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Microwave and Optical Technology Letters

Observing and interpreting superluminal behaviors in microwave and optical experiments

Ilaria Cacciari, Daniela Mugnai, Anedio Ranfagni, Andrea Petrucci

Published Jan 14, 2020• 2 citations
Publisher

Abstract

Abstract Some typical cases of tunneling and near‐field propagation experiments, carried out mainly in the microwave, but also in the optical range, giving rise to superluminal behavior, are re‐examined. The several types of superluminal behavior are classified and an attempt is made searching for a unified interpretation. Within this context, the role played by the “weak measurement” theory is also evidenced.

DOI
10.1002/mop.32250

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Abstract Some typical cases of tunneling and near‐field propagation experiments, carried out mainly in the microwave, but also in the optical range, giving rise to superluminal behavior, are re‐examined. The several types of superluminal behavior are classified and an attempt is made searching for a unified interpretation. Within this context, the role played by the “weak measurement” theory is also evidenced.
Since 1992 experimental evidence of superluminal (faster than light, FTL) signals are causing much excitement in the physical community and in the media. Superluminal signal velocity and zero time tunneling was first observed in an analog tunneling experiment with microwaves. Recently, the conjectured zero time of electron was claimed to be observed in ionizing helium. The FTL signal velocity was reproduced with infrared light and with various tunneling barriers in several laboratories worldwide. Remarkable, it was shown that the tunneling time is a universal quantity for elastic and for electromagnetic fields. Many theoretical physicists predicted this FTL nature of the tunneling process. However, even with this background many members of the physics community did not accept the superluminal signal velocity interpretation of the experimental results and they also ignored the universal tunneling time. The predicted and measured zero tunneling time was taken as a fantastic nonsense. A brief explanation of the misleading theoretical approaches against FTL signals due to the tunneling process is presented eventually.
Abstract Contrary to mechanical waves, the two‐slit interference experiment of single photons shows that the behavior of classical electromagnetic waves corresponds to the quantum mechanical one of single photons, which is also different from the quantum‐field‐theory behavior such as the creations and annihilations of photons, the vacuum fluctuations, etc. Owing to a purely quantum effect, quantum tunneling particles including tunneling photons (evanescent modes) can propagate over a spacelike interval. With this picture we conclude that the superluminality of evanescent modes is a quantum mechanical rather than a classical phenomenon.
We exploit the analogy between the transfer of a pulse across a scattering medium and Aharonov's weak measurements to resolve the long standing paradox between the impossibility to exceed the speed of light and the seemingly ``superluminal'' behavior of a tunneling particle in the barrier or a photon in a ``fast-light'' medium. We demonstrate that superluminality occurs when the value of the duration $\ensuremath{\tau}$ spent in the barrier is uncertain, whereas when $\ensuremath{\tau}$ is known accurately, no superluminal behavior is observed. In all cases only subluminal durations contribute to the transmission which precludes faster-than-light information transfer, as observed in a recent experiment.
Abstract The observation of superluminal transmission of photons created discussions of their reality and significance. To analyze the possibility of such phenomena in general, the most general method is employed based on the covariant theory of dispersion relations. It is shown that superluminal phenomena can only be realized as tunneling of excitations within the near field zone. On this basis, the main experimental data can be described as sequential processes of single photon scattering. Their temporal distribution is estimated by taking into account the scattering time. Thus, the difference is explained between the advance of the superluminal part of the light pulse (i.e., the transition time of single photons) and the signal velocity of the macroscopic pulse of light.