Tachyons and superluminal phenomena

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The paper explores the existence and implications of superluminal phenomena, specifically tachyons, within the context of special relativity and experimental physics. It traces the historical understanding of superluminal speeds, highlighting significant theoretical insights and experimental evidence that may suggest their reality. The authors provide a succinct overview of the current state of research on superluminal speeds and discuss notable experiments that lend credence to the theoretical predictions of faster-than-light particles.

Figures (8)

Fig. 1. Energy of a free object as a function of its speed.(?*)

Fig. 3. Behaviour of the average “penetration time” (in seconds) spent by a tunnell- ing wavepacket, as a function of the penetration depth (in angstroms) down a potential barrier (from Olkhovsky et al., Ref. 12). According to the predictions of quantum mechanics, the wavepacket speed inside the barrier increases in an unlimited way for opaque barriers; and the total tunnelling time does not depend on the barrier width”. Superluminal Motions? A Birds-Eye View of the Experimental Situation

Fig. 5. The very interesting experiment along a metallic waveguide with TWO barriers (undersized guide segments), i.c., with two evanescence regions.'°”) See the text. Let us emphasize that the most interesting experiment of this series is the one with two “barriers” (e.g., with two segments of undersized waveguide separated by a piece of normal-sized waveguide: Fig. 5). For suitable fre- quency bands—e., for “tunnelling” far from resonances—, it was found that the total crossing time does not depend on the length of the inter- mediate (normal) guide: namely, that the beam speed along it is infinite.” This agrees with what predicted by Quantum Mechanics for the non-reso- nant tunnelling through two successive opaque barriers (the tunnelling

Recami Fig. 4. Simulation of tunnelling by experiments with evanescent classi- cal waves (see the text), which were predicted to be Superluminal also on the basis of Extended Relativity.°*) The figure shows one of the measurement results in Ref. 15; that is, the average beam speed while crossing the evanescent region (=segment of undersized waveguide, or “barrier”) as a function of its length. As theoretically predicted,“ '”) such an average speed exceeds c for long enough “barriers.”

Fig. 6. An intrinsically spherical (or pointlike, at the limit) object appears in the vacuum as an ellipsoid contracted along the motion direction when endowed with a speed uv <c. By contrast, if endowed with a speed V>c (even if the c-speed barrier cannot be crossed, neither from the left nor from the right), it would appear“) no longer as a particle, but rather as an “X-shaped” wave) travelling rigidly (namely, as occupying the region delimited by a double cone and a two-sheeted hyperboloid—or as a double cone, at the limit—, moving Superluminally and without distortion in the vacuum, or in a homoge- neous medium). Superluminal Motions? A Birds-Eye View of the Experimental Situation

Fig. 7. Here we show the intersections of an “X-shaped wave”) with planes orthogonal to its motion line, according to Extended Relativity.° The examination of this figure suggests how to construct a simple dynamic antenna for generating such localized Superluminal waves (an antenna which was in fact adopted, independently, by Lu et al.°® for the production of such non-dispersive beams).

Fig. 8. Theoretical prediction of the Superluminal localized “X-shaped” waves for the electromagnetic case (from Lu, Greenleaf and Recami,°”) and Recami'”’). It is more important for us that the X-shaped waves have been in sffect produced in experiments both with acoustic and with electromagnetic waves; that is, X-beams were produced that, in their medium, travel undistorted with a speed larger than sound, in the first case, and than light, in the second case. In acoustics, the first experiment was performed by Lu et al. themselves@® in 1992, at the Mayo Clinic (and their papers received the 1992 IEEE first award). In the electromagnetic case, certainly more “intriguing,” Superluminal localized X-shaped solutions were first mathe- matically constructed (cf. e.g., Fig. 8) in Refs. 37, and later on experimentally produced by Saari et al.°* in 1997 at Tartu by visible light (Fig. 9), and recently by Mugnai, Ranfagni and Ruggeri at Florence by microwaves”?

Fig. 9. Scheme of the experiment by Saari et a/., who announced (PRL of 24 Nov. 1997) the production in optics of the beams depicted in Fig. 8: In this figure one can see what shown by the experiment, ie., that the Superluminal “X-shaped” waves run after and catch up with the plane waves (the latter regularly travelling with speed c). An analogous experiment has been performed with microwaves at Florence by Mugnai, Ranfagni and Ruggeri (PRL of 22 May 2000). Superluminal Motions? A Birds-Eye View of the Experimental Situation