[Paper Review] A homage to E.C.G.Sudarshan: Superluminal objects and waves (An updated overview of the relevant experiments)

Erasmo Recami|ArXiv.org|Apr 9, 2008

Quantum Mechanics and Applications3 citations

TL;DR

This paper presents an updated experimental overview of superluminal phenomena, focusing on tachyons and localized superluminal waves (SLS), such as X-shaped pulses, as predicted by extended special relativity. It reviews experiments with evanescent waves, photonic bandgap structures, and microwave/optical setups demonstrating group velocities exceeding c, with tunnelling times independent of barrier width—confirming the generalized Hartman effect and supporting Sudarshan's early theoretical framework on faster-than-light motion.

ABSTRACT

This writing has been prepared on the occasion of the 75th birthday of E.C.George Sudarshan, who (besides the originator of the V-A theory for weak-forces, of quantum optics --the quantum representation of coherent light--, of dynamical maps for open quantum systems, of the Zeno effect, etc.) was a pioneer, already in the sisties, also of the theory of the so-called "tachyons". This paper wishes to be a homage to E.C.G.Sudarshan, in connection with the last-mentioned pioneering work of his. After a brief theoretical introduction (based on the standard postulates of Special Relativity, and therefore extending it --or rather non-restricting it-- without any violations: for example, without any violations of the so-called Einstein causality), the main aim of this article is an updated presentation of the status-of-the-art of the "superluminal" experiments. In particular, we devote such a review to the phenomena met in tunneling through quantum (and classical) barriers [e.g., referring to the Genelalized Hartman Effect]; and in connection with the "Localized (nondiffracting) Solutions" of the wave equantions [e.g., of Maxwell equations]: especially of the "X-shaped" ones. The interested reader is provided with an extended Bibliography.

Motivation & Objective

• To review and update experimental evidence for superluminal group velocities in classical and quantum systems.
• To validate theoretical predictions of superluminal localized solutions (SLS), such as X-shaped waves, in electromagnetic and optical systems.
• To demonstrate the persistence of superluminal signal propagation in tunnelling through double-barrier structures, independent of barrier separation.
• To reconcile apparent superluminal motion with relativity via extended relativity, including tachyons and retrocausal interpretations.
• To counter pseudoscientific misuse of 'tachyon' terminology by grounding the concept in rigorous experimental and theoretical physics.

Proposed method

• Analysis of experimental data from microwave and optical waveguides using evanescent waves and photonic bandgap structures (e.g., FBGs) to probe tunnelling times.
• Numerical simulation of wave propagation using Maxwell’s equations to model superluminal pulse transmission through classical barriers.
• Use of group delay measurements to determine tunnelling times in double-barrier configurations, comparing with theoretical predictions.
• Construction and analysis of X-shaped localized solutions (SLS) to the wave equation, derived from relativistic field theory and extended relativity.
• Comparison of experimental results—such as pulse advancement and amplitude preservation—with theoretical models of superluminal wavepackets.
• Use of time-domain measurements in setups like metallic waveguides and optical fibers to observe superluminal signal propagation without distortion.

Experimental results

Research questions

• RQ1 Can superluminal group velocities be experimentally observed in classical wave systems such as evanescent waves and photonic bandgap structures?
• RQ2 Do tunnelling times through double barriers remain constant as barrier separation increases, confirming the generalized Hartman effect?
• RQ3 Can localized superluminal solutions (SLS), such as X-shaped waves, be generated and observed in optical and microwave experiments?
• RQ4 How do superluminal wavepackets preserve their shape and amplitude during propagation through barriers, despite energy attenuation?
• RQ5 Is the apparent superluminal motion consistent with relativistic causality, and can it be reconciled with extended relativity theory?

Key findings

• Experiments with double-barrier photonic structures (e.g., FBGs) show that tunnelling time remains constant regardless of barrier separation, confirming the generalized Hartman effect.
• In microwave and optical setups, superluminal X-shaped waves were generated and observed, with group velocities up to 5c, as predicted by extended relativity.
• Pulse shape preservation during tunnelling—despite significant amplitude attenuation—was confirmed, suggesting potential for information transmission via Morse-like encoding.
• Numerical simulations based on Maxwell’s equations reproduced superluminal signal propagation with zero time delay, indicating non-localized but rigidly propagating waveforms.
• Theoretical models confirm that superluminal charges in vacuum do not radiate energy, and their electric fields form two-sheeted hyperboloids, distinct from Cherenkov radiation.
• Experiments by Saari et al. and Ranfagni et al. successfully demonstrated X-shaped waves catching up with and overtaking co-propagating light-speed plane waves in vacuum and homogeneous media.