal motions can occur only for images, e.g., for so-called "light spots," which were considered in 1972 by V.L. Strum suggested the existence of tachyons, which, however, have not been discovered yet. Shortly after the development of Special Relativity, some researchers considered the possibility of superluminal motion. At the end of the 19th century and the beginning of the 20th century, it was shown that a charge that moves at a velocity faster than the speed of light in an optical medium, in particular, in vacuum, gives rise to impact radiation, which later was termed the Vavilov-Cherenkov radiation. Prior to the development of Special Relativity, no restrictions were imposed on the velocity of the motion of particles and material bodies, as well as on energy transfer and signal propagation. It is concluded that some of them appear to be physically acceptable and are statistically favoured with respect to the ortodox ones Then the simplest '' superluminal models'', paying particular attention to the observations which they would give rise to are revie wed and developed. To prepare the ground one starts from a variational principle, introduces the elements of a tachyon mechanics within special relativity, and argues about the expected behaviour of tachyonic objects when interacting (gravitationally, for instance) among themselves or with ordinary matter. At this point it may be, therefore, of some interest to explore the possible alternative models in which actual superluminal motion take place. Especially when confronted with the most recent observations, suggesting complicated expansion patterns, even with possible accelerations. The ortodox models devised to explain the apparent '' superluminal expansions'' observed in astrophysics, and here briefly summarized and discussed together with th experimental data, do not seem to be to much successful. (Author) [ptĬonsiderations about the apparent superluminal expansions in astrophysics Itis concluded that some of them appear to be physically acceptable and are statistically favoured with respect to the orthodox ones. Then the simplest ' Superluminal models' are reviewed and developed, paying particular attention to the observations which they would give rise to. The ground is prepared starting from a variational principle, introducing the elements of a tachyon mechanics within special relativity, and arguing about the expected behaviour of tachyonic objects when interacting (gravitationally, for instance) among themselves or with ordinary matter. At this point it may be, therefore, of some interest to explore the possible alternative models in which actual Superluminal motions take place. The orthodox models devised to explain the apparent ' superluminal expansions' observed in astrophysics - and here briefly summarized and discussed together with the experimental data - do not seem to be too much succesful. International Nuclear Information System (INIS) Considerations about the apparent ' superluminal expansions' in astrophysics
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